Anish Dasgupta, Haoran He, Rushi Gong, Shun-Li Shang, Eric K Zimmerer, Randall J Meyer, Zi-Kui Liu, Michael J Janik, Robert M Rioux. Nature Chemistry 14 (2022) 523-529. Highlighted by C&E News (February 27, 2022)

Abstract: Intermetallic compounds offer unique opportunities for atom-by-atom manipulation of catalytic ensembles through precise stoichiometric control. The (Pd, M, Zn) γ-brass phase enables the controlled synthesis of Pd–M–Pd catalytic sites (M = Zn, Pd, Cu, Ag and Au) isolated in an inert Zn matrix. These multi-atom heteronuclear active sites are catalytically distinct from Pd single atoms and fully coordinated Pd. Here we quantify the unexpectedly large effect that active-site composition (that is, identity of the M atom in Pd–M–Pd sites) has on ethylene selectivity during acetylene semihydrogenation. Subtle stoichiometric control demonstrates that Pd–Pd–Pd sites are active for ethylene hydrogenation, whereas Pd–Zn–Pd sites show no measurable ethylene-to-ethane conversion. Agreement between experimental and density-functional-theory-predicted activities and selectivities demonstrates precise control of Pd–M–Pd active-site composition. This work demonstrates that the diversity and well-defined structure of intermetallics can be used to design active sites assembled with atomic-level precision.

P. Agarwal, D. Jensen, C.-H. Chen, R. M. Rioux, T. Matsoukas. ACS Applied Energy Materials (2022).

L. Qi, W. A. Elliott, J. Wohland, A. Kann, A. K. Kipshagen, T. Tabassum, X. Chen, N. Washton, A. Andersen, R. M. Rioux, S. L. Scott. JACS Au (2022).

J. W. Chang, Y. Mu. A. Armaou, R. M. Rioux. Journal of the American Chemical Society (2022).

Ji Woong Chang, Antonios Armaou, Robert M Rioux. Journal of Physical Chemistry B 125 (2021) 8075-8087. 

Abstract: We utilize a continuous injection approach (CIA) rather than the traditional incremental injection approach (IIA) to deliver ligand (or receptor) to the calorimeter cell to evaluate thermodynamic binding parameters for three common ligand–receptor binding models—single independent, competitive, and two independent binding sites—using isothermal titration calorimetry (ITC). A general mathematical expression for the binding isotherm for any binding stoichiometry under continuous delivery of ligand (or receptor) resulting in an analytical solution for the thermodynamic binding parameters is presented. The advantages of CIA include reduction in experimental time, estimation of thermodynamic binding parameter values, and automation of the experiment since thermodynamic parameters are estimated in situ. We demonstrate the inherent advantages of CIA over IIA for the three binding models. For the single independent site model, we utilized the binding of Ba²⁺ ions to ethylenediaminetetraacetic acid (EDTA), while competitive binding was captured by titration of Ca²⁺ ions into a buffered solution of Ba²⁺ and EDTA. We experimentally simulated a two independent binding site system by injecting Ca²⁺ into a solution of EDTA and 1,3-diaminopropane-N,N,N′,N′-tetraacetic acid (DPTA). The results demonstrate estimation of thermodynamic parameters with greater confidence and simultaneous reduction in the experimental time of 83% and titrating reagent of 50%, as compared to IIA.

Haoran He, Randall J Meyer, Robert M Rioux, Michael J Janik. ACS Catalysis 11 (2021) 11831-11842.

Abstract: Cyclohexene is a chemical intermediate produced through catalytic partial hydrogenation of benzene. Density functional theory calculations and microkinetic modeling (MKM) are used to illustrate that the binding energy of benzene is a predictor of a catalyst’s cyclohexene selectivity. Brønsted–Evans–Polanyi (BEP) and scaling correlations are developed to correlate elementary reaction energetics on 3-fold active metal ensemble sites. Based on thermochemical linear relationships and BEP correlations, only benzene binding energies and H₂ dissociation energies are needed in MKM to predict benzene hydrogenation activity and selectivity. MKM demonstrates that an intermediate benzene binding energy leads to an optimal balance of activity and selectivity toward cyclohexene formation. Uncertainties in the slope and intercept of BEP and scaling relationships, estimated by a Bayesian inference approach, were propagated in the MKM to quantify uncertainty in catalytic performance. Ni₅Ga₃ and Ni₃Ga intermetallic compounds are predicted to be highly selective catalysts for benzene hydrogenation to cyclohexene.

Tianze Xie, Robert M Rioux. Catalysis Today 371 (2021) 29-39. 

Abstract: The alloying of metals with foreign elements is a common approach for manipulating catalytic properties of active sites. For palladium-based catalysts, incorporation of light elements receives far less attention than Pd-metal alloys even though incorporated light element are catalytically significant. Due to their small size, light elements (H, B, C, and O) can occupy octahedral sites in Pd to form an interstitial phase, while palladium adjusts its lattice to accommodate the incorporation of larger light elements (S and P) to form distinct phases. The incorporated light element modifies the electronic structure of palladium, influencing the binding energy of substrates and reactive intermediates. Pd hydride and oxide often form in situ during hydrogenation or oxidation reactions, respectively, representing the (more, less) active phase of the catalyst. The reactive interstitial hydrogen dissolved in Pd tends to hydrogenate reactants completely and unselectively. Pd oxide can be the active phase for oxidation reactions, and the incorporated oxygen or oxygen vacancies may participate in the reaction directly. Incorporated light elements change geometric aspects of Pd ensembles, serving the role of site blocking, ensemble isolation, or serve as an additional active site, endowing Pd with bifunctional character. We review the synthetic approaches for incorporating light elements into the palladium lattice. We summarize the catalytic influence of light element incorporation into the palladium lattice with respect to electronic and geometric modification, in situ incorporated hydrogen and oxygen, and discuss the influence of light element incorporation into Pd on specific reactions (i.e., selective hydrogenation, oxidation, electrocatalysis, and CC cross coupling).

Suprita Jharimune, Ajay A Sathe, Robert M Rioux. Journal of Physical Chemistry C 125 (2021) 12792-12801.

Abstract: Cation exchange reactions in nanoparticles are widely used to synthesize nanostructures that cannot be synthesized via direct colloidal synthesis approaches. Cation exchange in chalcogenide nanocrystals has been utilized to produce a massive library of derivative nanostructures. Understanding the thermodynamic feasibility of these reactions is often based on qualitative considerations (i.e., Pearson’s acid–base theory). In our previous publication [Jharimune, S.; Nano Lett. 2018, 18 (11), 6795−6803], we demonstrated isothermal titration (ITC) is a robust method to quantify the thermodynamics of cation-exchange reactions in nanocrystals and examined the influence of nanocrystal diameter, identity of capping ligands, and temperature. We extend the use of ITC to study the influence of anions accompanying the foreign cation, solvents, and foreign cation identity on the thermodynamics of cation-exchange reaction in CdSe nanocrystals. Results indicate a strong correlation between the identity of the anion and the apparent enthalpy of the reaction, whereby harder anions favor the cation exchange reaction of CdSe with Ag⁺ salts in acetonitrile. When an identical exchange is conducted in a harder solvent, water, the strong impact of anions on the thermodynamics of cation exchange disappears. We examined the exchange of CdSe nanocrystals with Cu²⁺ that has a comparable hardness to the parent cation (Cd²⁺). This study probes the impact and interplay of solvent and accompanying anion with exchanging cation on energetics in cation-exchange reactions.

Kristen A Fichthorn, Zihao Chen, Zhifeng Chen, Robert M Rioux, Myung Jun Kim, Benjamin J Wiley. Langmuir 37 (2021) 4419-4431. 

Abstract: In this feature article, we provide an account of the Langmuir Lecture delivered by Kristen Fichthorn at the Fall 2020 Virtual Meeting of the American Chemical Society. We discuss how multiscale theory and simulations based on first-principles DFT were useful in uncovering the intertwined influences of kinetics and thermodynamics on the shapes of Ag and Cu cubes and nanowires grown in solution. We discuss how Ag nanocubes can form through PVP-modified deposition kinetics and how the addition of chloride to the synthesis can promote thermodynamic cubic shapes for both Ag and Cu. We discuss kinetic factors contributing to nanowire growth: in the case of Ag, we show that high-aspect-ratio nanowires can form as a consequence of Ag atom surface diffusion on the strained surfaces of Marks-like decahedral seeds. On the other hand, solution-phase chloride enhances Cu nanowire growth due to a synergistic interaction between adsorbed chloride and hexadecylamine (HDA), which leaves the {111} nanowire ends virtually bare while the {100} sides are fully covered with HDA. For each of these topics, a synergy between theory and experiment led to significant progress.

Akbar Mahdavi‐Shakib, KB Sravan Kumar, Todd N Whittaker, Tianze Xie, Lars C Grabow, Robert M Rioux, Bert D Chandler. Angewandte Chemie 133 (2021) 7814-7822.

Abstract: H₂ adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H₂ adsorption on Au/TiO₂ catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H₂ adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H₂ adsorption kinetics measured with the BBA show a lower E_app value (23 kJ mol⁻¹) for H₂ adsorption than previously reported from proxy H/D exchange (33 kJ mol⁻¹). We also identify a previously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites, providing new insight into the nature and dynamics of H₂ adsorption at the Au/TiO₂ interface.

Yang Guo, Haoran He, Xu Liu, Zhifeng Chen, Robert M Rioux, Michael J Janik, Phillip E Savage. Chemical Engineering Science 406 (2021) 126853.

Abstract: The activity and selectivity of activated-carbon-supported Ni, Pt, Ru, and Ni-Ru bimetallic catalysts was examined for hydrothermal denitrogenation of indole. The molar yield of pyrrole ring-opening compounds, without an added hydrogen source, are in the order: Ni < Pt < Ni₉₀Ru₁₀ < Ni₇₅Ru₂₅ < Ni₅₀Ru₅₀ ~ Ni₂₅Ru₇₅ ~ Ru. Ru-containing catalysts facilitated production of hydrocarbons (hydrodenitrogenation (HDN) products) when used with added formic acid (hydrogen source). We elucidated catalytic hydrothermal HDN pathways for indole based on the product distributions and the variation of their yields with time. Hydrogenation of indole to indoline is the primary pathway and ring-opening of indoline to form alkyl anilines is faster than forming HDN products (alkyl benzenes). DFT calculations confirmed experimental activity trends, showing Ru is more active than Ni for indole ring opening and for o-toluidine deamination. If no hydrogen source is present, directly breaking the N-C bond in the pyrrole ring is more favorable than breaking the C-N bond with an aromatic carbon. If a H source is provided, the pyrrole ring hydrogenates first, forming indoline, followed by cleavage of the C-N bond.

Prawal PK Agarwal, Devon Jensen, Chien-Hua Chen, Robert M Rioux, Themis Matsoukas. ACS Applied Materials & Interfaces 13 (2021) 6844-6853.

Abstract: The development of an in situ nonthermal plasma technology improved the oxidation and energy release of boron nanoparticles. We reduced the native oxide layer on the surface of boron nanoparticles (70 nm) by treatment in a nonthermal hydrogen plasma, followed by the formation of a passivation barrier by argon plasma-enhanced chemical vapor deposition (PECVD) using perfluorodecalin (C₁₀F₁₈). Both processes occur near room temperature, thus avoiding aggregation and sintering of the nanoparticles. High-resolution transmission electron microscopy (HRTEM), high-angular annular dark-field imaging (HAADF)-scanning TEM (STEM)-energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) demonstrated a significant reduction in surface oxide concentration due to hydrogen plasma treatment and the formation of a 2.5 nm thick passivation coating on the surface due to PECVD treatment. These results correlated with the thermal analysis results, which demonstrated a 19% increase in energy release and an increase in metallic boron content after 120 min of hydrogen plasma treatment and 15 min of PECVD of perfluorodecalin. The PECVD coating provided excellent passivation against air and humidity for 60 days. We conclude in situ nonthermal plasma reduction and passivation lead to the amelioration of energy release characteristics and the storage life of boron nanoparticles, benefits conducive for nanoenergetic applications.

Suprita Jharimune, Rueben Pfukwa, Zhifeng Chen, Justin Anderson, Bert Klumperman, Robert M Rioux. Journal of the American Chemical Society 143 (2021) 184-195.

Abstract: Ag nanocubes (AgNCs) are predominantly synthesized by the polyol method, where the solvent (ethylene glycol) is considered the reducing agent and poly(N-vinylpyrrolidone) (PVP) the shape-directing agent. An experimental phase diagram for the formation of Ag nanocubes as a function of PVP monomer concentration (C_m) and molecular weight (M_w) demonstrated end groups of PVP impact the final Ag product. Measured rates of the initial Ag⁺ reduction at different PVP C_m and M_w confirmed the reducing effect originates from end-groups. PVP with well-defined aldehyde and hydroxyl end groups lead to the formation of Ag nanocubes and nanowires respectively, indicating the faster reducing agent formed kinetically preferred nanowires. We demonstrate PVP end-groups induce initial reduction of Ag⁺ to form seeds followed by autocatalytic reduction of Ag⁺ by ethylene glycol (and not solvent oxidation products) to form Ag nanostructures. The current study enabled a quantitative description of the role of PVP in nanoparticle shape-control and demonstrates a unique opportunity to design nanostructures by combining nanoparticle synthesis with polymer design to introduce specific physicochemical properties.

Rui Li, William A Elliott, R John Clark, James G Sutjianto, Robert M Rioux, Jeremy C Palmer, Jeffrey D Rimer. Physical Chemistry Chemical Physics. 23 (2021) 18610-18617.

Abstract: Interactions between organic molecules and inorganic materials are ubiquitous in many applications and often play significant roles in directing pathways of crystallization. It is frequently debated whether kinetics or thermodynamics plays a more prominent role in the ability of molecular modifiers to impact crystal nucleation and growth processes. In the case of nanoporous zeolites, approaches in rational design often capitalize on the ability of organics, used as either modifiers or structure-directing agents, to markedly impact the physicochemical properties of zeolites. It has been demonstrated for multiple topologies that modifier-zeolite interactions can alter crystal size and morphology, yet few studies have distinguished the roles of thermodynamics and kinetics. We use a combination of calorimetry and molecular modeling to estimate the binding energies of organics on zeolite surfaces and correlate these results with synthetic trends in crystal morphology. Our findings reveal unexpectedly small energies of interaction for a range of modifiers with two zeolite structures, indicating the effect of organics on zeolite crystal surface free energy is minor and kinetic factors most likely govern growth modification.

Linxi Wang, Shyam Deo, Kerry Dooley, Michael J Janik, Robert M Rioux. Chinese Journal of Catalysis. 41 (2020) 951-962.

Abstract: The redox properties of ceria make it suitable as a catalyst or support in oxidation reactions. Ceria-supported transition metal nanoparticles or isolated single atoms provide a metal-support interface that reduces the energy cost to remove interfacial oxygen atoms, providing active oxygen species that can participate in Mars van Krevelen oxidation processes. COoxidation is a key probe reaction to test the reducibility of ceria-supported catalysts and is also practically important in the elimination of CO at relatively low temperatures in various applications. Preferential oxidation of CO (PROX) in excess H₂ controls the CO concentration to ultra-low levels to prevent poisoning of hydrogen oxidation electrocatalysts. The reactivity of catalysts in CO oxidation and selectivity towards CO over H₂ in PROX is dependent on the type and dispersion of metal species, the structural and chemical properties of CeO₂, and the synthetic preparation methods of the catalysts. In this review, we summarize recently published works on catalytic CO oxidation and PROX reactions on ceria-supported metal nanoparticles and single atoms. We summarize the reactivity on different supported metals, and on different CeO₂ surfaces with the same metal. We summarize the most likely reaction mechanisms as suggested by density functional theory calculations. The factors contributing to selectivity towards CO oxidation in PROX reactions on various supported metals are also discussed.

Zhenshu Wang, Aaron Garg, Linxi Wang, Haoran He, Anish Dasgupta, Daniela Zanchet, Michael J Janik, Robert M Rioux, Yuriy Román-Leshkov. ACS Catalysis. 10 (2020) 6763-6770.

Abstract: We demonstrate that atomically thin Pt shells deposited on transition metal carbide or nitride cores induce up to a 4-fold enhancement in C₂H₄selectivity during the partial hydrogenation of acetylene compared with commercial carbon-supported Pt (Pt_comm) nanoparticles. While Pt typically catalyzes the complete hydrogenation of alkynes to alkanes, a catalyst comprising a nominal one monolayer (ML) Pt shell on titanium tungsten nitride cores (Pt/TiWN) is capable of net C₂H₄ generation under industrial front-end reaction conditions featuring a large excess of C₂H₄ and H₂. Microcalorimetry measurements are consistent with a change in the Pt electronic structure that decreases C₂H₄ binding strength, thus increasing partial hydrogenation selectivity. Density functional theory (DFT) calculations and X-ray absorption near edge structure (XANES) both indicate broadening of the Pt d-band and concomitant down-shifting of the d-band center. The ability to control shell coverage and core composition opens up extensive opportunities to modulate the electronic and catalytic properties of noble metal-based catalysts.

Yunwen Zhou, Yanyu Mu, Ming-Feng Hsieh, Bernd Kabius, Carlos Pacheco, Carol Bator, Robert M Rioux, Jeffrey D Rimer. Journal of the American Chemical Society. 142 (2020) 8211-8222.

Abstract: The synthesis of two-dimensional (2D) zeolites has garnered attention due to their superior properties for applications that span catalysis to selective separations. Prior studies of 2D zeolite catalysts demonstrated enhanced mass transport for improved catalyst lifetime and selectivity. Moreover, the significantly higher external surface area of 2D materials allows for reactions of bulky molecules too large to access interior pores. There are relatively few protocols for preparing 2D materials, owing to the difficultly of capping growth in one direction to only a few unit cells. To accomplish this, it is often necessary to employ complex, commercially unavailable organic structure-directing agents (OSDAs) prepared via multistep synthesis. However, a small subset of zeolite structures exist as naturally layered materials where postsynthesis steps can be used to exfoliate samples and produce ultrathin 2D nanosheets. In this study, we selected a common layered zeolite, the MWW framework, to explore methods of preparing 2D nanosheets via one-pot synthesis in the absence of complex organic templates. Using a combination of high-resolution microscopy and spectroscopy, we show that 2D MMW-type layers with an average thickness of 3.5 nm (ca. 1.5 unit cells) can be generated using the surfactant cetyltrimethylammonium (CTA), which operates as a dual OSDA and exfoliating agent to affect Al siting and to eliminate the need for postsynthesis exfoliation, respectively. We tested these 2D catalysts using a model reaction that assesses external (surface) Brønsted acid sites and observed a marked increase in the conversion relative to three-dimensional MWW (MCM-22) and 2D layers prepared from postsynthesis exfoliation (ITQ-2). Collectively, our findings identify a facile and effective route to directly synthesize 2D MWW-type materials, which may prove to be more broadly applicable to other layered zeolites.

James E Bruno, Nicolas S Dwarica, Todd N Whittaker, Emily R Hand, Clemente S Guzman IV, Anish Dasgupta, Zhifeng Chen, Robert M Rioux, Bert D Chandler. ACS Catalysis. 10 (2020) 2565-2580.

Abstract: Bimetallic NiAu catalysts have garnered broad interest for a variety of reactions including automotive emissions, selective hydrogenation, selective oxidation, hydrodechlorination, and biomass conversion. However, the bulk immiscibility of the two metals, complicating catalyst synthesis, has limited studies of this bimetallic system. We report a solution-phase synthesis for Ni and bimetallic NiAu heterogeneous catalysts. Using oleylamine as a capping agent, an optimized synthesis for Ni catalysts led to supported particles with a narrow size distribution (4.7 ± 0.4 nm). Gold was added to the Ni nanoparticles via galvanic displacement of Ni in organic solution, the particles were deposited onto commercial alumina, and oleylamine capping agent was removed. The catalytic activity of the bimetallic materials in 1-octyne partial hydrogenation was in between the activity of monometallic Ni and Au catalysts. At high space velocity, the bimetallic catalysts largely maintained the high alkene selectivity associated with Au catalysts (>90% alkene selectivity at a 95% conversion). At lower space velocities, the NiAu catalysts also had a reduced propensity to overhydrogenate the alkene (relative to Ni). A simple catalyst performance parameter, which combined activity, selectivity, and space velocity, was developed and used to describe the overall performance of each catalyst under varying reaction conditions. By this metric, the bimetallic catalysts had considerably better performance than monometallic Ni. The most active bimetallic catalyst was examined with a week-long stability test; it showed no activity loss with a 100% carbon balance. Catalysts were characterized by transmission electron microscopy, X-ray diffraction, H₂ and N₂ adsorption, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The reactivity and characterization studies suggest the active catalysts are likely composed of bimetallic NiAu surfaces. The incorporation of Au into the catalysts suppresses H₂ adsorption on Ni, leading to lower hydrogen coverage during catalysis; this contributes to slowing undesirable alkene hydrogenation and improving catalyst selectivity.

Shahzahan Mia, Shelton JP Varapragasam, Aravind Baride, Choumini Balasanthiran, Balamurugan Balasubramanian, Robert M Rioux, James D Hoefelmeyer. Nanoscale Advances. 2 (2020) 4853-4862. 

Abstract: Cobalt(II) ions were adsorbed to the surface of rod-shape anatase TiO₂ nanocrystals and subsequently heated to promote ion diffusion into the nanocrystal. After removal of any remaining surface bound cobalt, a sample consisting of strictly cobalt-doped TiO₂ was obtained and characterized with powder X-ray diffraction, transmission electron microscopy, UV-visible spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy, SQUID magnetometry, and inductively-coupled plasma atomic emission spectroscopy. The nanocrystal morphology was unchanged in the process and no new crystal phases were detected. The concentration of cobalt in the doped samples linearly correlates with the initial loading of cobalt(II) ions on the nanocrystal surface. Thin films of the cobalt doped TiO₂nanocrystals were prepared on indium-tin oxide coated glass substrate, and the electrical conductivity increased with the concentration of doped cobalt. Magnetic measurements of the cobalt-doped TiO₂ nanocrystals reveal paramagnetic behavior at room temperature, and antiferromagnetic interactions between Co ions at low temperatures. Antiferromagnetism is atypical for cobalt-doped TiO₂ nanocrystals, and is proposed to arise from interstitial doping that may be favored by the diffusional doping mechanism.

S.J.P. Varapragasam, S. Mia, C. Wieting, C. Balasanthiran, M.Y. Hossan, A. Baride, R.M. Rioux, J.D. Hoefelmeyer. ACS Appl. Energy Mater. 2 (2019) 8274-8282.

Abstract: We report a thermolytic reduction of silver precursors in the presence of anatase TiO₂ nanorods to form Ag–TiO₂ hybrid nanocrystals (HNCs). Upon changing the reaction conditions, the size and number density of Ag on the HNCs could be adjusted. The size and number density of Ag on the HNCs were found to have an inverse relation. We assess the hydrogen evolution of TiO₂ nanorods, P25 TiO₂, and Ag–TiO₂ HNCs in methanol/water under xenon lamp irradiation. The turnover frequency for hydrogen evolution on silica-supported Ag–TiO₂ was 1.4 × 10^–4 s^–1, greater than that of the anatase TiO₂ nanorods (9.8 × 10^–6 s^–1) or the coupled anatase/rutile TiO₂(P25 catalyst; 5.2 × 10^–5 s^–1).

Z. Chen, J.W. Chang, C. Balasanthiran, S.T. Milner, R.M. Rioux. J. Am. Chem. Soc. 141 (2019) 4328-4337.

Abstract: Polyvinylpyrrolidone (PVP) is used in the synthesis of Ag nanoparticles (NPs) with controlled shape, most commonly producing cubes. The mechanism for shape control is unclear but believed by many to be caused by preferential binding of PVP to Ag(100) facets compared to Ag(111) facets and assumed by most to be the result of thermodynamic control, whereby facets with lower interfacial free energy predominate. To investigate this mechanism, we measured adsorption isotherms of PVP on different-shaped Ag NPs, to determine the thermodynamics of PVP adsorption to Ag(100) and Ag(111) facets. The equilibrium adsorption constant is independent of PVP molecular weight and depends only weakly on NP shape (and thus Ag facet). The equilibrium adsorption constant for PVP on Ag(111) (2.8 M^–1) is about half that on Ag(100) (5 M^–1). From a Wulff construction, this difference is not nearly enough to produce cubes via thermodynamic control. This result indicates the importance of kinetic control of the Ag nanoparticle shape by PVP, as has recently been proposed.

L. Wang*, M. Al-Aufi*, L. Xie*, R. M. Rioux. ACS Sustainable Chem. Eng. 7 (2019) 14785-14795.

Abstract: Amine-impregnated sorbents can be applied in CO₂ sequestration, but they suffer from low amine utility due to slow diffusion of CO₂, leading to reduced accessibility of buried amines. We examined silica-supported polyethylenimine (PEI) sorbents and blended them with polyethylene glycol (PEG) as an additive to study the role of PEG in modifying sorption performance. We report an increase in amine efficiency, a decrease in the heat of sorption and regeneration temperature, and a change CO₂ amine speciation with the addition of PEG. The increase in viscosity due to CO₂sorption leads to greater diffusion resistance; increases in viscosity were lower during CO₂ sorption in blends with higher PEG composition. ¹³C NMR results on CO₂-saturated PEI–PEG blends revealed the formation of carbamic acid and carbamate and showed a larger fraction of carbamic acid in PEG-rich samples. We propose PEG increases the amine efficiency by enabling the diffusion of CO₂ due to viscosity reduction and changes in the CO₂-amine speciation (carbamate versus carbamic acid), which modifies the theoretical CO₂/amine stoichiometry.

A. Garg, D. Goncalves, Y. Liu, Z. Wang, L. Wang*, J. S. Yoo, A. Kolpak, R. M. Rioux, D. Zanchet, Y. Román-Leshkov. ACS Catal. 9 (2019) 7090-7098.

Abstract: Atomically thin platinum (Pt) shells on titanium tungsten carbide (TiWC) and titanium tungsten nitride (TiWN) core nanoparticles display substantially modified catalytic performance compared to commercial Pt nanoparticles. In situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicate these differences are primarily caused by ligand effects from the hybridization of Pt and W d states at the core–shell interface. The heterometallic bonding between the shell and the core elements leads to broadening of the Pt valence d-band, a downshift of the d-band center, and greatly reduced adsorbate binding energies, as verified by density functional theory calculations and microcalorimetry of CO adsorption. In situ XANES measurements during reduction treatment demonstrated how surface oxides disrupt the bonding interactions between Pt and W. Changes to the Pt electronic structure from different core materials correlated with ethylene hydrogenation reactivity, where increased Pt d-band broadening was associated with weaker adsorbate binding and consequently lower turnover frequency. The significant electronic structure modification of Pt by the TiWC and TiWN cores exemplifies how core–shell nanoparticle architectures can be used to tune catalyst reactivity.

A. Dasgupta*, E. Zimmerer*, R. J. Meyer, R. M. Rioux. Catal. Today. 334 (2019) 231-242.

Abstract: The existing literature suggests it is particularly difficult to access the catalytically relevant, and relatively complex, intermetallic γ-brass crystal structure through traditional nanoparticle (NP) synthesis techniques. We introduce a simple and rational approach to access this phase in M-Zn (M = Pd, Cu, Ni) systems as silica-supported single-phase nanocrystals. This hybrid approach involves the initial synthesis of supported M/SiO₂ through traditional approaches (dry impregnation and strong electrostatic adsorption) followed by heating to high temperatures in the presence of a stoichiometric amount of metallic Zn in an evacuated closed system for several hours. We demonstrate the generality of this method with three different catalytically important bimetallic systems: Pd-Zn, Ni-Zn and Cu-Zn. Of these three, Pd-Zn is by the far the most popular in terms of catalytic applications and yields the smallest particle size (∼8 nm). We tested the influence of various synthesis parameters on phase purity and particle size distribution in case of the synthesized γ-brass Pd-Zn/SiO₂supported catalysts and provide general guidelines towards optimization of synthesis. Upon transformation of Pd/SiO₂ to γ-brass Pd-Zn/SiO₂, a precipitous drop in CO adsorption and a 25 kJ/mol increase in the ethylene hydrogenation barrier is observed, indicating the catalytic active sites are significantly modified as a result of alloying. We anticipate these catalysts may find applications in various Pd-catalyzed chemistries.

Dasgupta*, R. M. Rioux. Catal. Today . 330 (2019) 2-15.

Abstract: In this article, we highlight the growing application of intermetallic compounds in heterogeneous catalysis. We clearly discuss intermetallics as distinct from other multi-metallic systems (such as alloys, surface modifiers and dopants). Intermetallics possess a number of attributes over random alloys (i.e., solid solutions) and serve as model catalysts for a number of industrially relevant reactions. We review a variety of methods used for the synthesis of a large number of intermetallic compounds and discuss how standard material characterization techniques can be extended to intermetallics to gain important insight regarding active site morphology, a critical metric in heterogeneous catalyst design. We further summarize the use of intermetallics in understanding changes in reactivity and selectivity due to geometric and electronic effects, with emphasis on determining compositional and structural factors of the active site. We conclude with a brief summary of avenues for future advances in the field and its potential to contribute to environment and economy.

M A. Mellmer, C. Sanpitakseree, K. Ma, B. Demir, W. A. Elliott*, P. Bai, R. L. Johnson, B. H. Shanks, R. M. Rioux, M. Neurock, J. A. Dumesic. Nature Comm. 2019.

Abstract: The use of polar aprotic solvents in acid-catalyzed biomass conversion reactions can lead to improved reaction rates and selectivities. We show that further increases in catalyst performance in polar aprotic solvents can be achieved through the addition of inorganic salts, specifically chlorides. Reaction kinetics studies of the Brønsted acid-catalyzed dehydration of fructose to hydroxymethylfurfural (HMF) show that the use of catalytic concentrations of chloride salts leads to a 10-fold increase in reactivity. Furthermore, increased HMF yields can be achieved using polar aprotic solvents mixed with chlorides. Ab initio molecular dynamics simulations (AIMD) show that highly localized negative charge on Cl⁻ allows the chloride anion to more readily approach and stabilize the oxocarbenium ion that forms and the deprotonation transition state. High concentrations of polar aprotic solvents form local hydrophilic environments near the reactive hydroxyl group which stabilize both the proton and chloride anions and promote the dehydration of fructose.

Z. Chen*, T. Balankura, K. A. Fichthorn, R. M. Rioux. ACS Nano. 2019.

Abstract: Chloride (Cl^–) is often used together with polyvinylpyrrolidone (PVP) in the polyol synthesis of Ag nanocubes. In the literature, shape control is attributed predominantly to the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets, whereas the role of Cl^– has not been well studied. Several hypotheses have been proposed regarding the role of Cl^–; however, there is still no consensus regarding the exact influence of Cl^– in the shape-controlled synthesis of Ag nanocubes. To examine the influence of Cl^–, we undertook a joint theoretical–experimental study. Experimentally, we examined the influence of Cl^– concentration on the shape of Ag nanoparticles (NPs) at constant H⁺ concentration. In the presence of H⁺, in situ formed HNO₃ etches the initially formed Ag seeds and slows down the overall reduction of Ag⁺, which promotes the formation of monodisperse Ag NPs. Ex situ experiments probed the evolution of Cl^–during the growth of Ag nanocubes, which involves the initial formation of AgCl nanocubes, and their subsequent dissolution to release Cl^–, which adsorbs onto the surfaces of single crystal seeds to impact shape evolution through apparent thermodynamic control. The formation of cubes is independent of the source of AgCl, indicating temporal control of the Cl^– chemical potential in solution leads to high-yield synthesis of Ag nanocubes. Increasing the concentration of Cl^– alone leads to a progression in shape from truncated octahedra, to cuboctahedra, truncated cubes, and ultimately cubes, directly demonstrating the importance of Cl^– in Ag NP shape control. We used ab initio thermodynamics calculations based on density functional theory to probe the role of Cl^– in directing shape control. With increasing Cl chemical potential (surface coverage), calculated surface energies γ of Ag facets transition from γ₁₁₁ < γ₁₀₀ to γ₁₀₀ < γ₁₁₁ and predict Wulff shapes terminated with an increasing (100) contribution, consistent with experimental observations. The combination of theory and experiment is beneficial for advancing the understanding of nanocrystal formation.

J. E. Bruno, N. S. Dwarica, A. Hüther, K. B. Sravan Kumar, Z. Chen*, C. S. Guzman IV, E. R. Hand, W. C. Moore, R. M. Rioux, L. C. Grabow, B. D. Chandler. ChemCatChem. 2019.

Abstract: We report a quantitative kinetic evaluation and study of support effects for partial alkyne hydrogenation using oleylamine-capped Au colloids as catalyst precursors. The amine capping agents can be removed under reducing conditions, generating supported Au nanoparticles of ∼2.5 nm in diameter. The catalysts showed high alkene selectivity (>90 %) at all conversions during alkyne partial hydrogenation. Catalytic activity, observed rate constants, and apparent activation energies (25–40 kJ/mol) were similar for all Au catalysts, indicating support effects are relatively small. Alkyne adsorption, probed with FTIR and DFT, showed adsorption on the support was associated with hydrogen-bonding interactions. DFT calculations indicate strong alkyne adsorption on Au sites, with the strongest adsorption sites at the metal-support interface (MSI). The catalysts had similar hydrogen reaction orders (0.7–0.9), and 1-octyne reaction orders (∼−0.2), suggesting a common mechanism. The reaction kinetics are most consistent with a mechanism involving the non-competitive activated adsorption of H₂ on an alkyne-covered Au surface.

H. He, A. Dasgupta*, R. M. Rioux, R. J. Meyer, M. J. Janik. J. Phys. Chem. 2019.

Abstract: Selective hydrogenation of linear alkenes in the presence of aromatics is desired to prevent gum formation in pyrolysis gasoline (PYGAS) upgrading. To examine the competitive hydrogenation between linear alkenes and aromatics, we investigate ethylene and benzene competitive hydrogenation on different catalysts. Through density functional theory (DFT) calculations, we show the adsorption energies of benzene and ethylene correlate on monometallic close-packed surfaces, with benzene binding stronger for the same C to surface metal atom ratio. DFT calculations demonstrate Brønsted–Evans–Polanyi and scaling relationships hold, and these are fed into microkinetic modeling to predict the rate of ethylene and benzene hydrogenation with only ethylene and hydrogen binding energies as the surface descriptors. Due to stronger binding, benzene adsorption will dominate the surface. Higher barriers for benzene hydrogenation versus ethylene hydrogenation lead to benzene poisoning at temperatures at which ethylene hydrogenation would otherwise have been fast. Experimental studies using a Pd foil catalyst agree with microkinetic model predictions that benzene will poison the surface during ethylene hydrogenation while not being hydrogenated itself. Computational results predict bimetallic surfaces can avoid benzene poisoning during ethylene hydrogenation with the addition of an inert metal to disrupt the binding of benzene on 3-fold sites.

Y. Guo, Z. Chen*, R. M. Rioux, P. E. Savage.  J. Supercrit Fluids.143 (2019) 336-345.

Abstract: The major products from hydrothermal reactions of tryptophan at 350 and 400 °C in the absence of catalyst were indole and methyl indoles, indicating simultaneous deoxygenation and deamination. In the presence of carbon-supported Ni and NiM catalysts (M = Ru, Cu, Pd, Pt) primary aromatic amines also appeared, via cleavage of the heterocyclic ring of the indoles. Ni and NiCu catalysts have the lowest selectivity for the formation of this family of compounds, while NiRu had the highest selectivity. Catalysts containing noble metals also produced monosubstituted alkylaromatics, due to the deamination of the aromatic amines, with the NiPt catalyst providing the highest molar yield (∼7%). We propose catalytic and non-catalytic hydrothermal reaction pathways for tryptophan based on the observed product distributions. The bimetallic particles were smaller than the pure Ni particles and the surfaces of NiRu and NiPd particles were enriched in Ni, relative to the nominal bulk composition.

A. Dasgupta*, E. K. Zimmerer*, R. J. Meyer, R. M. Rioux. Catal. Today.

Abstract: The existing literature suggests it is particularly difficult to access the catalytically relevant, and relatively complex, intermetallic γ-brass crystal structure through traditional nanoparticle (NP) synthesis techniques. We introduce a simple and rational approach to access this phase in M-Zn (M = Pd, Cu, Ni) systems as silica-supported single-phase nanocrystals. This hybrid approach involves the initial synthesis of supported M/SiO₂ through traditional approaches (dry impregnation and strong electrostatic adsorption) followed by heating to high temperatures in the presence of a stoichiometric amount of metallic Zn in an evacuated closed system for several hours. We demonstrate the generality of this method with three different catalytically important bimetallic systems: Pd-Zn, Ni-Zn and Cu-Zn. Of these three, Pd-Zn is by the far the most popular in terms of catalytic applications and yields the smallest particle size (∼8 nm). We tested the influence of various synthesis parameters on phase purity and particle size distribution in case of the synthesized γ-brass Pd-Zn/SiO₂supported catalysts and provide general guidelines towards optimization of synthesis. Upon transformation of Pd/SiO₂ to γ-brass Pd-Zn/SiO₂, a precipitous drop in CO adsorption and a 25 kJ/mol increase in the ethylene hydrogenation barrier is observed, indicating the catalytic active sites are significantly modified as a result of alloying. We anticipate these catalysts may find applications in various Pd-catalyzed chemistries.

1. S. Jharimune*, A. A. Sathe*, R. M. Rioux. Nano Letters. 18 (2018) 6795-6803.

Abstract: Among the various reported post synthetic modifications of colloidal nanocrystals, cation exchange (CE) is one of the most promising and versatile approaches for the synthesis of nanostructures that cannot be directly synthesized from their constitutive precursors. Numerous studies have reported on the qualitative analysis of these reactions, but rigorous quantitative study of the thermodynamics of CE in colloidal nanoparticles is still lacking. We demonstrate using isothermal titration calorimetry (ITC), the thermodynamics of the CE between cadmium selenide (CdSe) nanocrystals and silver in solution can be quantified. We survey the influence of CdSe nanocrystal diameter, capping ligands and temperature on the thermodynamics of the exchange reaction. Results obtained from ITC provide a detailed description of overall thermodynamic parameters—equilibrium constant (K_eq), enthalpy (ΔH), entropy (ΔS) and stoichiometry (n)—of the exchange reaction. We compared the free energy change of reaction (ΔG) between CdSe and Ag⁺ obtained directly from ITC for both CdSe bulk and nanoparticles with values calculated from previously reported methods. While the calculated value is closer to the experimentally obtained ΔG_rxn for bulk particles, nanocrystals show an additional Gibbs free energy stabilization of ∼−14 kJ/mol Se. We discuss a thermochemical cycle elucidating the steps involved in the overall cation exchange process. This work demonstrates the application of ITC to probe the thermochemistry of nanoscale transformations under relevant solution conditions.

A. Dasgupta*, R. M. Rioux. Catal. Today.

Abstract: In this article, we highlight the growing application of intermetallic compounds in heterogeneous catalysis. We clearly discuss intermetallics as distinct from other multi-metallic systems (such as alloys, surface modifiers and dopants). Intermetallics possess a number of attributes over random alloys (i.e., solid solutions) and serve as model catalysts for a number of industrially relevant reactions. We review a variety of methods used for the synthesis of a large number of intermetallic compounds and discuss how standard material characterization techniques can be extended to intermetallics to gain important insight regarding active site morphology, a critical metric in heterogeneous catalyst design. We further summarize the use of intermetallics in understanding changes in reactivity and selectivity due to geometric and electronic effects, with emphasis on determining compositional and structural factors of the active site. We conclude with a brief summary of avenues for future advances in the field and its potential to contribute to environment and economy.

Y. Yang+, J. W. Chang*, R. M. Rioux. J. Catal. 365 (2018) 43-54.

Abstract: The local structures of rhodium complexes derived from the immobilization of Wilkinson’s complex, RhCl(PPh₃)₃, on SBA-15 silica functionalized with primary–amine, secondary–amine, or diphenylphosphine groups within the mesoporous channels were characterized by a series of techniques including XRD, HR-TEM, multinuclear (¹³C/²⁹Si/³¹P) solid-state NMR, 2D ³¹P{¹H} HETCOR NMR, XPS, and Rh K-edge EXAFS. Immobilization of RhCl(PPh₃)₃ through covalent bond formation with different functional groups grafted to the silica surface lead to variations in the local structure of the Rh center that has important implications for catalysis. The immobilized Rh complexes demonstrated high activity for the addition of alkynes with thiols (hydrothiolation) or sulfonic acids (hydrosulfonation) with excellent regio- and stereoselectivity under mild reaction conditions. This work demonstrates the elucidation of the local structure of the immobilized Rh complexes requires a complimentary multi-technique characterization approach that probes both the metal center itself and surrounding ligands.

M. D. Yang, L. X. Wang*, S. M. K. Shahri*, R. M. Rioux, A. Armaou. Ind. Eng. Chem. Res. 57 (2018) 10303-10314.

Abstract: The behavior of an isothermal packed bed sorption column for CO₂ is investigated based on combined mass spectrometry and calorimetry temporal measurements at different temperatures. The inclusion of the calorimetry data stream at multiple temperatures accentuates the limitations of our previous spatiotemporal model with a simple sorption mechanism to describe the experimental observations. We propose four models, which consider dispersion and convection with different descriptions of the sorption mechanism. The unknown parameters in the models are determined by minimizing the integral in time of the squared difference between model prediction and the experimental measurements. Analyzing the simulation results for thermodynamic consistency and site density, we conclude physical sorption followed by chemical sorption is a probable mechanism to describe the specific experiment data. We also conclude a combination of multiple measurement streams is required to verify the consistency of proposed mechanisms and establish a clear(er) macroscopic description of the underlying physicochemical phenomena.

R. M. Rioux, S. H. Kim. Molecular Surface Science. Top. Catal. 61 (2018) 711-713.

L. Wang*, S. M. K. Shahri*, R. M. Rioux. J. Phys. Chem. C 122 (2018) 11442-11449.

Abstract: Amine-based solid sorbents represent promising replacements for aqueous alkanolamines for CO₂ capture because they reduce significantly the sorbent regeneration energy and avoid instrument corrosion encountered with aqueous amine solutions. CO₂ capture by solid amine sorbents is often achieved by pressure–temperature-swing adsorption (PTSA) technology, which selectively sorbs CO₂ from a gas mixture and desorbs concentrated CO₂ by pressure swing and/or heating. CO₂ capacity and heat of sorption on different types of amine-based solids have been evaluated predominantly under equilibrium conditions using thermogravimetric analysis (TGA), with emphasis on parametric studies of temperature, amine loadings, and supporting substrates on CO₂ sorption kinetics and capacity. Owing to differences in the configuration between a TGA apparatus and a PTSA column, CO₂ capacity and heat of sorption may vary under transient sorption condition versus equilibrium sorption condition. In this work, we constructed a laboratory-scale breakthrough reactor (BTR) to simulate the industrial PTSA process, evaluated CO₂ capacity and heat of sorption at different temperatures under transient sorption conditions, and compared these values with equilibrium values determined by volumetric sorption analysis. We found that the temperature effects on CO₂ capacity and the heat of sorption differ under the two sorption modes. CO₂ capacity and heat of sorption data not only provide useful information for the design of sorption and regeneration processes, but also enable spatiotemporal modeling of the CO₂ sorption process in a packed bed.

C. Balasanthiran+, S. Jensen, C. S. Spanjers*, S. J. P. Vararagasam, R. M. Rioux, D. Kilin, J. D. Hoefelmeyer. Langmuir. 34 (2018) 5422-543.

Abstract: We report the sequential, quantitative loading of transition-metal ions (Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺) onto the surface of rod-shaped anatase TiO₂ nanocrystals in bimetallic combinations (₆C₂ = 15) to form M,M′-TiO₂ nanocrystals. The materials were characterized with transmission electron microscopy (TEM), powder X-ray diffraction (XRD), elemental analysis, X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. TEM and XRD data indicate that the sequential adsorption of metal ions occurs with the retention of the phase and morphology of the nanocrystal. Atomistic models of the M,M′-TiO₂nanocrystals were optimized with density functional theory calculations. Calculated UV–visible absorption spectra and partial charge density maps of the donor and acceptor states for the electronic transitions indicate the importance of metal-to-metal charge transfer (MMCT) processes.

G. Kumar, L. Tibbitts, J. Newell, B. Panthi, A. Mukhopadhyay*, R. M. Rioux, C. J. Pursell, M. Janik, B. D. Chandler. Nature Chem. 3 (2018) 268-274.

Abstract: Supported metal catalysts, which are composed of metal nanoparticles dispersed on metal oxides or other high-surface-area materials, are ubiquitous in industrially catalysed reactions. Identifying and characterizing the catalytic active sites on these materials still remains a substantial challenge, even though it is required to guide rational design of practical heterogeneous catalysts. Metal–support interactions have an enormous impact on the chemistry of the catalytic active site and can determine the optimum support for a reaction; however, few direct probes of these interactions are available. Here we show how benzyl alcohol oxidation Hammett studies can be used to characterize differences in the catalytic activity of Au nanoparticles hosted on various metal-oxide supports. We combine reactivity analysis with density functional theory calculations to demonstrate that the slope of experimental Hammett plots is affected by electron donation from the underlying oxide support to the Au particles.

S. J. P. Varapragasam, C. Balasanthiran+, A. Gurung, Q. Q. Qiao, R. M. Rioux, J. D. Hoefelmeyer. J. Phys. Chem. C 121 (2017) 11089-11099.

Abstract: We report the formation of high surface area hollow Mn₃O₄ nanoparticles that form as a result of the galvanic reaction of Cu²⁺ with MnO nanocrystals concomitant with a nanoscale Kirkendall effect. The MnO nanocrystals were prepared according to the ultralarge scale synthesis reported by Hyeon, which allowed the preparation of hollow Mn₃O₄ in multigram quantities. Ex-situ analyses with transmission electron microscopy and powder X-ray diffraction show the morphology and phase stability of the hollow particles correlate with DSC-TGA data and show collapse of the hollow particles at temperatures greater than 200 °C. Electrodes fabricated from hollow Mn₃O₄ exhibited excellent initial Li ion storage capability (initial discharge capacity = 1324 mAh/g) but poor cycling performance (97% loss of discharge capacity after 10th cycle), whereas Mn₃O₄-MWCNT electrodes exhibited good reversibility and discharge capacity of 760 mAh/g after 100 cycles.

Y. Noda+, H. B. Zhang+, R. Dasari, R. Singh, C. Ozmeral, Y. Roman-Leshkov, R. M. Rioux. Ind. Eng. Chem. Res. 56 (2017) 5843-585.

Abstract: Catalytic dehydration of lactic acid in the presence of water is a potentially green, synthetic approach for the production of acrylic acid, and development of a highly selective catalyst is a primary challenge, leading to a resurgence in catalyst exploration and discovery. However, because the complexity in the analytical assessment of the efficiency of catalysts stemming from the possible presence of dimers in lactic acid feedstocks has often been neglected in the literature, we demonstrate, without consideration of the dimer during catalyst evaluation, that they can have a substantial influence on the determination of conversion of lactic acid and selectivity to acrylic acid in aqueous solution. In one example of a modified zeolite catalyst, a true acrylic acid of selectivity of 64% was overestimated to be 80% if the dimers in the feed solution were neglected in the analytical analysis. A survey of the literature demonstrated very few researchers account for the possible presence of lactic acid dimers in the lactic acid solution; therefore, the reported catalyst performance should be carefully considered in light of the potentially significant impact of lactic acid dimers. We further demonstrate that the heat treatment of a lactic acid feed solution prior to the reaction can hydrolyze dimers back to monomers, avoiding analytical misinterpretation and providing an accurate measure of the catalytic performance.

L Qi, R. Alamillo, W. A. Elliott*, A. Andersen, D. W. Hoyt, E. D. Walter, K. S. Han, N. M. Washton, R. M. Rioux, J. A. Dumesic, S. L. Scott. ACS Catal. 7 (2017) 3489-3500. 

Abstract: In the liquid-phase catalytic processing of molecules using heterogeneous catalysts—an important strategy for obtaining renewable chemicals from biomass—many of the key reactions occur at solid–liquid interfaces. In particular, glucose isomerization occurs when glucose is adsorbed in the micropores of a zeolite catalyst. Since solvent molecules are coadsorbed, the catalytic activity depends strongly and often nonmonotonically on the solvent composition. For glucose isomerization catalyzed by NaX and NaY zeolites, there is an initial steep decline when water is mixed with a small amount of the organic cosolvent γ-valerolactone (GVL), followed by a recovery as the GVL content in the mixed solvent increases. Here we elucidate the origin of this complex solvent effect using operando solid-state NMR spectroscopy. The glucopyranose tautomers immobilized in the zeolite pores were observed and their transformations into fructose and mannose followed in real time. The microheterogeneity of the solvent system, manifested by a nonmonotonic trend in the mixing enthalpy, influences the mobility and adsorption behavior of the carbohydrates, water, and GVL, which were studied using pulsed-field gradient (PFG) NMR diffusivity measurements. At low GVL concentrations, glucose is depleted in the zeolite pores relative to the solution phase, and changes in the local structure of coadsorbed water serve to further suppress the isomerization rate. At higher GVL concentrations, this lower intrinsic reactivity is largely compensated by strong glucose partitioning into the pores, resulting in dramatic (up to 32×) enhancements in the local sugar concentration at the solid–liquid interface.

C. S. Spanjers*, A. Dasgupta*, M. Kirkham, B. Burger*, G. Kumar, M. J. Janik, R. M. Rioux. Chem. Mater. 29 (2017) 504-512.

Abstract: Previous attempts to characterize the γ-brass crystal structure of Ni–Zn (15.4–24% Ni) have failed to identify the location of the Ni and Zn atoms in the crystal lattice for more than 15.4% Ni content (Ni₈Zn₄₄) due to the similar X-ray diffraction cross sections of Ni and Zn. Ni₈Zn₄₄ is known to have a typical γ-brass crystal structure (space group 217, I4̅3m, 52 atom unit cell with four distinct symmetry positions: inner tetrahedral, outer tetrahedral, octahedral, and cuboctahedral) where Ni atoms reside in outer tetrahedral sites completely isolated from each other and coordinated by 12 Zn atoms. We utilize neutron diffraction to identify the substitution positions of Zn by Ni when the Ni content is increased above 15.4% and up to 19.2% (Ni₁₀Zn₄₂). Upon increasing the Ni content above 15.4% (Ni₉Zn₄₃and Ni₁₀Zn₄₂), Zn in the γ-brass octahedral positions are replaced by Ni leading to the formation of Ni–Ni–Ni trimers, which are absent in Ni₈Zn₄₄. Density functional theory (DFT) calculations confirm our neutron diffraction results regarding the optimal position of excess Ni in the γ-brass unit cell. The well-defined atomic site distribution in γ-brass Ni–Zn provides an excellent opportunity for producing site-isolated base metal catalysts that may find application in selective semihydrogenation. We investigated the presence of Ni–Ni–Ni trimers on the surface using H–D exchange and ethylene hydrogenation as probe reactions, observing the influence of Ni concentration on catalysis. We conclude the catalytic performance is insensitive to Ni content. We provide a possible explanation for this observation using DFT calculations, which demonstrate that surface containing trimer sites are energetically unfavorable and therefore not exposed on Wulff reconstructions of γ-brass phase Ni–Zn particles.

A. Sathe*, A. Nambiar*, R. M. Rioux. Catal. Sci. Technol. 7 (2017) 84-89.

Abstract: Methodology for direct catalytic conversion of olefins into cyclic carbonates using peroxide and carbon dioxide under relatively mild conditions is demonstrated. The protocol utilizes packed bed flow reactors in series to couple rhenium catalyzed olefin epoxidation and aluminum catalyzed epoxide carboxylation in a single sequence.

C. S. Spanjers*, P. Guillo, M. J. Janik, T. D. Tilley, R. M. Rioux. J. Phys. Chem. A121 (2017) 162-16.

Abstract: X-ray absorption near-edge structure (XANES) is a common technique for elucidating oxidation state and first shell coordination geometry in transition metal complexes, among many other materials. However, the structural information obtained from XANES is often limited to the first coordination sphere. In this study, we show how XANES can be used to differentiate between C, Si, and Ge in the second coordination shell of Ti–O–(C, Si, Ge) molecular complexes based on differences in their Ti K-edge XANES spectra. Experimental spectra were compared with theoretical spectra calculated using density functional theory structural optimization and ab initio XANES calculations. The unique features for second shell C, Si, and Ge present in the Ti K pre-edge XANES are attributed to the interaction between the Ti center and the O–X (X = C, Si, or Ge) antibonding orbitals.

C. S. Sievers, Y. Noda+, L. Qi, E. M. Albuquerque, R. M. Rioux, S. L. Scott. ACS Catal. 6 (2016) 8286-8307.

Abstract: Interest in liquid-phase reactions over heterogeneous catalysts is growing rapidly, partially because of the desire to find efficient methods for biomass conversion to renewable fuels and chemicals. The presence of a solvent can affect reactions at surfaces by competing with reactants and products for adsorption sites and solvating adsorbed species. Mass transport limitations can also have a pronounced effect on liquid-phase reaction rates. Because many heterogeneous catalysts were designed to be stable under gas-phase reaction conditions, their operation in liquid reaction media at moderately elevated temperatures can result in unexpected structural changes. In some cases, components derived from the evolving catalyst contribute significantly to the catalytic activity. Solvents, as well as byproducts from biomass feedstocks, can also act as homogeneous catalysts to alter the intrinsic reactivity of the heterogeneous catalyst. In this contribution, we discuss each of these phenomena and provide illustrative examples.

Y. Noda+, K. Li*, A. Engler*, W. A. Elliott*, R. M. Rioux. Catal. Sci. Technol. 6 (2016) 5961-5971.

Abstract: In catalytic applications of surface-modified mesoporous silica materials, distinguishing and quantifying different types of functional groups on the surface is crucial for enabling accurate evaluation of catalytic activity and possible cooperativity among mixed functional groups. We investigated sulfonic acid groups supported on SBA-15 mesoporous silica as a promising organic–inorganic hybrid solid acid catalyst and developed a robust technique for sulfur quantification and speciation after oxidation of grafted thiol groups to sulfonic acid. We combined SH-selective titration (Ellman’s test), acid–base titration and ICP-OES analysis to quantitatively monitor S speciation at each step of the preparation of these catalysts. Ellman’s test confirmed the successful grafting of SH groups on SBA-15, which is in good agreement with ICP-OES total S determination (∼0.8 mmol g_cat⁻¹). These SH groups were converted in a subsequent 30% H₂O₂ treatment to SO₃H groups, while significant leaching of S during oxidation limited the yield of grafted SO₃H (∼0.3 mmol g_cat⁻¹) in the final catalyst. The influence of grafting and oxidation conditions on S speciation was investigated, and the presence of partially oxidized S on the surface was quantified. Despite the loss of a fraction of grafted S, the prepared sulfonic acid-functionalized SBA-15 catalysts exhibited comparable catalytic activity per H⁺ with homogeneous sulfonic acid catalysts in esterification reactions. Re-grafting of SH groups after oxidation was shown to be effective in achieving the coexistence of SH and SO₃H groups on the surface, and a catalytic study revealed no cooperativity between SH and SO₃H for the esterification reaction.

D. B. Pourkargar, S. M. K. Shahri*, R. M. Rioux, A. Armaou. Ind. Engr. Chem. Res. 55 (2016) 6443-6453.

Abstract: This paper focuses on the development of a rigorous model for isothermal CO₂ adsorption columns which describes the spatiotemporal dynamics of CO₂ concentrations in the bulk and solid bed by a set of partial and location-varying ordinary differential equations. By considering both dispersion and convection phenomena, the model provides the spatiotemporal behavior of the adsorption rate and circumvents the unphysical simplifying assumptions of linear driving force and uniform adsorption rates through the column length invoked in previous modeling efforts. The proposed model is then employed to compute physical quantities originating from material conservation laws such as the adsorption rate constant and CO₂ adsorption capacity from a set of experimental data without using empirical parameter assumptions invoked in previous research. The spatiotemporal dynamics of CO₂ adsorption in an aminosilica packed bed are successfully predicted by the proposed model. The adsorption rate constant and capacity of the bed are then identified using a set of experimental CO₂ concentration measurements at the adsorption column outlet by solving a dynamic optimization problem using a shooting method formulation. Finally, the adsorption enthalpy is computed by employing the heat of adsorption data to validate the estimated parameters of the system.

J. Saavedra, T. Whittaker, Z. Chen*, C. J. Pursell, R. M. Rioux, B. D. Chandler. Nature Chem. 8 (2016) 585-590.

Abstract: Industrial hydrogen production through methane steam reforming exceeds 50 million tons annually and accounts for 2–5% of global energy consumption. The hydrogen product, even after processing by the water–gas shift, still typically contains ∼1% CO, which must be removed for many applications. Methanation (CO + 3H₂ → CH₄ + H₂O) is an effective solution to this problem, but consumes 5–15% of the generated hydrogen. The preferential oxidation (PROX) of CO with O₂ in hydrogen represents a more-efficient solution. Supported gold nanoparticles, with their high CO-oxidation activity and notoriously low hydrogenation activity, have long been examined as PROX catalysts, but have shown disappointingly low activity and selectivity. Here we show that, under the proper conditions, a commercial Au/Al₂O₃ catalyst can remove CO to below 10 ppm and still maintain an O₂-to-CO₂ selectivity of 80–90%. The key to maximizing the catalyst activity and selectivity is to carefully control the feed-flow rate and maintain one to two monolayers of water (a key CO-oxidation co-catalyst) on the catalyst surface.

A. Gellman, R. M. Rioux, P. Stair. Surf. Sci. 648 (2016) 1.

S. G. Turney, M. Ahmed, I. Chandrsekar, R. B. Wysolmerski, Z. M. Goeckeler, R. M. Rioux, G. M. Whitesides, P. C. Bridgman. Molecular Biology of the Cell 27 (2016) 500-51.

Abstract: Nerve growth factor (NGF) promotes growth, differentiation, and survival of sensory neurons in the mammalian nervous system. Little is known about how NGF elicits faster axon outgrowth or how growth cones integrate and transform signal input to motor output. Using cultured mouse dorsal root ganglion neurons, we found that myosin II (MII) is required for NGF to stimulate faster axon outgrowth. From experiments inducing loss or gain of function of MII, specific MII isoforms, and vinculin-dependent adhesion-cytoskeletal coupling, we determined that NGF causes decreased vinculin-dependent actomyosin restraint of microtubule advance. Inhibition of MII blocked NGF stimulation, indicating the central role of restraint in directed outgrowth. The restraint consists of myosin IIB- and IIA-dependent processes: retrograde actin network flow and transverse actin bundling, respectively. The processes differentially contribute on laminin-1 and fibronectin due to selective actin tethering to adhesions. On laminin-1, NGF induced greater vinculin-dependent adhesion–cytoskeletal coupling, which slowed retrograde actin network flow (i.e., it regulated the molecular clutch). On fibronectin, NGF caused inactivation of myosin IIA, which negatively regulated actin bundling. On both substrates, the result was the same: NGF-induced weakening of MII-dependent restraint led to dynamic microtubules entering the actin-rich periphery more frequently, giving rise to faster elongation.

M. E. Strayer, T. P. Senftle, J. P. Winterstein, N. M. Vargas-Barbosa, R. Sharma, R. M. Rioux, M. J. Janik, T. E. Mallouk. J. Am. Chem. Soc. 137 (2015) 16216-16224.

Abstract: Interfacial interactions between late transition metal/metal oxide nanoparticles and oxide supports impact catalytic activity and stability. Here, we report the use of isothermal titration calorimetry (ITC), electron microscopy and density functional theory (DFT) to explore periodic trends in the heats of nanoparticle–support interactions for late transition metal and metal oxide nanoparticles on layered niobate and silicate supports. Data for Co(OH)₂, hydroxyiridate-capped IrO_x·nH₂O, Ni(OH)₂, CuO, and Ag₂O nanoparticles were added to previously reported data for Rh(OH)₃grown on nanosheets of TBA_0.24H_0.76Ca₂Nb₃O₁₀ and a layered silicate. ITC measurements showed stronger bonding energies in the order Ag < Cu ≈ Ni ≈ Co < Rh < Ir on the niobate support, as expected from trends in M–O bond energies. Nanoparticles with exothermic heats of interaction were stabilized against sintering. In contrast, ITC measurements showed endothermic interactions of Cu, Ni, and Rh oxide/hydroxide nanoparticles with the silicate and poor resistance to sintering. These trends in interfacial energies were corroborated by DFT calculations using single-atom and four-atom cluster models of metal/metal oxide nanoparticles. Density of states and charge density difference calculations reveal that strongly bonded metals (Rh, Ir) transfer d-electron density from the adsorbed cluster to niobium atoms in the support; this mixing is absent in weakly binding metals, such as Ag and Au, and in all metals on the layered silicate support. The large differences between the behavior of nanoparticles on niobate and silicate supports highlight the importance of d-orbital interactions between the nanoparticle and support in controlling the nanoparticles’ stability.

C. S. Spanjers*, R. S. Sim*, N. P. Sturgis*, B. Kabius, R. M. Rioux. ACS Catal. 5 (2015) 3304-3315.

Abstract: The structures of ZnO-supported Ni catalysts were explored with in situ X-ray absorption spectroscopy, temperature-programmed reduction, X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy, and electron energy loss spectroscopy. Calcination of nickel nitrate on a nanoparticulate ZnO support at 450 °C results in the formation of Zn-doped NiO (ca. Ni_0.85Zn_0.15O) nanoparticles with the rock salt crystal structure. Subsequent in situ reduction monitored by X-ray absorption near-edge structure (XANES) at the Ni K edge reveals a direct transformation of the Zn-doped NiO nanoparticles to a face-centered cubic alloy, Ni_1–xZn_x, at ∼400 °C with x increasing with increasing temperature. Both in situ XANES and ex situ HRTEM provide evidence for intermetallic β₁-NiZn formation at ∼550 °C. In comparison to a Ni/SiO₂ catalyst, Ni/ZnO necessitates a higher temperature for the reduction of Ni^II to Ni⁰, which highlights the strong interaction between Ni and the ZnO support. The catalytic activity for acetylene removal from an ethylene feed stream is decreased by a factor of 20 on Ni/ZnO in comparison to Ni/SiO₂. The decrease in catalytic activity of Ni/ZnO is accompanied by a reduced absolute selectivity to ethylene. H–D exchange measurements demonstrate a reduced ability of Ni/ZnO to dissociate hydrogen in comparison to Ni/SiO₂. These results of the catalytic experiments suggest that the catalytic properties are controlled, in part, by the zinc oxide support and stress the importance of reporting absolute ethylene selectivity for the catalytic semihydrogenation of acetylene in excess ethylene.

B. Panthi, A. Mukhopadhyay*, L. Tibbitts, C. Pursell, R. M. Rioux, B. D. Chandler. ACS Catal. 5 (2015) 2232-2241.

Abstract: Two techniques to study the surface chemistry of supported gold nanoparticles were developed. First, phenylethyl mercaptan (PEM) adsorption from hexane solution was followed with UV–vis spectroscopy to evaluate the total amount of surface Au available. Two catalysts, Au/Al₂O₃and Au/TiO₂, were found to have Au:S surface stoichiometries of ∼2:1, whereas a Au/SiO₂ catalyst had a Au:S surface stoichiometry of ∼1:1. The room temperature equilibrium binding constants for PEM adsorption on the Au/Al₂O₃ and Au/TiO₂ catalysts were similar (∼3 × 10⁵ M^–1; ΔG ≈ −31 kJ/mol); the PEM–Au/SiO₂ binding constant was somewhat larger (∼2 × 10⁶ M^–1; ΔG ≈ −36 kJ/mol). XPS data for all of the catalysts showed no observable changes in the Au oxidation state upon adsorption of the thiol. Implications of these experiments regarding self-assembled monolayers and thiol-stabilized Au nanoparticles are discussed. Second, kinetic titrations (i.e., controlled thiol-poisoning experiments) were developed as a method for evaluating the number of active sites for selective 4-methoxybenzyl alcohol oxidation. These experiments suggested only a fraction of the surface Au (∼10–15% of the total Au) was active for the reaction. When thiol was added with the 4-methoxybenzyl alcohol substrate, more thiol was required to poison the catalyst, suggesting that the thiol and substrate compete for initial adsorption sites, possibly at the metal–support interface. These two methods were combined to evaluate the magnitude of the support effect on selective 4-methoxybenzyl alcohol oxidation. Correcting the catalytic activity of the catalysts to the number of sites determined by thiol titration provided clear evidence that the support has a strong influence on the catalytic activity of Au in benzyl alcohol oxidation.

R. M. Rioux, F. D. Toste. Catal. Sci. Technol. 5 (2015) 1357-1359.