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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 C C cross coupling).

H 2 adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H 2 adsorption on Au/TiO 2 catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H 2 adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H 2 adsorption kinetics measured with the BBA show a lower E app value (23 kJ/mol) for H 2 adsorption than previously reported from proxy H/D exchange (33 kJ/mol). 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 H 2 adsorption at the Au/TiO 2 interface.

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 (Cm) and molecular weight (Mw) demonstrated end groups of PVP impact the final Ag product. Measured rates of the initial Ag+ reduction at different PVP Cm and Mw 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.

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. CO oxidation 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.

We demonstrate that atomically thin Pt shells deposited on transition metal carbide or nitride cores induce up to a 4-fold enhancement in C2H4 selectivity during the partial hydrogenation of acetyl...

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 associate...

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/SiO2 supported 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.

Atomically thin platinum (Pt) shells on titanium tungsten carbide (TiWC) and titanium tungsten nitride (TiWN) core nanoparticles display substantially modified catalytic performance compared to com...

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.

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., Despite the potential advantages of using polar aprotic solvents for biomass upgrading reactions, fundamental understanding of these solvation effects is limited at present. Here, the authors show that further increases in catalyst performance in polar aprotic solvents can be achieved through the addition of inorganic salts.

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.

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 Bronsted–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 hydroge...

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) Bronsted 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.

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.

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.

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 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, mani...

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.

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 illustrati...

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 ...

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

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 gcat−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 gcat−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.

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. Ni0.85Zn0.15O) 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, Ni1–xZnx, 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 NiII to Ni0, which highlights the strong interaction between Ni and...

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/Al2O3 and 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 nu...

Silica-supported Pd and PdZn nanoparticles of a similar size were evaluated for neopentane hydrogenolysis/isomerization and propane hydrogenolysis/dehydrogenation. Monometallic Pd showed high neopentane hydrogenolysis selectivity. Addition of small amounts of Zn to Pd lead Pd–Zn scatters in the EXAFS spectrum and an increase in the linear bonded CO by IR. In addition, the neopentane turnover rate decreased by nearly 10 times with little change in the selectivity. Increasing amounts of Zn lead to greater Pd–Zn interactions, higher linear-to-bridging CO ratios by IR and complete loss of neopentane conversion. Pd NPs also had high selectivity for propane hydrogenolysis and thus were poorly selective for propylene. The PdZn bimetallic catalysts, however, were able to preferentially catalyze dehydrogenation, were not active for propane hydrogenolysis, and thus were highly selective for propylene formation. The decrease in hydrogenolysis selectivity was attributed to the isolation of active Pd atoms by inactive metallic Zn, demonstrating that hydrogenolysis requires a particular reactive ensemble whereas propane dehydrogenation does not.

Isotopic labeling and density functional theory (DFT) were used to determine the mechanism for acetylene hydrogenation and oligomerization on well-defined intermetallic nickel–zinc catalysts. The primary benefit of adding zinc to nickel is a reduction in oligomeric species formation which leads to higher ethylene selectivity. The production of ethane is not highly dependent on zinc content; therefore, ethane production is not a good descriptor of ethylene selectivity since acetylene may also be converted to higher molecular weight products. Analysis using DFT and Langmuir–Hinshelwood kinetics shows that the large decrease in the adsorption energy of acetylene on intermetallic NiZn compared to pure Ni is responsible for the observed increase in ethylene selectivity. The adsorption energy of acetylene appears to be a descriptor for carbon–carbon bond formation since a high adsorption energy leads to an increased coverage of C2 species and an increased rate of carbon–carbon bond formation.

Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and α-Zr(HPO4)2·H2O. High surface area SiO2 and γ-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ΔH values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and γ-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports.

A practical, efficient, and low-cost heterogeneous catalyst consisting of a Cu-NHC (N-heterocyclic carbene) complex grafted to SBA-15 silica for the catalytic hydrothiolation of alkynes and electron-deficient alkenes under mild reaction conditions has been developed. The heterogeneous catalyst displays higher activity and stereoselectivity to Z-anti-Markovnikov isomers compared with the homogeneous analog under otherwise identical reaction conditions. The catalytic system is applicable to a broad range of alkynes and thiols and is recyclable without significant loss in catalytic performance. High activity and perfect selectivity to alkyl sulfides formed by the addition of electron-deficient alkenes to various thiols catalyzed by the supported Cu-NHC complex were also realized.

Silica-supported Pd and shell/core PdAu nanoparticles of a similar size were evaluated for neopentane conversion. Monometallic Pd exhibited poor neopentane isomerization selectivity in favor of high selectivity to primary and secondary hydrogenolysis products. Similarly sized PdAu catalysts of increasing Pd weight loading were synthesized to evaluate the effect of increasing Pd monolayers on neopentane conversion. All PdAu catalysts had neopentane conversion selectivity within the range of monometallic Pd catalysts from previous work (~5–30%). However, there was an inverse relationship between Pd weight loading and neopentane isomerization selectivity. The increase in isomerization selectivity did not correlate to a decrease in heats of adsorption as seen with monometallic Pd catalysts, but was correlated with the catalyst surface structure which suggests a geometric effect as the cause for changes in catalytic performance rather than an electronic effect.

We present a thermodynamic study of the chelate effect for a series of P–P, N–N, and P–N bidentate ligands binding to PdCl2(MeCN)2 in acetonitrile (MeCN), using isothermal titration calorimetry (ITC) to measure the solution-phase binding thermodynamics. The chelate effect is considered to be an entropic effect, that is, the enthalpic contributions attributed to binding for comparable monodentate and bidentate ligands are nearly identical, meaning that changes in ΔG are due to favorable changes in ΔS upon displacement of bound solvent molecules. However, our results demonstrate that enhanced enthalpic contributions (i.e. large and exothermic) control the stability of the chelated complexes and generally are accompanied by large entropic penalties. We discuss the contribution of solvent reorganization and its role in enthalpy–entropy compensation for the binding equilibria studied.

Editorial for the themed issue “Catalysis in the USA”.

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted uti...

The complex Ti[OGeiPr3]4 (1), prepared via the reaction of Ti(OiPr)4 with iPr3GeOH, represents a useful structural and spectroscopic model for titanium–germanium species dispersed onto silica. This precursor was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA15 support via the thermolytic molecular precursor method. The local environments of the supported materials (TiGe3SBA15 and calcined TiGe3SBA15-O2) were studied by various spectroscopic methods, including X-ray absorption spectroscopy. These materials are active catalysts for the epoxidation of cyclic and terminal olefins with alkyl hydroperoxides under anhydrous conditions. Compared to catalysts synthesized from siloxide-only precursors, the new catalysts produce 2–3 times more product after 9 h under identical reaction conditions for the epoxidations of cyclohexene and 1-octene. The new materials did not significantly leach under reaction conditions.

A Rh-catalyzed process for the regioselective formation of vinyl sulfonate esters in moderate to high yields through the hydrosulfonation of alkynes with sulfonic acids has been developed. Our synthetic approach is capable of adding a variety of alkynes to a set of sulfonic acids and is amenable to different phosphorus ligands, solvents, and Rh precursors. We control the chemo- and regioselectivity of the reaction to the Markovnikov vinyl sulfonate ester by tuning the concentration of the exogenous ligand, PPh3, which forms the active RhP species in situ. At lower PPh3 concentrations, the reaction favors the formation of vinyl sulfonate esters, whereas at higher PPh3 concentrations, the reaction favors the formation of vinylphosphonium salts. We perform Hammett analyses and kinetic isotope effect experiments to determine the steps of the catalytic mechanism. Reaction data for substituted acetylenes indicates syn and anti addition occur across the triple bond. Experiments performed with the vinylphosphonium salt as the exogenous ligand determined that the formation of the salt was not a necessary step in the catalytic mechanism for the direct formation of the vinyl sulfonate esters.

A model catalytic system of a monolayer consisting of 9-nm average size, cubic, single-crystal Pt nanoparticles and poly(vinylpyrrolidone) (PVP) polymer capping agent deposited on a sapphire prism was investigated by sum-frequency generation (SFG) vibrational spectroscopy in total internal reflection (TIR) geometry. Exposure of a clean nanoparticle monolayer after removal of PVP by cyclic oxidation–reduction treatment to high-pressure ethylene at room temperature led to the formation of ethylidyne and di-σ bonded ethylene. Low-pressure ethylene adsorption on a pseudohexagonal reconstructed Pt(100) single crystal resulted only in the formation of di-σ bonded ethylene. High-pressure adsorption of ethylene on Pt nanoparticle monolayers and Pt(100) led to the formation of both ethylidyne and di-σ bonded ethylene and stabilized the pseudohexagonal reconstruction of Pt(100) on both the single crystal and the surface of clean cubic nanoparticles. Restructuring of the PVP layer caused by CO adsorption indicated a...

The interfacial interactions between late transition metal/metal oxide nanoparticles and oxide supports impact catalysts’ 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 IrOx.nH2O, Ni(OH)2, CuO, and Ag2O nanoparticles were added to previously reported data for Rh(OH)3 grown on nanosheets of TBA0.24H0.76Ca2Nb3O10 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 as revealed by temperature resolved images recorded using transmission electron microscopy. 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 surface-bound 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.

Two novel tris(tert-butoxy)siloxy palladium(II) complexes of the form (4,4′-di-tert-butyl-2,2′-bipyridyl)Pd[OSi(OtBu)3](R) were synthesized (1, R = OSi(OtBu)3 and 2, R = CH3). The structures of these compounds were determined by multinuclear NMR spectroscopy and single crystal X-ray diffraction. The solid-state thermolytic chemistry of 1 and 2 was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). These compounds were covalently grafted onto the surface of mesoporous silica (SBA15) under mild, nonaqueous conditions to generate supported Pd(II) centers. Reactions of 1 and 2 with the surface Si–OH groups occurs selectively through the -OSi(OtBu)3 ligand with elimination of HOSi(OtBu)3. The new materials, designated Pd(1)SBA15 and Pd(2)SBA15, were characterized using N2 porosimetry, TGA, powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). The coordination environments of the supported Pd centers were investiga...

The reaction kinetics of hydrodechlorination (HDCl) for a series of CH(4−x)Cl(x) (x = 1−4) compounds were measured on a Pd/carbon catalyst. The rate of HDCl correlated with the C−Cl bond energy, suggesting scission of this bond in the molecularly adsorbed molecule is rate-determining. The measured reaction kinetics of the CH(4−x)Cl(x) compounds support a previously proposed Langmuir−Hinshelwood type reaction mechanism. Kinetic and isotope exchange experiments demonstrated the following: gas phase H2 and HCl are in equilibrium with surface H and Cl; adsorbed Cl is the most abundant surface intermediate; and irreversible scission of the first C−Cl bond is rate-determining. The overall hydrodechlorination reaction rate can be written as kK(R−Cl)[R−Cl]/(1 + K(HCl)[HCl]/K(H2)(1/2)[H2](1/2)). The activation energy of the rate-determining step was related linearly to the dissociation energy of the first C−Cl bond broken in a Brönsted−Evans−Polanyi relationship. This behavior is in agreement with a previous study of CF(3)CF(3−x)Cl(x) compounds. During the reaction of CH3Cl, CH2Cl2, and CHCl3 with deuterium, H−D exchange occurred in only 2%, 6%, and 9% of products, respectively. The increasing H−D exchange with Cl content suggests the steps which determine selectivity in these multipath, parallel reactions. The density functional theory (DFT)-calculated activation energies for the dissociation of the first C−Cl bond in the family of chlorinated methane compounds are in good agreement with the values extracted from kinetic modeling, suggesting that parameters estimated from DFT calculations may be used to estimate the reactivity of a particular chlorinated compound within a family of chlorocarbons.

The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by alcohol reduction of a Pt salt in the presence of poly(vinylpyrrolidone) (PVP), followed by incorporation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. With an increase in particle size from 1.7 nm to 7.1 nm, the selectivity towards crotyl alcohol increases from 13.7% to 33.9% (8 Torr crotonaldehyde, 160 Torr H2 and 353 K). The turnover frequency increases from 2.1 × 10−2 s−1 to 4.8 × 10−2 s−1 with increasing particle size. Additionally, the decarbonylation pathway to form propene and CO is enhanced over smaller nanoparticles. The apparent activation energy remains constant (~16 kcal mol−1 for the formation of butyraldehyde and ~8 kcal mol−1 for the formation of crotyl alcohol) as a function of particle size as does the reaction order in H2, which is unity. In the presence of 130–260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of −1 to −1.4 for crotyl alcohol and butyraldehyde. Hydrogen reduction at 673–723 K results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with respect to the organic capping agent, PVP.

The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C6H10, 200–600 Torr H2, and 273–650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H2 (H2: C6H10 ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature ( 573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.

A procedure has been developed for the selective etching of silver from platinum nanoparticles of well-defined shape, resulting in the formation of nearly elementally pure Pt cubes, cuboctahedra, or octahedra, with a largest vertex-to-vertex distance of ∼9.5 nm from Ag-modified Pt nanoparticles. The characterization of mesoporous silica-supported Pt nanoparticles by XRD, TEM, and N2 adsorption measurements demonstrated that the structure of the nanoparticles and the mesoporous support was conserved after etching in concentrated nitric acid. Both elemental analysis and ethylene hydrogenation indicated that etching of Ag is only effective when [HNO3] ≥ 7 M; below this concentration, the removal of Ag is limited to ∼10%. The activity for ethylene hydrogenation increased by four orders of magnitude after etching Pt octahedra containing the highest fraction of silver. High-resolution transmission electron microscopy of the unsupported particles after etching demonstrated that etching does not alter the surface...

The influence of particle size on the poisoning of ethylene hydrogenation by CO was studied over a series of catalysts composed of nearly monodisperse Pt nanoparticles (1.7–7.1 nm) encapsulated in mesoporous silica (SBA-15). The turnover frequency at 403 K in the presence of 0.5 Torr CO was ∼2 × 10−2 s−1 (compared with ∼102 s−1 in the absence of CO). The apparent activation energy in the absence and presence of 0.2 Torr CO was ∼10 and 20 kcal mol−1, respectively. The pressure dependency changes significantly in the presence of CO; reaction orders in hydrogen were 1/2 in the presence of CO at 403 K and noncompetitive with regard to co-adsorption with C2H4. In the absence of CO at similar temperatures, H2 adsorption was primarily irreversible (first-order dependence), and H2 and C2H4 compete for the same sites. Ethylene orders at 403 K were first order in the presence of 0.2 Torr CO and remained unity with increasing CO pressure. At similar reaction conditions in the absence of CO, ethylene had an inhibitory effect (negative reaction order) on the overall hydrogenation reaction. The change in C2H4 and H2 kinetics suggests strong competitive adsorption between C2H4 and CO for the same type of site, whereas H2 apparently adsorbs on distinct surface sites due either to steric hindrance or H2-induced CO desorption. Incorporation of a quasi-equilibrated CO adsorption step into a noncompetitive Langmuir–Hinshelwood mechanism predicts the experimentally observed pressure dependencies and a doubling of the apparent activation energy. Hydrogenation of ethylene in the presence of 1 Torr CO was examined under reaction conditions at 403 K by infrared spectroscopy; the only surface species identified under reaction conditions was linear-bound CO. The hydrogenation of ethylene on clean Pt catalysts was structure-insensitive and remains insensitive in the presence of CO; rates decreased only by a factor of two with increasing particle size.

The oxidation and reforming kinetics of methane by O 2 , CO 2 and H 2 O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO 2 and H 2 O reforming. Total oxidation of methane to CO 2 and H 2 O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO 2 or H 2 O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO 2 and H 2 O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes.

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.

Monodisperse platinum nanoparticles with well-defined faceting have been synthesized by a modified polyol process with the addition of silver ions. Pt nanoparticles are encapsulated in mesoporous silica during in situ hydrothermal growth of the high surface area support. Removal of the surface regulating polymer, poly(vinylpyrrolidone), was achieved using thermal oxidation-reduction treatments. Catalysts were active for ethylene hydrogenation after polymer removal. Rates for ethylene hydrogenation decreased in accordance with the amount of Ag retained in the Pt nanoparticles after purification. Ag is most likely present on the Pt particle surface as small clusters. Future prospects for these catalysts for use in low temperature selective hydrogenation reactions are discussed.