Approaches for Understanding and Controlling Interfacial Effects in Oxide-Supported Metal Catalysts

Ro I., Aragao I.B., Brentzel Z.J., Liu Y., Rivera-Dones K.R., Ball M.R., Zanchet D., Huber G.W., Dumesic J.A.

Bimetallic PtFe/SiO2 and PtMo/SiO2 catalysts were prepared using controlled surface reactions (CSR) of (cyclohexadiene)iron tricarbonyl and (cycloheptatriene)molybdenum tricarbonyl on a Pt/SiO2 parent material. These catalysts were studied for the hydrogenation of ketone and aldehyde groups. Selective deposition of Fe onto Pt nanoparticles via the CSR method was confirmed by UV–vis absorption spectroscopy, scanning transmission electron microscopy, and inductively coupled plasma absorption emission spectroscopy. The oxidation states of the Pt and Fe species for PtFe catalysts were determined following treatment in H2 at 573 K using X-ray photoelectron spectroscopy, showing that the dominant Pt phase is metallic Pt, while Fe is present in the metallic and +2 oxidation states, and Mo is present as a mixture of metallic, +4, and +6 oxidation states. The turnover frequency (TOF) of a Pt site for acetone (ketone) hydrogenation at 353 K and atmospheric pressure is 0.9 min−1, whereas the TOF values of Pt-FexOy and Pt-MoOx sites are 93 and 76 min−1, respectively. For the hydrogenation of 2-hydroxytetrahydropyran (2-HY-THP, aldehyde) at 393 K and 30 bar pressure, the TOF of a Pt site is 7.8 min−1, while the TOF values of Pt-FexOy and Pt-MoOx sites are 480 and 830 min−1, respectively. The order of magnitude enhancement of the TOF on the Pt-FexOy and Pt-MoOx interfacial sites compared to that of the Pt site suggests that the Pt-metal oxide interface created on Pt catalysts by selective addition of Fe and Mo are active sites for both acetone and 2-HY-THP hydrogenation reactions. The presence of interfacial sites may enhance the catalytic activity over PtFe/SiO2 and PtMo/SiO2 catalysts by stabilization of adsorbed reactive intermediates through bonding with C O groups.

Resasco J., Dai S., Graham G., Pan X., Christopher P.

The elucidation of structure–property relationships for supported metal catalysts requires atomic-scale descriptions and quantitative measurements of the relative population of various exposed active sites under reaction conditions. The requirement of describing catalyst structures under reaction conditions stems from the potential physical and chemical reconstruction that can be induced by changes in environment. Here we highlight our recent work, where catalyst characterization via a combination of in-situ transmission electron microscopy and infrared spectroscopy is used to provide sample-averaged, site-specific atomic level information on supported metal catalysts under reaction conditions. We show two illustrative examples using different reaction systems: the oxidation of CO over supported Pt and the reduction of CO2 over supported Rh. Using these examples we demonstrate differentiation of the catalytic reactivity of small supported metal clusters from isolated atoms and interrogate the catalytic co...

Li T., Liu F., Tang Y., Li L., Miao S., Su Y., Zhang J., Huang J., Sun H., Haruta M., Wang A., Qiao B., Li J., Zhang T.

Guo Y., Zhang Y.

Heterogeneous metal catalysts nowadays adopt more well-defined and complex structures to enhance their utility in industrial catalysis. The metal-support interactions (MSI) play the key role in the structure–activity relationship of the metal catalysts, among which oxide supported metal clusters have greatly magnified MSI during nanocatalysis and exhibit plenty of potential in building the model of interfacial effects associated with MSI at atomic/molecular levels. As the MSI can be readily affected by the properties of both constituents in supported assemblies, at least one structural factor of the constituents has to be controlled so as to elucidate the specific interfacial effects. Accordingly, when the size of nanoclusters is controlled to high level of uniformity, the influence of oxide supports on the MSI can be uncovered and a mechanistic model of MSI in oxide supported catalysts can be established. Starting with the summary of the preparation techniques of size-controlled supported metal clusters, this review pays close attention to the variation and regulation of MSI in metal clusters supported by oxides with different reducibilities and conductivities, and proposes the specific mechanistic models of MSI on different supports.

Pacchioni G., Freund H.

Model systems are very important to identify the working principles of real catalysts, and to develop concepts that can be used in the design of new catalytic materials.

Gänzler A.M., Casapu M., Maurer F., Störmer H., Gerthsen D., Ferré G., Vernoux P., Bornmann B., Frahm R., Murzin V., Nachtegaal M., Votsmeier M., Grunwaldt J.

Pt-CeO2-Al2O3 catalysts play an important role in diesel oxidation and three-way catalysis. In this study, the fast structural dynamics of both platinum and ceria in a 1 wt %Pt/5 wt %CeO2-Al2O3 catalyst prepared by flame spray pyrolysis have been systematically investigated under reducing and oxidizing conditions to elucidate the role of the Pt–CeO2 interface for CO oxidation and fast oxygen storage/release of ceria. The catalyst showed enhanced catalytic activity, particularly after application of a reducing/oxidizing conditioning step at 250 °C, with a pronounced dependence on the reducing agent (C3H6 < H2 < CO). In situ time-resolved X-ray absorption spectroscopy (XAS) at the Ce L3-edge unraveled a dependence of the reduction extent of ceria during temperature-programmed reduction on the noble metal constituent and the applied reducing agent. Dynamic reducing/oxidizing cycling (2% H2 ↔ 10% O2 or 2% CO ↔ 10% O2) at various temperatures (150, 250, and 350 °C) showed that the reducibility of ceria increas...

Zhai H., Alexandrova A.N.

Subnano surface-supported catalytic clusters can be generally characterized by many low-energy isomers accessible at elevated temperatures of catalysis. The most stable isomer may not be the most catalytically active. Additionally, isomers may interconvert across barriers, i.e., exhibit fluxionality, during catalysis. To study the big picture of the cluster fluxional behavior, we model such a process as isomerization graph using bipartite matching algorithm, harmonic transition state theory, and paralleled nudged elastic band method. All the minimal energy paths form a minimum spanning tree (MST) of the original graph. Detailed inspection shows that, at temperatures typical for catalysis, the cluster geometry changes frequently within several regions in the MST, while transition across regions is less likely. As a further confirmation, the structural similarity analysis was additionally performed based on molecular dynamics trajectories. This local fluxionality picture provides a new perspective on understanding finite-temperate catalytic processes.

Kumar G., Nikolla E., Linic S., Medlin J.W., Janik M.J.

There has been a recent surge of interest in multicomponent catalysts that combine properties of chemically diverse materials. A major factor in this increased interest is the widespread recognition that the scaling relationships for adsorption and transition state energies of reactions place significant constraints on making step-change improvements in catalyst performance using monofunctional catalysts. In this perspective, we review the fundamental rationale for multicomponent materials and describe several classes of materials that offer promise for improving activity and selectivity in catalysis. Our focus is on illustrating how recent advances in the ability to prepare precisely controlled multicomponent nanostructures have the potential to enhance the capability to design highly active and selective catalysts.

Aragao I.B., Ro I., Liu Y., Ball M., Huber G.W., Zanchet D., Dumesic J.A.

FePt bimetallic catalysts with intimate contact between the two metals were synthesized by controlled surface reactions (CSR) of (cyclohexadiene)iron tricarbonyl with hydrogen-treated supported Pt nanoparticles. Adsorption of the iron precursor on a Pt/SiO2 catalyst was studied, showing that the Fe loading could be increased by performing multiple CSR cycles, and the efficiency of this process was linked to the renewal of adsorption sites by a reducing pretreatment. The catalytic activity of these bimetallic catalysts for the water gas shift reaction was improved due to promotion by iron, likely linked to H2O activation on FeOx species at or near the Pt surface, mostly in the (II) oxidation state.

Saavedra J., Pursell C.J., Chandler B.D.

The mechanism of CO oxidation over supported gold catalysts has long been debated, with two prevailing mechanisms dominating the discussion: a water-assisted mechanism and a mechanism involving O-defect sites. In this study, we directly address this debate through a kinetic and mechanistic investigation of the role of water in CO oxidation over Au/TiO2 and Au/Al2O3 catalysts; the results clearly indicate a common water-assisted mechanism to be at work. Water adsorption isotherms were determined with infrared spectroscopy; the extracted equilibrium constant was essentially the same for both catalysts. Added water decreases CO adsorption on Au/TiO2, likely by blocking CO binding sites at the metal-support interface. Reaction kinetics (CO, O2, and H2O reaction orders) were essentially the same for both catalysts, as were measured O-H(D) kinetic isotope effects. These data indicate that the two catalysts operate by essentially the same mechanism under the conditions of these experiments (ambient temperature, significant amounts of water available). A reaction mechanism incorporating the kinetic and thermodynamic data and accounting for different CO and O2/COOH binding sites is proposed. The mechanism and kinetic data are treated with an active site (Michaelis-Menten) approach. This indicated that water adsorption does not significantly affect reaction rate constants, only the number of active sites available at a given water pressure. Extracted water and O2 binding constants are similar on both catalysts and consistent with previous DFT calculations. Water adsorption constants are also similar to independently determined equilibrium constants measured by IR spectroscopy. The likely roles of water, surface carbonates, and oxygen vacancies at the metal-support interface are discussed. The results definitively show that, at least in the presence of added water, O vacancies cannot play an important role in the room-temperature catalysis, and that the water-assisted mechanism is far more consistent with the preponderance of the kinetic data.

Sun G., Sautet P.

Reactivity studies on catalytic transition metal clusters are usually performed on a single global minimum structure. With the example of a Pt13 cluster under a pressure of hydrogen, we show from first-principle calculations that low energy metastable structures of the cluster can play a major role for catalytic reactivity and that hence consideration of the global minimum structure alone can severely underestimate the activity. The catalyst is fluxional with an ensemble of metastable structures energetically accessible at reaction conditions. A modified genetic algorithm is proposed to comprehensively search for the low energy metastable ensemble (LEME) structures instead of merely the global minimum structure. In order to reduce the computational cost of density functional calculations, a high dimensional neural network potential is employed to accelerate the exploration. The presence and influence of LEME structures during catalysis is discussed by the example of H covered Pt13 clusters for two reactions of major importance: hydrogen evolution reaction and methane activation. The results demonstrate that although the number of accessible metastable structures is reduced under reaction condition for Pt13 clusters, these metastable structures can exhibit high activity and dominate the observed activity due to their unique electronic or structural properties. This underlines the necessity of thoroughly exploring the LEME structures in catalysis simulations. The approach enables one to systematically address the impact of isomers in catalysis studies, taking into account the high adsorbate coverage induced by reaction conditions.

Ro I., Aragao I.B., Chada J.P., Liu Y., Rivera-Dones K.R., Ball M.R., Zanchet D., Dumesic J.A., Huber G.W.

Supported Pt catalysts with different Fe/Pt atomic ratios were synthesized using controlled surface reactions to deposit (cyclohexadiene) iron tricarbonyl onto Pt/SiO2 to create Pt-FexOy interfacial sites. X-ray photoelectron spectroscopy measurements show that Pt and Fe species exist as metallic Pt and Fe oxides phases, respectively, after treatment in H2 at 573 K, whereas Fe becomes more oxidized under reaction conditions for CO oxidation at 313 K (CO:O2 = 1:1). The addition of Fe increases the turnover frequency of Pt1Fex/SiO2 at 313 K and atmospheric pressure by up to two orders of magnitude compared to Pt/SiO2. The reaction order with respect to the O2 partial pressure suggests that O2 adsorption on the surface is likely to be a rate controlling step for both Pt/SiO2 and Pt1Fe0.2/SiO2. The enhanced activity over Pt1Fex/SiO2 catalysts compared to Pt/SiO2 can be associated with a lower energy barrier for O2 adsorption and activation over Pt-FexOy interfacial sites.

Bo Z., Ahn S., Ardagh M.A., Schweitzer N.M., Canlas C.P., Farha O.K., Notestein J.M.

Controlling metal nanoparticle size and preserving metal dispersion at elevated temperature remain key challenges in designing new supported metal catalysts. Many methods have been proposed to stabilize metal nanoparticles for catalysis, but the use of specialized equipment or metal precursors can limit the application of these methods for scalable production. Here, we demonstrate a synthesis strategy to improve the dispersion and thermal stability of Pt nanoparticles on an oxide support. A thin SiO2 coat ( 45% by CO chemisorption even after prolonged heating at 500 °C, whereas Pt nanoparticles on unmodified TiO2 are less dispersed (∼33%) and their dispersion falls further upon prolonged heating. Ethylene hydrogenation demonstrates that the Pt nanoparticles on modified TiO2 preserve the catalytic activities of Pt on unmodified TiO2. The use of wet chemistry-based oxide modification and wetness impregnation makes this strategy a scalable and generalizable synthesis method to prepare other supported metal nanoparticles for catalysis applications.

Singh J.A., Thissen N.F., Kim W., Johnson H., Kessels W.M., Bol A.A., Bent S.F., Mackus A.J.

Area-selective atomic layer deposition (ALD) is envisioned to play a key role in next-generation semiconductor processing and can also provide new opportunities in the field of catalysis. In this work, we developed an approach for the area-selective deposition of metal oxides on noble metals. Using O2 gas as co-reactant, area-selective ALD has been achieved by relying on the catalytic dissociation of the oxygen molecules on the noble metal surface, while no deposition takes place on inert surfaces that do not dissociate oxygen (i.e., SiO2, Al2O3, Au). The process is demonstrated for selective deposition of iron oxide and nickel oxide on platinum and iridium substrates. Characterization by in situ spectroscopic ellipsometry, transmission electron microscopy, scanning Auger electron spectroscopy, and X-ray photoelectron spectroscopy confirms a very high degree of selectivity, with a constant ALD growth rate on the catalytic metal substrates and no deposition on inert substrates, even after 300 ALD cycles. We demonstrate the area-selective ALD approach on planar and patterned substrates and use it to prepare Pt/Fe2O3 core/shell nanoparticles. Finally, the approach is proposed to be extendable beyond the materials presented here, specifically to other metal oxide ALD processes for which the precursor requires a strong oxidizing agent for growth.

Kumar G., Tibbitts L., Newell J., Panthi B., Mukhopadhyay A., Rioux R.M., Pursell C.J., Janik M., Chandler B.D.

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. Understanding how a supporting material can change the surface chemistry of the nanoparticle catalysts that it hosts is critical to tuning catalytic properties. Experimental Hammett studies and density functional theory calculations show that differences in reactivity can be attributed to differences in the electron density at metal active sites, which arises from differences in electron donation from the support.

Gu J., Zhang H., Guo M., Hu Y.

Bono R., Uglietti R., Scheuer A., Keitl G., Wen F., Dreizler A., Votsmeier M.

Resasco J., Albanese J.F., Alvarez W., Crossley S., Haller G.L., Jacobs G., Lercher J., Li G., Lobban L., Noronha F.B., Ruiz M.P., Sooknoi T., Wang B.

Jing P., Yang J., Chu X., Chen Z., Gan T., Zhang P., Yan W., Wang D., Liu G.

Gebreeyessus G.D., Tamirat A.G., Habtu N.G., Chebude Y.

Yang Z., Shui Z., Zhao M., Wei Z., Zhang F., Duan X., Niu B., Li B., Jiang G., Hao Z.

Tu Z., Wu G., Zheng C., Wu X., Wan J., Liu S.

Solís-García A., Portillo-Cortez K., Domínguez D., Fuentes-Moyado S., Díaz de León J.N., Zepeda T.A., Caudillo-Flores U.

This study reports the synthesis, characterization, and catalytic performance of a series of catalysts of Ru supported on CeO2-Y2O3 composites (Ru/CeYX; X = 0, 33, 66, and 100 wt.% Y2O3) for CO2 hydrogenation. Supported material modification (Y2O3-CeO2), by the Y2O3 incorporation, allowed a change in selectivity from methane to RWGS of the CO2 hydrogenation reaction. This change in selectivity is correlated with the variation in the physicochemical properties caused by Y2O3 addition. X-ray diffraction (XRD) analysis confirmed the formation of crystalline fluorite-phase CeO2 and α-Y2O3. High-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed the formation of a homogeneous CeO2-Y2O3 nanocomposite. As the Y2O3 content increased, the specific surface area, measured by BET, showed a decreasing trend from 106.3 to 51.7 m2 g−1. X-ray photoelectron spectroscopy (XPS) of Ce3d indicated a similar Ce3+/Ce4+ ratio across all CeO2-containing materials, while the O1s spectra showed a reduction in oxygen vacancies with increasing Y2O3 content, which is attributed to the decreased surface area upon composite formation. Catalytically, the addition of Y2O3 influenced both conversion and selectivity. CO2 conversion decreased with increasing Y2O3 content, with the lowest conversion observed for Ru/CeY100. Regarding selectivity, methane was the dominant product for Ru/CeY0 (pure CeO2), while CO was the main product for Ru/CeY33, Ru/CeY66, and Ru/CeY100, indicating a shift towards the reverse water–gas shift (RWGS) reaction. The highest RWGS reaction rate was observed with the Ru/CeY33 catalyst under all tested conditions. The observed differences in conversion and selectivity are attributed to a reduction in active sites due to the decrease in surface area and oxygen vacancies, both of which are important for CO2 adsorption. In order to verify the surface species catalytically active for RWGS, the samples were characterized by FTIR spectroscopy under reaction conditions.

Chowdhury C., Studt F., Jelic J.

Cheng J., Nie J., Li X., Huang J., Zhang Z., Feng Z., Zhang G., Wu R., Shen S., Wei G., Zhang J.

Lu X., Gao T., Hu Q., Dong Z., Kong J., Mao L., Wang X., Bai Y., Xu J.

Chen J., Zhu H., Ma T., Li X.

We theoretically demonstrated that RhnV2O3–5– (n = 2–5) clusters can catalytically reduce NO into N2 selectively by CO, and a size-dependent behavior of NO reduction was discovered and rationalized.

Li S., Liu X., Ma J., Xu F., Lyu Y., Perathoner S., Centi G., Liu Y.

Zhang S., Wang P., Xu Z., Gong M., Cao J., Shen J., Fang X., Xu X., Xu J., Wang X.

K C B.R., Kumar D., Bastakoti B.P.

Abstract Electrochemical water splitting presents the ultimate potential of hydrogen and oxygen production; however, regulating the rate and efficiency of water splitting is highly dependent on the accessibility of extremely efficient electrode materials for slow performance kinetics and large overpotential of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Ruthenium oxide (RuO2) based materials display high performance for OER and HER because of their capacity to bind oxygen, eminent catalytic activity, low cost compared to other precious metals, and stability in a wide pH range. However, there is still much space to promote the OER and HER activity and stability of RuO2 to fulfill the necessity for practical applications in water splitting. Different researchers applied multiple approaches that boosted the catalytic performance of RuO2-based electrocatalysts toward overall water splitting. Herein, this review provides a comprehensive overview of recent advancements in RuO2-based materials in the field of water electrolysis for the generation of alternative energies. It gives a general description of water splitting in acidic and alkaline settings, including reaction mechanisms as well as common evaluation elements for the catalytic function of the materials. Most of the reviews reported based on RuO2 materials are only focused on OER performance, but this review highlighted comprehensive ideas on different strategies like morphology design, electronic structure, electrolytes, and compositions for optimizing both electrocatalytic HER and OER functioning of RuO2-based electrocatalysts.