Perimeter Interface Research Articles

Supported Inverse MnOx/Pt Catalysts Facilitate Reverse Water Gas Shift Reaction

Catalytic conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction has been identified as a promising approach for CO2 utilization and mitigation of CO2 emissions. Bare Pt shows low activity for the RWGS reaction due to its low oxophilicity, with few research works having concentrated on the inverse metal oxide/Pt catalyst for the RWGS reaction. In this work, MnOx was deposited on the Pt surface over a SiO2 support to prepare the MnOx/Pt inverse catalyst via a co-impregnation method. Addition of 0.5 wt% Mn to 1 wt% Pt/SiO2 improved the intrinsic reaction rate and turnover frequency at 400 °C by two and twelve times, respectively. Characterizations indicate that MnOx partially encapsulates the surface of the Pt particles and the coverage increases with increasing Mn content, which resembles the concept of strong metal–support interaction (SMSI). Although the surface accessible Pt sites are reduced, new MnOx/Pt interfacial perimeter sites are created, which provide both hydrogenation and C-O activation functionalities synergistically due to the close proximity between Pt and MnOx at the interface, and therefore improve the activity. Moreover, the stability is also significantly improved due to the coverage of Pt by MnOx. This work demonstrates a simple method to tune the oxide/metal interfacial sites of inverse Pt-based catalyst for the RWGS reaction.

Thermal Shock Synthesis for Loading Sub-2nm Ru Nanoclusters on Titanium Nitride as a Remarkable Electrocatalyst toward Hydrogen Evolution Reaction.

Heterogeneous catalysts embracing metal entities on suitable supports are profound in catalyzing various chemical reactions, and substantial synthetic endeavors in metal-support interaction modulation are made to enhance catalytic performance. Here, it is reported the loading of sub-2nm Ru nanocrystals (NCs) on titanium nitride support (HTS-Ru-NCs/TiN) via a special Ru-Ti interaction using the high-temperature shock (HTS) method. Direct dechlorination of the adsorbed RuCl3, ultrafast nucleation process, and short coalescence duration at ultrahigh temperatures contribute to the immobilization of Ru NCs on TiN support via producing the Ru-Ti interfacial perimeter. HTS-Ru-NCs/TiN shows remarkable activity toward hydrogen evolution reaction (HER) in alkaline solution, yielding ultralow overpotentials of 16.3 and 86.6mV to achieve 10 and 100mAcm-2, respectively. The alkaline and anion exchange membrane water electrolyzers assembled using HTS-Ru-NCs/TiN yield 1.0Acm-2 at 1.65 and 1.67V, respectively, which validate its applicability in the hydrogen production industry. Theoretical simulations reveal the favorable formation of Ru─O and Ti─H bonds at the interfacial perimeters between Ru NCs and TiN, which accelerates the prerequisite water dissociation kinetics for enhanced HER activity. This exemplified work motivates the design of specific interfacial perimeters via the HTS strategy to improve the performance of diverse catalysis.

Tuning the strong metal support interaction of the Fischer-Tropsch synthesis silica-coated cobalt-based nano-catalyst

The catalyst is crucial in enhancing productivity within the Fischer-Tropsch synthesis (FTS). The metal support interaction (MSI) plays a pivotal role in the FTS catalyst performance, impacting activity, stability, and selectivity. he configuration and composition of the catalyst dramatically impact MSI, with a strong MSI (SMSI) specified for the core-shell structure. In this study, two methods consisting of chemical treatment of etching and adding a promoter (Ru) for tuning the SMSI of the FTS catalyst are presented, and their footprints on the catalysts’ performance are screened by utilizing various characterization tests, molecular dynamic simulation, and catalytic tests. It is recognized that the silica interfacial perimeter acts as a barrier that restricts the charge transfer and boosts the SMSI in the silica-coated catalyst. Conversely, the etched catalyst exhibits a moderate MSI, contributing to increased reducibility, stability, and activity. Furthermore, the Ru-doped etched catalyst represents the optimum MSI, resulting in the highest overall performance.

Tuning Strong Metal‐Support Interactions for Enhancing Direct Deoxygenation of Biomass‐Lignin Derived Phenolics

AbstractCatalytic direct deoxygenation (DDO) of phenolic compounds to aromatics is an appealing approach for utilization of biomass‐lignin with minimal amount of H2 consumption. Metal/reducible metal oxide catalysts provide two functionalities, i. e., C−H bond formation (hydrogenation) and C−O bond breakage (deoxygenation), required for this reaction that takes place at the interfacial perimeter sites. Strong metal‐support interactions (SMSI) can profoundly alter the density and property of such sites. This short review summarized recent advances in tuning SMSI for enhancing DDO of phenolics. The approaches of varying reduction temperature, modulation of crystal facets and crystal phases of metal oxide, and reduction of mixed metal oxide were discussed for the origin for formation of SMSI, the degree of SMSI, and its consequence on the DDO performance. The analysis revealed that intermediate degree of SMSI with maximal density of metal/oxide interfacial perimeter site enhances DDO while minimizing unfavorable side reactions. This short review highlights the concept of tuning the interfacial perimeter sites via SMSI for reactions which require multiple functionalities and take place at the metal/oxide interface.

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity.

The implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface-induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA-15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched-row structure in softer Pt nanoparticles, which was identified by microscopic and surface-sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt-CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Pt/MnO Interface Induced Defects for High Reverse Water Gas Shift Activity

AbstractThe implementation of supported metal catalysts heavily relies on the synergistic interactions between metal nanoparticles and the material they are dispersed on. It is clear that interfacial perimeter sites have outstanding skills for turning catalytic reactions over, however, high activity and selectivity of the designed interface‐induced metal distortion can also obtain catalysts for the most crucial industrial processes as evidenced in this paper. Herein, the beneficial synergy established between designed Pt nanoparticles and MnO in the course of the reverse water gas shift (RWGS) reaction resulted in a Pt/MnO catalyst having ≈10 times higher activity compared to the reference Pt/SBA‐15 catalyst with >99 % CO selectivity. Under activation, a crystal assembly through the metallic Pt (110) and MnO evolved, where the plane distance differences caused a mismatched‐row structure in softer Pt nanoparticles, which was identified by microscopic and surface‐sensitive spectroscopic characterizations combined with density functional theory simulations. The generated edge dislocations caused the Pt lattice expansion which led to the weakening of the Pt−CO bond. Even though MnO also exhibited an adverse effect on Pt by lowering the number of exposed metal sites, rapid desorption of the linearly adsorbed CO species governed the performance of the Pt/MnO in the RWGS.

Reducing the prosthesis modulus by inclusion of an open space lattice improves osteogenic response in a sheep model of extraarticular defect.

Introduction: Stress shielding is a common complication following endoprosthetic reconstruction surgery. The resulting periprosthetic osteopenia often manifests as catastrophic fractures and can significantly limit future treatment options. It has been long known that bone plates with lower elastic moduli are key to reducing the risk of stress shielding in orthopedics. Inclusion of open space lattices in metal endoprostheses is believed to reduce the prosthesis modulus potentially improving stress shielding. However, no in vivo data is currently available to support this assumption in long bone reconstruction. This manuscript aims to address this hypothesis using a sheep model of extraarticular bone defect. Methods: Initially, CT was used to create a virtual resection plan of the distal femoral metaphyses and to custom design endoprostheses specific to each femur. The endoprostheses comprised additively manufactured Ti6Al4V-ELI modules that either had a solid core with a modulus of ∼120GPa (solid implant group) or an open space lattice core with unit cells that had a modulus of 3-6GPa (lattice implant group). Osteotomies were performed using computer-assisted navigation followed by implantations. The periprosthetic, interfacial and interstitial regions of interest were evaluated by a combination of micro-CT, back-scattered scanning electron microscopy (BSEM), as well as epifluorescence and brightfield microscopy. Results: In the periprosthetic region, mean pixel intensity (a proxy for tissue mineral density in BSEM) in the caudal cortex was found to be higher in the lattice implant group. This was complemented by BSEM derived porosity being lower in the lattice implant group in both caudal and cranial cortices. In the interfacial and interstitial regions, most pronounced differences were observed in the axial interfacial perimeter where the solid implant group had greater bone coverage. In contrast, the lattice group had a greater coverage in the cranial interfacial region. Conclusion: Our findings suggest that reducing the prosthesis modulus by inclusion of an open-space lattice in its design has a positive effect on bone material and morphological parameters particularly within the periprosthetic regions. Improved mechanics appears to also have a measurable effect on the interfacial osteogenic response and osteointegration.

Surface-dependent CO oxidation over Au/ZnO nanopyramids and nanorods

The catalytic activity of metals is very sensitive to the surface orientation of supporting oxides but rather little is known about ZnO in this regard. Nanostructured support morphologies present an attractive research strategy there because they allow probing the catalytic impact of well-defined surfaces at ambient pressure. The predominant {101̅0} side planes of hexagonal ZnO nanorods (NR) are terminated with the same number of O and Zn atoms while both the (0001) basal and {101̅1̅} side planes of hexagonal ZnO nanopyramids (NPy) are Zn-terminated. STEM analyses reveal two Au//ZnO interfacial configurations for each system after deposition of ∼3 nm Au nanoparticles. CO oxidation proceeds 15 times faster over the Au/ZnO NR than the Au/ZnO NPy catalyst at room temperature. The difference in catalytic activity can be traced to facilitated CO activation at the interfacial sites of Au/ZnO NR while it is retarded due to co-adsorption of CO with CO32- at undercoordinated Zn2+ ions of the interfacial perimeter of Au/ZnO NPy according to CO DRIFTS analyses. These results demonstrate that the catalytic behavior of metal/ZnO systems are very sensitive to ZnO surface terminations and the associated metal//ZnO interface configurations.

Influence of fibre cross-section profile on the multi-physical properties of uni-directional composites

The present work comprehensively examines the influence of fibre-matrix interface perimeter on the multi-physical properties of uni-directional composite materials. Three-dimensional microstructures (containing fibres of triangular, elliptical, rectangular, C-shape and two-lobe cross-section shape) are analysed to evaluate effective thermal conduction, thermo-elastic and piezo-electric properties. Each of these properties is normalised with the respective property of RVE with circular fibre cross-section; These normalised properties are, objectively, compared with the normalised fibre cross-section perimeter (shape factor). For all the considered properties and fibre cross-sectional shapes, a novel observation is that the property experiences a drop (or rise) in the initial range of the shape factor but rises (or drops) monotonically afterwards. This is in contrast with the existing literature observations, where properties are understood to have only a monotonic increase/decrease with the shape factor. Further, it is observed that the magnitude of the initial drop (or rise) is sensitive to the fibre volume fraction and the fibre-matrix property contrast. In accordance with the literature, a strong correlation is observed among the variation of in-plane shear moduli, transverse thermal conduction and transverse dielectric constants.

Design and Characterization of Bivalent Molecules Against SARS-CoV2

Abstract The pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has compelled extraordinary attention to the development of therapeutic reagents. The favorable physical properties of nanobodies make them attractive candidates for investigations of their ability to block viral entry. By X-ray crystallography we determined the structural basis of the interaction of a panel of synthetic nanobodies (Sb14, Sb16, Sb45 and Sb68) with the SARS-CoV-2 receptor binding domain (RBD): binary complexes of Sb16–RBD and Sb45–RBD; a ternary complex of Sb14–RBD–Sb68, and Sb45–RBD–Sb68. Sb16 and Sb45 bind the RBD with identical footprints with ACE2, but their complementarity determining regions (CDR)2 and CDR3 are oriented diametrically oppositely. The ternary structure of the Sb45-RBD-Sb68 complex indicates that Sb45 binds squarely at the ACE2 interface, whereas Sb68 attaches toward the ACE2 interface's perimeter. The ternary configuration suggests that several sybodies may capture a sizable portion of the RBD's surface. These sybody structures provided structural insights that were used to create novel, highly potent biparatopic nanobodies and bivalent molecules with ACE2 and ACE2 binding scaffold mimicking miniproteins that bind the RBD with high affinity and neutralize SARS-CoV-2 and variants. Supported by the Intramural research program of the NIAID/NIH

Origin of strong metal-support interactions between Pt and anatase TiO2 facets for hydrodeoxygenation of m-cresol on Pt/TiO2 catalysts

Metal/reducible metal oxide catalysts are widely used in hydrodeoxygenation (HDO) reactions for upgrading bio-oil to produce chemicals and fuel components. Under such strong reducing condition, the strong metal-support interaction (SMSI) should take place, however, few work has been devoted to understand the effect of facet of metal oxide on the SMSI and its consequence on HDO performance. Herein, Pt supported on anatase TiO2 with varying dominative exposed facets of (101), (100) and (001) were prepared and tested in HDO of m-cresol at 350 °C and atmosphere H2 pressure. The degree of SMSI of Pt/TiO2 is strongly dependent on the facet of TiO2 when reduced at 350 °C, and the degree of encapsulation decreases following the order of Pt/TiO2-100 > Pt/TiO2-001 > Pt/TiO2-101. The encapsulation correlates well with the reduction degree of TiO2 and the location of oxygen vacancy, which are resulted from varied oxygen vacancy formation and anisotropic diffusion rates in different facets. The intermediate SMSI is achieved on TiO2-001, which creates the maximal density of Pt-Ov-Ti3+ sites at the interfacial perimeter of TiOx/Pt for deoxygenation of m-cresol, resulting in the highest reaction rate and turnover frequency (TOF) while the lowest activation energy (Ea). In contrast, the maximum SMSI on TiO2-100 leads to the lowest deoxygenation rate while the highest Ea. This work provides insight into facet dependent SMSI of Pt/TiO2 and indicates the HDO activity can be finely tailored by tuning the facet of TiO2.

Role of MoOx/Ni(111) interfacial sites in direct deoxygenation of phenol toward benzene

The structural evolution of MoOx/Ni(111) accompanied by direct deoxygenation of phenol at the interfacial perimeter sites have been investigated by DFT. The coordinatively unsaturated Mo activates the C–OH bond, and the Ni–MoOx enables its breakage.

Size-Dependent Strong Metal–Support Interactions of Rutile TiO2-Supported Ni Catalysts for Hydrodeoxygenation of m-Cresol

A series of rutile TiO2-supported Ni catalysts with varying Ni sizes were prepared and reduced at 650 °C to explore the effect of Ni size on the strong metal–support interactions (SMSI) and its consequences on the hydrodeoxygenation (HDO) of m-cresol at 350 °C and atmospheric pressure. When the Ni size increases from 4 to 29.1 nm, the SMSI becomes stronger, e.g., the thickness of the TiOx overlayer and the coverage extent of TiOx on the Ni particle surface increase. Direct deoxygenation to toluene is the dominant pathway on Ni/TiO2 catalysts with varying Ni loadings, with almost no CH4 being formed. These results indicate that the TiOx overlayer significantly alters the property of Ni. That is, the C-C hydrogenolysis activity on bare Ni is completely inhibited due to SMSI, while the deoxygenation activity is improved at the Ni-TiOx interfacial perimeter sites. Meanwhile, the turnover frequency of HDO on small Ni particles of 4 nm is > 2 times higher than that on large Ni particles of 29.1 nm, indicating that the small Ni particle with moderate SMSI appears to be optimal for the direct deoxygenation of m-cresol to toluene. The results suggest HDO activity may be enhanced by tuning the metal particle size and SMSI degree.

Pre- and post-punching failure performances of flat slab-column joints with drop panels and shear studs

To investigate the pre- and post-punching failure performances of flat slab-column joints with drop panels and shear studs, static loading tests were conducted on three joint specimens subjected to in-plane restraints. The pre- and post-punching failure modes and load capacities of the joints were analysed. The tests identified successive punching failures at two critical regions (column perimeter and slab-drop panel interface) in all three joints. Compared with the specimen with a drop panel only, installing shear studs within the drop panels contributed to improving the punching shear strength. Although the post-punching capacity was not significantly improved, utilisation of shear studs effectively increased the ductility of the joints, rendering the punching failure around the column area more ductile. Punching shear strength validation against the international design codes reveals that they were conservative for the specimen with a drop panel only but overestimated for specimens with drop panels and shear studs.

Surface Oxygen Vacancies Confined by Ferroelectric Polarization for Tunable CO Oxidation Kinetics.

Surface oxygen vacancies have been widely discussed to be crucial for tailoring the activity of various chemical reactions from CO, NO, to water oxidation by using oxide-supported catalysts. However, the real role and potential function of surface oxygen vacancies in the reaction remains unclear because of their very short lifetime. Here, it is reported that surface oxygen vacancies can be well confined electrostatically for a polarization screening near the perimeter interface between Pt {111} nanocrystals and the negative polar surface (001) of ferroelectric PbTiO3. Strikingly, such a catalyst demonstrates a tunable catalytic CO oxidation kinetics from 200°C to near room temperature by increasing the O2 gas pressure, accompanied by the conversion curve from a hysteresis-free loop to one with hysteresis. The combination of reaction kinetics, electronic energy loss spectroscopy (EELS) analysis, and density functional theory (DFT) calculations, indicates that the oxygen vacancies stabilized by the negative polar surface are the active sites for O2 adsorption as a rate-determining step, and then dissociated O moves to the surface of the Pt nanocrystals for oxidizing adsorbed CO. The results open a new pathway for tunable catalytic activity of CO oxidation.

Effect of electropulsing on the solidification of mould flux

The effect of electropulsing on the microstructure formation in solidification of mould flux has been investigated. Experimental characterization and image analysis reveals that the electropulsing treatment reduces 46% porosity and increases the interfacial perimeter to area ratio by 100%. The latter provides strong evidence that electric current influences interfacial energy. Dendrite arm spacing is reduced considerable. Electropulsing promotes the anisotropic crystal growth and retards the formation of interconnected crystal network. The latter is favourable to the current flow, liquid refilling and lubrication. The work demonstrates that electropulsing can be used to modify the microstructure of slag and hence to control the horizontal heat transfer rate in the mould, which is desirable in continuous casting of advanced high strength steels.

Au Nanoparticles Anchored on Sulfonated Carbon Nanotubes for Benzyl Alcohol Oxidation

Promotion of gold (Au)–support synergy provides considerable opportunities to enhance the thermal stability of nanoscale gold catalysts. In this work, we developed an anchorage strategy for Au on sulfonated carbon nanotubes (CNTs), inspired by the mysterious Au–S synergy. S species existed mainly as sulfonyl groups with a tiny quantity of S atoms inserted in the carbon skeleton. The geometric and electronic structures of gold particles were modified, rendering them efficient for the selective oxidation of benzyl alcohol. Using integrated experimental and theoretical approaches, we explicitly show that Au particles of around 2.6 nm were highly dispersed at a S/C ratio of 1.4 at. %. Electrons were delivered from Au to the support with a maximized Au–C interfacial perimeter and promoted adsorption ability with the reactant. However, high S/C atomic ratios (≥3.9%) resulted in increased surface acidity and self-rearrangement of particles with abundant high-coordinated Au3+ sites at the expense of conversion loss. Under appropriate sulfonation, Au/S1CNT exhibited a TOF value of 2294 h–1 at only 50 °C. This work disentangled the structure manipulation of Au catalyst with a suitable action mode between Au and S species to afford a better Au catalyst with resistance to coarsening.

Active Cu0–Cuσ+ Sites for the Hydrogenation of Carbon–Oxygen Bonds over Cu/CeO2 Catalysts

CeO2-supported copper species have been reported as an active catalyst for the hydrogenation of carbon–oxygen bonds (CO, CO2, furfural, esters, etc.). However, the identification of active sites remains challenging. Herein, we prepared a series of rod-shaped ceria-supported copper catalysts with different copper sizes (single-atom, 1.4 nm nanoclusters, 3.0 nm and 6.8 nm nanoparticles) and applied them for methyl acetate (MA) hydrogenation. The structure and chemical environment of copper species were detected, and the surface Cu0 and Cuσ+ species and defects (oxygen vacancy and M–[Ox]–Ce solid solution) were quantitatively measured. To identify the active sites for MA hydrogenation, we also prepared contrast samples with increased surface defects or with reduced Cu0–Cuσ+ species. It is demonstrated that the Cu0–Cuσ+ species rather than oxygen vacancies or M–[Ox]–Ce solid solution are the primary active sites for MA hydrogenation. From the results of in situ experiments and various chemisorption and density functional theory calculations, the Cu0–Cuσ+ interface located at the surface Cu deposits is evidenced to play the key role in enhancing the adsorption and activation of MA. The turnover frequency of Cu/CeO2 catalysts for MA hydrogenation is linearly increased with the increase of the Cu0–Cuσ+ interfacial perimeter. This insight into active sites for carbon–oxygen bond hydrogenation may provide guidance for high-performance catalyst design.

Restructuring of the gold-carbide interface for low-temperature water-gas shift.

A passivated Au/α-MoC catalyst, containing 2-4 layered Au clusters of 1.6 nm, was re-activated by CH4/H2 at 590 °C, during which the structure of the gold-carbide interface changed considerably. The partially-oxidized surface Mo species were carburized to MoC, while the Au clusters dispersed into smaller ones, accompanied by the coating of carbide thin layers on Au. This restructuring promoted charge transfer from Au to MoC and extended the Au-MoC interfacial perimeter, which was largely responsible for the activity in the low-temperature water-gas shift reaction.

Water-gas shift reaction over supported Au nanoparticles

The water–gas shift (WGS) reaction rates per total mole of Au at 120 °C, 7% CO, 22% H2O, 8.5% CO2, 37% H2 decrease in the order Au/Anatase ∼ Au/Anatase001 (uniform anatase TiO2 single crystals with 64 per cent of the more {001} facets) ∼ Au/P25 ∼ Au/P25-WGC (obtained from the World Gold Council) ∼ Au/Rutile > Au/ZrO2 > Au/CeO2 > Au/ZnO when compared at the same number average Au particle size (d) and vary as ∼ d-3. From high resolution transmission electron microscopy images, the geometry of Au nanoparticles on these catalysts resembled truncated cubo-octahedra. A physical model of Au nanoparticles as truncated cubo-octahedra was used to calculate that the fractions of surface, perimeter and corner sites to the total Au sites vary as d-0.7, d-1.8 and d-2.9, respectively. Thus, the variation in the WGS reaction rate per total mole of Au (∼d-3) correlates well with the corner sites (d-2.9) allowing us to determine that the dominant active sites for these catalysts are the low coordinated metallic corner Au sites. In addition, as the apparent H2O order increases and the apparent activation energy decreases, the WGS reaction rate per total mole of Au systematically decreases for Au nanoparticles supported on anatase, anatase001, P25, rutile, ZrO2, CeO2, ZnO and Al2O3 at near 120 °C. Density functional theory calculations were carried out over Au nano-rods supported on rutile TiO2(100) and α-Al2O3(0001) surfaces to elucidate the differences in reactivity for the most reactive (Au/TiO2) and least reactive (Au/Al2O3) catalysts. Water preferentially adsorbs and dissociates at the Lewis acid- Lewis base Ti4+-Ob and Al3+-Ob site pairs at the Au/TiO2(110) and Au/α-Al2O3 interfaces with activation energies of 0.25 eV and 0.20 respectively. These barriers are significantly lower than those to activate water on the corner sites of unsupported Au nanoparticles (1.48 eV). The prediction that both the TiO2 and Al2O3 readily dissociate water supports the experimental findings that the support plays an important role in activating the water in the WGS reaction. The subsequent oxidation of CO appears to proceed at the Au/support interface via the reaction of CO adsorbed on Au with the OH on the support. DFT results indicate that OH binds 0.4 eV stronger to Al3+ sites at the Au/Al2O3 interface perimeter than to Ti4+ sites at the Au/TiO2 interface perimeter. As such, the barrier for OH to react with CO at an adjacent Au site is 0.52 eV higher for Au/Al2O3 (0.85 eV) than for Au/TiO2 (0.33 eV). The theoretical results suggest that the oxidation of CO is rate-limiting on Al2O3 whereas both the water dissociation as well as CO oxidation limit the rate on TiO2. This is consistent with the near first order CO dependence of the measured rates over both Au/TiO2 and Au/Al2O3 as well as the changes in the water reaction orders over Au/TiO2 (−0.2 to −0.5) and Au/Al2O3 (0.5 to 0.8).