H-assisted CO Research Articles

Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation.

The mechanism of CO hydrogenation to CH4 at 260 °C on a cobalt catalyst is investigated using steady-state isotopic transient kinetic analysis (SSITKA) and backward and forward chemical transient kinetic analysis (CTKA). The dependence of CHx residence time is determined by 12CO/H2 → 13CO/H2 SSITKA as a function of the CO and H2 partial pressure and shows that the CH4 formation rate is mainly controlled by CHx hydrogenation rather than CO dissociation. Backward CO/H2 → H2 CTKA emphasizes the importance of H coverage on the slow CHx hydrogenation step. The H coverage strongly depends on the CO coverage, which is directly related to CO partial pressure. Combining SSITKA and backward CTKA allows determining that the amount of additional CH4 obtained during CTKA is nearly equal to the amount of CO adsorbed to the cobalt surface. Thus, under the given conditions overall barrier for CO hydrogenation to CH4 under methanation condition is lower than the CO adsorption energy. Forward CTKA measurements reveal that O hydrogenation to H2O is also a relatively slow step compared to CO dissociation. The combined transient kinetic data are used to fit an explicit microkinetic model for the methanation reaction. The mechanism involving direct CO dissociation represents the data better than a mechanism in which H-assisted CO dissociation is assumed. Microkinetics simulations based on the fitted parameters confirms that under methanation conditions the overall CO consumption rate is mainly controlled by C hydrogenation and to a smaller degree by O hydrogenation and CO dissociation. These simulations are also used to explore the influence of CO and H2 partial pressure on possible rate-controlling steps.

CO Dissociation Mechanism on Pd-Doped Fe(100): Comparison with Cu/Fe(100)

Spin-polarized density functional theory computations have been used to investigate the CO dissociation mechanisms and the different catalytic activities of the reaction on Fe(100) surfaces with different Pd coverages. CO can dissociate on Pd/Fe surfaces via three different mechanisms: direct and H-assisted mechanisms via HCO intermediate or COH intermediate. In our calculation, it was found that the activation barriers of direct CO and COH dissociation mechanisms on pure and Pd-doped Fe(100) surfaces were higher than that of the HCO dissociation mechanism. Besides, energy barriers for the identical reaction pathway on Fe-rich Fe(100) surfaces were lower than those on Pd-rich Fe(100) surfaces, namely, CO dissociation mainly occurs via the HCO intermediate pathway and the catalytic activity becomes lower with Pd coverage increasing toward CO dissociation in both direct CO and H-assisted CO dissociation mechanisms. As a result, CO dissociation mainly occurs on Fe-rich Pd/Fe surfaces, leading to the formatio...

Insight into CHx formation in Fischer–Tropsch synthesis on the hexahedron Co catalyst: Effect of surface structure on the preferential mechanism and existence form

Spin-polarized density functional theory calculations have been performed to investigate the preferential mechanism of CHx(x=1–3) formation in Fischer-Tropsch synthesis on only the hexahedron Co(101¯0)-A and Co(10 1¯0) surfaces. Our results show that CO hydrogenation to CHO is favored compared to CO direct dissociation and hydrogenation to COH on these two surfaces. Starting from C and CHO, we further seek out the optimal pathways of CHx formation, suggesting that CHx is mainly formed through H-assisted CO dissociation pathways on Co(101¯0)-A surface, in which CH is form via CHO dissociation, CH2 and CH3 are formed through CH2O with the direct and H-assisted dissociation, respectively; meanwhile, CH2 hydrogenation also contributes to CH3 formation; CH2 and CH3 are the surface abundant species on Co(101¯0)-A surface. However, on Co(101¯1) surface, CHx species is formed through CO direct dissociation into C, followed by C successive hydrogenation, C and CH are the surface abundant species. Therefore, Co surface structure can affect the preferential formation pathways and the dominant existence form of CHx species. Moreover, CH3OH formation cannot compete with CHx formation on Co(101¯0)-A and Co(101¯1) surfaces, considering both surfaces covering 63% of the total surface area exposed of hexahedron Co surfaces, which depends on the reaction conditions, particle size, catalyst support, carbon deposition and many other factors, the contribution to the overall CHx sources from Co(101¯0)-A and Co(101¯1) surfaces even surpasses that of other hexahedron Co surfaces under the certain realistic conditions. As a result, the hexahedron Co surfaces exhibit high catalytic selectivity for CHx formation, and provide more CHx sources to participate into the F-T synthesis.

Fischer-Tropsch Synthesis, Catalysts, and Catalysis: Advances and Applications (Chemical Industries 142)

Spin-polarized density functional theory calculations have been performed to investigate the preferential mechanism of CHx(x = 1–3) formation in Fischer-Tropsch synthesis on only the hexahedron Co(101¯0)-A and Co(101¯0) surfaces. Our results show that CO hydrogenation to CHO is favored compared to CO direct dissociation and hydrogenation to COH on these two surfaces. Starting from C and CHO, we further seek out the optimal pathways of CHx formation, suggesting that CHx is mainly formed through H-assisted CO dissociation pathways on Co(101¯0)-A surface, in which CH is form via CHO dissociation, CH2 and CH3 are formed through CH2O with the direct and H-assisted dissociation, respectively; meanwhile, CH2 hydrogenation also contributes to CH3 formation; CH2 and CH3 are the surface abundant species on Co(101¯0)-A surface. However, on Co(101¯1) surface, CHx species is formed through CO direct dissociation into C, followed by C successive hydrogenation, C and CH are the surface abundant species. Therefore, Co surface structure can affect the preferential formation pathways and the dominant existence form of CHx species. Moreover, CH3OH formation cannot compete with CHx formation on Co(101¯0)-A and Co(101¯1) surfaces, considering both surfaces covering 63% of the total surface area exposed of hexahedron Co surfaces, which depends on the reaction conditions, particle size, catalyst support, carbon deposition and many other factors, the contribution to the overall CHx sources from Co(101¯0)-A and Co(101¯1) surfaces even surpasses that of other hexahedron Co surfaces under the certain realistic conditions. As a result, the hexahedron Co surfaces exhibit high catalytic selectivity for CHx formation, and provide more CHx sources to participate into the F-T synthesis.

Development of a macro-micro kinetic model for CO hydrogenation over Co–Ni catalyst

A novel approach in kinetic study of CO hydrogenation reaction over Co–Ni/Al2O3 catalyst was described. The experimental data were collected using a fixed bed micro reactor under next conditions: pressure 2–12 bar, temperature 230–280°C, molar ratio H2: CO = 1–3, space velocity 2000–2700 h–1. A new Langmiur–Hinshelwood (LH) kinetic model on the basis of formyl mechanism was developed. The apparent activation energy of LH kinetic model, bond energy of carbon, oxygen, hydrogen and some surface intermediate on catalyst surface, activation energies of some surface reactions and catalyst surface coverage were estimated from a new combined LHHW/UPI–QEP methodology. It was postulated that under reaction conditions used, CH2 formation through H-assisted CO dissociation is probably dominant mechanism of hydro-genation over Co–Ni catalyst.

Methane, formaldehyde and methanol formation pathways from carbon monoxide and hydrogen on the (0 0 1) surface of the iron carbide χ-Fe5C2

Formation of CHx(O) monomers and C1 products (CH4, CH2O, and CH3OH) on C-terminated χ-Fe5C2(001) (Hägg carbide) surfaces of different carbon contents was investigated using periodic DFT simulations. Methane (CH4) as well as monomer (CHx) formation follows a Mars–van Krevelen-like cycle starting with the hydrogenation of surface carbidic carbon, which is regenerated by subsequent CO dissociation, while oxygen is removed as H2O. In cases where surface carbon is readily available, the apparent barrier for CH4 formation was found to be ∼95kJ/mol. However, different rate-determining steps show that different propagation mechanisms may be possible for actual chain growth, depending on the carbon content of the surface. Hydrogen addition to CO forms formyl (HCO), which is a precursor for both H-assisted CO activation and oxygenate formation. Further hydrogenation of HCO yields adsorbed formaldehyde and methoxy, rather than hydroxymethyl (HCOH) that would give C–O bond splitting. Full hydrogenation to gas-phase methanol faces a high barrier, suggesting that CHxO species may be involved in higher oxygenate formation in a full Fischer–Tropsch mechanism or that the C–O bond does not break until the CHO fragment has been incorporated in a C2 species, a route for which precedents are available in the literature.

Direct versus hydrogen-assisted CO dissociation over stepped Ni and Ni3Fe surfaces: a computational investigation.

The adsorption and dissociation of CO over stepped Ni and Ni3Fe surfaces were systematically studied using density functional theory slab calculations. Both (211)-like surface structure terminations (NiNi step and NiFe step, denoted as Ni3Fe(211)-AA and Ni3Fe(211)-AB) are considered for Ni3Fe. Direct scission of the C-O bond in CO is identified as the least likely one among the three proposed dissociation pathways and CO dissociation via a CHO intermediate appears to be most feasible at low CO coverage on pure and alloyed Ni(211) surfaces. The priority of H-assisted CO dissociation might originate from the more activated C-O bond in COH and CHO. Compared to Ni(211), the Ni3Fe(211)-AB surface could facilitate CO activation especially for the most possible CHO intermediate mechanism, whose rate-limiting step is found to be altered. The d-band center theory and Mulliken charge analysis are also employed to explain the activity difference between Ni3Fe(211)-AB and Ni3Fe(211)-AA. The significant structural sensitivity of CO dissociation highlights the importance of Fe locating in the step edge and the high reactivity of Ni3Fe(211)-AB is largely ascribed to the synergistic effect between Ni and Fe at the step edge.

Elementary reactions of CO and H2 on C-terminated χ-Fe5C2(0 0 1) surfaces

CO and H2 (co-)adsorption, direct and H-assisted CO activation, and surface carbon hydrogenation were investigated on C-terminated χ-Fe5C2(001) surfaces. Periodic DFT simulations at different surface carbon contents on the carbide surface showed that CO adsorbs preferably linearly on Fe top sites; CO and H2 adsorptions being stable. The perfect carbide surface favors carbidic carbon hydrogenation (i.e. CH formation), whereas carbon-free surface favors direct CO dissociation and restoration of the carbide structure. In partially carbon-vacant intermediate situations, both direct and H-assisted CO activations are energetically feasible, the latter being the preferred path. Considering CHx and CHxO species as initiators for different product types can explain the catalytic behavior and selectivity patterns of iron carbide catalysts. The catalytically active surfaces are concluded to be dynamic, where carbon atoms of the carbide surface participate in the surface reactions, and CO dissociation on vacant sites leads to restoration of the carbide structure.

CO Activation Pathways of Fischer–Tropsch Synthesis on χ-Fe5C2 (510): Direct versus Hydrogen-Assisted CO Dissociation

Iron carbides, especially χ-Fe5C2, among the active iron species in Fischer–Tropsch synthesis (FTS), are considered to be responsible for high FTS activity. CO activation pathways as the initial steps of FTS over χ-Fe5C2 were explored by spin-polarized density functional theory calculations. Surface energies of χ-Fe5C2 facets observed from the XRD patterns were first calculated, and then the corresponding equilibrium χ-Fe5C2 shape was obtained by Wulff construction. The thermodynamically stable (510) surface was predicted to have the largest percentage among the exposed crystal facets. Subsequently, the adsorption properties of CO on χ-Fe5C2 (510) were studied. Despite exhibiting lower binding energy than that at the 3F-4 site as the most stable configuration, CO adsorption at the 4F-1 site led to significant weakening of the C–O bond from both the structural and electronic properties’ points of view. Furthermore, two kinds of CO activation mechanisms (i.e., the direct and H-assisted CO dissociation) and ...

CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces

Periodic density functional theory calculations were carried out to investigate CO dissociation pathways on the Fe(100) surfaces covered with up to one monolayer of Cu atoms, which serve as the simple models for the Cu/Fe catalysts for higher alcohol synthesis (HAS) from syngas. For all the model catalyst surfaces, H-assisted CO dissociation was predicted to have lower energy barriers than direct CO dissociation. The difference in the energy barriers between the two dissociation pathways increases as Cu surface coverage increases, suggesting reduced contribution of direct CO dissociation on Cu-rich surfaces. A further thermodynamic analysis also reaches the same conclusion. Several reaction properties for CO dissociation, including CO physisorption and chemisorption energies, and energy barriers for direct and H-assisted CO dissociations, were found to scale linearly with Cu surface coverage, and these reaction properties were predicted to depend largely on the structure of the surface layer, which can be...

Preparation of Fe-Mn/K/Al2O3 Fischer-Tropsch Catalyst and Its Catalytic Kinetics for the Hydrogenation of Carbon Monoxide

A K promoted iron-manganese catalyst was prepared by sol-gel method, and subsequently was tested for hydrogenation of carbon monoxide to light olefins. The kinetic experiments on a well-characterized Fe-Mn/K/Al2O3 catalyst were performed in a fixed-bed micro-reactor in a temperature range of 280-380 °C, pressure range of 0.1-1.2 MPa, H2/CO feed molar ratio range of 1-2.1 and a space velocity range of 2000-7200 h−1. Considering the mechanism of the process and Langmuir-Hinshelwood-Hogan-Watson (LHHW) approach, unassisted CO dissociation and H-assisted CO dissociation mechanisms were defined. The best models were obtained using non-linear regression analysis and Levenberg-Marquardt algorithm. Consequently, 4 models were considered as the preferred models based on the carbide mechanism. Finally, a model was proposed as a best model that assumed the following kinetically relevant steps in the iron-Fischer-Tropsch (FT) synthesis: (1) CO dissociation occurred without hydrogen interaction and was not a rate-limiting step; (2) the first hydrogen addition to surface carbon was the rate-determining steps. The activation energy and adsorption enthalpy were calculated 40.0 and −30.2 kJ·mol−1, respectively.

CO Chemisorption and Dissociation at High Coverages during CO Hydrogenation on Ru Catalysts

Density functional theory (DFT) and infrared spectroscopy results are combined with mechanism-based rate equations to assess the structure and thermodynamics of chemisorbed CO (CO*) and its activation during Fischer-Tropsch synthesis (FTS). CO* binding becomes weaker with increasing coverage on Ru(0001) and Ru201 clusters, but such decreases in binding energy occur at higher coverages on Ru201 clusters than on Ru(0001) surfaces (CO*/Ru = 1.55 to 0.75); such differences appear to reflect weaker repulsive interactions on the curved surfaces prevalent on small Ru201 clusters. Ru201 clusters achieve stable supramonolayer coverages (CO*/Ru > 1) by forming geminal dicarbonyls at low-coordination corner/edge atoms. CO* infrared spectra on Ru/SiO2 (~7 nm diameter) detect mobile adlayers that anneal into denser structures at saturation. Mechanism-based FTS rate equations give activation energies that reflect the CO*-saturated surfaces prevalent during catalysis. DFT-derived barriers show that CO* predominantly reacts at (111) terraces via H-assisted reactions, consistent with measured effects of H2 and CO pressures and cluster size effects on rates and O-rejection selectivities. Barriers are much higher for unassisted CO* dissociation on (111) terraces and low-coordination atoms, including step-edge sites previously proposed as active sites for CO* dissociation during FTS. DFT-derived barriers indicate that unassisted CO* dissociation is irreversible, making such steps inconsistent with measured rates. The modest activation barriers of H-assisted CO* dissociation paths remove a requirement for special low-coordination sites for unassisted CO* activation, which is inconsistent with higher rates on larger clusters. These conclusions seem generally applicable to Co, Fe, and Ru catalysts, which show similar FTS rate equations and cluster size effects. This study also demonstrates the feasibility and relevance of DFT treatments on the curved and crowded cluster surfaces where catalysis occurs.

A new insight into the initial step in the Fischer–Tropsch synthesis: CO dissociation on Ru surfaces

In the present work, we have investigated the CO dissociation on corrugated Ru(1121) and the stepped Ru(0001) surfaces by means of density functional theory with slab models. Our results show that, while the direct CO dissociation is preferred on the six-fold site of Ru(1121), the H-assisted CO dissociation is found to be favored on the B5 site of the stepped Ru(0001) surface. Furthermore, we have studied the effects of co-adsorbed spectator species on the CO dissociation mechanisms. Our results demonstrate that spectators can change the potential energy landscape dramatically, such that different reaction mechanisms can be favored in the presence of different spectators. Neither the H-assisted CO dissociation mechanism nor the CO direct dissociation mechanism should be overlooked at authentic ambient conditions. This work emphasizes a dynamic picture of the reaction mechanisms due to the inherent structural and compositional inhomogeneity on surfaces. Different mechanisms can work together as different active sites will co-exist on a real catalyst surface, and the reaction preferences on an active site can vary as the adsorbate compositions on surfaces are varying during the course of the reactions.

Detailed Kinetics of the Fischer–Tropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

A complete mechanistic kinetic model of the Fischer–Tropsch synthesis (FTS) over a Co/Al2O3 state-of-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H2 consumption and n-paraffins, α-olefins and H2O formation are derived. Such expressions are functions of the molar fraction of CO, H2 and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by non-linear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8–25 bar, T = 210–235 °C, H2/CO feed ratio = 1.8–2.7 mol mol−1, GHSV = 2,000–7,000 cm3(STP) h−1 gcat−1). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer–Tropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.

Density Functional Theory Study on Surface CxHy Formation from CO Activation on Fe3C(100)

Spin-polarized density functional theory calculations have been performed to investigate the mechanisms for CxHy formation on Fe3C(100). It is found that H-assisted CO dissociation (CO + H → CHO; CHO → CH + O) has lower barrier than CO direct dissociation (CO → C + O), but surface Cs atom hydrogenation to form surface CsH is the most favored pathway. As the first C2 surface species, surface ketenylidene CsCO rising from CO adsorption is an important intermediate for C2Hx formation. Initial surface C2Hx forms from CsCO hydrogenation instead of direct dissociation. The formation of CsCH, CsHCH and CsH2CH has close effective barriers and depends on the CO/H2 ratio. In addition, surface vacancy can activate CO strongly and lower the CO dissociation barrier considerably, and this regenerates the carburized active surface and closes a catalytic cycle.

Kinetically Relevant Steps and H2/D2Isotope Effects in Fischer−Tropsch Synthesis on Fe and Co Catalysts

H2/D2 isotope effects on Fischer−Tropsch synthesis (FTS) rate and selectivity are examined here by combining measured values on Fe and Co at conditions leading to high C5+ yields with theoretical estimates on model Fe(110) and Co(0001) surfaces with high coverages of chemisorbed CO (CO*). Inverse isotope effects (rH/rD < 1) are observed on Co and Fe catalysts as a result of compensating thermodynamic (H2 dissociation to H*; H* addition to CO* species to form HCO*) and kinetic (H* reaction with HCO*) isotope effects. These isotopic effects and their rigorous mechanistic interpretation confirm the prevalence of H-assisted CO dissociation routes on both Fe and Co catalysts, instead of unassisted pathways that would lead to similar rates with H2 and D2 reactants. The small contributions from unassisted pathways to CO conversion rates on Fe are indeed independent of the dihydrogen isotope, as is also the case for the rates of primary reactions that form CO2 as the sole oxygen rejection route in unassisted CO dissociation paths. Isotopic effects on the selectivity to C5+ and CH4 products are small, and D2 leads to a more paraffinic product than does H2, apparently because it leads to preference for chain termination via hydrogen addition over abstraction. These results are consistent with FTS pathways limited by H-assisted CO dissociation on both Fe and Co and illustrate the importance of thermodynamic contributions to inverse isotope effects for reactions involving quasi-equilibrated H2 dissociation and the subsequent addition of H* in hydrogenation catalysis, as illustrated here by theory and experiment for the specific case of CO hydrogenation.

CO activation pathways and the mechanism of Fischer–Tropsch synthesis

Unresolved mechanistic details of monomer formation in Fischer–Tropsch synthesis (FTS) and of its oxygen rejection routes are addressed here by combining kinetic and theoretical analyses of elementary steps on representative Fe and Co surfaces saturated with chemisorbed CO. These studies provide experimental and theoretical evidence for hydrogen-assisted CO activation as the predominant kinetically-relevant step on Fe and Co catalysts at conditions typical of FTS practice. H 2 and CO kinetic effects on FTS rates and oxygen rejection selectivity (as H 2O or CO 2) and density functional theory estimates of activation barriers and binding energies are consistent with H-assisted CO dissociation, but not with the previously accepted kinetic relevance of direct CO dissociation and chemisorbed carbon hydrogenation elementary steps. H-assisted CO dissociation removes O-atoms as H 2O, while direct dissociation forms chemisorbed oxygen atoms that desorb as CO 2. Direct CO dissociation routes are minor contributors to monomer formation on Fe and may become favored at high temperatures on alkali-promoted catalysts, but not on Co catalysts, which remove oxygen predominantly as H 2O because of the preponderance of H-assisted CO dissociation routes. The merging of experiment and theory led to the clarification of persistent mechanistic issues previously unresolved by separate experimental and theoretical inquiries.

Surface Species Formed During CO and CO 2 Hydrogenation over Rh/TiO 2 (W6+) Catalysts Investigated by FTIR and Mass-Spectroscopy

Surface species formed over Rh/TiO 2 (W 6+) catalysts during CO and CO 2 hydrogenation were investigated by in situ FTIR spectroscopy and transient techniques. It was found that at least four carbon-containing species, namely, linear CO, bridged CO, active carbon (C α) and less-active carbon (C β), exist on Rh crystallites, while formate and/or carbonate and/or hydrocarbonate species are on the TiO 2 support. A reduction of CO coverage by 25 ∼ 40% and a shift of the linear CO band to higher wavenumber values by 10 ∼ 14 cm −1 were observed under CO and CO 2 hydrogenation upon doping the TiO 2 carrier with small amounts of W 6+ cations (< 1 at.%). This implies that the Rh-CO bond is significantly weakened by doping, presumably due to an alteration of the electronic structure of the TiO 2 carrier, which modifies the electronic state of surface Rh via electronic interactions at the metal-support interface. The concentrations of the C α and C β species on Rh crystallites and carbon-containing species on the support were also found to be influenced by doping. While the concentration of the C α species (including CH x species) was reduced and that of the C β species was enhanced with increasing W 6+ dopant content, the concentration of the carbonate and/or hydrocarbonate species on the carrier exhibited a maximum at a W 6+ content of 0.11 ∼ 0.22 at.%. It is reasoned that hydrogen chemisorption is favored on the doped catalysts, as the CO coverage and the strength of the Rh-CO bond are significantly reduced. This has twofold consequences which result in a large increase of the CO and CO 2 hydrogenation activity, as previously observed over the Rh/TiO 2 (W 6+) catalysts: there is an increase in the concentration of surface hydrogen participating in the rate-determining step and an enhancement of the formation of Rh carbonyl hydride species, through which the reaction proceeds via a route of lower activation energy (H-assisted CO dissociation).

SMSI effects on CO adsorption and hydrogenation on Pt catalysts: Part II. Influence of support and crystallite size on the kinetics of methanation

Platinum was dispersed on η-Al 2O 3, SiO 2, SiO 2Al 2O 3, and TiO 2, and its fraction exposed was varied from near unity to 0.007. The turnover frequency (TOF) for CO hydrogenation to CH 4 was constant over this range of Pt crystallite size on a given support. However, the support utilized had a major influence on specific activity and TOF values on Pt TiO 2 catalysts were 10 times greater than those on Pt Al 2O 3 and Pt SiO 2Al 2O 3 and 100-fold higher than the TOF on Pt SiO 2 . A model for methanation invoking H-assisted CO bond rupture as the rate determining step correlated the different kinetic parameters measured over these Pt catalysts. Analysis of this model using transition state theory provided an estimate of the maximum active site density and, in agreement with in situ ir spectroscopic studies, indicated that only a small fraction (0.01 or less) of the surface Pt atoms constitute active sites on TiO 2-supported Pt. The alteration in catalytic behavior for TiO 2-supported Pt is attributed to a change in structure of the Pt crystallites caused by a strong interaction with a partially reduced titania surface. The observed enhancement in activity after reduction at only 473 K implies that this effect is localized and not dependent on bulk electronic properties.