Article pubs.acs.org/JPCC
Fischer−Tropsch Synthesis over Supported Pt−Mo Catalyst: Toward Bimetallic Catalyst Optimization Sergey N. Rashkeev*,† and Michael V. Glazoff‡ †
Center for Advanced Modeling & Simulation, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States Advanced Process & Decision Systems, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States
‡
ABSTRACT: The product distribution of the Fischer−Tropsch (FT) process demonstrates a strong dependence upon the choice of catalyst, catalytic support, and reaction temperature. To develop understanding of the factors that underpin catalytic activity, we performed density-functional-theory (DFT)-based first-principles calculations for syngas reaction over bimetallic (Pt−Mo) catalysts including bimetallic surfaces and alloyed nanoparticles (NPs) positioned on a top of γ-Al2O3 substrate. It was found that catalytic activity of the (Pt−Mo) nanoparticles depends upon (i) the selectivity and reactivity of different atomic sites at the surface that may significantly affect the kinetics of different stages of the FT synthesis and (ii) the optimal composition of the NP allowing increasing the methane production at the first stage of the FT synthesis. This work highlights the main mechanisms that govern bimetallic catalyst activity for the FT synthesis. Similar considerations could be developed for any bimetallic catalytic system and any catalytic reactions. The results presented here should help to provide a solid basis for the rational design and/or improvement of many bimetallic catalysts.
1. INTRODUCTION The Fischer−Tropsch synthesis has been well-known for about a century. However, it continues to inspire a significant body of research due to the increase in the price of oil and the abundance of coal and biofeeds that could be used to generate liquid fuel. The FT process involves the catalytic conversion of CO and molecular hydrogen into chain hydrocarbons which can be converted to diesel fuel and other commercially important products. The FT synthesized hydrocarbons are virtually free of sulfur, nitrogen, and metallic contaminants which make them more environmentally friendly. Despite being an established industrial technology since 1926, the complex chemistry of FT synthesis is still not fully understood.1−5 Product distribution of the FT process demonstrates a strong dependence upon the choice of a catalyst, catalytic support, and reaction temperature. The overall FT reaction consists of a complex sequence of the bond-making and bond-breaking elementary steps. First, adsorbed CO and H2 are activated upon the catalyst surface; second, carbon-containing surface intermediates get hydrogenated; and finally, carbon species react with each other to form complex chain hydrocarbons. A delicate balance between the rates of these reactions controls the reactivity and selectivity of the process. Advances in current catalyst technology require that a more complete understanding of the elementary atomic level transformations involved in the FT synthesis should be developed. Commercially, the FT process is conducted at temperatures around 250 °C, with syngas (CO + H2) pumped through a reactor containing supported a transition-metal-based catalyst. The choice of a catalyst is critically important for the product © 2013 American Chemical Society
distribution. A variety of catalysts can be used for the Fischer− Tropsch process, but the most common are the transition metals such as Co, Fe, and Ru. Nickel can also be used but tends to favor methane formation. Co-based catalysts are highly active, although iron may be more suitable for low-hydrogencontent synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal, the catalysts typically contain a number of “promoters”, including potassium and copper. Group I alkali metals, including potassium, are poisons for cobalt catalysts but serve as promoters for iron catalysts.6 Catalysts are supported on a high-surface-area support (silica, alumina, or zeolites).7 Cobalt catalysts are more active for the FT synthesis when the feedstock is natural gas while iron catalysts are preferred for lower quality feedstocks such as coal or biomass. Unlike the other metals used for this process (Co, Ni, Ru), which remain in the metallic state during synthesis, iron catalysts tend to form a number of phases, including various oxides and carbides during the reaction. This is not necessarily bad. Recent spinpolarized DFT calculations for the carbon pathways and hydrogenation mechanism for CH4 formation on Fe2C(011), Fe5C2(010), Fe3C(001), and Fe4C(100) surfaces showed that with the formation of vacancy sites by C atoms escaping from the FexCy surface the CO dissociation barrier decreases largely.8 As a consequence, the active carburized surface is maintained. However, control of the phase transformations should be important for maintaining catalytic activity and preventing the Received: May 10, 2012 Revised: February 5, 2013 Published: February 14, 2013 4450
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hydrogenation than supported monometallic catalysts of Mo or Pt. The structure of the Mo−Pt bimetallic catalysts, which were prepared by incipient wetness impregnation of γ-alumina with the variation of the order of Mo and Pt addition, was studied by EXAFS spectroscopy and CO chemisorptions technique.17 It was demonstrated that the bonds between Mo and Pt are formed for all bimetallic catalysts and that molybdenum segregates to the surface. Regarding the process of CO hydrogenation, it was concluded that the reduced Mo sites were responsible for the high activity and that Pt enhanced the reduction of the Mo sites. Thus, it is expected that understanding of the structure of Pt−Mo bimetallic catalysts would help clarify the role of each metal. In this study we employed first-principles, density-functionaltheory (DFT) calculations to elucidate the main processes that contribute into the catalytic activity of the Mo−Pt bimetallic catalysts for the first stage of the FT processmethane formation. We did not consider later stages at which carbon species react with each other to form complex chain hydrocarbons. Calculations for chemical reactions at bimetallic (Mo−Pt) nanoparticles supported at the γ-Al2O3 substrate surface were performed in a way similar to the first-principles calculations of CO oxidation at rutile supported gold nanoparticles.18 The Mo−Pt system was chosen mainly because of the availability of synchrotron EXAFS data providing information on the structure of such catalytic clusters.17 We confirmed the conclusion made in EXAFS experiments regarding segregation of molybdenum to the alumina surface. We also found that at both monometallic and bimetallic NPs the dissociation of the C−O bond still remains the main ratelimiting step in the methanation processall the barriers for subsequent hydrogenation reactions (i.e., CH*, (CH2)*, (CH3)*, and CH4 formation) are lower than the barrier for the initial CO molecule decomposition. We showed that the C−O bond decomposition barrier is always lower at the NP’s surface Pt−Mo bond than at both Pt−Pt and Mo−Mo bonds. This result is also valid for the hydrogen-assisted CO decomposition. Our calculations explain the efficiency of alloyed NPs. The developed methodology could be applied to any types of alloyed nanoparticles and may provide guidance for development of bimetallic catalytic systems for FT processes.
breakdown of the catalyst particles. Also, practically all Fischer−Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds (which is a big problem in using natural gas as a feedstock). The sensitivity of the catalyst to sulfur is greater for cobalt-based catalysts than for their iron counterparts. For a rational design of catalysts for the FT synthesis based on the use of other metals, it is necessary to choose metals with certain desired functionality. For example, transition metals to the left side of the periodic table will readily dissociate the adsorbed CO molecules. However, the products, namely surface chemisorbed carbon (C*) and oxygen (O*), can strongly inhibit the rate of hydrogenation reactions and hydrocarbon coupling reactions (i.e., intermediate products are slowing down the process of interest). On the other hand, C* and O* on the transition metals to the right side of the periodic table can readily undergo hydrogenation and coupling reactions. The barrier for CO activation, however, is very high.9,10 The optimal material should readily dissociate CO* yet not be poisoned by strongly adsorbed carbon or oxygen. In addition, an ideal catalyst would have the proper balance of hydrogenation and carbon−carbon coupling reactions to promote high molecular weight hydrocarbon products. The products that form under reaction conditions may be controlled by the species present at the surface as the reaction proceeds. Surface conditions such as roughness and presence of defects should also contribute to the rate of the FT process. A combination of extensive DFT calculations, ultrahigh-vacuum experiments on well-defined single crystals, and catalytic activity measurements on supported catalysts have been performed to study the dissociation mechanism of CO on Ni surfaces.11 It was found that this process is highly structure sensitivethe dissociation proceeds through a direct route in which only undercoordinated sites (e.g., steps) are active. Also, the process is sensitive to the presence of hydrogen, and under methanation conditions, the dissociation also proceeds most favorably over undercoordinated sites, but through a formation of COH species. Another promising area in developing catalytic systems for the FT process is employing bimetallic catalysts. These systems are often used because they may have significantly different catalytic properties than either of the parent metals. Also, bimetallic catalysts, in which the metal type and concentration can be used to “tune” catalytic performance, offer unlimited opportunities for any chemical process. A detailed DFT calculation-based analysis of the catalytic activity of a large group of transition-metal-based bimetallic catalysts for the methanation reaction was recently reported.12 Bimetallic systems that consist of Mo and Pt have shown high activity (higher than any monometallic catalyst) in hydrogenolysis of alkanes13−15 and dehydrogenation of cyclohexane.16 It was suggested that the higher activity of the Mo− Pt catalyst for hydrogenolysis of ethane is due to the change in electronic properties of platinum caused by its interaction with molybdenum on a support.13 Later it was concluded that addition of Mo made initially electron-deficient Pt particles be similar to bulk Pt and that the enhanced activity is due to the complementary role of Pt and Mo in the adsorption of reactants.14,15 A more detailed absorption and XPS measurements related the higher activity to molybdenum, modified by addition of Pt in the bimetallic catalyst.16 It was also shown that molybdenum−platinum bimetallic catalysts supported on alumina exhibited high activities in CO
2. COMPUTATIONAL APPROACH In order to investigate the role of different nanoparticle features in the FT process, we constructed a large ensemble of Mo−Pt nanoparticles of different size and shape on γ-alumina substrates and optimized their geometries using densityfunctional theory. Most of the calculations were performed for γ-Al2O3 five-atomic-layer-thick slabs that were cut from fully relaxed bulk structures19 to expose the (110C) surface, which is energetically preferred.20 We performed convergence tests for slabs with larger number of γ-Al2O3 layers (seven and nine) and found that total energy differences and energy barriers of interest do not significantly differ from five-layer-thick calculations. Large, periodically repeated alumina supercells were used (15.0 Å−10.5 Å− 22.0 Å). The vacuum layer between slabs was >14 Å. The positions of atoms in the lower layer of the slab (opposite to the surface at which nanoparticles were accommodated) were fixed; i.e., only the “functional surface” (and several atomic layers under it) was relaxed. Small transition-metal (TM) particles (below 1 nm in diameter) containing between 5 and 40 atoms with different 4451
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basis was set at 500 eV, and all integrations over the Brillouin zone were done using the Monkhorst−Pack scheme with 4 k points in the relevant irreducible wedge.26 Inclusion of additional k points was found to have minimal effect on the total energy differences of interest here. The total number of atoms varied between 200 and 300 atoms for different periodic supercells. For each supercell, we relaxed all atoms until the quantum-mechanical force on each atom became smaller than 0.02 eV/Å. All the calculations were spin-polarized. Activation barriers were calculated using the nudged-elastic-band (NEB) method.27 In some cases (when the potential energy curve is very steep around the transition state) we had to employ the adaptive NEB approach28 because the conventional NEB sampling did not provide satisfactory results. We did not calculate the pre-exponential factors and reaction rates for the considered reactions. Although such calculations could be performed,29 they are very difficult and time-consuming. Comparison between the activation barriers at different surface sites provides only qualitative information which, however, is quite sufficient for most of the practical purposes.
Mo/Pt ratio on oxide slabs were constructed as follows. First, we tried to find the anchoring site for a single transition metal (Mo or Pt) atom at the oxide surface. Nanoparticles with larger number of atoms were constructed by repeatedly adding a single metal atom to an existing particle and allowing the structure to relax. The supercells are sufficiently large so that even 40-atom nanoparticles interact with their neighbors only weakly. In calculations we did not perform a systematic search for the lowest energy structure. Instead, we found that it is more useful to construct an ensemble of nanoparticles with diverse local bonding and coordination numbers. Once we had a relaxed nanoparticle configuration with some Mo/Pt ratio, we permuted the TM atoms within the particle without changing the numbers of Mo and Pt atoms and relaxed the structures again. Typically, such permutation modifies only the interatomic distances within the particle while the site coordination for most of the atoms remains unchanged. However, the nanoparticle bonding to the substrate may change because Mo and Pt bind to different sites at the alumina surface. For simulation of processes on larger nanoparticles, quasione-dimensional periodic rodlike structures containing a welldefined boundary between the nanoparticle and the substrate were used. For modeling the processes on the surfaces of larger particles, we simply used surfaces of “bulk” transition metals which is face-centered cubic (fcc) for Pt and body-centered cubic (bcc) for Mo. For understanding the role of Pt−Pt, Mo− Mo, and Mo−Pt bonds we also performed “hybrid surfaces” by substituting some Pt atoms at the Pt surface by Mo atoms (and vice versa). Although the particles constructed in such a way cannot be taken exactly of the size of metal nanoparticles used in real catalytic processes (some of these particles may contain thousands of atoms, and the first-principles calculations for so big systems are not affordable at the present), we have a possibility to model the local structure and electronic properties of the most catalytically active sites. Typically, these sites are closely related to the structural defects at the surface of the particle (edges, vertices, etc.) and are represented by lowcoordinated atoms. This fact gives us a possibility to relate calculations for small particles to experiments performed on larger particles that still contain the same types of defect sites. Although the number of different supercells one could construct for such systems is very large (especially, if one assumes an existence of a disordering in Mo−Pt particles and different possibilities for their accommodation at the substrate), an important fact is that FT chemical reaction barriers are defined mainly by the local structure of the surrounding network, i.e., by 1−2 nearest-neighboring layers of metal atoms. Therefore, we did not perform a full systematic study of the statistics for all the possible supercells with different topology. Instead, we formed different local structures around reacting sites oxygen at the nanoparticle surface by simple permutation of nearest-neighboring metal atoms with further structure relaxation. We found such a strategy most reasonable for searching local structures that provide the most favorable conditions for catalytic activity. The calculations were based on the generalized gradient approximation (GGA) for exchange and correlation and plane waves.21 We used the GGA functional of Perdew, Burke, and Ernzerhof (PBE),22 which gives good results for chemisorption of molecules at transition-metal surfaces. Projected augmented wave (PAW) scalar relativistic pseudopotentials23,24 and the VASP code25 were used. The energy cutoff for the plane-wave
3. RESULTS AND DISCUSSION 3.1. Pt−Mo Nanoparticles Structure. The structure of air-formed aluminum oxide films is predominantly noncrystalline but has short-range cubic order,30,31 for which the cubic γand η-transition alumina polymorphs could be reasonable models to consider. These two forms are structurally closely related and both contain undercoordinated oxygen atoms by virtue of the presence of cation vacancies. However, in some cases they could exhibit very different catalytic behavior.32 Here we consider the γ-alumina structure only; schematics of its energetically preferable (110C) surface is shown in Figure 1a.
Figure 1. (a) Schematics of γ-alumina surface with (110C) orientation. (b) A typical Pt−Mo nanoparticle at γ- alumina surface configurations with Mo atoms at the bottom binding to surface oxygen at the surface are energetically preferable. Al atoms are shown in brown, O in red, H in white, Mo in cyan, and Pt in blue.
This surface consists of alternating oxygen and aluminum rows. The oxygen atoms at the exposed γ-Al2O3 (110C) surface are inequivalent, and the binding energy of adsorbed surface impurity atoms atomic hydrogen to O varies over some range. Most of the surface oxygen atoms are three-coordinated (O(3)); the other (minority type) of O atoms at the surface, the four-coordinated atoms (O(4)) located in the so-called “trench” on the relaxed γ-alumina surface,33 should bind impurity atoms weaker than O(3). The model used for γ4452
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alumina also contains some number of hydrogen atoms which is consistent with experimentally observed presence of hydrogen in this material.19 Individual Pt atoms adsorbed at γ-alumina surface always bind to two surface aluminum atoms (between them); i.e., the Pt atom has a tendency to oxidize aluminum rather than reduce oxygen, and the Pt atom binding energy is about 4.5 eV. Mo atom always binds between two surface oxygen atoms. Its binding energy to two O(3) atoms is 5.1 eV while the Mo binding energy to two O(4) atoms is 4.8 eV. These energies give an idea about the binding mechanism of Pt−Mo nanoparticles to the γ-alumina surface. EXAFS and XANES measurements indicated strong interaction of Mo atoms and alumina support; i.e., for all bimetallic Pt−Mo catalysts, Mo atoms segregate to the surface irrespective of the sequence of the impregnation.17 These calculations confirm such a picture. Namely, there are two reasons for Mo segregation to the surface: (i) a thermodynamic reason, namely, each Mo atoms binds to the surface stronger than any Pt atom; (ii) a “structure” reasonthe number of O atoms at the surface (to which Mo binds) is higher than the number of Al atoms (to which Pt binds). These two factors work together and make Mo appearance at the surface more likely than Pt appearance. However, it is still likely that some Pt atoms also go to the Al surface and bind to surface Al atoms. This binding will increase to total binding energy of the nanoparticle to the surface, i.e., the stability of the bimetallic catalyst with respect to small particle detaching and their further conglomeration into larger particles which should reduce the catalytic activity. A typical 14atom (8 Pt atoms and 6 Mo atoms, or (8 + 6) particle) Pt−Mo nanoparticle at the γ-alumina surface is shown in Figure 1b. This configuration was chosen by minimization of the total energy of (8 + 6) particles attached to the surface with respect to the transmutation of atoms within the particle (with a subsequent structure relaxation). Although most of the atoms attached to the surface are Mo atoms, there is one Pt atom that attaches to two aluminum atoms at the substrate surface. It is also clear that Pt atoms positioned within nanoparticle will partly oxidize Mo atoms (take away electrons), i.e., become more electronegative which increases activity in attaching carbon atom of CO molecules at the initial stage of the FT process. In order to have a better understanding of the phenomenological aspects of the methanation reaction on the supported Mo−Pt catalysts, we also constructed the Mo−Pt binary phase diagram using the ThermoCalc AB software and TTNI8 thermodynamic database (Figure 2).34 The diagram agrees well with the most up-to-date experimental data.35 In the range of the studied Pt−Mo cluster compositions, one could expect that a solid solution on the basis of the MoPt2 compound will be formed. While the Pt−Mo phase diagram constructed for the bulk phases does not exactly correspond to a composition of the nano Pt−Mo clusters, it nevertheless provides a selfconsistent model of phase equilibria and thermodynamic properties of alloys in this system. We used this information for assessing the potential mechanisms of the Pt−Mo catalyst degradation (e.g., the formation of Mo2C and/or MoC in different alloys of the Pt−Mo system). The methodology of such thermodynamic calculations is beyond the scope of the present work and could be found, e.g., in ref 34. 3.2. CO Decomposition at Pt−Mo Surfaces. The requirements for the catalyst for the FT process are very strict and somewhat contradictory. The Fischer−Tropsch synthesis
Figure 2. Pt−Mo phase diagram constructed by using the ThermoCalc AB software and the TTNI8 thermodynamic database.34
process is initiated by activation of CO and H2 bonds and continues by the subsequent “propagation steps” including hydrogenation and carbon−carbon coupling, followed by chain termination reactions. However, the barrier for CO activation is quite high.9,10 In addition to that, the material that successfully dissociates adsorbed CO molecule should not get poisoned by adsorbed molecule fractures (carbon or oxygen atoms). Then, an ideal catalyst should show a proper balance of hydrogenation and carbon−carbon coupling reactions to provide synthesis of hydrocarbon products with required molecular weight. The process as a whole is quite complicated, and the products that form under reaction conditions at any given moment depend on the concentration of different molecular species that are present at the surface at this moment. A delicate balance of surface species hydrogenation reactions and hydrocarbon coupling reactions is critical to produce longer chain hydrocarbon products. In other words, the FT process is very sensitive to the reaction kinetics features. While this reaction has been studied for many years, there is still a rather poor fundamental understanding of how the atomic surface structure influences catalytic performance. At the present, we do not study the kinetics of reactions involved in the FT process. We investigate only the initial stages of the FT synthesis (CO and H2 activation with a subsequent methane formation) and try to find the main features of Pt−Mo alloy bimetallic nanoparticles that are responsible for the control of the main rate-limiting steps of the process. In particular, small nanoparticles naturally contain many undercoordinated atomic sites that are favorable for CO dissociation mechanism as in was shown for Ni surfaces.11 Also, presence of Pt−Mo metal−metal bonds at the particle surface adds an additional degree of freedom allowing to split different functions (oxidation and reduction) between atoms of different sort. The local atomic structures at the particle surface (local environment of a given active site) should also affect the efficiency of catalytic processes. So far we did not calculate the barriers as a function of surface coverage that should indicate the dependence of barriers on lateral repulsive surface interactions of adsorbed species as well as the effects related to a lack of availability of surface sites. 4453
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The initiation reactions consist of the adsorption of gas phase CO and H2 and their subsequent dissociation to generate the hydrocarbon precursors, C*, O*, and H*. To understand the role of Pt and Mo in bimetallic system, we start with calculations for supercell with slab geometry that simulates an energetically favorable Pt (111) surface (Figure 3). This
Table 1. Reaction Energies (ΔE, the Difference between the Energy of the System with Attached C and O Fragments and the Energy of the System with Absorbed CO Molecule) and Reaction Barriers (Eb) for CO Decomposition at Hydrogen Free Mo-Modified Pt (111) Surface for Different Channels (Pt−Pt, Mo−Mo, and Pt−Mo; See Also Figure 3) channel type
ΔE (eV)
Eb (eV)
Pt−Pt Mo−Mo Pt−Mo
2.2 −0.9 −1.1
4.5 1.95 1.5
remains at the same site) while O* attaches to three Mo atoms. For Pt−Pt channel, the process is endothermic (the energy of the system with C and O fragments is higher than the energy of the system with absorbed CO molecule), while for Mo−Mo and Pt−Mo dissociation channels, the process is slightly exothermic (with the energies −0.9 and −1.1 eV). This is mainly related to the fact that oxygen binding to Mo atoms is stronger than to Pt atoms. Another important feature is that CO decomposition barrier is lowest for the Pt−Mo channel (about 1.5 eV). It means that “mixed” parts of the surface that contain both Pt and Mo atoms are most efficient for the CO decomposition process. The CO dissociation barrier at a clean Pt(111) surface is in good agreement with previous calculations.36 These results are also consistent with recent DFT-based computational study for binary methanation catalysts which indicated that lower CO dissociation energy corresponds to lower dissociation activation energy.12 Further evolution of the system depends on several factors such as surface coverage with CO molecules and their fragments, the relation between the rates of CO dissociation and recombination processes, the presence of other surface defects such as roughness and steps, etc. More sophisticated and timeconsuming statistical simulations (e.g., based on kinetic Monte Carlo methods) should be performed to understand the role of all of these factors in the process. When adsorbed hydrogen is also present at the surface, the energetics of the CO decomposition is modified. Figures 4a and 4b show that when CO molecule tilts and O atom approaches toward the surface, it may capture H* and eventually form an OH* group attached to the surface by the oxygen atom. Calculations indicate that in the presence of hydrogen the conclusions made for Figure 3c are still valid; i.e., (i) the dissociation process in Pt−Pt channel is endothermic while the dissociation in Mo−Mo and Pt−Mo channels is exothermic; (ii) the CO decomposition barrier is lowest for Pt−Mo bond (1.3 eV). It means that even in the presence of H*, the Pt−Mo channel is most efficient for the CO decomposition process (Figure 4c and Table 2). However, hydrogen may facilitate the CO decomposition by lowering the decomposition barrier in comparison with the case when hydrogen species are not adsorbed at the surface. Surprisingly, in spite of the many undercoordinated atoms at small Pt−Mo nanoparticles, the CO dissociation mechanism at such a particle is not very different from the case of a “flat” surface considered above (Figure 5). First, a CO molecule binds to one of the particle sites. Most energetically preferable configurations correspond to C binding to three metal atoms positioned at the particle. After the tilting, oxygen atom attracts to the NP surface, and CO molecule dissociates. Finally, O atom occupies one of the NP surface positions (again, the lowest energy positions correspond to oxygen attachment to
Figure 3. Schematics of CO decomposition at Pt (111) surface with the surface atomic layer partly substituted by Mo. (a) An initial possible configuration where CO is attached to three Pt atoms (hcp site) at the surface. (b) Final possible atomic configuration after decomposition (C is attached to three Pt atoms, O to two Pt atoms and one Mo atom). (c) Reaction energies and reaction barriers for the three channels. Pt is shown in blue, Mo in cyan, C in gray, and O in red. Zero energy for all three configurations corresponds to the initial structure with CO adsorbed at the surface.
structure may be considered as a realistic imitation for a large Pt nanoparticle where the surface consists of Pt bulk crystalline facets. Two rows of Pt atoms at the surface layer were substituted by Mo (with further relaxation). The purpose of this substitution is to consider CO decomposition at “pure” Pt parts of the surface, “pure” Mo parts of the surface, and “mixed” parts that include lateral surface Pt−Mo bonds. For such a partial Pt to Mo substitution in the surface layer, we did not observe any significant relaxation at the surface except for some insignificant corrugation in the Mo part of the layer. When we substituted Pt by Mo in two or more Pt layers of the supercell (near the free surface), surface relaxation was larger but the CO dissociation energy and dissociation barrier at the surface Pt− Mo bond were still comparable to those for one layer partial Pt to Mo substitution. Therefore, most of our calculations for this sort of systems were performed for surface layer substitution. Figures 3a and 3b illustrate the schematics of the CO decomposition process at the Mo-modified Pt (111) surface. When a CO molecule adsorbs at the surface, it performs tilting oscillating motions before the oxygen atom gets close enough to another metal atom at the surface and gets “captured”. Further evolution of the system consists of the dissociation of CO molecule and attachments of both of its fragments (C* and O*) to the surface. Figure 3c and Table 1 show the reaction energies and activation barriers for different possible dissociation channels. The Pt−Pt and Mo−Mo channels correspond to the dissociation at the “pure” Pt and Mo parts of the surface (the CO molecule as well as final C* and O* products are attached to Pt or Mo atoms only). For the Pt−Mo channel, CO is initially attached to two Pt and one Mo atom (and C* 4454
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processes of adsorbed species at the nanoparticle surface which should affect the dependence of processes on the surface coverage. 3.3. Hydrogenation Reactions. Further steps of the FT process consist of hydrogenation reactions and carbon−carbon coupling reactions. The hydrogenation reactions can produce strongly adsorbed radical-like intermediate hydrocarbons. In the hydrogenation reactions, surface hydrocarbons react with surface hydrogen atoms ((CxHy)* + H* → (CxHy+1)*), and other strongly adsorbed radical-like intermediate hydrocarbons are produced. The coupling reactions can result in the formation of more weakly bound closed-shell hydrocarbon products. This process is typically referred to as termination. The closed-shell species can desorb from the surface. The balance between all of these reactions is what determines the product distribution. The hydrogenation process includes dissociative adsorption of H2 molecules which are activated and then dissociate at the catalyst surface. Calculations indicate that H2 decomposition and migration at Pt−Mo nanoparticles have the following features: (i) H2 may split at any bond (Pt−Pt, Mo−Mo, and Pt−Mo) at the NP surface with rather low barrier (
