镍族金属团簇在催化加氢过程中的应用

Nanoparticles of precious metals play an important role in many heterogeneous catalytic reactions due to their excellent catalytic performance. As an idealized model, gas phase metal clusters have been extensively utilized to understand catalytic mechanisms at a molecular level. Here we provide an overview of our recent studies on H₂ dissociative chemisorption on nickel family clusters. The structure evolution and the stability of the metal clusters were first compared. H₂ dissociation on the clusters was then carefully addressed to understand the capability of metal clusters to break the H-H bond. Two key parameters, the dissociative chemisorption energy (ΔE_CE) and the H sequential desorption energy (ΔE_DE), were employed to characterize the catalytic activity of metal clusters. Our results show that both ΔE_CE and ΔE_DE decline significantly as the H coverage increases. Since the catalyst is in general covered entirely by H atoms and H₂ molecules in a typical hydrogenation process, and maintained at a pre-determined pressure of H₂ gas, we can rationally use the calculated ΔE_CE and ΔE_DE values at full H saturation to address the activity of metal clusters. Our results suggest that at full H coverage, each Pt atom is essentially capable of adsorbing 4 H atoms, while each Ni or Pd atom can only accommodate 2 H atoms. Considering the similar values of H desorption energies on Pt and Pd clusters, the higher average H capacity per Pt atom could probably lead to a faster reaction rate because more active H atoms are produced on the Pt catalyst particles in the hydrogenation process. Finally, the charge sensitivity of the key catalytic properties of Pt clusters for hydrogenation was systematically evaluated. The results show that the dissociation of H₂ and H desorption are strongly correlated to the charge state of the Pt clusters at low H coverage. However, at high H-capacities, both ΔE_CE and ΔE_DE fall into a narrow range, suggesting that the charge can be readily dispersed and that the Pt-H bonds average the interaction between clusters and H atoms. As a result, the H-capacities on charged clusters were found to be similar as the cluster size increased; in case of sufficiently large clusters, the reactivity of a fully saturated cluster was no longer sensitive to its charge state.     

Effect of Co doping on catalytic activity of small Pt clusters

Platinum is the most widely used catalyst in fuel cell electrodes. Designing improved catalysts with low or no platinum content is one of the grand challenges in fuel cell research. Here, we investigate electronic structures of Pt(4) and Pt(3)Co clusters and report a comparative study of adsorption of H(2), O(2), and CO molecules on the two clusters using density functional theory. The adsorption studies show that H(2) undergoes dissociative chemisorption on the tetrahedral clusters in head on and side on approaches at Pt centers. O(2) dissociation occurs primarily in three and four center coordinations and CO prefers to adsorb on Pt or Co atop atoms. The adsorption energy of O(2) is found to be higher for the Co doped cluster. For CO, the Pt atop orientation is preferred for both Pt(4) and Pt(3)Co tetrahedral clusters. Adsorption of CO molecule on tetrahedral Pt(3)Co in side on approach leads to isomerization to planar rhombus geometry. An analysis of Hirshfeld charge distribution shows that the clusters become more polarized after adsorption of the molecules.     

Nanoclusters Au_(19)Pd and Au_(19)Pt Catalyzing CO Oxidation: a Density Functional Study

The gold atoms on the Au20 cluster had been substituted by the palladium and platinum atoms to obtain the doped clusters with more stable geometries as a function of the bind energy and interaction energy in the previous study. Therefore, we investigated the catalytic activities of the Au_(19)Pd and Au_(19)Pt clusters for CO oxidation along the Langmuir-Hinshelwood mechanism. It is found that the coadsorption of CO and O2 on the doped clusters is obviously stronger than on the Au20 cluster, especially on the doped atom, which makes potential energy of the transition state lower than the total energy of the reactants so that it can promote CO oxidation. The reaction on these doped clusters with the heteroatom on the vertex is more difficult. However, the Au_(19)Pd(S) is more prone to catalyzing the CO oxidation, in which the rate-limiting step has the lower energy barrier of 38.84 kJ/mol for this study. Therefore, the single atom can be modified to change the catalytic activity of the cluster for the CO oxidation. Meanwhile, the different sites on the clusters have different strengths of activity for the reaction.     

Binary clusters AuPt and Au6Pt: structure and reactivity within density functional theory

Within density functional theory with the general gradient approximation for the exchange and correlation, the bimetallic clusters AuPt and Au(6)Pt have been studied for their structure and reactivity. The bond strength of AuPt lies between those of Au(2) and Pt(2), and it is closer to that of Au(2). The Pt atom is the reactive center in both AuPt and AuPt(+) according to electronic structure analysis. AuPt(+) is more stable than AuPt. Au(6)Pt prefers electronic states with low multiplicity. The most stable conformation of Au(6)Pt is a singlet and has quasi-planar hexagonal frame with Pt lying at the hexagonal center. The doping of Pt in Au cluster enhances the chemical regioselectivity of the Au cluster. The Pt atom essentially serves as electron donor and the Au atoms bonded to the Pt atom acts as electron acceptor in Au(6)Pt. The lowest triplet of edge-capped rhombus Au(6)Pt clusters is readily accessible with very small singlet-triplet energy gap (0.32 eV). O(2) prefers to adsorb on Au and CO prefers to adsorb on Pt. O(2) and CO have stronger adsorption on AuPt than they do on Au(6)Pt. CO has a much stronger adsorption on AuPt bimetallic cluster than O(2) does. The adsorption of CO on Pt modifies the geometry of AuPt bimetallic clusters.     

A small paramagnetic platinum cluster in an NaY zeolite: characterization and hydrogen adsorption and desorption

A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt13Hm, possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m=30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.     

DFT studies of oxidation routes for Pd9 clusters supported on γ-alumina

This research is focused on the analysis of adsorbed bare and oxidized Pd(9) nanoparticles supported on γ-alumina. From first-principle density functional theory calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Studies of Pd/PdO nanoparticles prove that they are stable at γ-alumina supports. It is shown that the Pd(9) nanoparticle favors dissociative adsorption of oxygen molecules. The most energetically preferable sites for adsorption are close to the contact between the cluster and the support, where one oxygen atom interacts with a 5-coordinated aluminium atom, and the remaining oxygen is in contact with the closest palladium atom. After first dissociation, one oxygen atom creates a bridge between the palladium atom and the 5-coordinated aluminium atom and the second oxygen atom moves to the top of the Pd(9) cluster, making a bridge between two palladium atoms. Subsequent dissociations arise analogously, with the difference that oxygen atoms in the second layer of the palladium cluster occupy hollow sides of the cluster. Investigation of the charge distribution in each oxidation step reveals that charge transfer increases towards the Pd/PdO nanoclusters. The electronic density of states indicates that gradual oxygen molecule adsorption and dissociation shift the highest states of the Pd/PdO nanoparticles in different ways. The overall investigation is found to be beneficial for studying methane oxidation.     

Structural, energetic, and electronic properties of hydrogenated titanium clusters

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti-Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti-Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed.     

Cluster size selectivity in the product distribution of ethene dehydrogenation on niobium clusters

Ethene reactions with niobium atoms and clusters containing up to 25 constituent atoms have been studied in a fast-flow metal cluster reactor. The clusters react with ethene at about the gas-kinetic collision rate, indicating a barrierless association process as the cluster removal step. Exceptions are Nb8 and Nb10, for which a significantly diminished rate is observed, reflecting some cluster size selectivity. Analysis of the experimental primary product masses indicates dehydrogenation of ethene for all clusters save Nb10, yielding either Nb(n)C2H2 or Nb(n)C2. Over the range Nb-Nb6, the extent of dehydrogenation increases with cluster size, then decreases for larger clusters. For many clusters, secondary and tertiary product masses are also observed, showing varying degrees of dehydrogenation corresponding to net addition of C2H4, C2H2, or C2. With Nb atoms and several small clusters, formal addition of at least six ethene molecules is observed, suggesting a polymerization process may be active. Kinetic analysis of the Nb atom and several Nb(n) cluster reactions with ethene shows that the process is consistent with sequential addition of ethene units at rates corresponding approximately to the gas-kinetic collision frequency for several consecutive reacting ethene molecules. Some variation in the rate of ethene pick up is found, which likely reflects small energy barriers or steric constraints associated with individual mechanistic steps. Density functional calculations of structures of Nb clusters up to Nb(6), and the reaction products Nb(n)C2H2 and Nb(n)C2 (n = 1...6) are presented. Investigation of the thermochemistry for the dehydrogenation of ethene to form molecular hydrogen, for the Nb atom and clusters up to Nb6, demonstrates that the exergonicity of the formation of Nb(n)C2 species increases with cluster size over this range, which supports the proposal that the extent of dehydrogenation is determined primarily by thermodynamic constraints. Analysis of the structural variations present in the cluster species studied shows an increase in C-H bond lengths with cluster size that closely correlates with the increased thermodynamic drive to full dehydrogenation. This correlation strongly suggests that all steps in the reaction are barrierless, and that weakening of the C-H bonds is directly reflected in the thermodynamics of the overall dehydrogenation process. It is also demonstrated that reaction exergonicity in the initial partial dehydrogenation step must be carried through as excess internal energy into the second dehydrogenation step.     

Three-site model for hydrogen adsorption on supported platinum particles: influence of support ionicity and particle size on the hydrogen coverage

Pt L(3) X-ray absorption edge data on small supported Pt particles (N < 6.5) reveals that at very low H(2) pressure or high temperature the strongest bonded H is chemisorbed in an atop position. With decreasing temperature or at higher H(2) pressure only n-fold (n = 2 or 3) sites are occupied. At high H(2) pressure or low temperature, the weakest bonded H is positioned in an "ontop" site, with the chemisorbing Pt already having a stronger bond to a H atom in an n-fold site. DFT calculations show that the adsorption energy of hydrogen increases for Pt particles on ionic (basic) supports. The combination of the DFT calculations with hydrogen chemisorption data and the analysis of the Pt L(3) X-ray absorption spectra implies that both the H coverage and/or the type of active Pt surface sites, which are present at high temperature catalytic reaction conditions, strongly depend on the ionicity of the support. The consequences for Pt catalyzed hydrogenolysis and hydrogenation reactions will be discussed.     

An embedded atom method study of the equilibrium shapes of small platinum and palladium clusters

The embedded atom method (EAM) was used to determine the equilibrium shapes of small platinum and palladium clusters (N=5–60 atoms). The stability of various polyhedral symmetries was performed at 0K. A search for the presence of any other lower energy structures was also carried out. Pt clusters reconstructed to lower energy clusters even at the magic numbers. Pd also reconstructed to lower energy structures except at the magic number sizes where the icosahedron was found to be the most stable.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au

Structures of small clusters of Ni, Pd, Pt, Cu, Ag, and Au with n=2–34 and n=55 atoms are calculated as functions of number of atoms and temperature by the simulated annealing Monte Carlo method using an embedded atom potential.     

Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters

The identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steady-state vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.     

Growth mechanism of palladium clusters on rutile TiO2 （110） surface

Oxide-supported transition metal systems have been the subject of enormous interest due to the improvement of catalytic properties relative to the separate metal.Thus in this paper,we embark on a systematic study for Pd n (n=1-5) clusters adsorbed on TiO2 (110) surface based on DFT-GGA calculations utilizing periodic supercell models.A single Pd adatom on the defect-free surface prefers to adsorb at a hollow site bridging a protruded oxygen and a five-fold titanium atom along the [110] direction,while Pd dimer is located on the channels with the Pd-Pd bond parallel to the surface.According to the transition states (TSs) search,the adsorbed Pd trimer tends to triangular growth mode,rather than linear mode,while the Pd4 and Pd5 clusters prefer three-dimensional (3D) models.However,the oxygen vacancy has almost no influence on the promotion of Pd n cluster nucleation.Additionally,of particular significance is that the Pd-TiO2 interaction is the main driving force at the beginning of Pd nucleation,whereas the Pd-Pd interaction gets down to control the growth process of Pd cluster as the cluster gets larger.It is hoped that our theoretical study would shed light on further designing high-performance TiO2 supported Pd-based catalysts.     

H2 dissociation on individual Pd atoms deposited on Cu(111)

We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.     

氢原子在金属锂表面台阶附近吸附和表面扩散行为的ab initio研究

取Li7H和Li9H两个原子簇模拟氢原子与含台阶的金属锂表面的相互作用, 以小基组用ab initip方法计算了体系的吸附和表面扩散势能面(或势能曲线)。结果表明: (1)对Li7H体系, 台阶面附近沿垂直边棱方向存在三种不同的桥位吸附位, 最稳定的吸附位在上台面接近台阶边棱处, 台阶面显著地改变了表面扩散活化能, 台阶边棱处有一个较高的势垒。于是, 迁移原子将会在台阶边棱处受到反射, 并可被捕获于台阶面上及其附近。由势能面确定了最低能量表面扩散途径。(2)对Li9H体系, 在Li7H原子簇基础上增加次表面层两个锂原子后, 表面扩散活化能略有减小, 氢原子在上台面的桥位吸附更趋稳定, 各吸附位相对稳定性及势垒内何位置几无改变, 这些结果显示了台阶面对氢原子的化学吸附和表面扩散发生扰动, 台阶边棱对表面扩散起着重要作用。     

CO与Pdn(n=1-8)团簇的相互作用

采用密度泛函理论对CO与钯团簇的相互作用进行了系统研究. 结果表明, PdnCO(n=1-8)体系的最低能量结构是在Pdn(n=1-8)团簇最低能量结构或亚稳态结构的基础上吸附CO生长而成; CO的吸附以端位吸附为主, 其吸附没有改变Pdn团簇的结构; CO分子在Pdn团簇表面发生的是非解离性吸附. 与优化的CO键长(0.1166 nm)相比, 除了n=2, 团簇PdnCO的C—O键长为0.1167-0.1168 nm, 吸附后C—O键长变化较小, CO分子被活化程度较小. 电荷集居数分析表明, CO的吸附对Pdn团簇的影响比较小; 二阶能量差分表明, n=4,6的团簇是相对稳定的团簇.     

Kinetics of hydrogen production upon reduction of aqueous TiO2 nanoparticles catalyzed by Pd(0), Pt(0), or Au(0) coatings and an unusual hydrogen abstraction; steady state and pulse radiolysis study

Reduction of H(+) by TiO(2) electrons (e(TiO)(2)(-)) in aqueous colloidal solution takes place in the presence of surface metal catalysts. The catalytic reduction gives rise to adsorbed hydrogen atoms. In the presence of Pd(0) or Pt(0), material balance shows that most of the adsorbed H atoms combine to molecular hydrogen. When the TiO(2) nanoparticles are partially coated with Au(0) instead of Pd(0) or Pt(0), a higher than expected molecular hydrogen level is observed, attributed to a short chain reaction involving hydrogen abstraction from 2-propanol. This unusual hydrogen abstraction reaction has not been reported before. The mechanism and energy balance are discussed. The surface modification of TiO(2) nanoparticles was carried out by reduction of K(2)PdCl(4), H(2)PtCl(6), or HAuCl(4) with e(TiO)(2)(-). The latter had been generated through electron injection from hydrated electrons, hydrogen atoms, or 2-propanol radicals, produced by gamma or pulse radiolysis prior to the addition of the metal compounds. Upon addition of the metal compounds, immediate reactions take place producing metals clusters (M(0)) by multistep reductions reactions on the TiO(2) surface. The chemical kinetics involving the different metals and the reaction rate constant of e(aq)(-) and e(TiO)(2)(-) with AuCl(4)(-) is also reported.     

Pt原子在γ-Al2O3(001)表面的吸附及迁移

采用密度泛函理论(DFT)中广义梯度近似(GGA)方法, 对Pt原子与γ-Al₂O₃(001)面的相互作用及迁移性能进行了研究. 分析了各种可能吸附位及吸附构型的松弛和变形现象, 吸附能和迁移能垒的计算结果表明: Pt团簇能够稳定吸附在该表面. Pt原子在表面O位的吸附能明显较高, 这主要是由Pt向基底O原子转移了电子所致. 电荷布居分析表明, Pt原子显电正性, Pt和Al原子之间存在排斥作用, 导致与Al原子产生较弱相互作用. 计算的平均吸附能大小依赖于Pt团簇的大小和形状, 总体趋势是随着Pt原子数增多, 吸附能降低. Pt原子在γ-Al₂O₃(001)表面迁移过程所需克服的迁移能垒最高值为0.51 eV. 随着吸附的Pt原子数增多,更倾向于形成Pt团簇. 因此, Pt原子在γ-Al₂O₃(001)表面的吸附演变不可能形成光滑、均匀平铺的吸附构型, 而在一定条件下容易出现团聚.     

Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH(x) and C(2)H(y) intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt(14.13.8) cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt(3))CH prefers a mu(3) hollow site (fcc), (Pt(2))CH(2) prefers a mu(2) bridge site, and PtCH(3) prefers mu(1) on-top sites. Vinyl leads to (Pt(2))CH-CH(2)(Pt), which prefers a mu(3) hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt(2))C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a pi bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH(x) species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH(3), BE = 134.83 kcal/mol) among the C(2)H(y) molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.