Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211)

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.     

Surface Buckling and Subsurface Oxygen: Atomistic Insights into the Surface Oxidation of Pt(111)

Platinum is a catalyst of choice in scientific investigations and technological applications, which are both often carried out in the presence of oxygen. Thus, a fundamental understanding of platinum’s (electro)catalytic behavior requires a detailed knowledge of the structure and degree of oxidation of platinum surfaces in operando. ReaxFF reactive force field calculations of the surface energies for structures with up to one monolayer of oxygen on Pt(111) reveal four stable surface phases characterized by pure adsorbate, high‐ and low‐coverage buckled, and subsurface‐oxygen structures, respectively. These structures and temperature programmed desorption (TPD) spectra simulated from them compare favorably with and complement published scanning tunneling microscopy (STM) and TPD experiments. The surface buckling and subsurface oxygen observed here influence the surface oxidation process, and are expected to impact the (electro)catalytic properties of partially oxidized Pt(111) surfaces.     

氢原子在Ni(111)次表面和Ni(211)、(533)台阶面上的吸附与振动

应用原子与表面簇合物相互作用的五参数Morse势(5-MP)方法对氢原子在Ni(111)表面和次表面以及Ni(211), (533)台阶面进行了系统研究, 得到了氢原子在上述各面的吸附位、吸附几何、结合能和本征振动频率. 计算结果表明, 在Ni(111)面上, 氢原子优先吸附在三重位, 随着覆盖度的增加会吸附在次表面八面体位和四面体位. Ni(211), (533)的最优先吸附位都是四重位, 当氢原子的覆盖度增大时占据(111)平台的三重吸附位. 靠近台阶面的吸附位受台阶和平台高度的影响很大. 此外, 我们计算了氢原子在各表面的不同吸附位的扩散势垒, 获得氢原子在各表面的最低能量扩散通道.     

Dynamics of CO at the solid/liquid interface studied by modeling and simulation of CO electro-oxidation on Pt and PtRu electrodes

We consider theoretical models for CO monolayer oxidation on stepped Pt single-crystal electrodes and Ru-modified Pt(111) electrodes. For both systems, our aim is to assess the importance of CO surface diffusion in reproducing the experimental chronoamperometry or voltammetry. By comparing the simulations with the experimental chronoamperometric transients for CO oxidation on a series of stepped Pt surfaces, it was concluded that mixing of CO on the Pt(111) terrace is good, implying rapid diffusion (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. van Santen, J. Phys. Chem. B, submitted). We discuss here a more detailed model in which the CO adsorbed on steps is converted into CO adsorbed on terraces as the oxygen-containing species occupy the steps (as observed experimentally on stepped Pt in UHV), followed by a subsequent oxidation of the latter, to reproduce the observed chronoamperometry on stepped surfaces with a higher step density. On Ru-modified Pt(111), the experimentally observed splitting of the CO stripping voltammetry into two stripping peaks, may suggest a slow diffusion of CO on Pt(111). This apparent contradiction with the conclusions of the experiments on stepped surfaces, is resolved by assuming a weaker CO binding to a Pt atom which has Ru neighbors than to "bulk" Pt(111), in agreement with recent quantum-chemical calculations. This makes the effective diffusion from the uncovered Pt(111) surface to the perimeter of the Ru islands, which are considered to be the active sites in CO oxidation electrocatalysis on PtRu surfaces, very slow. Different models for the reaction are considered, and discussed in terms of their ability to explain experimental observations.     

Computational study of the adsorption and dissociation of phenol on Pt and Rh surfaces

The adsorption of phenol on flat and stepped Pt and Rh surfaces and the dissociation of hydrogen from the hydroxyl group of phenol on Pt(111) and Rh(111) were studied by density functional calculations. On both Pt(111) and Rh(111), phenol adsorbs with the aromatic ring parallel to the surface and the hydroxyl group tilted away from the surface. Furthermore, adsorption on stepped surfaces was concluded to be unfavourable compared to the (111) surfaces due to the repulsion of the hydroxyl group from the step edges. Transition state calculations revealed that the reaction barriers, associated with the dissociation of phenol into phenoxy, are almost identical on Pt and Rh. Furthermore, the oxygen in the dissociated phenol is strongly attracted by Rh(111), while it is repelled by Pt(111).     

Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.     

Tellurium adatoms as an in-situ surface probe of (111) two-dimensional domains at platinum surfaces

Irreversibly adsorbed tellurium has been studied as a probe to quantify ordered domains in platinum electrodes. The surface redox process of adsorbed tellurium on the Pt(111) electrode and Pt(111) stepped surfaces takes place around 0.85 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the tellurium atoms involved in the redox process are only those deposited on the (111) terrace sites. Moreover, the corresponding charge density is proportional to the number of sites on (111) ordered domains (terraces) that are, at least, three atoms wide. Hence, this charge density can be used to measure the number of (111) terrace sites on any given platinum sample. Structural information about tellurium adsorption is obtained from atomic-resolution STM images for the Pt(111) and Pt(10, 10, 9) electrodes. A rectangular structure (2 x radical 3) and a compact hexagonal structure (11 x 8) were identified. However, the redox peak for adsorbed tellurium on (100) domains at 1.03 V overlaps with peaks arising from steps and (110) sites. Therefore, it cannot be used without problems for the determination of (100) sites on a platinum sample. On the (100) terraces, the surface structure of the adsorbed tellurium is c(2 x 2), as revealed by STM. Finally, tellurium irreversible adsorption has been used to estimate the number of (111) ordered domains terrace sites on different polycrystalline platinum samples, and the results are compared to those obtained with bismuth irreversible adsorption.     

Ammonia selective oxidation on Pt(100) sites in an alkaline medium

The oxidation of ammonia on platinum surfaces is a structure sensitive reaction that takes place almost exclusively on Pt(100) sites. This report shows how dependent the activity is on different arrangements of (100) sites of platinum. The effect of two-dimensional domains has been addressed by using stepped surfaces having terraces with (100) geometry, either with (111) and (110) steps. The results were compared with those obtained from stepped surfaces having terraces with (111) or (110) symmetry and monatomic (100) steps, thus representing monodimensional (100) domains. The observed behavior confirms the extreme sensitivity of the reaction to the different arrangement of this type of square sites.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO₄+0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.     

First-principles study of some factors controlling the rate of ammonia decomposition on Ni and Pd surfaces

Using the plane-wave pseudopotential method within the density-functional theory with the generalized gradient approximation for exchange and correlation potential, we have calculated adsorption energies (E(ad)), diffusion barrier, and the first dissociation barrier (E(1)) for NH(3) on Ni and Pd surfaces. While the top site is found to be preferred for NH(3) adsorption on both Ni(111) and Pd(111), its calculated diffusion barrier is substantially higher for Pd(111) than for Ni(111). We also find that during the first dissociation step (NH(3)-->NH(2)+H), NH(2) moves from the top site to the nearest hollow site on Ni(111) and Pd(111) and on the stepped surfaces, Ni(211) and Pd(211), it moves from the initial top site at the step edge to the bridge site in the same atomic chain. Meanwhile H is found to occupy the hollow sites on all four surfaces. On Ni(111), E(1) is found to be 0.23 eV higher than E(ad), while at the step of Ni(211), E(1) and E(ad) are almost equal, suggesting that the probability for the molecule to dissociate is much on the step of Ni(211). In the case of Pd(211), however, we find that the dissociation barrier is much higher than E(ad). These trends are in qualitative agreement with the experimental finding that ammonia decomposition rate is much lower on Pd than on Ni.     

Development and application of a ReaxFF reactive force field for hydrogen combustion

To investigate the reaction kinetics of hydrogen combustion at high-pressure and high-temperature conditions, we constructed a ReaxFF training set to include reaction energies and transition states relevant to hydrogen combustion and optimized the ReaxFF force field parameters against training data obtained from quantum mechanical calculations and experimental values. The optimized ReaxFF potential functions were used to run NVT MD (i.e., molecular dynamics simulation with fixed number of atoms, volume, and temperature) simulations for various H(2)/O(2) mixtures. We observed that the hydroperoxyl (HO(2)) radical plays a key role in the reaction kinetics at our input conditions (T ≥ 3000 K, P > 400 atm). The reaction mechanism observed is in good agreement with predictions of existing continuum-scale kinetic models for hydrogen combustion, and a transition of reaction mechanism is observed as we move from high pressure, low temperature to low pressure, high temperature. Since ReaxFF derives its parameters from quantum mechanical data and can simulate reaction pathways without any preconditioning, we believe that atomistic simulations through ReaxFF could be a useful tool in enhancing existing continuum-scale kinetic models for prediction of hydrogen combustion kinetics at high-pressure and high-temperature conditions, which otherwise is difficult to attain through experiments.     

氢原子在Pt及Pt系双金属催化表面吸附的密度泛函理论研究

采用密度泛函理论(dFT)考察了Pt(100)、(110)、(111)三种表面氢原子的吸附行为, 计算了覆盖度为0.25 ML时氢原子在Pt 三种表面和M-Pt(111)双金属(M=Al, Fe, Co, Ni, Cu, Pd)上的最稳定吸附位、表面能以及吸附前后金属表面原子层间弛豫情况. 分析了氢原子在不同双金属表面吸附前后的局域态密度变化以及双金属表面d 带中心偏离费米能级的程度并与氢吸附能进行了关联. 计算结果表明, 在Pt(100), Pt(110)和Pt(111)表面, 氢原子的稳定吸附位分别为桥位、短桥位和fcc 穴位. 三种表面中以Pt(111)的表面能最低, 结构最稳定. 氢原子在不同M-Pt(111)双金属表面上的最稳定吸附位均为fcc 穴位, 其中在Ni-Pt 双金属表面的吸附能最低, Co-Pt 次之. 表明氢原子在Ni-Pt 和Co-Pt 双金属表面的吸附最稳定. 通过对氢原子在M-Pt(111)双金属表面吸附前后的局域态密度变化的分析, 验证了氢原子吸附能计算结果的准确性. 掺杂金属Ni、Co、Fe 的3d-Pt(111)双金属表面在吸附氢原子后发生弛豫, 第一层和第二层金属原子均不同程度地向外膨胀. 此外, 3d金属的掺入使得其对应的M-Pt(111)双金属表面d带中心与Pt 相比更靠近费米能级, 吸附氢原子能力增强, 表明3d-Pt系双金属表面有可能比Pt具有更好的脱氢活性.     

Catalytic oxidation activity of Pt3O4 surfaces and thin films

The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of platinum oxide surfaces and thin films and their reactivities toward oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive toward either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favored at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than that on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.     

ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation

To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT-MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C-H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We observed that the predominant initiation reaction for oxidation of methane, propene, and o-xylene under fuel lean conditions involved hydrogen abstraction of the methyl hydrogen by molecular oxygen forming hydroperoxyl and hydrocarbon radical species. While under fuel rich conditions with a mixture of these hydrocarbons, we observed different chemistry compared with the oxidation of isolated hydrocarbons including a change in the type of initiation reactions, which involved both decomposition of the hydrocarbon or attack by other radicals in the system. Since ReaxFF is capable of simulating complicated reaction pathways without any preconditioning, we believe that atomistic modeling with ReaxFF provides a useful method for determining the initial events of oxidation of hydrocarbons under extreme conditions and can enhance existing combustion models.     

DFT investigation of CO adsorption on Pt(211) and Pt(311) surfaces from low to high coverage

Adsorption of CO on Pt(211) and Pt(311) surfaces has been investigated by the density functional theory (DFT) method (periodic DMol3) with full geometry optimization. Adsorption energies, structures, and C-O stretching vibrational frequencies are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The calculated C-O stretching frequencies agree well with the experimental ones, and precise determination of adsorption sites can be carried out. For Pt(211), CO adsorbs at the atop site on the step edge at low coverage, but CO adsorbs at the atop and bridge sites simultaneously on both the step edge and the terrace with further increasing CO coverage. The present results interpret the reflection adsorption infrared (RAIR) spectra of Brown and co-workers very well from low to high coverage. For Pt(311), CO adsorbs also at the atop site on the step edge at low coverage. The lifting of reconstruction by CO adsorption occurs also for Pt(311), whereas the energy gain for lifting the reconstruction of the Pt(311) surface is smaller than that for Pt(110). The largest difference between the stepped Pt(211)/Pt(311) and Pt(110) surfaces is the occupation on the edge sites at higher coverage. For the stepped surfaces, the bridge site begins to be occupied at higher coverage, whereas the atop site is always occupied for the Pt(110) surface.     

Dehydroxylation of Glycerol on Pt Surfaces: Ab Initio Molecular Dynamics Study

Glycerol is an important raw material in the chemical industry, and dehydroxylation of glycerol would produce 1, 2-propanediol and 1, 3-propanediol. Here we studied glycerol dehydroxylation with ab initio molecular dynamics simulations on Pt(111) and Pt(211) surfaces at 453 K. The free energies obtained on Pt show that dehydroxylation is more likely to occur at the terminal carbon than the central carbon, and 1, 2-propanediol would be produced preferentially, which is consistent with the selectivity observed experimentally. We found a linear relationship between the free energy barrier and the difference of average distances between O atoms at the initial state and transition state. Although a high correlation between the stability of gaseous glycerol and the number of formed hydrogen bonds is determined from density functional theory calculations, the hydrogen bonds formed within surface structures play a negligible role in determining the free energy barriers of dehydroxylation.