Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks: I. Configurations,energies, and hydrogen bonding

Adsorption and rotation of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum were studied by density functional theory calculations using the PW91 approximation to the energy functional. On the (111) terrace, water monomer and the donor molecule of the dimer and trimer adsorb at atop sites. The per-molecule adsorption energies of the monomer, dimer, and trimer are 0.30, 0.45, and 0.48 eV, respectively. Rotation of monomers, dimers, and trimers on the terrace is facile with energy barriers of 0.02 eV or less. Adsorption on steps and kinks is stronger than on the terrace, as evidenced by monomer adsorption energies of 0.46 to 0.55 eV. On the (221) stepped surface the zigzag extended configuration is most stable with a per-molecule adsorption energy of 0.57 eV. On the (322) stepped surface the dimer, two configurations of the trimer, and the zigzag configuration have similar adsorption energies of 0.55 ± 0.02 eV. Hydrogen bonding is strongest in the dimer and trimer adsorbed on the terrace, with respective energies of 0.30 and 0.27 eV, and accounts for their increased adsorption energies relative to the monomer. Hydrogen bonding is weak to moderate for adsorption at steps, with energies of 0.04 to 0.15 eV, as the much stronger water–metal interactions inhibit adsorption geometries favorable to hydrogen bonding. Correlations of hydrogen bond angles and energies with hydrogen bond lengths are presented. On the basis of these DFT/PW91 results, a model for water cluster formation on the Pt(111) surface can be formulated where kink sites nucleate chains along the top of step edges, consistent with the experimental findings of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

S在Pt皮肤Pt3Ni(111)面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理方法,计算并分析了S原子在 Pt皮肤Pt3Ni(111)面不同位置的吸附特性.结果表明：S原子在Pt皮肤Pt3Ni的fcc位吸附最强,吸附能为5. 49 eV;与S原子在纯净Pt(111)表面的吸附相比,S原子在Pt皮肤Pt3Ni表面相应吸附位置的吸附能变小、与近邻的Pt原子形成的S-Pt键变长,表明掺杂的Ni会减小相应位点S原子的吸附能,降低体系对S原子的吸附能力,进而减弱S吸附对体系催化能力的影响;态密度分析发现, S原子的吸附使得Pt基催化剂的催化活性降低,主要是S的2p电子引起的;这些结果将为后续研究Pt基合金电极抗S中毒效果以及探究S原子吸附后Pt3Ni的活性位提供依据.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Effect of the electronic structure of a substrate on the photoinduced behavior of adsorbed NO and CO molecules

Photoprocesses in systems produced by adsorption of NO and CO molecules on the Pt(111) and Ni(111) surfaces, as well as on the (111) surface of Pt-Ge alloy, is studied by the IR absorption spectroscopy, resonant multiphoton ionization, and UV photoelectron spectroscopy methods. The energy of photons varies between 2.3 and 6.4 eV. The character of the processes depends on the type of the metallic substrate. On the Pt(111) surface, NO molecules dissociate or are desorbed, depending on the degree of coverage. On the Ni(111) surface, the molecules only dissociate. Conversely, NO molecules adsorbed on the (111) surface of the Pt-Ge alloy are only desorbed from the surface. In the CO/Pt(111) and CO/Pt(111)-Ge systems, CO molecules adsorbed on on-top adsorption sites are desorbed under the action of the photons, while those occupying bridging adsorption sites change their properties insignificantly. A model of photoinduced processes is suggested. According to this model, the lifetime of a state excited by charge transfer between the valence band of the metal and the 2π-antibonding molecular orbital plays a decisive part in the occurrence of one or the other of these processes.     

The effect of defects on the hydrogenation in Mg (0 0 0 1) surface

Density-functional theory was presented to investigate the hydrogen dissociation on a pure, Pt-doped, vacancy and oxide Mg(0 0 0 1) surface. Our results show that the energy barriers are 1.05, 0.39, 0.93 and 1.33 eV for H₂ dissociation on the pure, Pt-doped, vacancy and oxide Mg surface, respectively. The calculation results imply that the initial dissociation of H₂ is enhanced significantly for the Pt-doped Mg(0 0 0 1) surface, negligible for the vacancy model and weekend for the oxide model. The density of state results shows that, following the dissociation reaction coordinate, the H–H interactions are weeker for the Pt-doped model while interactions become stronger for the oxide model. It is suggested that the dissociation process is facilitated when Pt atom acts as catalyst and oxide overlayers delay hydrogen adsorption on the Mg layer. The present study will help us understand the defect role being played for the improvement or opposition effect in absorption kinetics of H₂ on the Mg(0 0 0 1) surface.     

Adsorption and diffusion of a single Pt atom on γ-Al2O3 surfaces

Motivated to better understand the interactions between Pt and γ-Al₂O₃ support, the adsorption and diffusion of a single Pt atom on γ-Al₂O₃ was studied using density functional theory. Two different surface models with atoms of various coordination (3–5) were used, one derived from a defected spinel structure, and another derived from the dehydration of boehmite (AlOOH). Adsorption energies are similar for the two surfaces, about −2 eV for the most stable sites, and involve Pt binding to surface O atoms. An unusually strong trapping geometry whereby Pt moves into the surface was identified over the boehmite-derived surface. In all cases the surface transfers 0.2–0.3 e⁻ to the Pt atom. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. The similarity of the two surfaces is attributed to the similar local environments of the surface atoms, as corroborated by geometry analysis, density of states, and Bader charge analysis. Calculated activation barriers (0.3–0.5 eV) for the defected spinel surface indicate fast diffusion and a kinetic Monte Carlo model incorporated these barriers to determine exact diffusion rates and behavior. The kinetic Monte Carlo results indicate that at low temperatures (<500 K) the Pt atom can become trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. Finally we modeled the adsorption of Pt on hydrated surfaces and found adsorption to be weaker due to steric repulsion and/or decreased electron-donating ability of the surface.     

NO-methanol interaction on the surface of Pt(3 3 2)

The interaction between NO and CH₃OH on the surface of stepped Pt(3 3 2) was investigated using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). At 90 K, pre-dosed CH₃OH molecules preferentially adsorb on step sites, suppressing the adsorption of NO molecules on the same sites. However, due to a much stronger interaction with Pt, at 150 K and higher, the adsorption of NO molecules on step sites is restored, giving rise to peaks closely resembling those of NO molecules adsorbed on clean Pt(3 3 2) surface. Adsorbed CH₃OH is very reactive on this surface, and is readily oxidized to formate in the presence of O₂, even at 150 K. In contrast, reactions between CH₃OH and co-adsorbed NO are slight to non-existent. There are no new peaks in association with intermediates resulting from CH₃OH-NO interactions. It is concluded that the reduction of NO with CH₃OH on Pt(3 3 2) does not proceed through a mechanism of forming intermediates.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

Effects of water and electric field on atomic oxygen adsorption on Pt–Co alloys

Density functional theory is used to study the effect of atomic oxygen adsorption at various coverages with and without the presence of water on ordered and Pt-segregated PtCo surfaces. The strength of O adsorption, as well as surface reconstruction effects due to the adsorbate are strongly influenced by the presence of the oxygen-philic transition metal on the surface or subsurface. At high O coverage, buckling of the Co atom on PtCo surfaces is much smaller than that of Pt on Pt(1 1 1) surfaces, and buckling of Pt atoms on Pt-skin surfaces is negligible. Also, the effect of an electric field perpendicular to the surface on adsorbed water and atomic oxygen is investigated. Spontaneous water dissociation is not found on the ordered and segregated alloy surfaces within the entire applied electric field range (−0.51 to 0.51 V/Å). Water changes orientation under strong negative fields, switching from a metal–O to a metal–H interaction, and the effect is much more pronounced in the low-coordination sites of cluster models.     

TPD study of the adsorption and reaction of nitromethane and methyl nitrite on ordered Pt–Sn surface alloys

The adsorption and reaction of the isomers nitromethane (CH₃NO₂) and methyl nitrite (CH₃ONO) on two ordered Sn/Pt(111) surface alloys were studied using TPD, AES, and LEED. Even though the Sn–O bond is stronger than the Pt–O bond and Sn is more easily oxidized than Pt, alloying with Sn reduces the reactivity of the Pt(111) surface for both of these oxygen-containing molecules. This is because of kinetic limitations due to a weaker chemisorption bond and an increased activation energy for dissociation for these molecules on the alloys compared to Pt(111). Nitromethane only weakly adsorbs on the Sn/Pt(111) surface alloys, shows no thermal reaction during TPD, and undergoes completely reversible adsorption under UHV conditions. Methyl nitrite is a much more reactive molecule due to the weak CH₃O–NO bond, and most of the chemisorbed methyl nitrite decomposes below 240 K on the alloy surfaces to produce NO and a methoxy species. Surface methoxy is a stable intermediate until 300 K on the alloys, and then it dehydrogenates to evolve gas phase formaldehyde with high selectivity against complete dehydrogenation to form CO on both alloy surfaces.     

Comparison of sulfur interaction with hydrogen on Pt(1 1 1), Ni(1 1 1) and Pt3Ni(1 1 1) surfaces: The effect of intermetallic bonding

Adsorption strengths of hydrogen and sulfur both individually and together as co-adsorbates were investigated on Pt(1 1 1), Ni(1 1 1) and Pt₃Ni(1 1 1) surfaces using density functional theory in order to determine the effect of metal alloying on sulfur tolerance. The adsorption strengths of H and S singly follow the same trend: Ni(1 1 1) > Pt(1 1 1) > Pt₃Ni(1 1 1), which correlates well with the respective d-band center positions of each surface. We find that the main effect of alloying is to distort both the sub-layer structure and the Pt overlayer resulting in a lowered d-band. For all three surfaces, the d-band shifts downward non-linearly as a function of S coverage. Nearly identical decreases in d-band position were calculated for each surface, leading to an expectation that subsequent adsorption of H would scale with surface type similarly to single species adsorption. In contradiction to this expectation, there was no clearly discernable difference between the energies of coadsorbed H on Pt(1 1 1) and Ni(1 1 1) and only a slightly lowered energy on Pt₃Ni(1 1 1). This provides evidence that coadsorbed species in close proximity interact directly through itinerant mobile electrons and through electrostatic repulsion rather than solely through the electronic structure of the surface. The combination of the lowered d-band position (arising from distorted geometry) and direct co-adsorbate interactions on Pt₃Ni(1 1 1) leads to a lower energy barrier for H₂S formation on the surface compared to pure Pt(1 1 1). Thus, alloying Pt with Ni both decreases the likelihood of S adsorption and favors S removal through H₂S formation.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

Photoemission from Co on Pt(111) using synchrotron radiation in the range 40 eV ω 150 eV

Photoelectron spectra for CO adsorbed on the (111)-face of Pt have been measured using synchrotron radiation of energy 40 eV ω 150 eV. A dramatic increase of the molecular orbital (MO) intensity relative to the intensity of the Pt 5d valence band (VB) is observed for ω > 100 eV, to a ratio at that is a factor of three higher than at . The energy variation of the Pt 5d VB photoemission peak intensity has been derived independently in the 40–200 eV range from measurements on clean Pt. The 5d peak intensity is found to decrease steeply (by more than an order of magnitude) between 100 and 150 eV. The observed increase of the MO peak intensity relative to that of the 5d VB is attributed to this cross-section effect. The Pt VB peak nearest the Fermi energy which is mainly t_2g in character, is found to decrease in intensity on adsorption of CO. In the present case synchrotron radiation in the ω > 100 eV range appears to be especially valuable for studies of adsorbates.     

Flash desorption and equilibration of H2 and D2 on single crystal surfaces of platinum

The adsorption and flash desorption of hydrogen and the equilibration of H₂ and D₂ has been studied on the (110), (211), (111) and (100) planes of platinum. Desorption from Pt (211), a stepped surface composed of (111) and (100) ledges, yields a desorption spectrum which apparently is a composite of desorption from the individual ledges. Pt (110) is quite similar to the tungsten structural analog, W (211), in that both yield two-peak desorption spectra, and on both planes adsorption kinetics are dramatically different for filling of the two states. On all four planes adsorption kinetics are apparently proportional to (1 ? θ)², and estimates of the initial sticking probabilities show them to decrease in the order: (110) > (211) > (100) > (111). Equilibration activity follows approximately the same order [(110) > (211) > (111) > (100)] with a factor of ~ 5 difference between the most and least active planes; no extraordinary activity is observed for the stepped surface, Pt(211). Below ~ 570 K equilibration of H₂ and D₂ is activated by less than 2 kcal/mole with the magnitude dependent on the specific face, and above this temperature the reaction is nonactivated. The non-activated case apparently results from absorption followed by statistical mixing on the surface. Calculated rates for HD production per cm² based on this model are in excellent agreement with the experimental values for Pt(110) and Pt(211), and in somewhat poorer agreement in the case of Pt (111) and Pt (100). This latter is probably due to the greater inaccuracy in the values of the sticking coefficients on these planes.     

Surface chemistry studied by in situ X-ray photoelectron spectroscopy

Time-dependent X-ray photoelectron spectroscopy is used to study the kinetics and dynamics of simple surface reactions. Combining high-resolution core level spectroscopy with a supersonic molecular beam in one experimental setup, processes such as the dissociative adsorption of methane on both Pt(111) and Ni(111), the coadsorption of water and CO on Pt(111), and the oxidation of CO on Pt(111) have been studied. In the case of methane, the observed vibrational fine structure in C 1s spectra is used to identify the adsorbed species (CH₃) and further thermal dehydrogenation steps. While simple dehydrogenation via CH is observed on Pt(111), a C–C coupling reaction to acetylene is found on Ni(111). In the coadsorbate phase, CO is found to be able to replace predosed water from the bilayer into multilayers. Water, in turn, leads to a site change of the CO molecules, which are preferably adsorbed at bridge sites in the presence of water, as opposed to on-top adsorption on clean Pt(111). For the truly bimolecular surface reaction, the CO oxidation on Pt(111), the ability of the molecular beam to create a relatively high CO pressure was found essential to study the kinetics of the basic step (CO+OCO₂) without influence of adsorption or diffusion rate. An activation energy of 0.53 eV and a preexponential factor of 5×10⁶ s^-1 are found. PACS 68.43.Mn; 79.60.Dp; 82.20.Pm     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

Initial stage of the adsorption of fluorofullerene molecules on Si surface

Spatially resolved images of an individual C₆₀F₁₈ fluorofullerene molecule on Si(100) − 2 × 1 surface have been obtained using scanning tunneling microscopy. Scanning tunneling microscopy results and ab initio calculations show that the fluorofullerene molecules interact with the Si(100) − 2 × 1 surface with F atoms pointing down towards the surface. The adsorption energy of a C₆₀F₁₈ molecule on Si(100) − 2 × 1 surface is ∼12.1 eV, which is much higher than the adsorption energy of the same molecule on Si(111) − 7 × 7 surface (6.65 eV). C₆₀F₁₈ molecules are located in the troughs in-between the dimer rows occupying the four-dimer site on Si(100) − 2 × 1 surface.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

Mechanistic study of the CO oxidation reaction on the CuO (111) surface during chemical looping combustion

Cu-based oxides oxygen carriers and catalysts are found to exhibit attractive activity for CO oxidation, but the dispute with respect to the reaction mechanism of CO and O₂ on the CuO surface still remains. This work reports the kinetic study of CO oxidation on the CuO (111) surface by considering the adsorption, reaction and desorption processes based on density functional theory calculations with dispersion correction (DFT-D). The Eley–Rideal (ER) CO oxidation mechanism was found to be more feasible than the Mars-van-Krevelen (MvK) and Langmuir–Hinshelwood (LH) mechanisms, which is quite different from previous knowledge. The energy barrier of ER, LH, and MvK mechanisms are 0.557, 0.965, and 0.999 eV respectively at 0 K. The energy barrier of CO reaction with the adsorbed O species on the surface is as low as 0.106 eV, which is much more active in reacting with CO molecules than the lattice O of CuO (111) surface (0.999 eV). A comparison with the catalytic activity of the perfect Cu₂O (111) surface shows that the ER mechanism dictates both the perfect Cu₂O (111) and the CuO (111) surface activity for CO oxidation. The activity of the perfect Cu₂O (111) surface is higher than that of the perfect CuO (111) surface at elevated temperatures. A micro-kinetic model of CO oxidation on the perfect CuO (111) surface is established by providing the rate constants of elementary reaction steps in the Arrhenius form, which could be helpful for the modeling work of CO catalytic oxidation.