Water desorption from an oxygen covered Pt(111) surface: multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.     

Dynamical study of H2 and D2 desorbing from a Cu(111) surface

A theoretical study of H(2) and D(2) desorbing from Cu(111) is reported. The study makes use of the LEPS PES of Dai and Zhang [J. Chem. Phys. 1995, 102, 6280]. The LEPS parameters have been modified in order to lower the barrier threshold in conformity with accurate ab initio electronic structure calculations. The topological study of the modified PES by the CHAIN method reveals unambiguously that the transition state (TS) is located at the top of a unique early barrier along the desorption path. The adsorbed H atoms are supposed to be in thermal equilibrium with the metal surface. Batches of classical trajectories (CT) are then carried out from the TS onto the products with their initial conditions canonically distributed and the effect of their possible recrossing of the TS taken into account according to Keck's method [Discuss. Faraday Soc. 1962, 33, 173]. Product state distributions are then calculated using the Gaussian weighting procedure [Chem. Phys. Lett. 2004, 397, 106] to account for the quantization of the vibration motion of the desorbed diatom. These distributions are in overall good agreement with experimental measurements. On average, the early barrier to desorption results in a significant vibrational excitation of the final diatom and a strong deexcitation of its rotational angular momentum J from the TS onto the products. Moreover, the orientation of the rotation plane is roughly random for low values of J (both cartwheel and helicopter motions are observed) while it is more likely parallel to the metal surface for large values of J (predominance for helicopter motion). These findings are analyzed in some details.     

Effect of subsurface oxygen on the reactivity of the Ag(111) surface

Periodic, self-consistent, density functional theory calculations have been performed to demonstrate that subsurface oxygen (O(sb)) dramatically increases the reactivity of the Ag(111) surface. O(sb) greatly facilitates the dissociation of H2, O2, and NO and enhances the binding of H, C, N, O, O2, CO, NO, C2H2, and C2H4 on the Ag(111) surface. This effect originates from an O(sb)-induced upshift of the d-band center of the Ag surface and becomes more pronounced at higher O(sb) coverage. Our findings point to the important role that near-surface impurities, such as O(sb), can play in determining the thermochemistry and kinetics of elementary steps catalyzed by transition metal surfaces.     

Adsorption and reaction of cyclohexene on a Ni(111) surface

We studied the adsorption and reaction of cyclohexene (C6H10) on Ni(111) at different temperatures with high-resolution in-situ X-ray photoelectron spectroscopy (HR-XPS). For exposure at 125 K, we find intact cyclohexene with two distinct C 1s signals at 283.3 and 284.2 eV, due to the nonequivalent carbon atoms in the molecule. The energetic separation is significantly increased relative to the gas-phase value, due to the interaction with the substrate. Upon exposure at 210 K, complete dehydrogenation of cyclohexene to benzene (C6H6) and hydrogen is observed; coverage-dependent changes of the benzene adsorption site occur in a way similar to those for pure benzene layers, which indicates a phase separation in benzene and hydrogen islands. The thermal evolution of the adsorbed layers was studied by temperature-programmed (TP-) XPS and temperature-programmed desorption spectroscopy (TPD). Upon heating, the benzene + hydrogen layer formed at 210 K shows a coverage-dependent reorientation of the benzene molecules during partial desorption. The cyclohexene layer adsorbed at 125 K only shows partial conversion of cyclohexene to benzene and hydrogen upon heating to 185 or 210 K, with the remaining cyclohexene being stable up to approximately 300 K. We propose that upon heating these molecules are stabilized by coadsorbed benzene and hydrogen; furthermore, the mobility of benzene and hydrogen in this coadsorbed layer is reduced, so that no phase separation can occur.     

First-principles study of the mechanism of carbon deposition on Fe(111) surface and subsurface

使用密度泛函方法对C原子在Fe(111)表面吸附团聚和次表层的吸附扩散进行了研究.在炭覆盖度θc＜1ML时,C主要以孤立的原子态存在并导致表面重构;1ML≤θc≤2ML,“mC2+nC”为主要的吸附形式;θc≥2ML时,复杂的吸附形态比如碳链和岛状碳团簇开始生成.这些复杂岛状碳团簇是Fe(111)表面石墨沉积或碳纳米管生长的成核中心.在次表层,C原子在八面体位稳定存在.C在表面的迁移能垒为0.45eV,由表面迁移到次表面的的能垒为0.73eV.虽然C2团簇的生成是热力学有利的,但是C向次表层的迁移动力学上占优.     

第一性原理研究C在Fe(111)表面和次表面的积炭机理

使用密度泛函方法对C原子在Fe(111)表面吸附团聚和次表层的吸附扩散进行了研究。在炭覆盖度θ_C <1 ML时,C主要以孤立的原子态存在并导致表面重构;1 ML≤θ_C ≤2 ML,"mC₂+nC"为主要的吸附形式;θ_C≥2 ML时,复杂的吸附形态比如碳链和岛状碳团簇开始生成。这些复杂岛状碳团簇是Fe(111)表面石墨沉积或碳纳米管生长的成核中心。在次表层,C原子在八面体位稳定存在。C在表面的迁移能垒为0.45 eV,由表面迁移到次表面的的能垒为0.73 eV。虽然C₂团簇的生成是热力学有利的,但是C向次表层的迁移动力学上占优。     

Static surface temperature effects on the dissociation of H(2) and D(2) on Cu(111)

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H(2) and D(2) on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (T(s) = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found.     

State-to-state scattering of D2 from Cu(100) and Pd(111)

Results from state resolved experiments are presented for the interaction of D2(v=1,J=2) with Cu(100) and Pd(111). The reflected molecules were probed using quantum state specific spectroscopy. For D2 scattered from Cu(100) the vibrational survival probability and some transition inelastic probabilities were measured for incident energies from 70-200 meV. The survival probability was found to be larger then that found previously for H2(v=1) scattered from the same surface; these differences are discussed in terms of the lower zero point energy and smaller vibrational energy spacings of D2. D2 translational energy exchange was studied for several different scattering channels and interpreted using simple classical calculations. The survival probability was also measured for D2(v=1) scattered from Pd(111) at one incident energy. Pd is reactive for D2 dissociation and this survival probability was measured to be small and also to be much smaller than that for H2(v=1) under similar conditions. Vibrational relaxation channels were studied for D2 scattering from both Cu(100) and Pd(111). The vibrational relaxation probability on both surfaces was also found to be smaller than that measured for comparable channels for H2. The smaller survival probability and vibrational relaxation probability for D2 on Pd(111) cannot be easily accounted for by the difference in zero point energy and vibrational energy spacings.     

The electronic structure and orientation of the surface methoxy species on Ni(111)

Photoelectron spectra of the methoxy species formed by the decomposition of methanol on nickel(111) indicate that the C-O-nickel bond is normal or nearly normal to the metal surface. This structural assignment for the adsorbate complex is consistent with new photoelectron data using linearly polarized synchrotron radiation.     

Molecule-substrate interaction channels of metal-phthalocyanines on graphene on Ni(111) surface

Molecule-substrate interaction channels of metal-phthalocyanines (MPcs, including NiPc, CuPc, ZnPc, FePc, and CoPc) on graphene on Ni(111) were investigated by employing high-resolution electron energy loss spectroscopy (HREELS). Except the expected IR-active modes, some Raman-active modes were also observed in all of MPcs, which are considered in this study. From the origination of the Raman-active features, it was deduced that MPcs are coupled with the substrate mainly through their central metal atom. The Raman-active modes appear as symmetric peaks in the HREELS in the case of MPcs with Ni, Cu, and Zn, whereas they are asymmetric and appear as a Fano line shape in the case of MPcs with Fe and Co. This spectroscopic difference indicates that the molecule-substrate coupling is completely different in the two cases mentioned above. The molecule-substrate interaction strength is considerably weak and comparable with the π-π interaction between molecules in the case of MPcs with Ni, Cu, and Zn, whereas it is much stronger in the case of MPcs with Fe and Co. From the HREELS observations, it can be suggested that the whole molecule can be effectively decoupled from the underneath Ni(111) by inserting a single layer of graphene between them in the case of MPcs with Ni, Cu, and Zn, whereas only benzene rings can be completely decoupled in the case of MPcs with Fe and Co.     

Isolated heterometallic Cr7Ni rings grafted on Au(111) surface

A study of the deposition of heterometallic antiferromagnetically coupled rings onto gold surfaces is reported. Two new {Cr7Ni} rings, [NH2nPr2][Cr7NiF8(3-tpc)16] (1) (where 3-tpc=3-thiophenecarboxylate) and [nBuNH2CH2CH2SH] [Cr7NiF8(O2CtBu)16] (2) have been made and structurally characterized. They have been deposited from the liquid phase on Au(111) and the adsorbed molecules compared by means of scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). In both cases a two-dimensional distribution of individually accessible {Cr7Ni} heterometallic rings on the gold surface has been obtained, exploiting the direct grafting of sulfur-functionalized clusters. There is a competition between the chemisorption of the {Cr7Ni} clusters and a thiolic self-assembled monolayer (SAM) formed by free ligands. In 2, the presence of a single sulfur ligand should force the molecule to graft with the ring axis normal to the surface. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both functionalizations, the strength of the grafting with the gold surface.     

Adsorption and desorption of HCl on Pt(111)

The adsorption and desorption of HCl on Pt(111) is investigated by temperature programmed desorption, infrared reflection absorption spectroscopy, and low energy electron diffraction. Five peaks are identified in the desorption spectra prior to the onset of multilayer desorption. At low coverage ( < 0.25 monolayers (ML)), desorption peaks at approximately 135 and 200 K are observed and assigned to recombinative desorption of dissociated HCl. At higher coverages, desorption peaks at 70, 77, and 84 K are observed. These peaks are assigned to the desorption of molecularly adsorbed HCl. The infrared spectra are in agreement with these assignments and show that HCl deposited at 20 K is amorphous but crystallizes when heated above 60 K. Kinetic analysis of the desorption spectra reveals a strong repulsive coverage dependence for the desorption energy of the low coverage features ( < 0.25 ML). The diffraction data indicate that at low temperature the adsorbed HCl clusters into ordered islands with a (3 x 3) structure and a local coverage of 4/9 with respect to the Pt(111) substrate.     

Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals

The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.     

Manipulating the activation barrier for H2(D2) desorption from potassium-modified palladium surfaces

In this work the permeation and desorption of hydrogen (deuterium) from potassium-modified Pd(111) and polycrystalline palladium surfaces have been studied in the temperature range from 350 to 523 K. Time-of-flight spectroscopy has been used to determine the translational energy distributions of associatively desorbing H(2)(D(2)) molecules as a function of the potassium coverage and additional isotropic O(2) and CO background pressures. It turned out that the energy distribution of the hydrogen desorption flux is thermalized for the clean Pd surfaces but hyperthermal for the potassium-covered surfaces. The activation barrier for adsorption was found to increase with the potassium coverage but to decrease again in the presence of coadsorbates such as O(2) or CO. Especially by choosing different isotropic CO pressures, the effective desorption barrier for hydrogen could be reversibly decreased and increased, which resulted in the equivalent changes of the mean kinetic energies of the desorbing H(2) molecules.     

The interaction of water with Ni(111) and H/Ni(111) studied by TPD and HREELS

We have used temperature-programmed desorption in combination with specular and off-specular high resolution electron energy loss spectroscopy to study the interaction of H(2)O and D(2)O with the bare and hydrogen-covered Ni(111) surface. Our results for the bare metal surface agree with previous reports and we are able to relate two prominent features in vibrational spectra to nuclear motions at the surface. Pre-covering Ni(111) with hydrogen alters both adsorption and desorption of water significantly. The strong H-Ni bond does not allow for isotopic exchange with co-adsorbed D(2)O. Strong resemblance of desorption traces and vibrational spectra of submonolayer coverages on H-covered Ni(111) and multilayers on bare Ni(111) suggests that adsorption of hydrogen makes this nickel surface hydrophobic.     

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.     

High translational energy release in H2 (D2) associative desorption from H (D) chemisorbed on C(0001)

Highly energetic translational energy distributions are reported for hydrogen and deuterium molecules desorbing associatively from the atomic chemisorption states on highly oriented pyrolytic graphite (HOPG). Laser assisted associative desorption is used to measure the time of flight of molecules desorbing from a hydrogen (deuterium) saturated HOPG surface produced by atomic exposure from a thermal atom source at around 2100 K. The translational energy distributions normal to the surface are very broad, from approximately 0.5 to approximately 3 eV, with a peak at approximately 1.3 eV. The highest translational energy measured is close to the theoretically predicted barrier height. The angular distribution of the desorbing molecules is sharply peaked along the surface normal and is consistent with thermal broadening contributing to energy release parallel to the surface. All results are in qualitative agreement with recent density functional theory calculations suggesting a lowest energy para-type dimer recombination path.     

O2和CO在Ni(111)表面的吸附活化

采用高分辨电子能量损失谱(HREELS)、俄歇电子能谱(AES)和低能电子衍射(LEED)研究镍单晶表面氧物种及CO与O₂的共吸附。实验结果表明,Ni(111)表面氧化后存在两种氧物种,位于54 meV能量损失峰的表面化学吸附氧物种和位于69 meV能量损失峰的表面氧化镍。首先,随着暴露氧量的增加,表面化学吸附氧物种的能量损失峰蓝移至58 meV;其次,通过真空退火及与CO相互作用考察,发现表面化学吸附氧物种较不稳定。在室温条件下,表面预吸附形成的表面化学吸附氧物种与CO共吸附,导致端位吸附CO增多,表明氧优先吸附在穴位上,随着CO暴露量的增加化学吸附氧物种与CO反应脱去;而表面氧化镍需在较高温度和较高CO分压下才能被CO还原。预吸附CO可被氧逐渐移去。