Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

合成氨钌催化剂的晶体和形貌敏感性

哈伯-博施法合成氨反应是高温高压的耗能过程,因此降低该过程的能量消耗及开发温和条件下合成氨反应催化剂具有重要意义. 金属钌是合成氨反应中最有前途的催化剂之一,一直备受广泛关注. 确定金属钌催化剂的结构敏感性并提高其比质量活性是多相催化中亟待解决的重要问题. 氮气(N₂)活化是合成氨反应中的关键步骤. 本文通过第一性原理理论计算和微观动力学模拟方法系统研究了具有六方密排和面心立方晶体结构的钌催化剂上N₂活化过程和N₂解离反应速率. 理论计算研究表明,在六方密排Ru形貌中,{2130}晶面具有最高的N₂解离活性,其次是{0001}台阶面,它们比六方密排Ru其他表面上N₂解离反应速率高3个数量级以上;在面心立方Ru形貌中,{211}和{311}表面上N₂解离活性最高. 这些结果都表明台阶面/台阶位对氮气活化至关重要. 虽然六方密排Ru {2130}晶面具有最低的N₂解离能垒,然而由于面心立方Ru上可以暴露更高密度的活性位点,使得面心立方Ru比六方密排Ru具有更高的N₂转化速率. 本研究深入理解了N₂解离过程中,金属Ru 催化剂形貌和晶相结构敏感性,这为设计和优化高活性的合成氨Ru催化剂提供了理论基础.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

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.     

An XPS study of formaldehyde and related molecules adsorbed on the surface of a tungsten (100) single crystal

X-ray photoelectron spectroscopy has been used to study the adsorption and catalytic decomposition of formaldehyde on a W(100) single crystal. Comparison with the O(1s) spectra of CO(ads), CO₂(ads) and O(ads) has been carried out in an attempt to understand the surface complexes formed from H₂CO. It has been shown that H₂CO dissociates at 100 K upon adsorption up to ca. 1/2 monolayer. Above this coverage, condensation of undissociated H₂CO occurs. A surface complex leading to the liberation of CO₂ from the formaldehyde layer has been detected by XPS. However, no complex uniquely related to an intermediate which yields a small quantity of CH₄ has been detected by XPS.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

GROWTH MODEL OF TEXTURED DIAMOND (111) FILM IN CH4/O2/H2 ATMOSPHERE

Partially oriented and highly textured diamond films on Si( 111 ) substrates were achieved by hot-filament chemical vapor deposition(HFCVD). High nucleation density greater than 5×10⁸cm^-2 was realiged in 3 min by near-surface glow discharge. The os-grown films were characterized by scanning electron microscopy(SEM), X-ray diffraction(XRD) and Raman spectroscopy. It was found that by adding a small amount of oxygen to the mixture of CH₄/H₂, the appearance of facet(111) was well controlled, and the secondary nucleation on the facet(111) was suppressed greatly. Growth feature of homoepitaxy on diamond (111) surface was demonstrated to be in Stranski-Krastanov model by SEM.     

MD study on high-energy reactive cluster impact on diamond (111) and (100) surfaces

Molecular dynamics (MD) simulations of single argon, CO₂ and O₂ cluster impacts on diamond (100) and (111) surfaces are performed in order to investigate the surface erosion process. The transient crater on the (100) surface seems rather unpherical and skew compared to the typical hemispherical crater appeared on the (111) surface due to the orientation-dependent hardness. Argon cluster impacts on the diamond (100) surface resulted in a slightly higher erosion rate than on the (111) surface while it is lowered on the (111) surface for CO₂ cluster impacts. The difference in the susceptibility to the physical erosion appears in the rim or the crater.     

基于反应力场的分子动力学方法研究氢在钯表面的分解

基于二阶矩近似反应力场方法构建的全维度势能面研究了氢分子及其同位素分子在钯表面的分解过程.在构建势能面的过程中数据库中只包含了氢分子与钯（111）表面相互作用的相关信息,该势能面在研究氢分子在钯（100）表面上的分解过程中表现出了非常好的可转移性.结果表明,氢分子及其同位素分子在钯（111）与钯（100）表面上的分解系数S₀均随着入射能量的增加呈现非单调变化,并且通过固定分子取向的方法发现同核分子（H₂、D₂和T₂）最有利分解取向角为90°,而异核分子（HD、HT和DT）受质心偏移的影响,其最有利分解取向角向大角度偏移.     

The initial oxidation of molybdenum: IV. Epitaxial growth of oxide on molybdenum

The oxide which grows in low oxygen pressure and at temperatures between 700 and 1000 K on molybdenum is shown to be MoO₂. The epitaxial relationships between the oxide and the metal (100), (110) and (111) surfaces are given. The epitaxial relationships of oxide on the molybdenum (100) and (110) surfaces are geometrically equivalent. The oxide grows on the (111) molybdenum surface with no major oxide plane parallel to the substrate. It is suggested that the epitaxy of MoO₂ on the (111) surface is a consequence of growth on {211} molybdenum facets. The atomic positions in the pairs of interfacial planes found are given. There is little agreement between the positions of ions in the oxide and substrate lattice sites. Only in the postulated case of MoO₂ on {211} Mo facets is a small misfit found.     

The decomposition of formic acid on Ni(100)

The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D₂O, CO₂, CO, and H₂. The D₂O was evolved below room temperature. CO₂ and H₂ were evolved in coincident peaks at a temperature above that at which h₂ desorbed following H₂ adsorption and well above that for CO₂ desorption from CO₂ adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H₂, CO, and CO₂ within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO₂ was given by:

$\text{Rate = 6 × 10}{}^{15}\text{exp{?[25,500 + ω(}\text{c}\text{c}{}_{\mathrm{sat}}\text{)]/RT} × c}$

, where c is the local surface concentration, c_sat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O.     

A LEED study of UO2(∼100) vicinal surfaces

The ordering and faceting properties of UO₂(～100) vicinal surfaces have been studied via LEED and Auger measurements. The measurements have demonstrated a reduced tendency for step ordering on UO₂(～100) vicinal surfaces when compared to step ordering on UO₂(～111) vicinal surfaces. The UO₂(～100) vicinal surfaces were observed to decompose irreversibly into low-index facets, including prominent (100) facets, at temperatures below those needed for creation of lowest index faceting on UO₂(～111) vicinal surfaces. These properties suggest that (100) terraces, in contrast to (111) terraces, act as surface diffusion barriers that limit longrange surface communication while growing at the expense of intermediate faceting stages.     

Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt

Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H₂SO₄ using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with (110) step, whereas the asymmetric bridge H is observed on Pt(211) = 3(111)–(100) and Pt(311) = 2(111)–(100) that have (100) step. The absence of the atop H on Pt(100), Pt(211), and Pt(311) can be attributed to the relative stability of the bridge site.     

The three-dimensional equilibrium crystal shape of Pb: Recent results of theory and experiment

The three-dimensional equilibrium crystal shape (ECS) is constructed from a set of 35 orientation-dependent surface energies of fcc Pb which are calculated by density functional theory in the local-density approximation and distributed over the [110] and [001] zones of the stereographic triangle. Surface relaxation has a pronounced influence on the equilibrium shape. The (111), (100), (110), (211), (221), (411), (665), (15,1,1), (410) and (320) facets are present after relaxation of all considered surfaces, while only the low-index facets (111), (100) and (110) exist for the unrelaxed ECS. The result for the relaxed Pb crystal state is in support of the experimental ECS of Pb at 320–350 K. On the other hand, approximating the surface energies of vicinal surfaces by assuming a linear relationship between the Pb(111) first-principles surface energy and the number of broken bonds of surface atoms leads to a trivial ECS that shows only (111) and (100) facets, with a sixfold symmetric (111) facet instead of the correct threefold symmetry. It is concluded that the broken bond rule in this simple linear form is not a suitable approximation for obtaining the proper three-dimensional ECS and correct step formation energies. PACS 05.70.Np; 61.50.Jr; 68.35.Md; 71.15.Mb     

Temperature programmed desorption study of acetylene on a clean,H-covered,and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH₄, C₂H₆, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO₂, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.     

Studies of high-pressure n-butane oxidation with CO2 dilution up to 100 atm using a supercritical-pressure jet-stirred reactor

A novel supercritical-pressure jet stirred reactor (SP-JSR) is developed to operate up to 200 atm. The SP-JSR provides a unique platform to conduct kinetic studies at low and intermediate temperatures at extreme pressures under uniform temperature distribution and a short flow residence time. n-Butane oxidations with varying levels of CO₂ dilutions at pressures of 10 and 100 atm and over a temperature range of 500-900 K were conducted using the SP-JSR. The experiment showed that at 100 atm, a weak NTC behavior is observed and the intermediate temperature oxidation is shifted to lower temperatures. Furthermore, the results showed that CO₂ addition at supercritical conditions slows down the fuel oxidation at intermediate temperature while has little effect on the low temperature oxidation. The Healy model under-predicts the NTC behavior and shows little sensitivity of the effect of CO₂ addition on the n-butane oxidation. Reaction pathway and sensitivity analyses exhibit that both the low and intermediate temperature chemistries are controlled by RO₂ consumption pathways. In addition, the reactions of CH₃CO (+ M) and CH₃CO + O₂ become important at 100 atm. The results also revealed that fuel oxidation kinetics is insensitive to the third body effect of CO₂. The kinetic effect of supercritical CO₂ addition may come from the reactions involving H₂O₂, CO, CH₂O, and CH₃CHO, especially for the reactions of CO₂ + H and CO₂ + OH.     

Catalytic activity of Pd ensembles incorporated into Au nanocluster for CO oxidation: A first-principles study

We investigated the catalytic activity of Pd atoms incorporated into Au(111) facet through first-principles calculations, and found that the Pd monomer, dimer, and trimer are highly reactive for the reaction of CO+O₂→CO₂+Ovia association mechanism, in which an intermediate state (OOCO) is formed. Significantly, a low energy barrier (0.19-0.32 eV) was found for the formation of OOCO. The atomic oxygen left by CO+O₂→CO₂+O reaction can be removed by another CO on Pd-decorated Au cluster via Langmuir-Hinshelwood or Eley-Rideal mechanism. Our studies indicate Pd ensembles incorporated into Au(111) facet markedly improve the catalytic activity of gold nanocluster.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

三甲基镓在Pd(111)表面吸附解离及表面预吸附H和O的影响

利用X-射线光电子能谱（XPS）和程序升温脱附谱（TPD）研究了三甲基镓在Pd（111）表面的吸附和解离行为,并考察了表面预吸附H和O的影响。结果表明,在吸附温度为140 K时,三甲基镓在Pd（111）上主要为解离吸附,此时表面物种为Ga（CH₃）_x （x=1,2,3）和CH_x物种。加热将导致Ga的甲基化合物中的Ga-C键发生分步断裂,在不同温度下产生CH₄和H₂从表面脱附。同时,XPS结果证实了在275~325 K的温度区间内存在Ga甲基化合物的分子脱附。退火至更高温度,表面只观察到积碳和金属Ga物种,这二者随着温度的继续升高逐渐向体相扩散。在Pd（111）表面预吸附O和H对上述吸附和解离行为存在显著的影响。当表面预吸附H时,脱附产物CH₄和H₂的脱附主要位于315 K,可归属为一甲基镓的解离脱附。当表面预吸附O时,只在258 K观察到CH₄和H₂的脱附峰,可能来自于Pd-O-Ga（CH₃）₂吸附结构的解离.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.