Adsorption and reaction of methanol on clean and oxygen modified rhodium/vanadium surface alloys

The dehydrogenation of methanol on Rh(111), on a Rh(111)/V subsurface alloy and on Rh(111) with V islands has been studied with and without preadsorbed oxygen using a supersonic molecular beam and temperature programmed desorption. The reactivity is highest for the V islands surface without oxygen. But this surface is deactivated due to CO dissociation. The subsurface alloy is less reactive than the islands, but still more active than the Rh(111) surface. The reaction products are carbon monoxide and hydrogen only. With preadsorbed oxygen Rh(111) is the most active surface, but a strong dependence of the activity on the amount of preadsorbed oxygen is found for all three surfaces. The reaction products with preadsorbed oxygen are water, hydrogen, carbon dioxide and carbon monoxide. The reactions follow the same mechanism on all surfaces, but the activation energy of the individual reaction steps is different leading to significant changes in the thermal desorption spectra and in King and Wells-type experiments.     

Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.     

Analysis of methane-to-methanol conversion on clean and defective Rh surfaces

We investigate by density-functional theory simulations several elementary reactions associated to direct methane-to-methanol conversion on clean Rh(111) surfaces and on Rh adatoms on Rh(111). Energy barriers and reaction paths have been determined by the nudged elastic band method. The rate-limiting step in the process, C-O bond formation, has higher activation energy than the one for complete methane dehydrogenation. Our analysis enables us to understand the effect of defects on the reactivity and rules out Rh as candidate catalyst for methanol synthesis.     

甲氧基在Rh(111)表面吸附的密度泛函研究

采用密度泛函理论(DFT)的B3LYP方法,以原子簇Rh13(9,4)为模拟表面,在6-31G(d,p)与Lanl2dz基组水平上,对甲氧基在Rh(111)表面的四种吸附位置(fcc、hcp、top、bridge)的吸附模型进行了几何优化、能量计算、Mulliken电荷布局分析以及前线轨道的计算。结果表明,当甲氧基通过氧与金属表面相互作用时,在bridge位的吸附能最大,吸附体系最稳定,在top位转移的电子数最多;吸附于Rh(111)面的过程中C—O键被活化,C—O键的振动频率发生红移。     

Water adsorption on Rh(111) at 20 K: from monomer to bulk amorphous ice

The adsorption of water (D(2)O) molecules on Rh(111) at 20 K was investigated using infrared reflection absorption spectroscopy (IRAS). At the initial stage of adsorption, water molecules exist as monomers on Rh(111). With increasing water coverage, monomers aggregate into dimers, larger clusters (n = 3-6), and two-dimensional (2D) islands. Further exposure of water molecules leads to the formation of three-dimensional (3D) water islands and finally to a bulk amorphous ice layer. Upon heating, the monomer and dimer species thermally migrate on the surface and aggregate to form larger clusters and 2D islands. Based on the temperature dependence of OD stretching peaks, we succeeded in distinguishing water molecules inside 2D islands from those at the edge of 2D islands. From the comparison with the previous vibrational spectra of water clusters on other metal surfaces, we conclude that the number of water molecules at the edge of 2D islands is comparable with that of water molecules inside 2D islands on the Rh(111) surface at 20 K. This indicates that the surface migration of water molecules on Rh(111) is hindered as compared with the cases on Pt(111) and Ni(111) and thus the size of 2D islands on Rh(111) is relatively small.     

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.     

Decomposition and oxidation of methanol on platinum: A study by in situ X-ray photoelectron spectroscopy and mass spectrometry

The reactions of the catalytic oxidation and decomposition of methanol on the atomically smooth and high-defect Pt(111) single-crystal surfaces were studied using in situ temperature-programmed reaction and X-ray photoelectron spectroscopy. It was found that the decomposition of methanol on both of the surfaces occurred via two reaction pathways: complete dehydrogenation to CO and decomposition with the C-O bond cleavage. Although the rate of reaction via the latter pathway was lower than the rate of dehydrogenation by three orders of magnitude, the carbon formed as a result of the C-O bond cleavage can be accumulated on the surface of platinum to prevent the further course of the reaction. It was shown that oxygen exhibits high activity toward the formed carbon deposits. As a result, the rate of methanol conversion in the presence of oxygen in a gas phase increased by one or two orders of magnitude; in this case, CO₂ and water appeared in the composition of the reaction products as a result of the oxidation of CO and hydrogen, respectively. The high-defect surface of platinum was more active in the reactions of methanol decomposition and oxidation than the atomically smooth Pt(111) single-crystal surface. On the former, selectivity for the formation of methanol dehydrogenation products in oxygen deficiency was higher than on the latter. The main reaction pathways of the decomposition and oxidation of methanol on platinum were considered.     

Analysis of the influence of rhodium addition to platinum on its activity towards methanol electrooxidation by EIS

Influence of rhodium addition to platinum on the activity of the alloy in methanol electrooxidation has been studied using Pt–Rh/Au limited volume electrodes with various surface compositions including the pure Pt and Rh metals. Electrochemical impedance spectroscopy (EIS) was used in the study. In the case of the Pt–Rh alloy, the impedance picture of methanol oxidation is qualitatively the same as for the pure Pt electrode. However, impedance spectra strongly depend on alloy composition. Equivalent circuits suitable for methanol oxidation on Pt were also used in the case of Pt–Rh and similar fitting results were obtained. A reaction mechanism suggested in the literature for Pt, which involves two strongly adsorbed intermediates competing for the same adsorption sites, is likely also for the Pt–Rh alloys. However, fittings with a corresponding impedance model were unsuccessful for both Pt and Pt–Rh because of mathematical caveats, such that quantitative comparisons were not possible. Nevertheless, EIS results suggest that Rh inhibits the kinetics of formation of reactive oxygen species at the surface of the alloy.     

铑催化转化合成气活性中间体的量子化学研究

采用DV-Xα-SCC方法研究用铑催化剂在有助剂MnO和无助剂两种情况下转化合成气产生的甲酰基中间体,讨论了它在催化剂表面的稳定状态、电子结构及成键性质.结果表明,吸附态甲酰基在催化剂表面形成金属氧卡宾结构,稳定了体系.与无助剂的情况相比,助催剂MnO可使体系更稳定,并使C-O键受到更大程度的削弱,同时它还对中间体的反应行为产生影响.     

Full hemispherical photoelectron diffraction and Fermi surface mapping

The routine measurement of full hemispherical photoemission intensity maps gives us the possibility for the combined investigation of structural and electronic phenomena at surfaces. As an example the growth of ultrathin films of Co on Cu(111) is studied as a function of film thickness. While X-ray photoelectron diffraction (XPD) shows the early appearance of stacking faults as a precursor of the hcp structure, Fermi surface maps reveal the very fast evolution of the Co Fermi surface that can be compared to measurements on a clean Co(0001) crystal. For the system O/Rh(111), XPD brings up important structural clues, relating changes in surface reactivity to small amounts of subsurface oxygen, which forces adjacent oxygen atoms to occupy new and more reactive adsorption sites. In the course of this last study we observed for the first time the weak backscattering signals in the angular pattern of adsorbate emission. These cone-like features are extremely sensitive to the adsorbate–substrate bond length.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

Activation of the C-O bond on the surface of palladium: An <Emphasis Type="Italic">In situ</Emphasis> study by X-ray photoelectron spectroscopy and sum frequency generation

An in situ study of the adsorption of CO on atomically smooth and defect Pd(111) surfaces was performed over wide ranges of temperatures (200–400 K) and pressures (10^–6-1 mbar) by X-ray photoelectron spectroscopy and sum frequency generation. Both of the techniques indicated that CO was adsorbed as three-fold hollow, bridging, and terminal species to form well-known ordered structures on the surface. In the course of the in situ experiments, no signs of CO dissociation or of the formation of carbonyl structures (Pd(CO)_n, n > 1) were detected. The mechanism of C-O bond activation in the course of methanol decomposition on the surface of palladium was considered. It was found that the adsorption of methanol on the surface of palladium essentially depends on pressure. Along with the well-known reaction path of methanol dehydrogenation to CO and hydrogen, a slow process of methanol decomposition with C-O bond cleavage was observed at elevated pressures. In this case, the formation of carbon deposits finally resulted in the carbonization and complete deactivation of the surface. A mechanism for C-O bond activation on the surface of palladium was proposed; the geometry of adsorption complexes plays an important role in this mechanism.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 288–301.Original Russian Text Copyright © 2005 by Kaichev, Bukhtiyarov, Rupprechter, Freund.     

铑催化合成气制乙醇反应中CO断键途径的研究

利用程序升温表面反应─红外（ＴＰＳＲ－ＩＲ）动态技术考察ＣＯ吸附物种对氢的反应性能并检验表面反应生成的中间物，结果表明线式ＣＯ对氢的反应性能高于桥式ＣＯ，即线式ＣＯ更可能是活性吸附态；表面反应生成了ＨＣＯ、ＣＨ２等中间物．用键级守恒（ＢＯＣ）－Ｍｏｒｓｅ势方法计算比较了ＣＯ→ＣＨ２过程中各可能基元步骤在Ｒｈ（１１１）面上的反应活化能和反应热，结果表明ＣＯ经其部分氢化物种（如Ｈ２ＣＯ、ＨＣＯＨ）的氢解反应断裂Ｃ─Ｏ键在能量上最有利．根据这些实验结果，提出铑基催化剂上合成气转化反应主要按缔合式机理进行；ＣＯ的优势断键途径为先部分氢化，而后氢助断键．     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

Palladium monolayer and palladium alloy electrocatalysts for oxygen reduction

We investigated the oxygen-reduction reaction (ORR) on Pd monolayers on various surfaces and on Pd alloys to obtain a substitute for Pt and to elucidate the origin of their activity. The activity of Pd monolayers supported on Ru(0001), Rh(111), Ir(111), Pt(111), and Au(111) increased in the following order: Pd/Ru(0001) < Pd/Ir(111) < Pd/Rh(111) < Pd/Au(111) < Pd/Pt(111). Their activity was correlated with their d-band centers, which were calculated using density functional theory (DFT). We found a volcano-type dependence of activity on the energy of the d-band center of Pd monolayers, with Pd/Pt(111) at the top of the curve. The activity of the non-Pt Pd2Co/C alloy electrocatalyst nanoparticles that we synthesized was comparable to that of commercial Pt-containing catalysts. The kinetics of the ORR on this electrocatalyst predominantly involves a four-electron step reduction with the first electron transfer being the rate-determining step. The downshift of the d-band center of the Pd "skin", which constitutes the alloy surface due to the strong surface segregation of Pd at elevated temperatures, determined its high ORR activity. Additionally, it showed very high methanol tolerance, retaining very high catalytic activity for the ORR at high concentrations of methanol. Provided its stability is satisfactory, this catalyst might possibly replace Pt in fuel-cell cathodes, especially those of direct methanol oxidation fuel cells (DMFCs).     

Zn modification of the reactivity of Pd(111) toward methanol and formaldehyde

The adsorption and reaction of methanol and formaldehyde on two-dimensional PdZn alloys on a Pd(111) surface were studied as a function of the Zn content in the alloy in order to understand the role of Zn in Pd/ZnO catalysts for the steam reforming of methanol (SRM). Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) data show that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity and alter the preferred bonding sites for adsorbed CO, CH3O, and CH2O intermediates. The experimental results obtained in this study are consistent with previous theoretical studies of this system and provide new insight into how Zn alters the reactivity of Pd.     

Initial oxidation of the Rh(110) surface: ordered adsorption and surface oxide structures

The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10x2) one-dimensional surface oxide and (2x1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2x4) periodicity develops. The experimental and theoretical results reveal that the c(2x4) structure is a "surface oxide" very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3.     

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。     

Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.     

Effect of steps on the decomposition of CH3O at PdZn alloy surfaces

The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.