Electron spectroscopic studies of the adsorption and decomposition of methanol on transition metals. A review

Results of investigations of the adsorption and decomposition of methanol on the surface of transition metals such as Fe, Ni, Cu, Pd, Ag, Mo, W and Pt byuv and x-ray photoelectron spectroscopy, electron energy loss spectroscopy, Auger electron spectroscopy and thermal desorption spectroscopy have been reviewed. The first step in the decomposition of CH₃OH on these metal surfaces is the formation of the methoxy species, OCH₃ radical. In the case of Fe, Mo and W, complete decomposition of CH₃OH occurs leaving CO(β), H₂ and CH₄ on the surface. Dissociation proceeds upto CO(α) and H₂ on the surface of Ni, Pd and Pt whereas on Ag and Cu, selective oxidation of CH₃OH to H₂CO is preferred. The difference in the reactivity of metals towards CH₃OH is rationalised from the heats of adsorption of O₂, CO and H₂ on these metals. Contribution No. 253 from the Solid State and Structural Chemistry Unit.     

Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt,Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (～10³ Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O_2ads, CO_ads) on Pt(100)-hex and dissociative adsorption (O_ads, CO_ads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (O_ads) and the concentration of CO_ads molecules was revealed for the Pd(110) surface. Studies using the isotope label ¹⁸O_ads demonstrated that the low-temperature formation of CO₂ at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (O_ads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (O_hot) appearing on the surface at the instant of dissociation of O_2ads molecules was studied by the molecular beam techniques.     

NO2在Ag/Pt(110)双金属表面上的吸附和分解

利用俄歇电子能谱(AES)和程序升温脱附谱(TDS)研究了NO₂在Ag/Pt(110)双金属表面的吸附和分解.室温下NO₂ 在Ag/Pt(110)双金属表面发生解离吸附, 生成NO(ads)和O(ads)表面吸附物种. 在升温过程中NO(ads)物种发生脱附或者进一步分解. 500 K时NO₂在Ag/Pt(110)双金属表面发生解离吸附生成O(ads)表面吸附物种. Pt 向Ag传递电子, 从而削弱Pt－O键的强度, 降低O(ads)从Pt 表面的并合脱附温度. 发现能够形成具有稳定组成的Ag/Pt(110)合金结构, 其表现出与Pt(110)-(1×2)相似的解离吸附NO₂能力, 但与O(ads)的结合明显弱于Pt(110)-(1×2). 该AgPt(110)合金结构是可能的低温催化直接分解氮氧化物活性结构.     

Simple Electrodeposition of Dendritic Pd Without Supporting Electrolyte and Its Electrocatalytic Activity Toward Oxygen Reduction and H2O2 Sensing

Metallic palladium (Pd) electrocatalysts for oxygen reduction and hydrogen peroxide (H₂O₂) oxidation/reduction are prepared via electroplating on a gold metal substrate from dilute (5 to 50 mM) aqueous K₂PdCl₄ solution. The best Pd catalyst layer possessing dendritic nanostructures is formed on the Au substrate surface from 50 mM Pd precursor solution (denoted as Pd‐50) without any additional salt, acid or Pd templating chemical species. The Pd‐50 consisted of nanostructured dendrites of polycrystalline Pd metal and micropores within the dendrites which provide high catalyst surface area and further facilitate reactant mass transport to the catalyst surface. The electrocatalytic activity of Pd‐50 proved to be better than that of a commercial Pt (Pt/C) in terms of lower overpotential for the onset and half‐wave potentials and a greater number of electrons (n) transferred. Furthermore, amperometric i–t curves of Pd‐50 for H₂O₂ electrochemical reaction show high sensitivities (822.2 and ?851.9 µA mM^?1 cm^?2) and low detection limits (1.1 and 7.91 µM) based on H₂O₂ oxidation H₂O₂ reduction, respectively, along with a fast response (<1 s).     

Variations in the Overall Charge and Open-Circuit Potential of a Palladium Electrode during the Adsorption of Iodine and Iodide Anions

Transients of the current and the open-circuit potential during the adsorption of I^– and I₂ on Pd/Pt (supporting electrolyte 0.5 M H₂SO₄) are measured and so are potentiodynamic and galvanostatic charging curves, in the presence and absence of adlayers. Dependences of the overall electrode charge on potential are constructed. After the adsorption of both I^– and I₂, values of the electrode surface charges at potentials of the double-layer region turn negative and considerably differ in their magnitude. The latter is connected with different character of adlayers, which is shown by experiments on the adsorption of silver atoms at underpotentials after a preliminary adsorption of I^– and I₂ on the electrode surface. Certain discrepancies between experimentally-found and theoretically-expected values of transients are attributed to a substantial increase in the irreversibility of sorption and desorption of hydrogen and oxygen in the presence of iodine atoms adsorbed on the surface.     

An exploration of the surfaces of some Pt/SiO2 catalysts using CO as an infrared spectroscopic probe

Infrared spectra in the bond-stretching, ν CO, region have been measured for CO adsorbed on an impregnated Pt/SiO₂ catalyst (16% Pt) with the aim of characterizing the adsorption sites present. This catalyst has previously been widely used for the study of the spectra from adsorbed hydrocarbons. It has relatively large metal particles, typically in the diameter range of 5 to 15 nm. Samples were studied which were hydrogen-covered, hydrogen-depleted, oxygen-covered and “mature”, the latter in the sense that the catalyst had undergone a number of repeated adsorption/desorption/re-reduction cycles. The spectra were interpreted in terms of the adsorption sites available by using recent correlations between ν CO wavenumbers and different patterns of CO metal interactions in metal clusters of structures known from X-ray crystallography.The strongest ν CO absorptions were from linearly adsorbed OCPt species, but these were relatively uninformative (maxima in the region 2087 to 2084 cm⁻¹ in all cases at saturation coverage) because strong dipolar coupling causes intensity-distortion and the merging-together of absorptions from different types of sites. Small proportions of sites, probably of an atomically rough nature from curved areas of the crystallites, gave weaker absorptions near 2050 and 2030 cm⁻¹. On the oxygen-covered surface a sharp absorption at 2099 cm⁻¹ denoted CO adsorption adjacent to sites of adsorbed oxygen.The weaker absorption bands in the ν CO bridged region were more informative. The strongest, near 1850 cm⁻¹ was correlated with 2-fold bridged species adsorbed on (111) surfaces. Weaker, overlapping features near 1880 and 1795 cm⁻¹ (separated from an overall contour with the aid of computer analysis) were correlated, respectively with 2-fold bridged species on (100) or (110) planes, and 3-fold bridged species on (111) planes. Weaker absorptions near 1700 cm⁻¹ were considered to correspond to different types of adsorption sites involving unequal interactions of CO with at least two metal atoms.The “mature” Pt/SiO₂ samples gave notably different spectra in both regions and this was tentatively attributed to the effect of residual carbon atoms on or near the Pt surface.A comparison was made of the present spectra with those previously published from similar experiments on a small-particle (ca 2 nm) Pt/SiO₂ catalyst, EUROPT-1. The spectral differences could be well accounted for in terms of the reported raft-like (111)-based morphology of the small metal particles of the EUROPT-1 catalyst.     

Surface-enhanced Raman scattering characteristics of 4-nitrobenzenethiol adsorbed on palladium and silver thin films

Palladium is an important catalytic metal, and it is desirable to develop a surface-enhanced Raman scattering (SERS) technique to investigate the reagent and product species adsorbed on its surface. Unfortunately, Pt-group metals, e.g., Pt and Pd, have been commonly considered as non- or weak-SERS-active substrates. In this work, Ag and Pd thin films were deposited very efficiently and evenly onto the surface of glass substrates by using only corresponding metal nitrate salts (AgNO₃ and Pd(NO₃)₂) with butylamine in ethanolic solutions. In this process, pure ethanol was used for Ag deposition, while an ethanol–water (8:2) mixture was used for Pd deposition. The as-prepared Ag and Pd films exhibited SERS activity over a large area. The surface-induced photoconversion capabilities of these Ag and Pd films were then tested on 4-nitrobenzenethiol by means of SERS. It was found that at least under visible laser irradiation, the surface-catalyzed photoreaction occurs more readily on a Ag film than on a Pd film for the conversion of 4-nitrobenzenethiol to 4-aminobenzenethiol, even though Pd is known to be an important transition metal with high catalytic activity.     

Effect of metals on the catalytic activity of sulfated zirconia for light naphtha isomerization

We studied on the function of the metal in the sulfated zirconia(SO₄^2–/ZrO₂) catalyst for the isomerization reaction of light paraffins. The addition of Pt to the SO₄^2–/ZrO₂ carrier could keep the high catalytic activity. The improvement in this isomerization activity is because Pt promotes removal of the coke precursor deposited on the catalyst surface. Though this catalytic function was observed in other transition metals, such as Pd, Ru, Ni, Rh and W, Pt exhibited the highest effect among them. It was further found that the Pd/SO₄^2–/ZrO₂–Al₂O₃ catalyst possessed a catalytic function for desulfurization of sulfur-containing light naphtha in addition to the skeletal isomerization. The sulfur tolerance of catalyst depended on the method of adding Pd, and the catalyst prepared by impregnation of the SO₄^2–/ZrO₂–Al₂O₃ with an aqueous solution of Pd exhibited the highest sulfur tolerance.Further, we investigated the improvement in sulfur tolerance of the Pt/SO₄^2–/ZrO₂–Al₂O₃ catalyst by impregnation of Pd. The results of EPMA analysis indicated that this catalyst was a hybrid-type one (Pt/SO₄^2–/ZrO₂–Pd/Al₂O₃) in which Pt/SO₄^2–/ZrO₂ particles and Pd/Al₂O₃ particles adjoined closely. This hybrid catalyst possessed a very high sulfur tolerance to the raw light naphtha that was obtained from the atmospheric distillation apparatus, although this light naphtha contained much sulfur. We assume that such a high sulfur tolerance in the hybrid catalyst is brought about by the isomerization function of Pt/SO₄^2–/ZrO₂ particles and the hydrodesulfurization function of Pd/Al₂O₃ particles. Besides, since the hybrid catalyst also provides high catalytic activity in the isomerization of HDS light naphtha, we suggest that the Pd/Al₂O₃ particles supply atomic hydrogen to the Pt/SO₄^2–/ZrO₂ particles by homolytic dissociation of gaseous hydrogen and also enhance the sulfur tolerance of Pt/SO₄^2–/ZrO₂ particles. Finally, we also propose the most suitable location of Pd and Pt in the metal-supported SO₄^2–/ZrO₂–Al₂O₃ catalyst.     

Adsorption and Reactions of N2O on Transition Metal Surfaces

Data on the interaction of nitrous oxide with polycrystalline and low-index single crystal surfaces of transition metals (Cu, Ag, Pt, Pd, Ni, W, Ir, Rh, and Ru) are reviewed. The kinetics of N₂O adsorption, desorption, and dissociation N₂O on these surfaces, as well as the energetics and mechanisms of these processes, are considered. New calculated data on the energetics of nitrous oxide transformations on (111) single crystalline transition metal surfaces are reported.     

Multi-d Electron Synergy in LaNi1-xCoxRu Intermetallics Boosts Electrocatalytic Hydrogen Evolution

Despite the fact that d-band center theory links the d electron structure of transition metals to their catalytic activity, it is yet unknown how the synergistic effect of multi-d electrons impacts catalytic performance. Herein, novel LaNi_1-xCo_xRu intermetallics containing 5d, 4d, and 3d electrons were prepared. In these compounds, the 5d orbital of La transfers electrons to the 4d orbital of Ru, which provides adsorption sites for H*. The 3d orbitals of Ni and Co interact with the 5d and 4d orbitals to generate an anisotropic electron distribution, which facilitates the adsorption and desorption of OH*. The synergistic effect of multi-d electrons ensures efficient catalytic activity. The optimized LaNi_0.5Co_0.5Ru has an overpotential of 43mV at 10 mA cm⁻² for alkaline electrocatalytic hydrogen evolution reaction. Beyond offering a variety of new electrocatalysts, this work reveals the multi-d electron synergy in promoting catalytic reaction.     

糠醛在Pt(111)表面的吸附和脱碳反应

采用广义梯度近似的密度泛函理论并结合平板模型的方法, 优化了糠醛分子在Pt(111)面的吸附模型,并探究了糠醛脱碳反应形成呋喃的机理. 结果表明: 吸附后糠醛分子环上的C―H(O)键及支链―CHO相对于金属表面倾斜上翘, 分子平面被扭曲, 易于呋喃的形成; 同时, 糠醛分子向Pt表面转移电子0.765e, 环中的大π键与Pt(111)表面的d轨道发生较强的相互作用, 使得糠醛的芳香性被破坏, 环上的碳原子呈现准sp³杂化. 此外, 对糠醛脱碳反应中的各反应步骤进行过渡态搜索, 通过比较各步骤的活化能, 得出糠醛更易先失去支链上的H形成酰基中间体(C₄H₃O)CO, 中间体继续脱碳加氢形成产物呋喃. 该过程的控速步骤为(C₄H₃O)CO^*+^*→C₄H₃O^*+CO^* (^*为吸附位),活化能为127.65 kJ·mol^-1.     

Interaction of SO2 with single crystal metal surfaces

Studying the interaction of SO₂ with metal surfaces under UHV conditions, a question of central interest is whether the molecule dissociates (leaving back the catalyst poison sulphur on the surface) or not. A spontaneous or a thermally activated dissociation of SO₂ occurs on Fe, Rh, W, Ni, Pd and Pt. On Cu and Ag a strong chemisorption, but only a partial dissociation induced by defects or coadsorbed alkalis, and on Au no chemisorption at all were observed.

In this paper a comparison of our results obtained for the chemisorption and multilayer adsorption of SO₂ on Cu(111), Ag(111), Ag(100) and Ag(110) in the temperature range between 80 K and 900 K is given. By combining highly resolved TPD-measurements, isothermal and temperature-programmed ΔΦ-experiments after different stages of exposure and molecular beam backscattering measurements (MBBS) —assisted by LEED, AES and isotope mixing experiments — a destinction between ordinary desorption and desorption after a reorientation process during the heating procedure could be made. Whereas on clean Ag surfaces adsorption and desorption of SO₂ are observed only below 300 K, on Cs-precovered Ag desorption of SO₂ takes place even above 600 K.

Finally, results concerning the different stages of SO₂ multilayer adsorption (bi-, tri-, multilayers) are presented showing a characteristic dependence of the layer growth on the adsorption temperature, the impinging SO₂ flux density and on the surface structure.     

Neutron activation analysis of platinum metals in airborne particulate matter

A method is described for the determination of Au, Pt, Pd, Ag and Ir in two atmospheric aerosol samples, namely in Ghent and in the Milanese intercomparison sample. After neutron irradiation the samples are fused with Na₂O₂. Gold is extracted with ethylacetate, Pt precipitated as (NH₄)₂PtCl₆ Pd as dimethylglyoximate, Ag as chloride and Ir separated by anion-exchange adsorption and batch extraction. Ge(Li) gamma-spectrometry is applied for all determinations. The concentrations in ng·g⁻¹ in the samples are respectively: Au: 49 and 3000; Pt: below 100 for both samples; Pd: 7 and 28; Ag: 6000 and 14 000; Ir: 2.5 and 1.3.     

Elucidating the Doping Effect on the Electronic Structure of Thiolate‐Protected Silver Superatoms by Photoelectron Spectroscopy

Gas‐phase photoelectron spectroscopy (PES) was conducted on [XAg₂₄(SPhMe₂)₁₈]^? (X=Ag, Au) and [YAg₂₄(SPhMe₂)₁₈]^2? (Y=Pd, Pt), which have a formal superatomic core (X@Ag₁₂)⁵⁺ or (Y@Ag₁₂)⁴⁺ with icosahedral symmetry. PES results show that superatomic orbitals in the (Au@Ag₁₂)⁵⁺ core remain unshifted with respect to those in the (Ag@Ag₁₂)⁵⁺ core, whereas the orbitals in the (Y@Ag₁₂)⁴⁺ (Y = Pd, Pt) core shift up in energy by about 1.4 eV. The remarkable doping effect of a single Y atom (Y=Pd, Pt) on the electronic structure of the chemically modified (Ag@Ag₁₂)⁵⁺ superatom was reproduced by theoretical calculations on simplified model systems and was ascribed to 1) the weaker binding of valence electrons in Y@(Ag⁺)₁₂ compared to Ag⁺@(Ag⁺)₁₂ due to the reduction in formal charge of the core potential, and 2) the upward shift of the apparent vacuum level due to the presence of a repulsive Coulomb barrier between [YAg₂₄(SPhMe₂)₁₈]^? and electron.     

噻吩在M(111)(M=Pd,Pt, Au)表面的吸附

采用密度泛函理论(DFT), 选取DMol³程序模块, 对噻吩在M(111) (M=Pd, Pt, Au)表面上的吸附行为进行了探讨. 通过对噻吩在不同底物金属上的吸附能、吸附构型、Mulliken 电荷布居、差分电荷密度以及态密度的分析发现, 噻吩在Pd(111)面上的吸附能最大, Pt(111)面次之, Au(111)面最小. 吸附后, 噻吩在Au(111)面上的构型几乎保持不变, 最终通过S端倾斜吸附于top 位; 噻吩在Pd(111)及Pt(111)面上发生了折叠与变形, 环中氢原子向上翘起, 最终通过环平面平行吸附于hollow 位. 此外, 噻吩环吸附后芳香性遭到了破坏, 环中碳原子发生sp³杂化, 同时电子逐渐由噻吩向M(111)面发生转移, M(111)面上的部分电子也反馈给了噻吩环中的空轨道, 这种协同作用最终导致了噻吩分子稳定吸附于M(111)面.     

IR spectroscopic and thermal desorption studies of the interaction of the SO2 + O2 mixture with the 9% Ni-Cu-Cr/2% Ce/(θ + α)-Al2O3 catalyst

Data on the formation of surface structures during the adsorption of sulfur dioxide on alumina-supported metal and oxide catalysts in the presence of oxygen were reviewed. The results of our IR spectroscopic and thermal desorption studies of the interaction of the SO₂+O₂ mixture with the 9% Ni-Cu-Cr/2% Ce/(θ + α)-Al₂O₃ catalyst are presented. At 673 K chemisorption of SO₂ takes place (absorption band 1165 cm^?1, T _des ^m = 673 K), and SO₂ is oxidized to form sulfate structures (absorption bands 1235, 1140, 1100, 985, and 615 cm^?1, T _des ^m = 973 K), which decompose into SO₂ and O₂ at 973 K and decay when reduced in H₂.     

The effect of alloying on the H-atom adsorption on the (100) surfaces of Pd-Ag,Pd-Pt,Pd-Au,Pt-Ag,and Pt-Au. A theoretical study

The effect of alloying on the adsorption of atomic hydrogen was studied using density functional theory (DFT). In the study the (100) surfaces of Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag, and Pt-Au alloys were considered by means of a cluster model. The structural and energetic properties of the H atom adsorbed on the Pd₄Me (Me = Ag, Pt, Au) and Pt₄Me (Me = Pd, Ag, Au) clusters were calculated and compared with the H-atom adsorption on monometallic clusters. The effect of alloying on the H-atom adsorption is evident for all the investigated bimetallic systems. However, it strongly depends on the second metal atom, Me, is placed in the surface layer or in the subsurface one. In general, the H atom adsorbed in a site containing the second metal exhibits different properties from those characteristic of its adsorption on Pd(100) and Pt(100). Hence, the modified interaction between atomic hydrogen and the alloyed surfaces may increase the selectivity of the catalytic hydrogenation reactions on such surfaces.     

Investigation of the state of palladium in the Pd/SO4/ZrO2 system by diffuse-reflectance IR spectroscopy

The Pd/ZrO₂ and Pd/SO₄/ZrO₂ systems were investigated by diffuse-reflectance IR spectroscopy using CO as the probe molecule. For the Pd/ZrO₂ system, the behavior of the metal is characteristic of the weak metal-support interaction. Intense bands attibuted to the bridging CO species indicate the formation of large metal particles in the reduced systems. Modification of the ZrO₂ support with SO₄ ^2? anions leads to an increase in the metal—support interaction and makes the metal more resistant to reduction. On the surface promoted by SO₄ ^2? anions, metal particles with a positive charge (Pd⁺ and Pd^δ+) were observed. The smaller the size of the metal clusters and the higher degree of oxidation of sulfur, the stronger the influence of acidic protons and surface sulfur compounds on the metal.     

负载金属对WO3-TiO2光催化剂结构与催化性能的影响

用溶胶-凝胶和浸渍-还原相结合的方法制得M/WO₃-TiO₂ (M＝Pd, Cu, Ni, Ag)光催化剂. 利用X射线衍射(XRD)、程序升温还原(TPR)、红外(IR)、程序升温脱附(TPD)、紫外-可见漫反射光谱(UV-Vis-DRS)和光反应器等技术研究了复合半导体负载金属的物相结构、光吸收性能和光催化反应性能. 结果表明: 金属负载在复合半导体上延迟了TiO₂由锐钛矿向金红石相转化, 增强W与载体TiO₂的相互作用, 使TiO₂光吸收限发生蓝移, 对可见光部分的吸收明显增加; 固体材料吸光性能强弱顺序Pd/WO₃-TiO₂＞Cu/WO₃-TiO₂＞Ag/WO₃-TiO₂＞Ni/WO₃-TiO₂; 金属Pd对CO₂吸附能力过强, 卧式吸附态脱附温度高, 光催化效率不高; 金属Cu对CO₂吸附能力适中, CO₂与C₃H₆脱附温度较接近, 实现了“光-表面-热”协同作用, 光量子效率最高, 达到19.7%.     

Correlations between hydrogen electrosorption properties and composition of Pd-noble metal alloys

Hydrogen adsorption on and absorption into Pd alloys with other noble metals was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry. Correlations were found between the potentials of adsorbed/absorbed hydrogen oxidation peaks and surface/bulk compositions of Pd–Rh alloys. The potential of the α–β-phase transition depends linearly on Pd bulk content in Pd–Au, Pd–Rh, Pd–Pt and Pd–Pt–Rh alloys. The obtained relationships can be utilized for the determination of the composition of homogeneous Pd-noble alloys from hydrogen electrosorption experiments.