Active surfaces for CO oxidation on palladium in the hyperactive state

Hyperactivity was previously observed for CO oxidation over palladium, rhodium, and platinum surfaces under oxygen-rich conditions, characterized by reaction rates 2-3 orders higher than those observed under stoichiometric reaction conditions [Chen et al. Surf. Sci. 2007, 601, 5326]. In the present study, the formation of large amounts of CO(2) and the depletion of CO at the hyperactive state on both Pd(100) and polycrystalline Pd foil were evidenced by the infrared intensities of the gas phase CO(2) and CO, respectively. The active surfaces at the hyperactive state for palladium were characterized using infrared reflection absorption spectroscopy (IRAS, 450-4000 cm(-1)) under the realistic catalytic reaction condition. Palladium oxide on a Pd(100) surface was reduced eventually by CO at 450 K, and also under CO oxidation conditions at 450 K. In situ IRAS combined with isotopic (18)O(2) revealed that the active surfaces for CO oxidation on Pd(100) and Pd foil are not a palladium oxide at the hyperactive state and under oxygen-rich reaction conditions. The results demonstrate that a chemisorbed oxygen-rich surface of Pd is the active surface corresponding to the hyperactivity for CO oxidation on Pd. In the hyperactive region, the CO(2) formation rate is limited by the mass transfer of CO to the surface.     

Pd/WO3-ZrO2催化剂的结构对乙烯直接氧化制乙酸的影响

利用浸渍法制备了一系列具有不同W/Zr比和不同WO3-ZrO2焙烧温度的1%pd/WO3-ZrO2催化剂.通过X射线衍射、N2吸附、氨程序升温吸附、吡啶吸附红外光谱和程序升温还原方法表征了催化剂的晶体结构、表面状态以及酸性.结果表明,制备条件对W和Pd的表面状态具有很人的影响.随着W/Zr比的增加和焙烧温度的提高,催化剂表而的WOx经历了从聚钨酸物种到聚钨酸/WO3晶体共存再到WO3品体颗粒的转化.W/Zr比为0.2且焙烧湍度为1073K的催化剂具有最多的聚钨酸物种,而该催化剂具有最好的活性.因此,聚钨酸物种的最决定了催化剂的活性.Pd的分散状态只依赖于WO3-ZrO2的焙烧温度.Pd物种分散性好,则乙酸选择性高,而分散性差的人颗粒Pd会导致乙烯的燃烧反应.     

Reactions of carbon monoxide with free palladium oxide clusters: strongly size dependent competition between adsorption and combustion

The gas phase reactions of carbon monoxide with small mass-selected clusters of palladium, Pd(x)(+) (x = 2-7), and their oxides, Pd(x)O(+) (x = 2-7) and Pd(x)O(2)(+) (x = 4-6), have been investigated in a radio frequency ion trap operated under multi-collision conditions. The bare palladium clusters were found to readily adsorb CO yielding a highly size dependent product pattern. Most interestingly, the reactions of the pre-oxidized palladium clusters with CO lead to very similar product distributions of Pd(x)(CO)(z)(+) complexes as in the case of the corresponding pure Pd(x)(+) clusters. Consequently, it has been concluded that the investigated palladium oxide clusters efficiently oxidize CO under formation of the bare clusters, which further adsorb CO molecules yielding the previously observed Pd(x)(CO)(z)(+) product complex distributions. This CO combustion reaction has been observed even at temperatures as low as 100 K. However, for Pd(2)O(+), Pd(6)O(+), Pd(6)O(2)(+), and Pd(7)O(+) a competing reaction channel yielding palladium oxide carbonyls Pd(x)O(CO)(z)(+) could be detected. The latter adsorption reaction may even hamper the CO combustion under certain reaction conditions and indicates enhanced activation barriers involved in the CO oxidation and/or the CO(2) elimination process on these clusters.     

Low-temperature oxidation of carbon monoxide over (Mn1 − x M x )O2 (M = Co, Pd) catalysts

(Mn_{1 ? x} M _x )O₂ (M = Co, Pd) materials synthesized under hydrothermal conditions and dried at 80°C have been characterized by X-ray diffraction, diffuse reflectance spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, and adsorption and have been tested in CO oxidation under CO + O₂ TPR conditions and under isothermal conditions at room temperature in the absence and presence of water vapor. The synthesized materials have the tunnel structure of cryptomelane irrespective of the promoter nature and content. Their specific surface area is 110–120 m²/g. MnO₂ is morphologically uniform, and the introduction of cobalt or palladium into this oxide disrupts its uniformity and causes the formation of more or less crystallized aggregates varying in size. The (Mn,Pd)O₂ composition contains Pd metal, which is in contact with the MnO₂-based oxide phase. The average size of the palladium particles is no larger than 12 nm. The initial activity of the materials in CO oxidation, which was estimated in terms of the 10% CO conversion temperature, increases in the following order: MnO₂ (100°C) < (Mn,Co)O₂ (98°C) < (Mn,Co,Pd)O₂ (23°C) < (Mn,Pd)O₂ (?12°C). The high activity of (Mn,Pd)O₂ is due to its surface containing palladium in two states, namely, oxidized palladium (interaction phase) palladium metal (clusters). The latter are mainly dispersed in the MnO₂ matrix. This catalyst is effective in CO oxidation even at room temperature when there is no water vapor in the reaction mixture, but it is inactive in the presence of water vapor. Water vapor causes partial reduction of Mn⁴⁺ ions and an increase in the proportion of palladium metal clusters.     

Comparison of the reactivity of different Pd-O species in CO oxidation

The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.     

单原子催化剂活化氢气:Pt1/WOx催化氢解反应（英文）

单原子催化剂具有独特的结构位点,能最大化利用贵金属原子,在一系列化学转化反应中具有优异的活性和选择性.但单原子的稳定性是单原子催化剂应用的一个挑战,特别是还原气氛下单原子的稳定性,这极大地限制了单原子催化剂在加氢、脱氢和氢解反应中的应用.理解还原气氛下单原子的稳定机制和单原子催化剂活化氢气的反应机理对于扩大单原子催化剂的应用非常重要.Pt/WOx(2Pd>Au.氢气能在Pd和WOx界面非均相解离,而Au/WOx不能活化、解离氢气.我们进一步采用实验表征验证了DFT理论计算的结果.实验合成了WOx负载的Pt、Pd、Au三种催化剂,X射线衍射(XRD)和透射电子显微镜(TEM)结果表明,Pt能在WOx表面原子级分散和稳定,而Pd在WOx表面形成较小的纳米颗粒,Au形成较大的纳米颗粒.采用氢气化学吸附研究了三种催化剂对氢气的活化能力,结果表明三种催化剂的氢气活化能力顺序为Pt/WOx(137μmol/g-Pt)>Pd/WOx(43μmol/g-Pd)>>Au/WOx(4μmol/g-Au).将三种催化剂用于甘油选择性氢解制备1,3-丙二醇的反应中,只有Pt/WOx催化剂对甘油氢解具有优异的活性和选择性.从而实验证实了氢气气氛下原位产生的Bronsted酸具有关键作用和Pt1/WOx催化剂具有双功能催化性质.我们的研究不仅解释了还原气氛下金属单原子在氧化物表面的稳定机理,而且对单原子催化剂活化解离氢气提供了新的认识.     

Ultrathin metals films on W(221): Structure, electronic properties and reactivity

Nanoscale pyramidal facets with (211) faces are formed when W(111) surface is covered by monolayer film of certain metals (including Pt, Pd and Au) and annealed to T ≥ 750 K. In the present work, we focus on the structure, electronic properties and reactivity of planar W(211) covered by ultrathin films of platinum and palladium. The measurements include soft X-ray photoelectron spectroscopy using synchrotron radiation, Auger electron spectroscopy, low energy electron diffraction (LEED) and thermal desorption spectroscopy. The metal film growth and evolution during annealing has been investigated for coverages ranging from 0 to 8 monolayers. The films grow initially in a layer-by-layer mode at 300 K. LEED, Auger, and Surface Core Level Shift (SCLS) measurements reveal that for coverages of one monolayer, the films are stable up to temperatures at which desorption occurs. In contrast, at higher coverages, SCLS data indicate that surface alloys are formed upon annealing films of Pt and Pd; surface alloy formation is not seen for Au overlayers. These findings are discussed in terms of structural and electronic properties of these bimetallic systems. Relevance to catalytic properties for acetylene cyclization over Pd/W(211) is also discussed.     

Effect of Structure of Pd/WO3-ZrO2 Catalyst on Its Activity for Direct Oxidation of Ethylene to Acetic Acid

A series of 1%Pd/WO₃-ZrO₂ catalysts with different W/Zr ratios and calcination temperatures of WO₃-ZrO₂ were prepared by an impregnation method. Their crystal structure, surface state, and acidity were determined using X-ray diffraction, N₂ adsorption, NH₃ temperature-programmed desorption, pyridine infrared spectroscopy, and temperature-programmed reduction. Special attention was paid to the surface states of tungsten and palladium under different preparation conditions. The results revealed that WO_x surface species underwent a transformation from polytungstate species to coexistent polytungstate/crystalline WO₃ and further to crystalline WO₃ particles with increase of W/Zr ratio and calcination temperature. The W/Zr = 0.2 sample calcined at 1 073 K showed the maximum amount of polytungstates, which were responsible for the excellent activity. Moreover, the state of palladium was only dependent on the calcination temperature. Well-dispersed Pd species were responsible for high selectivity to acetic acid, and large metallic Pd particles were favorable for ethylene combustion.     

The study of the active surface for CO oxidation over supported Pd catalysts

CO oxidation was investigated on various powder oxide supported Pd catalysts by temperature-programmed reaction.The pre-reduced catalysts show significantly higher activities than the pre-oxidized ones.Model studies were performed to better understand the oxidation state,reactivities and stabilities of partially oxidized Pd surfaces under CO oxidation reaction conditions using an in situ infrared reflection absorption spectrometer(IRAS).Three O/Pd(100)model surfaces,chemisorbed oxygen covered surface,surface oxide and bulk-like surface oxide,were prepared and characterized by low-energy electron diffraction(LEED)and Auger electron spectroscopy(AES).The present work demonstrates that the oxidized palladium surface is less active for CO oxidation than the metallic surface,and is unstable under the reaction conditions with sufficient CO.     

TiCl4(THF)2 impregnation on a flat SiOx/Si(1 0 0) and on polycrystalline Au foil: determination of surface species using XPS

TiCl₄(THF)₂ was impregnated by spin-coating on a Si(1 0 0) wafer covered with a thin SiO_x layer and on a polycrystalline Au foil. The nature of the surface species was determined at room temperature and after annealing, by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). A mixed Si:O:Ti interfacial layer was formed on the silicon substrate while in the case of Au, TiOCl_x and TiO_x were the main surface species at room temperature. Annealing at 723 K leads to the total desorption of the Cl atoms, and in both cases a significant amount of Ti atoms was reduced to the Ti³⁺ state. AFM measurements revealed a homogenous distribution of nano-sized TiO_x clusters with semi-ellipsoid shape and increased contact area with the underlying silica.     

Ionization and Coulomb explosion of small group 10 transition metal oxide clusters in strong light fields

The ionization properties of small group 10 metal oxide clusters are explored using ultrafast pulses centered at 624 nm. Maximum atomic charge states resulting from Coulomb explosion were observed to be Ni(3+), Pd(3+), Pt(5+), and O(2+) species with similar ionization potentials ～30-35 eV. Ion signal as a function of laser intensity of each charge state of Ni, Pd, Pt, and O resulting from Coulomb explosion was mapped and compared to that predicted from semi-classical tunneling theory using sequential ionization potentials to quantify observed enhancements in ionization. The saturation intensity (I(sat)) of each charge state is measured and compared to previous studies on group 5 transition metal oxides. The atomic charge states of nickel showed a large enhancement in ionization compared to palladium and platinum, reflective of the differing bonding properties of each metal with oxygen. Results indicate that nickel oxide clusters undergo a greater extent of ionization enhancement as a result of multiple ionization mechanisms. The ionization enhancement behavior of each metal oxide species is explored herein.     

钯团簇形成和增长机理的Monte Carlo研究

利用Monte Carlo（MC）方法和Lennard-Jones加Axilord-Teller （LJ＋AT）势能函数,研究了气相中钯团簇的形成过程和增长机理.发现具有二十面体结构的Pd13团簇可以在气相中自发形成,较大的团簇通过在Pd13二十面体结构的表面添加原子组成四面体的方式形成.通过分析团簇结构和能量之间的关系,发现除了Pd13和Pd55以外,Pd19和Pd39团簇也具有五次对称性,都是比较稳定的结构.     

Untersuchungen an Metallkatalysatoren. XVII. Phasenaufbau,Dispersität und Dehydrieraktivität Molybdän- und wolframmodifizierter Palladiumkatalysatoren

Investigations on Metal Catalysts. XVII. Phase Structure, Dispersity, and Dehydrogenation Activity of Palladium Catalysts Modifided by Molybdenum and Tungsten Molybdenum and tungsten containing palladium catalysts were prepared by reduction of mixtures from Pd(NO₃)₂ with MoO₃ and WO₃, respectively, with hydrogen at 600°C and 800°C. The powders were characterized by means of several methods: Determination of the oxidation state for molybdenum and tungsten, X-ray measurements, N₂ adsorption, CO chemisorption, H₂ sorption, dehydrogenation of cyclohexane. The properties of the samples (heated at 600°C) are determined to a high degree by the co-existence of the palladium phase as well as the molybdenum and tungsten oxide, respectively, in the mean oxidation state +4. The after-reduction at 800°C leads to a great portion of metallic molybdenum and tungsten in the concerned catalysts. There are references that the treatment at 800°C in the presence of hydrogen causes for the Pd? Mo catalysts an increase of the palladium content in the surface of the crystallites.     

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)₂ and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd₃P_0.8 → Pd₅P₂ → PdP₂. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.     

Vacuum annealing phenomena in ultrathin TiD y /Pd bi-layer films evaporated on Si(100) as studied by TEM and XPS

Using a combination of TEM and XPS, we made an analysis of the complex high-temperature annealing effect on ultrathin titanium deuteride (TiD _y ) films evaporated on a Si(100) substrate and covered by an ultrathin palladium layer. Both the preparation and annealing of the TiD _y /Pd bi-layer films were performed in situ under UHV conditions. It was found that the surface and bulk morphology of the bi-layer film as well as that of the Si substrate material undergo a microstructural and chemical conversion after annealing and annealing-induced deuterium evolution from the TiD _y phase. Energy-filtered TEM (EFTEM) mapping of cross-section images and argon ion sputter depth profiling XPS analysis revealed both a broad intermixing between the Ti and Pd layers and an extensive inter-diffusion of Si from the substrate into the film bulk area. Segregation of Ti at the Pd top layer surface was found to occur by means of angle-resolved XPS (ARXPS) and the EFTEM analyses. Selected area diffraction (SAD) and XPS provided evidence for the formation of a new PdTi₂ bimetallic phase within the top region of the annealed film. Moreover, these techniques allowed to detect the initial stages of TiSi phase formation within the film–substrate interlayer.     

Phenomenological aspects related to the electrochemical behaviour of smooth palladium electrodes in alkaline solutions

The voltammetric behaviour of smooth palladium electrodes in 1 M NaOH is studied in the potential range related to the thermodynamic stability of water. The electrosorption of H atoms on bulk Pd appears as a reversible reaction coupled to a diffusion process which occurs within bulk Pd. The voltammetric electrodesorption of H from bulk Pd is a process under mixed control, i.e. the diffusion from the bulk and the surface oxidation of H atoms. Fast pseudocapacitive reactions are detected in the range 0.2–0.4 V associated with the adsorption of H atoms at the submonolayer level. The initial stages of Pd oxide layer formation, at ca. 0.68 V, involves two reversible stages. The Pd oxide monolayer formation is achieved at 1.25 V/RHE and is followed by the formation of a third reversible system. This system is enhanced by an excursion in the potential range of the oxygen evolution reaction. This reversible system is probably a redox system involving Pd(II)/Pd(IV) species. The voltammetric electroreduction of the Pd oxide film shows rather irreversible behaviour. Inhibition effects on the reversible adsorption of H atoms due to residual oxide species were observed as well as inhibition on loading the Pd electrode with hydrogen to form the (α + β)-PdH phase. Rotating ring-disc experiments demonstrate that Pd electrodissolution in basic solutions is much smaller than in acid solutions. However, soluble palladium species are detected, especially during the formation of the fast redox systems, in the potential range related to Pd oxide layer growth.     

XPS and XANES studies of SnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanolayers

This paper presents the results of our XPS (X-ray photoelectron spectroscopy) and XANES (X-ray absorption near edge structure) studies of tin oxide nanolayers obtained by magnetron spraying of the metal and its further oxidation in air at different temperatures. It was shown that at 240°C (annealing temperature), tin monoxide was dominant in the surface layer of the samples. When the temperature was increased to 450°C, the phase composition corresponded to tin dioxide. Increased sorption ability was found for the samples oxidized at 450°C. The band structure model of SnO _x nanolayers obtained by superposition of the XANES and XPS data revealed cross transitions with energy ～3.7 eV in the presence of the SnO and SnO₂ phases. Surface doping of nanolayers with palladium gave the Pd, PdO, and PdO₂ components, among which PdO was most intense. Alternate treatments with O₂ and H₂ gases led to the disappearance of palladium dioxide and the reduction of PdO to the Pd metal. After the volume doping of nanoplayers with palladium, the surface layer contained PdO and PdO₂; the latter was represented by two types of particles with different sizes.     

Palladium(II)-substituted tungstosilicate [Cs2K(H2O)7Pd2WO(H2O)(A--SiW9O34)2]9-

The palladium(II)-substituted tungstosilicate [Cs(2)K(H(2)O)(7)Pd(2)WO(H(2)O)(A-alpha-SiW(9)O(34))(2)](9)(-) (1) has been synthesized and characterized by IR, elemental analysis, and electrochemistry. Single-crystal X-ray analysis was carried out on Cs(3)K(2)Na(4)[Cs(2)K(H(2)O)(7)Pd(2)WO(H(2)O)(A-alpha-SiW(9)O(34))(2)].5H(2)O (1a), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 16.655(3) A, b = 19.729(4) A, c = 25.995(5) A, beta = 95.46(3) degrees , and Z = 4. Polyanion 1represents the first structurally characterized palladium(II)-substituted tungstosilicate. The title polyanion consists of two (A-alpha-SiW(9)O(34)) Keggin moieties linked via a [WO(H(2)O)](4+) group and two equivalent, square-planar Pd(2+) ions leading to a sandwich-type structure with C(2)(v) symmetry. The central belt of 1 contains also one potassium and two cesium ions. Polyanion 1 was synthesized by reaction of Pd(CH(3)COO)(2) with K(10)[A-alpha-SiW(9)O(34)] in aqueous acidic medium (pH 4.8). A cyclic voltammetry study of polyanion 1 in a pH 5 medium shows a Pd(0) deposition process on the glassy carbon electrode surface. The corresponding wave and that of tungsten redox processes could be separated clearly during the first few runs before their merging into a broad composite wave. The film thickness increases with the number of potential cycles or the duration of potentiostatic electrolysis. As judged from hydrogen sorption/desorption pattern, the quality of the film deposited from polyanion 1 is better than that of a film deposited directly from Pd(2+) solutions.     

Dissolution of oxygen in polycrystalline palladium at $${P_{{O_2}}}$$= 100 Pa and temperatures of 500 to 950 K

The dissolution of oxygen in polycrystalline palladium Pd(poly) at an O₂ pressure of 100 Pa and temperatures of 500–950 K has been investigated by temperature-programmed desorption. At 500 K, the process yields a surface palladium film that includes an oxide-like reconstructed structure on a rarefied metal surface layer. At this temperature, palladium sorbs ~2 monolayers (ML) of oxygen. At 600–800 K, palladium dissolves up to ~140 ML of oxygen as a result of O₂ chemisorption on the surface of the oxide film, penetration of O_ads atoms under the oxide film, and their diffusion into the metal bulk. The dependence of the amount of oxygen sorbed by Pd(poly) (n) on the time of exposure to an O₂ atmosphere is described by a nearparabolic function, n = at^b, indicating that oxygen atoms diffuse in the metal lattice. The activation energy of this diffusion, Е _dif, is ~83.5 kJ/mol. At high temperatures (800–950 K), palladium sorbs much less oxygen (≤10 ML). This is due to the complete decomposition of the surface oxide film, a process that markedly hampers the insertion of Oads atoms under the surface layer of the metal.