Ge2H2 a π-ligand in organometallic chemistry

η² π-Complexes of Ge₂H₂ with the organometallic fragments V(PH₃)₂(I)(CO)₂, Cr(CO)₄, Co(PH₃)₂(Cl) and M(PH₃)₂ (M = Ni, Pd, Pt) have been studied at the B3LYP level using the SBKJC relativistic effective core potentials and their associated basis sets on metals and iodine, and the 6-31G(d) basis set on all other elements. The transition metal fragments of V, Cr, Co, Ni, Pd and Pt were chosen based on known alkyne compounds. All the complexes are local minima for both the HGeGeH and GeGeH₂ isomers of the Ge₂H₂ ligand. The complexes containing GeGeH₂ isomer as a ligand are lower in energy than those with the HGeGeH ligand (except in the V complex in which the difference is only 1.0 kcal/mol). There is a net charge transfer from ligand to metal in complexes V-Co and from metal to ligand in late transition metal complexes (Ni-Pt).     

X-ray photoelectron study of the interaction of H<Subscript>2</Subscript> and H<Subscript>2</Subscript>+O<Subscript>2</Subscript> mixtures on the Pt/MoO<Subscript>3</Subscript> model catalyst

The effects of H₂ and H₂ + O₂ gas mixtures of varying composition on the state of the surface of the Pt/MoO₃ model catalyst prepared by vacuum deposition of platinum on oxidized molybdenum foil were investigated by X-ray photoelectron spectroscopy (XPS) at room temperature and a pressure of 5–150 Torr. For samples with a large Pt/Mo ratio, the XP spectrum of large platinum particles showed that the effect of hydrogen-containing mixtures on the catalyst was accompanied by the reduction of molybdenum oxide. This effect results from the activation of molecular hydrogen due to the dissociation on platinum particles and subsequent spill-over of hydrogen atoms on the support. The effect was not observed at low platinum contents in the model catalyst (i.e., for small Pt particles). It is assumed for the catalyst that the loss of its hydrogen-activating ability is a consequence of the formation of platinum hydride. Possible participation of platinum hydride as intermediate in hydrogen oxidation to H₂O₂ is discussed.     

Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O = C (H)– O – C (H₂)–CH₃ and trans (staggered) configuration of the O= C (H)– O – C (H₂)– C H₃ dihedral angles. We observed the following ethyl formate pyrolysis products: CH₃CH₂OH, CH₃CHO, C₂H₆, C₂H₄, HC(O)OH, CH₂O, CO₂, and CO, with HC(O)OH and C₂H₄ pyrolyzing further, forming CO + H₂O and C₂H₂ + H₂. The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH₃CH₂O + CHO and CH₃CH₂ + HC(O)O, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.     

Surface Chemistry of Ga(CH3)3 on Pd(111) and Effect of Pre-covered H and O

The adsorption and decomposition of trimethylgallium (Ga(CH₃)₃, TMG) on Pd(111) and the effect of pre-covered H and O were studied by temperature programmed desorption spectroscopy and X-ray photoelectron spectroscopy. TMG adsorbs dissociatively at 140 K and the surface is covered by a mixture of Ga(CH₃)_x (x=1, 2 or 3) and CHx(a) (x=1, 2 or 3) species. During the heating process, the decomposition of Ga(CH₃)₃ on clean Pd(111) follows a progressive Ga-C bond cleavage process with CH₄ and H₂ as the desorption products. The desorption of Ga-containing molecules (probably GaCH₃) is also identi ed in the temperature range of 275-325 K. At higher annealing temperature, carbon deposits and metallic Ga are left on the surface and start to di use into the bulk of the substrate. The presence of precovered H(a) and O(a) has a signi cant effect on the adsorption and decomposition behavior of TMG. When the surface is pre-covered by saturated H₂, CH₄, and H₂ desorptions are mainly observed at 315 K, which is ascribed to the dissociation of GaCH₃ intermediate. In the case of O-precovered surface, the dissociation mostly occurs at 258 K, of which a Pd-O-Ga(CH₃)₂ structure is assumed to be the precusor. The presented results may provide some insights into the mechanism of surface reaction during the lm deposition by using trimethylgallium as precursor.     

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.     

Methanol oxidation on the PtPd(111) alloy surface: A density functional theory study

The mechanisms of methanol (CH₃OH) oxidation on the PtPd(111) alloy surface were systematically investigated by using density functional theory calculations. The energies of all the involved species were analyzed. The results indicated that with the removal of H atoms from adsorbates on PtPd(111) surface, the adsorption energies of (i) CH₃OH, CH₂OH, CHOH, and COH increased linearly, while those of (ii) CH₃OH, CH₃O, CH₂O, CHO, and CO exhibited odd‐even oscillation. On PtPd(111) surface, CH₃OH underwent the preferred initial C H bond scission followed by successive dehydrogenation and then CHO oxidation, that is, CH₃OH → CH₂OH → CHOH → CHO → CHOOH → COOH → CO₂. Importantly, the rate‐determining step of CH₃OH oxidation was found to switch from CO → CO₂ on Pt(111) to COOH → CO₂ + H on PtPd(111) with a lower energy barrier of 0.96 eV. Moreover, water also decomposed into OH more easily on PtPd(111) than on Pt(111). The calculated results indicate that alloying Pt with Pd could efficiently improve its catalytic performance for CH₃OH oxidation through altering the primary pathways from the CO path on pure Pt to the non‐CO path on PtPd(111).     

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.     

A study of Co/Mo/Al2O3 hydrodesulphurization catalysts and related model compounds byxps and x-ray absorption spectroscopy (xanes andexafs )

A detailed investigation of sulphided Co/Mo/Al₂O₃ catalysts, their oxide precursors and several model oxides and sulphides of cobalt and molybdenum has been carried out using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (xanes andexafs). Octahedrally coordinated Co(II) and Mo(IV) are shown to be present in a sulphidic environment on the surfaces of these catalysts. The surface species contain an excess of sulphur, probably involving disulphide linkages. The surface compositions of the catalysts examined conform to the general formula Co¹¹ Mo _2n ^IV (2n + 3)S ₂ ²⁻ (2n -2)S²⁻.     

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H₂SO₄ + O₂ (saturated), 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH, and 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH + O₂ (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH₃OH and O₂ individual solutions. The surface coverages with the CH₃OH and O₂ adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.     

Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH₃OC(O)H, were elucidated to be CH₃OH + CO, 2 CH₂O and CH₄ + CO₂ as in part distinct from the dissociation of the radical cation (CH₃OH^+• + CO and CH₂OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.     

Reducibility of Mo species in Pt/MoO3 and PtMo/Al2O3 catalysis

Recucibility of Mo species in Pt/MoO₃ and PtMo/Al₂O₃ has been investigated by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (H₂-TPD) and temperature programmed electronic conductivity (TPEC) techniques. In Pt/MoO₃ at H₂ atmosphere, it was found by TPEC and TPR that, a slight amount of Pt could activate the transfer of the species and H atoms between H₂ and MoO₃, and thus accelerate the reduction of MoO₃. In PtMo/Al₂O₃, TPR and H₂-TPD revealed that the reduction of surface Mo species could also be facilitated by Pt. Two kinds of hydrogen molybdenum species were proposed on the surface of the catalyst after prereduction.     

Effect of Pt,Pd, and Cs<Superscript>+</Superscript> Additives on the Surface State and Catalytic Activity of WO<Subscript>3</Subscript> in Oxidation of Hydrogen

We have shown that additions of Pt(Pd) and Cs⁺ to WO₃ significantly increase its specific surface area and catalytic activity in H₂ oxidation. After reduction, the promoted specimens contain the phases WO₃, WO_2.9, H_xWO₃; and in the case of Cs⁺ additions, Cs_xWO₃. According to X-ray photoelectron spectroscopy (XPS), the Pt and Pd have an oxidation state close to 0, while tungsten has a +5 oxidation state. The W:O ratio indicates the content of oxygen vacancies in the surface layer. The data are explained taking into account hydrogen spillover from Pt(Pd) to the support.__________Translated from Teoreticheskaya i Eksperimental’naya Khimiya, Vol. 41, No. 2, pp. 126–129, March– April, 2005.     

Thermal characterisation of molybdenum and platinum-molybdenum catalysts

Alumina supported Mo and Pt-Mo catalysts was subject to temperature programmed reduction (TPR) using H₂ and CO. After earlier oxidation step TPR–H₂ profiles shows different surface species, which depends on the composition of the catalysts and reduction temperature. Change in reducing gas from H₂ into CO results in significant changes in catalyst system. Hydrogen causes a decrease in oxidation number of metals, while carbon monoxide reacts with chemisorbed chemicals.     

A new simple preparation of platinum-nickel alloy nanoparticles and their characterization as an electrocatalyst for methanol oxidation

Pt-Ni alloy nanoparticles were produced by casting 2 or 10 mM H₂PtCl₆ solutions on a Ni column. The apparent particle size for the resultant Pt-Ni alloys increased with the concentration of the H₂PtCl₆ solution, while the content of Pt in the alloy decreased. The potential sweeps of 5 cycles in an H₂SO₄ aqueous solution for Pt-Ni (2 mM)/Ni and Pt-Ni (10 mM)/Ni electrodes led to electrochemical behavior similar to a polycrystalline Pt electrode, suggesting the formation of a few thin Pt layers on each Pt-Ni alloy surface. In electrochemical measurements, both Pt-Ni/Ni electrodes showed more negative onset potential of methanol oxidation and slower degradation of oxidation current of methanol than the polycrystalline Pt electrode. X-ray photoelectron spectroscopy of both Pt-Ni/Ni electrodes showed the shift of Pt4f peaks to a higher binding energy, suggesting that the increase in the d vacancy in the balance band 5d orbital of Pt contributed to the improved electrocatalytic activity and durability of the Pt-Ni/Ni electrodes.     

Influence of Metallic Modification on Ethylbenzene Dealkylation over ZSM‐5 Zeolites

A series of metal‐modified HZSM‐5 catalysts were prepared by impregnation and were used for ethylbenzene dealkylation of the mixed C8 aromatics (ethylbenzene, m‐xylene and o‐xylene). The effects of different supported metals (Pt, Pd, Ni, Mo) on catalytic performance, including reaction conditions, were investigated. The physicochemical properties of catalysts were characterized by means of XRD, BET, TEM and NH₃‐TPD. Experimental results showed that metallic modification obviously increased the ethylbenzene conversion and reduced the coke deposition, greatly improving the catalyst stability. The distinction of ethylbenzene conversion depended on the interaction between hydrogenation reactivity and acidic cracking of bifunctional metal‐modified zeolites. Compared with Pt and Ni, Pd and Mo were easier to disperse into HZSM‐5 micropores during loading metals. The acidic density of different metal‐modified HZSM‐5 declined in the following order: HZSM‐5>Pt/HZSM‐5>Pd/HZSM‐5>Ni/HZSM‐5>Mo/HZSM‐5. The activity of ethylene hydrogenation decreased with Pt/HZSM‐5>Pd/HZSM‐5>Ni/HZSM‐5>Mo/HZSM‐5. In comparison, Pd/HZSM‐5 showed the best catalytic performance with both high activity and high selectivity, with less cracking loss of m‐xylene and o‐xylene. Moreover, the following reaction conditions were found to be preferable for ethylbenzene dealkylation over Pd/HZSM‐5: 340°C, 1.5 MPa H₂, WHSV 4 h^?1, H₂/C8 4 mol/mol.     

H2 production with low CO selectivity from photocatalytic reforming of glucose on metal/TiO2 catalysts

H₂ with low CO concentration is produced via photocatalytic reforming of glucose (as a representative of biomass component) on metal/TiO₂ catalyst (metals: Pt, Rh, Ru, Ir, Au, Ni, Cu). It is shown that the loaded metals generally enhance the rate of H₂ production, while they depress the CO selectivity. Both H₂ production and CO selectivity are strongly dependent on the kind of deposited metals on TiO₂. For example, Rh/TiO₂ catalyst is found to be most active for H₂ production while with the most extremely low CO concentration from the photocatalytic reforming of glucose. Supported by the National Natural Science Foundation of China (Grant No. 20403018, 20503034), National Basic Research Program of China (Grant No. 2003CB214504), and Knowledge Innovation Program of the Chinese Academy of Sciences (DICP K2006E2)     

Enhancing effect of water vapor on the reduction of lean NOx by ethanol over an Ag/Al2O3 catalyst supported on cordierite honeycomb

The Ag/Al₂O₃ catalyst supported on cordierite honeycomb (Ag/Al₂O₃/ cordierite) is highly active forthe reduction of lean NOx by ethanol. Addition of H₂O enhances the NOx reduction to CO₂ and N₂, and suppresses the formation of by-products such as CO, CH₃CHO and C₂H₄.     

Electrospray mass spectrometric study of homogeneous catalytic system Pd(CF3COO)2-Ph2P(CH2)3PPh2-MeOH/Me2CO-H2O for copolymerization of ethylene and carbon monoxide

The catalytic system Pd(CF₃COO)₂-Ph₂P(CH₂)₃PPh₂-MeOH/Me₂CO was studied by electrospray mass spectrometry. The {[Pd(dPPP)₂]²⁺ [(dppp)₂Pd(CF₃COO)]⁺, [(dppp)Pd(CF₃COO)]⁺, and [(dppp)Pd(CF₃COO)₂Pd(dppp)]²⁺} cations were found in the system. The addition of H₂O to the system resulted in the formation of binuclear bicharged ions [(dppp)Pd(OH)]₂ ²⁺ and their associates with water.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 494–496, February, 1996     

Formation and electrocatalytic properties of Pd deposits on Mo obtained by galvanic displacement

A Pd-Mo electrocatalytic system was obtained by forming palladium particles on the Mo surface that contacted a PdCl₂ solution under open-circuit conditions. The state of palladium on the electrode surface depended on the contact displacement time. Palladium particles 5–10 nm in size formed on the surface of the Pd(Mo) electrode after palladium deposition for 1 min. The specific rates of formic acid oxidation on the Pd(Mo) electrode were smaller than those on the Pd/Pt electrode. On the Pd(Mo) electrode, anode currents of methanol oxidation were recorded at a potential of 0.4 V. The difference in the effects of the Mo substrate on the activity of Pd particles in the electrooxidations of HCOOH and CH₃OH was explained by the difference in the mechanisms of these reactions.     

相转移法制备PdMo催化剂及其氧还原活性

以四丁基氢氧化铵作为相转移剂,以硼氢化钠为还原剂,利用相转移法在二氯甲烷中制备了一系列不同比例的Pd_xMo/C(Pd/Mo的原子比x=1、2、3、4、5)催化剂。透射电镜(TEM)图像显示,Pd_x Mo/C是呈2～4 nm的圆形颗粒,尺寸均匀、分散性良好。X射线衍射(XRD)结果表明,加入第二组元Mo后,Pd的晶格发生扩张,调节了 Pd的几何结构。此外,X射线光电子能谱(XPS)结果表明,相对于Pd/C,Pd_4Mo/C的Pd3d_(5/)2结合能负移了 0.50 eV,说明电负性较大的Pd从Mo吸电子,电子结构发生改变。氧还原反应(ORR)结果表明,不同比例的Pd_xMo/C催化剂活性均优于Pd/C,其中当x=4时,ORR活性最佳,其起始电位和半波电位分别为0.876和0.813 V,高于商业Pt/C的0.870和0.810 V。此外,在经过3 h的运行之后电流密度仍保留82.9%,与商业Pt/C相比具有明显的优势。