Methanol decomposition over supported palladium and platinum

Methanol decomposition over supported Pt and Pd catalysts was investigated in the temperature range from 453 to 573 K with the partial pressure of methanol up to 0.8 arm. The specific activity of Pd was higher than that of Pt, and alumina-supported Pd was more active than silicasupported Pd, although aluminasupported catalysts produced dimethyl ether as a by-product. A silica-supported Pd catalyst prepared from a tetraamine salt solution with a proper pH exhibited higher activity than the other silica-supported Pd catalysts. Dependence of the decomposition rate on the partial pressure of methanol was similar regardless of the metal, the support or the preparation method. The apparent reaction orders were near 0.3 at low pressures below about 0.4 atm and became near zero or slightly negative at higher pressures. The apparent activation energies were about the same for most of the catalysts and were in the range from 66 to 77 kJ/g-mol.     

Preparation of ordered macroporous platinum and palladium powders

Ordered macroporous platinum and palladium metal powders were prepared with a combination of silica template and hydrogen reduction methods. The effects of species of metal precursor and calcination of silica templates on the structure of final metal powders were investigated. Precursor of platinum or palladium was infiltrated into closely packed silica templates prepared from tetraethyl orthosilicate and vacuum dried at room temperature following hydrogen reduction to obtain metal–silica complexes. After the removal of the silica templates from the complexes by hydrofluoric acid treatment, metal powders were obtained. Ordered macroporous platinum and palladium powders were obtained by using hydrogen hexachloroplatinate (IV) hexahydrate with uncalcined silica template and palladium (II) chloride with calcined templates, respectively. Composition analyses indicated that silica templates were completely removed from the ordered metal powder. Sizes of the voids in the ordered macroporous metal powders were controlled in the range of 240–520 nm corresponding to those of the silica spheres used.     

Epitaxial overgrowth of platinum on palladium nanocrystals

This paper describes a systematic study on the epitaxial overgrowth of Pt on well-defined Pd nanocrystals with different shapes (and exposed facets), including regular octahedrons, truncated octahedrons, and cubes. Two different reducing agents, i.e., citric acid and L-ascorbic acid, were evaluated and compared for the reduction of K?PtCl? in an aqueous solution in the presence of Pd nanocrystal seeds. When citric acid was used as a reducing agent, conformal overgrowth of octahedral Pt shells on regular and truncated octahedrons of Pd led to the formation of Pd-Pt core-shell octahedrons, while non-conformal overgrowth of Pt on cubic Pd seeds resulted in the formation of an incomplete octahedral Pt shell. On the contrary, localized overgrowth of Pt branches was observed when L-ascorbic acid was used as a reducing agent regardless of the facets expressed on the surface of Pd nanocrystal seeds. This work shows that both the binding affinity of a reducing agent to the Pt surface and the reduction kinetics for a Pt precursor play important roles in determining the mode of Pt overgrowth on Pd nanocrystal surface.     

Theoretical and computational chemistry

Computer-based and theoretical approaches to chemical problems can provide atomistic understanding of complex processes at the molecular level. Examples ranging from rates of ligand-binding reactions in proteins to structural and energetic investigations of diastereomers relevant to organo-catalysis are discussed in the following. They highlight the range of application of theoretical and computational methods to current questions in chemical research.     

Selective hydrogenation catalyzed by polymeric palladium and platinum complexes

An organic polymer containing a diphenylbenzyl-phosphine functional group, combined with PtCl₂ or PdCl₂, gives a heterogeneous hydrogenation catalyst which is analogous to the homogeneous catalysts PtCl₂(Pø₃)₂ and PdCl₂(Pø₃)₂. In the hydrogenation of soybean methyl ester, this catalyst is highly selective, the products being monoene and diene, with almost no increase in the content of saturated ester.     

用X射线荧光光谱法测定催化剂中铂钯含量

X射线荧光光谱法是一种无损检测的方法,对于粉末样品,可以通过直接压片的方式进行测量,无需对样品进行消解。它具有分析速度快、测定范围广、自动化程度高的特点。采用X射线荧光光谱法,模拟样品制备的工艺条件和组分含量自制了一系列标样,绘制了标准曲线,并利用经验系数方程矫正基体效应,测定了催化剂中钯、铂含量。通过与电感耦合等离子体发射光谱法和分光光度法进行实验方法的比对,得到的测试结果基本一致,表明此方法具有很好可靠性。     

Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al₂O₃ catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al₂O₃ catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al₂O₃ catalyst. Virtually no coke was detected on the Pd/α-Al₂O₃ catalyst. The formation of coke on Pd/α-Al₂O₃ may be slower than on the Pd/γ-Al₂O₃ owing to the carrier’s smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al₂O₃ catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.     

Oxygen reaction on titanium catalysed by gold,platinum and palladium

Electrocatalytic properties of titanium surfaces coated with small amounts (0.025–0.6 mg cm^–2) of Au, Pt, Pd and their mixtures, with respect to both oxygen evolution and reduction in alkaline solution have been investigated. It was found that all the samples exhibited higher electrochemical activity than the corresponding compact metal surfaces. This could be ascribed to a larger specific surface area of the dispersed catalysis than that of the smooth metal surfaces. The best performance was obtained with dispersed palladium, the potential gap between oxygen evolution and reduction at 1 mA cm^–2 being reduced to 0.66 V.Reported in part at the 31st Meeting of ISE, Venice, September 1980.     

Extraction of platinum and palladium from hydrochloric solutions by trioctymethylammonium dinonylnaphthalenesulfonate

We investigated the phase distribution of chloride complex platinum and palladium acids in the presence of the binary extracting agent trioctylmethylammonium dinonylnaphthalenesulfonate. During the extraction of H₂PtCl₆ and H₂PdCl₄, the distribution coefficients were found to decrease as long as the pH value was growing, in full accordance with the known general trend of binary extraction. In contrast to the extraction variant using quaternary ammonium bases with inorganic counterions, a binary system, which was based on dinonylnaphthalenesulfonic acid, could be used for the further aqueous reextraction of platinum metals from the organic phase because of the higher thermodynamic stability of the binary reagent in the two-phase system.     

Biosorption of palladium and platinum by sulfate‐reducing bacteria

The biosorption capacities of palladium and platinum were studied in three different species of Desulfovibrio: Desulfovibrio desulfuricans, Desulfovibrio fructosivorans and Desulfovibrio vulgaris. The influence of several parameters such as pH, acidic background and competitor anions on biosorption equilibria and biosorption kinetics were evaluated. Differences were observed between the three strains of Desulfovibrio with respect to the optimum biosorption parameters of both metals, suggesting differences in the metal speciation–dependent sorption mechanisms involved. The most promising Pd and Pt biosorption results were obtained using D desulfuricans with rapid achievement of equilibrium (90% of total sorption was achieved in 5–15 min) and a maximum value of 190 mg g^?1 dry biomass and 90 mg g^?1 dry biomass for Pd and Pt accumulation respectively, at pH 3. Copyright © 2003 Society of Chemical Industry     

Matching chemistry and shape in molecular docking

We have added a chemical filter to the ligand placement algorithmof the molecular docking program DOCK. DOCK places ligands inreceptors using local shape features. Here we label these shapefeatures by chemical type and insist on complementary matches.We find fewer physically unrealistic complexes without reducingthe number of complexes resembling the known ligand–receptorconfigurations. Approximately 10-fold fewer complexes are calculatedand the new algorithm is correspondingly 10-fold faster thanthe previous shape-only matching. We tested the new algorithm'sability to reproduce three known ligand–receptor complexes:methotrexate in dihydrofolate reductase, deoxyuridine monophosphatein thymidylate synthase and pancreatic trypsin inhibitor intrypsin. The program found configurations within 1 AÅof the crystallographic mode, with fewer non-native solutionscompared with shape-only matching. We also tested the program'sability to retrieve known inhibitors of thymidylate synthaseand dihydrofolate reductase by screening molecular databasesagainst the enzyme structures. Both algorithms retrieved manyknown inhibitors preferentially to other compounds in the database.The chemical matching algorithm generally ranks known inhibitorsbetter than does matching based on shape alone.     

Catalytic dehydrogenation of propane on chromia,palladium and platinum supported catalysts

The dehydrogenation of propane to propylene over Cr₂O₃/Al₂O₃, Pd/Al₂O₃ and Pt/SiO₂ has been investigated in the temperature range 580–618°C. Runs were performed on propane, alone or in the presence of nitrogen (as a diluent), with complete analysis of the reaction products. The reaction was carried out in a fixed bed reactor at space velocities from 450–800 h^?1 which are close to industrial values and at pressures from 0.3 to 1 atm. A set of runs was made over a commercial chromia-alumina catalyst (10% Cr₂O₃) and over a promoted catalyst prepared in the laboratory by impregnation (16.8% Cr₂O₃ + 2% K₂O). The latter catalyst showed high selectivity and stability even when subjected to continuous cycles of dehydrogenation, regeneration and purging. Of the two noble metal supported catalysts used, reduced Pd/Al₂O₃ showed higher activity than Pt/SiO₂ at 618°C. The former catalyst gave a propylene yield of around 98% at 20% conversion level.     

Approaches to bi- and trimetallic platinum and palladium complexes using the DPPEPM ligand

Bis{(diphenylphosphinoethyl)phenylphosphino}methane (DPPEPM) reacts with [PtR₂(cod)] in 1:1 ratio to give [PtR₂(DPPEPM-PP)] (2a, R=Me; 2b, R=Ph), whereas with [PtCl₂(cod)] or [PdCl₂(cod)] it yields the ionic species [M(DPPEPM-PP)₂]²⁺ (3 and 4). With [MClMe(cod)], the product is [MMe(DPPEPM-PPP)]⁺ (5, M=Pt; 6, M=Pd), in which one of the internal P atoms of the ligand is uncoordinated. These complexes undergo oxidation of the free P atom to give 7 and 8 on standing in solution. Complexes 2–4 may be used to construct bimetallic and trimetallic mixed metal complexes. The molecular structures of 7 and [PtMe₂(μ-DPPEPM)PdCl₂] (11) are reported.     

Regioselective carbon-carbon bond formation in proteins with palladium catalysis; new protein chemistry by organometallic chemistry

Palladium-catalyzed reactions have contributed to the advancement of many areas of organic chemistry, in particular, the synthesis of organic compounds such as natural products and polymeric materials. In this study, we have used a Mizoroki-Heck reaction for site-specific carbon-carbon bond formation in the Ras protein. This was performed by the following two steps: 1) the His6-fused Ras protein containing 4-iodo-L-phenylalanine at position 32 (iF32-Ras-His) was prepared by genetic engineering and 2) the aryl iodide group on the iF32-Ras-His was coupled with vinylated biotin in the presence of a palladium catalyst. The biotinylation was confirmed by Western blotting and liquid chromatography-mass spectrometry (LC-MS). The regioselectivity of the Mizoroki-Heck reaction was furthermore confirmed by LC-MS/MS analysis. However, in addition to the biotinylated product (bF32-Ras-His), a dehalogenated product (F32-Ras-His) was detected by LC-MS/MS. This dehalogenation resulted from the undesired termination of the Mizoroki-Heck reaction due to steric and electrostatic hindrance around residue 32. The biotinylated Ras showed binding activity for the Ras-binding domain as its downstream target, Raf-1, with no sign of decomposition. This study is the first report of an application of organometallic chemistry in protein chemistry.     

Deuteration of methyl linoleate with nickel,palladium, platinum and copper-chromite catalysts

Samples taken during deuteration of methyl linoleate with the title catalysts were separated into saturate, monoene and diene fractions. Monoenes were further separated intocis andtrans fractions. A comparison of the double bond distribution in monoenes with those from hydrogenation of alkaliconjugated linoleate indicated that up to 59% of the linoleate was reduced through a conjugated intermediate with nickel catalyst. The respective percentages for palladium and platinum catalysts were 51 and 23. Copper catalysts have previously been shown to reduce linoleate solely through conjugated intermediates. Copper-chromite catalyst showed infinite selectivity for the reduction of linoleate, because stearate did not form. The decreasing order of various catalysts for the selective reduction was copper-chromite>>>Ni at 195 C>Pd>Ni at 100 C>Pt. Computer simulation of platinum reduction indicated that ca. 20% of the linoleate was directly reduced to stearate through a shunt. Geometrical isomers of linoleate were formed during reduction with all catalysts except copper-chromite. Nickel catalyst formed bothtrans,trans- andcis,trans-isomers, as well as nonconjugatable dienes. These isomers were favored at the higher temperature and deuterium was incorporated into them. Palladium and platinum did not isomerize linoleate to nonconjugatable dienes. Because conjugated dienes are more reactive than linoleate, they were not found in appreciable amounts during reduction. Conjugated dienes were the only isomers formed with copper-chromite catalyst. Deuterium was found in these conjugated dienes, which were also extensively isomerized. As a result of isomerization and exchange during reduction of linoleate-as well as further exchange between deuterium and monoenes-a wide distribution of isotopic isomers in monoenes was found with nickel, palladium and platinum catalysts. Since isomerization of monoenes with copper-chromite is negligible, the isotopic distribution of monoenes must be due to exchange of intermediate conjugated dienes followed by addition. Presented at the AOCS Meeting, Ottawa, September 1972. ARS, USDA.     

压力消解ICP-AES法对润滑油异构脱蜡催化剂中铂、钯含量的测试

采用高压消解技术,使用盐酸、硝酸和氢氟酸对润滑油异构脱蜡催化剂进行处理,制备测定样品水溶液,选择Pt波长214.423 nm和Pd波长340.458 nm作分析线,电感耦合等离子体原子发射光谱仪(ICP-AES)进行测定,对压力消解条件进行确定,对待测元素的分析谱线选择进行分析。结果表明,各元素校准曲线线性相关系数均不小于0.999 00,检出限Pt为0.030 12 mg·L^-1,Pd为0.015 21 mg·L^-1。按照实验方法测定润滑油异构脱蜡催化剂样品中Pt、Pd结果的相对标准偏差(RSD,n=6)均小于1%,回收率98.0%~103.0%,测试方法准确可靠。     

Studies of platinum electroplating baths Part I: The chemistry of a platinum tetrammine bath

The chemistry of a new commercial electroplating bath for platinum, based on a Pt(NH₃) ₄ ²⁺ salt in a phosphate buffer, is investigated using¹⁹⁵Pt NMR and microelectrode techniques. It is shown that the maximum rate of deposition is determined by a homogeneous chemical reaction preceding the electron transfer step; the nature of this homogeneous chemical reaction is discussed.