摘要:

AbstractTo produce reaction products with 100% chemical selectivity, and to attain much higher turnover rates and number of turnovers, the development of new methods of catalyst preparation are necessary. Precise control of particle size, the separation between catalyst particles and their superior chemical and thermal stability must be achieved. To this end, five new methods of preparation of high‐technology catalysts are being studied. These are: (A) the method successfully used byW.M.H. Sachtlerfor metal particle deposition in zeolite cages, (B) the deposition of metal particles by atomic layer epitaxy, (C) controlled nucleation and growth of metal particles by substrate spinning, (D) deposition of metals on oxides by evaporation, (E) preparation of ordered metal arrays by electron‐beam lithography using microelectronic techniques. These different methods are described and discussed, and their present states of development are asses

ISSN:0165-0513    

DOI:10.1002/recl.19941131002

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA simple analytical expression for theEvanswindow in the infrared absorption spectrum of hydrogen‐bonded basic molecules in protonic zeolites is presented. The expression shows that the essential parameters are the position of the shifted “uncoupled” OH stretching frequency, its width, the position of the in‐plane bending overtone and the coupling of this mode with the OH stretching mode. Analysis of the Fermi resonances in the OH stretching band profile provides a method to determine whether adsorbed molecules are hydrogen bonded or pro

ISSN:0165-0513    

DOI:10.1002/recl.19941131003

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractAs a model reaction for the selective hydrogenation of carboxylic acids towards aldehydes, the reaction of acetic acid towards acetaldehyde has been investigated. Various oxides are used as catalysts, including platinum‐enriched oxides. It appeared that oxides with a moderate metal‐oxygen bond strength are good catalysts, provided that a source of activated hydrogen is present. On exposure to molecular hydrogen zero‐valent metals can produce activated hydrogen. Addition of precious metals, such as Pt, or completely reducing part of the oxidic catalyst can provide the zero‐valent metal. The hydrogenation probably occurs on the oxidic surface by a Mars and Van Krevelen type of me

ISSN:0165-0513    

DOI:10.1002/recl.19941131004

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractPure Pd/SiO2, pure Rh/SiO2, and bimetallic PdRh/SiO2catalysts, with Pd and Rh in the atomic ratios of 75/25 at% and 50/50 at%, were studied by means of infrared (IR) spectroscopy. Spectra of adsorbed CO and NO were recorded in the temperature range from 298 to 573K. The IR spectra obtained for the bimetallic catalysts exhibit characteristic features of both Pd and Rh. The results point to the presence of alloy crystallites with properties between those of the constituent metals. Most likely the PdRh(1:1)/SiO2catalyst consists of small Rh clusters or atoms, surrounded by Pd, whereas for the PdRh(3:1)/SiO2catalyst the surface consists primarily of Pd. However, in the presence of NO, Rh migrates to the surface, due to the dissociation of NO and the concomitant oxidation of the metal particles. No indication of mixed sites, consisting of both Pd and Rh atoms, was found. The results are compared with those previously reported for Pt‐Rh catalyst

ISSN:0165-0513    

DOI:10.1002/recl.19941131005

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe application of field‐emission microscopy (FEM) and field‐ion microscopy (FIM) to surface studies related to catalysis (symmetry of atomic‐like states localized at the surface atoms of transition metals, spillover effect, dissipation of energy, etc.) is discussed together with a new approach to modeling of supported bimetallic catalysts in these de

ISSN:0165-0513    

DOI:10.1002/recl.19941131006

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA field‐ion microscope (FIM) was used in the investigation of hydrogen oxidation at temperatures between 300 K and 430 K and pressures ofp(O2) = 5 · 10−4mbar,p(H2) = 3 · 10−4to 7 · 10−4mbar. The H2O molecules produced act as an imaging gas and display the catalytically active surface area. The structure‐sensitive H2O formation was studied under steady‐state conditions or in the oscillatory regime or in form of chemical waves. The displacement of Pt lattice atoms is spectacular in the Pt(100) plane, where the transformation from (1 × 1) Pt(100) to hex Pt(100) can be

ISSN:0165-0513    

DOI:10.1002/recl.19941131007

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractBiacetyl (butane‐2,3‐dione) adsorbs to silica via H bonding. Splitting of the central C‐C bond occurs on adsorption on Rh particles in Rh/SiO2catalysts. The resulting surface‐acetyl species seems to undergo rapid decomposition even at temperatures below 300K and leads to the formation of chemisorbed CO and methyl fragments. Based on the principle of microscopic reversibility, it is concluded that acetyl species may be formed under catalytic conditions from chemisorbed CO and methyl species by migratory insertion of CO into the metal‐carbon bond of a chemisorbed methyl

ISSN:0165-0513    

DOI:10.1002/recl.19941131008

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractIn situ13C MAS NMR was used to investigate the influence of total and partial pressure of propane on the initial stages of its conversion over a Ga/H‐MFI catalyst (2‐13C). Propane was the labelled reactant. Different pressures were achieved by varying the amounts of propane and of nitrogen as diluent in the cell. The nature of primary and secondary labelled reaction products depends on total pressure. High total and partial pressures of propane enhance bimolecular primary formation of n‐butane and isobutane via a BREST (Bifunctional Reaction Step) mechanism1. Low total pressure leads to methane and polymeric fragments as primary products. A reaction pathway including the formation of a polymeric hydrocarbon‐chain intermediate is proposed to account for secondary isobutane formation at low pressure. The influence of pressure on reaction equilibria and kinetics and on adsorption and exchange processes is di

ISSN:0165-0513    

DOI:10.1002/recl.19941131009

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractRecent results on surface reaction steps in the oxidative coupling of methane (OCM) obtained from (1) transient experiments and (2) a microkinetic analysis are summarized. The interaction of methane and oxygen with MgO and Sm2O3surfaces was investigated by applying H/D‐ and oxygen‐isotope‐exchange reactions. The role of short‐lived adsorbed oxygen species in methane activation and product formation over MgO and Sm2O3catalysts is discussed. Furthermore, elementary reaction steps and their rate constants are derived for the oxidative conversion of methane to COxand ethane from kinetic data for different (CaO)x(CeO2)1−xcatalysts; the rate constants are related to the solid's properties,i.e., electron and O2−c

ISSN:0165-0513    

DOI:10.1002/recl.19941131010

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA LEED and XPS study of the adsorption of naphthalene, quinoline, and 10,11‐dihydrocinchonidine on Pt(111) at 300K has shown that only naphthalene forms an ordered ad‐layer, and that quinoline and the alkaloid adsorb in a disordered state and without decomposition. These experiments do not support the hypothesis of ordered adsorption of alkaloid that forms the basis of the template model for the interpretation of enantioselectivity in Pt‐catalysed pyruvate hydrogenation. The model is accordingly reviewed. Molecular modelling studies show that a highly specific 1:1 interaction between cinchonidine (or cinchonine) and pyruvate interprets the observed sense of the enantioselectivity, provided relative energy relationships derived for purely inter‐molecular interactions are valid for the same molecules in the adsorbed state. Moreover, the ‘product’ of this 1:1 interaction is a satisfactory precursor to the H‐bonded state considered responsible for the greatly enhanced rate that always accompanies enantioselective reaction over cinchona‐modified Pt. The previously published dependencies of optical yield on (a) surface concentration of adsorbed cinchonidine modifier, and (b) modifier composition for mixtures of quinine and quinidine, are shown to be in quantitative agreement with the proposed 1:1 interaction model and at variance with the ordered adsorption model. Catalysts modified and used under strictly anaerobic conditions show negligible activity and enantioselectivity demonstrating that oxygen plays a crucial role in successful catalyst preparation. XPS experiments confirm that adsorption of cinchonidine from air‐saturated ethanolic solution on Pt(111) provides an adlayer containing both alkaloid and adsorbed oxygen. (S)‐(‐)‐1‐benzyl‐pyrrolidine‐2‐methanol, various configurations of ephedrine, D‐and L‐histidine and the methyl esters of D‐ and L‐tryptophan have been examined as modifiers for supported Pt. Although there is evidence that these compounds can provide chiral direction to pyruvate hydrogenation, rate enhancement is slight and en

ISSN:0165-0513    

DOI:10.1002/recl.19941131011

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY