Colin H. Rochester

Hydrogenation of but-2-enal over supported Au/ZnO catalysts

The hydrogenation of but-2-enal over supported Au catalysts is discussed, together with a detailed characterisation study using X-ray diffraction, infrared spectroscopy and transmission electron microscopy. Au/ZnO catalysts are found to be selective for the formation of the unsaturated alcohol, but-2-en-1-ol rather than the saturated aldehyde, butanal. In general, the addition of thiophene is found to enhance the yield of the unsaturated alcohol. Detailed transmission electron microscopy and infrared spectroscopy studies show that thiophene modification of Au/ZnO catalysts does not affect the Au-particle size or morphology; rather, thiophene undergoes irreversible dissociative adsorption giving a surface in which the Au sites are electronically promoted by sulfur. It is observed that thiophene modification does not give any marked effect on catalyst performance for the catalysts that contain large Au-particles (10 nm) and, hence, it is considered that the sulfur promotion observed is associated with smaller Au nanoparticles. The highest but-2-en-1-ol selectivities (∽80%) are observed for 5 wt.% Au/ZnO catalysts reduced at 400°C prior to reaction. It is proposed that the origin of high selectivity is associated with large Au particles (10–20 nm in diameter) that are present in this catalyst.

In situ infrared study of the surface oxidation of activated carbon in oxygen and carbon dioxide

The surface oxidation of a wood-based activated carbon by heat treatment in oxygen and carbon dioxide has been studied by diffuse reflectance Fourier transform infrared spectroscopy and compared with the results of aqueous oxidation by hydrogen peroxide and nitric acid. Untreated carbon gave infrared bands due to CO, C—O and aromatic species. Gaseous oxidation generated cyclic acid anhydride groups, CO groups associated with polynuclear aromatic systems and several types of C—O species. The formation of C—O species was independent of whether oxygen or carbon dioxide was used as oxidant but CO species were less readily formed in carbon dioxide than in oxygen. Aqueous oxidation followed by drying at 393 K gave similar surface groupings to those resulting from the gas-phase treatments.

Infrared and Raman study of the adsorption of NH3, pyridine, NO and NO2 on anatase

An anatase precursor of V2O5/TiO2 catalysts contains nitrate and sulphate impurities, the former being destroyed by calcination at 673 K but surface sulphate remaining after calcination at 723 K. Two types of Lewisacidic site were distinguished by ammonia and pyridine adsorption. Slight surface Bronsted acidity was considerably enhanced by deliberate doping with additional surface sulphate. Both nitric oxide and nitrogen dioxide were adsorbed to form bridged and bidentate nitrato species, with monodentate nitrate also formed from the dioxide. Ammonia did not react with the nitrogen oxides on the anatase surface. Co-adsorption was noncompetitive, the two gases being adsorbed at different surface sites.

Infrared study of surface hydroxyl groups on goethite

Infrared spectra of goethite (α-FeOOH) exhibited bands which have been assigned to OH stretching vibrations of bulk or surface hydroxyl groups. On exposure of goethite to deuterium oxide vapour the latter underwent rapid isotopic exchange to give the corresponding surface deuteroxyl groups. The conversion of bulk hydroxyl groups to bulk deuteroxyl groups by contact between deuterium oxide vapour and goethite was comparatively slow. Analogous conclusions were drawn from spectra of deuterated goethite (α-FeOOD) before and after treatment with water vapour. The decomposition of geothite to haematite (α-Fe2O3) by heat treatment in vacuo or in an oxygen atmosphere has been briefly investigated.

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO2 catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO2 and H2 on ZnO/SiO2 catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO2 and H2 with Cu/ZnO/SiO2 catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.

Infrared study of CO adsorption on reduced and oxidised silica-supported copper catalysts

FTIR spectra are reported of CO adsorbed on silica-supported copper catalysts prepared from copper(II) acetate monohydrate. Fully oxidised catalyst gave bands due to CO on CuO, isolated Cu2+ cations on silica and anion vacancy sites in CuO. The highly dispersed CuO aggregated on reduction to metal particles which gave bands due to adsorbed CO characteristic of both low-index exposed planes and stepped sites on high-index planes. Partial surface oxidation with N2O or H2O generated Cu+ adsorption sites which were slowly reduced to Cu° by CO at 300 K. Surface carbonate initially formed from CO was also slowly depleted with time with the generation of CO2. The results are consistent with adsorbed carbonate being an intermediate in the water-gas shift reaction of H2O and CO to H2 and CO2.

Surface interactions of NO and CO with LaMO3 oxides

The surface interactions of NO and CO with LaMO3 perovskites (M = Cr, Mn, Fe, Co, Ni) have been studied by volumetric adsorption and infrared spectroscopy. NO adsorption at 200 mm Hg on LaNiO3 was found to be constant over the temperature range 200–750 K. NO adsorption on LaMO3 oxides at room temperature as a function of the atomic number of M showed maxima for LaMnO3 and LaCoO3 while CO adsorption exhibited one maximum for LaFeO3. The inhibiting effect of NO preadsorption on the subsequent adsorption of CO was found to be larger than the inhibiting effect of CO on NO adsorption. NO is apparently more strongly adsorbed than CO on the surface of these oxides. Infrared spectra after simultaneous adsorption of NO + CO on LaFeO3 contained bands between 1650 and 850 cm−1 due to species formed by the interaction of NO and CO with oxygen ions in the oxide surface, and a band at 2170 cm−1 assigned to isocyanate species. Spectra in the region 2300-1900 cm−1 after simultaneous adsorption of NO + CO on LaCoO3 contained bands at 2175 and 2000 cm−1, the latter being assigned to NO adsorbed via a donor-type or coordinative bond. The numbers of NO molecules adsorbed at monolayer coverage on the LaMO3 oxides did not equal estimated numbers of transition metal ions exposed in the oxide surfaces. The observed constancy of NO adsorption on LaFeO3 and LaNiO3 over a wide range of temperatures could provide a method for oxide characterization.

Evidence for the adsorption of molecules at special sites located at copper/zinc oxide interfaces: part 1.—A Fourier-transform infrared study of formic acid and formaldehyde adsorption on reduced and oxidised Cu/ZnO/SiO2 catalysts

Fourier-transform infrared (FTIR) spectra are reported of formic acid and formaldehyde on ZnO/SiO2, reduced Cu/ZnO/SiO2 and reoxidised Cu/ZnO/SiO2 catalyst. Formic acid adsorption on ZnO/SiO2 produced mainly bidentate zinc formate species with a lesser quantity of unidentate zinc formate. Formic acid on reduced Cu/ZnO/SiO2 catalyst resulted not only in the formation of bridging copper formate structures but also in an enhanced amount of formate relative to that for ZnO/SiO2 catalyst. Formic acid on reoxidised Cu/ZnO/SiO2 gave unidentate formate species on copper in addition to zinc formate moieties.The interaction of formaldehyde with ZnO/SiO2 catalyst resulted in the formation of zinc formate species. The same reaction on reduced Cu/ZnO/SiO2 catalyst gave bridging formate on copper and a remarkable increase in the quantity of formate species associated with the zinc oxide. Adsorption of formaldehyde on a reoxidised Cu/ZnO/SiO2 catalyst produced bridging copper formate and again an apparent increase in the concentration of zinc formate species. An explanation in terms of the adsorption of molecules at special sites located at the interface between copper and zinc oxide is given.

Infrared study of crotonaldehyde and CO adsorption on a Pt/TiO2 catalyst

A Pt/TiO2 catalyst has been subjected to reduction in hydrogen at 473, 573 and 773 K and the various degrees of metal-support interaction (SMSI) confirmed by means of CO and H2 chemisorption, FTIR of CO and the hydrogenation of crotonaldehyde. Coadsorption of CO and crotonaldehyde were performed to identify the preferred adsorption site and mode of adsorption of the unsaturated aldehyde. Results which appear to suggest shifts to lower frequencies of bands due to adsorbed carbonyls are not due to electronic effects induced by coadsorption, but rather indicate displacement of CO from the weaker bonding sites which eliminate dipole coupling effects between different carbonyl clusters, and consequently removes intensity transfer phenomena leading to enhancement in intensity at lower frequencies.

Infrared study of the adsorption of formic acid on silica-supported copper and oxidised copper catalysts

Infrared spectra are reported of formic acid adsorbed at 300 K on a reduced copper catalyst (Cu/SiO2) and a copper surface which had been oxidised by exposure to nitrous oxide. Formic acid was weakly adsorbed on the silica support. Ligation of formic acid to the copper surface occurred only on the reduced catalyst. Dissociative adsorption resulted in the formation of unidentate formate on the oxidised catalyst. The presence of reduced copper metal instigated a rapid reorientation to a bidentate formate species.