Stephen J. Gentry

Study of the promoting influence of transition metals on the reduction of cupric oxide by temperature programmed reduction

Temperature programmed reduction (t.p.r.) has been used to study the hydrogen reduction of cupric oxide and the influence on the reduction of transition metals added to the bulk oxide. X.r.d. and X.p.s. were used to characterise the samples.For cupric oxide containing 2 mol% metal additive, three types of reduction behaviour were indicated. (i)For Cr, Mn, Fe, Co and Ni, a single reduction process some 30 K below that observed for pure CuO. (ii) For Pd and Ru, two reduction processes some 100 K below that observed for pure CuO. (iii) For Pt, Rh and Ir, complex reduction behaviour with the bulk of the reduction occurring some 100 K below that for CuO. For Ag and Au, similar complex behaviour with the main reduction some 30 K below that for CuO.A detailed study was made of the reduction of CuO containing Pd (0.05–4.0 mol%) and Pt (0.05–10 mol%).Measured activation energies were essentially independent of the metal additive indicating that rate limiting steps for the reductions involve the growth of metal nuclei.The promoting effects of metal additives are discussed in terms of their ability (i) to remain within the CuO lattice, (ii) to produce extra nucleation sites and (iii) to increase the concentration of surface hydrogen. Thus the first row transition metals produce extra copper nucleation sites by non-specific distortion of the CuO lattice. For Group VIII and Ib additives, the nucleation sites are provided by the preferential reduction of the additive ion. The Group VIII metals further promote the reduction of CuO by mechanisms involving hydrogen adsorption and spillover.

Temperature programmed reduction of copper ions in zeolites

Temperature programmed reduction (t.p.r.) has been used to study the redox behaviour of copper ions in X and Y zeolites. Two distinct CuII to CuI reduction processes are discernible in both X and Y zeolites. These two processes have been attributed to the reduction of CuII ions occupying sodalite and supercage sites; a small fraction of the CuII ions, assumed to occupy hexagonal prism sites, remain unreduced. At 773 K in nitrogen, autoreduction of both sodalite and supercage CuII occurs in X zeolite, while only supercage CuII ions are autoreduced in Y zeolite. The reductions are reversible in Y zeolite, but some CuO is formed in X which is subsequently reduced by a third process. Activation energies (kJ mol–1) were obtained as follows: CuII to CuI(sodalite cages) 84 ± 13, CuII to CuI(supercages) 64 ± 10 and CuO to Cu0(X-type zeolite) 49 ± 7.

Catalytic oxidation of hydrogen over platinum

The oxidation of hydrogen on platinum has been studied using a microcalorimetric technique. The reaction takes place by two mechanisms depending on whether the surface is in an oxidised or reduced state. On the reduced surface, hydrogen is dissociatively adsorbed and competes with adsorbed molecular oxygen for adsorption sites. The reaction rate is higher on the oxidised surface where hydrogen molecules are weakly adsorbed and react with strongly adsorbed oxygen.

Dehydration of propan-2-ol on X zeolites

The dehydration of propan-2-ol on moderately protonated X zeolites leads to the formation of both propene and di-isopropyl ether at temperatures in the range 383–430 K. Unexchanged zeolite was considerably less active, and only yielded propene at temperatures in the range 444–450 K. With protonated X zeolites the rates of formation of both products were investigated as a function of temperature, pressure of propan-2-ol and the extent of protonation, which lay between 8.6 and 28.4%. Decomposition at 411 K was completely poisoned by pyridine vapour. The Bronsted acidity of the accessible hydroxyl groups within the zeolites is considered to be the source of activity, and a mechanism involving carbonium and oxonium ions is proposed.

Influence of silica and alumina supports on the temperature-programmed reduction of copper(II) oxide

Temperature-programmed reduction has been used to study the reduction of copper oxide supported on silica and alumina. It has been shown that copper oxide exists in two distinct phases when supported on silica, dispersed cupric oxide and a copper silicate species. The relative amounts of the two phases depend on surface area and copper loading, while the reducibility of the silicate phase is dependent on calcination temperature. No evidence of support interaction is found for α-Al2O3, while the complex t.p.r. profiles obtained for copper on γ-Al2O3 suggest complex support interaction.

Computer-Aided Video Exposure Monitoring

A computer-aided video exposure monitoring system was used to record exposure information. The system comprised a handheld camcorder, portable video cassette recorder, radio-telemetry transmitter/receiver, and handheld or notebook computers for remote data logging, photoionization gas/vapor detectors (PIDs), and a personal aerosol monitor. The following workplaces were surveyed using the system: dry cleaning establishments--monitoring tetrachoroethylene in the air and in breath; printing works--monitoring white spirit type solvent; tire manufacturing factory--monitoring rubber fume; and a slate quarry--monitoring respirable dust and quartz. The system based on the handheld computer, in particular, simplified the data acquisition process compared with earlier systems in use by our laboratory. The equipment is more compact and easier to operate, and allows more accurate calibration of the instrument reading on the video image. Although a variety of data display formats are possible, the best format for videos intended for educational and training purposes was the review-preview chart superimposed on the video image of the work process. Recommendations for reducing exposure by engineering or by modifying work practice were possible through use of the video exposure system in the dry cleaning and tire manufacturing applications. The slate quarry work illustrated how the technique can be used to test ventilation configurations quickly to see their effect on the workers personal exposure.

Kinetics of methanol oxidation over platinum wire catalysts

The oxidation of methanol at atmospheric pressure over platinum wire catalysts has been studied over a wide range of experimental conditions (T= 310–660 K, PO2/PCH3OH= 2–100). Two experimental techniques were used. In the conventional flow-reactor experiments kinetic parameters were obtained from measurement of gas-phase concentrations, while in the microcalorimeter experiments kinetic parameters were derived from the rates of heat generation at the catalyst surface.At 373 K, for PO2/PCH3OH= 25, a point within the experimental range common to both techniques, the specific rates of oxidation determined by each method were 120 nmol s–1 cm–2(conventional flow reactor) and 100 nmol s–1 cm–2(microcalorimeter). The use of the microcalorimeter enabled kinetic measurements to be made over the full experimental range. It was found that the observed kinetics were dependent on both temperature and oxygen : methanol ratio. Thus in the general rate expression: R=kPmCH3OHPnO2 the values of m and n were found to vary between 0 and ½(PO2/PCH3OH⩽ 5, T 405 K, all ratios).Non-steady-state kinetics were also observed for PO2/PCH3OH= 5 at temperatures between 380 and 405 K.A mechanism, which is consistent with the observations, is proposed involving the interaction of gaseous methanol with adsorbed atomic oxygen to produce adsorbed methoxy species. These species react with further oxygen to produce formaldehyde or carbon dioxide.

Decomposition of propan-2-ol on pure and doped rutile

The decomposition of propan-2-ol on pure rutile and rutile doped with 0.1 mol% Cr2O3, Fe2O3, NiO, Nb2O5 or WO3 has been studied between 438 and 519 K using a flow system. Rates of dehydration to propene and dehydrogenation to acetone were independent of propan-2-ol pressure between 2.9 × 102 and 4.4 × 103 N m–2, whereas rates of partial dehydration to di-isopropyl ether exhibited a low pressure dependence. Arrhenius parameters for the formation of all three products have been determined. Poisoning experiments indicate that dehydration proceeds by an acid-base mechanism, in which the effects of doping are “geometric” rather than “electronic”. Although doping influences dehydrogenation, the effects cannot be correlated with “geometric” or “electronic” factors.

Oscillations of the platinum-catalysed oxidation of carbon monoxide induced by inhibitors

Isothermal oscillations in the heterogeneous oxidation of carbon monoxide on platinum have been induced in an open system by the addition of low concentrations of organophosphorus compounds to the system.