Platinum Metals Rev., 1993, 37, (4), 210
Limiting Environmental Pollutants
Catalytic Control of Air Pollution: Mobile and Stationary Sources ACS Symposium Series 495 Edited By R. G. Silver, J.E. Sawyer and J.C. Summers, American Chemical society, Washington D.C., 1992, 176 pages, ISBN 0-8412-2455-2, U.S.$49.95
To improve the quality of the air has become a growing concern around the world. The use of catalysis to limit the amount of pollution released into the atmosphere is by now well established, and the importance of platinum group metals in this activity is fully recognised. Nevertheless, as the regulations are progressively tightened, there is an increasing need to maximise the efficiency of existing catalysts and to develop new catalysts.
This volume encapsulates papers from a symposium, sponsored by the Division of Colloid and Surface Chemistry of the American Chemical Society, held in New York, in 1991. The first part of the volume deals with emission control for mobile sources, while the second is devoted to stationary sources. With few exceptions, the papers contained describe investigations based on platinum group metal catalysts.
The first section begins with a review by J. C. Summers and R. G. Silver of Allied Signal on legislation in the U.S. and elsewhere and the catalyst technologies which are being investigated to meet the legislative requirements. This theme is expanded in a subsequent paper by J. C. Summers, W. B. Williamson and J. A. Scaparo of Allied Signal, who discuss specific catalyst technologies based on palladium.
The beneficial effects of ceria on platinum-based catalysts is reported by J. G. Nunan, R. G. Silver of Allied Signal and S. A. Bradley of UOP, who contribute to the by now substantial literature on this subject. They state that after reduction in hydrogen or rich exhaust gas there appears to be a promotional effect of cerium which extends beyond oxygen storage effects, water gas shift promotion and platinum dispersion. This effect relates to a platinum-cerium interaction and the synergistic reduction of platinum and surface cerium.
Topics of much current interest are the platinum group metal interaction with support materials and the interaction of nitric oxide with platinum group metals. These factors, and the influence of metal particle size, are the subject of a paper by G. Zafiris, S. I. Roberts and R. J. Gorte of the University of Pennsylvania, who have examined nitric oxide adsorption on rhodium supported on zirconia and α-alumina, and platinum on ceria, α-alumina and zinc oxide. For rhodium, nitric oxide reduction appears to be structure sensitive due to changes in nitrogen desorption rates; whereas, for platinum, nitric oxide decomposition limits reaction rates and this may be affected by metal-support interaction. The use of steady state isotopic transient kinetic analysis in the study of carbon monoxide oxidation and the carbon monoxide-nitric oxide reaction of a platinum supported on rhodium three-way catalyst is reported by R. Oukaci, D. G. Blackmond, J. G. Goodwin and G. R. Gallaher from the University of Pittsburgh. They suggest that the distribution of sites for carbon dioxide formation is bimodal, with less than 30 per cent of the active sites accounting for 75 per cent of the catalyst activity. Since a similar bimodal activity was also found for rhodium supported on La2 O3, the authors conclude that low activity (but more stable) sites could be associated with a metal-rare earth interaction.
The oxidation of hydrocarbons and oxygenated compounds commonly found in exhaust is examined by J. M. Bart, M. F. Prigent of Institut Français du Pétrole and A. Pentenero of the Université de Nancy. They find that as the oxygen concentration is increased the rate of removal of hydrocarbon is dependent upon the hydrocarbon species. In the case of oxygenated compounds, the extent of removal appears to be independent of the number of carbon atoms in the molecule. Addition of sulphur dioxide into the gas mix generally inhibits conversion of hydrocarbons but, under stoichiometric and lean conditions, hydrocarbons with a carbon number less than 3 show increased activity.
Natural gas is being considered as an alternative fuel for automotive applications in many parts of the world. However, methane is a powerful greenhouse gas, so emissions of unburnt methane are of particular concern. The paper by S. H. Oh, P. J. Mitchell and R. M. Siewert, from General Motors, examines the relative performance of platinum, palladium and rhodium on alumina for methane oxidation in a simulated exhaust gas. The results show the order of activity to be palladium > rhodium > platinum, and that optimum performance is obtained slightly rich of stoichiometry. The authors consider some of the mechanistic implications of these results, and conclude that high surface concentrations of oxygen inhibit methane adsorption and reaction, and that water gas shift reactions are unimportant in the system, except possibly for rhodium catalysts.
The second section of this volume deals with pollution control of stationary sources and also begins with a review by J. C. Summers, J. E. Sawyer and A. C. Frost of Allied Signal, who examine the implications of the U.S. 1990 Clean Air Act. Two other papers in this section deal with laboratory investigations of the catalytic oxidation of trichloroethylene (TCE) over platinum and PdO on alumina. TCE is typical of a range of widely used industrial solvents. The principal products over both catalysts are carbon dioxide, chlorine and hydrogen chloride; the production of chlorine arises from each TCE molecule having a hydrogenxhlorine ratio of 1:3. However, if water or methane is introduced into the reaction system, sufficient hydrogen is provided to increase the proportion of hydrogen chloride, with a corresponding reduction in chlorine. In addition, these reductants lower the proportion of the intermediate tetrachloroethylene appearing in the gas stream.
Thus the emphasis on catalysis, particularly using the platinum group metals, to control gaseous emissions continues to be very great. The full understanding of how these metals can be promoted to achieve extra performance remains elusive. By sponsoring and publishing symposia of this kind a gradual understanding is being built up.