The Specific Activity of Silica Supported Platinum for the Catalysis of Hydrogen–Methane Deuterium Exchange

Abstract

Rate data are reported as a function of temperature for the exchange of D2 with CH4 over a number of platinum–silica catalysts. Two different methods of preparing catalysts, impregnation and ion exchange, were used to get two series of metal particle sizes. Within each series, the particle size was increased by sintering batches in air at temperatures to 800 °C. These methods produced platinum particles ranging from 0.9 to 10 nm diameter. The metal surface areas were measured by hydrogen chemisorption and, where possible, by X-ray diffraction and electron microscopy.The specific rate coefficients or rate coefficients per unit surface area of platinum varied by a factor of up to 40 within each series and reached a maximum for sintering temperatures of about 500 °C. The maximum specific rate coefficients for CH4 conversion at 200 °C, and total pressure of 60 kN m−2, were 1.3 and 1.9 × 10−6 mol CH4 m−2 s−1 for the ion-exchanged and impregnated catalysts respectively. Activation energies were in the range 60 to 100 kJ mol−1. Since a relatively small change in specific rate coefficient was noted between the two series as compared to the changes within each series, no large effect of particle size on the specific rate coefficient was demonstrated. The different specific rate coefficients within the series are ascribed to other effects of sintering, particularly the formation of the lower energy Pt(111) surface. Some evidence was noted for a particle size effect in the distribution of products, with more multiple exchange being observed for the smallest particles.