THE INFLUENCE OF PLATINUM CRYSTALLITE SIZE ON H-2 AND CO HEATS OF ADSORPTION AND CO HYDROGENATION

Integral heats of adsorption for H, and CO at 300 K were determined on SiO2-supported Pt crystallites over a range of 1.3 to 22.3 nm and compared to those on Pt powder. No significant trend with crystallite size was observed. For H2, the average value of 13.5 ± 2.9 kcal/mol for saturation coverage was the same as that on Pt powder, while the average value of 17 ± 5 kcal/mol for irreversible hydrogen chemisorption on the small particles was slightly lower than that on Pt powder (19.8 ± 0.8 kcal/mol). The average Qad value of 29.4 ± 3.8 kcal/mol for CO on the dispersed Pt crystallite was slightly higher than that of 27.0 ± 1.0 kcal/mol for the Pt powder, and although somewhat higher values occurred for the smallest Pt particles, the distinct increase in Qad values previously found for Pd crystallites smaller than 3 nm was not observed with Pt. No trend in the variation of the turnover frequency (TOF) for methanation with crystallite size was observed under our reaction conditions; thus this reaction is structure insensitive. However, low Pt loadings gave catalysts with lower TOFs and higher activation energies and replicated the variations in TOF reported in UHV studies of Pt. The reasons for this are not known, but the possibilities exist that either two reaction pathways may exist or the active sites on the Pt are significantly modified. © 1991.