Temperature programmed desorption of carbon monoxide adsorbed on supported platinum catalysts

Abstract

Temperature programmed desorption (TPD) measurements have been made for CO from Pt/aerosil ( d Pt ≈ 2.5 nm), Pt γ-Al 2O 3 ( d Pt, 2.4 nm) and Pt-Au/aerosil ( d Pt, 1.5–2.0 nm) catalysts. CO was adsorbed at room temperature, with TPD at 35 K min −1. On Pt/aerosil and Pt/γ-Al 2O 3 the TPD profile showed a weak maximum at 388–398 K, with the rest of the profile continuining with very substantial strength out to about 800 K. Increasing the gold content caused an enhancement of the peak at around 390 K relative to the rest of the profile. Adsorbed CO was recovered quantitatively by 860–870 K, while on Pt/aerosil a second adsorption/TPD sequence without intervening catalyst clean-up gave an identical profile, confirming negligible dissociative chemisorption of CO on Pt under these conditions. A small amount of CO 2 was desorbed along with the CO (CO 2 ≈ 6–7% of CO on Pt/ and Pt-Au/aerosil; CO 2 ≈ 12% of CO on Pt γ-Al 2O 3 ). Evidence is given for believing that the CO 2 originated from a catalysed reaction between CO and traces of residual H 2O (the latter from the support) rather than from CO disproportionation. Monolayer uptake measurements with CO and H 2 gave values for the ratio CO( ads)/H( ads) : Pt/aerosil, 0.75; Pt γ-Al 2O 3 , 0.84; Pt76-Au24/aerosil, 0.98; Pt15-Au85/ aerosil, 1.3.1. Reasons for the effect of Au on CO adsorption on Pt-Au are discussed.