Short-Chain Alkane Activation

Abstract

SO 2 chemisorption on oxygenated Pt(111) enormously enhances the dissociative chemisorption and subsequent combustion of propane. This activation of the metal surface is induced by an adsorbed sulfoxy species which is formed > 220 K. In the absence of adsorbed SO 2 the sticking probability of propane is immeasurably small. However in the presence of SO 2 , the precursor-mediated initial sticking probability rises from ~0.02 at 300 K to ~ 0.15 at 160 K. XPS and HREELS measurements identify the active species as SO4 2-, and also demonstrate the consumption of SO 4 during the oxidation reaction. Coincident CO 2 and SO 2 formation suggest decomposition of a complex reaction intermediate: this is supported by isotope experiments involving CO adsorption onto SO 2 / 18 O2 precovered Pt(111). Propane oxidation over sulphated AlO X films on Pt (111) is also reported, with increased activity for submonolayer oxidised Al films being observed compared to clean Pt(111).The activation of short chain alkanes is often the rate limiting step in catalytic oxidation of hydrocarbons [1]. In particular, the slow oxidation of alkanes, namely propane and methane, is a technological problem faced by the automotive exhaust catalyst industry [2]. Reactor studies of Pt/Al203 and Pt-Rh/Al203/Ce02 catalysts have shown that the inclusion of SO2 in the gas feed significantly promotes the oxidation of propane [3,4], whilst inhibiting reactions of CO, NO and propene. It has also been observed that in the presence of SO2, the activity of Pt/Al203 catalysts for propane oxidation becomes independent of Pt loading. In the absence of SO2, the activity usually increases with Pt loading, a result which suggests the formation of different active sites on inclusion of SO2 in the gas feed. Infra-red analysis of these catalysts following exposure to O2 and SO2, indicates the presence of S04^" on the support, which is thought to be involved in the activation process. Enhanced C-H bond dissociation is often attributed to catalyst acidity [5]; for example homogeneous Pt(III) solutions display increased oxidative capacity in the presence of sulphuric acid.

SO2-Promoted Chemisorption and Oxidation of Propane over Pt (111)

“…The identity of the SO, species and further aspects of the mechanism of propane adsorption and reaction on Pt(lll) are currently under investigation by HREELS and XPS. 19 Conclusions A Pt(lll) surface precovered by O2 and SO2 at 300 K is highly active for the dissociative chemisorption and subsequent combustion of propane. The precursor-mediated initial sticking probability of propane on the SO,-activated surface varies between ~0.02 (300 K) and ~0.15 (160 K).…”

Electronic, Structural, and Reactive Properties of Ultrathin Aluminum Oxide Films on Pt(111)

“…S 2p XP spectra were recorded after 300 K exposure of AlO x films to 100 L of O 2 and 50 L of SO 2 as a function film thickness (Figure ). On clean Pt(111), surfaces pretreated in this way are active for C 3 H 8 dissociative chemisorption, an effect that is induced by the presence of SO 4 . ,, However, this species is unstable with respect to electron-stimulated dissociation 9,19 so that XP spectra of sulfated clean Pt(111) show only a weak emission at 164.5 eV BE, similar to the S 2p state resulting from 300 K adsorption of H 2 S . This emission is attributed to atomic sulfur (S a ) resulting from electron-stimulated decomposition (ESD) of adsorbed SO 4 , which occurs on the metal surface during the time scale of the XPS experiments. , However, in the presence of 0.5 ML of AlO x , two S species were observed at ∼164.5 and ∼170 eV BE, respectively.…”

“…The key activating species on Pt(111) has been identified as SO 4 . This increases the dissociative sticking probability of propane, probably by H abstraction and formation of an adsorbed alkyl species that undergoes combustion. , On powdered alumina, SO 2 readily forms Al 2 (SO 4 ) 3 at ∼420 K .…”