Mechanisms behind sulfur promoted oxidation of methane

Abstract

The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied using a flow-reactor, in situ infrared spectroscopy and in situ high-energy X-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in the absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SO(x) species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporarily enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed.

Sulfur Poisoning Effects on Modern Lean NOx Trap Catalysts Components

“…These bands increased in intensity and shifted to higher frequencies with extended SO 2 exposure. The observed shift has been previously reported by several authors [30][31][32], ascribed to either pure frequency shift of a single band caused by the change in the strength of molecular interactions or the combined effect of two overlapping bands that differ in relative intensity. The shift in frequency is observed only after 90 min of sulfur poisoning.…”

“…In contrast, ceria-containing catalysts required higher temperatures for sulfur removal compared to other samples, probably caused by a large formation of very stable bulk sulfates during SO 2 poisoning procedure, as reported from DRIFTS measurements. Bounechada et al [32] indeed suggested that the high dynamic oxygen storage capacity (OSC) of ceria-containing materials leads to the diffusion of surface sulfates into the bulk, blocking some pores and inhibiting the access to active sites. Meanwhile, the remaining sulfur species on Ce-containing catalysts certainly affected the NO x stored stability by reducing the amount of NO x released after sulfation.…”

“…According to the literature, the bands at 1346 and 1156 cm −1 correspond to surface and bulk sulfates [30,32,49,50], respectively. The surface sulfate can be either formed on Al 2 O 3 and/or ceria sites [29,30,32,34,49], whereas bulk sulfate is mostly associated with ceria sites [32,49]. The 1156 cm −1 band can be also associated with bulk MgSO 4 [48], however, this is not likely since it was not observed for Mg-Al 2 O 3 samples (see Figure 5).…”

Vibrational Study of SOx Adsorption on Pt/SiO2

“…The details on the preparation and characterisation of the Pt/SiO 2 sample can be found elsewhere 16 and thus we give only a brief description here. Silica powder (Kromasil Silica KR-300-10, Akzo…”

Unsteady-state operation of supported platinum catalysts for high conversion of methane

“…The samples are from here referred to as Pt/Al, Pt/AlCe and Pt/Ce, respectively. Transient activity tests were performed with monolith samples in a continuous gas-flow reactor, which has been described in detail elsewhere [21]. The monolith samples were prepared by coating 200 mg of the respective powder catalyst onto cordierite monolith substrates (L=15 mm, ?=12 mm) as described in reference [20].…”