Promotional effect of Sn addition to sulfated Pt/γ-Al₂O₃ catalysts on CH₄ combustion. Effect of C₃H₈ addition

Abstract

The effect of Sn addition to Pt/γ-Al₂O₃ and the effect of sulfating the bimetallic Pt–Sn/γ-Al₂O₃ catalysts are investigated for the combustion of methane in the presence and in the absence of C₃H₈. Sulfated Pt–Sn/γ-Al₂O₃ catalyst showed a higher thermal stability and higher activity on CH₄–O₂ reaction related to pre-sulfated 1%Pt/γ-Al₂O₃. C₃H₈ addition to the reaction feed resulted in a further strong promotional effect on methane oxidation over sulfated Pt–Sn/γ-Al₂O₃ catalysts. Reasons for these different effects are discussed. Results suggest that a sulfated Pt–Sn/γ-Al₂O₃ catalyst may eliminate methane emissions at low temperature without losing Pt active area, for these reactions and without being deactivated by sulfur.

Introduction

The activation of hydrocarbons is of interest because of the desire to functionalize inexpensive feedstocks and also because catalytic combustion offers a possible means to generate power without creating excessive amounts of nitrogen oxides. Natural gas provides an attractive source of energy for various purposes. For instance, it is used to fire gas turbine combustion chambers [1] and more recently has been reported as an alternative fuel for automotive applications [2]. The main advantages are lower levels of particulate matter and nitrogen oxides in lean burn combustion [3]. The high H/C ratio reduces the net carbon dioxide emissions when compared to other fossil fuels. Methane is the most difficult hydrocarbon to oxidize since it contains no C–C bond, but only C–H bonds, which are more difficult to break [4]. Although methane does not contribute to tropospheric ozone production, it is a potent greenhouse gas estimated to have a 20-year global warming potential 21 times that of carbon dioxide at equivalent emission rates [5]. The complete oxidation of methane can be performed over either noble metals or transition metal oxides [6], [7], [8], [9]. Few studies were made on platinum catalysts [10], [11], [12], [13] compared to palladium ones [14], [15], [16], [17], [18]. Burch et al. observed T₅₀ (temperature at 50% conversion) for Pt or Pd supported on high surface area γ-Al₂O₃ catalysts within 100 °C of each other, Pd having higher activity [13]. Natural gas vehicles exhaust contains sulfur derived from the gas and engine lubricating oil, although at ppm concentrations [19], and it is the sulfur in the exhaust that strongly deactivates Pd catalysts for methane oxidation. However, Pt based catalysts are not deactivated by sulfur in the exhaust for methane combustion. A mechanism for the oxidation of methane on Pt suggests that the metal would activate the almost non-polar C–H bonds of methane through a homolytic mechanism (dissociate adsorption of CH₄ at free metal sites), oxygen species acting as an inhibitor for the reaction at full coverage [20].

In a recent paper [21], we showed that 2 wt% tin addition to a 1%Pt/γ-Al₂O₃ catalyst, prevents Pt particles from sintering during high temperature processes, thus, Pt surface remained unchanged for oxidation reactions. Now, D. Roth et al. [7] found that SnO₂ strongly promoted the oxidation of methane over Pt catalysts. However, when Pt was supported on SnO₂ grafted on Al₂O₃ (15 wt% Sn), the activity was found after ageing, lower than Pt/Al₂O₃. Thus, on basis of these results, we investigated in this work, the effect of lower amounts of tin addition (2 wt% Sn) on Pt/Al₂O₃ activity for CH₄ oxidation.

On the other hand, in recent investigations [22], we found a strong promotional effect of the presence of 1000 ppmV C₃H₈ in the reaction feed on CH₄–O₂ reaction over pre-sulfated 1%Pt/γ-Al₂O₃. This promotion is explained in terms of the high propane combustion heat which is transferred, to activate the abstraction, of the first hydrogen of the adsorbed methane molecule. This abstraction is assumed to be the rate determining step, in the methane oxidation mechanism [20], [23]. Thus, in this work, we also investigated the effect of the presence of C₃H₈ in the reaction feed on CH₄–O₂ reaction over pre-sulfated 1%Pt/γ-Al₂O₃.