Effect of Ti(IV) loading on CH₄ oxidation activity and SO₂ tolerance of Pd catalysts supported on silica SBA-15 and HMS

Abstract

Pure silica SBA-15 and HMS and corresponding Ti(IV) modified mesoporous silica, with 5 and 10 wt% of TiO₂, were prepared and used as support for palladium (1 wt%) catalysts. The materials, analysed by XPS, XRD, BET, NH₃-TPD and FT-IR techniques, were tested in the total oxidation of methane. The catalytic activity was measured in lean conditions at WHSV = 60,000 ml g⁻¹ h⁻¹ in the absence and presence of 10 vol. ppm SO₂. Moreover, the effect of a prolonged reaction aging and severe SO₂ poisoning on the catalytic performance of the best performing catalyst was investigated. The addition of TiO₂ improved the catalytic performance of the SBA-15 supported catalysts by increasing the sulfur tolerance and most importantly by favoring the regeneration of the catalyst in subsequent SO₂-free runs. An opposite behavior was observed with the palladium supported on Ti(IV)-modified HMS support which exhibited lower activity and a substantial worsening of the SO₂ tolerance as compared to palladium supported on pure HMS. On the bases of the structural and chemical investigation, the differences between the two series of catalysts were ascribed to the distinct structural and acidic properties of the supports. In particular, the good performance of the Ti(IV) doped SBA-15 supported catalysts was due to the combination of Ti(IV) structurally incorporated into the silica lattice and present as surface dispersed TiO₂ particles. The negative effect of the Ti(IV) over the HMS supported catalysts was related to the high acidity induced by the more homogeneous incorporation of Ti(IV) into the silica structure.

Introduction

Natural gas, mainly consisting of methane, is increasingly replacing gasoline or diesel as fuel for transportation vehicles [1], [2]. The reason for this change is the decline of the petroleum reserves and the lower emission of pollutants associated with the combustion of methane. Since the natural gas fuelled vehicles (NGV) typically run at low temperature (320–420 °C) the emission of NO_x is lower and due to the high H:C ratio also the produced CO₂ is less as compared to the other fuel driven vehicles [2]. Nevertheless a major concern related to the use of natural gas is the emission of the unburned methane which has an even stronger greenhouse effect as compared to the CO₂ [3]. Since methane is an almost inert molecule, it requires high temperature for its complete oxidation. A crucial step in the achievement of the total combustion process is the activation of the first C–H bond [4], [5]. Either homolytic bond cleavage with the formation of radicals, or heterolytic C–H bond cleavage at the acid–base pair of sites is generally considered. In any case, in order to increase the efficiency of the methane combustion at low temperature (below 600 °C) it is necessary to use a suitable catalyst. For this specific reaction, two classes of catalysts are currently investigated, those based on transition metal oxides as solid solution oxides [6], [7], perovskites [8], [9], hexaaluminate [10], and those based on noble metals [11], [12], [13], [14]. Among this latter class, Pd-based catalysts are the most active for the methane total oxidation at low temperatures. As pointed out in several studies, their catalytic activity depends strongly on the nature of the support [12], on the palladium precursors [15], [16] and on the size of the PdO particles [17]. Their major drawback is represented by their easy poisoning by sulfur derived from the gas and engine lubricating oil [11]. According to the literature, when a sulfating support like Al₂O₃ is used, palladium deactivates slowly, due to the preferential interaction of SO_x with the support, at variance with palladium over the inert SiO₂ deactivating quickly because of the direct interaction between palladium and SO_x. Nevertheless, the use of silica, in the absence of particle sintering, allows an easier regeneration of the sulfur-poisoned catalyst through the thermal decomposition of the palladium sulfate occurring at temperature above 600 °C [1], [11]. Recent studies in our group have shown that both methane conversion activity and sulfur tolerance could be improved substantially by using high surface area silica with specific characteristic of acidity and morphology. In particular, supporting palladium on mesoporous silica HMS yielded more efficient and more sulfur resistant catalysts as compared to a lower surface area silica supported catalyst [18]. Incorporation of a precise amount (10 wt%) of TiO₂ during the sol–gel preparation of amorphous silica produced an additional improvement of the sulfur tolerance during the methane oxidation in the presence of SO₂, still allowing the complete catalyst regeneration typical of a silica support [19], [20]. The positive behavior was attributed to the combination of the high surface area support and the scavenger action of the sulfating oxide like TiO₂. Within this frame, the present study intends to further improve the catalytic behavior of palladium for the oxidation of methane by using titania-doped silica mesoporous materials as catalyst supports. To this aim two series of palladium catalysts supported on Ti(IV)-modified HMS and Ti(IV)-modified SBA-15 were prepared. In order to compare with the previous sol–gel prepared titania modified silica [19], Ti(IV) was incorporated using the one step synthesis in which titanium precursor was added during the preparation of the mesoporous materials. To evaluate the effect of the support structure on the Pd activity and stability, analyses by XPS, XRD, BET, NH₃-TPD and FT-IR techniques were performed.