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.