The stability of Pd/TS-1 and Pd/Silicalite-1 for catalytic oxidation of methane - understanding the role of titanium
The stability of Pd/TS-1 and Pd/Silicalite-1 catalysts was assessed for catalytic combustion of fugitive methane emissions at low concentrations, aiming to understand the role titanium plays for the stability of the catalyst. The long-term stability of both catalysts is attributed to both hydrophobicity and low acidity of the aluminium-free zeolites. The structure and bonding of Pd deposited on the TS- 1 catalyst was fully evaluated using a number of techniques including EXAFS, XPS and TEM. Pd/Silicalite-1, in contrast to Pd supported on TS-1, displayed a gradual deactivation over 30 h of time-on-stream. This deactivation occurred coincident with the agglomeration of palladium atoms on the surface of this catalyst as well as carbon deposition, whereas this phenomenon was not observed for the Pd/TS-1 material. It is concluded that the hydrophobicity and low acidity is the reason for the high stability and activity of Pd supported on TS-1. In contrast, the dispersion of palladium in Pd/Silicalite-1 (and indeed Pd on alumina) decreased remarkably and carbon deposits were formed on the catalyst, contributing to the deactivation We showed by both, Ti K and Pd K EXAFS analysis an anchoring role of TI for Pd and confirmed the location of the Pd active in the combustion. It is suggested that the Ti in T positions of the TS-1 framework form the sites for palladium particles at an average distance of 2.5 Å and inhibits the sintering of palladium particles. In contrast to acidic supports, we did not observe deactivation by deposition of carbon containing species. Hence, the notable stability of Pd/TS-1 over 1900 h of reaction results in Pd/TS-1 being the most promising catalyst for methane combustion in industrial applications.