Further insights on the physico-chemical aspects of PdIn-Hmordenite catalysts

Abstract

The addition of Pd improves the catalytic performance of InH mordenite for the SCR of NO with methane, but this effect depends on the In/Al ratio. We studied two PdInMordenite samples, one with In/Al = 1/3 (PdInHM(1/3)) and the other with In/Al = 1/6 (PdInHM(1/6)). While in the former the presence of Pd in the InHM(1/3) catalyst increased the activity, in the sample with lower In content this effect was not observed. In order to explain these observations, the catalysts were characterized with several techniques. TPR results showed that the presence of palladium shifted the reduction peak of the solid PdInHM(1/3) towards higher temperatures, while on solid PdInHM(1/6), the shift was negligible. This effect could be ascribed to a higher Pd-In interaction in the PdInHM(1/3) catalyst. FTIR of CO adsorbed showed the presence of Pd2+, Pd+ and Pd0 adsorption sites in the calcined PdInHM(1/3). However, in the case of PdInHM(1/6), there was no evidence of Pd2+ species. The TEM images of used PdInHM(1/3) catalysts suggested the mobilization of indium to inner positions into the matrix, thus increasing the interactions with Pd. In the fresh PdInHM(1/6) the indium particles could not be detected but after reaction some agglomeration of particles could be observed, which is in agreement with CO-FTIR and XPS results. In brief, the results indicate that in the PdInHM(1/3) catalyst, Pd and In interact closely, giving place to a promotional effect of Pd, which prevents the agglomeration of In species.