Revisiting the Problem of Active Sites for Methane Combustion on Pd/Al2O3 by Operando XANES in a Lab-Scale Fixed-Bed Reactor

The behavior of Pd in 2 wt % Pd/Al 2 O 3 (a commercially relevant catalyst) is monitored under genuine conditions by operando X-ray absorption spectroscopy during the methane combustion cycle between ca. 400 and 1130 K in a lab-scale fixed-bed reactor connected to a mass spectrometer for product analysis. This operando setup allows us to circumvent intrinsic limitations of in situ reaction cells and thus to obtain more accurate and reliable data on the structure−activity relationships. The catalyst is characterized before (fresh) and after the reaction cycle (used) by N 2 physisorption, CO chemisorption, HAADF-STEM, and in situ EXAFS to study the influence of the reaction on the nature and distribution of Pd species in the catalyst. The nature of active Pd species for the combustion of methane reaction is temperature-dependent, as evident from operando XANES. Pd oxide species are essential for the reaction, especially at reaction temperatures below 950 K, and above this temperature, reduced Pd can also catalyze the reaction. Characterization of fresh and used catalysts reveals that the dispersion of Pd decreases and that the mean Pd oxide particle size increases after treating the catalyst to 1130 K, which in turn leads to the increased combustion activity at low reaction temperatures (<650 K) during cooling from 1130 K. This suggests that the reaction is dependent on Pd oxide particle size.