Mechanistic understanding of methane combustion over H-SSZ-13 zeolite encapsulated palladium nanocluster catalysts

Catalytic methane (CH 4 ) combustion to CO 2 and H 2 O is of great practical significance and an important prototype catalytic reaction. Extensive experimental studies have suggested that zeolite supported palladium (Pd) catalysts are very active for catalytic CH 4 oxidation, while the structure-performance relationship is still not clear. Herein, using H-SSZ-13 zeolite encapsulated Pd nanoclusters as a demonstration case, reaction mechanisms and kinetics of complete CH 4 combustion were systematically investigated using first-principles density functional theory (DFT) calculations combined with atomistic thermodynamic analysis and the energetic span model (ESM). Four H-SSZ-13 zeolite encapsulated Pd Ⅱ x O m H n model catalysts, i.e., Pd Ⅱ /H-SSZ-13, Pd Ⅱ O/H-SSZ-13, Pd Ⅱ 2 O/H-SSZ-13, and Pd Ⅱ 3 O 3 H/H-SSZ-13 were studied. The encapsulated [Pd Ⅱ 2 O] 2+ and [Pd Ⅱ 3 O 3 H] + nanoclusters were identified as the most stable binuclear and trinuclear Pd structures under experimental oxidative conditions. DFT calculations indicated that the most kinetically relevant step in the complete CH 4 oxidation reaction over four Pd model catalysts is different. The first, third, third, and fourth C–H bond cleavage were identified as the most kinetically relevant steps over Pd Ⅱ /H-SSZ-13, Pd Ⅱ O/H-SSZ-13, Pd Ⅱ 2 O/H-SSZ-13, and Pd Ⅱ 3 O 3 H/H-SSZ-13, respectively. Using the energetic span model, the relative turnover frequencies of CH 4 oxidation over four Pd Ⅱ x O m H n /H-SSZ-13 model catalysts were calculated. Consistent with recent experimental observation, the encapsulated Pd Ⅱ 3 O 3 H/H-SSZ-13 nanocluster was found to be the most active catalyst for complete CH 4 oxidation. On the basis of DFT results and the ESM model analysis, it is noted that the reaction rate of complete CH 4 oxidation over the Pd Ⅱ x O m H n /H-SSZ-13 catalysts is not completely dependent on one “highest” activation barrier, i.e., the barrier for the most kinetically relevant step. Neither one specific transition state nor one reaction step carried all the kinetics information that determines the catalytic performance of the Pd Ⅱ x O m H n /H-SSZ-13 catalysts. The present work sheds light on the structure-performance relationship for CH 4 combustion over the zeolite supported noble metal catalysts.