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Because of their heterogeneous nature, supported metal catalysts always contain metal centers in a rather broad dispersion range, and the presence of even atomically dispersed metals has been reported on oxide supports. The role of the atomically dispersed metal centers in the overall catalytic performances of these supported metal catalysts, however, has not been addressed to date. In this study, temperature programmed reaction and scanning transmission electron microscopy experiments were applied to show the fundamentally different reactivity patterns exhibited by Pd metal in atomically dispersed and traditional 3D clusters in the demanding reaction of CO2 reduction. The requirement for two different catalyst functionalities in the reduction of CO2 with hydrogen on Pd/Al2O3 and Pd/MWCNT catalysts was also substantiated. The results obtained clearly show that the oxide support material, even when it is considered inert like Al2O3, can function as a critical, active component of complex catalyst systems.
Kwak et al. (2013) studied this question.