Oxidation State and Symmetry of Magnesia-Supported Pd₁₃O_xNanocatalysts Influence Activation Barriers of CO Oxidation

Abstract

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd 13 clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd 13 cluster, leading to facile partial oxidation to form Pd 13 O 4 clusters with C 4v symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd 13 O 6 clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd 13 O 4 cluster. Subsequent reactions on the resulting lower-symmetry Pd 13 O x (x < 4) clusters entail lower activation energies. The nonsymmetric Pd 13 O 6 clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.

Reprint of “Mass-selected supported cluster catalysts: Size effects on CO oxidation activity, electronic structure, and thermal stability of Pd /alumina (n≤ 30) model catalysts”

“…We also recently observed a very strong size-dependent correlation between Pt electronic structure and electrocatalytic activity for Pt/glassy carbon, where some sizes showed the expected Pt redox and oxygen reduction activity, but other sizes efficiently catalyzed electro-oxidation of carbon by water [2]. The Heiz group has reported numerous studies of catalysis with recent work including CO oxidation kinetics over Pd clusters deposited on MgO [13,14,[58][59][60][61], optical spectroscopy of deposited clusters and adsorbates [62,63], studies of oxide support thickness effects on cluster activity [15,64], Pt n deposition on CdS nanorod films for photocatalysis studies [65], and STM studies of cluster mobility on graphene/Ru(0001) Moiré supports [66]. In collaboration with Arenz, this group has also reported studies of electrocatalytic activity of supported Pt clusters [67][68][69][70][71].…”

“…Catalysts are important in 35% of the world GDP [1], thus, it is not surprising that there is extensive fundamental and applied research aimed at improving catalytic activity and selectively [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Typical supported catalysts are quite complex and difficult to characterize in detail, and an important research thrust uses model catalysts to simplify the problem and allow important properties to be varied and measured in detail, to extract mechanistic insight.…”

Spectroscopy for model heterogeneous asymmetric catalysis

“…Our understanding of heterogeneous catalysis in general (not only for asymmetric catalysis) is still lagging behind compared with homogeneous catalysis. However, the latest studies on model heterogeneous catalysts under controlled conditions have been gaining enormous momentum [48][49][50][51][52] and have led to speeding advances in the field even under more realistic conditions. [53][54][55][56] This progress has only been possible as a result of the collaboration between theoretical researchers and experimentalists.…”

Application of Computational Methods to Supported Metal–Oxide Catalysis

“…Notable contributions have been made by Landman and coworkers, who applied DFT in conjunction with isotopic labeling experiments to determine the oxidation state of Pd clusters supported on MgO during CO oxidation. 104 This allowed them to determine that partially oxidized Pd clusters are active toward CO oxidation, along with the temperature range in which the clusters will remain oxidized. In another study, these authors used quantum (QM) calculations in tandem with infrared spectroscopy to demonstrate that charging effects lead to enhanced CO oxidation activity over Au/MgO catalysts.…”