Superior Catalytic Performance of Atomically Dispersed Palladium on Graphene in CO Oxidation

Abstract

Although substantial advances have been made in a few reactions of industrial significance over single-atom catalysts (SACs), the origin of the superior catalytic performance, the nature of the active sites, and the reaction pathways are still the subject of debate. Even for CO oxidation over SACs on nonreducible substrates, the understanding is limited. We investigated the performance of Pd atoms monodispersed on graphene (PdGr) in CO oxidation. Combining firstprinciples-based thermodynamics calculations and microkinetics modeling, we showed that the positively charged PdGr can exhibit a rather high low-temperature activity in CO oxidation. Under reaction conditions, the Pd atom binds strongly with O 2 , acting as the reactive species to convert CO. A comparison of the conversion rates of steps along different potential reaction pathways provides direct evidence that CO oxidation mainly proceeds through revised Langmuir−Hinshelwood pathways, and the dissociation of the peroxide intermediate (O−O−CO) is the ratelimiting step. The predicted catalytic performance was attributed to the specific electronic structure of PdGr with the positively charged Pd on graphene monovacancy exposing sp-type frontier states. We expect these findings to help in understanding the performance of SACs and to guide the design and fabrication of SACs with superior catalytic performance.

Engineering the Coordination Sphere of Isolated Active Sites to Explore the Intrinsic Activity in Single-Atom Catalysts

“…The authors attribute the exceptional catalytic performance to the obviously reduced capability of the Au 1 SACs to dissociate H 2 in the catalytic process. Meng and colleagues have also tracked the origin of the superior CO oxidation performance of the isolated Pd atoms decorated graphene (PdGr) [ 101 ]. Combining DFT simulations and micro-kinetics modeling, the authors have confirmed that the positively charged PdGr can bind strongly with O 2 , acting as the reactive species to convert CO.…”

The formation and evolution of carbonate species in CO oxidation over mono-dispersed Fe on graphene

“…This barrier is already more significant than the calculated E ad of O 2 (À1.81 eV); therefore, O 2 may desorb prior to dissociation. 34,39,78 As O 2 adsorption is more plausible than CO adsorption, Eley-Rideal-type pathways can only be initiated with the reaction between gaseous CO or CO in van der Waals complexes formed between CO and preadsorbed O 2 . 23 According to the symmetry of the molecular states of gaseous CO and O 2 , only the interactions between 5s of CO gaseous with the p* states of O 2 in the direction with O(O 2 )-C-O of B1201 is allowed, and the reaction may lead to direct formation of CO 2 .…”

“…8,9 Recently, considerable attention has been continuously paid to the controlled synthesis and application of heterogeneous transition metal SACs that inherit not only the merits of enzymes and metal complexes with each mono-dispersed transition metal atom as a reaction center for efficient chemical conversion, but also the advantages of heterogeneous catalysts for the reusability of the catalyst and easy product separation. [10][11][12][13][14][15][16] Many graphene-derived SACs of Fe, 8,9,[17][18][19][20][21][22][23][24][25][26][27][28] Ni, 29 Co, 18 Mn, 30 Pd, [31][32][33][34][35][36] Au, [37][38][39][40] Pt, [41][42][43] Ru, 44 etc., have been investigated and were proposed to be efficient in many processes of industrial significance. 45 Fe-Based SACs on graphene and graphenerelated materials were shown to be effective for the oxidation of arenes, 8 oxygen reduction, 9,[17]…”

“…The dominant role of the electronic structure in the performance of a specific catalyst has been well recognized. 34,45,46 Considering the monodispersed under-coordinated nature of metals in SACs and the dynamic adsorption and conversion of substrates in reaction conditions, the impact of reaction species on metal centers would be more pronounced, and it may stabilize or even drive further evolution of metal centers, switch the reaction mechanisms and contribute to the conversion and product selectivity. [47][48][49][50][51] To this end, mechanism investigations on SAC-catalyzed chemical conversion in conditions relevant to experiments would be more helpful to rationalize the understanding of the observed superior performance of SACs.…”

“…[47][48][49][50][51] To this end, mechanism investigations on SAC-catalyzed chemical conversion in conditions relevant to experiments would be more helpful to rationalize the understanding of the observed superior performance of SACs. 34,45 CO oxidation is one of the most investigated heterogeneous reactions. Due to the strong affinity of conventional transition metals to CO, O 2 activation and dissociation in CO oxidation conditions are demanding.…”