Research Article pubs.acs.org/acscatalysis
CO2 Electroreduction Performance of Transition Metal Dimers Supported on Graphene: A Theoretical Study Yawei Li,†,‡ Haibin Su,†,§ Siew Hwa Chan,†,∥ and Qiang Sun*,†,‡ †
Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602 ‡ Department of Materials Science and Engineering, Peking University, Beijing 100871, China § School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 ∥ School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 S Supporting Information *
ABSTRACT: Graphene-based materials are being hotly pursued for energy and environment applications. Inspired by the recent experimental synthesis of Fe2 dimer supported on graphene (He, Z.; He, K.; Robertson, A. W.; Kirkland, A. I.; Kim, D.; Ihm, J.; Yoon, E.; Lee, G.-D.; Warner, J. H. Nano Lett. 2014, 14, 3766−3772), here using large-scale screening-based density functional theory and microkinetics modeling, we have identified that some transition metal dimers (Cu2, CuMn, and CuNi), when supported on graphene with adjacent single vacancies (labeled as XY@2SV), perform better in CO2 electroreduction with reduced overpotental and enhanced current density. Specifically, Cu2@2SV is catalytically active toward CO production, similar to Au electrodes but distinct from bulk Cu; MnCu@2SV is selective toward CH4 generation, while NiCu@2SV promotes CH3OH production because of the difference in oxophilicity between incorporated Mn and Ni. The advantages of the outstanding selectivity of products, the high dispersity of spatial distribution, and the reduced overpotentials allow these new systems to be promising catalysts, which will motivate more experimental research in this direction to further explore graphenebased materials for CO2 conversion. KEYWORDS: CO2 reduction, graphene-supported metal dimers, density functional theory, microkinetics modeling, electrocatalysis
1. INTRODUCTION CO2 catalytic reduction has been hotly pursued for a long period of time because of its practical importance in alleviating the greenhouse effect as well as generating artificial fuels and chemicals.1 Among the various methods for achieving CO2 conversion, electrocatalytic CO2 reduction has received much attention because of its ambient aqueous operation environment. Furthermore, the reaction process can be readily controlled by changing the applied potential.2 In particular, heterogeneous electrocatalytic CO2 reduction that occurs on a solid electrode−liquid electrolyte interface not only benefits the efficient generation of desired products with a longer catalyst life cycle but also allows us to investigate the catalytic mechanism of the surface and electrolyte, as well as the interaction between them. A detailed understanding of the catalytic mechanism may help us to improve the catalyst performance with a clear materials design strategy. In recent years, much experimental and theoretical effort has been devoted to the area of heterogeneous electrocatalytic CO2 reduction,3−9 and most of it has focused on electrodes composed of extended metal surfaces. However, theoretical investigations reveal that intrinsic scaling relationships between © XXXX American Chemical Society
CO and other carbon-end intermediates as well as OH and other oxygen-end intermediates exist universally among these extended metal surfaces because of the well-known d-band theory.10,11 These scaling relationships hamper further enhancement of the catalytic performance in CO or CH4 production from CO2 from the perspective of both overpotential reduction and current density development. In contrast to the cases on single-crystal metal surfaces, scaling relationships between crucial adsorption intermediates on metal nanostructures are found to be broken because of the versatile geometrical flexibility and electronic structure effects.12 In recent years, major progress has been made in the area of nanocatalysts for CO2 electroreduction, with the reports that Cu, Au, and Pd nanoparticles, nanowires, and nanocrystals exhibit considerable improvements in catalytic activities.13−19 Nevertheless, the activity of these materials is strongly dependent on size and shape, and only partial edge, corner, or grain boundary surface atoms are liable for this activity Received: June 5, 2015 Revised: September 27, 2015
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DOI: 10.1021/acscatal.5b01165 ACS Catal. 2015, 5, 6658−6664
Research Article
ACS Catalysis enhancement. Furthermore, the ideal nanoparticle sizes as well as the catalytically active sites vary substantially with respect to the constitution of different metals, imposing extra difficulties in synthetic methods and mechanism analyses.16,19 Therefore, downsizing these metal nanostructures to singly dispersed metal atoms is highly desirable for maximizing the efficiency of catalytically active metal sites.20,21 In recent years, single metal atoms like Pt and Au anchored on various oxides with fine dispersion have been found to be highly active in preferential CO oxidation and water−gas shift (WGS) reaction, and their catalytic mechanisms are welldocumented.22−27 However, because of the fact that the oxygen vacancies on the supporting oxides are highly oxidative,21,25 application of these single-atom catalysts to CO2 reduction has been scarce. The question of whether dispersed atomic sites can be tuned to reductive conversion of CO2 using a different supporting material then arises. Here, inspired by the successful fabrication of graphene-supported Fe dopant pairs,28 we systematically investigate CO2 electroreduction on various graphene-supported first-row transition metal dopant pairs. The adjacent graphene single vacancy supported Cu2, MnCu, and NiCu pair dopants are identified as promising candidates for catalyzing CO2 electroreduction to CO, CH4, and CH3OH, respectively, with a substantially reduced overpotential. Furthermore, the unique geometry of the metal dopant pair provides extra stabilization of COOH and CHO intermediates, while the positively charged metal sites as well as the electronic interaction between Cu and Mn/Ni weaken CO adsorption. The combination of these effects is able to account for the enhanced catalytic activity.
Figure 1. Schematics for Fe2@TV, Fe2@QV, and MN@2SV structures (M or N = Mn−Cu).
periodic images. The reciprocal space was sampled using a 5 × 5 × 1 Monkhorst−Pack32 mesh, and the geometries were fully relaxed until the maximal residual force was
