Article pubs.acs.org/JPCC
Electrocatalysis on Shape-Controlled Palladium Nanocrystals: Oxygen Reduction Reaction and Formic Acid Oxidation Minhua Shao,*,† Jonathan Odell,† Michael Humbert,‡ Taekyung Yu,§,∥ and Younan Xia§ †
UTC Power, South Windsor, Connecticut 06074, United States United Technologies Research Center, East Hartford, Connecticut 06108, United States § Department of Biomedical Engineering, Washington University in St. Louis, Saint Louis, Missouri 63130, United States ‡
S Supporting Information *
ABSTRACT: A systematic study was conducted on small Pd nanocrystals (5−6 nm) to understand the effects of catalyst structure and electrolyte on the oxygen reduction reaction (ORR) and formic acid oxidation (FAO). The ORR activities of Pd catalysts strongly depended on their structure and the electrolyte used. It was found that Pd cubes were 10 times more active than Pd octahedra for ORR in an aqueous HClO4 solution due to higher onset potential of OHad formation on the cubic surface. In the case of a H2SO4 solution, the ORR activity of Pd cubes was 17 times higher than that of Pd octahedra due to the stronger adsorption of (bi)sulfate on the surface of octahedral nanocrystals in addition to OHad. In alkaline solutions, however, no structure dependence was observed for ORR due to the outer-sphere electron-transfer mechanism in the potential region for Pd oxide formation. For FAO, no advantage was observed on shape-controlled Pd nanocrystals in comparison to conventional Pd catalysts. The FAO current densities, both at peak current and at 0.4 V, followed the order of conventional Pd > octahedral Pd > cubic Pd. It was hypothesized that steps and defects were more active for FAO than terraces, which could be used to explain why the shape-selective materials were less active than conventional Pd because they contained fewer defects and edge sites. been shown with superior activity for FAO.9 In addition, the cost of Pd is only 1/4 to 1/3 that of Pt, historically.6 Thus, study of ORR and FAO on Pd nanocatalysts is of great importance in understanding the structure effects at the nanometer scale, which will facilitate the design of more active catalysts. Several studies reported the shape-dependent ORR activities on Pd nanocatalysts. The specific activity of Pd nanorods prepared by Xiao et al. was 10 times higher than that of conventional nanoparticles in a HClO4 solution.10 The authors attributed the high activity to the contribution from (110) sites on the nanorods. Shao et al.11 reported the structural dependence of ORR on Pd nanocrystals with a small particle size (5−6 nm). The activity of Pd cubes was about 10 times higher than that of Pd octahedra and even more active than state-of-the-art Pt/C catalysts with an average particle size of 2.8 nm. This result demonstrated that Pd(100) sites are much more active than Pd(111) at the nanometer scale, consistent with the study involving extended surfaces.4 Erikson et al.12,13 demonstrated that cubic Pd particles covered by {100} facets and with an average size of ∼27 nm had a higher ORR activity than spherical Pd particles (2.8 nm) in 0.5 M H2SO4 and 0.1 M NaOH solutions. The higher activity of large cubic Pd (27, 48, 63 nm) in comparison to conventional nanoparticles ( octahedral Pd > cubic Pd. The conventional Pd was found to be the most active. More steps and defect sites on conventional Pd surfaces may be responsible for the higher activity compared to the nanocrystals enclosed with more terraces.
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ASSOCIATED CONTENT
S Supporting Information *
TEM images and cyclic voltammetry curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel (+1) 860-727-7251; Fax (+1) 860-660-7384. Present Address ∥
Department of Chemical Engineering, College of Engineering, Kyung Hee University, Youngin 446-701, Korea. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Use of the National Synchrotron Light Source at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. Beamline X18A at the NSLS is supported in part by the Synchrotron Catalysis Consortium (DOE BES grant DE-FG02-03ER15688). Part of the work was performed at the Nano Research Facility (NRF), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF under Award ECS-0335765.
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