Article pubs.acs.org/cm
Controllable Increase of Boron Content in B‑Pd Interstitial Nanoalloy To Boost the Oxygen Reduction Activity of Palladium Jun Li,† Junxiang Chen,† Qiang Wang,† Wen-Bin Cai,*,‡ and Shengli Chen*,† †
Hubei Key Laboratory of Electrochemical Power Sources, Department of Chemistry, Wuhan University, Wuhan 430072, China Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China
‡
S Supporting Information *
ABSTRACT: Boron doping can boost the catalytic activity of palladium for diverse reactions. Precise control of the doping content is crucial but remains difficult in current synthesis, which generally involves the use of instable and costly borane-organic compounds. Herein, by taking advantage of the relatively strong solvation of N,N-dimethylformamide (DMF) to Na+ and the increased stability BH4− in DMF, we synthesize B-Pd interstitial nanocrystals in DMF, with NaBH4 acting as a reductant and boron source. The boron content, which can be readily tuned by changing the reaction time and NaBH4 concentration, can reach up to 20 at. %. Such a high boron doping results in a great beneficial effect on the catalytic capability of Pd toward the oxygen reduction reaction (ORR). The synthesized B-Pd nanoalloy exhibits a mass and specific activity for ORR that are, respectively, ca. 14 and 14.6 times higher than those of the stateof-the-art commercial Pt catalyst in alkaline solution. Density functional theory (DFT) calculations reveal three types of surface sites that are responsible for the enhanced activity, namely, Pd-BO2 assemblies, Pd atoms neighbored by the assemblies, and the Pd atoms modified with subsurface B atoms. The Pd-BO2 assembly has a Pt-like activity, while the neighboring Pd-BO2 assembly and subsurface Bmodified Pd atoms could catalyze ORR much more efficiently than Pt. The facile and controllable boron doping in palladium should strengthen the power of Pd-based catalysts and, therefore, provides great prospects for their widespread application.
■
INTRODUCTION Alloying is a general yet efficient route to enhance the catalytic performance of metals.1−5 Bimetallic nanoalloys have been a main focus of study in recent years. Recently, there has been a growing interest in the use of nonmetal elements to promote the catalytic capability of metal nanocrystals.6−13 Boron-doped palladium (B-Pd) has been shown to exhibit versatile catalytic capabilities for reactions such as the hydrogenation of alkyne,14,15 furfural hydrogenation,16 H2 production,17,18 the electro-oxidation of ethanol and formic acid,19−21 and oxygen reduction.22,23 In considering the structure−property relationship, one of the urgent challenges in extending the applications of B-Pd catalysts is to develop a simple and facile route that enables precise control of boron doping in palladium over a wide range. Historically, borane was used to deposit B atoms on metal surfaces, based on the adsorption and decomposition of borane on metal surfaces.24−27 As for metallic Pd, the maximal B/Pd atom ratio up to 20% can be reached.28 Recently, researchers have reported the doping of boron in metal nanocrystals in solution using borane-organic compounds (BOCs) as dopants. Tsang and co-workers15 prepared B-Pd catalyst with the high boron content using borane tetrahydrofuran (THF) to deposit boron onto as-synthesized Pd/C in THF. Cai et al.19 reported the in situ doping of boron during the growth of Pd nanocrystals, with dimethylamine borane as a © 2017 American Chemical Society
reductant and dopant in aqueous solution, and a boron content of ca. 7 at. % was achieved. It has been shown that BOCs can release borane in solutions. Given the cost, instability, and safety issues of BOCs, the current solution synthesis has the limitations in precisely controllable doping of the boron content, as well as scale-up catalyst preparation and application. NaBH4 is a commonly used reductant for synthesizing metal nanocrystals in aqueous system. Although it has been used as a boronizing reagent in the synthesis of transition-metal (e.g., Fe and Co) borides in aqueous solution29,30 or under hightemperature conditions,31 NaBH4 is not effective for the boronization of noble metals, because of the rapid hydrolysis of BH4−. As for Pd, the use of NaBH4 usually results in a B uptake of
