We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at -0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at -0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic reduction of CO2 to CO.

Monodisperse nickel nanoparticles are prepared from the reduction of Ni(acac)(2) with borane tributylamine in the presence of oleylamine and oleic acid. Without any special treatment to remove the surfactants, the as-synthesized Ni nanoparticles supported on the Ketjen carbon support exhibit high catalytic activity in hydrogen generation from the hydrolysis of the ammonia-borane (H(3)NBH(3)) complex with a total turnover frequency value of 8.8 mol of H(2) x (mol of Ni)(-1) x min(-1). Such catalysis based on Ni nanoparticles represents a promising step toward the practical development of the H(3)NBH(3) complex as a feasible hydrogen storage medium for fuel cell applications.

Formic acid (FA, HCOOH) is a common small organic acid with a melting point of 8.4 8C and boiling point of 100.8 8C. It can undergo a dehydrogenation reaction, HCOOH!H 2 + CO 2 , releasing H 2 that will be important for hydrogen-based energy applications. [1] Traditionally, the dehydrogenation of FA is catalyzed by metal complexes dissolved in an organic solvent and the catalysis is enhanced by adding an additive, such as sodium formate or amine adducts. [2] To make more practical catalyst for the dehydrogenation reaction of FA, heterogeneous catalysts based on metal nanoparticles (NPs) have been developed. These catalysts are generally more stable but much less active than the homogeneous ones. [3] Recently, bimetallic NP catalysts were found to be more active than their single component counterparts for the dehydrogenation of FA. [4] For example, AgPd NPs supported on cerium oxide or AuPd NPs immobilized in a metal-organic framework showed an enhanced FA dehydrogenation catalysis with the initial turnover frequency (TOF) reaching 210 h À1 or 192 h À1 at 90 8C, respectively. [5] However, the high rate of hydrogen generation observed from these heterogeneous catalysts could only be achieved when an additive was present and the reaction was maintained at temperatures close to 100 8C. [6] Under these "harsh" conditions, HCOOH was also subject to an undesired dehydration reaction, HCOOH!H 2 O + CO. [7] Interestingly, Ag/Pd core/shell NPs were found to be promising in catalyzing the dehydrogenation of FA in an aqueous FA solution at lower temperatures (up to 50 8C) without any additive. [8] But their initial TOFs were in the range of 125-252 h À1 at temperatures between 25-50 8C.Considering the limitation seen from the previous syntheses in controlling the NP size and composition, we decided to re-evaluate the binary alloy NPs on their catalysis for the dehydrogenation of FA. Our very recent report showed that monodisperse 4 nm AuPd NPs were more active in catalyzing the dehydrogenation of FA in water at 50 8C without using any additive and their initial TOF reached 230 h À1 . [9] Encouraged by this result, we further improved our solution phase synthesis and produced monodisperse 2.2 nm AgPd NPs with the desired composition controls. We found that these monodisperse 2.2 nm AgPd alloy NPs were a highly active heterogeneous catalyst for the dehydrogenation of FA. In water without any additive, the Ag 42 Pd 58 NPs showed the highest catalytic activity among all AgPd NPs tested with their initial TOF reaching 382 h À1 at 50 8C and apparent activation energy at 22 AE 1 kJ mol À1 . These are the best values ever reported by a heterogeneous catalyst for the dehydrogenation of FA in aqueous solution. It demonstrates the great potential of binary alloy NPs as a more practical catalyst for the dehydrogenation of FA and hydrogen generation.The 2.2 nm AgPd alloy NPs were synthesized by coreduction of silver(I) acetate, Ag(Ac), and palladium(II) acetylacetonate, Pd(acac) 2 , in oleylamine (OAm), oleic acid (OA) and 1-octadecen...

Monodisperse CoPd nanoparticles (NPs) were synthesized and studied for catalytic formic acid (HCOOH) oxidation (FAO). The NPs were prepared by coreduction of Co(acac)(2) (acac = acetylacetonate) and PdBr(2) at 260 °C in oleylamine and trioctylphosphine, and their sizes (5-12 nm) and compositions (Co(10)Pd(90) to Co(60)Pd(40)) were controlled by heating ramp rate, metal salt concentration, or metal molar ratios. The 8 nm CoPd NPs were activated for HCOOH oxidation by a simple ethanol wash. In 0.1 M HClO(4) and 2 M HCOOH solution, their catalytic activities followed the trend of Co(50)Pd(50) > Co(60)Pd(40) > Co(10)Pd(90) > Pd. The Co(50)Pd(50) NPs had an oxidation peak at 0.4 V with a peak current density of 774 A/g(Pd). As a comparison, commercial Pd catalysts showed an oxidation peak at 0.75 V with peak current density of only 254 A/g(Pd). The synthesis procedure could also be extended to prepare CuPd NPs when Co(acac)(2) was replaced by Cu(ac)(2) (ac = acetate) in an otherwise identical condition. The CuPd NPs were less active catalysts than CoPd or even Pd for FAO in HClO(4) solution. The synthesis provides a general approach to Pd-based bimetallic NPs and will enable further investigation of Pd-based alloy NPs for electro-oxidation and other catalytic reactions.

Monodisperse 8 nm CoPd nanoparticles (NPs) with controlled compositions were synthesized by the reduction of cobalt acetylacetonate and palladium bromide in the presence of oleylamine and trioctylphosphine. These NPs were active catalysts for hydrogen generation from the hydrolysis of ammonia borane (AB), and their activities were composition dependent. Among the 8 nm CoPd catalysts tested for the hydrolysis of AB, the Co(35)Pd(65) NPs exhibited the highest catalytic activity and durability. Their hydrolysis completion time and activation energy were 5.5 min and 27.5 kJ mol(-1), respectively, which were comparable to the best Pt-based catalyst reported. The catalytic performance of the CoPd/C could be further enhanced by a preannealing treatment at 300 °C under air for 15 h with the hydrolysis completion time reduced to 3.5 min. This high catalytic performance of Co(35)Pd(65) NP catalyst makes it an exciting alternative in pursuit of practical implementation of AB as a hydrogen storage material for fuel cell applications.

We report a facile synthesis of monodisperse NiPd alloy nanoparticles (NPs) and their assembly on graphene (G) to catalyze the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of R-NO 2 and/or R-CN to R-NH 2 in aqueous methanol solutions at room temperature. The 3.4 nm NiPd alloy NPs were prepared by coreduction of nickel(II) acetate and palladium(II) acetlyacetonate by borane-tert-butylamine in oleylamine and deposition on G via a solution phase self-assembly process. G-NiPd showed composition-dependent catalysis on the tandem reaction with G-Ni 30 Pd 70 being the most active. A variety of R-NO 2 and/or R-CN derivatives were reduced selectively into R-NH 2 via G-Ni 30 Pd 70 catalyzed tandem reaction in 5−30 min reaction time with the conversion yields reaching up to 100%. Our study demonstrates a new approach to GNiPd-catalyzed dehydrogenation of AB and hydrogenation of R-NO 2 and R-CN. The G-NiPd NP catalyst is efficient and reusable, and the reaction can be performed in an environment-friendly process with short reaction times and high yields.

Monodisperse 4 nm AuPd alloy nanoparticles with controlled composition were synthesized by co-reduction of hydrogen tetrachloroaurate(III) hydrate and palladium(II) acetylacetonate with a borane-morpholine complex in oleylamine. These NPs showed high activity (TOF = 230 h(-1)) and stability in catalyzing formic acid dehydrogenation and hydrogen production in water at 50 °C without any additives.

No abstract