Abstract
This work presents the catalysts of Pt-Bi shells on Au nanoparticle cores and Pt overlayers on the Pt-Bi shells toward formic acid oxidation (FAO). Pt and Bi were co-deposited on Au nanoparticles (Au NP) via the irreversible adsorption method using a mixed precursor solution of Pt and Bi ions, and the amount of the co-deposits was controlled with the repetition of the deposition cycle. Rinsing of the co-adsorbed ionic layers of Pt and Bi with a H2SO4 solution selectively removed the Bi ions to leave Pt-rich and Bi-lean (<0.4 atomic %) co-deposits on Au NP (Pt-Bi/Au NP), conceptually similar to de-alloying. Additional Pt was deposited over Pt-Bi/Au NPs (Pt/Pt-Bi/Au NPs) to manipulate further the physicochemical properties of Pt-Bi/Au NPs. Transmission electron microscopy revealed the core–shell structures of Pt-Bi/Au NPs and Pt/Pt-Bi/Au NPs, whose shell thickness ranged from roughly four to six atomic layers. Moreover, the low crystallinity of the Pt-containing shells was confirmed with X-ray diffraction. Electrochemical studies showed that the surfaces of Pt-Bi/Au NPs were characterized by low hydrogen adsorption abilities, which increased after the deposition of additional Pt. Durability tests were carried out with 1000 voltammetric cycles between −0.26 and 0.4 V (versus Ag/AgCl) in a solution of 1.0 M HCOOH + 0.1 M H2SO4. The initial averaged FAO performance on Pt-Bi/Au NPs and Pt/Pt-Bi/Au NPs (0.11 ± 0.01 A/mg, normalized to the catalyst weight) was higher than that of a commercial Pt nanoparticle catalyst (Pt NP, 0.023 A/mg) by a factor of ~5, mainly due to enhancement of dehydrogenation and suppression of dehydration. The catalytic activity of Pt/Pt-Bi/Au NP (0.04 ± 0.01 A/mg) in the 1000th cycle was greater than that of Pt-Bi/Au NP (0.026 ± 0.003 A/mg) and that of Pt NP (0.006 A/mg). The reason for the higher durability was suggested to be the low mobility of surface Pt atoms on the investigated catalysts.
Highlights
Recent technical developments in the conversion of CO2 to formic acid have directed the attention of researchers toward the development of direct formic acid fuel cells (DFAFCs) for portable devices and power sources at off-grid locations [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
The particle sizes of Pt-Bi/Au nanoparticles (Au NP) certainly increase as the co-deposition cycle is repeated
Corresponds to three atomic layers approximately, while that on Pt/Pt-Bi(5) Au NP is slightly less than two layers
Summary
FAO takes place on such metal surfaces in a dual path mechanism [29,30,31,32,33]: a dehydrogenation path to directly oxidize formic acid to CO2 and H+ to produce two electrons (HCOOH → CO2 + 2H+ + 2e) and a dehydration path to dissociate formic acid to poisonous CO (HCOOH → H2 O + CO), which is electrochemically oxidized to CO2 (CO + H2 O → CO2 + 2H+ + 2e) at a high potential. The development of more efficient FAO electrochemical catalysts has been focused on the suppression of dehydration and enhancement of dehydrogenation. The general way to achieve this specific goal is to manipulate the physicochemical properties of Pt or Pd metals by alloying with other secondary metals such as Ni, Au, Bi, Pb, As and Sb [34,35,36,37,38,39,40,41,42,43,44] or by attaching secondary metals on their surfaces [45,46,47,48,49,50,51,52,53,54,55,56]
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