Abstract
The formic acid oxidation reaction (FAOR) is one of the key reactions that can be used at the anode of low-temperature liquid fuel cells. To allow the knowledge-driven development of improved catalysts, it is necessary to deeply understand the fundamental aspects of the FAOR, which can be ideally achieved by investigating highly active model catalysts. Here, we studied SnO2-decorated Pd nanocubes (NCs) exhibiting excellent electrocatalytic performance for formic acid oxidation in acidic medium with a SnO2 promotion that boosts the catalytic activity by a factor of 5.8, compared to pure Pd NCs, exhibiting values of 2.46 A mg–1Pd for SnO2@Pd NCs versus 0.42 A mg–1Pd for the Pd NCs and a 100 mV lower peak potential. By using ex situ, quasi in situ, and operando spectroscopic and microscopic methods (namely, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption fine-structure spectroscopy), we identified that the initially well-defined SnO2-decorated Pd nanocubes maintain their structure and composition throughout FAOR. In situ Fourier-transformed infrared spectroscopy revealed a weaker CO adsorption site in the case of the SnO2-decorated Pd NCs, compared to the monometallic Pd NCs, enabling a bifunctional reaction mechanism. Therein, SnO2 provides oxygen species to the Pd surface at low overpotentials, promoting the oxidation of the poisoning CO intermediate and, thus, improving the catalytic performance of Pd. Our SnOx-decorated Pd nanocubes allowed deeper insight into the mechanism of FAOR and hold promise for possible applications in direct formic acid fuel cells.
Highlights
Fuel cells are efficient power sources that generate electric energy from chemical reactions.[1]
The shape and the elemental distribution of both catalysts were tracked by highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping in the as-prepared state, after cycling 20 times in the presence of formic acid and after 3 h of reaction at a constant working potential
In the present “synthesis-by-design” approach, SnO2decorated Pd (SnO2@Pd) nanocubes and Pd nanocubes were prepared via a hydrothermal chemical synthesis, supported on carbon powder, and studied for the electrochemical oxidation of formic acid in an acidic medium
Summary
Fuel cells are efficient power sources that generate electric energy from chemical reactions.[1]. A few examples of the use of Pd−Sn systems for FAOR applications have been previously reported, all demonstrating that CO oxidizes more on the Sn-containing samples, leading to an improved catalytic activity for FAOR.[29−34] The particular interest in Sn arises from its ability to oxidize potentially poisoning intermediates such as CO on Pd or Pt at low overpotentials, through an electronic effect,[29,30,37] a bifunctional mechanism,[32,38] or a third-body effect.[9,39,40] Regarding the electronic effect, alloying Sn with Pd modifies the electronic structure of the catalyst, lowering the adsorption energy of the intermediates. We found that the addition of SnO2 improves the catalytic activity of Pd NCs toward FAOR in acidic media, because it leads to a decrease in the oxidizing potential of CO intermediates, keeping the active Pd sites free of poisoning intermediates
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