International audienceInsights into the ethanol electro-oxidation reaction mechanism on palladium in alkaline media are presented combining polarization modulation infrared reflec- tion absorption spectroscopy (PM-IRRAS) and density functional theory (DFT) calculations. The synergy between PM-IRRAS and DFT calculations helps to explain why the C− C bond is not broken during ethanol electro-oxidation, and the reaction stops at acetate. Coupling chronoamperometry (CA) with in situ PM-IRRAS enables us to simultaneously identify ethanol electro-oxidation products on the catalyst surface and in the bulk solution. We show that, at lower potential, it is possible to break the C−C bond on Pd/C in alkaline media to form CO2. However, the selectivity is poor, because of competition with the formation of acetate and other side products, which gets worse at higher potentials. DFT computations complete the picture using the computational hydrogen electrode approach. The computations highlight the pivotal role of the CH3CO intermediate that can either undergo a C−C bond scission yielding CO and then CO2 or that can be oxidized toward CH3COO−. The latter is a dead end in the reaction scheme toward CO2 production, since it cannot be easily oxidized nor broken into C1 fragments. However, CH3CO is not the most favored intermediate formed from ethanol electro-oxidation on Pd, hence limiting the production of CO2

The promotional role of oxide supports (CeO2, SnO2, TiO2) on ethanol electrooxidation in alkaline media over Pd nanoparticles (NPs) is presented and compared to Pd on carbon. XPS revealed a shift to lower binding energy of the Pd 3d peak when Pd NPs were deposited on metal oxides, implying a charge transfer from the oxides to the Pd. The catalytic activity of the supported NPs for ethanol electrooxidation was assessed by using cyclic voltammetry and chronoamperometry. The electrooxidation products were monitored in situ, using polarization modulation–infrared reflection absorption spectroscopy (PM‐IRRAS), which revealed that the supports influence the selectivity of reactions on Pd. Pd/CeO2 has superior selectivity towards breaking the C−C bond to produce CO2 compared to the other three supports. Acetate, as a product, was evident on all of the catalysts, but at different ratios. Pd supported on metal oxides showed higher activity and, in particular, CeO2 and SnO2 stand out as the best supports.

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The efficiency of direct ethanol fuel cells suffers from the partial oxidation of ethanol into acetic acid as opposed to the complete oxidation of this fuel to CO 2 . Herein, we support the quest for a selective catalyst for ethanol electro-oxidation to CO 2 , building on our previous mechanistic hypothesis based on experimental insight and DFT computations. We derive a simple descriptor of the expected selectivity towards full oxidation, Ω, as a function of the adsorption energy of atomic C and O. Three different families of catalyst surfaces are screened using this descriptor: monometallics, bimetallics and conducting metal oxides, totaling to 600 surfaces. In agreement with available experimental data, no single metal surface is more selective for total oxidation than platinum and palladium. While the selected conducting oxides were not predicted to be selective towards splitting the C-C bond, structurally-controlled monometallics (such as Pd (100)) or some bimetallics (Pd 3 Ag) are found to be competitive with the most stable facet, (111), of Pd and Pt. Despite this very extensive screening, no very promising catalyst has been identified. This highlights the need to identify catalysts for acetate oxidation or to exploit support effects and electrolyte engineering to profit from the full power of direct ethanol fuel cells.

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Infrared spectroscopy is a powerful non-destructive technique for the identification and quantification of organic molecules widely used in scientific studies. For many years, efforts have been made to adopt this technique for the in situ monitoring of reactions. From these efforts, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was developed three decades ago. Unfortunately, because of the complexity of data processing and interpretation, PM-IRRAS had been avoided in lieu of the single potential alteration infrared spectroscopy (SPAIRS) and subtractively normalized interfacial Fourier transform infrared (SNIFTIR). In this work, we present a new approach for PM-IRRAS data processing and presentation, which provides more insight into in situ and surface studies besides dramatically improving the S/N. In this new approach, we recommend three complementary methods of data treatment (eqn (7), (9) and (10)) as the new protocols for presenting PM-IRRAS data. These equations are robust in visualising the surface processes at the solid-liquid and solid-gas interphases. Eqn (7) contrasts the surface adsorbed species with respect to the isotropic background with or without the influence of the applied potential. Eqn (9) highlights the surface potential-driven changes between the sample and the reference spectra. Eqn (10) focuses on the bulk-phase (solution/gas and surface species) potential-driven changes between the sample and the reference spectra, and hence it can be used to track the production of species, which desorb from the surface upon their formation. Examples of ethanol electro-oxidation reaction are provided as a test system for in situ studies and PVP deposited on glassy carbon for thin-film studies to illustrate the utility of the new PM-IRRAS data handling protocol, which is poised to improve the understanding of the chemistry and physics of surface processes.

SummaryThree-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.