In the last few years, transition metal carbides have emerged as novel materials with promising catalytic properties toward important practical reactions. In this work, cubic and hexagonal molybdenum carbides are synthesized and evaluated as carbon‐supported catalysts and as support materials for Pt nanoparticles for the electrochemical oxygen reduction reaction (ORR). The catalysts are characterized by XRD, energy‐dispersive X‐ray spectroscopy, TEM, XPS, and cyclic voltammetry on stationary and rotating ring‐disk electrodes. The results suggest different reactivity of the molybdenum carbide phases as both catalysts and supports for the ORR. Enhanced mass and specific ORR activities at 0.9 V are calculated for Pt–molybdenum carbide‐derived composites compared to commercial Pt and Pt/C catalysts prepared by depositing Pt by the same method. The origin of the improved ORR activity is discussed in terms of the synergistic effect between Pt and the carbide‐derived support and a decrease in the adsorption strength of oxygen‐containing species on the Pt surface, similar to that proposed for Pt–metal alloys. Additionally, the possible formation of a Pt–Mo alloy on the catalyst surface is proposed.

Modification of electronic and chemical properties of a material by the introduction of another element into its lattice is one of the most common methods for designing new catalysts for different applications. In this work the effect of modifying molybdenum carbide with transition metals (Fe, Co, Ni, Cu), TM−Mo 2 C composites, upon the catalytic activity toward hydrogen evolution reaction (HER) in mild acidic and alkaline media has been studied. Catalysts were prepared by carbothermal reduction of molybdenum and TM oxides precursors and were characterized by different physicochemical techniques. Results evidenced a strong pH effect on the catalytic performance of TM−Mo 2 C, while, at pH = 5, inclusion of TM into the Mo 2 C lattice has a deleterious effect on the HER activity and, at pH = 9, a promoting effect was observed, highlighting the importance of considering specific operation conditions during the catalyst design process. Analysis of in situ near-edge X-ray adsorption data reveals a decrease on the oxidation state and average bond ionicity of dopant metal upon a pH increase, shedding light of the different effects of TMs on the resulting HER activity in acidic and alkaline media. Finally, stability tests demonstrated no deterioration on catalysts' performance after 8 h of continuous cycling within the HER working range, confirming the suitability of Mo 2 C materials as promising HER catalysts.

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In the last decade, transition metal carbides, particularly based of molybdenum and tungsten, have emerged as novel materials with promising catalytic properties toward several important practical electrochemical reactions. In this presentation, the activity of such materials will be discussed for the following electrochemical approaches/reactions: i) the electrocatalysis of oxygen reduction reaction (ORR) in acid media on Pt- and Pt2Ni-nanoparticles supported on carbon forming composites with tungsten and molybdenum carbides with cubic (α) and hexagonal (β) structures; ii) the hydrogen evolution reaction (HER) in acid and alkaline media on TM-modified molybdenum carbide (TM-Mo2C) catalysts, where TM is a transition metal, such as, Fe, Co, Ni and Cu; iii) the CO2 and CO electro-reduction reactions in 0.1 M NaClO4 and 0.5 and 1 M KHCO3 solutions on carbon supported tungsten and molybdenum carbides. Syntheses of the carbide-based catalysts have been conducted employing different procedures, after which the produced material are characterized by X-Ray diffraction (XRD), energy dispersive X-ray spectroscopy, (EDS), X-ray photoelectron spectroscopy (XPS), in situ X-ray absorption near edge structure (XANES), and transmission electron microscopy (TEM). Cyclic voltammetry and polarization measurements on stationary and rotation disk electrodes have been employed for the electrochemical investigations. For the CO/CO2 reduction, reaction products were analyzed by differential electrochemical mass spectrometry (DEMS) and high performance liquid chromatography (HPLC) analyses. Enhanced mass and specific ORR activities are seen for Pt–molybdenum carbide-derived composites compared to Pt-α-WC/C, which in turn presents specific activity for the ORR similar to that of a standard carbon supported Pt catalyst (Pt/C). For Pt-β-WC1-x/C composites, the specific activity is also higher than that of Pt/C when a carbide-to carbon load of 40 wt.% is used. In the case of the HER, high activities are observed for all TM-Mo2C catalysts in acid and alkaline media. Nonetheless, while in acid there is a decrease in the activity upon metal doping, with an activity trend of α-Mo2C > Fe-Mo2C > Co-Mo2C > Ni-Mo2C > Cu-Mo2C, the opposite tendency is found in alkaline solutions. In situ near-edge X-ray adsorption analysis reveals different oxidation states of the TM inside the carbide in both solutions. In acid, a positive charge of the TM in the materials in the electrochemical environment is possibly at the origin of the deleterious effect of Fe, Co, Ni and Cu. In addition, catalysts stability against dissolution in acid media is increased by doping of molybdenum carbide with TM. Regarding of CO and CO2 electro-reductions in 0.1 M NaClO4 and 0.5 and 1 M KHCO3 solutions, results suggest that neither molybdenum nor tungsten carbides can be considered as active catalysts, in contrast to reported results in gas phase. It appears that both the aqueous environment and their activity toward the HER surpass the reactivity toward these latter reactions. Acknowledgements: Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP - Procs. 2013/16930-7 and 2014/23486-9) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (Procs. 1423454), Brazil, for financial supports.