Different approaches to enhancement of electrocatalytic activity of noble metal nanoparticles during oxidation of small organic molecules (namely potential fuels for low-temperature fuel cells such as methanol, ethanol and formic acid) are described. A physical approach to the increase of activity of catalytic nanoparticles (e.g. platinum or palladium) involves nanostructuring to obtain highly dispersed systems of high surface area. Recently, the feasibility of enhancing activity of noble metal systems through the formation of bimetallic (e.g. PtRu, PtSn, and PdAu) or even more complex (e.g. PtRuW, PtRuSn) alloys has been demonstrated. In addition to possible changes in the electronic properties of alloys, specific interactions between metals as well as chemical reactivity of the added components have been postulated. We address and emphasize here the possibility of utilization of noble metal and alloyed nanoparticles supported on robust but reactive high surface area metal oxides (e.g. WO3, MoO3, TiO2, ZrO2, V2O5, and CeO2) in oxidative electrocatalysis. This paper concerns the way in which certain inorganic oxides and oxo species can act effectively as supports for noble metal nanoparticles or their alloys during electrocatalytic oxidation of hydrogen and representative organic fuels. Among important issues are possible changes in the morphology and dispersion, as well as specific interactions leading to the improved chemisorptive and catalytic properties in addition to the feasibility of long time operation of the discussed systems.

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Unique hybrid systems for electroreduction of CO 2 under both conventional and visible-light-induced conditions are proposed and designed here by over-coating copper(I) oxide with tungsten(VI) oxide nanowires. When Cu 2 O and WO 3 nanostructures are sequentially deposited on glassy carbon substrate, the resulting system has exhibited high electrocatalytic activity toward reduction of CO 2 in phosphate buffer of pH = 6.1. By introducing WO 3 , the Cu-based system becomes more selective against the competitive hydrogen evolution and exhibits higher CO 2 -reduction currents relative to the performance of single components. During electroreduction, highly catalytic Cu sites are generated or intercalated within WO 3 nanowires partially reduced to mixed-valence hydrogen-absorbing tungsten(VI,V) oxide bronzes, H x WO 3 , co-existing with sub-stoichiometric tungsten(VI,IV) oxides, WO 3-y . Strong adsorption and activation of CO 2 molecule has been demonstrated at the WO 3 -decorated Cu 2 O interface. Under photoelectrochemical conditions involving illumination with sun-light, the proposed hierarchical system of Cu 2 O semiconductor deposited onto the transparent fluorine-doped conducting glass electrode and decorated with WO 3 nanowires is well-behaved and active toward CO 2 -reduction in the Na 2 SO 4 neutral medium. In addition to the Cu 2 O stabilization effect, the heterojunction formed by p-type Cu 2 O and n-type WO 3 semiconductors seems to facilitate charge distribution and separation at the photoelectrochemical interface.

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There has been growing interest in the electrochemical reduction of carbon dioxide (CO 2 ), a potent greenhouse gas and a contributor to global climate change, and its conversion into useful carbon-based fuels or chemicals [1][2][3][4][5]. Numerous homogeneous and heterogeneous catalytic systems have been proposed to induce the CO 2 reduction and, depending on the reaction conditions (applied potential, choice of buffer, its strength and pH, local CO 2 concentration, or the catalyst used) various products that include carbon monoxide, oxalate, formate, carboxylic acids, formaldehyde, acetone, or methanol, as well as such hydrocarbons as methane, ethane, and ethylene, are typically observed at different ratios. These reaction products are of potential importance to energy technology, food research, medical applications, and fabrication of plastic materials.Given the fact that the CO 2 molecule is very stable, its electroreduction processes are characterized by large overpotentials, and they are not energy efficient. To produce highly efficient and selective electrocatalysts, the transitionmetal-based molecular materials are often considered [6][7][8]. The latter systems are capable of driving multi-electron transfers and, in principle, produce highly reduced species. In reality, such multi-electron charge transfer catalysts tend to effectively induce the two-electron reduction of CO 2 to CO rather than yield highly reduced products in large amounts. Metallic copper electrodes are unique in this respect because they can drive multi-electron transfers. Mechanisms of the successful electrochemical reductions of CO 2 to methane and ethylene can be interpreted in terms of complex processes occurring at copper electrodes [9,10]. It is believed that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate [11]. Significant decrease of the reaction overpotentials can be achieved with the use of the metal complex modified electrodes capable of both mediating electron transfers and stabilizing the reduced products [12].Because reduction of CO 2 can effectively occur by hydrogenation [13], in the present work, we concentrate on such a model catalytic system as nanostructured metallic palladium capable of absorbing reactive hydrogen in addition to the ability to adsorb monoatomic hydrogen at the interface [14][15][16]. Under such conditions, the two-electron reduction of CO 2 typically to CO [12] is favored. When the reaction proceeds on palladium in aqueous KHCO 3 solutions, carbon monoxide together with hydrogen and small amounts of formate are produced [17][18][19]. Further, it has been postulated that CO and COOH adsorbates are expected to be formed at the surfaces of Pd electrodes at −1.0 V (vs. Ag/AgCl) and, subsequently, desorbed at even more negative potentials [20].To produce highly dispersed and stabilized palladium nanoparticles (as for Fig. 1a), we have generated them by electrodeposition (through consecutive potential cycling) from the thin film of...

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Poly(4‐vinylpyridine)‐modified copper(I) oxide photocathodes, bare or decorated with palladium nanoparticles, can be used for the visible‐light‐driven selective reduction of CO2, mostly to methanol and carbon monoxide (in the absence of Pd cocatalyst) or formic acid and carbon monoxide (in the presence of Pd cocatalyst). The photocathode materials, which are composed of hierarchically deposited (onto transparent fluorine‐doped conducting glass electrode) Cu2O (inner layer) and poly(4‐vinylpyridine) (P4VP) polyelectrolyte film (outer‐layer), with or without dispersed Pd nanoparticles, have been fabricated and examined using electrochemical methodology, atomic force microscopy, and various spectroscopic techniques, including the Raman approach. While the physicochemical characteristics of Cu2O, including the oxide identity and its semiconducting properties, are not largely affected by the interfacial modification with P4VP, the photostability of the hybrid photocathode is significantly improved. The P4VP‐modified Cu2O exhibits reasonable durability during photoelectrochemical reduction of CO2 upon illumination with sunlight in semi‐neutral medium (Na2SO4). While Cu2O can be identified using Raman spectroscopy as the sole bulk component of the semiconducting film, the XPS data imply that the Cu2O surface is likely to be partially reduced during prolonged operation. Introduction of palladium cocatalyst seems to improve distribution (collection and transport) of photoelectrons at the photoelectrochemical interface and affects the CO2‐electroreduction mechanism.