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Thermal decomposition of glycine molecules on cerium oxide films is defined by the type of cerium cations on the surface.

International audience; Carbon materials have attracted increasing attention as supports for metal catalysts. Ironcontaining carbon nanotubes often promoted with copper have found application in Fischer-Tropsch synthesis, which provides an alternative way for conversion of renewable feedstocks to chemicals and fuels. In carbon nanotubes, the active phase can be nanoconfined inside the channels or localized on the outer surface. In most of previous work, the distribution of metal nanoparticles inside or outside carbon nanotubes is considered to be immobile during the catalyst activation or catalytic reaction. In this paper, we uncovered remarkable mobility of both iron and copper species in the bimetallic catalysts between inner carbon nanotube channels and outer surface, which occurs in carbon monoxide and syngas, while almost no migration of iron species proceeds in the monometallic catalysts. This mobility is enhanced by noticeable fragility and defects in carbon nanotubes, which appear on their impregnation with the acid solutions of metal precursors and precursor decomposition. Remarkable mobility of iron and copper species in bimetallic catalysts affects the genesis of iron active sites, and enhances interaction of iron with the promoter. In the bimetallic iron-copper catalysts, the major increase in the activity was attributed to higher reaction turnover frequency over iron surface sites located in a close proximity with copper.

Dependence on noble metal catalysts is considered to be the main factor which hinders wider commercialization of proton exchange membrane fuel cells (PEM-FCs) and water electrolyzers (PEM-WEs). One way of lowering the loading of Pt and Ir is by using thin-film techniques for their deposition onto the high-surface conductive nanoparticles. Another approach, which is convenient in applications where the complete cycle of electricity - > H2 - > electricity takes place, is merging the PEM-WEs and PEM-FCs into one bi-functional system – the unitized regenerative fuel cell (PEM-URFC). In accordance with the above mentioned conception, this paper revolves around unconventionally prepared bi-functional magnetron sputtered lower-loading Pt-Ir catalysts for the anode side of PEM-URFC. Two geometries of catalyst coated membranes (CCM) were compared, differing in relative positioning of individual Pt and Ir thin films sputtered on TiC-based support sublayer; the sandwich-like Ir/TiC/Pt structure and the co-sputtered Pt-Ir/TiC structure. Wide arsenal of analytical methods, ranging from photoelectron spectroscopy to electrochemical atomic force microscopy determined that co-sputtering of Pt and Ir leads to alloy formation, thus preventing iridium to fully electro-oxidize to IrOx which in turn helps to explain why sandwich-like Ir/TiC/Pt structure, with no alloy, outperforms the co-sputtered Pt-Ir/TiC CCM in both operational regimes despite having the exactly same noble metal loading. The PEM-URFC single cell with sandwich-like bi-functional anode catalyst yielded 31.8% of round-trip efficiency at 1 A cm−2 in comparison to 34.2% achieved by combination of single-purpose cells with more than double the loading of noble metals.

The interplay between the metal-support interaction and stability with respect to sintering has been investigated for Pt nanoparticles supported on well-ordered Co3O4(111)/Ir(100) films in UHV and under oxidizing conditions by means of synchrotron radiation photoelectron spectroscopy (SRPES) and near ambient pressure X-ray photoelectron spectroscopy (NAP XPS). The electronic metal-support interaction between Pt and Co3O4(111) associated with charge transfer results in partial reduction of Co3O4(111) yielding partially oxidized Ptδ+ species at the interface. The stability of the supported Pt particles is coupled with the oxidation state of Ptδ+ species, which can be reduced or oxidized depending on the Pt coverage and reactive environment. Annealing of Pt/Co3O4(111)/Ir(100) in UHV triggers the reduction of Ptδ+ species. At higher temperature, reverse spillover of oxygen to the Pt nanoparticles is accompanied by reduction of Co3O4(111). Under these conditions, the oxidation state of Ptδ+ species depends strongly on Pt coverage. Thus, at low Pt coverage (0.3 ML Pt), Ptδ+ is converted to Pt4+, at intermediate coverage (1.3 ML Pt), Ptδ+ remains stable, and at high Pt coverage (1.93 ML), Ptδ+ is reduced to Pt0. Sintering of Pt particles is associated with the reduction of the Ptδ+ species. This process is prevented under oxidizing conditions due to the formation of an interfacial oxide PtOx. The formation of an interfacial PtOx is observed at 300 K under exposure to 1 × 10−6 mbar O2 at Pt coverages below 1.3 ML. Using NAP XPS, we observe the formation of an interfacial PtOx at high Pt coverage (2.0 ML) in an oxygen atmosphere (1 mbar) at 300 K while the formation of surface PtOx is kinetically hindered and occurs above 550 K only.

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO2(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO2(111) system are accompanied by charge transfer and partial reduction of CeO2(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO2(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO2(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO2(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO2(111) represents the dominant reaction channel upon annealing in H2 above 450 K. The interaction of Ir/CeO2(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H2. The oxidation of Ir nanoparticles under NAP conditions yields stable IrOx nanoparticles. The stability of Ir and IrOx nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

Polycrystalline cerium oxide thin films (15 nm) deposited on a glassy carbon substrate were used as an electrode in a mediator-free, non-enzymatic electrochemical sensor for hydrogen peroxide. The electrode surface was characterized by X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. The electrode sensitivity, detection limit and pH range of sensor stability were determined by applying electrochemical techniques: cyclic voltammetry and chronoamperometry. It was found that the sensor reactivity to H2O2 is directly related to the presence of electroactive cerium centres of 3+ character on the electrode surface. The Michaelis–Menten mechanism of catalase-like activity of ceria film is suggested as an explanation of the data and discussed. The results confirmed the sensing abilities of technologically well-accessible nanostructured cerium oxide films for hydrogen peroxide detection without using a mediator, i.e. the enzymatic properties of CeO2/GC electrode.

Ever increasing amount of installed power capacity from the volatile renewable sources (e.g. wind and solar) puts high demand on a reliable and scalable system, capable of storing the energy during the overproduction and releasing it in times of the deficit. Utilization of hydrogen as an energy vector proves to be an interesting option in this context. Overproduced electrical energy can be electrochemically converted to hydrogen by means of the water electrolyzers (WEs). Generated hydrogen can be either injected into the existing natural gas pipelines or stored for later conversion back to electricity via the fuel cells (FCs). The most suitable WEs and FCs for this task are arguably the proton exchange membrane WEs and FCs (PEM-WEs, PEM-FCs). Major factor currently hindering wider commercialization of these technologies is the high price and scarcity of noble metals, namely Pt and Ir which serve the role of a catalyst within the individual cells. One way of lowering the loading of noble metals is by using thin-film techniques for their deposition onto the high-surface conductive nanoparticles. Another approach, which is convenient in applications where the complete cycle of electricity->H2->electricity takes place, is merging the PEM-WEs and PEM-FCs into one bi-functional system – the PEM unitized reversible fuel cell (PEM-URFC). Such unification may potentially lead to significant cost savings not only on the catalyst but on the overall hardware as well. In this work, we present and discuss unorthodoxly prepared bi-functional thin-film low-loading Pt-Ir catalysts for the anode side of PEM-URFC (i.e. the oxygen evolution/hydrogen oxidation side). Two different geometries of the catalyst coated membranes (CCM) were studied; the CCM with magnetron co-sputtered Pt-Ir catalyst on top of the TiC-based support sublayer and the CCM with sandwich-like design where Pt is magnetron sputtered on the bottom and Ir on the top side of the TiC-based support sublayer. Although the loading of noble metals within both tested CCMs and the corresponding membrane electrode assemblies (MEAs) was similar, the obtained in-cell efficiency varied significantly. The sandwich-like CCM performed better in both electrolyzer and fuel cell regimes. Combination of data obtained from the in-cell performance measurements, X-ray photoelectron spectroscopy (XPS) and electrochemical atomic force microscopy (EC-AFM) helped us to identify the reasons why sandwich-like CCM performed better in both operational regimes. We concluded that thorough oxidation of Ir to IrO2, which did not occur when Ir was within Pt-Ir alloy after co-sputtering, was responsible for higher efficiency of the sandwich-like CCM in PEM-WE regime. On the other hand, the less prominent increase of efficiency in PEM-FC mode seemed to be due to the swapping of Pt thin film from position "top" (i.e. further from the PEM) to position "bottom" (i.e. closer to the PEM) which allowed for better ionic conductivity. Considering relatively high efficiencies obtained by the sandwich-like MEA with just a fraction of conventional noble metal loading, we believe that continuing in the research of thin-film, segmented catalysts is the step in the right direction.