CONICET | Buscador de Institutos y Recursos Humanos

One of the main challenges in the catalytic production of hydrogen from ethanol steam reforming (ESR) lies in the development of active catalysts that inhibit coke formation and production of CO. It has been reported that cerium oxide can be a suitable support for ESR catalysts due to its high oxygen mobility that can improve the catalytic stability by preventing metal particles sintering and suppressing the formation of carbonaceous species1. In particular, the incorporation of Ga3+ cations in the fluorite structure of ceria markedly improves the reducibility and acid-base properties of CeO2.2,3 Another important aspect of those Ce-Ga mixed oxides is their ability to facilitate H2 dissociation into Ga-H surface species, or otherwise favor the recombination of Ga-H to release molecular H2.3,4 As a result, it is expected that the resulting catalytic properties will benefit from the use of cerium-gallium mixed oxides as new supports for ESR. In this work, the adsorption and temperature programmed surface reaction (TPSR) of ethanol on pure CeO2, Ga2O3 and Ce-Ga mixed oxides (CeGaOx) was studied by diffuse reflectance infrared spectroscopy (TPSR-IR) and mass spectrometry (TPSR-MS). The correlation of the experimental results obtained by these techniques allowed us to propose a mechanism for the reaction of ethanol on the surface of the supports. Ethanol is dissociatively adsorbed as ethoxy species at 373 K. These species decompose upon heating the oxides above 423 K to form acetate groups as evidenced by the TPSR-IR experiments. Above 463 K, ethylene was released in the case of CeO2 and Ga2O3, but not for Ce-Ga mixed oxides. It is suggested that ethylene formation is due to the β-CH and C-O bond breaking of the ethoxy species.5 The release of CH4 and CO2 was evidenced by mass spectrometry on CeO2 and Ce-Ga mixed oxides above 523 K and can be associated to the decomposition of adsorbed acetate species, leaving oxygen vacancies on the surface.5 The unexpected non-detection of ethylene in the case of the Ce-Ga mixed oxides can be explained in terms of the ability of these oxides to dissociate H2 more easily than gallia (and ceria) to form Ga-H species5 together with the oxygen bond lability in Ce-O-Ga compared to the pure oxides.3 TPSR-IR results showed the formation of Ga-H species begins along with the decomposition of the ethoxy species, favoring in turn the reduction of Ce4+. Thus, it is possible that the dehydrogenation, followed by oxidation, of ethoxy to acetate species is favored on CeGaOx, leading to the non-detection of ethylene in the gas phase. In addition, it is suggested that the decomposition of the acetate species is enhanced by the increased oxygen mobility of the Ce-Ga mixed oxides, reflected by the lower desorption temperature of CO2 compared to CeO2 and Ga2O3. After the decomposition, the remaining carbonaceous fragments were oxidized by heating under O2, and it was observed that the CO2 evolution was at least 40% higher in the CeO2 with respect to the mixed oxides. Our whole set of results suggest that ethylene could be one of the sources of coke formation on ceria-based ESR catalysts. It is proposed that the incorporation of Ga3+ cations to ceria improves its oxidative behavior, which would result in a greater stability of the catalysts based on Ce-Ga in the hydrogen production by ethanol steam reforming.