RESUMEN
Supported gold on co-precipitated nanosized NiAl layered double hydroxides (LDHs) was studied as an effective catalyst for medium-temperature water-gas shift (WGS) reaction, an industrial catalytic process traditionally applied for the reduction in the amount of CO in the synthesis gas and production of pure hydrogen. The motivation of the present study was to improve the performance of the Au/NiAl catalyst via modification by CeO2. An innovative approach for the direct deposition of ceria (1, 3 or 5 wt.%) on NiAl-LDH, based on the precipitation of Ce3+ ions with 1M NaOH, was developed. The proposed method allows us to obtain the CeO2 phase and to preserve the NiAl layered structure by avoiding the calcination treatment. The synthesis of Au-containing samples was performed through the deposition-precipitation method. The as-prepared and WGS-tested samples were characterized by X-ray powder diffraction, N2-physisorption and X-ray photoelectron spectroscopy in order to clarify the effects of Au and CeO2 loading on the structure, phase composition, textural and electronic properties and activity of the catalysts. The reduction behavior of the studied samples was evaluated by temperature-programmed reduction. The WGS performance of Au/NiAl catalysts was significantly affected by the addition of CeO2. A favorable role of ceria was revealed by comparison of CO conversion degree at 220 °C reached by 3 wt.% CeO2-modified and ceria-free Au/NiAl samples (98.8 and 83.4%, respectively). It can be stated that tuning the properties of Au/NiAl LDH via CeO2 addition offers catalysts with possibilities for practical application owing to innovative synthesis and improved WGS performance.
RESUMEN
The impact of ceria doping by Zn (atomic ratio Zn/(Zn + Ce) = 0.05) on the structural and catalytic properties of Au/CeO(2) catalyst was studied. The ceria modification influenced the catalytic activity toward purification of hydrogen via water-gas shift (WGS) and preferential CO oxidation (PROX) reactions in a different way: it diminished the WGS activity and improved the PROX performance. A characterization by FTIR spectroscopy was conducted to explain differences in the catalytic performance. The nature of gold active species after different pretreatments, under different atmospheres (H(2), D(2)), and after admission of CO and its subsequent interaction with (18)O(2) was investigated. Evidence has been found of the dissociation of hydrogen at room temperature on gold, producing on the oxidized sample a broad absorption assigned to Au-OH vibrations, whereas on the reduced one, bands at 3200 and 1800 cm(-1) ascribed, respectively, to Au-OH and Au-H species have been detected. For the first time, the formation of Au-hydride on supported heterogeneous catalysts was proposed. These features were stronger on the Au/CeO(2) sample than on the Au/Zn-CeO(2) sample. The availability of highly dispersed gold clusters in contact with oxygen vacancies on the ceria surface could contribute to higher WGS activity, whereas the steps of small gold particles are the active sites for both CO and oxygen activation during the PROX reaction.