ESTUDIO DE CATALIZADORES DE ORO PARA LA PRODUCCIÓN DE HIDRÓGENO

Abstract

Our present civilization strongly depends upon fossil fuels such as petroleum, natural gas and coal. The increase in the concentration of CO2, a major green house effect gas in the atmosphere, is due to the continuous burning of such fossil fuels. Hydrogen is a clean fuel that emits no CO2 when it is burned or used in H2–O2 fuel cells. One of the realistic methods to produce hydrogen with less CO2 emission is to utilize methane as the source of hydrogen through a catalytic approach. In the present work, the catalytic decomposition of methane for hydrogen production was investigated by using monometallic Au and bimetallic Au-Ni catalysts. The support materials used in the catalysts preparation were activated carbon, Al-MCM-41 and SBA-15. Two synthesis methods (impregnation and deposition by precipitation) were applied to uptake the active phases on the support. All the catalysts were characterized using various techniques such as X-ray diffraction, N2 adsorption isotherms, UVvis spectroscopy, temperature-programmed reduction, H2 chemisorption, transmission electron microscopy, scanning electron microscopy, electron dispersive spectroscopy and Raman spectroscopy. The methane decomposition reaction was carried out in a microreactor system at a temperature range from 500 to 700 C. The product effluent was analyzed by gas chromatography. In general, the catalysts Au/C and Au/Al-MCM-41 synthesized by the impregnation technique gave gold particle sizes greater than 100 nm and the catalysts Au/C synthesized by the deposition-precipitation technique showed particle sizes from 5 up to 10 nm. Although these results were interesting, the catalytic activity was not correlated with the Au nanoparticles. On the other hand, all the bimetallic Au-Ni catalysts were more active than the monometallic catalysts. The most effective catalysts for methane decomposition were the series of catalysts Au-Ni/SBA-15. The methane conversions over the Au- Ni/SBA-15 catalysts were approximately 15 % at a reaction temperature of 600 C. All the experiments, hydrogen was the gaseous product, no CO or CO2 was detected. It is assumed that Au addition into Ni catalysts might improve dehydrogenation of methane and increased the concentration of CHx species in the surface of the catalysts. Therefore, Au addition significantly enhanced the catalyst activity and stability. During a period of 15 hr evaluation under different space velocity, no obvious catalytic deactivation was observed on our catalysts, which showed much better stability in comparison with the Cu-Ni/Al2O3 and Ni-Cu/SiO2 catalysts. Raman spectroscopy analysis confirmed that two kinds of carbon materials were formed on the catalysts after reaction: one was carbon nanotube and the other was amorphous carbon. The amorphous carbon usually covers on the surface of the catalysts, leading to active phase encapsulation. In the bimetallic catalysts, amorphous carbon materials were much less than that formed on the monometallic catalysts. This is one of the reasons responsible for the enhancement of the catalytic activity of the bimetallic catalysts. Hydrogen chemisorption experiments showed that before the reaction, Au- Ni/SBA-15 catalysts have large capacity for hydrogen adsorption and possible storage; however, after reaction, their capacity for hydrogen adsorption was reduced because of the formation of amorphous carbon and increment of active particle size. This results indicated that it is possible to use Au-Ni/SBA-15 catalysts for both hydrogen production and hydrogen storage.