Thesis
ESTUDIO DE CATALIZADORES DE ORO PARA LA PRODUCCIÓN DE HIDRÓGENO
Autor
I.Q. HERNÁNDEZ RAMÍREZ, EDGAR
Institución
Resumen
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.