Cu and Ni Catalysts Supported on γ-Al2O3 and SiO2 Assessed in Glycerol Steam Reforming Reaction

A http://dx.doi.org/10.5935/0103-5053.20140209 J. Braz. Chem. Soc., Vol. 26, No. 1, 22-31, 2015. Printed in Brazil - ©2015 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Article Cu and Ni Catalysts Supported on g-Al2O3 and SiO2 Assessed in Glycerol Steam Reforming Reaction Vivian V. Thyssen, Thaisa A. Maia and Elisabete M. Assaf* Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador Sãocarlense 400, 13560-970 São Carlos-SP, Brazil Catalisadores de Cu e Ni suportados em g-Al2O3 e SiO2 comerciais foram estudados frente a reforma a vapor de glicerol. Os catalisadores foram preparados pelo método da impregnação e caracterizados por espectrometria de emissão atômica por plasma acoplado indutivamente, fisissorção de nitrogênio, redução a temperatura programada com H2 e difração de raios X. A caracterização mostrou a presença de espécies com diferentes interações com os suportes: CuO, NiO e NiAl2O4. Os testes catalíticos foram realizados a 600 °C e, para o catalisador que apresentou o melhor desempenho, foram realizadas reações a 500 °C e a 700 °C. Os testes mostraram que a presença de NiAl2O4 desfavorece a deposição de carbono, provavelmente devido à maior dispersão do Ni na superfície do catalisador. Observou-se também que o catalisador NiSi foi o que apresentou a maior atividade para a formação de H2, porém este apresentou a maior deposição de carbono ao longo da reação. Cu and Ni catalysts supported on commercial g-Al2O3 and SiO2 were tested in the glycerol steam reforming. The catalysts were prepared by the impregnation method and characterized by inductively-coupled plasma atomic emission spectrometry, nitrogen physisorption, temperature programmed reduction with H2 and X-ray diffraction. The characterization indicated several catalytic species in the samples interacting differently with the supports: NiO, CuO and NiAl2O4. The catalytic tests were performed at 600 °C, and the best catalyst was also tested at 500 °C and 700 °C. The results showed that the presence of NiAl2O4 did not favor the deposition of carbon during the reaction, probably due to the greater dispersion of Ni on the catalyst surface. It was also observed that, although NiSi was the most active catalyst for H2 production, it also showed the highest carbon deposit during the reaction. Keywords: nickel, copper, catalysts, glycerol steam reforming Introduction Most of the world’s energy demand is supplied by fossil fuel burning. Besides subjecting countries to economic instability due to price fluctuations, these resources generate waste products that seriously endanger the environment, for example by global warming. Moreover, it is also known that one day these resources will be exhausted. Thus, alternative, clean and sustainable energy sources are vitally important. One such alternative source of energy that stands out is H2, which can be produced from renewable raw material and whose combustion in fuel cells involves little waste and practically no pollution. H2 can be produced by steam *e-mail: reforming of hydrocarbons and alcohols, such as methane, naphtha, methanol, ethanol and glycerol.1 Glycerol is produced as a byproduct in the transesterification reaction used to make biodiesel (equation 1), which is increasingly used as fuel, and the increase in its production results in a large supply of glycerol on the market. One possible way of using this excess glycerol is in the production of H2 by glycerol steam reforming reaction (GSR), given that, from 1 mol of glycerol, up to 7 mol of H2 can be produced (equation 2).1 C3H5(OOC)3(Rn)3 + 3R’OH D 3RnCOOR’ + C3H5(OH)3 (1) C3H5(OH)3 + 3H2O D 3CO2 + 7H2 DHf25 °C = +123.00 kJ mol-1 (2) Vol. 26, No. 1, 2015 Thyssen et al. Buffoni et al.2 observed that the conversion of glycerol into gaseous products depends greatly on temperature, with a minimum temperature of 550 °C to achieve H2 selectivity. According to Adhikari et al.,1 the conversion of glycerol into H2 and CO2 involves preferential C–C bond cleavage as opposed to the breaking of C–O bonds. It is well established that catalysts with a noble metal in their compositions have excellent properties in various reactions.3 However, these catalysts are expensive and highly susceptible to deactivation by poisons such as sulfur. Thus, there is a need to develop catalysts that are specifically suited to each purpose. On account of their low cost and good activity, supported Cu and Ni systems are good alternatives to noble metals. Carrero et al.4 studied Cu and Ni supported systems and chose Ni as one of the active metals for their catalysts, owing to its good activity in the steam reforming processes. Researchers suggest that Ni catalysts favor the C–C cleavage of the alcohols, leading to CH4, CO and H2 formation, but these catalysts are also known to favor the deposition of carbon, which may impair the performance of the catalyst.5-7 CuNi bimetallic catalysts have been studied, in order to avoid deactivation by carbon formation, due to the ability of Cu to inhibit such formation.4 g-alumina (g-Al2O3) and silica (SiO2) are widely used as supports, owing to their high specific areas, which provide a greater contact area and possibly a higher number of active sites for the desired reactions to occur. Therefore, considering the need to seek new energy sources and knowing the benefits of using H2, this article focuses on the study of Cu, Ni and CuNi catalysts supported on commercial g-Al2O3 and on SiO2, applied to the generation of H2 by the steam reforming of glycerol. Experimental Preparation The catalysts were prepared by the wet impregnation method, with mass contents of 10% Cu and 10% Ni for monometallic catalysts and 5% Cu plus 5% Ni for bimetallic catalysts, supported on the commercial g-Al2O3 and SiO2. Commercial SiO2 (Degussa, Aerosil 200) and g-Al2O3 (Alfa-Aesar) supports were treated at 600 °C for 2 h under a flow of synthetic air, for thermal stabilization and surface water removal. The supports were impregnated with the metals wetting them with aqueous solutions of Cu(NO3)2.3H2O and Ni(NO3)2.6H2O, the impregnation of the bimetallic catalysts being performed simultaneously with the two metals; the excess water was removed by rotary evaporation at 80 °C for 6 h and the samples were 23 dried at 80 °C for 12 h. The catalytic precursor oxides were then prepared by calcination for 3 h at 600 °C, under synthetic air flowing at 30 mL min-1. The samples obtained were: 10% Cu/g-Al2O3 (CuAl), 10% Ni/g-Al2O3 (NiAl), 5% Cu 5% Ni/g-Al2O3 (CuNiAl); 10% Cu/SiO2 (CuSi), 10% Ni/SiO2 (NiSi) and 5% Cu 5% Ni/SiO2 (CuNiSi). Characterization In order to measure the metal content of each sample, it was analyzed by atomic emission spectrometry of inductively-coupled plasma (ICP), with a Perkin Elmer ICP Optima 3000 DV. Ni and Cu standards were used to determine the calibration curve from which the concentrations of the metals in the catalysts were read. The nitrogen physisorption (...truncated)