Synthesis of Pd/ Al2 O3 catalysts using different techniques and its catalytic activity for acetylene hydrogenation recation

Abstract

The ĐatalLJtiĐ perforŵaŶĐe of Pd ŶaŶopartiĐles supported oŶ ϒ-Al2O3 aŶd α-Al2O3 have ďeeŶ iŶǀestigated iŶ the seleĐtiǀe hLJdrogeŶatioŶ of aĐetLJleŶe. ϒ-Al2O3 aŶd α-Al2O3 were synthesized by solution combustion using Al(NO3)3.9H2O precursor as well as sol gel using Al(OC4H9)3 and AlCl3.6H2O precursors. Pd dispersion on the Al2O3 support (1wt% Pd/ Al2O3) was accomplished via chemical reduction method using formaldehyde as reducing agent at 80°C. Low temperature synthesis of Pd/ Al2O3 catalyst prevented the doping of the compound and ensured the presence of Pd inmetallic state. Effect of temperature and flow rate on the catalytic activity of the synthesized Pd/ Al2O3 catalysts have been explored in the present work.

It was observed that ethylene selectivity increases as surface area decreases in the Pd/ Al2O3 catalysts synthesized by solution combustion and sol gel (using Al(OC4H9)3 precursor) technique. Lower is the surface area, lower is the Pd dispersion on the support and lower are the active sites available for direct ethane formation. However, this is not universally true as can be noticed in the case of Pd/ Al2O3 catalysts synthesized by sol gel technique using AlCl3.6H2O precursor. In this case, irrespective of the surface area ethylene yield remained zero. Preparation method of support modified the surface nature of the catalyst which played another crucial role in ethylene selectivity. CO2 temperature programmed desorption (TPD) studies suggested that lower basicity of the Al2O3 surface resulted in better ethylene selectivity for both solution combustion and sol gel method. Moreover, the crystallite size of α-Al2O3 particles is not an important parameter affecting ethylene selectivity. Pd/ α-Al2O3 catalyst synthesized by solution combustion method using Al(NO3)3.9H2O precursor at 1200°C shows the best ethylene selectivity during the entire temperature range from RT to 125°C. 100% conversion as well as good selectivity is simultaneously obtained at 125°C which is a superior performance as compared to most of the existing catalysts in literature.

Pd/ ϒ-Al2O3 catalyst synthesized by solution combustion method using Al(NO3)3.9H2O precursor at 700°C is a moderately good catalyst in terms of ethylene selectivity. It shows a decrease in conversion with the increase in total flow rate of reactant gases. However, flow rate seems to have a negligible impact on ethylene selectivity as well as yield. We may extend this result to other catalysts as well.