Chemisorption and TPD studies of hydrogen on Ni/Al2O3 Original Research Article

The activities of two supported nickel catalysts, a commercial (17 wt.-% Ni/Al2O3) and a non-commercial (10 wt.-% Ni/Al2O3) catalyst, were investigated in gas phase toluene hydrogenation. Both catalysts were active in hydrogenation, exhibiting rate maxima at about 443 K. The catalysts were characterized using hydrogen chemisorption and temperature programmed desorption (TPD) techniques. Decreasing hydrogen adsorption capacity was generally found in the temperature interval 298–423 K, the capacity of both the commercial and the non-commercial Ni-catalysts being about 20 cm3/gNi at 423 K. No effect on the total adsorption capacity was found by increasing the pretreatment temperature from 503 K to 773 K on the commercial catalyst. Three adsorption states of hydrogen (I–III) were resolved from the TPD-spectra of both catalysts. Hydrogen desorption was modelled with peak shape analysis as a second order process with free readsorption, giving hydrogen adsorption enthalpies ranging from −108 to −124 kJ mol−1 for adsorption state I. The kinetic data and the TPD studies indicate that the decrease of the toluene hydrogenation activity at temperatures above 443 K is due to the decay of adsorption state I.