In studying the HDS catalysts, two kinds of catalysts were investigated. The one is carbon supported CoMo catalysts (CoMo/C) and the other is the unsupported NdNi catalysts. The synergistic effects of thiophene HDS reaction were studied over CoMo/C catalysts having 7 different metal compositions and the unsupported NdNi catalysts were studied as a candidate for new HDS catalysts.
For the CoMo/C catalysts the heats of adsorption for each of thiophene and hydrogen sulfide($H_2S$) were obtained through temperature programmed desorption(TPD) experiments by measuring the appearance of peak maxima with different heating rates. It was found that $H_2S$ is more strongly adsorbed on the CoMo/C catalysts than thiophene. Assuming the competitive adsorption between thiophene and $H_2S$, the synergistic effect of HDS reaction in CoMo/C catalysts was explained by correlating the activity of thiophene HDS with the difference in the heat of adsorption between thiophene and $H_2S$. It should be important to develop catalysts having a high HDS activity, which could control the adsorption strength of $H_2S$.
The conventional HDS catalysts for heavy oil have relatively short life time, while the demand in hydrotreating the heavy oil is ever increasing. For the development of more stable HDS catalysts, new species of active catalysts should be pursued. In this regard, two types of unsupported neodymium-nickel(NdNi) catalysts were investigated, i.e., intermetallic $NdNi_5$ and coprecipitated bimetallic NdNi catalysts. Unsupported NdNi catalysts undergo drastic physical and chemical changes during calcination, presulfidation and reaction. After calcination each metal in the catalysts exists as metal oxide, and after presulfidation or reaction nickel exists nickel sulfide while neodymium as oxysulfide. Even intermetallic $NdNi_5$ lost its intermetallic properties.
Although Ni alone showed very low HDS activity, the addition of small amount of Nd to Ni inhanced HDS activity remarkably. This seems to be due to the increase of surface area with Nd addition. It seems that the $Ni_3S_2$ is active metal and Nd works as a structural promoter preventing nickel from sintering. The thiophene HDS activity was well correlated with the surface area of unsupported catalysts after reaction.
From additional experiments using La and Ce, it was found that these metals also show similar behaviours of Nd.
