In general, it is known that the reactivity can be promoted as the concentration of hydroxyl groups increases, and various studies have been reported to increase the hydroxyl group as the adsorption site of reactants[1]. Herein, we investigated the influence of HCl treatment on the acidity of the TiO
2 surface. The catalysts were prepared by the treated TiO
2 and abbreviated as Pd/XH-T, where X is represents the molarity of HCl. From NH
3-TPD result, it is observed that the concentration of the weak acid site increases as the concentration of HCl increases, whereas TiO
2 treated with above 4 M HCl showed the reduced acidic concentration. This was ascribed to the presence of an excessive chlorine on the surface of TiO
2. The excessive chlorine on the surface inhibits the adsorption of ammonia, due to the reduction in surface hydroxyl groups[2]
. The catalytic activity of Pd/TiO
2 catalysts was tested for hydrogenation of furfural to tetrahydrofurfuryl alcohol. As a result, all the catalysts showed more than 94% conversion. Among them, Pd/2H-T with the highest amount of acid sites showed the highest selectivity of 91.5%, while the catalyst with the least amount of acid sites showed the lowest selectivity of 42.8%. The surface acidity on TiO
2 was varied by HCl concentration that increases the concentration of OH groups on surface. From this result, it was confirmed that the selectivity was influenced by the amount of weak acid sites which act as an adsorption site for reactants. Thus, we suggest that selectivity of tetrahydrofurfuryl alcohol was affected by the amount of weak acid sites.
[1]J.E. De Vrieze. et.al., âRole of surface hydroxyl species in copper catalyzed hydrogenation of ketonesâ, ACS Catal., 8, 7539â7548, (2018).
