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Quantification of active sites is indispensable for understanding a catalytic material, ultimately enabling meaningful comparisons across different catalysts and discerning the underlying intrinsic kinetics. At present, however, few methods are available for quantitatively probing the catalytic surface of transition metal sulfides (TMS) during catalysis. TMS catalysts expose Bronsted acidic (sulfhydryls, SH) and Lewis acidic sites (coordinatively unsaturated sites, CUS) at their surfaces and, thus, are conceptually amenable to in situ acid-base titration with judicious choices of bases that strongly bind to the acidic entities. In this proof-of-concept study, in situ titration using several N-containing organic bases allowed us to determine the surface concentration of active sites and their specific activities for polyaromatics hydrogenation on unsupported TMS catalysts. Ni-promoted sulfide catalysts were more active than the unpromoted ones, because of a modest increase in the site density and a 2- to 3-fold enhancement in the TOF averaged over all titrated sites. By further discriminating between the titrated CUS and SH with site-selective titrants, a substantial difference in the TOF was found between CUS and SH among the titrated pool of sites. In addition to the titrated sites that contributed to the majority (80–95%) of the hydrogenation activity, some active sites remained unoccupied by the studied base molecules. The contribution of these residual sites, likely inaccessible to large-sized and less mobile titrants, depended on the temperature, H2 pressure and the catalyst, but was unaffected by the space time, titrant concentration, and the type or concentration of the polyaromatic reactant, for a given catalyst. This work leads to several emerging opportunities to better understand complex TMS catalysts. |
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