Hydrogenation and C[sbnd]S bond activation pathways in thiophene and tetrahydrothiophene reactions on sulfur-passivated surfaces of Ru, Pt, and Re nanoparticles

Abstract

Thiophene-H2 reactions proceed via sulfur removal and hydrogenation routes on dispersed metal nanoparticles that become decorated by refractory S adlayers during catalysis. The identity and kinetic relevance of the required elementary steps are described here based on rates measured at S-chemical potentials set by H2S/H2 ratios similar to those prevalent during practical catalysis on Re, ReSx, Ru, and Pt catalysts. The free energies of formation of S adatoms from H2S/H2 reactants are large and negative at low coverages (< −50 kJ mol−1 on Pt(111) and < −150 kJ mol−1 on Re(0001) and Ru(0001)), but strong repulsion among adatoms prevents full saturation, leading to sub-monolayer coverages and to passivated surfaces that expose uncovered interstices. The residual interstitial spaces (*) within adlayers of unreactive S-atoms (S′) bind intermediates and transition states reversibly, thus allowing these passivated surfaces to carry out catalytic turnovers. These interstices are larger on Pt than on Ru or Re surfaces, leading to concomitantly higher turnover rates for thiophene hydrogenation and desulfurization routes. The identity and kinetic relevance of elementary steps for desulfurization (to form C4 hydrocarbons) and hydrogenation (to form tetrahydrothiophene; THT) are similar among these catalysts; they involve kinetically-relevant H-addition steps of thiophene-derived intermediates that cleave their CS bonds or “over-hydrogenate” to THT in one surface sojourn. THT undergoes CS bond cleavage in secondary reactions that correct this over-hydrogenation to form the more unsaturated species that cleave CS bonds. THT/C4 product ratios are insensitive to H2S/H2 ratios and thiophene pressure, even though active interstitial spaces are covered by kinetically-detectable S* and bound thiophene; therefore, primary and secondary reactions must occur on the same active ensembles. The observed increase in THT/C4 ratios with H2 pressure shows that THT formation transition states involve a larger number of H-atoms than for CS cleavage. CS bonds cleave in species with partial unsaturation (H-contents between those in THT and thiophene), in processes that are reminiscent of the requirement to add or remove H-atoms from C and O atoms in cleaving CC and CO bonds in alkanes and alkanols, as well as CO bonds in CO hydrogenation reactions. The rate and selectivity data and the mechanistic conclusions described here show that thiophene hydrogenation and desulfurization routes require similar binding interstices within refractory S adlayers and that metal-sulfur bond energies act as indirect descriptors of reactivity because they determine the number, size, and binding properties of the exposed weakly-binding ensembles that stabilize the relevant transition states and enable catalytic turnovers.