Hydrogen on unsupported ruthenium sulfide. Thermodesorption and 1H NMR studies

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Hydrogen on Unsupported Ruthenium Sulfide: Thermodesorption and 1H NMR Studies
The interaction of transition metal sul fi des with hydrogen is a complex process.Understanding this process is important due to the very large number of industrial pro cesses dealing with these systems.The main utilization of these catalysts concerns cata lytic hydrotreating which involves hydroge nation reactions and C-S, C-N, and C-0 bond cleavage reactions carried out in the presence of high partial pressures of hydro gen.The complexity of the interaction of hydrogen with metal sulfi des is related to its double role, as reactant and as modifi er of the concentration of active sites.As a matter of fact, the interaction of hydrogen with the surface of the catalyst provokes its partial reduction, leading to coordinatively unsatu rated metal ions which are directly involved in the adsorptive and catalytic properties (1).
As molybdenum sulfi de is the essential component of industrial hydrotreating cata lysts, its interaction with hydrogen has been the subject of many investigations.The re view of Moyes (2) and the recent paper of Hall and Komatsu (3) have shown that the amount of adsorbed hydrogen changes dras tically from one study to another.In order to compare easily this large number of litera ture data the amount of adsorbed hydrogen is often expressed using the nomenclature HxMoS 2 • Using the same preparation proce dure (decomposition of ammonium tetrathi omolybdate) the values of x vary between 0.012 and 0.37.This important variation of x has probably to be ascribed to different catalysts pretreatment prior to adsorption measurements, leading to different number of vacancies.This hypothesis has been demonstrated by Jalowiecki et al. (4), who have reported that the activity and the hy drogen adsorption capacity of a molybde num-based catalyst is greatly dependent on the sulfur-to-molybdenum ratio.Concern ing the chemical nature of the adsorbed species, inelastic neutron scattering studies have evidenced that dissociated hydrogen resides on sulfur anions in the form of SH groups (5,6).This result has been recently confi rmed by solid NMR spectroscopy (3).Although these techniques have never shown the presence of adsorbed hydride type hydrogen, its presence on molybdenum or rhenium sulfides has been assumed for explaining the mechanisms of thiophene or hydrothiophene hydrodesulfurization and alkenes hydrogenation (7,8).
The relation between the sulfur-to-metal ratio and the catalytic activity was also stud ied for other sulfi des.We have recently re ported that the H2-D2 exchange activity of a model ruthenium sulfi de is drastically en hanced by sulfur removal (9).As ruthenium sulfi de is one of the most active chalcogen ide (13 times more active than molybdenum sulfi de in the hydrodesulfurization of thio phene and 10 times in the hydrogenation of biphenyl), its activity could be related to a large adsorption capacity or to the presence of adsorbed hydrogen species different from those already detected on lamellar sulfi des.To tackle this fundamental problem of hy drogen adsorption on ruthenium sulfi de, we have taken the fully sulfi ded state as starting point and evaluated the infl uence of progres sive desulfurization on the amount of ad sorbed hydrogen.The nature of the ad sorbed species was investigated using thermodesorption and 1H NMR.
Ruthenium sulfi de was prepared by pre- lated to H1 species follows a completely dif ferent trend, i.e., its amount increases with desulfurization, reaches a maximum for a = 0.3-0.4,and then decreases above.
This different behavior of H1 and H 2 to ward sulfur removal suggests that different adsorbing sites are required for hydrogen chemisorption.As the concentration of H 2 decreases when the desulfurization of the solid increases, this type of hydrogen does not seem to be adsorbed on anionic vacanc ies but in interaction with sulfur anions as SH groups.This species is probably the one observed in Heise 's INS work on ruthenium sulfide (10).By contrast, the amount of H1 hydrogen increases with desulfurization up to a-0.4.At this stage of the reduction, the surface of the catalyst contains a very low concentration of sulfur anions.Conse quently, these experimental data suggest that H1 is adsorbed on coordinatively unsat urated ruthenium ions.The comparison of these adsorption results with those obtained previously for the Hi-D 2 exchange reaction (also reported in Fig. 2) and the 1-butene hydrogenation reaction (9, 11) evidences a striking similarity in their variations with a.This strongly favours the hypothesis that H1 is the active species responsible for the hydrogenation reaction.For a > 0.5, the diminution of H1 adsorption can be ascribed to surface area collapsing and to the forma tion of a surface-poisoned ruthenium metal-lie phase inactive for hydrogen activation, as already proposed for the exchange reac tion (9,(12)(13)(14).
To obtain more information about the chemical nature of H1 and H 2 two samples were studies by solid 1H NMR spectros copy.Sample 1 was poorly desulfurized and presented both kinds of adsorbed hydrogen (a�0.15),whereas sample 2 contained mostly type-1 hydrogen (a = 0.35).Both magic-angle-spinning (MAS) spectra were recorded in sealed tubes on a Bruker MSL apparatus at 400 MHz at room temperature.A weak rotation speed (1 kHz) was enough to observe the signals (Fig. 3), the small widths of which characterize species mov ing during the time characteristic of the ex periment 00-9 sec).
Figure 3 shows the results of the 1H NMR.For both solids, the chemical shift relative to tetramethylsilane could be deduced with fair accuracy.Sample 1 exhibits two signals at -7.4 ppm and 5 .1 ppm.The value of the later is consistent with an acidic character of the adsorbed species, in agreement with the hypothesis of the formation of superfi cial SH groups (15).The other signal, -7.4 ppm (upfi eld), can be most likely ascribed to a hydride-type species (16)(17)(18)(19)(20).
To sum up, the characterization of hydro gen adsorbed on partially desulfurized ru thenium sulfi de has evidenced two different species: one leading to the formation of SH groups and the other with hydridic character being chemisorbed on coordinatively unsat urated ruthenium cations.The latter, related to the hydrogenating properties of the cata lyst, probably explains the high activity of ruthenium sulfi de.

ppm - 7 . 4 Fm. 3 .
Solid 1H NMR of adsorbed hydrogen.The degree of reduction is 0.15 for sample 1 and 0.35 for sample 2.