Natural
gas is often sour, containing varying
amounts of H2S and CO2. Gas sweetening processes
that are routinely used rely on processes such as amine-based adsorption–desorption
cycles, membrane separation, and cryogenic technologies which are
energy-intensive and reject CO2. This work explores the
mechanism and nature of active sites for hydrogenation (HYD) of CO2 in the presence of H2S on MoS2, a sulfur-tolerant
catalyst, using steady-state kinetics studies and density functional
theory calculations. In the absence of H2S, this reaction
is positive order in CO2 and H2 and negative
order in CO and H2O; the system, further, shows no H/D
kinetic isotope effect, thereby indicating that C–O scission
is rate-controlling. The specific mechanism at play, namely, redox
or associative, could, however, not be adequately resolved with the
available data. In the presence of H2S, the rate first
drops (until about 60 ppm co-feed) and, then, increases (at >200
ppm),
indicating a complex functional relationship of the reaction system
with H2S. A combination of the ab initio phase diagram,
kinetic experiments, and H2S/D2 scrambling studies
indicate that (i) the edge structure of the molybdenum sulfide catalyst
varies substantially with reaction conditions and co-feeds, (ii) multiple
types of sites are at play, in particular, a sulfur-poor site that
is active but prone to H2S poisoning and a sulfur-rich
site that is less active but tolerant to sulfur with a preponderance
of data pointing to these being coordinative unsaturated (CUS) and
brim sites, respectively, on the metal edge, and (iii) H2S inhibits CO2 HYD by dissociatively adsorbing on these
CUS sites at low co-feeds (0–60 ppm), reacts with CO2 to produce COS and CO at high co-feeds (>1000 ppm) and assists
(or
co-catalyzes) HYD by probably acting as a hydrogen shuttle at intermediate
co-feeds.