Inhibitor, Co-Catalyst, or Co-Reactant? Probing the Different Roles of H₂S during CO₂ Hydrogenation on the MoS₂ Catalyst

Abstract

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.

Sulfur Tolerant Subnanometer Fe/Alumina Catalysts for Propane Dehydrogenation

“…The temperature was ramped at 20 °C/min to 600 °C. Typically, the catalyst was pretreated in a stream of H 2 S ( P H 2 S = 0.01 atm, the balance N 2 ) at 600 °C for 4 h. More details regarding the experimental setup can be found in the previous work. , The number of reducible [Fe] sites was determined using H 2 -TPR. The rate (based on C 3 H 8 conversion per second per gram of catalyst) and selectivity were calculated according to eqs –.…”

Machine-Learned Corrections to Mean-Field Microkinetic Models at the Fast Diffusion Limit

“…To further demonstrate the importance of geometry factor and prove the power of the method, we compare various lattice geometries, i.e., square, hexagonal, one-dimensional (1D) band with width = 1, and 1D band (with square arrangement) with width = 2. We include the 1D band lattices as representatives of the active site on the edges of 2D catalysts such as MoS 2 , , where the active region is confined to the perimeter of the catalyst particle. 1D band has a closed boundary on its shorter dimension, while being periodic along its length.…”

Atomically Dispersed Tin-Modified γ-alumina for Selective Propane Dehydrogenation under H2S Co-feed

“…Alternative earth-abundant catalysts, such as metal oxides, are active and selective catalysts for alkane activation; − however, the exact influence of H 2 S on these catalysts is poorly understood, and they are likely unstable and get potentially converted into the respective sulfides . In this context, developing earth-abundant, sulfur-tolerant catalysts for shale gas upgrading to produce valuable liquid products can preclude extensive pretreatment . As a first step toward this, inexpensive, regenerable catalysts are sought that can dehydrogenate light alkanes selectively to the corresponding olefins, which are important precursors in the chemical industry.…”

“…18 In this context, developing earth-abundant, sulfurtolerant catalysts for shale gas upgrading to produce valuable liquid products can preclude extensive pretreatment. 19 As a first step toward this, inexpensive, regenerable catalysts are sought that can dehydrogenate light alkanes selectively to the corresponding olefins, which are important precursors in the chemical industry.…”