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One of the most useful techniques to obtain valuable information on catalyzed heterogeneous reactions at, or near to, molecular level is the Steady-State Isotopic Transient Kinetic Analysis (SSITKA). Kinetic parameters of catalyst-surface reaction intermediates, such as concentration, site coverage, reactivity, and rate constants can be obtained and processed to provide valuable information about the reaction mechanism. This technique has been extensively tested in a wide range of different surface-catalyzed reactions, where the influence of different parameters on the intermediates has been studied (i.e., supports, active phases, particle size, addition of promoters). Progresses in the coupling of spectroscopic techniques and advanced modeling could greatly improve the understanding of the surface reaction mechanism and provide more reliable kinetic models. This review compiles the main goals achieved up to date in heterogeneous catalytic systems using SSITKA and analyzes the perspectives of this technique in the near future.

This review describes the recent progress in computational catalysis that has addressed the C–H bond activation of light alkanes.

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The fundamentals of structure sensitivity and promoter effects in the Fischer−Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12 CO to 13 CO and H 2 to D 2 , was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2−7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH 4 response curves revealed the presence of parallel side-pools of reacting carbon. CH x coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CH x coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppressed upon addition of promoters in line with lower methane selectivity and higher lower olefin selectivity. Density functional theory (DFT) was applied on H adsorption for a fundamental understanding of this promoter effect on the selectivities, with a special focus on counterion effects. Na 2 S is a better promoter than Na 2 O due to both a larger negative charge donation and a more effective binding configuration. On the unpromoted Fe 5 C 2 (111) surface, H atoms bind preferably on C after dissociation on Fe. On Na 2 S-promoted Fe 5 C 2 surfaces, adsorption on carbon sites weakens, and adsorption on iron sites strengthens, which fits with lower H coverage, less CH 4 formation, and more olefin formation.

Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importancewith the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH4 formation and C1–C1 coupling on a thermodynamically stable, terraced-like χ-Fe5C2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH4 formation and C1–C1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C2+ selectivity. This indicates that manipulating the crystal facet of χ-Fe5C2 catalyst can effectively tune the FTS selectivity, which will open a new avenue for highly selective Fe-based FTS catalysts.