DFT and microkinetic modeling study for the catalytic CO2 hydrogenation on small Ru clusters encapsulated in Silicalite

The increasing concentration of atmospheric CO 2 has intensified efforts to develop efficient strategies for carbon capture and utilization. Among the most promising approaches is the catalytic hydrogenation of captured CO 2 into value-added chemicals and fuels, that provides a pathway towards a sustainable carbon-neutral economy. In this context, single-atom and few-atom metal clusters encapsulated in zeolites have emerged as highly attractive catalysts due to their unique combination of activity, selectivity and stability. Based on our previous investigation of CO 2 hydrogenation over the single-atom catalyst Ru 1 @S-1, the present study explores the catalytic performance of small ruthenium clusters encapsulated in Silicalite (Ru 2–4 @S-1) to elucidate the influence of cluster size on the activity and the product formation pathways. To this end, Density Functional Theory (DFT) calculations combined with Microkinetic Modeling (MkM) simulations were carried out at temperatures in the range 600–800 K and in the range of pressures 1–10 bars. The results reveal that CO (g) and H 2 O (g) are the dominant products for all catalysts. Compared with SAC, the few-atom Ru clusters exhibit a substantial enhancement in turnover frequencies (TOF). Notably, Ru 2 @S-1 shows a higher tendency to CH 2 O (g) formation, whereas Ru 4 @S-1 shows an enhanced tendency to HCOOH (g) reaching TOFs comparable to those of CO (g) and H 2 O (g) at 800 K. However, the calculated TOFs reported correspond to upper-bound estimates since product re-adsorption are not included in the MkM reactor model considered. These findings demonstrate that tailoring the size of the encapsulated clusters provides an effective strategy to tune both catalytic activity and product distribution.