Plasma Catalysis Modeling: How Ideal Is Atomic Hydrogen for Eley–Rideal?

Plasma catalysis is an emerging technology, but a lot of questions about the underlying surface mechanisms remain unanswered. One of these questions is how important Eley–Rideal (ER) reactions are, next to Langmuir–Hinshelwood reactions. Most plasma catalysis kinetic models predict ER reactions to be important and sometimes even vital for the surface chemistry. In this work, we take a critical look at how ER reactions involving H radicals are incorporated in kinetic models describing CO2 hydrogenation and NH3 synthesis. To this end, we construct potential energy surface (PES) intersections, similar to elbow plots constructed for dissociative chemisorption. The results of the PES intersections are in agreement with ab initio molecular dynamics (AIMD) findings in literature while being computationally much cheaper. We find that, for the reactions studied here, adsorption is more probable than a reaction via the hot atom (HA) mechanism, which in turn is more probable than a reaction via the ER mechanism. We also conclude that kinetic models of plasma-catalytic systems tend to overestimate the importance of ER reactions. Furthermore, as opposed to what is often assumed in kinetic models, the choice of catalyst will influence the ER reaction probability. Overall, the description of ER reactions is too much “ideal” in models. Based on our findings, we make a number of recommendations on how to incorporate ER reactions in kinetic models to avoid overestimation of their importance.


S.1 Convergence of computational parameters for DFT calculations
The adsorption energy of species, Eads, is defined as: Where Eadsorbate+surface, Esurface, and Eadsorbate are the total energies of the adsorbate on the slab, the clean slab and the gaseous adsorbate, respectively.

Figure S1 :
Figure S1: Convergence of the H adsorption energy on Cu(111) as a function of the cutoff.The different lines show the convergence for different electronic SCF convergence criteria.

Figure S2 :
Figure S2: Convergence of the H adsorption energy on Ni(111) as a function of the cutoff.The different lines show the convergence for different electronic SCF convergence criteria.

Figure S3 :
Figure S3: Convergence of the H adsorption energy on Ru(0001) as a function of the cutoff.The different lines show the convergence for different electronic SCF convergence criteria.

Figure S4 :
Figure S4: Convergence of the H adsorption energy as a function of the K-point grid.The different lines show the convergence for different metals.

Figure S. 5 :
Figure S.5: Illustrative picture on how the PES intersections are constructed for H(g) + C* at Ni(111).The distance between the H atom positions is not to scale.The different positions of the H atom are represented by white spheres.

Figure
Figure S.7: PES intersection for H(g) + N* at Ru(0001) surface along the hcp-top-fcc line (top panel) and the hcpbridge-fcc line (bottom panel).

Figure
Figure S.8: PES intersection for H(g) + O* at Ni(111) surface along the fcc-top-hcp line (top panel) and the fccbridge-hcp line (bottom panel).

Figure S. 12 :
Figure S.12: PES intersection for H(g) + OH* at Ni(111) surface along the fcc-top-hcp line (top panel) and the fcc-bridge-hcp line (bottom panel).

Figure S. 19 :
Figure S.19: PES intersection for H(g) + N* at Ru(0001) surface along the hcp-top-fcc line (top panel) and hcpbridge-fcc line (bottom panel) for a high coverage of N.

Figure S. 20 :
Figure S.20: PES intersection for H(g) + O* at Ni(111) surface for high O* coverage along the fcc-top-hcp line (top panel) and the fcc-bridge-hcp line (bottom panel).

Figure S. 21 :
Figure S.21: PES intersection for H(g) + NH* at Ru(0001) surface along the hcp-bridge-fcc line for a high coverage of NH*.

Figure S. 23 :
Figure S.23: PES intersection for H(g) + CH* at Ni(111) surface along the fcc-bridge-hcp line for a high coverage of CH*.
Table S.1: C adsorption energies in eV for the different metal surfaces.