Lowering Energy Barriers in Surface Reactions through Concerted Reaction Mechanisms

Abstract

Any technologically important chemical reaction typically involves a number of different elementary reaction steps consisting of bond-breaking and bond-making processes. Usually, one assumes that such complex chemical reactions occur in a step-wise fashion where one single bond is made or broken at a time. Using first-principles calculations based on density functional theory we show that the barriers of rate-limiting steps for technologically relevant surface reactions are significantly reduced if concerted reaction mechanisms are taken into account.

Novel insight into the hydrogen absorption mechanism at the Pd(110) surface

“…Thereby this concerted process may require considerably less activation energy than the monatomic migration of chemisorbed H atoms into the subsurface. This was recently demonstrated by Groß and co-workers, 45,48 who traced the events following H 2 dissociation on H-precovered Pd(100) with ab initio molecular dynamics simulations on DFT potential energy surfaces. According to their results, 45 the energy barrier for an excess H atom to settle from a bridge position into a neighboring fourfold hollow site while the H atom chemisorbed at that site simultaneously penetrates into the subsurface amounts to only 0.06 eV.…”

“…For 0.5 ML H on Pd(100) in a (2 × 2) arrangement that provides no neighboring vacancies, H 2 dissociates through a molecular precursor near the H-vacancy over a barrier of 0.1 eV, placing one H atom into the empty fourfold hollow chemisorption site and an excess H atom into the adjacent bridge position. [45][46][47][48] Activation barriers for H 2 dissociation also develop at high H coverages on Pd(111). 49,50 These theoretical results indicate that (in contrast to bare Pd surfaces) the dissociation of H 2 in high coverage regimes becomes impeded by small activation barriers that are well comparable to the experimental E abs values.…”

“…39 Since P diss is less than 1, the model predicts a maximum P abs of 2.5 × 10 −8 , which, however, is much smaller than the experimental value of ∼5 × 10 −4 at 145 K. This may indicate that H 2 dissociation involves mobile H 2 precursors, which diffuse across the surface and thereby increase the probability of H 2 to find vacant chemisorption sites. A H 2 precursor with an adsorption energy of E a (H 2 ) ∼ −0.16 eV such as described 48 on H-precovered Pd(100) at θ = 0.5 will have a rather short surface residence time (3.6 × 10 −8 s at 145 K), yet if its surface diffusion velocity is high (i.e., gas phase-like, ∼1000 m/s), the resulting large number (∼10 4 ) of encounters with surface Pd sites may suffice to compensate for the very small H vacancy coverage.…”

“…45,46,48 If the pre-adsorbed H atom transits below the surface while the excess H atom simultaneously reoccupies the chemisorption site that is being vacated, we refer to this process as "concerted penetration." Due to the concerted motion the energy gain in stabilizing the excess H atom in the deep chemisorption well can compensate a large part of the energy cost to transport the pre-chemisorbed H atom below the surface.…”

Reaction Network of Methanol Synthesis over Cu/ZnO Nanocatalysts

“…This demonstrates an additional active role of CO in the C 1 reduction process over 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 approaches, carefully narrowing down transition states, and finally estimating thermodynamic corrections by virtue of harmonic analysis using the optimized structures. 6,[81][82][83][84][85]88 Applying this traditional approach to low-index Cu surfaces and unsupported Cu clusters, three different reaction mechanisms were proposed: the so-called HCOO -, CO and COOH + based mechanisms (for a recent review see Ref. 77).…”