Catalyzed Surface-Aligned Reaction, H(ad) + H₂(ad) = H₂(g) + H(ad) on Coinage Metals

Abstract

The first calculation of reaction dynamics was made in 1931 by H. Eyring and M. Polanyi for the gas-phase exchange reaction H(g) + H 2 (g) = H 2 (g) + H(g). Here we use Density Functional Theory to explore the dynamics of an analogous exchange reaction, H(ad) + H 2 (ad) = H 2 (g) + H(ad), taking place at coinage metal surfaces; Cu(100), Ag(100) and Au(100). The attacking atom, H(ad), recoils in a preferred direction toward the physisorbed H 2 (ad), as envisioned in other studies of 'Surface Aligned Reaction' (SAR). The dynamics of SAR for the above reaction is compared with gas-phase reaction, employing the same initial state geometry in the absence of the surface. The effect of weak physisorption at the metal surface is to approximately halve the bond dissociation energy of H 2 (ad) as compared with H 2 (g). As a result, the surface markedly catalyzes the exchange reaction, decreasing the energy barrier due to increased heat of reaction in the adsorbed state, in accord with the Brønsted, Evans and Polanyi (BEP) relationship.

Retention of Bond Direction in Surface Reaction: A Comparative Study of Variously Aligned p-Dihalobenzenes on Cu(110)

“…The finding in every case is that the observed initial bond directions provide a useful guide to the product angular distributions. This finding is shown to conform to the expectation from MD computations, employing an impulsive model of electron-induced reaction taking place across a multilayer slab corresponding in composition to the substrate. − …”

“…These included the ground state of the physisorbed pDIB and pDCB, the products of the electron-induced reaction, the molecular dynamics trajectories, the electronic structures, and the STM image simulations. The parameters employed here are similar to those previously reported, − namely, the general gradient approximation (GGA) for the exchange-correlation potential, , the projector-augmented wave (PAW) method, , and the Perdew–Bruke–Ernzerhof (PBE) functional. The second version of Grimme’s dispersion correction was applied to PBE (PBE-D2) to include the van der Waals interactions.…”

Vibrational Excitation Induces Double Reaction

“…Plane-wave-based density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP 5.2), , installed at the SciNet supercomputer . The parametrizations of geometry and MD calculations are identical to our earlier work, ,− using the general gradient approximation with the Perdew–Burke–Ernzerhof functional and the projector-augmented wave scheme. , The Cu(110) surface was represented by a (6 × 6) supercell that consisted of 180 Cu atoms in five layers and a vacuum gap of ∼15 Å. The surface Brillouin zone was sampled at the gamma point only.…”

How Adsorbate Alignment Leads to Selective Reaction

“…In geometry optimizations, all atoms except those for the bottom two Cu layers were fully relaxed until the residual force per atom was less than 0.02 eV/Å. The I2S model, − ,− described elsewhere in detail, incorporates an anionic state of duration t * simulated by a pseudopotential approach. ,, The molecular dynamics in the anionic state were calculated until t *. The system was then returned to the ground state with atomic momenta and positions taken from the excited state.…”

“…The dynamics of the electron-induced reaction are well-described by an “impulsive two-state” (I2S) model introduced in earlier work, − ,− in which the C–I bond becomes an antibond for some femtoseconds with the result that the I atom and the radical recoil in opposite directions along the prior C–I axis. In the present example, the direction of the C–I axis can be chosen by the experimenter by selecting the physisorbed m -IPy and placing the STM tip that delivers electrons over it to induce reaction either along “A” or “P”, with markedly different outcomes.…”