Fingerprints of energy dissipation for exothermic surface chemical reactions: O2 on Pd(100)

Abstract

We present first-principles calculations of the sticking coefficient of O 2 at Pd(100) to assess the effect of phononic energy dissipation on this kinetic parameter. For this we augment dynamical simulations on sixdimensional potential energy surfaces (PESs) representing the molecular degrees of freedom with various effective accounts of surface mobility. In comparison to the prevalent frozen-surface approach energy dissipation is found to qualitatively affect the calculated sticking curves. At the level of a generalized Langevin oscillator (GLO) model we achieve good agreement with experimental data. The agreement is similarly reached for PESs based on two different semi-local density-functional theory functionals. This robustness of the simulated sticking curve does not extend to the underlying adsorption mechanism, which is predominantly directly dissociative for one functional or molecularly trapped for the other. Completely different adsorption mechanisms therewith lead to rather similar sticking curves that agree equally well with the experimental data. This highlights the danger of the prevalent practice to extract corresponding mechanistic details from simple fingerprints of measured sticking data for such exothermic surface reactions.

Coverage-dependent adsorption and desorption of oxygen on Pd(100)

“…However, a recent O 2 /Pd(100) adsorption dynamics study that used classical trajectory calculations on a PES calculated with an Revised Perdew-Burke-Ernzerhof (RPBE) functional challenges this idea. 15 A 0.4 eV barrier between the O 2,chem and O ads states is found. Trapping into the molecular state on the bare surface becomes the dominant process when incorporating a generalized Langevin oscillator (GLO) model to allow for energy transfer to the surface.…”

“…15 On a PBE-based PES, the majority of molecules were found to stick through direct dissociation. The energy dependence to sticking and the surface-temperature independence agree reasonably well with our experimental data.…”

“…[13][14][15] Barriers are mostly reported to be on the order of 0.2 eV. As the transition from O 2 to the O 2,chem state is highly exothermic, the small a) l.juurlink@chem.leidenuniv.nl barrier to the dissociated state is generally not expected to hamper dissociation.…”

“…For example, the dissociative adsorption and recombinative desorption of O 2 on Pd(100) has been extensively studied experimentally and theoretically. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] In a previous study, we have argued that O 2 dissociation on clean Pd(100) proceeds mostly via a direct mechanism. 16 Incident kinetic energy modestly increases the dissociation probability.…”

“…The reaction energy is efficiently dumped into phonons. 14,15 An early highresolution electron energy loss spectroscopy (HREELS) study showed that O 2 dissociates on the bare Pd(100) surface at T s = 80 K 17,18 and indicated it even to occur prior to the associative adsorption at 10 K. 18 In agreement with most theoretical studies, this suggests that any barrier between the O 2,phys , O 2,chem , and O ads states on clean Pd(100) is easily surmounted by the energy released during adsorption and dissociation. However, a recent O 2 /Pd(100) adsorption dynamics study that used classical trajectory calculations on a PES calculated with an Revised Perdew-Burke-Ernzerhof (RPBE) functional challenges this idea.…”