A note on the vibrational efficacy in molecule-surface reactions

The effectiveness of vibrational energy in promoting dissociation of molecules colliding with surfaces can be measured through the so-called vibrational efficacy. It is by many thought to be a pure “energetic” measure and therefore believed to be limited from below by zero (in the case that there is no increase in dissociation probability upon vibrational excitation) and from above by one (in the case that all of the vibrational excitation energy is used to promote reaction). However, the quantity vibrational efficacy is clearly linked to the detailed dynamics of the system, and straightforward considerations lead to the conclusion that it is not limited either from below or above. Here we discuss these considerations together with a quasi-classical dynamics study of a molecule-surface system, N2/Ru(0001), for which a vibrational efficacy bigger than one has been found both experimentally and theoretically. We show that an analysis of the vibrational efficacy only in terms of energy transfer from vibration to translation can be too simple to describe the behavior of systems for which the potential energy surfaces present (high) reaction barriers, potential corrugation and anisotropy, and curved reaction paths

Mechanisms of H2 dissociative adsorption on the Pt(211) stepped surface

We utilize classical trajectory calculations to study the reaction dynamics of the dissociative adsorption of H2 on the stepped Pt(211) surface. The potential-energy surface has been obtained through an accurate interpolation of density-functional theory data at the generalized gradient approximation level, using the corrugation reduction procedure. New techniques for visualizing the collective dynamics of trajectories are introduced to elucidate the reaction mechanisms involved. Reaction exhibits a nonmonotonic dependence on collision energy, first decreasing with energy, and then increasing. A strong component of direct nonactivated reaction exists at the top edge of the step over the entire range of energies. The inverse relationship between reaction and collision energy at low energies is attributed to trapping in weak chemisorption wells. These wells also influence the direct reaction at the step, leading to a strong asymmetric dependence on incidence angle. Reaction on the terrace is activated, and only contributes significantly at high energies. Agreement with experiments on Pt(533) [A. T. Gee, B. E. Hayden, C. Mormiche, and T. S. Nunney, J. Chem. Phys. 112, 7660 (2000); Surf. Sci. 512, 165 (2002)] is good, and we are able to suggest new interpretations of the experimental data. © 2005 American Institute of Physics

Effect of transition metal dopants on initial mass transport in the dehydrogenation of NaAlH4: Density functional theory study

Sodium alanate (NaAlH4) is a prototype system for storage of hydrogen in chemical form. However, a key experimental finding, that early transition metals (TMs) like Ti, Zr, and Sc are good catalysts for hydrogen release (and reuptake) whereas traditional hydrogenation catalysts like Pd and Pt are poor catalysts for NaAlH4, has so far received little attention. We performed density functional theory (DFT) calculations at the PW91 generalized gradient approximation level on Ti, Zr, Sc, Pd, and Pt interacting with the (001) surface of nanocrystalline NaAlH4, employing a cluster model of the complex metal hydride to study the initial mass transport in the dehydrogenation process. A key difference between Ti, Zr, and Sc on one hand and Pd and Pt on the other is that exchange of the early TM atoms with a surface Na ion, whereby Na is pushed on to the surface, is energetically preferred over surface absorption in an interstitial site, as found for Pd and Pt. These theoretical findings are consistent with a crucial feature of the TM catalyst being that it can be transported with the reaction boundary as it moves into the bulk, enabling the starting material to react away while the catalyst eats its way into the bulk and affecting a phase separation between a Na-rich and an Al-rich phase. Additional periodic DFT/PW91 calculations in which NaAlH 4 is modeled as a slab to model dehydrogenation of larger NaAlH 4 particles and which only consider adsorption and absorption of Ti suggest that Ti prefers to absorb interstitially but with only a small energy preference over a geometry in which Ti has exchanged with Na. Additional nudged elastic band calculations based on periodic DFT show only a small barrier (0.02 eV) for exchange of Ti with a surface Na atom. The mechanism inferred from the cluster calculations is therefore consistent with the slab calculations and may well be important. © 2012 American Chemical Society.The work presented here has been supported by a grant from the Dutch research council NWO under the ACTS Hydrogen programme and by a grant of computer time by the Dutch National Computing facilities Foundation (NCF).Peer Reviewe

Effect of Transition Metal Dopants on Initial Mass Transport in the Dehydrogenation of NaAlH₄: Density Functional Theory Study

Sodium alanate (NaAlH₄) is a prototype system for storage of hydrogen in chemical form. However, a key experimental finding, that early transition metals (TMs) like Ti, Zr, and Sc are good catalysts for hydrogen release (and reuptake) whereas traditional hydrogenation catalysts like Pd and Pt are poor catalysts for NaAlH₄, has so far received little attention. We performed density functional theory (DFT) calculations at the PW91 generalized gradient approximation level on Ti, Zr, Sc, Pd, and Pt interacting with the (001) surface of nanocrystalline NaAlH₄, employing a cluster model of the complex metal hydride to study the initial mass transport in the dehydrogenation process. A key difference between Ti, Zr, and Sc on one hand and Pd and Pt on the other is that exchange of the early TM atoms with a surface Na ion, whereby Na is pushed on to the surface, is energetically preferred over surface absorption in an interstitial site, as found for Pd and Pt. These theoretical findings are consistent with a crucial feature of the TM catalyst being that it can be transported with the reaction boundary as it moves into the bulk, enabling the starting material to react away while the catalyst eats its way into the bulk and affecting a phase separation between a Na-rich and an Al-rich phase. Additional periodic DFT/PW91 calculations in which NaAlH₄ is modeled as a slab to model dehydrogenation of larger NaAlH₄ particles and which only consider adsorption and absorption of Ti suggest that Ti prefers to absorb interstitially but with only a small energy preference over a geometry in which Ti has exchanged with Na. Additional nudged elastic band calculations based on periodic DFT show only a small barrier (0.02 eV) for exchange of Ti with a surface Na atom. The mechanism inferred from the cluster calculations is therefore consistent with the slab calculations and may well be important