Accurately simulating heterogeneously catalyzed reactions requires reliable barriers for molecules reacting at defects on metal surfaces, such as steps. However, first-principles methods capable of computing these barriers to chemical accuracy have yet to be demonstrated. We show that state-resolved molecular beam experiments combined with ab initio molecular dynamics using specific reaction parameter density functional theory (SRP-DFT) can determine the molecule-metal surface interaction with the required reliability. Crucially, SRP-DFT exhibits transferability: the functional devised for methane reacting on a flat (111) face of Pt (and Ni) also describes its reaction on stepped Pt(211) with chemical accuracy. Our approach can help bridge the materials gap between fundamental surface science studies on regular surfaces and heterogeneous catalysis in which defected surfaces are important.

Stepped transition metal surfaces, including the reconstructed Pt(110)-(2 × 1) surface, can be used to model the effect of line defects on catalysts. We present a combined experimental and theoretical study of CHD 3 dissociation on this surface. Theoretical predictions for the initial sticking coefficients, S 0 , are obtained from ab initio molecular dynamics calculations using the specific reaction parameter (SRP) approach to density functional (DF) theory, while the measured sticking coefficients were obtained using the King and Wells method. The SRP DF used here had been previously derived for methane dissociation on Pt(111) so that the experiments test the transferability of this SRP DF to methane + Pt(110)-(2 × 1). The agreement between the experimental and calculated S 0 is poor, with the average energy shift between the theoretical and measured reactivities being 20 kJ/mol. There are two factors which may contribute to this difference, the first of which is that there is a large uncertainty in the calculated sticking coefficients due to a large number of molecules being trapped on the surface at the end of the 1 ps propagation time. The second is that the SRP32-vdW functional may not accurately describe the Pt(110)-(2 × 1) surface. At the lowest incident energies considered here, Pt(110)-(2 × 1) is more reactive than the flat Pt(111) surface, but the situation is reversed at incident energies above 100 kJ/mol.

, 2018 --Transition-metal catalysts, such as nickel and cobalt, are widely used in industry to produce hydrogen and other useful compounds from natural gas. Researchers achieve this transformation through steam reforming, which is the process of heating methane with steam in the presence of the catalyst, thus producing hydrogen and carbon monoxide.Transition metals are known for their superior catalytic capabilities and researchers know that the most significant reactions occur at the surface of the catalysts. So far, the search for even better catalysts has been largely based on trial and error, and on the assumption that catalyzed reactions take place on step edges and other atomic defect sites of the metal crystals.An international research team from Switzerland, the Netherlands, and the United States has combined experiments using advanced infrared techniques with quantum theory to explore methane dissociation reactions in minute detail. For the first time, their research shows exactly where the most significant reactions occur on the catalyst's surface. The researchers focused on platinum (Pt) as the catalyst to break down methane, but the model can be applied to other transition-metal catalysts, such as nickel. They report their findings this week in The Journal of Chemical Physics, from AIP Publishing."A tested predictive theory with chemical accuracy could change the way one searches for new catalysts and make the search more efficient and cheaper," said Rainer Beck, co-author of the paper and professor of chemical science and engineering at École Polytechnique Fédérale de Lausanne (EPFL).At the atomic scale, the surface of a platinum catalyst (as well as other metal crystals) can consist of steps, terraces, and other defects that are seen as important "sites" in the catalytic process.The Pt(211) surface has three-atom-wide terraces and one-atom-high steps. The researchers labeled the row of atoms on the step edge as "step" (red), the middle row as "terrace" (black) and the final row as "corner" (gray).

We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and Pt( 110)-(1 × 2). Reflection absorption infrared spectroscopy (RAIRS) was used to detect chemisorbed methyl species formed by dissociative chemisorption of CH 4 on specific surface sites and to measure surfacesite-specific sticking coefficients of CH 4 on the terrace, step, and ridge sites as function of incident translational energy. Methane dissociation is observed to be direct on all sites and diffusion of the chemisorbed methyl species is absent for surface temperature below 150 K. The experimental data are compared with the results of density functional (DFT) calculations that give minimum energy barriers for CH 4 chemisorption that properly account for the experimental relative site-specific reactivities. Also in agreement with experiments, DFT results predict a negligible effect of co-adsorbed H and CH 3 species on the vibrational frequency of a methyl group chemisorbed on terrace and step sites of Pt(211). However, the origin of the red-shift of the RAIRS peak of CH 3 chemisorbed on terrace sites compared with that on step sites of Pt(211) remains elusive and still demands further investigation.

The dissociation of methane on transition metal surfaces is not only of fundamental interest but also of industrial importance as it represents a rate-controlling step in the steam-reforming reaction used commercially to produce hydrogen. Recently, a specific reaction parameter functional (SRP32-vdW) has been developed, which describes the dissociative chemisorption of CHD3 at normal incidence on Ni(111), Pt(111), and Pt(211) within chemical accuracy (4.2 kJ/mol). Here, we further test the validity of this functional by comparing the initial sticking coefficients (S0), obtained from ab-initio molecular dynamics calculations run using this functional, with those measured with the King and Wells method at different angles of incidence for CHD3 dissociation on Pt(211). The two sets of data are in good agreement, demonstrating that the SRP32-vdW functional also accurately describes CHD3 dissociation at off-normal angles of incidence. When the direction of incidence is perpendicular to the step edges, an asymmetry is seen in the reactivity with respect to the surface normal, with S0 being higher when the molecule is directed toward the (100) step rather than the (111) terrace. Although there is a small shadowing effect, the trends in S0 can be attributed to different activation barriers for different surface sites, which in turn is related to the generalized co-ordination numbers of the surface atom to which the dissociating molecule is adsorbed in the transition state. Consequently, most reactivity is seen on the least co-ordinated step atoms at all angles of incidence.