Transferability of the SRP32-vdW specific reaction parameter functional to CHD3 dissociation on Pt(110)-(2 × 1)

Stepped transition metal surfaces, including the reconstructed Pt(110)-(2 × 1) surface, can be used to model the effect of line defects on cata- lysts. We present a combined experimental and theoretical study of CHD3 dissociation on this surface. Theoretical predictions for the initial sticking coefficients, S0, 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 S0 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.

Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

Rainer Beck, Li Chen, Hirokazu Ueta

The dissociative chemisorption of CH4 on Pt(111) was studied using quantum state-resolved methods at a surface temperature (Ts) of 150 K where the nascent reaction products CH3(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S0 on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH3(ads) uptake during molecular beam deposition as a function of incident translational energy (Et) and vibrational state (n3 anti-symmetric C–H stretch of CH4) to obtain the initial sticking probability S0, the coverage dependence of the sticking probability S(y) and the CH3(ads) saturation coverage ysat. We observe that both S0 and ysat increase with increasing Et as well as upon n3 excitation of the incident CH4 which indicates a coverage dependent dissociation barrier height for the dissociation of CH4 on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.

Royal Society of Chemistry2013

Surface Reaction Barriometry: Methane Dissociation on Flat and Stepped Transition-Metal Surfaces

Rainer Beck, Helen Jane Chadwick, Ana Gutiérrez González, Francesco Nattino

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.

Amer Chemical Soc2017

Incident Angle Dependence of CHD3 Dissociation on the Stepped Pt(211) Surface

Rainer Beck, Helen Jane Chadwick, Ana Gutiérrez González

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.

2018