Quantum state resolved molecular beam reflectivity measurements: CH4 dissociation on Pt(111)

Abstract

The King and Wells molecular beam reflectivity method has been used for a quantum state resolved study of the dissociative chemisorption of CH4 on Pt(111) at several surface temperatures. Initial sticking coefficients S0 were measured for incident CH4 prepared both with a single quantum of ν3 antisymmetric stretch vibration by infrared laser pumping and without laser excitation. Vibrational excitation of the ν3 mode is observed to be less efficient than incident translational energy in promoting the dissociation reaction with a vibrational efficacy ην3 = 0.65. The initial state resolved sticking coefficient S0ν3 was found to be independent of the surface temperature over the 50 kJ/mol to 120 kJ/mol translational energy range studied here. However, the surface temperature dependence of the King and Wells data reveals the migration of adsorbed carbon formed by CH4 dissociation on the Pt(111) surface leading to the growth of carbon particles.

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

“…13, the data for Pt(111) 80 they compared to in Ref. 39 (black) are systematically higher than the sticking coefficients measured by Bisson et al 81 (blue), Luntz and Bethune 62 (green), and Chadwick et al 60 (red) for the same system, with the reactivities from the latter three studies being in reasonable agreement (noting again that the differences in TS are not expected to significantly affect the measured S 0 60 ). This implies that the sticking coefficients for Pt(111) used by McMaster and Madix in their comparison of the reactivities of CH 4 on the two surfaces are too large and that this is the reason for their different conclusion.…”

“…A double exponential was used for the fits because the dissociative chemisorption of methane on a Pt(111) surface at a range of surface temperatures between 500 and 800 K had previously been shown to be governed by two processes: a fast initial dissociation of the CH 4 and a slower growth of carbon particles on the surface. 60 Fitting S(t) to a double exponential decay takes into account both processes.…”

“…Figure 12 presents a comparison of the experimental [panel (a)] and calculated [panel (b)] sticking coefficients for CHD 3 dissociation on Pt(111) 23 (black), Pt(211) 23 (red), and Pt(110)-(2 × 1) (blue). The Pt(111) data are for a surface temperature of 500 K, whereas the Pt(110)-(2 × 1) and Pt(211) data are for a surface temperature of 650 K. Sticking coefficient measurements for CH 4 dissociation on Pt(111) have shown that the reactivity does not change significantly between surface temperatures of 500 K and 800 K, 60 and therefore, the difference in surface temperature is unlikely to affect the reactivity trends for CHD 3 dissociation shown in Fig. 12.…”

“…Whilst the Pt(211) and Pt(110)-(2 × 1) surfaces have lower activation barriers for CHD 3 dissociation than Pt(111), the atomic density of the sites with the lowest activation barrier (given in the fifth column of Table III) is lower for the stepped surfaces than for Pt(111). Additionally, the transition states at alternative sites on the stepped 80 at T S = 500 K (black), Bisson et al 81 at T S = 600 K (blue), Chadwick et al 60 at T S = 650 K (red), and Luntz and Bethune 62 at T S = 800 K (green).…”

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

“…Their values are shown in Figure 2A. S ( t ) is fit using a double exponential decay 28 to obtain the initial sticking coefficient S 0 , as shown in Figure 2B. The baseline of the K&W trace when the flag is shut ( t < 0 s, t > 15 s) is not zero, as the QMS current was seen to increase when the beam flag is opened under conditions where no reactivity was observed.…”