Influence of Steps on the Adsorption of Methane on Platinum Surfaces

Abstract

The activated adsorption of methane was investigated on two stepped Pt(355) and Pt(322) surfaces by timedependent in situ high-resolution X-ray photoelectron spectroscopy (XPS) combined with a supersonic molecular beam and was compared to corresponding results on Pt(111). Both stepped surfaces have five atom row wide (111) terraces but a different step orientation, namely, ( 111) and ( 100), respectively. Independent of the kinetic energy (0.45-0.83 eV) of impinging methane (CH 4 or CD 4 ), methyl is formed on all surfaces upon adsorption at 120 K. For the stepped surfaces, two different methyl species are identified from the XP spectra, which are attributed to adsorption at terrace sites and at step sites. The total initial sticking coefficients (for terrace + step sites) and the total coverages are very similar for all three surfaces. At low coverages, diffusion of methyl from the terraces to the steps is significantly stronger on Pt(355) than on Pt(322), and the step saturation coverage is higher on Pt(355). The thermal evolution of methyl was also investigated by in situ temperature-programmed XPS. Overall, an enhanced reactivity toward dehydrogenation to methylidyne is found for both terrace and step sites on the stepped surfaces, with the (111) steps of Pt(355) exhibiting the highest activity.

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

“…For other species such as CH, C, and H, the effect is rather mild, that is below 7 %, of the adsorption energy. Current results agree with experimental results of Papp et al, [62] who found after methane dissociation on a PtA C H T U N G T R E N N U N G (355) single crystal a higher methyl coverage at steps of the PtA C H T U N G T R E N N U N G (355) surface than on terraces locally featuring a (111) structure. DFT results on a Pd 79 cluster also showed much stronger stabilization at nanoparticle edges for CH 3 and CH 2 , 23 and 27 kJ mol…”

“…It should be pointed out, however, that this effect is moderate, thus elevated temperatures should still be required for carbon formation. In agreement with these results, CH decomposition on stepped surfaces was observed in the temperature range 420-450 K. [62] Dehydrogenation of CH 4 on Pt nanoparticles supported on CeO 2 : Experiment: To experimentally corroborate the theoretically predicted effect of particle edges and corner sites on methane activation and dehydrogenation, SSMB experiments were performed on the dissociation of methane on supported Pt nanoparticles. The reaction intermediates were detected in a time-resolved mode using HR-PES.…”

“…Methyl formation should be feasible by employing lower kinetic energies than on PtA C H T U N G T R E N N U N G (111), and once formed the species would undergo rapid decomposition to follow the downhill route to methylidyne. DFT calculations explain why, on stepped Pt surfaces, [62] methyl dehydrogenation occurs in the 160-220 K temperature range, which is noticeably lower than on PtA C H T U N G T R E N N U N G (111). Once methylidyne is formed, the back reaction to methane on nanoparticles is unlikely due to rapid H 2 desorption (at least in the case of UHV experiments).…”

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

“…At these surface temperatures, any hydrogen in the molecular beam that dissociates rapidly recombinatively desorbs, [50][51][52][53] not affecting the measurement of S(t). Within the experimental error, the values of S ν 3 0 obtained are independent of the temperature of the surface.…”

Active Site Elucidation in Heterogeneous Catalysis via In Situ X-Ray Spectroscopies

“…composition and oxidation state) of the outermost 1-10 atomic layers of catalyst surfaces under ultra-high vacuum (10 À13 mbar). Over the past 15 years, time-resolved XPS [29] has helped unravel surface reaction mechanisms in precious metal catalyzed alcohol [30,31] and alkene [32,33] selective oxidation, C-C coupling, [34,35] dehalogenation, [36,37] and thermal [38][39][40] and photochemical [41] C-H activation, but has been restricted to studies of strongly-bound reactants over pristine model systems in the absence of solvents. Recent advances in surface science instrumentation, notably access to 3rd-generation synchrotron light sources and differentially-pumped, electron optics, [42] are now helping bridge the so-called 'pressure gap' in catalysis [12] and facilitate time-resolved XPS measurements at pressures up to 1 mbar, [43] sufficient to stabilize weakly bound adsorbates e.g.…”