Physisorption-Induced C-H Bond Elongation in Methane

Abstract

Physisorption of methane to a Pt surface was studied by x-ray absorption spectroscopy in combination with density functional theory spectrum calculations. The experiment shows new electronic states appearing upon physisorption. We find that these states are due to orbital mixing causing charge polarization as a means to minimize Pauli repulsion. The results can be explained by elongation of 1 C-H bond by 0.09 A in the physisorbed state even though no covalent chemical bond is formed.

Analysis of silica-supported vanadia by X-ray absorption spectroscopy: Combined theoretical and experimental studies

“…The present computational approach has been used successfully to yield highly accurate results for X-ray absorption of light atoms in gas phase molecules [30,32,33]. It has also been used to study properties of hydrogenbonded systems [30,34,35], of adsorbates on metal surfaces [36,37], and of differently coordinated oxygen sites near vanadium and molybdenum oxide surfaces [38][39][40]. Further details concerning the methods can be found in the corresponding references.…”

Activated Methane on Small Cationic Platinum Clusters

“…[3] Despite these challenges, Yoshinobu et al have used infrared reflection absorption spectroscopy to show that CH 4 adsorbed on Pt(111) has at most C 3v symmetry. [4] Östrçm et al have determined the adsorption geometry of methane on Pt(977) using X-ray absorption spectroscopy, [5] reporting methane to bind by a single hydrogen atom, though they were unable to determine whether it was bound atop or in hollow sites. Partially dehydrogenated reaction intermediates/products, including methyl, methylene, and methylidyne, have been extensively studied (see for example Ref.…”

“…In both cases, h 2 -CH 4 molecularly adsorbed species provide the best matches to the experimental spectra. In the trigonal bipyramid Pt 5 + there are two distinct types of Pt atom, that is, three-and four-coordinate, to which the CH 4 can bind. The calculated total energies in these sites differ (CH 4 on a four-coordinate Pt atom is 0.29 eV higher in energy) but the calculated frequencies and relative intensities are rather similar, making it difficult to determine which site is favored.…”

“…[7] In the case of small ionic clusters reacting with CH 4 under single collision conditions, Pt n [C,2H] + complexes were found to be the favored products. [8][9][10] There have been a number of computational studies of the interactions of platinum clusters [11][12][13] and surfaces [5,14,15] with methane, primarily using density functional theory (DFT). Such calculations are challenging, owing to the large system size, number of electrons and possible paths, and the fact that several electronic states and crossings between them may need to be treated.…”

The local structure of molecular reaction intermediates at surfaces

“…A nice recent example is an investigation of methane physisorbed on Pt(977), in which information on the C-H bondlength was obtained in this way [12]; this is a particularly striking result because H is a weak electron scatterer and in general contributes minimally to LEED, PhD or SEXAFS. Note also that while LEED and PhD involve electron detection, and thus are generally only applicable in UHV, the X-ray absorption in NEXAFS and SEXAFS can be detected by X-ray fluorescence (resulting from refilling of the core hole), so in this X-ray-in/X-ray-out configuration, these methods can be applied under 'high pressure' conditions.…”