Infrared Reflection Absorption Spectroscopic and DFT Calculation Studies on the Adsorption Structures of Dimethyl Ether on Ag(110), Cu(110), and Their Atomic Oxygen-Reconstructed Surfaces

Abstract

Infrared spectra were measured at 80 K for dimethyl ether (CH3OCH3, DME) and dimethyl ether-d6 (CD3OCD3, DME-d6) with increasing amounts of exposures to metal substrates, Ag(110), Cu(110), and their atomic oxygen-reconstructed surfaces, p(2 × 1)O−Ag(110) and p(2 × 1)O−Cu(110). At relatively lower surface coverages, the IR spectra of DME on Ag(110) and Cu(110) in the 1500−800 cm-1 region give rise to IR bands mainly ascribable to A1 species, including symmetric COC stretching (νs(COC)) bands at 903 cm-1 on Ag(110) and 895 cm-1 on Cu(110), while at nearly saturation coverages, the adsorbate gives IR bands ascribable to B1 and/or B2 species in addition to the A1 bands with the νs(COC) band discretely sifted to 915 cm-1 on Ag(110) and to 901 cm-1 on Cu(110). Similar distinct spectral changes were observed also for DME and DME-d6 on the reconstructed surfaces. The stepwise spectral changes were interpreted in terms of a conversion from a state of DME with the C 2 axis almost perpendicular to the surfaces to a state with the C 2 axis tilted away from the perpendicular orientation. Fermi resonance effects cause stepwise but complicated spectral changes in the CH3 stretching vibration region of DME during the conversion. The changes strongly depend on the kind of the substrates, in contrast to the spectral changes in the 1500−800 cm-1 region, suggesting that the analyses of Fermi resonances can delineate subtle differences in the DME/substrate interaction modes among the substrates. Density functional theory (DFT) molecular orbital calculations were carried out on the cluster models of DME/Cu(110), where the oxygen atom of DME is coordinated to two copper atoms in the surface of metal clusters consisting of 12 copper atoms in the first layer and six copper atoms in the second layer. The results of calculations reproduce the observed frequencies appreciably well, substantiating the coordination interaction model.

Structure and Reactivity of Alkyl Ethers Adsorbed on CeO2(111) Model Catalysts

“…The strong absorption at 2878 cm -1 is assigned to the m s (CH 3 ) fundamental enhanced by a Fermi resonance with a CH 3 bending mode [43]. To interpret the lower wavenumber region, it is instructive to compare our results with those obtained by Kasahara et al by RAIRS of DME adsorbed on Cu(111), Ag(111), Cu(110) and Ag(110) [30,31]. Based upon their assignments, the peak at 919 cm -1 is assigned to t s (COC) and the peak at 1452 cm -1 is assigned to d a (CH 3 ).…”

“…Further explanation on these controversial results stated that probably the different experimental conditions and preparation of the high surface area catalysts were responsible for the different results obtained. [26] Alkyl ethers have also been of interest due to their role as lubricants leading to studies to elucidate the reactivity of alkyl and perfluoro-alkyl ethers on metals [27][28][29][30][31] and on oxides [13,14,[32][33][34][35][36][37][38][39]. It is interesting to note that several of these studies showed reaction of the ethers adsorbed on the oxides (Al 2 O 3 , SiO 2 , TiO 2 and ZrO 2 ) to form alkoxides, potentially due to the presence of hydroxyls and also further conversion of these alkoxides to carboxylates.…”

Infrared Reflection Absorption Spectroscopic Study of the Rotational Isomerism of Methyl Ethyl Ether on Cu(110) and Ag(110)

“…As can be seen from Figure 8B, the orientation of MEE in the G form on Cu(110) is favorable for the oxygen atom to coordinate to a bridge site of the surface metal atoms through the two lone pair electrons as in the case of DME on Cu(110). 7 On the other hand, MEE in the T form cannot take on the bridge coordination site on Ag(110) because of the steric repulsion between the CH 3 (-CH 2 ) group and the surface. Presumably, one of the lone pair electrons of the oxygen atom participates in a coordination interaction, as shown in Figure 8A.…”

“…As for the factor i, it has been known that the T form of MEE in the gaseous state is more stable than the G form with the enthalpy difference of 5.65 kJ/ mol. 3 The results of the IRA study on dimethyl ether (DME) adsorbed on Cu(110) and Ag(110) 7 suggests that the intrinsic coordination interaction between the oxygen atom of MEE and the surface metals on Cu(110) is stronger than that on Ag(110). Presumably, this is one of the main reasons for the fact that MEE takes the G form on Cu(110), even if it is energetically unfavorable compared to the T form.…”

“…The T form of MEE in the gaseous state is more stable than the G form with the enthalpy difference of 5.65 kJ/mol. 3 Our IRA spectroscopic study 7 has indicated that dimethyl ether (CH 3 -OCH 3 , DME) adsorbs molecularly on Cu(110) and Ag(110) at 80 K at relatively lower surface coverages with the C 2 axis almost perpendicular to the surfaces. The shifts to the lower frequency side observed for the ν s (COC) band compared to that of DME in the gaseous state (920 cm -1 ), -25 cm -1 for DME on Cu(110) and -17 cm -1 for DME on Ag(110), suggest a weak coordination interaction of the oxygen atoms of DME to the substrate copper and silver atoms.…”