Temperature effects on adsorption and diffusion dynamics of CH_3CH_(2(ads)) and H_3C–C≡C_((ads)) on Ag(111) surface and their self-coupling reactions: Ab initio molecular dynamics approach

摘要

Density functional theory (DFT)-based molecular dynamics (DFTMD) simulations in combination with a Fourier transform of dipole moment autocorrelation function are performed to investigate the adsorption dynamics and the reaction mechanisms of self-coupling reactions of both acetylide (H_3C–C_((β))≡C_((α) (ads))) and ethyl (H_3C_((β))–C_((α))H_(2(ads))) with I_((ads)) coadsorbed on the Ag(111) surface at various temperatures. In addition, the calculated infrared spectra of H_3C–C_((β))≡C_((α)(ads)) and I coadsorbed on the Ag(111) surface indicate that the active peaks of –C_((β))≡C_((α))– stretching are gradually merged into one peak as a result of the dominant motion of the stand-up – C–C_((β))≡C_((α))– axis as the temperature increases from 200 K to 400 K. However, the calculated infrared spectra of H_3C_((β))–C_((α))H_(2(ads)) and I coadsorbed on the Ag(111) surface indicate that all the active peaks are not altered as the temperature increases from 100 K to 150 K because only one orientation of H_3C_((β))–C_((α))H_(2(ads)) adsorbed on the Ag(111) surface has been observed. These calculated IR spectra are in a good agreement with experimental reflection absorption infrared spectroscopy results. Furthermore, the dynamics behaviors of H_3C–C_((β))≡C_((α)(ads)) and I coadsorbed on the Ag(111) surface point out the less diffusive ability of H_3C–C_((β))≡C_((α)(ads)) due to the increasing s-character of C_α leading to the stronger Ag–C_α bond in comparison with that of H_3C_((β))–C_((α))H_(2(ads)) and I coadsorbed on the same surface. Finally, these DFTMD simulation results allow us to predict the energetically more favourable reaction pathways for self-coupling of both H_3C–C_((β))≡C_((α)(ads)) and H_3C_((β))–C_((α))H_(2(ads)) adsorbed on the Ag(111) surface to form 2,4-hexadiyne (H3C–C≡C–C≡C–CH3(g)) and butane (CH3–CH2–CH2–CH3(g)), respectively. The calculated reaction energy barriers for both H_3C–C≡C–C≡C–CH_(3(g)) (1.34 eV) and CH_3–CH_2–CH_2–CH_(3(g)) (0.60 eV) are further employed with the Redhead analysis to estimate the desorption temperatures approximately at 510 K and 230 K, respectively, which are in a good agreement with the experimental low-coverage temperature programmed reaction spectroscopy measurements.