DFT study of surface reactivity of CX₃I (XH and F) with CH₂I₂ to form CH₂CX₂ on the Ag(111) surface

Abstract

ABSTRACT:Recently, Chiang's research group successfully carried out the temperature programmed reaction (TPR) spectroscopy under ultrahigh-vacuum conditions to probe the reaction pathways of adsorbed methyl (CH 3(ads) ) and trifluoromethyl (CF 3(ads) ) with coadsorbed methylene (CH 2(ads) ) via the CH 2 insertion reaction, leading to the formation of adsorbed 1,1,1-trifluoro-ethyl (CF 3 CH 2(ads) ) and ethyl (CH 3 CH 2(ads) ) on the Ag(111) surface. Additionally, the authors found it feasible for CF 3 CH 2(ads) to proceed the subsequent ␤-fluoride elimination on the Ag(111) surface to produce 1,1 difluoroethylene (CF 2 ACH 2(g) ) but difficult for CH 3 CH 2(ads) to proceed ␤-hydride elimination to form ethylene (CH 2 ACH 2(g) ) on the Ag(111) surface. To elaborate on this noticeably different reactivity between ␤-hydride and ␤-fluoride eliminations on the Ag(111) surface we performed total energy calculations based on density functional theory in connection with ultrasoft pseudopotential and generalized gradient spin-polarized approximation, and partial structural constraint path minimization to establish the energetically more favorable pathways for the CH 2 insertion into AgOCX 3 (XAH and F) bonds followed by ␤-X elimination to generate an isolated CH 2 ACX 2(g) on the Ag(111) surface. Following our proposed reaction pathways, namely, the diffusion of the fcc-hollow site of CX 3(ads) toward the bridge site of CH 2(ads) through the CH 2 insertion and the subsequent ␤-X elimination to form both isolated CH 2 ACX 2(g) and hcp-hollow site of X (ads) on the Ag(111) surface, our calculated energy barrier for ␤-hydride elimination is significantly larger than (ϳ0.661 eV) that for ␤-fluoride elimination. This unusual high-energy barrier prohibits ␤-hydride elimination of CH 2 CH 3(ads) to form an isolated CH 2 ACH 2(g) on the Ag (111) surface and explains what we observed from the TPR experimental data. We also attribute this much higher energy barrier to forming a largely distorted seven-center ring transition state structure with a larger distortion of the AgOC (␣) H 2 C (␤) H 3 OAg and a less stable AgOH bond on the Ag(111) surface. Finally, our calculated energy barrier for ␤-fluoride elimination to form CH 2 ACF 2(g) is 0.687 eV, in very good agreement with the experimental data.

DFT Study of Selective α-Fluoride Elimination of Adsorbed CF3(ads) on Both Ag(111) and Cu(111) Surfaces

“…Thermal Stability of CF 3(ads) , CF 2(ads) , and F (ads) on both Ag(111) and Cu(111) Surfaces. We followed the same strategy, that is, to determine energetically and structurally all of the possible sites of CF 3(ads) , CF 2(ads) , and F (ads) on both Ag(111) and Cu(111) surfaces, as in our earlier work, , to design the energetically most favorable reaction pathway for the single α-fluoride elimination of adsorbed CF 3(ads) leading to adsorbed CF 2(ads) and F (ads) on both Ag(111) and Cu(111) surfaces. Our calculated results indicate that on the Ag(111) surface (1) both hcp-hollow and fcc-hollow sites of adsorbed CF 3(ads) are the least-stable sites but only ∼0.06 eV higher than the bridge site, and the top site of CF 3(ads) is ∼0.4 eV even lower than the bridge site, (2) the adsorbed CF 2(ads) was found to prefer the bridge site with at least ∼0.16 eV lower than the top site and both the hcp-hollow and fcc-hollow sites are ∼0.06 and ∼0.12 eV, respectively, higher than the top site, and (3) the adsorbed F (ads) was found to prefer the fcc-hollow site with ∼0.03 eV lower than the hcp-hollow site and ∼0.11 eV lower than the bridge site, and the top site is the least-stable site.…”

“…Both surfaces were constructed by a periodically repeated slab of silver and copper atoms (three layers in the unreconstructed geometry) with one side of two layers fixed and the other side of one layer followed by a vacuum region of approximately 16 Å. The cutoff of 300 eV is used throughout all of the calculations on the basis of the plane-wave convergence tests described in our earlier works. , For the Brillouin-zone integration, we used 2 × 2 × 1 grid of Monkhorst−Pack special points after convergence tests described in our previous works. , For the transition-state structure search, we apply PSCPM 21,22 with the chemical intuition to determine the possible transition-state structure and the corresponding energy barriers along energetically more favorable paths. To obtain the PDOS, we generated the total density of states (TDOS) of the whole system using a reasonable number of k points and bands (normally a few more than the occupied bands).…”

“…Although Chiang and his co-workers presented the above pioneering works [15][16][17][18] for illustrating the different reactivity for the single R-fluoride elimination of adsorbed CF 3(ads) on both Cu(111) and Ag(111) surfaces, the details of (1) the more favorable sites of adsorbed CF 3(ads) on both surfaces, (2) the reaction mechanism of C-F bond breaking and Ag-F/Cu-F bond forming involved in the single R-fluoride elimination of adsorbed CF 3(ads) on both surfaces, and (3) the corresponding kinetic and thermodynamic parameters still remain unanswered. In addition, our recent total-energy calculation results 19,20 based on DFT to establish an energetically more favorably pathway for the CH 2 insertion into the Ag-CF 3(ads) bond followed by the β-fluoride elimination to generate an isolated CH 2 dCF 2(g) only answer the question related to factors controlling the rate of forming the CH 2 dCF 2(g) . Other questions such as the effect of different surface electronic states of both Ag(111) and Cu-(111) surfaces on the reactivity of the single R-fluoride elimination of adsorbed CF 3 on these surfaces and on the different reactivity between the single R-fluoride elimination of adsorbed CF 3(ads) and the CH 2 insertion into the Ag-CF 3(ads) bond on the Ag(111) surface remain unclear.…”

“…The cutoff of 300 eV is used throughout all of the calculations on the basis of the plane-wave convergence tests described in our earlier works. 19,20 For the Brillouin-zone integration, we used 2 × 2 × 1 grid of Monkhorst-Pack special points after convergence tests described in our previous works. 19,20 For the transitionstate structure search, we apply PSCPM 21,22 with the chemical intuition to determine the possible transition-state structure and the corresponding energy barriers along energetically more favorable paths.…”

“…19,20 For the Brillouin-zone integration, we used 2 × 2 × 1 grid of Monkhorst-Pack special points after convergence tests described in our previous works. 19,20 For the transitionstate structure search, we apply PSCPM 21,22 with the chemical intuition to determine the possible transition-state structure and the corresponding energy barriers along energetically more favorable paths. To obtain the PDOS, we generated the total density of states (TDOS) of the whole system using a reasonable number of k points and bands (normally a few more than the occupied bands).…”