| The investigation that the first transition-metal forming ethylene catalyzed by ethane is very concerned by researchers. The mechanism of ethane decomposition is still ambiguous because experimental apparatuses are finite. In this research, the Dmol3 program module based on DFT has been used to compute and analyze the ethane adsorption and decomposition on low index transition metal surface. And improve the reaction mechanism.In this paper, the density functional theory (DFT) and self-consistent periodic calculation have been used to investigate that the species CxHy(x=0-2, y=1～6) are absorpt on the Fe(110), Co(111),and Ni(111) surface, respectively. The adsorption energy, the optimized geometry of the species CxHy(x=0-2,y=1-6) absorpt on the four possible sites(top, hcp, SB, and LB) of Fe(110), Co(111) and Ni(111) surfaces have been calculated and compared.Mulliken charges have been analyzed, then the most stable site have got. A possible decomposition mechanism for ethane absorpting on the Fe(110), Co(111),and Ni(111)surfaces have been investigated using DFT. The transition states have been defined using complete linear synchronous transit and quadratic synchronous transit (LST/QST) methods.The main results indicate the following:1.When the ethane decomposes on the Fe(110) surface, the most stable sites of the species C2H6,C2H5,C2H4,CH4,CH3,CH2,H2, and H are LB,LB,top,SB,top,hcp,LB, and hcp,respectively. The corresponding adsorption energy is-80.24,-178.89,-132.73,-38.14,-171.78,-342.43,-19.50, and-345.63 kJ·mol-1,respectively. The charges have transfered from the species C2H6, C2H5, C2H4, CH2, and H to Fe(110) surface during the adsorption process. However, the charges of the species CH4, CH3, and H2 have transfered from Fe(110) surface during the adsorption process. All species adsorptions are chemisorptions on the Fe(110) surface. In the C-C bond activation pathway, the activation energy of the rate-determining step is 122.9 kJ·mol-1.While the activation energy of the rate-determining step is 148.8 kJ·mol-1 during the C-H bond activation pathway, which suggests that the C-C bond activation pathway is preferred. The mainly pathway of ethane decomposing on the Fe(110)surface is Fe(110)+C2H6→Fe(110)-C2H6→TS3→IM2→TS4→P2,and the main products are CH4 and CH2.2.When the ethane decomposes on the Co(111) surface, the most stable sites of the species C2H6,C2H5,C2H4,CH4,CH3,CH2,H2, and H are bridge, bridge, top, top, top,bridge,bridge, and fcc, respectively.The corresponding adsorption energy is-89.39,-243.98,-133.32,-60.35,-227.61,-440.57,-40.24, and-338.46 kJ·mol-1, respectively. The charges have transfered from the species C2H6, C2H4, CH4, CH3, CH2, and H to Co(111) surface during the adsorption process. However, the charges of the species C2H5 and H2 have transferred from Co(111) surface during the adsorption process. All species adsorptions are chemisorptions on the Co(111) surface. In the C-C bond activation pathway, the activation energy of the rate-determining step is 223.8 kJ·mol-1.While the activation energy of the rate-determining step is only 180.9 kJ·mol-1 during the C-H bond activation pathway, which suggests that the C-H bond activation pathway is preferred. The mainly pathway of ethane decomposing on Co(111) surface is Co(111)+C2H6→Co(111)- C2H6→TS1→IM1→TS2→P1,and the main products are C2H4 and H2.3. When the ethane decomposes on Ni(111) surface,the most stable sites of the species C2H6,C2H5,C2H4,CH4,CH3,CH2,H2, and H are top,hcp,top,hcp,hcp,fcc,bridge, and fcc, respectively. The corresponding adsorption energy is 36.41,-100.21,-48.62,-16.13,-126.18,-296.60,-12.19,and-352.13 kJ·mol-1,respectively. The charges have transfered from the species C2H5, C2H4, CH3,CH2, and H to Ni(111) surface during the adsorption process. However, the charges of the species C2H6,CH4, and H2 have transfered from Ni(111) surface during the adsorption process.All species adsorptions are chemisorptions on the Ni(111)surface.In the C-C bond activation pathway, the activation energy of the rate-determining step is 257.9 kJ·mol-1.While the activation energy of the rate-determining step is only 159.8 kJ·mol-1 during the C-H bond activation pathway,,which suggested that the C-H bond activation pathway is preferred. The mainly pathway of ethane decomposing on Ni(111) surface is Ni(111)+C2H6→Ni(111)-C2H6→TS1→IM1→TS2→P1,and the main products are C2H4 and H2.In short, the nickel is the best catalyst in the first transition-metals catalyzed ethane forming ethylene. The cobalt is second. However, it is not advised to use iron as the catalyst in the reaction of ethane forming ethylene.
|
PDF Full Text Request