H覆盖对费托合成催化剂CO活化影响的理论计算研究

Theoretical calculation study on effects of H coverage on CO activation in Fischer-Tropsch synthesis catalysts

Fe基和Co基催化剂是最理想的费托合成催化剂,随着计算机模拟技术在催化领域快速地发展,深入理解Fe基和Co基催化剂表面的微观反应机理成为可能。采用理论计算方法,研究了不同H覆盖度的Fe基(反应条件:温度320℃、压力1 MPa以及n(H_(2)):n(CO)=1:1)、Co基(反应条件:温度240℃、压力1 MPa以及n(H_(2)):n(CO)=2:1)费托合成催化剂的表面结构以及H覆盖对CO活化与解离的影响。通过氢气化学势与氢吸附吉布斯自由能的线性关系确定了Fe基、Co基费托合成催化剂热力学最优的H覆盖表面模型,分别为H覆盖度为11/42的χ-Fe_(5)C_(2)(510)表面和H覆盖度为24/36的HCPCo(0001)表面。研究了洁净的催化剂表面与H覆盖的催化剂表面的CO解离机理及相关反应能垒,发现H覆盖会改变CO解离路径且增大CO解离能垒,相较于洁净的χ-Fe_(5)C_(2)(510)和HCPCo(0001)表面,H覆盖的χ-Fe_(5)C_(2)(510)和HCPCo(0001)表面的CO解离能垒分别升高了5.4%和20.3%。通过对表面态密度以及晶体轨道哈密顿布居的计算解释了H覆盖影响CO活化与解离的本质原因。

Fe-based and Co-based catalysts are the most ideal Fischer-Tropsch synthesis catalysts.With the rapid development of computer simulation technology in the field of catalysis,it is possible to deeply understand the microscopic reaction mechanism on the surface of Fe-based and Co-based catalysts.Theoretical calculation method was used to study the effects of surface structures of Fe-based(reaction conditions:temperature 320℃,pressure 1 MPa and n(H_(2)):n(CO)=1:1)and Co-based(reaction conditions:temperature 240℃,pressure 1 MPa and n(H_(2)):n(CO)=2:1)Fischer-Tropsch synthesis catalysts with different H coverages on the activation and dissociation of CO.The thermodynamically preferred H-covered surface models of Fe-based and Co-based Fischer-Tropsch synthesis catalysts were determined through the linear relationships between the hydrogen chemical potential and the Gibbs free energy of hydrogen adsorption,which is theχ-Fe_(5)C_(2)(510)surface with H coverage of 11/42 and HCP Co(0001)surface with H coverage of 24/36,respectively.The CO dissociation mechanism and corresponding reaction energy barriers on the clean surface and the H-covered surface of catalysts were studied,and it is found that H coverage can change the reaction pathways of CO dissociation and increase the dissociation energy barrier.Compared with the clean surfaces ofχ-Fe_(5)C_(2)(510)and HCP Co(0001),the dissociation energy barriers of CO on H-coveredχ-Fe_(5)C_(2)(510)and HCP Co(0001)surfaces increase by 5.4%and 20.3%,respectively.The origin reasons for the effects of H coverage on CO activation and dissociation were explained by calculating the density of states and crystal orbitals Hamiltonian population.