Theoretical Investigation On The Mechanism Of CO₂ Hydrogenation Catalyzed By Ni

Based on quantum chemistry approach, the gas-phase H2/CO2/Ni(1S/3D) and 4H2/CO2/Ni(1S/3D) systems have been systemically investigated at the CCSD(T)/ 6-311+G(2d,2p)//B3LYP/6-311+G(2d,2p) levels, in order to explore the mechanism of CO2 hydrogenation catalyzed by singlet and triplet state Ni atom. The reaction paths along which various intermediates transfer from one to another via transition states have also been rationalized by their energies, optimized geometrical structures, vibrational frequencies and bond orbital analysis. The main results are as the following: 1. Theoretical investigation on the gas-phase reaction of H2 + CO2 →CO + H2O catalyzed by Ni. The overall singlet and triplet reaction mechanisms are similar with each other. In the initial reaction step, H2 and CO2 co-attaches to Ni center with the formation of co-complex (H2)NiCO2. Next, the insertion of Ni into one C-O bond of CO2 proceeds and leads to the cleavage of CO2 with the formation of the intermediate (H2)NiOCO. Then, the cleavage of H2 happens, and one H atom moves to O atom, with the formation of the intermediate HONiHCO; this intermediate may release CO and leave the product HONiH behind. In the last reaction step, the cleavage of Ni-H bond and the migration of H atom take place and lead to the formation of (H2O)NiCO; this intermediate can continually decompose to the final products of Ni + H2O + CO. For the singlet state s-Ni(1S) + H2 + CO2 →s-NiCO + H2O reaction, the rate determining step (RDS) is predicted to be the Ni insertion step from the co-complex s-(H2)NiCO2 to s-(H2)NiOCO via transition state s-TS1, because the energy barrier for this step is calculated to be the highest in the entire singlet state reaction. For the triplet state t-Ni(3D) + H2 + CO2 →Ni(3D) +CO + H2O reaction, the RDS is predicted to be the H migration step from the intermediate t-HONiHCO to t-(H2O)NiCO via transition state t-TS3. Two possible spin crossings between singlet and triplet PESs occur near the exit channels, corresponding to the formations of products HONiH + CO and Ni + H2O + CO, respectively. Since the predicted energies for triplet state products are lower than those for singlet state products, the triplet state products may be liable to form for both singlet and triplet state reaction. 2. Theoretical investigation on the gas-phase reaction of 4H2 + CO2 →CH4 + 2H2O catalyzed by Ni. There exist some differences between singlet and triplet reactions in the 4H2/CO2/Ni(1S/3D) systems, but both reactions can be roughly divided into three reaction stages. In the first stage, the interaction between co-attached H2 and CO2 proceeds, and H2 and CO2 are co-activated via the formation of formic acid intermediate, which leads to the cleavage of C-O and H-H bonds. In the second stage, the intermediate ONiCHOH interacts further with two H2 molecules, forming the intermediate HONiCH3. In the third stage, another H2 molecule attaches to HONiCH3, forming H2O and yielding the product CH4. The third stage involves a competitive reaction, which corresponds to the migration of H atom from O atom to C atom via transition state (s-TS12 and t-TS10, respectively), with the formation of molecule-molecule complex ONi(CH4). The latter complex can decompose to produce NiO + CH4. Since the endothermicty and barrier height for this competitive reaction are much larger than those in former reaction, the reaction may proceeds along the former reaction path. Calculations show that the first and the third reaction stages are similar for both singlet and triplet reactions, but the second reaction stage varies. These discrepancies are found to be originated from the different reaction process ofoxygen-containing carbon species during the following hydrogenation reaction. Calculations also indicate that the energies barriers for the second and third reaction stages are lower than that for the first reaction stage, implying the second and third reaction stages may be easy to proceed. For the overall Ni + 4H2 + CO2 →Ni + 2H2O + CH4 reaction, the RDS is predicted to be the step of Ni insertion into C=O bond of formic acid species. Meanwhile, four possible spin crossings between singlet and triplet PESs may exist, all of which are related to the intermediate HONiCH3. Because of these spin crossings, the state of the final products may be more complicated. In addition, comparisons of the calculated results with those from experiments are made. The IR spectrum of some stable species predicted by quantum chemistry investigation coincide well with those observed experimentally, and the overall reaction mechanism obtained in the present work could well explain the experimental results. Methane activation by Li2O has also been investigated at the CCSD(T)/ 6-311++G(2d,2p)//B3LYP/6-311++G(3df,3pd) levels. The main results are as the following: Methane activation by Li2O involves two distinct reactions. One of which produces CH3OH + Li2, and the other produces CH3Li + LiOH. Calculations predict the endothermicty for the former reaction to be 219.1 kJ mol-1, and the RDS to be the step from complex CH4-OLi2 to CH3OLi2H via transition state a-TS1, which corresponds to the insertion of Li2O into one C-H bond of methane. Calculations also predict the endothermicty for latter reaction to be 111.1 kJ mol-1, and the RDS to be the step from intermediate CH3Li2OH to CH3Li-LiOH via transition state b-TS2, which corresponds to the cleavage of the Li-O bond. Because the energy barrier height for the former reaction (322.3 kJ mol-1) is significantly lager than that for the latter reaction (157.4 kJ mol-1), the reaction may be liable to produce the product CH3Li.