Sulfite-Promoted Photochemical Reduction Of CO₂ And N₂

Significant economic and environmental missions motivate the CO2 reduction or N2 fixation to promote the resource utilization and solve the energy crisis. Thus, scientists have developed various routes and methods to fulfil these targets, such as electrocatalytic reduction, enzyme-catalytic reduction and photocatalytic reduction, et.al. However, almost all the reduction technologies are encountered with the difficulty of lack of enough reduction potential to overcome the kinetics barriers and activate the inert molecule efficiently, which leading to a "bottleneck restriction" in the reduction of CO2 or N2. It has been reported that the photolysis of sulfite is able to release hydrated electrons, whose reduction potential is as high as-2.9 V, holding the potential to activate inert CO2 or N2 molecule directly. In this article, we thus introduced a sulfite-mediated photochemical system which could directly activate the inert molecules with active species which induced by photo-excitation process in this photochemical system. Then, the roles of succeeding successive hydrogen atom transfer (HAT), electron transfer (ET) or proton transfer (PT) in this system were also recognized to illuminate the inner reduction mechanisms. In addition, the another S(IV) species in this system, sodium bisulfite, contributed a lot to the proton or hydrogen atom transfer in the later reduction process for its weak proton affinity, which accelerated the overall reduction efficiency.In the first section, we demonstrate that atmospheric CO2 could be efficiently reduced to HCOO- by sulfite-mediated photochemical system in the absence of any organic sacrificial agent. At the beginning, the optimal pathway for CO2 activation and HCOO- production were simulated by Gaussian Program. According to the reaction-kinetics theory, HAT was predicted as the most feasible pathway to convert CO2·- into HCOO- for the characteristic spin charge distribution of CO2·-. Then, the ability of hydrogen atom generation in this photochemical system was verified by the Transient State and IRC theory, revealing that the sulfite could free the proton from the water by hydrolysis and significantly promote the generation of hydrogen atom in this system. Thus, we deduced that the sulfite-mediated photochemical system might provide an excellent CO2 reduction performance due to its abundant of hydrated electron and hydrogen atom. We also designed an experimental sulfite-mediated photochemical system and validated the validity of our assumption. In this part, HCOO was selected as the target products to evaluate the CO2 reduction performance of this system. Advanced instruments and equipment were introduced to monitor the generation of active species and clarify the roles of these species respectively during the CO2 reduction. Based on the theoretical and experimental study, we finally explained the inner mechanism of CO2 reduction in this sulfite-mediated photochemical system in detail and gave possible schematic diagram of CO2 reduction pathway. This work may open a new avenue to the non-biomimetic photosynthesis of formate through the coupling of water splitting and CO2 reduction.When it comes to the second section, we selected the N2 as target molecular, which possesses an inerter physical and chemical property than CO2. The overall N2 fixation process was divided into two stages, constituted by N2â†'N2H4 and N2H4â†'NH3 stages respectively, and the optimal path in each stage was predicted by DFT theory. The whole thermodynamics process manifested that HAT was capable of activating N2 as well as inducing conversion of N2 into N2H4 with high efficiency. While as for the second stage, PCET was considered as the most viable pathway to convert N2H4 into NH3, which means decent proton donor is significant for second reduction stage. Considering that bisulfite is a unique proton donor in the sulfite-mediated photochemical system and there also exist loads of high-energy species in this system, such as hydrated electron and hydrogen atom, we constructed the sulfite-mediated photochemical N2 fixation system practically and explained every step during the whole reduction process by introducing FT-IR, IC and so on. The results conformed our prediction that hydrogen atom was responsible for the first stage of N2 fixation, while hydrated electron was the indispensable participant in the second reduction stage. It was worth noting that bisulfite was regarded as an excellent proton donor in this photochemical system, which contributed significantly to the promotion of hydrogen atom generation and proton transfer process. In addition, the FTIR analysis proved that the N2 fixation in sulfite-mediated system was more energetically favorable in a symmetric alternating mode to produce N2H4 as the main intermediate. In conclusion, we predicted the inner mechanism of N2 fixation in this photochemical system and proposed possible N2 fixation schematic.