Mechanism Of Fe（OH）₂ Phase Transformation To Produce H₂ During Low Temperature Serpentinization

The CO₂ emission caused by the extensive use of fossil energy has caused the global environmental crisis,and the alternative clean energy-hydrogen energy cannot overcome the technical bottleneck of artificial hydrogen production in the short term.Therefore,low-cost natural hydrogen energy is regarded as an important part of the world’s hydrogen energy supply in the next three decades,and it is of great significance to control the CO₂ emission at the source.The natural hydrogen energy generated by the water-rock process represented by low temperature（<200 ^oC）serpentinization has great potential to be commercially developed,but the hydrogen production mechanism and influencing factors of this process are still controversial,which hinders the exploration and development of natural hydrogen energy.The transformation of Fe（OH）₂ into magnetite with hydrogen production was the core key step and has always been considered to proceed rapidly and spontaneously by geoscientists,but in other fields such as steel corrosion,it is considered that the reaction rate of this reaction is extremely limited.In order to solve the above research disputes and clarify the key factors affecting the phase transformation of Fe（OH）₂ to produce hydrogen during low-temperature serpentinization,this paper carried out the relevant mechanisms research from the microscopic perspectives of doping elements,exposed crystal planes,and defect sites.（1）The elemental composition of magnetite formed by hydrogen production from low-temperature serpentinization in the field mainly includes impurity elements such as Ni.Therefore,we synthesized pure Fe（OH）₂ and Ni²⁺coexisting Fe（OH）₂ by co-precipitation,and then quantitatively evaluated the effect of Ni²⁺on hydrogen production from the phase transition of Fe（OH）₂ at 90 ^oC.Compared with the limited hydrogen production capacity of pure Fe（OH）₂,the amount and rate of hydrogen production at 90 ^oC were increased by 134and 315 times,respectively,when 1%Ni²⁺was co-precipitated.In addition,Ni²⁺content,reaction temperature,Mg²⁺content and solution p H all have important effects on Ni²⁺-promoted Fe（OH）₂ phase transformation to produce hydrogen.The isotope labeling experiment combined with DFT theoretical calculation proposed and verified that the reduction of H₂O on the surface of Fe（OH）₂ was the reason for the generation of hydrogen,and it was clarified that Ni²⁺doping with Fe（OH）₂ resulted in stronger H₂O adsorption and dissociation capacity,so that the hydrogen production capacity of this system is greatly improved.Finally,combined with the chemical characteristics of the solution and the phase transition process in the low-temperature serpentinization process and the statistical analysis of the elemental composition of nearly 3,000 actual olivines,three favorable conditions for the low-temperature serpentinization hydrogen production were proposed:the presence of Ni²⁺,the higher reaction temperature and the high ratio of water to rock.（2）Based on the surface exposure features of Fe（OH）₂ and combined with the interface chemical model of H₂O reduction on Fe（OH）₂ surface proposed by the above research,the DFT theoretical calculation was used to systematically evaluated the effect of different crystal planes of Fe（OH）₂ on H₂O adsorption and dissociation ability.It is found that,compared with the（001）surface,the（100）plane with exposed Fe sites is the active plane for H₂O reduction.Furthermore,by analyzing the energy difference of the adsorption and dissociation of H₂O on the（100）,（101）and（102）crystal faces of Fe（OH）₂,it was found that although the（100）facet had the strongest H₂O adsorption(E_ads=-0.62 e V)and H₂O dissociation ability,but the energy barrier of the dissociation transition state is still high（2.79 e V）,so it is not conducive to the reduction of H₂O to produce hydrogen.Further,through the electronic structure analysis,it was revealed that the reason why the transition state energy barrier of H₂O dissociation at the（100）plane is still high,may be due to the co-action with the surface OH and Fe sites during the adsorption process of H₂O,which promotes the adsorption of H₂O in the（100）surface and leads to to high structural stability,so high energy is required to destroy the structure of this stable adsorbed H₂O.So far,a detailed mechanism explanation is provided for the reason why pure Fe（OH）₂ undergoes phase transformation was limited to produce hydrogen.（3）On the basis of the above research,the effect of surface OH defects on the adsorption and dissoiciation of H₂O on Fe（OH）₂（100）surface was further investigated by DFT theoretical calculation.The results show that although surface OH defects reduce the overall ability of Fe（OH）₂（100）surface to adsorb H₂O,it enhances the interaction between surface Fe sites and O atoms in H₂O.Bader charge analysis shows that H₂O can obtain more electrons when adsorbed on the surface of Fe（OH）₂-V_OH（100）surface with OH defects,which is beneficial to the reduction of H₂O.On the other hand,the transition state energy barrier（0.59e V）and the final product formation energy barrier（0.49 e V）of H₂O dissociation on the（100）surface of Fe（OH）₂-V_OH are lower when they are compared with those of intact Fe（OH）₂（100）surface（2.69 e V,2.25 e V）,thereby enhancing the ability of Fe（OH）₂ to undergo phase transformation to produce hydrogen.Combined with the above-mentioned mechanism explanation for the limited hydrogen production during phase transformation of complete Fe（OH）₂,considering that there may be a large number of OH defects on the surface of Fe（OH）₂ regenerated from Fe²⁺released by water-rock interaction,we believe that whether there are OH defects on the surface would be one of the keys to control whether pure Fe（OH）₂can undergo a rapid phase transition and generate hydrogen.As mentioned in the above,this thesis not only discovered the way that Ni²⁺promotes the rapid phase transition of Fe（OH）₂ with hydrogen production during low-temperature serpentinization,but also used the proposed model of the reduction of H₂O on the surface of Fe（OH）₂ to revealed the reason for the limited hydrogen production capacity of pure Fe（OH）₂.In addition,the surface OH defects were predicted to have the potential to enhance the Fe（OH）₂ phase,which would be beneficial to explain the research debate on whether pure Fe（OH）₂ could undergo rapid phase transformation and producting hydrogen.Our results provides a detailed theoretical basis for re-understanding the mechanism and influencing factors of hydrogen production from Fe（OH）₂ phase transformation transformation during low-temperature serpentinization,which is helpful for the exploration and development of natural hydrogen formed by low-temperature serpentinization.