Mass Transfer Enhancement Of Hydrogenation Product And CO₂ Hydrogenation Dynamics In Electrochemical Hydrogen Pump Reactor

With the tension of energy crisis and global warming,the refining of bio-oil and utilization of CO₂ become increasingly important.EHPR could activate reactant by electrical energy,transfer proton by proton exchange membrane and transfer reactant by gas diffusion electrode.Thus,it can high-efficiently and controllably hydrogenate CO₂ and bio-oil with hydrogen exhaust or H₂O as hydrogen source in mild condition,which makes EHPR a promising hydrogenation way.However,there are still some problems such as production inhibition and complex hydrogenation dynamics in cathode,which restrict the further improvement of hydrogenation efficiency.High resistance of hydrogenation products of bio-oil in EHPR cathode catalyst layer causes product inhibition and therefore affects EHPR hydrogenation performance.This article is based on our existing macroscopic experimental research,namely ethanol can effectively alleviate the product inhibition,and utilizes the molecular dynamics simulation method to explore the mechanism of ethanol in easing product inhibition from the microscopic view and to extract criteria for evaluating additive performance.By calculating the adsorption energy,interaction energy and equilibrium adsorption configuration in the heterogeneous system,it is found that the competitive adsorption and solvation of ethanol are helpful to reduce the adsorption of succinic acid by carbon carrier in the liquid phase.Based on the analysis of the mechanism of action of ethanol,several important factors for the evaluation of additives are extracted:amphiphilicity,polarity,molecular weight and hydrophobic end size.Combined with energy calculation,the mitigation effects of common alcohols and ketones were evaluated:ethylene glycol?EG?<methanol?MeOH?<ethanol?EtOH?<acetone?AC?<iso-propanol?iPrOH?.EHPR kinetic experiment verifies the predicted results,that is,isopropanol has the best eliminating effect on the product inhibition.After the addition of isopropanol,the hydrogenation conversion of maleic acid at 12 h increases by 64.3%compared with that without additives,and 21.7%compared with that with the addition of ethanol.The establishment of CO₂ hydrogenation kinetic model is of great significance for revealing experimental rules,optimizing operating parameters,improving current efficiency and productivity.However,there is still no research on the establishment of CO₂ hydrogenation kinetic model in EHPR.So in this paper,based on the Tafel plot,kinetics model is established considering the cathode potential,buffer concentration,reaction time and current density,and the source of CO₂ on the surface of the catalyst,competition between hydrogen evolution and hydrogenation,and the influence of reaction time on the reaction kinetics are also explored.The results showed that the adsorption equilibrium constant of CO₂ molecules in KHCO₃buffer solution on the Sn surface was 5.9 L mol^-1,and that of CO₂ molecules in the gas phase was 5.0×10^-4 L mol^-1,which means reactants CO₂ are mainly from the KHCO₃ buffer.CO₂ on Sn surface mainly comes from buffer solution,and a small part of it comes from gas phase CO₂.The hydrogen evolution reaction will not have an obvious inhibitory effect on the hydrogenation reaction.It can be attributed to the rate limiting step,the Volmer step,which results to the generation rate of in-situ adsorbed hydrogen is slow but the rate of consumption is fast,thus it will not accumulate on the surface of the catalyst to inhibit the adsorption of CO₂.When the concentration of product formic acid reaches about 3.0 mmol cm^-2,the reaction rate rapidly decline with time.This is because the adsorption of formic acid on the surface of the catalyst can not be ignored at this time,which reduces the number of empty adsorption sites in catalyst and inhibits the adsorption of reactant CO₂.At the same time,partial formic acid will decompose,resulting in product inhibition.Moreover,due to the large mass transfer resistance in the microchannel of EHPR catalytic layer,formic acid will accumulate in the catalytic layer,which is more serious than the product inhibition phenomenon in the traditional three-phase reactor.