| Nowadays, the declining of fossil reserves and the enhancement of greenhouse effect force people to pay more attention to the hydrogenation of bio-oils and CO2 to fuels. The hydrogenation in proton exchange membrane reactor (PEMR) can be operated at normal pressure and temperature, due to the ability of producing adsorbed hydrogen atoms readily in-situ on the cathode catalyst surface with controllable concentration. But the hydrogenation performance of bio-oils and CO2 in PEMR is not good because of the problems of the mass transfer in the diffusion layer and the structure of PEMR.Bio-oils usually have wide boiling range with complex components, and the mass transfer of bio-oil in diffusion layer is the important factor that affects PEMR hydrogenation performance. In this study, carbon papers treated with PTFE and stainless steel welded meshes (SSWMs) are used as hydrophobic and hydrophilic diffusion media, respectively. Through the cathode flow channel experiments, we intuitively investigated the mass transfer form of bio-oil model compounds, butanone (volatile) and maleic acid (non-volatile) aqueous solutions, in both hydrophobic and hydrophilic diffusion layers, and proposed the matching principle of diffusion layer’s hydrophobicity and reactant’s volatility. With hydrophilic SSWM as the diffusion layer, both butanone and maleic acid can transfer in the form of liquid. While with hydrophobic carbon paper as the diffusion layer, butanone can permeate through carbon paper in the form of vapor due to its high volatility. However, for the non-volatile maleic acid aqueous solution, mass transport through hydrophobic carbon paper is the rate-limiting step, resulting in an almost zero reaction rate of hydrogenation..According to the mass transfer mechanism of diffusion layer, hydrophobic carbon paper with large air permeability and hydrophilic SSWM were used as the diffusion layer of hydrogenation of butanone and maleic acid, respectively. The effects of current density, reactant concentration, reaction temperature and reaction time on the PEMR hydrogenation performance were investigated. The maximum reaction rate reaches around 340 nmol cm-2 s-1 for both hydrogenation systems (95 mA cm-2,1 mol L-1,40℃), and current efficiency reaches around 70%. The results are much better than that reported in the literature, which may be attributed to the optimization of diffusion layer. Besides, because of the swelling of Nafion/PTFE composite membrane and Nafion binder of catalyst layer in organics, the catalyst layer fractured and even fell off when PEMR was operated in high concentration and temperature.The hydrogenation performance of CO2 in traditional PEMR is very poor. Though the addition of a buffer layer between the membrane and cathode catalyst layer can obviously improve the current efficiency of CO2 hydrogenation, the acting mechanism is not clear. So we compared the CO2 hydrogenation performance in both traditional PEMR and PEMR with buffer layer, and investigated the effect of catalyst, cathode potential and buffer layer concentration on CO2 hydrogenation performance. It is found that buffer layer can inhibit the generation of by-product hydrogen and help the cathode potential to reach the required value, prompting the hydrogenation reaction of CO2. Precious metal catalysts are not effective for CO2 hydrogenation with hydrogen as the staple product, while Sn is suitable for electrochemical hydrogenation of CO2 to generate formic acid. In addition, the increasing of buffer layer concentration is beneficial to the increase of current density and production of formic acid, but inhibits the production of CO. |
PDF Full Text Request