Design And Regulation Of High Efficiency Catalytic System For Formic Acid Dehydrogenation

Hydrogen has been considered as one of the most promising energy carriers due to its high mass specific energy density and clean emission.Much attention has been paid to its preparation,storage and utilization and leaded to rapid development,while difficulties in compression,storage and transportation of hydrogen gas restrict the popularization of this method.As an organic hydrogen storage molecule,formic acid can supply hydrogen through catalytic decomposition at ambient temperature.It would play a positive role in improving the rapid evolution,efficient storage and transportation of hydrogen.In this paper,factors involved in the process of formic acid decomposition such as the design and preparation of catalysts,efficient expression and evaluation of catalytic performance,analysis of catalytic mechanism and optimization of catalytic environment were studied.Based on the confirmation of the qualitative and quantitative relationship between the size and valence composition of the catalyst and the performance,the activities of different catalysts could be evaluated more objectively and reliably by normalizing the turnover frequency(TOF),so that the optimal configuration of the catalyst could be confirmed.Based on these new understandings,Pd/C catalysts with higher activity and stability were synthesized.The relationship between the efficient dehydrogenation of formic acid and the surface activity of the catalyst was analyzed.The qualitative relationship between the process of formic acid decomposition on the catalyst surface and the catalytic environment was established,so as to realize the efficient expression of catalyst activity and construct an efficient hydrogen production system by formic acid decomposition.The details are as follows:1.Based on a series of catalysts with different particle sizes and the same valence composition,it could be confirmed that the size effect does not affect the intrinsic activity of formic acid dehydrogenation.However,due to the higher proportion of face sites in the large-size catalyst which made it easy to inactive,it could be indicated that the substrate adsorbed on the face sites was more prone to dehydrate.The results showed that the higher the content of Pd(Ⅱ)on the surface of the catalyst,the higher the activity and selectivity,indicating that Pd(Ⅱ)promoted the dehydrogenation process of formic acid and inhibited the dehydration process of formic acid.DFT and surface enhanced infrared experiments also supported this conclusion.2.The surface reconstruction of high Pd(Ⅱ)content catalyst was studied by slow in-situ reduction method.It was found that its activity increased with the reconstruction.Furthermore,the quantitative relationship between the rate of formic acid decomposition and the lower content components of Pd(0)and Pd(Ⅱ)on the catalyst surface is established,which shows that the real site of the catalyst is composed of Pd(0)-Pd(Ⅱ),while neither of these single components is active.Based on this,TOF was normalized for activity evaluation and it was found that the isokinetic temperature of formic acid decomposition(the temperature at which different kinds of catalysts showed similar intrinsic activity)was between 70-80℃.By combining normalized TOF with mass specific activity,it was found that the site utilization rate was near to 100%when the size of the catalyst was less than 2.26 nm.Therefore,it could be predicted that the small-size catalyst with 50%Pd(0)and Pd(Ⅱ)could exhibit the best formic acid decomposition performance.By coupling pre-nucleation with in-situ reduction,a high activity and high stability catalyst was successfully prepared with 1.8 nm particle size,50%Pd(0)and 50%Pd(Ⅱ)contents,which also verified this conclusion.3.The H-down(linear)adsorption configuration of the intermediate species in the dehydrogenation of formic acid was confirmed by ion dipole probe,which showed the inhibition effect of cations on its activity during the formic acid decomposition,indicating that cations with small charge density could greatly improve the activity.By fitting the reaction kinetic equation of formic acid and formate in dilute solution,the dehydrogenation order of formate could be confirmed as 1 for the first time,indicating that the process is a elementary reaction,and the apparent reaction order less than 1 could be contributed to the inhibition caused by the change of cation concentration.The fitted catalyst deactivation mechanism showed that the attenuation of catalyst activity is affected by both formic acid molecules and formte.The formic acid dehydration step(poisoning step)was a direct dehydration path of formic acid molecules with the promotion by ortho formate active hydrogen,which further verified the conclusion that the catalyst with more face sites is more likely to be inactivated.