Theoretical Studies On The First Dehydrogenation Of CH₃OH By The Charged And Defect Graphene-Pt Catalysts

In present,the direct methanol fuel cell(DMFC)is the most competitive in all the fuel cell.The DMFC products are only CO2 and H2 O,and less environmental pollution.The actual energy conversion efficiency can reach 30%-40%,which is much higher than the internal combustion engine(10%-20%).DMFC has simple operation and can work effectively under low temperature,which can get rid of the dependence on single petroleum reresources in a certain extent,and it is beneficial to realize the strategic transformation of diversified clean energy.DMFC has a wide range of applications in different field,such as small household appliances,sensors,cameras,laptops,mobile phones,military mobility instruments and so on.Thus,the DMFC has attracted great attention because of these adwantages in recent years.In the DMFC study,the preparation and properties of anodic catalysts are the focus of research in the central position.Platinum(Pt)is often used as an anode catalyst.Therefore,Pt catalyst has been playing an importan role in the electrochemical catalysis of methanol.However,methanol shows very poor electrooxidation activity on Pt electrode,which is mainly attributed to CO poisoning or other carbonaceous reaction intermediates on the electrode surface.At the same time,the cost of the Pt is too high,which limits the wide commercial application in DMFC.In rectent years,the preparation/modification of Pt based nanoscale catalysts have become a hot spot in the field of energy,caltalysis and material.Very recently,in order to promote the ca talytic activity of methanol,experimental chemists have synthesized many nanocomposite Pt catalysts with new chemical synthesis methods,e.g.,PDDA/Pt-CNT,Pt-Ce O2/CNTs,Pt-WC and Pt-Ce PO4,which have higher electrocatalytic activity and chemical stabilit y toward methanol oxidation than the bare Pt catalysts.In the work of Wang et al,the methanol electro-oxidizing current was increased from 383 to 638 A g-1 using cyclic voltammetry methods,after introducing Ce O2 into the Pt catalyst,which means that Pt-Ce O2/CNTs exhibits higher catalytic activity for methanol electro-oxidation than bare Pt and Pt/CNT.Park et al have shown that the Pt-Ce PO4 with Ce/Pt=0.15 exhibits the better catalytic activity of methanol oxidation compared to the bare Pt.By hydrogen underpotential deposition,the electrochemical surface area values form hydrogen oxidation is 1.27 and 2.1(cm2 per cm2 sample)for bare Pt and Ce/Pt=1.5,respectively,which means that the catalytic activity of methanol oxidation in Pt-Ce PO4 is almost two times as much as in bare Pt.Moreover,the experimental chemists have also found a phenomenon that the surface of nano-supports could provide stronger negative electronic property to Pt atoms through one electron donating effect.It seems that the electron-injection for improving the oxidation of methanol has very important influence.One key problem in the utilization of Pt eletrocatalyst is the tolerance of Pt to the CO-poisoning in the anode.Luckily,chemists have found very recently that the defects in graphenes can not only fix the Pt particles,but also greatly improve the CO-tolerance of Pt.This is very pleasing since the defects are usually produced inevitably during the preparation or growth procedure of graphene,e.g.,single vacancy(SV)defect,Stone-Wales(SW)defect,double vacancy(DV)defect.The defective graphene can not only increase the fixing the Pt catalyst,but also improve the anti-CO poisoning.In this paper,we presented important information for the reaction of CH3 OH and Pt catalyst in DMFC,enriched the research on the mechanism of anode Pt catalyst,and provides an important theoretical bases for synthesis,preparation,design and so on in experiment.The main results are summarized as follows:(1)Abstract Understanding the bond-cleavage ability of metal clusters is very important in various fields,such as catalysis and surface science.In this work,we performed density functional theory calculations on the first dehydrogenation process(also the key step)of methanol on Ptn q(n = 1–3,q = 0,+1,-1)clusters in varied charge states using quantum chemical calculations.It is shown that methanol is adsorbed much more easily to the cationic Ptn+ than to the neutral and anionic Ptn0/-.By contrast,the intrinsic bond cleavage barriers of both C–H and O–H on the cationic Ptn+ are significantly higher than on Ptn0/-(the only exception is the C–H bond cleavage on Pt+).Promisingly,injecting an electron to the neutral Ptn0 to give Ptn-can greatly reduce the C–H/O–H bond scission barrier while maintaining appreciable adsorption energy.The charging effect can be nicely interpreted by the nature of the frontier orbitals of Ptn q.On all Ptnq,the first dehydrogenation barrier for the C–H scission was shown to be much lower than the barrier energy in the path of O –H scission.So the C–H bond scission may be the main reaction pathway of the first dehydrogenation.Ionization(Ptn0?Ptn+)can much increase the adsorption energy(favorably),while a t the same time increase the intrinsic X-H scission barrier(unfavorably).By contrast,electron-injection(Ptn0?Ptn-)can,in general,reduce the X–H scission barrier to a large extent(favorably),while retaining the considerable adsorption energy(favorably).Thus,electron-injection might be a suitable strategy to help design platinum catalysts with favorable activity.(2)Platinum(Pt)is a star catalyst that is now widely used in electrode reactions.Very recently,growing experimental evidences have shown that injection of electrons to the Pt catalyst could improve the surface catalytic reactivity to reactant(e.g.the CH3OH).However,the underneath molecular mechanism remains unclear.In this work,by performing density functional theory calculations,we for the first time studied the methanol dehydrogenation on the single layer of the Pt lattice(Pt13)under the conditions of electronic neutrality and injection of one electron.On neutral Pt13,the O-site is the more preferred adsorption than the methyl-site,though the latter can kinetically induce more facile bond cleavage.By sharp contrast,on anionic Pt13-,the methyl-site is both the more preferred adsorption and can induce the more facile bond cleavage.Thus,the cooperation of the adsorption and bond cleavage caused by the electron injection is responsible for the enhanced catalytic ability of the negatively charged Pt catalyst.It was shown that the energetic preference of the entrance adsorption and the subsequent bond cleavage is quite different for the CH3 OH oxidation on neutral and anionic Pt13 lattices,i.e.,disoperative in the former and cooperative in the latter.The injected electron is responsible for the disoperative! cooperative transition that can greatly improve the catalytic activity of Pt towards CH3 OH.It is our optimistic suggestion that one can effectively tune the catalytic activity by manipulating the electric property of Pt catalyst via suitably choosing substrate or metal ligand for Pt-electrocatalysis in anode.(3)Graphene has been increasingly used as an effective support for Pt-electrocatalyst in the direct methanol fuel cells(DMFCs).Very recently,it has been found that introducing the defects in graphenes can not only stabilize the Pt-particles on graphene,but also greatly increase the tolerance to CO-poisoning.Here we explored for the first time the influence of graphene-defects on the Pt-electrocatalytic behavior.Therefore,in addition to studying the charge effect,the effect of the defective graphene supporter on the Pt electrode is discussed.At the B3 LYP and M06(single-point)levels,the adsorption and the first dehydrogenation(C-H/O-H)of methanol on Pt supported by three types of graphenes,i.e.,pristine,in Stone-Wales(SW)defect and in single-vacancy(SV)defect.The calculations revealed that the SV-defect has the most stabilization and CO-tolerance for Pt,compared to SW-and pristine graphenes.However,meanwhile,the SV-defect significantly increases the intrinsic C-H/O-H cleavage barrier,which would deteriorate the electro-catalytic activity of Pt.It is our suggestion that in choosing the substrate for Pt-electrocatalysis in anode,it is very desirable to consider the controlling effects that sometimes may be contrastive in a balanced manner.