Catalytic Effect Of Niobium-based MXene Catalysts On Carbon Monoxide Oxidation

The emergence of proton exchange membrane fuel cells(PEMFCs)has greatly improved the consumption of non-renewable energy and the environmental pollution in traditional batteries.However,due to the extensive use of noble metals,the cost of conventional electrode materials for PEMFCs is relatively high.In addition,the traditional electrode materials are easily poisoned by carbon monoxide,which leads to the deactivation of catalysts.A variety of problems make PEMFCs not only can not be widely used,but also have the problem of low energy conversion efficiency.Therefore,it is desired to find electrode materials with low cost and high CO-tolerance to replace the traditional Pt/C electrode material.To improve the CO-tolerance of the electrode material,the surface must first be protected from CO poisoning,and the efficiency of CO oxidation should be higher.In this paper,Nb-based MXenes was used as the substrate,and the CO oxidation reaction was explored systematically through different surface modifications.The first principles method based on the density functional theory was used to explore the adsorption of CO and the catalysis of CO oxidation reaction on Nb-based MXene catalyst under different surface modification.The specific research contents and results are as follows:(1)The adsorption and oxidation of CO on Pd/Ov-Nb2CO2.Firstly,the migration of single palladium atom on the perfect Nb2CO2 surface was explored.It was found that single palladium atom could easily migrate on the perfect Nb2CO2 surface.After the introduction of oxygen vacancy,we explored the migration of single palladium atom near the oxygen vacancy and found that single palladium atom could migrate to the oxygen vacancy on the surface of Nb2CO2(Ov-Nb2CO2).By comparing the adsorption energy of palladium atoms on Ov-Nb2CO2 with the cohesion energy of palladium dimer,we found that single palladium atom tends to be adsorbed on Ov-Nb2CO2.We concluded that the single palladium atom can be stabilized at the oxygen vacancy.Then we analyzed the adsorption energy of the small molecules involved in different reactions on Pd/Ov-Nb2CO2.According to the adsorption characteristics of small molecules,we found that the presence of carbon monoxide can promote the adsorption of oxygen on Pd/Ov-Nb2CO2.Next,we studied the three mechanisms of carbon monoxide oxidation on Ov-Nb2CO2.It was found that,the oxidation reaction of CO tended to be carried out through the termolecular-Eley-Rideal(TER)mechanism on Ov-Nb2CO2,with an energy barrier of 0.42 eV.(2)The adsorption and oxidation of CO on MML/Nb2C.We first determined the most stable adsorption configurations of single atoms of different transition metals(M=Rh,Ir,Pd,Pt,Ag,Au)on Nb2C.Then the transition metal monolayer(MML/Nb2C)model was constructed.To verify the stability of the system,we compared the average adsorption energy and cohesion energy of metal atoms on Nb2C,and found that all transition metal atoms tended to form monolayer rather than cluster or metal bulk on Nb2C.By comparing and screening the adsorption energy of small gas molecules in different composite systems,we selected AgML/Nb2C system which has an appropriate adsorption strength for O2 and CO,and further tested the CO oxidation reaction.By comparing the energy barriers of different reaction pathways,we concluded that the oxidation of CO at AgML/Nb2C tended to occur along the Langmuir-Hinshelwood(LH)mechanism.The rate-limiting step is the oxidation of a CO by the adsorbed oxygen atom,which needs to overcome an energy barrier of 0.35 eV.In conclusion,AgML/Nb2C has a high catalytic activity for CO oxidation reaction.