Theoretical Insights Of H₂ Oxidation And Coking Tolerance Mechanism On Sm/Gd Doped CeO₂（111） Surface

Solid oxide fuel cell（SOFC）are considered one of the most promising alternative energy technologies because they can highly efficient and environmentally-friendly convert chemical energy into electrical energy.The anode provides active sites for fuels oxidation reaction.However,the high fuel adaptability requirements entail tremendous challenges for the selection of anode materials.At present,nickel and alloys are considered as ideal catalysts for fuel oxidation due to their low price and high catalytic activity.However,many problems such as carbon deposition caused by high catalytic activity hinders commercialization process.Therefore,it is an urgent problem for the solid oxide fuel cell industry to find an efficient carbon resistant anode material.Ceria has long been regarded as a good electrolyte materials of fuel cell because of high oxygen ionic conductivity.At the same time,Ce⁴⁺is always prone to be reduced into Ce³⁺,bringing a specific good electrical conductivity.Such mixed ion-electronic conductor is considered to be the ideal material to replace nickel and used as anode material for SOFCs.However,due to the low catalytic activity of pure ceria to hydrogen oxidation,the application of ceria in the anode of solid oxide fuel cell is critically limited.In this paper,CeO₂（111）surface was doped with two different rare earth metal atoms,Sm-doped CeO₂（SDC）and Gd-doped CeO₂（GDC）modification models were designed,and further demonstrated their stabilities by the phonon dispersion spectra and cohesive energy.Furthermore,the hydrogen oxidation mechanism on the designed SDC and GDC was investigated by periodic density functional theory（DFT）calculations and microkinetic modeling.Then,the coking tolerance mechanism of Gd-doped CeO₂ was further studied.Firstly,the mechanism of H₂ oxidation on the Sm doped CeO₂（111）surface was investigated.The results showed that the oxygen vacancy formation energy is dramatically decreased and the surface oxygen activity was enhanced due to the introduction of doped atom Sm.Combined with the analysis of the microkinetic model,it is believed that the secondary bond between Sm atom and H atom makes the H₂ oxidation reaction mechanism is Sm-mediated pathway（SMP）,and the rate-determing step is the dissociation process of H₂.Meanwhile,the maximum energy barrier is reduced from 3.54 e V on pure ceria surface to 0.532e V.In addition,the enormous improvement of H₂ oxidation reaction activity on SDC is primary based on the modification of electronic structure by the introduction of Sm³⁺ions,mirrored by the calculated density of states and bader charge of before and after doping and the co-adsorption of double H atoms.Secondly,the mechanism of H₂ oxidation on the Gd-doped CeO₂（111）surface was also studied.The results showed that the reaction mechanism of H₂ oxidation was Gd-mediated pathway（GMP）,unexpectively,the rate-determing step was the H₂ dissociation process,but the maximum energy barrier was reduced from 0.532 e V of Sm doped CeO₂ to 0.301 e V.At the same time,the H₂oxidation rate on Sm doped CeO₂（111）surface was compared at the range of temperature from 300 K to 1500 K,suggesting that CeO₂ doped with Gd has a good catalytic effect for H₂ oxidation in low temperature.In addition,studies on the CO oxidation reaction on Gd doped CeO₂（111）surface show that the introduction of Gd avoids the generation of crooked CO₂ and thus effectively avoids carbonate deposition.