Density Functional Theory Study Of Small Molecule Adsorption On Perovskites-type Catalyst Surface

In this paper, the adsorptions of NO on surface of LaCoO3and SO3on surface ofLa1-xSrxCoO3(x=0or0.25) were investigated by density functional theory methodwith the VASP package. The different conformations and the adsorption sites wereoptimized and the most stable structure was simulated according to the calcinationresults of adsorption energies. The distribution of the Bader charge was analyzedbased on the calculation results of the electron transfer direction and number.The results of NO molecule adsorbed on the LaCoO3perovskite surface showedthat there were two different adsorption conformations: one was the N downward andthe other one was the O downward. We found that the adsorption energy of Ndownward was always greater than that of O downward on the two terminated surface.It indicated that the N downward adsorption on the surface was the main adsorptionform. Comparing with the LaO-terminated surface, the NO molecule preferentiallyadsorbed on the CoO2-terminated surface, especially on the Co top site. The Badercharge results showed that the NO molecule obtained electrons from both theterminated surface in the N downward adsorption form. Electrons transferred from thesurface to the NO molecule. Wherein the N atom electron was reduced and transferredtoward the metal surface, while the O atom electron obtained electrons and thenumber was greater than the lost, leading that the whole of NO molecule expressed apulling electron performance.We created four initial conformations for the adsorption of SO3on theLa0.75Sr0.25CoO3(001) surface:1O-M,2O-M, S-M, S-O. Four adsorption sites onLaSrO-terminated surface and two adsorption sites on CoO2-terminated surfrace wereexamined. Through the optimization of the system and the calculation of theadsorption energy, the results indicated that the adsorption energy on the LaSrO-terminated surface was always larger than on the CoO2-terminated surface.Furthermore, the energy of the SO3molecule binding with surface by S-O was thehighest, demonstrating that this adsorption was the most stable form. Compared withthe LaCoO3adsorption system, the Sr doping increased the LaO-terminatedadsorption energy, but decreased the CoO2-terminated adsorption energy. Thus, theLaO-terminated surface absorbed SO3molecule more easily after the Sr doping, andthe sulfur resistance was weakened. According to the results of the Bader charge analysis for the SO3molecule adsorption, it strongly suggested that the electrondensity of the LaSrO-terminated surface decreased because the SO3molecule pulledelectrons from the surface. However, the migration of electron on the CoO2-terminated surface was the opposite, the electron of surface was increased and the SO3molecule provided electrons to the surface. In addition, the Bader charge of the SO3molecule adsorption on LaCoO3(001) surface was also investigated. In the LaCoO3system, the Bader charge transfer trend and the number were very similar to theLa0.75Sr0.25CoO3systems. It indicated that the similar sulfate was generated when theSO3molecular adsorption on the LaO-terminated surface.