Adsorption And Oxidation Of Formaldehyde On Gold/Ceria Catalyst: A Density Functional Theory Study

Formaldehyde is one of the most concerned indoor pollutants among volatile organic compounds (VOCs) since it is harmful for human health. Therefore, how to eliminate formaldehyde is an urgent work. Formaldehyde can be completely oxidized into CO2 and H2O over Au/CeO2 catalysts, which has been widely investigated. Compared with the extensive experimental studies, seldom theoretical investigation of formaldehyde oxidation on Au/CeO2 catalysts was reported. To obtain a systematic understanding of formaldehyde oxidation reaction mechanism, we firstly investigate the adsorption and reaction behaviors of O2 and CH2O on Au cluster, and then try to explore the possible mechanism of formaldehyde oxidation on Au/CeO2 surface, using density functional theory (DFT). Our result might be helpful to explore the new catalysts with high reactivity of CH2O oxidation at low temperature. The detailed results are as follows:The adsorption behaviors of O and O2 on the Au19 and Au20 clusters with and without charge were systematically investigated by density functional theory method. These results indicate that the adsorption energies of O on the hollow site of Au19 are higher than those on Au20; while those are similar on the side-bridge site of Au1g and Au20. When the cluster is negatively charged, the adsorption energies of O and O2 are higher than those on neutral and positive clusters. The 0-0 bond lengths of the adsorded O2 on Au19 and Au20 clusters with different charges show the similar trend as the adsorption energy, that is, the 0-0 bond lengths on Au19-1 is longer than those on Au19 and Au19+1 clusters. Population analysis shows that more electrons transfer to the adsorded O and O2 from Au19-1 and Au20-1 clusters, indicating their stronger interactions than the neutral and positive ones. Charge density difference (CDD) analysis of O2 on Au19 and Au20 clusters suggests that the electrons of Au19 and Au20 clusters transfer to theπ* orbital of O2, then the O-O bond are activated.Similar to O2 adsorption on Aun clusters, the adsorption behaviors of CH2O on the Au19 and Au20 clusters with and without charge were also systematically investigated. Our results suggest that the adsorption energies of CH2O on the top of Au19+ and Au20+ clusters are higher than those on other clusters. When the cluster is positive charged, the oxygen atom of CH2O interacts with Au cluster; while the hydrogen of CH2O interacts with Au cluster when the cluster is negatively charged. The lengths of C-O bond of formaldehyde on the positive clusters are longer than those on negatively and neutral ones. The dissociation reaction barrier of CH2O on Au19+ is 4.87 eV, which is higher than those on Au19-(1.05 eV) and Au19 (0.97 eV).The adsorption behaviors of O2 and CH2O on the periodical Au(111) steps surfaces were calculated by DFT method. The result shows that the adsorption energy of O2 on the top Au(111) surface is -0.46 eV, which is higher than the second layer. Similar results were observed on the first and second edges of Au(111) steps. This results are also consistent with CH2O adsorption on the periodical Au(111) steps surface. Transition state calculations indicate that the dissociation reaction barrier of O2 on Au(111) steps surface is 0.50 eV, which is lower than that of CH2O on the top surface (1.27 eV).Finally, the adsorption and oxidation of CH2O on Au/CeO2(111) surface was tentatively calculated using density functional theory. It indicates that Au atom on the top site of oxygen on CeO2(111) surface is more stable than that on the hollow site. The adsorption energies of CH2O on Au/CeO2 is -1.81 eV, which is much higher than that on the stoichiometic CeO2(111) surface. The C atom of CH2O with sp3 hybridization interact with O of CeO2(111) surface, while the O atom of CH2O bands to Au. After the hydrogen dissociation of CH2O on the Au/CeO2(111) surface, the OH species forms. Our calculation suggests that the dissociation of CH2O on Au/CeO2(111) surface is energetically favorable in thermodynamics. On the other hand, the adsorption energies and structures of Au on CeO2(111) surface and CH2O on Au/CeO2(111) are almost similar by standard DFT and DFT+U methods.