A DFT+U Study Of CO Oxidation And NO_x Reduction Over Rare Earth CeO₂Catalyst

Rare earth oxide ceria (CeO2) has been widely applied in industrial catalysis, environmental protection, electronic device, ceramics, alloy, coating, and many other materials application fields. Among all these applications, CeO2is mostly used as three-way catalyst (TWC) for automobile exhaust control. The key pollutants of emission are carbon monoxide (CO) and complicated oxynitride (NOx) gas. Owing to CeO2has good performance of oxygen storing/releasing capacity (OSC) and its preparation and technology method has also been greatly improved, the experimental investigation of CO oxidation and NOx reduction over CeO2catalyst have obtained huge progress. However, many catalytic reaction micromechanism and surficial characteristic of CeO2are still unknown for the limitations of macroscopic experiment characterization methods. Therefore, systematic study of CO oxidation and NOx reduction over CeO2(110) system has been shown through density functional theory (DFT) method. The results achieved are listed as follows:1. From the point view of environment protection, CO+NOx redox reaction can simultaneously eliminate two exhaust pollutants at the same time. Hence, we firstly studied the adsorption of CO and NOx at CeO2(110) with supported Au nanoparticle and their reactions by using density functional theory calculations corrected by on-site Coulomb interactions (DFT+U). The results show that CO can strongly adsorb on the top site of Au nanoparticle with the adsorption energy of～1.2eV, while the adsorption of NO on both Au nanoparticle and the interface between it and the CeO2support is generally much weaker. However, at the interface, formation of N2O2dimer followed by cleavage of terminal N-0bond is an effective way to decompose NOx. For the whole process, the first step of CO+N2O2reaction can readily occur in Langmuir-Hinshelwood mode with activation energy of only-0.4eV, leading to the formation of N2O and CO2via an intermediate ONNOCO species. By contrast, the second step to eliminate N2O needs a rather high energy barrier of～1.8eV through a Eley-Rideal type collision reaction. Further analyses illustrate that the unique electronic properties of rare earth Ce can induce the electron transfer and localization from supported Au to surface Ce cations, which then promotes the formation of negative charged N2O2. Moreover, the structural flexibility of Au nanoparticle also facilitates the adsorbed CO to approach and react with the N2O2at the interface.2. At highly defected CeO2surface, NO elimination and N2production can be realized through cleavage of N-O bond and refill the vacancy with O atom. No other reductant addition is more friendly to environment. For NOx reduction reaction, adjacent oxygen vacancy pair is the key factor for N2O2formation and further dissociation to N2. Through the calculation of oxygen vacancy on CeO2(110), we found associated oxygen vacancy pair is not as stable as separated ones. The occurrence of adjacent oxygen vacancy pair must be realized through oxygen vacancy diffusion. Oxygen vacancy diffusion is very easy with0.4eV along [001] direction but is rather difficult along [110] direction for its barrier reaches up to1.5eV. Fortunately, NO can adsorb not only above the oxygen vacancy (Ead=0.8eV), but also react with the lattice O of the same or neighboring CeO2row (Ead=1.4eV) to form NO2. Therefore, oxygen vacancy diffusion can be realized by alternative NO oxidation and NO2reduction and meanwhile N2O2dimer species will be generated by two NO molecule adsorption side by side at the adjacent oxygen vacancy. NO diffusion barrier is calculated to be only0.9eV over the whole reduced CeO2(110) surface. Afterward, N2O2will be reduced to N2O and further reduced to N2with the reaction barrier of about1eV. Through further electronic analyses, we also found rare earth ceria plays an important catalytic role in both static and dynamic ways by tuning the electron distribution in adsorbates and reacting molecules.3. CeO2(110) surface will occur reconstruction in a vacuum under a condition of high temperature. From Scanning tunneling microscopy (STM) and Reflection High-Energy Electron Diffraction (RHEED) images, we can see (110) surface will lose a CeO2row and induce2×1reconstruction. Experiments results indicate that reconstructed surface will show better activity. Density functional theory calculations with on-site Coulomb interaction correction (DFT+U) have been performed to study the structures and catalytic activities of2×1reconstructed surface of CeO2(110). The reconstructed surface gives better thermal stability compared to the bulk truncated one and exhibits unique surface activity. We comprehensively calculated the O vacancy formation and diffusion on the reconstructed surface and found that the vacancy formation energy corresponding to the removal of one subsurface four-fold coordinated O is1.71eV only, which is smaller than that of the top-surface O vacancy or the sub-surface O vacancy at the bulk truncated surface. Accordingly, the O vacancy diffusion at2×1reconstructed surface is also much more feasible than that at the origin CeO2(110) with the highest diffusion barrier of only0.84eV. By calculating the detailed pathways of CO reaction with lattice O, we found that CO2can directly occur without forming a bent negatively charged CO2intermediate on the reconstructed surface, which may reduce the chance for the carbonate formation. It has also been clearly shown that strong localization characteristics of Ce4f orbital indeed favor electron transfer from reaction intermediates to the CeO2support.