Complete catalytic reaction of mercury oxidation on CeO2/TiO2 (001) surface: A DFT study

Insight into the performance of VOx-WOx/TiO2 catalysts modified by various cerium precursors: A combined study on synergistic NOx and chlorobenzene removal

Cerium is widely used as a modifier to enhance the catalytic performance of the selective catalytic reduction (SCR) catalysts due to its exceptional low-temperature properties. However, the effects of different cerium precursors on catalytic performance remains unclear. In this study, VOx-WOx/TiO2 catalysts are modified using Ce(NO3)3·6H2O (cata-N), CeO2 (cata-O), and Ce(OH)4 (cata-OH), and their synergistic removal of NOx and chlorobenzene (CB), as well as their resistance to water and sulfur poisoning, were systematically investigated. Among the tested catalysts, cata-N demonstrated superior CB (45.0-93.3 %) and NOx (31.9-90.37 %) removal efficiencies under synergistic conditions, along with excellent water resistance (T90 = 193 °C with 5 % H2O). In contrast, cata-OH exhibited the highest sulfur resistance, maintaining a denitrification efficiency of 20 % after 10 h of sulfur exposure, compared to 9 % for cata-N and 8 % for cata-O. Characterization revealed that Ce(NO3) 3·6H2O improved cerium dispersion, leading to enhanced the redox properties and acidity (especially Brønsted acid sites (BAS)) in cata-N. Density functional theory (DFT) calculations and In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTS) results revealed that the well-dispersed cerium atoms contributed additional BAS in the form of Ce-OH, while also forming Ti-O-Ce bonds. These Ti-O-Ce bonds facilitated the formation of Ti-OH on the TiO2 surface. Ti-OH significantly enhanced the adsorption of NH3 and CB, thereby promoting both the NH3-SCR and CB oxidation processes. This study offers new insights into the role of cerium precursors and provides a practical strategy for tuning BAS of catalysts in multiple pollutants removal.

Mechanisms for Modifying the Electronic and Spatial Distribution of the Single-Atom Ni/CeO2 Surface to Enhance CO-SCR Reactivity: Density Functional Theory Study

Modulating the intrinsic activity of heterogeneous catalysts at the atomic level is an effective strategy to improve the low-temperature CO-SCR (selective catalytic reduction) reaction activity and N2 selectivity, but it remains challenging by the experiment. In this paper, a single-atom-loaded surface generation strategy is developed to construct single-atom catalysts by density functional theory analysis, which will effectively reduce the reaction energy barriers in CO-SCR reaction. Specifically, the reaction of NO reduction by CO before and after Ni adsorption was thoroughly investigated and the reactivity was evaluated by using the CeO2 (1 1 1) surface as a carrier, with the application of density functional theory, electronic structure analysis, and transition state theory. The loading of Ni increases the energy barrier for the generation of N2O on the CeO2 (1 1 1) surface by 1.498 eV and decreases the energy barrier for the generation of N2 by 1.864 eV. This indicates that the adsorption of Ni inhibits the generation of N2O and promotes the generation of N2. After thermodynamics and kinetics analysis, the pathway of CeO2 (1 1 1)-Ot-Ni via O atoms filling O vacancies to generate N2 is a spontaneous unidirectional reaction when no nonvolumetric work is done at constant temperature and pressure. Theoretical calculations show that the modification of isolated Ni atoms on CeO2 induces electronic coupling and redistribution, which leads to the activation of neighboring O sites around Ni atoms. This study provides the strategy mechanism to enhance the activity and N2 selectivity of the low-temperature CO-SCR reaction at the atomic level and provides theoretical guidance for the theory of novel catalysts for synergistic removal of NO and CO.

Enhanced immobilization of Arsenic(III) and Auto-oxidation to Arsenic(V) by titanium oxide (TiO2), due to Single-Atom vacancies and oxyanion formation

Pollution control of As(III), a naturally occurring carcinogen, has recently gained a global attention, while due to the dominance of neutral H3AsO3 over a wide pH range, As(III) immobilization by most minerals is not efficient as As(V) immobilization. TiO2 shows promise for controlling As(III) pollution, and herein, a comprehensive study about As(III) adsorption by TiO2 and oxyanion formation is conducted by means of DFT + D3 methods. Both anatase and rutile are effective for As(III) adsorption, while As(III) adsorption affinities differ significantly and are -1.48 and -3.79 eV for pristine surfaces, ascend to -3.85 and -5.08 eV for O vacancies, and further to -5.37 and -5.26 eV for Ti vacancies, respectively. The bidentate binuclear complexes dominate for pristine surfaces, and O vacancies prefer OAs insertion into TiO2 lattice, while for Ti vacancies, all As(III) centers are auto-oxidized to As(V). Ti-3d, O-2p or/and As-4p rather than other orbitals contribute significantly to As adsorption, and O and Ti vacancies promote adsorption through stronger orbital hybridization. The superior adsorption for Ti vacancies originates from As(V) formation instead of bonding interactions. The formation of As oxyanions, which may occur spontaneously at pristine surfaces and is greatly promoted by O and Ti vacancies, enhances As(III) adsorption pronouncedly and becomes a viable strategy for As(III) immobilization. H2AsO3- and HAsO32- dominate for pristine surfaces and O vacancies, and for Ti vacancies, H2AsO4- and HAsO42- dominate over anatase whereas AsO43- also makes an important contribution over rutile. Results rationalize experimental observations available, and provide significantly new insights about the migration, bioavailability and fate of As(III) over TiO2 surfaces that facilitate the exploration of scavengers for As and other pollutants.

Ab initio calculation of the adsorption of As, Cd, Cr, and Hg heavy metal atoms onto the illite(001) surface: Implications for soil pollution and reclamation

Elucidating the mechanisms of heavy metal (HM) adsorption on clay minerals is key to solving HM pollution in soil. In this study, the adsorption of four HM atoms (As, Cd, Cr, and Hg) on the illite(001) surface was investigated using density functional theory calculations. Different adsorption configurations were investigated and the electronic properties (i.e., adsorption energy (E_ad) and electron transfer) were analyzed. The E_ad values of the four HM atoms on the illite(001) surface were found to be As > Cr > Cd > Hg. The E_ad values for the most stable adsorption configurations of As, Cr, Cd, and Hg were -1.8554, -0.7982, -0.3358, and -0.2678 eV, respectively. The As atoms show effective chemisorption at all six adsorption sites, while Cd, Cr, and Hg atoms mainly exhibited physisorption. The hollow and top (O) sites were more favorable than the top (K) sites for the adsorption of HM atoms. The Gibbs free energy results show that the illite(001) surface was energetically favorable for the adsorption of As and Cr atoms under the influence of 298 K and 1 atm. After adsorption, there was a redistribution of positions and reconfiguration of the chemical bonding of the surface atoms, with a non-negligible influence around the upper surface atoms. Bader charge analysis shows electrons were transferred from the surface to the HM atoms, and a strong correlation between the valence electron variations and the adsorption energy was observed. HM atoms had a high electronic state overlap with the surface O atoms near the Fermi energy level, indicating that the surface O atoms, though not the topmost atoms around the surface, significantly influence HM adsorption. The above results show illite(001) preferentially adsorbed As among all four investigated HM atoms, indicating that soils containing a high proportion of illite might be more prone to As pollution.