Study of the effect of ceria on the activity and selectivity of Co and Ce co-doped birnessite manganese oxide for formaldehyde oxidation

Facile synthesis of multi-layer Co(OH)2/CeO2-g-C3N4 ternary synergistic heterostructure for efficient photocatalytic oxidation of NO under visible light

In this work, we report a one-step synthesis of ternary Z-scheme Co(OH)2/CeO2-g-C3N4 (CoCe-CN) heterostructure via hydrothermal method. Owing to the modification of Co(OH)2 and CeO2, the existence of Co(OH)2 as an electron acceptor-donor center between CeO2 and g-C3N4 accelerates the electron transfer and provides extra OH- reaction pathway for photocatalytic oxidation of NO. As a result, 50CoCe-CN (Co and Ce accounting for 25% mass ratio separately) achieved a 53.5% conversion efficiency of NO at 600 ppb concentration, which is 1.82 times that of g-C3N4 under visible light. The results of the DFT analysis and element distribution of cobalt and ceria provide convincing evidence supporting the existence of a novel multi-layer structure in the CoCe-CN photocatalyst. This structure involves the loading of CeO2 and Co(OH)2 on the g-C3N4 surface, and Co(OH)2 as a co-catalyst introduced between CeO2 and g-C3N4 realizes the synergy between CeO2 and Co(OH)2 which further improve the photocatalytic properties. The higher photocatalytic efficiencies observed in the CoCe-CN photocatalysts compared to those containing only cobalt (Co-CN) or ceria (Ce-CN) provide further evidence of the synergistic effect of these two elements. This work demonstrates a more efficient and effective ternary photocatalytic system, with greater practical potential for photocatalytic oxidation of NO.

Formulating the Li sites of Li-CoOx composites for achieving high-efficiency oxidation removal of formaldehyde over the Ag/Li-CoOx catalyst under ambient conditions

Oxide supported noble metals are extensively investigated for ambient formaldehyde oxidation, and the Ag-CoOx complex is one promising combination in terms of cost and activity. Further, we previously observed that cooperating Ag with Li + greatly boosted formaldehyde degradation on CoOx. Yet, there is still room for improvement in removal efficiency, mineralization capacity and resistance to severe conditions. These objectives could be realized via strategically formulating the Li+ sites of Li-CoOx composite in this sister study. Three samples with Li + ---Co3+-O2- connections (L-CO), spinel Li+ (LCO-S) and layered Li+ (LCO-L) were obtained at low (300 °C), moderate (500 °C) and high (700 °C) temperatures, respectively. The specific Li+ positions and componential interaction were demonstrated by Hyperspectral imaging (HSI), XRD, SEM, TEM, HAADF mapping, UV-vis DRS and XPS. Moreover, the effect of reactive oxygen exposure on catalytic oxidation of formaldehyde (330-350 mg/m3) was disclosed through CO-TPR and O2-TPD. Compared with the LCO-S and LCO-L, L-CO exhibited dominant formaldehyde degradation due to the larger content of surface oxygen. After Ag decoration, the Li+---Co3+-O2- connections uniquely caused a strong binding of Ag species with catalyst host, which boosted the amount of reactive oxygen and finally resulted in an even higher elimination of ∼73% (CO2 yield = ∼21%), 47% higher than that of the L-CO (CO2 yield = ∼6%). But in contrast, the Ag@LCO-S only achieved ∼53% removal (CO2 yield = ∼9%) and Ag modification was powerless in altering the inertness of LCO-L, demonstrating that the chemical environment of alkali metal is crucial to effectively tuning the catalyst activity. The advantage of Ag@L-CO in formaldehyde depollution was further reflected from its much better resistance to moisture and aromatic compound omnipresent in indoor air. For the first time, this study extended the understanding of the alkali-metal-promoted formaldehyde oxidation reaction to an in-depth level.

Oxidation mechanism of HCHO on copper-manganese composite oxides catalyst

Formaldehyde (HCHO) is a typical air pollutant that severely endangers human health. The Cu-Mn spinel-structure catalyst exhibits good catalytic oxidation activity for HCHO removal. Theoretical calculation study of density functional theory (DFT) was performed to provide an atomic-scale understanding for the oxidation mechanism of HCHO over CuMn2O4 surface. The results indicate that the (110) surface containing alternating three-coordinated Cu atom and three-coordinated Mn atom is more active for HCHO and O2 adsorption than the (100) surface. The Mars-van-Krevelen mechanism is dominant for HCHO catalytic oxidation. This reaction pathway of MvK mechanism includes HCHO adsorption and dehydrogenation dissociation, CO2 formation and desorption, O2 adsorption, H2O formation and surface restoration. In the complete catalytic cycle of HCHO oxidation, the second dehydrogenation (CHO* → CO* + H*) shows the highest energy barrier and is recognized as the rate-limiting step. The relationship of temperature and reaction rate constant is found to be positive by the kinetic analysis. The minimum activation energy of the MvK mechanism via the direct dehydrogenation pathway is 1.29 eV. This theoretical work provides an insight into the catalytic mechanism of HCHO oxidation over CuMn2O4 spinel.

Room temperature removal of high-space-velocity formaldehyde boosted by fixing Pt nanoparticles into Beta zeolite framework

Catalytic oxidation of volatile organic compounds like formaldehyde (HCHO) over the noble metals catalysts at room temperature is among the most promising strategies to control indoor pollution but remains one challenge to maximize the efficiency of noble metal species. Herein, we demonstrated the straightforward encapsulation of highly dispersive Pt nanoparticles (NPs) within BEA zeolite and adjacent with the surface hydroxyl groups to reach the synergistic HCHO oxidation at 25 °C. High efficiency and long-term stability was reached under large space velocity (∼100% conversion at 180,000 mL (g_cat × h)^-1 and >95% at 360,000 mL (g_cat × h)^-1), affording rapid elimination rate of 129.4 μmol (g_Pt × s)^-1 and large turnover frequency of 2.5 × 10^-2 s^-1. This is the first synergy example derived from the hydroxyl groups and confined noble metals within zeolites that accelerated the rate-determining step, the formate transformation, in the HCHO elimination.

Unveiling the Position Effect of Ce within Layered MnO2 to Prolong the Ambient Removal of Indoor HCHO

The position of Ce doping has a significant effect on ambient HCHO storage and catalytic oxidation on layered MnO₂. By associating structure and performance, it is unveiled that doping Ce into the in-layered lattice of MnO₂ is favorable to the generation of high-valence Mn cations, enhancing the oxidizing ability and capacity, but an opposite influence is displayed by interlayered Ce doping. From the aspect of energy minimization calculated by DFT, in-layered Ce doping is also recommended due to the decreased energies for molecule adsorption and oxygen vacancy formation. As a result, in-layered Ce-doped MnO₂ displays exceptional activity in catalyzing the deep oxidation of HCHO and a fourfold higher capacity of ambient HCHO storage than pristine MnO₂. The optimal oxide is combined with electromagnetic induction heating to complete the "storage-oxidation" cycle as a promising approach absolutely depending on non-noble oxides and household appliances to realize the long-acting removal of indoor HCHO at room temperature.

Enhanced oxidative ability, recyclability, water tolerance and aromatic resistance of α-MnO2 catalyst for room-temperature formaldehyde oxidation via simple oxalic acid treatment

To obtain a versatile formaldehyde oxidation material, simultaneously increasing the oxidative ability, recyclability and deactivation repellence (e.g., enduring the interference from moisture and aromatic compound omnipresent in indoor air) is of great significance. Herein, the above properties of α-MnO₂ were synchronously updated via one step treatment in oxalic acid (H₂C₂O₄), and an in-depth understanding of the surface properties-performance relationship was provided by systematic characterizations and designed experiments. Compared with the pristine sample, XPS, ESR, O₂-TPD, CO-TPR and pyridine-IR reveal that H₂C₂O₄ created substantial Mn³⁺ species on surface, exposing a higher coverage of oxygen vacancies that actively participated in the dissociative activation of gas-phase O₂ into reactive chemically adsorbed oxygen (O_C), and the abundant Lewis acid sites further enabled the effective O₂ activation process. The large amount of oxygen O_C promoted the HCHO-to-CO₂ conversion and inhibited the accumulation of formate that required a high temperature of 170 °C to be eliminated, thus conspicuously improving the α-MnO₂'s thermal recovery. The combined H₂O-TPD, H₂O-preadsorbed CO-TPR, C₆H₆-TPD and C₆H₆-preadsorbed CO-TPR investigations shed light on the H₂C₂O₄-induced water and benzene resistance. The notably weakened water and benzene binding strength with the H₂C₂O₄-modified surface together with the unrestrained oxygen O_C accounted for the outstanding anti-deactivation performance.