Kezhi Li

Potassium associated manganese vacancy in birnessite-type manganese dioxide for airborne formaldehyde oxidation

As a strategy for regulating the electronic structure of metal oxides, defect engineering has been widely studied, and the concentrations and spatial distributions of metal vacancies in metal oxides have always resulted in unprecedented properties. Moreover, alkali metals exhibit a universal promotion effect on catalytic oxidation of formaldehyde (HCHO). Herein, a kind of birnessite-type manganese dioxide (MnO2) with many Mn vacancies was hydrothermally synthesized for catalytic oxidation of HCHO. The significant effect of the K+ content on the structure, morphology and catalytic activity of birnessite-type MnO2 for HCHO oxidation was systematically studied for the first time. Initially, the increasing content of K+ obviously improved the catalytic performance for HCHO oxidation due to the considerable enhancement of the lattice oxygen activity. However, due to interaction with the excess K atoms, the oxygen atoms nearest to the K atoms were more stable and their mobility decreased, which was confirmed by experimental characterization and DFT (density functional theory) calculation. Moreover, the excess K+ increased the amount of surface basic sites, making CO2 difficult to desorb. Thus, there was an optimal K+ content to promote the activity of birnessite-type MnO2. With moderate K+ content in the birnessite-type MnO2, excellent catalytic activity for HCHO oxidation was achieved (T50% = 56 °C; T90% = 82 °C) under 100 ppm of HCHO and ∼90 L gcat−1 h−1 of gas hourly space velocity (GHSV). The present work provided an insight into the structure–activity relationship between birnessite-type MnO2 and its catalytic activity.

The relationship between surface open cells of α-MnO2 and CO oxidation ability from a surface point of view

α-MnO2 is a widely reported material for catalysis reactions. However, due to its complicated structure with 2 × 2 tunnels, knowledge of the surface structure and its relationship with the catalytic performance for α-MnO2 is limited, even though the understanding of this is key for developing better catalysts. Herein, a surface-focused structure–performance relationship regarding the oxidation of CO is established in this work. It is shown that the 2 × 2 tunnels are open at the surface {100} planes via transmission electron microscopy (TEM), TEM simulation, and CeO2 probe deposition experiments. Since α-MnO2 is rod-like and wrapped with {100} planes, the surface of α-MnO2 consists mainly of this open cell structure. The open cells are rich in adsorption sites for O2. Furthermore, these surface sites are also highly oxidative in nature. Owing to the fact that CO can react with surface oxygen species to yield CO2, the promoted adsorption of O2 and the oxidative surface sites will benefit CO oxidation over α-MnO2. Thus, the superior performance for CO oxidation can be related to the open cell structure. This work not only establishes the surface structure–performance relationship, but also provides a new point of view for the understanding of the structure of α-MnO2 by emphasising the surface open cells.

Facile surface improvement method for LaCoO3 for toluene oxidation

The rational design of low-cost transition metal catalysts that exhibit high activity and selectivity may be the most significant area of investigation in heterogeneous catalysis. A selective dissolution method using acid solutions was previously reported to tune catalyst surfaces. In this work, LaCoO3 (LCO-0) perovskite catalysts were synthesized by the traditional citrate sol–gel method for toluene oxidation. The catalytic activity of the LaCoO3 treated with acetic acid (LCO-1) was significantly increased: the T90 of LCO-1 was 223 °C, which was 40 °C lower than that of the untreated catalyst (LCO-0) under a weight hourly space velocity (WHSV) of 60 000 mL g−1 h−1. The exposed A-site cations of perovskite were slightly etched, but still preserved the original framework according to XRD, TEM, SEM and ICP results. Moreover, LCO-1 exhibited excellent stability even after 500 °C calcination. The high catalytic performance was mainly associated with improvements in reducibility, surface oxygen vacancies and surface area. The high stability was due to the preservation of the perovskite structure after acetic acid treatment.

Selective Catalytic Reduction of NO x with Ammonia over Copper Ion Exchanged SAPO-47 Zeolites in a Wide Temperature Range

Copper‐exchanged H‐SAPO‐47 zeolites were synthesized by an aqueous solution ion exchange method and applied in the selective catalytic reduction of NO with NH3 (SCR). The microporous chabazite (CHA) structure of the catalysts was characterized and confirmed using synchrotron X‐ray diffraction and nitrogen adsorption–desorption experiments. The synthesized Cux‐SAPO‐47 catalysts exhibited excellent SCR activity and nearly 100 % N2 selectivity in a wide temperature range. The Cu0.1‐SAPO‐47 sample exhibited higher activity than the other samples below 250 °C. The weak Lewis acid sites originating from the introduction of Cu2+ were more active than the Brønsted acid sites of the zeolites framework at low temperature. Two types of isolated Cu2+ species with different coordination surroundings were found, namely, Cu2+ cations bonded with the six‐membered rings and those bonded in the CHA cages. The former species exhibited good stability and activity at high temperature, whereas the latter ones were more active at low temperature. The Cu0.1‐SAPO‐47 catalyst provided a considerable amount of isolated Cu2+ bonded in the CHA cages. These results indicated that active Lewis acid sites and isolated Cu2+ species are responsible for their excellent SCR activity.