Highly c-disordered birnessite with abundant out-of-layer oxygen vacancies for enhanced ozone catalytic decomposition

Highlights

• The in-layer oxygen vacancy (V_O) and the out-of-layer V_O were identified on the c-disordered birnessites.

• The out-of-layer V_O was more abundant with the increase of c-disordering.

• Compared to the in-layer V_O, the out-of-layer V_O was more advantageous to O₃ decomposition.

Abstract

Ozone pollution has long been a hazard to human health and ecosystems. Manganese oxides have been intensively investigated for O₃ decomposition, but improving the catalytic activity and stability of MnO_x remains challenging. Herein, hexagonal birnessites with varying degrees of c-disordering were synthesized and showed strikingly different catalytic activities. As the degree of c-disordering increase, the highly disordered birnessite (HDB) possessed the highest content of oxygen vacancies (V_O) and thus demonstrated the superior catalytic activity. The HDB remained a high O₃ conversion efficiency of 96% even after reaction for 24 h at a high space velocity of 600,000 mL·g⁻¹·h⁻¹ and under the relative humidity of 50%. At the same time, HDB could be easily regenerated after heating for 1 h at 200 °C under air atmosphere. We found two different types of V_O (the in-layer V_O and the out-of-layer V_O) on the c-disordered birnessites, and the out-of-layer V_O was more stable than the in-layer V_O. The out-of-layer V_O was beneficial to desorption of the intermediate O₂^2– and reduced the competitive adsorption of H₂O, which accounted for the enhanced catalytic stability of HDB. This work may provide guidance for the V_O engineering in metal oxides toward the catalytic purification of waste gases.

Introduction

Ground-level ozone (O₃), as one of the primary airborne pollutants, is detrimental to human health and ecological environment [1], [2], [3]. Catalytic removal of O₃ is the most promising method which can be applied in purification of O₃-containing exhaust gas, especially under mild environmental conditions [4], [5], [6]. Among the various catalysts, noble metal catalysts can be used for effective elimination of O₃, but high price limits their applications [7], [8], [9], [10]. Transition metal oxides, particularly manganese oxides (MnO_x), have been extensively studied for O₃ decomposition owing to their low cost, great potentiality and environmental friendliness [7], [11], [12]. However, the catalytic activity and stability of MnO_x still need to be improved.

Oxygen vacancies (V_O) have been identified as the active sites on MnO_x for catalytic decomposition of O₃ [13], [14], [15]. Thus, increasing the content of V_O can significantly improve the catalytic activity of MnO_x. The main strategies to increase the content of V_O on MnO_x include: deoxidizing by vacuum reduction; doping with other metallic elements; regulating the species and concentration of tunnel cations [4], [13], [15], [16], [17], [18]. These researches were primarily based on crystalline MnO_x due to the mature synthetic protocols, controllable crystal phases and well-tuned morphology. However, the relatively perfect surface of highly ordered crystalline MnO_x limits the quantity of V_O. Previously, we reported the amorphous MnO_x with disordered lattice and abundant grain boundaries which promoted the generation of V_O [2], [19], [20]. But the modulation of structural order–disorder and the insight into how disordered lattice affects the stability of V_O require deeper understandings.

In general, unstable V_O are easier to combine with the intermediate peroxide specie O₂^2–, thus causing the O₂^2– difficult to be released [21], [22], [23]. The enrichment of O₂^2– at V_O is the decisive factor for MnO_x deactivation [16], [21], [22], [23], [24]. Hence, improving the stability of V_O can significantly improve the catalytic stability of MnO_x. Li et al. proved that two kinds of V_O (the sp2-V_O and the sp3-V_O) with different coordination environments were present on α-MnO₂, and the sp3-V_O was more stable by showing the easier release of O₂^2– [25]. So improving the stability of V_O can be attempted by tuning the chemical coordination environment of V_O to accelerate the release of O₂^2–. However, so far the precisely structural modulation of V_O still demands further investigations. On the other hand, the competitive adsorption of H₂O molecules at V_O could also cause deactivation of the MnO_x. Enhancing the moisture resistance of MnO_x can be achieved by the introduction of noble metals or rare earth elements, which will raise the cost definitely [4], [14], [16], [17], [26]. In addition, a new type of O₃ decomposition catalysts (metal organic frameworks, MOFs) with excellent moisture resistance has been developed [27], [28], [29]. Nevertheless, MOFs are not easily accessible and limited to high costs and low yields. Moreover, some regeneration methods have already been invented to settle the deactivation problem, but the catalytic activity of the MnO_x may only be partially recovered due to the limited regenerative ability of the V_O [7], [18], [30]. Therefore, the effective regulation of V_O is crucial to O₃ decomposition but remains challenging.

In this work, layered hexagonal birnessite-type MnO_x with varying degrees of structural disorder were synthesized to obtain two different types of V_O (the in-layer V_O and the out-of-layer V_O). It was found that the content of the out-of-layer V_O was more abundant with the increase of c-disordering on the c-disordered birnessites. And theoretical calculations indicated that the out-of-layer V_O was more beneficial to O₃ decomposition by displaying the easier adsorption of O₃, the weaker adsorption of H₂O and the easier release of O₂^2–. Hence the highly c-disordered birnessite (HDB) possessing the most out-of-layer V_O exhibited the best catalytic stability and regenerative ability toward O₃ decomposition.