Research paper
Engineering of crystal phase over porous MnO2 with 3D morphology for highly efficient elimination of H2S

https://doi.org/10.1016/j.jhazmat.2021.125180Get rights and content

Highlights

  • Porous α-, β- and δ-MnO2 catalysts with different tunnel structures were synthesized.

  • δ-MnO2 rich in oxygen vacancies showed significant advantage among MnO2 catalysts.

  • Porous δ-MnO2 nanospheres displayed 100% H2S conversion and high stability at 210 °C.

  • The reaction pathways of MnO2 catalysts for H2S selective oxidation were proposed.

Abstract

In the present work, we report a facile oxalate-derived hydrothermal method to fabricate α-, β- and δ-MnO2 catalysts with hierarchically porous structure and study the phase-dependent behavior for selective oxidation of H2S over MnO2 catalysts. It was disclosed that the oxygen vacancy, reducibility and acid property of MnO2 are essentially determined by the crystalline phase. Systematic experiments demonstrate that δ-MnO2 is superior in active oxygen species, activation energy and H2S adsorption capacity among the prepared catalysts. As a consequence, δ-MnO2 nanosphere with a hierarchically porous structure shows high activity and stability with almost 100% H2S conversion and sulfur selectivity at 210 °C, better than majority of reported Mn-based materials. Meanwhile, hierarchically porous structure of δ-MnO2 nanosphere alleviates the generation of by-product SO2 and sulfate, promoting the adoptability of Mn-based catalysts in industrial applications.

Introduction

Hydrogen sulfide (H2S) is an extremely odorous and nocuous gas responsible for facilities corrosion and catalysts poisoning in industry applications (Shah et al., 2017, Chen et al., 2020, Cao et al., 2019). The Claus technology is a traditional utilized method for the removal of H2S. Nevertheless, 2–5% of H2S cannot be converted to S owing to thermodynamic limitations (Pan et al., 2020, Li et al., 2020). Among advanced desulfurization approaches, the selective oxidation process (2H2S+O2 → (2/n)Sn + 2H2O) is a highly appealing technology due to thermodynamic completeness and low capital requirement (Zheng et al., 2019, Zhang et al., 2015, Lei et al., 2019).

With abundant oxygen vacancies, excellent redox ability and specific physical/chemical properties including diverse oxidation states and crystal structures, manganese dioxide (MnO2) has been extensively investigated in the field of pollution control (Hayashi et al., 2019, Zhang et al., 2017). Mn-based materials can also be considered for the H2S selective oxidation. Shin et al. concluded that the considerable activities of H2S selective oxidation over MnVOx catalyst are due to the promoted redox property by adding MnO2 (Shin et al., 2001). Nevertheless, the Mn-based catalysts are unsatisfactory because of poor porosity and easy sulfation. During H2S oxidation reaction, the deposition of sulfur and sulfate on the catalyst with poor porosity would cause the blocking of active sites and pores, resulting in low stability and activity.

Crystal-phase engineering is regarded as a knob to induce various physico-chemistry properties in metal oxides (Hu et al., 2019, Cheng et al., 2019). This is particularly true for MnO2-based catalysts due to the crystal structure of MnO2 plays a crucial role in determining the defect sites (Chen et al., 2020). Intrinsically, crystalline MnO2 usually exists in many polymorphic forms, mainly including δ-, β-, and α-type. The MnO2 nanocrystals consist of [MnO6] octahedral units shared by edges or corners, which leads to different layered and tunnel structures. The synergism of defect sites and different structures promotes the crystal-phase dependence of catalytic reactivity over MnO2-based catalysts (Xu et al., 2017). Zhang et al. concluded that δ-MnO2 is superior to other crystal phase of MnO2 with respect to the formaldehyde oxidation due to special 2D layer tunnel structure (Zhang et al., 2015). Yan et al. reported that the interaction between Mn2+/Mn3+ and Hg2+ is responsible for the Hg0 oxidation, and the activities increased as follow: β-MnO2 < γ-MnO2 < α-MnO2 (Xu et al., 2015). As for the catalytic oxidation of H2S, oxygen vacancies and reducibility of MnO2 could be controlled by crystal-phase engineering, which leads to higher oxygen concentration and therefore promotes catalytic activity. In addition, the adsorption energy of by-product SO2 is highly dependent on the crystal types of MnO2, which is beneficial to alleviating the formation of sulfate and therefore improves the stability (Ye et al., 2020). Although various studies have focused on phase dependence of catalytic performance over MnO2, the application of MnO2 in H2S selective oxidation is rare and relationship between crystalline phases of manganese dioxide and its catalytic activity has not been explored.

Toward this end, α-, β- and δ-MnO2 samples with hierarchical porous structures were successfully prepared by a facile oxalate approach. The decomposition of oxalate during the calcination process leads to the generation of hierarchical porous structure. Compared with α- and β-MnO2, the porous δ-MnO2 nanospheres displays better activity, S yield, and stability in H2S oxidation. The hierarchical porous structure consisted of at least two levels of porosity is beneficial to the reaction. We elaborately explored the impact of crystal phase, redox ability, and oxygen vacancy on the catalytic performance of MnO2. Electron energy-loss spectroscopy (EELS) spectra were also conducted to get insight into intrinsic oxygen vacancy. Moreover, the reaction pathway and structure-activity relationship for catalytic oxidation of H2S over Mn-based catalysts were revealed by the combination of the advanced characterizations.

Section snippets

Synthesis of porous δ-MnO2

In a typical procedure, 40 mL of KMnO4 (0.16 M) aqueous solution and 90 mL of (NH4)2C2O4·H2O (0.036 M) were mixed in a beaker (150 mL) under magnetic stirring at 50 °C for 30 min. Then the solution was transformed into an autoclave (200 mL) and kept at 205 °C for 24 h. After cooled down, the precipitate was centrifuged and washed with distilled water for several times. Finally, the precipitate was dried under vacuum in an oven at 95 °C overnight with the calcination at 300 °C for 2 h.

Synthesis of porous α-MnO2

40 mL of

Structural properties and morphology

The XRD patterns displayed in Fig. 1A indicate that the MnO2 samples with different phase structures were successfully prepared by the adopted hydrothermal methods. δ-MnO2 shows diffraction peaks at 2θ = 12.2, 24.7, 36.4 and 65.4°, which are assigned to the (001), (002), (100), and (110) planes of birnessite structure of hexagonal phase (JCPDS 80-1098, C2/m) with poor crystallinity (Sun et al., 2016). The diffraction peaks at 2θ = 12.8, 18.1, 28.7, 37.6, 41.9, 49.8, 56.4, 60.4 and 69.5°

Conclusions

In summary, the crystal-phase dependence of H2S selective oxidation over porous α-, β- and δ-MnO2 catalysts was systematically studied. The structure-property relationships established in this work prove that the porous δ-MnO2 nanospheres possesses large number of oxygen vacancies (verified by XPS, EELS, and EPR spectra) that favors more oxygen to be adsorbed on the catalyst and further transforms to active oxygen. As a result, δ-MnO2 showed the highest reactivity for catalytic oxidation of H2S

CRediT authorship contribution statement

Shijing Liang, Lijuan Shen and Lilong Jiang conceived the research and supervised the project. Xiaohai Zheng, Zheng Yao and Yong Zheng performed the experiments, Xiaohai Zheng wrote the paper, Yihong Xiao and Yanning Cao analyzed the data. All authors discussed the results and commented on the manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are grateful to the financial support from National Natural Science Foundation of China (22078063, 21677036 and 21878052), the National Key Research and Development Program of China (2018YFA0209304) and the Natural Science Foundation of Fujian Province (2018J01693 and 2020H6007).

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