Mo掺杂CeO2催化剂上甲烷的无氧偶联: 表面和气相过程

doi:10.1016/S1872-2067(23)64440-7

摘要/Abstract

摘要：

甲烷无氧偶联是一种很有前景的甲烷转化利用的增值途径. 由于高温条件下催化剂表面反应与气相自由基反应共存且均对产物分布具有重要影响, 因而很难深入理解该反应的机理.

本文采用火焰喷雾热解方法制备了一系列具有高度分散Mo位点的Mo掺杂的CeO₂样品, 并用于催化甲烷无氧偶联反应. 结果表明, 在气相产物中, 附加值较高的C₂烃产物(乙烷和乙烯)的选择性可达98%. 反应过程中催化剂中高度分散的Mo-oxo物种被还原并转化为Mo(oxy-)碳化物物种, 它是甲烷活化的活性位点. 通过改变反应器内气相部分(即无催化剂部分)的体积, 研究了气相反应对于产物分布的贡献. 乙烷是甲烷无氧偶联的初级产物, 部分乙烯和大部分苯是通过气相化学形成的. 综上, 本文为甲烷无氧偶联高效催化剂的设计提供了参考, 并明确了减少反应器中自由体积以限制二次气相反应的重要性.

关键词: 甲烷, 无氧偶联, CeO₂, Mo, 活性位

Abstract:

Direct catalytic non-oxidative coupling is a promising route for the valorization of abundant methane. Understanding the mechanism is difficult because reactions at the surface of the catalyst and in the gas phase via radicals are important at the high temperatures employed. Herein, a series of Mo-doped CeO₂ samples with isolated Mo sites were prepared by flame spray pyrolysis method and screened for their performance in non-oxidative coupling of methane. The selectivity to value-added C₂ hydrocarbons (ethane and ethylene) among gas-phase products could reach 98%. During the reaction, the isolated Mo-oxo species in the as-prepared catalyst are reduced and convert into Mo (oxy-)carbide species, which act as the active sites for methane activation. By varying the available catalyst-free gas volume along the length of the reactor, we studied the contribution of gas-phase reactions in the formation of different products. Ethane is the primary product of non-oxidative methane coupling and, at least, a part of ethylene and most of benzene is formed through gas-phase chemistry. This work provides insights into the design of efficient catalysts for non-oxidative coupling of methane and highlights the importance of reducing the free volume in the reactor to limit secondary gas-phase reactions.

Key words: Methane, Non-oxidative coupling, CeO₂, Mo, Active site

张昊, 苏亚琼, Nikolay Kosinov, Emiel J. M. Hensen. Mo掺杂CeO₂催化剂上甲烷的无氧偶联: 表面和气相过程[J]. 催化学报, 2023, 49: 68-80.

Hao Zhang, Yaqiong Su, Nikolay Kosinov, Emiel J. M. Hensen. Non-oxidative coupling of methane over Mo-doped CeO₂ catalysts: Understanding surface and gas-phase processes[J]. Chinese Journal of Catalysis, 2023, 49: 68-80.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(23)64440-7

https://www.cjcatal.com/CN/Y2023/V49/I6/68

图/表 11

Fig. 1. Synchrotron XRD patterns (a) and Mo 3d XP spectra (b) of fresh Mo@CeO2 catalysts. The experimental data are labeled using circles. The fitting results and the background are shown in grey.

Table 1 Mo/Ce ratios and actual Mo loadings obtained by XPS and ICP-OES analysis.

Catalyst	Mo loading (wt%)	Surface Mo/Ce ratio	Total Mo/Ce ratio
1Mo	1.11	0.05	0.02
2Mo	1.97	0.11	0.04
3Mo	3.14	0.33	0.06
4Mo	4.19	0.40	0.08

Fig. 2. AC-TEM (a), HAADF-STEM (b) and STEM-EDX elemental mapping analysis (c?e) of 1Mo; AC-TEM (f), HAADF-STEM (g) and STEM-EDX (h?j) measurements of 2Mo; AC-TEM (k), HAADF-STEM (l) and STEM-EDX (m?o) measurements of 4Mo.

Fig. 3. k3-weighted FT EXAFS (a) and the surface and bulk Mo content (b) of fresh Mo@CeO2 catalysts. The FT EXAFS is shown without phase correction. The Mo content was calculated using the bulk Mo loading obtained by elemental analysis and the contribution of each Mo species. The percentages (p) of Mo-O4 and Mo-O8 sites were calculated using the equation CN = 4 × pMo-O4 + 8 × pMo-O8, where the coordination number (CN) was obtained by EXAFS fitting. The insets in panel (b) show the DFT-optimized structures of Mo centers on CeO2 (111) surface (marked by Mo-O4) and in the bulk of CeO2 (marked by Mo-O8). Red: O atoms; ivory: Ce atoms; cyan: Mo atoms. Oxygen atoms coordinated with Mo in Mo-O4 are shown with smaller size balls. The atoms on the top layer in Mo-O8 are shown using smaller balls.

Fig. 4. (a) Methane conversion and product distribution over Mo-doped CeO2 catalysts, the error bar indicates the standard deviation. (b) C2 hydrocarbon yield, overall selectivity and CH4 conversion over 2Mo during two reaction-regeneration cycles. Conditions: 50 mg of catalyst, 950 °C, 20 mL/min 95 vol% CH4 and 5 vol% Ar.

Fig. 5. Pulse reaction over 2Mo catalyst (a) and bare CeO2 support (b). Conditions: 950 °C, 50 mg catalyst, 2 mL CH4 pulses every 180 s.

Fig. 6. Operando Mo-K edge XAS results of Mo species in 2Mo with XANES spectra (a) and MS signals (b) (15 °C/min to 750 °C and dwell for 20 min; 15 mL/min CH4; 150 mg of 2Mo catalyst).

Fig. 7. (a) The fraction of three components obtained from MCR-ALS analysis during reaction. (b?d) Their corresponding XANES. The relevant XANES of reference samples are also included.

Fig. 8. AC-TEM image (a), HAADF-STEM image (b), and STEM-EDX elemental mapping analysis (c?e) of 2Mo_used. The inset in panel (a) shows the enlarged region marked by the white square.

Fig. 9. (a) XRD patterns of used Mo@CeO2 catalysts. (b) k3-weighted FT EXAFS of used catalysts. The spectrum of the Mo2C reference is shown for comparison. EXAFS spectra are displayed without phase correction.

Fig. 10. (a) Reactor configurations employed to study gas-phase contributions of the NOCM reaction. (b) The catalytic performance of 2Mo evaluated for different reactor configurations. (c) Ar physisorption isotherms of the 2Mo catalyst upon pressing at different loads, the inset shows the pore size distribution. (d) Catalytic performance of the 2Mo catalysts pressed at different loads, the error bar indicates the standard deviation of experiments.

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Mo掺杂CeO₂催化剂上甲烷的无氧偶联: 表面和气相过程

Non-oxidative coupling of methane over Mo-doped CeO₂ catalysts: Understanding surface and gas-phase processes

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