Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives

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Abstract

With 28–34 times the greenhouse effect of CO ₂ over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH ₃OH, CH ₃CH ₂OH, C ₂H ₆, and C ₂H ₄, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.

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g-C3 N4 -Based Heterostructured Photocatalysts

Jiaguo Yu, Bei Cheng, Chuanjia Jiang … (2018)

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The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts.

Malte Behrens, Felix Studt, Igor Kasatkin … (2012)

One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al(2)O(3) methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

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The Global Methane Budget 2000–2017

Marielle Saunois, Ann Stavert, Ben Poulter … (2020)

Abstract. Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning. The data presented here can be downloaded from https://doi.org/10.18160/GCP-CH4-2019 (Saunois et al., 2020) and from the Global Carbon Project.

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Journal

Journal ID (nlm-ta): ACS Environ Au

Journal ID (iso-abbrev): ACS Environ Au

Journal ID (publisher-id): vg

Journal ID (coden): aeacc4

Title: ACS Environmental Au

Publisher: American Chemical Society

ISSN (Electronic): 2694-2518

Publication date (Electronic): 20 June 2023

Publication date Collection: 20 September 2023

Volume: 3

Issue: 5

Pages: 252-276

Affiliations

[1] ^†State Key Laboratory of Pollution Control and Resource Reuse, Frontiers Science Center for Critical Earth Material Cycling, School of the Environment and ^‡State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University , Nanjing 210023, China

[§ ]Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NJUPT) , Nanjing 210046, China

[∥ ]State Key Laboratory of Natural Medicines, Key Laboratory of Drug Quality Control and Pharmacovigilance, Department of Pharmaceutics, China Pharmaceutical University , Nanjing 210009, China

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[* ]Email: wenleizhu@ 123456nju.edu.cn .

Author information

Xiaolian Sun https://orcid.org/0000-0001-9549-4741

Yuanyuan Wang https://orcid.org/0000-0003-3971-8362

Wenlei Zhu https://orcid.org/0000-0001-6110-993X

Article

DOI: 10.1021/acsenvironau.3c00002

PMC ID: 10515711

PubMed ID: 37743954

SO-VID: 07a8959b-3765-4d73-a46a-004b6c9a76cd

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Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works ( https://creativecommons.org/licenses/by-nc-nd/4.0/).

History

Date received : 14 February 2023

Date accepted : 02 June 2023

Date revision received : 24 May 2023

Funding

Funded by: Zijin Scholars Foundation, doi NA;

Award ID: 0205181022

Funded by: National Natural Science Foundation of China, doi 10.13039/501100001809;

Award ID: 22171132

Funded by: Research Funds for Jiangsu Distinguished Professor, doi NA;

Award ID: NA

Funded by: Fundamental Research Funds for the Central Universities, doi 10.13039/501100012226;

Award ID: 021114380189

Funded by: Fundamental Research Funds for the Central Universities, doi 10.13039/501100012226;

Award ID: 021114380199

Funded by: State Key Laboratory of Pollution Control and Resource Reuse, doi 10.13039/501100011150;

Award ID: PCRR-ZZ-202106

Funded by: Fundamental Research Funds for the Central Universities, doi 10.13039/501100012226;

Award ID: 021114380183

Funded by: Outstanding Youth Foundation of Jiangsu Province of China, doi 10.13039/501100010035;

Award ID: BK20220086

Funded by: Nanjing Science and Technology Innovation Project for Chinese Scholars Studying Abroad, doi NA;

Award ID: NJKCZYZZ2022-01

Funded by: Nanjing University, doi 10.13039/501100008048;

Award ID: NA

Funded by: Nanjing University of Posts and Telecommunications, doi 10.13039/501100005374;

Award ID: GZR2022010010

Funded by: Natural Science Foundation of Jiangsu Province, doi 10.13039/501100004608;

Award ID: SBK2022044384

Funded by: Natural Science Foundation of Jiangsu Province, doi 10.13039/501100004608;

Award ID: BK20210189

Funded by: China Pharmaceutical University, doi 10.13039/501100002857;

Award ID: CPUQNJC22_03

Funded by: National Natural Science Foundation of China, doi 10.13039/501100001809;

Award ID: 82272138

Funded by: National Natural Science Foundation of China, doi 10.13039/501100001809;

Award ID: 22276100

Funded by: National Natural Science Foundation of China, doi 10.13039/501100001809;

Award ID: 22176086

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Keywords: greenhouse effect,methane emission,photocatalytic conversion,photocatalyst design,semiconductor,cocatalyst,electron scavengers,application scenarios

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Keywords: greenhouse effect, methane emission, photocatalytic conversion, photocatalyst design, semiconductor, cocatalyst, electron scavengers, application scenarios

Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives

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Most cited references 173

g-C3 N4 -Based Heterostructured Photocatalysts

The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts.

The Global Methane Budget 2000–2017

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