Recent trends in CO₂ reduction through various catalytic methods to achieve carbon-neutral goals: A comprehensive bibliometric analysis

Abstract

The extensive utilization of fossil fuels has led to a significant increase in carbon dioxide (CO₂) emissions, contributing to global warming and environmental pollution, which pose major threats to human survival. To mitigate these effects, many researchers are actively employing state-of-the-art technologies to convert CO₂ into valuable chemicals and fuels, thereby supporting sustainable development. However, few studies have employed bibliometric methods to systematically analyze research trends in CO₂ reduction reaction (CO₂RR), resulting in limited macroscopic insights into this field. This study aims to conduct a scientometric analysis of academic literature on electrocatalytic, photocatalytic, and thermocatalytic CO₂RR from 2015 to 2023. Utilizing bibliometric analysis tools Citespace, Bibliometrix, and Vosviewer for data visualization, it establishes a knowledge framework for catalytic CO₂RR. The results show that China, the United States, and India are the top three countries with the highest number of published papers in this field, with China and the United States having the highest levels of collaboration. The journal Applied Catalysis B-Environmental published the most articles and received the highest citation count, with 3.4% of the articles in this field appearing in the journal and a total of 62526 citations. Keyword analysis revealed that terms like “CO₂RR,” “CO₂,” “conversion,” and “reduction” are the most frequently occurring, indicating key areas of focus. Additionally, “selectivity” and “heterojunction” emerged as prominent research hotspots. The discussion section highlights the current challenges in the field and proposes potential strategies to address these obstacles, providing valuable insights for research in the field of catalytic CO₂RR.

Graphical abstract

Keywords

CO₂ reduction / electrocatalysis / photocatalysis / thermal catalysis / bibliometrics

1 Introduction

To transition successfully into a carbon-neutral economic era, it is essential to replace the current dependence on fossil fuels with sustainable alternatives sourced from renewable resources. This shift not only reduces greenhouse gas emissions but also promotes the development of a more environmentally sustainable and economically viable energy sector [1,2]. A promising approach to addressing global challenges related to CO₂ reduction reaction (CO₂RR) is the utilization of various renewable energy sources. These sources include solar [3], wind [4,5], hydroelectric [6], geothermal [7], and biomass energy [8]. These abundant and clean resources can significantly decrease the carbon footprint while meeting the growing energy demand [9].

Electrocatalytic CO₂RR technology plays a pivotal role in the transition to a carbon-neutral economy [10–20]. It involves using electricity to drive chemical reactions that convert CO₂ into valuable products such as methane or ethylene [21–23]. This process not only reduces CO₂ emissions but also creates an opportunity to produce high-value chemicals that can be used in various industries [19,24–29].

Photocatalytic CO₂RR technology is another promising area of focus. It leverages light energy to activate catalysts that facilitate the conversion of CO₂ into useful compounds like formic acid or methanol [30–32]. By harnessing sunlight, an abundant source of energy, this technology holds great potential for large-scale implementation, contributing both to the reduction of greenhouse gas emissions and the production of valuable chemicals [18,33–35].

Thermocatalytic CO₂RR technologies are also under extensive investigation [36–39]. These technologies use heat from renewable sources or waste heat from industrial processes to drive chemical reactions that convert CO₂ into useful products like syngas or hydrogen fuel [40,41]. This approach not only addresses climate change concerns, but also offers opportunities for efficient use of excess heat generated by various industries [42–46].

It is crucial to systematically evaluate and reflect on the progress made in the field of CO₂RR, as both theoretical research and practical applications have advanced. Previous reviews have address various aspects of CO₂RR, including the development of green catalysts [47,48], challenges faced by high-emission industries [49], innovative carbon reduction technologies [50,51], and pathways for energy transition [52,53]. These reviews have significantly contributed to the advancement of CO₂RR research by offering comprehensive insights and summaries from diverse disciplinary perspectives [54]. However, this reviews lack a macro-level analysis and the ability to handle large data sets, offering limited objective insights, and missing the opportunity to detect emerging trends. The aim of this paper is to comprehensively summarize the literature on CO₂RR field, perform a bibliometric analysis using visualization tools, and address the following research questions:

(1) What are the trends in publications, keyword evolution, authorship, geographic distribution, institutional involvement, and collaboration networks in CO₂ RR research?

(2) What are the primary focus areas within the field of CO₂RR, and what methodologies are being developed to achieve effective CO₂ conversion?

(3) What future research directions can be anticipated in the context of advancing CO₂RR technologies?

In bibliometric research, CiteSpace has become one of the most widely adopted visualization tools [55]. Its functions, such as dual-map overlay, co-citation network clustering, and temporal visualization of keywords, provide in-depth insights into the evolution trajectories and thematic changes in scientific fields [56–58]. However, several limitations have been identified when using CiteSpace for exploration: first, it lacks a comprehensive multi-dimensional data association, which limits its ability to fully reveal the complex interrelationships between research themes. Second, its analysis of temporal data heavily relies on keyword timezone mapping, with potential for improvement in dynamic time-series representation. Additionally, its approach to keyword co-occurrence analysis is relatively simplistic, and its presentation of collaboration networks, especially between authors and institutions, is relatively basic, making it difficult to capture more intricate cooperation patterns [27,59–62].

Given these limitations of CiteSpace, this study incorporates Bibliometrix and VOSviewer to enhance the analysis. Bibliometrix excels in using Sankey diagrams to present cross-dimensional correlations between keywords, authors, and journals, providing researchers with an intuitive understanding of the interactions among these three elements. The keyword trend graphs generated Bibliometrix provide a detailed view of their evolution over time, improving the precision of time-series analysis [63,64]. VOSviewer is particularly effective in constructing co-occurrence maps, clearly visualizing keyword co-occurrence patterns and displaying content-rich, diverse elements such as core factors, key nodes, and researcher collaboration networks [65,66].

Therefore, this study utilizes three software tools to comprehensively analyze and complement the limitations of each other. It provides an in-depth bibliometric analysis of catalytic CO₂RR, including electrocatalytic CO₂ RR, photocatalytic CO₂RR, and thermocatalytic CO₂RR. Compared to previously published articles, it offers a more specific and detailed analysis.

2 Methodology

2.1 Data collection

The Web of Science (WoS) platform is a powerful tool for academic literature retrieval, widely used for exploring and evaluating interdisciplinary research fields [67]. By indexing both articles and their references, WoS creates a comprehensive citation network for each article in every journal. Google Scholar (GS) has also gained significant attention within the bibliometric and scientometric community, due to its capacity for gathering publication data, tracking citations, and generating metrics. This study uses the WoS and GS platform as data sources to conduct a bibliometric analysis of the collected literature [68,69]. The steps of this study are outlined below.

The first step involves in collecting and retrieving the necessary data. All data for this study were sourced from the WoS Core Collection and GS. In WoS, the search terms used were “electrocatalytic reduction of CO₂,” “thermocatalytic reduction of CO₂,” and “photocatalytic reduction of CO₂,” yielding 19377 results. After manually excluding non-article documents, such as books, book reviews, early access documents, and conference abstracts from 2015 to 2023, 1835 documents were removed, leaving 17542 relevant articles. During the export process, full records and cited references were selected, including information on authors, document types, WoS categories, keywords, publication years, publishers, affiliations, and countries/regions. A similar search in GS yielded 6946 articles. After deduplication, a total of 22107 articles from both WOS and GS were used for bibliometric analysis.

The second step is data analysis and visualization. The 22107 articles retrieved from WOS and GS were exported as plain text files. Due to platform limitations, which allow a maximum of 500 records per export, 45 text files were generated. The first file contains records 1–500, the second file contains records 501–1000, and so on, with the last file containing records 22001–22107. For the visualization analysis, three software tools were used: Bibliometrix (version R. 4.2.2), VOSviewer (version 1.6.20), and CiteSpace (version 6.1.R1). The flowchart of the research process is illustrated in Fig.1.

2.2 Parameter design and key metrics description

2.2.1 Visualization analysis

In this study, CiteSpace was utilized to visualize scientific publications. The analysis covered the period from January 2015 to December 2023, with each year as representing a time slice. The data included titles, abstracts, author keywords, and keywords plus. The top 50 most cited or frequent items from each time slice were selected and the pruning sliced network method was used. Other settings remained default. CiteSpace provides three key metrics: Q value, which measures the significance of clustering structure. The Q value ranges from 0 to 1, and a value greater than 0.3 indicates significant clustering, where nodes are more tightly connected than by random distribution; Centrality, which evaluates a node’s importance within the network. Nodes with high centrality often act as bridges in knowledge dissemination, connecting different research groups; and Burst detection, which identifies nodes with a sudden increase in citations during a specific period, signaling rapid development or innovative directions in the field.

2.2.2 VOSviewer analysis

VOSviewer offers three visualization views: network visualization, overlay visualization, and density visualization. These are included: network visualization, which displays connections between items, where the node size represents strength, link strength, and citation counts, while color indicates different clusters; overlay visualization, which is similar to network visualization but assigns different colors to nodes based on time series, allowing for the tracking of field evolution over time; and density visualization, which shows node density through color intensity, with high-density areas in red and low-density areas in blue, helping to quickly identify key themes and research intensity.

2.2.3 Bibliometrix analysis

Bibliometrix parameters are set via the default interface. Key metrics include annual publication volume, average annual citations, and a Sankey diagram of the relationships among three metrics. The analysis examines author collaboration networks and co-keyword analysis, with the results presented in predefined charts for improved clarity and academic presentation.

3 Results

3.1 Descriptive bibliometric analysis

After filtering out irrelevant data, the WoS database included a total of 22107 relevant articles from 2015 to 2023, as shown in Fig.2. CO₂RR research has experienced significant growth since 2015, with the largest increases occurring in 2017 and 2018, which saw publication increments of 41% and 42%, respectively. In recent years, the number of publications has grown at an average annual rate of 12.7%. Notably, 2023 marked the year with the highest number of publications, with 3341 articles, accounting for 19% of the total publications over the past decade.

Furthermore, as shown in Tab.1, a total of 29956 authors have contributed to the publication of 17542 articles across 784 journals, books, and other scholarly publications from 2015 to 2023. The literature has an average age of 3.17 years, with an annual average publication rate of 1754.2 articles and an average citation frequency of 46.81 times per article. These articles collectively cited 356544 references. Of the total articles, 116 were single-author works, while multi-author collaborative articles accounted for 17426. The collaboration index of the literature is calculated to be 1.72.

3.2 Classification of literature in catalytic CO₂ RR field

After removing duplicates, a total of 19611 documents related to catalytic CO₂RR were selected from the WoS database and classified into four categories, as shown in Fig.3(a). The largest category, with 12736 articles, focuses on photocatalytic CO₂RR. This is followed by electrocatalytic CO₂RR, which has 8423 articles, and thermocatalytic CO₂RR, with 649 articles. This distribution indicates that the areas of greatest interest to researchers are currently photocatalytic and electrocatalytic CO₂RR. The types of photocatalytic, electrocatalytic, and thermal catalytic CO₂RR, as well as the star materials used were first analyzed and detailed in Tab.2.

Photocatalysis: Photocatalysis harnesses solar energy to drive CO₂ reduction by exciting electrons and generating electron-hole pairs. Ultrathin 2D materials, such as molybdenum disulfide (MoS₂) and molybdenum selenide (MoSe₂), are particularly noteworthy due to their unique structure, superior light absorption, and efficient charge carrier separation. Semiconductor materials like titanium dioxide (TiO₂) and cadmium sulfide (CdS) are also commonly used because they produce abundant photoexcited electrons and holes, facilitating CO₂ reduction. Additionally, metal-organic frameworks (MOFs) such as UiO-66 can incorporate metal ions or nanoparticles, significantly enhancing photocatalytic performance. These frameworks have demonstrated remarkable efficiency, especially in hydrogenating CO₂ to methanol.

Electrocatalysis: Electrocatalytic CO₂ reduction primarily relies on electron transfer processes at the electrode surface, driven by the application of an external voltage. Commonly used materials include precious metal catalysts such as gold (Au), silver (Ag), and platinum (Pt), which offer high conductivity and chemical stability, effectively promoting the conversion of CO₂ into carbon monoxide (CO) or methane (CH₄). Non-precious metals like copper (Cu), nickel (Ni), and tin (Sn) serve as cost-effective alternatives while still demonstrating good catalytic activity. In particular, Cu-based catalysts have gained significant attention due to their ability to generate a variety of valuable hydrocarbons. Additionally, carbon-based materials like graphene and carbon nanotubes (CNTs) are known for excellent electron conductivity and large surface areas, which further enhance reaction efficiency.

Thermocatalysis: Thermocatalysis utilizes high-temperature conditions to convert CO₂ with the help of catalysts. Transition metal catalysts such as iron (Fe), cobalt (Co), and nickel (Ni) exhibit strong catalytic activity at elevated temperatures and are effective in promoting CO₂ conversion. Oxide catalysts, including zinc oxide (ZnO) and cerium oxide (CeO₂), provide high thermal stability and reliable performance under extreme conditions. Zeolites, such as ZSM-5 and Beta zeolites, feature unique porous structures that create excellent diffusion pathways, boosting catalytic efficiency in high-temperature reactions.

In the field of photocatalytic CO₂RR, 12736 documents were analyzed based on keyword clustering, as shown in Fig.3(b). The majority of the literature focuses on topics such as homogeneous catalysis, titanium dioxide, single-atom catalysts, covalent organic framework materials, and oxygen vacancies. These areas are critical as they relate to the catalysts and materials necessary for the CO₂RR process, playing a significant role in photocatalytic CO₂RR.

Fig.3(c) presents the keyword clustering analysis for research on electrocatalytic CO₂RR. Key terms in this cluster include {\mathrm{C}}_{2^{+}} products, single-atom catalysts, silver, formate, homogeneous catalysis, and cobalt phthalocyanine. These keywords highlight essential aspects of electrocatalytic CO₂RR, such as the diversity of target products, catalyst design and application, and the chemical intermediates and catalytic principles involved in the reaction process. These factors are shaping future research directions and technological advancements in the field.

Fig.3(d) shows the keyword clustering analysis for research on thermocatalytic CO₂RR. In this context, photo-driven thermocatalysis offers a more efficient method for energy utilization, and synergistic effects help optimize catalyst performance. Ethylene, one of the main products, demonstrates significant practical application value, while amide polymers play a unique role in the catalytic process. These areas represent promising research directions that are propelling the development of the field.

3.3 Co-country or region network analysis

From the table of the top ten countries and regions (Tab.3), it is evident that China has the highest total collaboration strength, with 11166 publications. The United States and Saudi Arabia follow, with 1526 and 865 publications, respectively. China also leads in the number of collaboration instances, with 67 connections, followed by India with 62 connections. Overall, China ranks first in total collaboration strength, while the United States holds the second position.

Between 2015 and 2023, the network of collaborating nations and regions, as depicted in Fig.4, consisted of 72 nodes and 76 linkages. Based on publication numbers, China emerged as the leading contributor, with 1782 papers, more than double that of the second-ranked United States (685). In terms of research development over the years, around 2021, CO₂RR research in China, the United States, and Japan experienced rapid growth, while countries like UK, France, Saudi Arabia, and Malaysia showed slower progress. From the analysis, it is evident that the most closely linked nations are China and India, with 67 and 62 connections, respectively. The United States ranks third with 61 connections. Notably, the map shows that the most significant collaboration occurs between China and the United States.

3.4 Disciplines and journal analysis

3.4.1 Category and disciplines

The dual-map overlay function in CiteSpace superimposes one map onto another, where the former is known as the overlay map [70]. This tool effectively reveals the relationship between citing and cited domains, uncovering interconnections across different scientific fields or disciplines. As shown in Fig.6, the left side represents the citing map, indicative of frontier research knowledge, while the right side shows the cited map, reflecting foundational research. The curves on the map illustrate citation pathways, demonstrating how knowledge flows across disciplines.

In the citing map on the left, the vertical axis length of the ellipses corresponds to the number of papers published in journals, while the horizontal axis length represents the number of authors. Fig.3 clearly shows that disciplines such as Physics, Materials, and Chemistry (discipline 5), Veterinary, Animal and Science (discipline 7), and Molecular, Biology, and Immunology (discipline 4) dominate the citing journals. This dominance is because literature related to catalytic CO₂RR is often published in journals focused on physics, materials, and chemistry. The cited journals, on the other hand, mainly fall within the disciplines of Chemistry, Materials, and Physics (discipline 4) and (Molecular, Biology, and Dynamics (discipline 8). This indicates that publications in the field of CO₂RR frequently cite foundational literature from chemistry, materials, physics, molecular biology, and dynamics, signaling a deeper exploration of catalytic CO₂RR research.

The dual-map overlay, therefore, serves as effective visualization of knowledge transfer and interconnections between different scientific fields, shedding light on the disciplinary influences and citation patterns with catalytic CO₂RR research.

3.4.2 Journals

The journal co-occurrence map, generated using Vosviewer software (Fig.6) and the ranking table of the journals with the highest number of publications (Tab.4) were used to analyze the productivity of journals. In this map, each node represents a journal and the strength of connections between nodes indicates how frequently two journals co-occur in the same publication, suggesting a shared research focus or thematic relevance. Nodes are color-coded, with journals of the same color exhibiting closer relationships, meaning they co-occur more frequently. The top 10 journals accounted for 24.83% of all publications. Between 2015 and 2023, a total of 17542 articles were published across 784 different journals.

The journal with the highest publication count was Applied Catalysis B: Environmental, with 603 articles, representing 3.4% of the total publications. It also had the highest citation count, with 62526 citations and an H-index of 328. Following this, Journal of Materials Chemistry A and Applied Surface Science, ranked second and third, with 576 and 511 articles, respectively, accounting for 3.3% and 2.9% of the total publications.

The fourth and fifth most productive journals were Chemical Engineering Journal and ACS Catalysis, publishing 490 and 473 articles, respectively. Additionally, journals ranked sixth to tenth each published over 200 articles, including Angewandte Chemie International Edition, ACS Applied Materials & Interfaces, Journal of the American Chemical Society, Journal of Colloid and Interface Science, and Journal of CO2 Utilization.

These rankings indicate that research on catalytic CO₂RR is primarily concentrated in journals dedicated to environmental science, chemical engineering, materials science, and physics, which aligns with the findings from the subject category analysis.

As shown in Fig.7 and Tab.5, the Sankey diagram, generated using Bibliometrix software, illustrates the relationships between keywords, authors, and journals. The leftmost column lists the keywords, the center column represents the authors, and the rightmost column shows the journals. The thickness of the connecting lines corresponds to the frequency of the links: thicker lines indicate more frequent connections.

In this diagram, it can be observed that Li Y has published papers in prominent journals such as Applied Catalysis B: Environmental, Angewandte Chemie International Edition, Chemical Engineering Journal, and Journal of Materials Chemistry A. The most frequent keywords associated with Li Y include “CO₂RR,” “photocatalysis,” “CO₂ photoreduction,” and “photocatalytic CO₂ reduction.” Similarly, Wang Y has published papers in “Applied Catalysis B: Environmental,” “Applied Surface Science”, and “Chemical Engineering Journal”, with associated keywords like “CO₂RR,” “photocatalyst,” and “carbon dioxide.”

Overall, researchers in the CO₂RR field frequently collaborate with journals such as “Applied Catalysis B: Environmental” and “Journal of Materials Chemistry A” for their publications, as reflected by the strong connections shown in the diagram.

3.4.3 Author analysis

Using VOSviewer software, a co-authorship network map was generated (Fig.8), which analyzes the collaborative relationships between authors in the field of catalytic CO₂RR research. This map helps to visualize the social network structure, revealing key research teams or groups and highlighting internal collaboration patterns within these teams. The co-authorship network can assist individual researchers in identifying potential collaborators. Additionally, journal publishers can use the data to form editorial teams based on expertise and collaborative ties in the field.

In the co-authorship network analysis, 116 closely collaborating authors were selected from a total of 29956 authors and divided into ten distinct clusters. When ranked by the number of publications, Jiaguo Yu, positioned at the central node of the deep yellow cluster, ranks first with 128 articles. The second-ranked author, Lei Wang, in the red cluster with 107 articles. However, when ranked by the number of connections, Lei Wang takes the lead, followed by Xin Li.

3.5 Reference co-citation network analysis

When two or more papers are cited by one or more other papers, they form a co-citation relationship, which is useful for analyzing the relatedness between documents [71]. Co-citation analysis makes it possible to identify highly cited and observe closely related literature, as papers that are frequently cited together often share common themes or research areas.

The co-citation network clustering view in Fig.9 was generated by selecting “reference” as the node type, setting the time slice to one year, and using the g-index (k = 2) with “Pathfinder” and “pruning sliced networks” for visualization. The modularity value of 0.7848, which exceeds the threshold of 0.3, indicates a strong fit.

The clusters identified in the co-citation network of catalytic CO₂RR research include: #0 electrochemical reduction, #1 single-atom catalyst, #2 copper-based catalyst, #3 photocatalytic reduction, #4 photocatalytic conversion, #5 iron porphyrin, #6 graphitic carbon nitride, #7 selective photoreduction, #8 metal-organic framework, #9 s-scheme heterojunction, and #11 copper-based electrocatalyst. The most cited paper, located in Cluster #2 is by Nitopi S (2019), with 1023 citations. The second most cited work is by Li X (2019) in Cluster #4, with 781 citations. Clusters #0, #1, #2, #8, and #11 are related to electrochemical CO₂RR, focusing on catalysts, electrolyzers, and reaction conditions. Clusters #3, #4, and #7 are associated with photocatalytic CO₂RR, while Clusters #5, #6, and #9 emphasize special catalysts, materials, and related research areas.

3.6 Keyword analysis

3.6.1 Keyword co-occurrence network analysis

“CO₂RR” is the most strongly connected keyword in Tab.7, with a connection strength of 28843. The second most connected keyword is “carbon-dioxide,” with a connection strength of 25818. In CiteSpace, “Centrality” measures the frequency with which a node appears on the shortest paths in network analysis, indicating the influence and importance of the node within the network. The centrality scores for “CO₂RR,” “carbon-dioxide,” and “conversion” are the highest, at 0.21, 0.18, and 0.11, respectively. These three keywords hold a crucial position in the research on catalytic CO₂RR.

In this study, VOSviewer was used to analyze 17542 documents, generating the keyword co-occurrence network shown in Fig.10. This network visualizes the frequently occurring terms and their relationships in the literature on catalytic CO₂RR through nodes and lines of different colors. In the diagram, nodes represent different keywords, and lines indicate co-occurrence relationships between them.

The field of catalytic CO₂ RR include multiple facets and research directions. The high frequency of keywords such as “catalyst,” “nanomaterials,” and “photocatalysis” highlights the importance of selecting appropriate catalysts and materials for the CO₂RR process. “CO₂,” as the primary target of this research, appears frequently, underscoring its significance. Keywords related to “electrocatalytic reduction” and “photocatalytic reduction” indicate that these are currently hot topics and key areas of focus within the field.

3.6.2 Keyword trend analysis

Fig.11, created using Bibliometrix software, visualizes the frequency changes of keywords related to catalytic CO₂ reduction from 2017 to 2023. This visualization highlights the evolution of thematic terms over this period. In recent years, primary keywords include “selectivity,” “heterojunction,” “carbon-dioxide,” “reduction,” and “TiO₂.” Earlier research was centered around terms such as “electrocatalytic reduction,” “anatase TiO₂,” “semiconductor,” “visible-light irradiation,” and “hydrocarbon fuels.” By examining the frequency shifts of these keywords, one can quickly identify the current research hotspots in the field of catalytic CO₂RR. “Selectivity” has emerged as an important measure of catalyst performance, affecting the efficiency of reactions and the choice of products. The growing significance of “heterojunction” likely reflects its enhanced electron transport and reaction interface properties, which contribute to improved catalytic performance.

3.6.3 Timezone visualization map analysis

This CiteSpace timezone visualization map in Fig.12 and Tab.8 illustrates the research trends in catalytic CO₂RR literature from 2015 to 2023. Each small square represents a publication, with its size reflecting the number of documents and citation counts. Lines between squares indicate the connections between different documents, with closer proximity signifying tighter keyword relationships [72].

As shown in Fig.12, the cluster modularity value (Q value) is 0.7845, suggesting a significant clustering structure, and the average silhouette value (S value) is 0.9499, which confirms that the clustering is well-defined and convincing. The timeline map shows that research on catalytic CO₂RR revolves around seven primary themes: #0 photocatalysis, #1 electrocatalysis, #2 g-c3n4, #3 homogeneous catalysis, #4 single-atom catalysts, #5 metal-organic frameworks, #6 photocatalytic CO₂ reduction, #7 s-scheme heterojunction, #8 CO₂ photoreduction, and #9 formate. These clusters can be categorized into four major research areas: electrochemical reduction research, materials related to catalytic CO₂RR, catalyst ligands and design, catalytic process and related products. These research themes and areas represent the current hotspots in the field.

3.6.4 Analysis of burst keywords

The burst analysis of keywords in the literature on catalytic CO₂RR from 2015 to 2023 is presented in Tab.9, generated using CiteSpace software. This table shows the most ten frequently cited keywords along with their corresponding occurrence periods, effectively highlighting the research hotspots and development trends within this field. The red lines indicate periods of keyword bursts, while the blue lines show intervals of occurrence.

1) Carbon dioxide (2015–2016, Strength 51.04): As the core keyword, carbon dioxide reflects the primary focus of catalytic CO₂RR research. As a major component of greenhouse gases, CO₂ significantly impacts global climate change. Research in this area is crucial for environmental protection and the conversion of sustainable energy sources [20,73–77].

2) Anatase TiO₂ (2015–2019, Strength 41.63): Anatase titanium dioxide is a commonly used photocatalyst, known for its strong oxidative capacity and chemical stability under UV light. It is widely employed in environmental remediation, solar energy conversion, and CO₂RR applications [48,61,62,78–83].

3) Artificial photosynthesis (2015–2018, Strength 40.58): Artificial photosynthesis mimics the natural process of photosynthesis by using light energy to convert CO₂ into organic compounds like methanol or methane. This research aims to develop efficient photoelectrochemical systems for converting solar energy into chemical energy [84–89].

4) Titanium dioxide (2015–2019, Strength 38.56): Titanium dioxide is a significant wide-bandgap semiconductor material widely used in photocatalytic and electrocatalytic reactions. TiO₂ is commonly employed as a photocatalyst or support material. These keywords indicate the evolving focus and significant advancements in the field of catalytic CO₂RR [90–92].

5) Semiconductor (2015–2019, Strength 33.56): Semiconductor materials play a crucial role in catalysis, especially in photoelectrocatalytic and electrocatalytic CO₂RR. These materials provide the necessary electron-hole pairs to drive catalytic reactions on the surface [93–98].

6) Photochemical reduction (2015–2019, Strength 32.86): Photochemical reduction refers to CO₂RR through chemically excited reactions by light energy. Sensitizers or semiconductor materials absorb light energy, which is then used to reduce CO₂ into valuable products [99,100].

7) Metal-electrodes (2016–2018, Strength 34.14): Metal electrodes serve as catalysts or current collectors and are a key component in the experimental setup for electrochemical CO₂RR [101–103].

8) Visible light (2016–2018, Strength 32.77): Visible light, ranging from approximately 400 to 700 nm, is a portion of the sunlight spectrum. Using visible light in CO₂RR offers a more efficient way to harness solar energy for CO₂ conversion [104–106].

9) Visible Light Irradiation (2017–2018, Strength 40.23): This keyword highlights the use of visible light sources to drive the reduction of CO₂, emphasizing its potential in improving the efficiency of the reduction process [107,108].

10) Molecular Catalysis (2017–2020, Strength 34.56): Molecular catalysis involves designing and synthesizing catalysts at the molecular level, often with metal complexes and organic molecule catalysts. This approach focuses on optimizing the structure of the catalytic active center and ligands to enhance the efficiency of CO₂RR [109–111]. These keywords reflect the key areas of focus and advancements in catalytic CO₂RR, underscoring the evolving research trends and the significance of each area within the field.

4 Conclusions

Using Bibliometrix, CiteSpace, and VOSviewer bibliometric tools, a comprehensive analysis of the literature on catalytic CO₂RR was conducted. The findings show a significant surge in interest in this research area, with the number of related publications increasing sharply in recent years. Based on these analyses, the following key conclusions are drawn:

1) Extensive research on catalytic CO₂RR has been conducted by scholars from China, the United States, Saudi Arabia, India, and Republic of Korea. Notably, China has emerged as the leading contributor in this field, with Chinese researchers playing a pioneering role in advancing catalytic CO₂RR technologies. The most frequently publishing journals in this domain include Applied Catalysis B: Environmental, Angewandte Chemie International Edition, and Chemical Engineering Journal.

2) Catalytic CO₂RR research encompasses various catalytic processes, including electrocatalysis, photocatalysis, and thermocatalysis, resulting in a wide diversity of research topics. The field has evolved into an interdisciplinary area that primarily spans multiple domains, particularly chemistry, materials science, physics, molecular biology, and kinetics. This multidisciplinary nature has facilitated significant advancements and the development of novel approaches across serval scientific domains.

3) Keyword cluster analysis reveals that catalytic CO₂RR research covers a broad spectrum of topics. With the advancement of research, traditional methods are gradually being replaced by new technologies and approaches. Early research focused mainly on chemical reduction, while more recent research has shifted toward photocatalysis and electrochemical reduction. This shift reflects scientific progress and highlights the growing pursuit of more efficient, sustainable, and environmentally friendly CO₂RR technologies. Keyword co-occurrence analysis highlights core research directions, including electrochemical reduction processes, materials research related to catalytic CO₂RR, catalyst ligands and design, reduction reaction mechanisms, and ammonia synthesis.

4) Over time, research on catalytic CO₂RR has become more systematic, with distinct focal clusters emerging in the literature. Current key clusters include electrocatalytic reduction, single-atom catalysts, copper-based catalysts, and photocatalytic reduction. Recent trends highlight new thematic areas, such as selectivity, heterojunction, carbon-dioxide, reduction, and TiO₂. These themes point to the increasing sophistication of research and the development of innovative strategies for enhancing the efficiency and selectivity of CO₂ reduction reactions.

5 Perspectives

The ongoing advancements in CO₂ reduction technologies highlight the critical role of diverse catalytic methods in achieving carbon neutrality. To ensure continued progress in this field, future research should focus on the following key areas.

1) Design and development of novel catalysts: Future research should prioritize the development of catalysts with enhanced selectivity and activity, particularly those featuring specific nanostructures or microstructures. These materials should be designed to optimize CO₂ adsorption and activation through improved surface area, porosity, and active site accessibility. Key strategies include hierarchical porous structures to facilitate mass transport and maximize active site exposure, defect-rich surfaces, such as oxygen vacancies to stabilize CO₂ intermediates, atomically dispersed catalytic sites for efficient CO₂ activation, exposing specific crystal facets (e.g., {111} or {110}) to enhance CO₂ binding and electron transfer, core-shell architectures combining materials with complementary properties to improve selectivity and stability. Such materials are expected to improve overall performance by increasing the number of active sites and enhancing the interaction efficiency between the catalyst and CO₂ molecules. Single-atom catalysts have demonstrated promise in maximizing the utilization of active sites and improving CO₂ conversion selectivity. Ongoing research could explore how the structure, size, and configuration of these materials affect performance, providing more refined design guidance for high-efficiency CO₂ reduction catalysts [112–114].

2) Enhancing photocatalytic spectral response and stability: While photocatalytic CO₂ reduction is promising, many photocatalysts still have a narrow spectral response range, particularly in fully utilizing the visible and infrared portions of sunlight. Future efforts should focus on broadening this spectral response through techniques such as elemental doping, surface modification, or combining photocatalysts with wide-bandgap materials to enhance light absorption capabilities. To address stability challenges in practical applications, efforts should also target developing photocatalyst materials with high corrosion resistance and long-term photostability, protective coatings or self-healing capabilities to enhance material durability in photocatalytic CO₂ reduction [115–119].

(3) Enhancing electrocatalytic efficiency and process optimization: In electrocatalysis, high overpotentials and low current densities limit the efficiency of CO₂ electrocatalytic reduction. Future research should focus on developing nanostructured electrode materials with high surface areas and dispersed active sites to significantly increase reaction rates, and exploring novel electrolytes, including unconventional solvents and ionic liquids to enhance electrocatalytic efficiency and reduce the generation of byproducts. These new electrolyte systems may offer enhanced ion transport, catalytic stability, and compatibility with specific catalytic systems. The application of in situ characterization techniques to monitor the electrocatalytic reaction in real-time could also provide valuable insights into reaction mechanisms, leading to further catalyst optimization [120–124].

4) Improving efficiency and lowering operating temperatures in thermocatalytic technology: While thermocatalytic CO₂ reduction is effective at high temperatures, it suffers from high energy consumption, limiting its scalability. Future research should focus on developing catalysts that maintain high activity under milder conditions by introducing metal-organic frameworks (MOFs) or other porous structures to enhance surface activity. Computational simulations, based on reaction kinetics models, could provide theoretical guidance for designing new catalysts, accelerating the development of more efficient catalysts [125–127].

5) Integration and synergistic effects of multicatalytic systems: Future research should explore the integration of multiple catalytic technologies, such as combining photocatalysis, electrocatalysis, and thermocatalysis, to achieve more efficient CO₂ reduction. By thoughtfully designing catalytic systems that leverage the synergistic effects of different catalytic technologies, it may be possible to achieve higher energy conversion efficiency than individual methods. Photo-electrocatalytic hybrid systems have already shown potential in improving reaction rates and selectivity, and future efforts should focus on optimizing the coupling effects within these hybrid systems and ensuring feasibility for industrial-scale applications to ensure their environmental sustainability and scalability [128–131].

In summary, further research should aim to refine and develop highly efficient and stable catalysts with novel microstructures or nanostructures to enhance CO₂ adsorption and activation. Photocatalysts with broadened spectral responses and materials promoting efficient charge separation and transfer will play a key role in improving the overall performance of CO₂ reduction processes. In electrocatalysis, optimizing electrode materials and exploring novel electrolytes will be crucial for improving CO₂ conversion efficiency. Additionally, integrating multiple catalytic systems, such as combining photochemical and electrochemical methods, could yield synergistic effects, significantly boosting CO₂ reduction efficiency. As new methodologies and technologies emerge, the field is poised for continued evolution, advancing toward a sustainable and carbon-neutral future.

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