Enabling Heterogeneous Catalysis to Achieve Carbon Neutrality: Directional Catalytic Conversion of CO$_2$ into Carboxylic Acids

The increase in anthropogenic carbon dioxide (CO$_2$) emissions has exacerbated the deterioration of the global environment, which should be controlled to achieve carbon neutrality. Central to the core goal of achieving carbon neutrality is the utilization of CO$_2$ under economic and sustainable conditions. Recently, the strong need for carbon neutrality has led to a proliferation of studies on the direct conversion of CO$_2$ into carboxylic acids, which could effectively alleviate CO$_2$ emissions and create high-value chemicals. The purpose of this review is to present the application prospects of carboxylic acids and the basic principles of CO$_2$ conversion into carboxylic acids through photo-, electric-, and thermal catalysis. Special attention is focused on the regulation strategy of the activity of abundant catalysts at the molecular level, inspiring the preparation of high-performance catalysts. In addition, theoretical calculation, advanced technologies, and numerous typical examples are introduced to elaborate on the corresponding process and influencing factors of catalytic activity. Finally, challenges and prospects are provided for the future development of this field. It is hoped that this review contributes to a deeper understanding of the conversion of CO$_2$ into carboxylic acids and inspires more innovative breakthroughs


General Background
With the intensification of the global energy crisis and environmental deterioration caused by the warming climate, aiming for carbon-neutral recycling is in imminent demand regardless of environmental protection or economic development(1, 2). The effective conversion of carbon dioxide (CO2) into fine chemicals has undoubtedly emerged as a powerful strategy to achieve carbon-neutral recycling due to its low cost and potentially low energy consumption (3)(4)(5). Extensive cost-effective conversion paths have been explored to realize such conversion practicably (5)(6)(7)(8). To date, various methods and strategies, including biological transformation, photocatalytic reduction, electrocatalytic reduction, organic transformation, dry reforming, and others, have been explored to convert CO2 into valuable chemicals based on continuing indepth studies over the past two decades (9)(10)(11).
In general, CO2 can be converted into numerous chemicals, such as CO, carboxylic acids, CH3OH, CH4, olefin, and so on, under the drivers of light, electricity, and heat (12,13). A large volume of CO2 emissions in the atmosphere is regarded as exhaust gas and the root cause of environmental deterioration, whereas these products could become valuable raw chemical materials with wide application value through catalytic conversion. A well-known application of CO is the synthesis of aldehydes through hydroformylation reaction, and high toxicity also poses challenges for its use and storage (14). Another product, CH3OH, can be widely used as

Introduction of Carboxylic Acid Compounds
Carboxylic acids encompassing formic, acetic, benzoic, acetylenic, amino, lactic, and other acids are essential components in fine chemicals and are regarded as core compounds in the natural carbon cycle (21). These chemicals have great application value in synthesizing chemical compounds and energy conversion. In all these carboxylic acids, formic acid has the smallest molecular formula and can be obtained in high yield with clean water or hydrogen as a reactant to react with CO2 over numerous heterogeneous catalysts (22). In addition, it is also regarded as a safe and convenient hydrogen storage carrier due to its high volumetric capacity of 53 g H2/L, which can be converted into clean electricity in direct formic acid fuel cells with appealing advantages, such as easy transportation and storage and high theoretical open-circuit voltage (23). Furthermore, it is an important chemical intermediate, which can be widely used in the leather, pesticide, antibacterial agent, medicine, livestock feed, dyestuff, and rubber industries (24). Moreover, other multi-carbon carboxylic acids have equivalent application prospects and economic value to formic acid. For example, they have been widely used as preservatives in pharmaceutical, cosmetic, agriculture, and polymer industries and are recognized as valuable platform chemicals for a growing nonfossil industry (25).
The industrial production of carboxylic acids has undergone a cumbersome and ineffective process that generates more energy consumption and threatens the current environment. For instance, one adopted route to synthesize formic acid undergoes the direct carboxylation of methanol and subsequent hydrolysis of methyl formate, which has evident shortcomings, such as high cost and low production efficiency (26). Another developed synthesis route using lignocellulosic biomass to produce formic acid still suffers from 5 complicated steps, such as acid hydrolysis, wet oxidation, and catalytic oxidation, leading to ultra-low biomass utilization and poor selectivity (27). Recently, the direct conversion of CO2 into formic acid has become widely appealing due to the drive for carbon neutrality and the economics of the synthetic route. The high yield and selectivity of formic acid are expected under a low energy consumption, profiting from a one-step synthesis path and well-developed heterogeneous CO2 activation process.
Likewise, the synthesis of other carboxylic acids, such as benzoic, acetylenic, amino, and lactic acids, faces vigorous challenges in activating inert CO2 and C-H bonds of organic substrates, leading to excessive energy consumption and possible impure products from competitive reactions (28). This outcome is evident in the synthesis of benzoic acid by the partial oxidation of toluene using a cobalt-manganese catalyst under high temperature.
Benzaldehyde forms as a side product, and the conversion rates of toluene are far from satisfactory. Another case is the preparation of acetylenic acid, where metal-organic reagents that are sensitive to water and air are often needed and operated under anhydrous and oxygenfree conditions, increasing the production cost and creating challenges for industrialized operations (29). In the case of producing amino and lactic acids, the original organic reactants containing multiple functional groups have the possibility of side reactions. The transformation of functional groups that may produce undesired products should be avoided.
The central goal is to prepare various carboxylic acids with CO2 with a high conversion rate and selectivity under mild conditions to overcome the above challenges and minimize carbon emissions. Thus far, significant progress has been reached in producing numerous kinds of carboxylic acids through photocatalytic reduction, electrocatalytic reduction, and thermally 6 driven organic transformation (Figure 1) (30)(31)(32). Despite the great achievements, the unsatisfactory utilization rate of catalytic sites, competitive side reactions, and unclear reaction mechanism pathways should still be addressed by developing efficient heterogeneous catalysts and revealing the inner relationship between the structure and activity.

Scope of this Review
The increasing literatures recognize the urgency of reducing CO2 emissions to achieve carbon neutrality. Most published reviews are focused on the specific materials and characterization techniques in catalyzing CO2 conversion or are limited to preparing formic acid from CO2 (33)(34)(35)(36). However, few reviews have systematically summarized the progress and challenges in producing carboxylic acids using CO2. A systematic understanding of how the catalyst structure contributes to the yield or selectivity of products is still lacking, and it is not clear which factors primarily contribute to the conversion efficiency and product selectivity.
Therefore, an opportune overview of construction strategies for advanced catalysts and an elaboration of the mechanical process in converting CO2 into carboxylic acids through the drivers of light, electricity, and heat are highly desirable and may provide important insight into advancing the development of practical strategies in mitigating carbon emissions and producing more high-value chemicals, such as carboxylic acids.
This review starts with a general introduction of the basic principles of catalyzing CO2 into various carboxylic acids through photocatalysis, electrocatalysis, and thermal catalysis.
Then, we highlight the adopted strategies, including composite catalysts, heteroatom doping, morphology control, and surface functionalization in rationally engineering heterogeneous catalysts to boost the conversion of CO2 into carboxylic acids. Furthermore, recent experimental and theoretical progress has revealed the reaction mechanism and presented rich examples to reveal the achievements in converting CO2 to carboxylic acids. Finally, the key challenges and prospects are presented for guiding the rationale design of heterogeneous catalysts and the improvement in the conversion efficiency of CO2 to carboxylic acids. We aim to illustrate the important relationship between the structure of the designed catalysts and reaction activity by discussing vital influencing parameters in the reaction process and providing fundamental principles and strategies for further developing superior heterogeneous catalysts.

Photocatalytic CO2 Conversion Process
Photocatalytic reduction of CO2 into highly valuable chemicals has been attracting considerable attention due to the economy of solar energy, which is expected to replace fossil fuels, and the utilization of CO2 with minimal carbon emissions ( Figure 2A). In this process, well-designed photocatalysts with specific bandgap sizes and positions of the conduction band (CB) and valence band (VB) occupy a core position in enhancing solar energy utilization due to their ability to harvest light and drive the following redox reactions (37,38). Typically, the photocatalytic CO2 reduction to carboxylic acids includes the following three steps: (1) lightharvesting, (2) generation and separation of a photogenerated charge carrier, and (3) surface redox reactions ( Figure 2B). The corresponding photocatalytic process involves harvesting the incident photons by generating photoexcited electrons (e -) and holes (h + ) over the traditional semiconductors. Light-harvesting determines the utilization rate of solar energy and facilitates the following CO2 photoreduction step. Considering that visible light and near-infrared (IR) 8 light account for more than 95% of natural light, many attempts have been made to enhance light-harvesting photocatalysts (39). For example, the visible-light absorption of graphitic carbon nitride can be increased due to the produced nitrogen vacancies and modified electron structure through polymerization of dicyandiamide using tartaric acid (40). Furthermore, another example demonstrates that the enhanced photocatalysis performance can be realized in Ti-doped SnS2 by forming an intermediate bond to extend the absorption of IR light (41).
In the second step, the formed electron and hole (e-h) pairs migrate to the surface of the designed catalysts. The photogenerated charge separation process is accompanied by recombination, and only a few photogenerated electrons participate in the following CO2involved reaction. Numerous strategies have been developed to suppress the recombination of charge carriers to overcome the inefficient utilization of photogenerated electrons. For instance, the photocatalytic activity of Bi2WO6 increased greatly by reducing the bulk Bi2WO6 into Bi2WO6 nanosheets (NSs) due to the minimized charge transport pathway, leading to the rapid migration of photoexcited electrons and holes to the surface of photocatalysts (42). In addition, combining the semiconductor photocatalyst with carbon-based materials to form composite catalysts efficiently boosts the carrier separation efficiency due to the improved conductivity with the aid of support. Composite catalysts, such as CdS@biomass porous carbons, can be used as electron reservoirs to boost the separation of charge carriers and the transportation of electrons owing to good conductivity, leading to excellent activity in CO2 photoreduction under visible irradiation (43).
After the charge migrates to the catalyst surface, it reacts with the CO2 adsorbed around the active site. Engineering active sites can stimulate the occurrence of surface redox reaction 9 and effectively regulate the activation energy barrier of the intermediates to produce carboxylic acids, leading to the desired products with high activity and selectivity (44,45). For instance, the selectivity of formic acid can be tuned by installing different metal active centers with variable electron structures into covalent organic frameworks (COF). The metal sites, as a poor π-donor, prefer to enhance the C-O bonding force of CO2, producing formic acid, whereas metal sites with an electron-rich coordination environment tend to weaken and break the C-O bond to form CO (46).
In the past few years, great progress has been made in boosting the conversion efficiency of CO2 into carboxylic acids through photocatalysis by optimizing the structure and composition of photocatalysts, such as adjusting the band structure and allowing the electron pumps to accelerate the surface charge separation and transfer (47). Numerous highly valuable carboxylic acids can be obtained through the photocatalytic conversion path with CO2 ( Figure 2C). Nevertheless, the process of producing carboxylic acids is often accompanied by producing a diverse range of products via proton-coupled multi-electron transfer processes with thermodynamic potentials between about -0.20 and -0.70 V. For instance, formic acid is produced through two electron transfer processes with a potential of -0.61 V, which faces competitive reactions, such as the hydrogen evolution reaction with a potential of -0.42 V, reduction of CO2 into CH4 with a potential of -0.24 V, and reduction of CO2 into CO with a potential of -0.52 V(48).
The unfavorable energy barrier for producing formic acid should be reduced by introducing efficient catalysts to enhance the reaction kinetics. Furthermore, some effective strategies, such as enhancing the hydrophobicity of the catalyst surface to inhibit the hydrogen evolution reaction and controlling the composition of the catalyst to suppress the occurrence of multiple electron transfer processes, should be considered to improve the selectivity of formic acid.

Electrocatalytic CO2 Conversion Process
Electrocatalytic CO2 reduction to valuable fuels represents an appealing way to make full use of CO2 as a C1 resource, driven by electricity generated through renewable energy sources, such as wind and hydropower ( Figure 2D) (49). As a result of the significant decline in the cost of electricity, electrocatalytic CO2 reduction exhibits great potential for future practical and large-scale industrial applications. The appealing advantages in this process include the highly selective preparation of chemicals, satisfactory electric-power utilization efficiency, and environmentally friendly reaction medium. Therefore, researchers are enthusiastic about exploring advanced devices, efficient catalysts, and green additives to realize high-efficiency electrocatalytic CO2.
The design of heterogeneous catalysts for electrocatalytic conversion of CO2 into formic acid with superior performance depends on the accurate understanding of the mechanical process ( Figure 2E) (50)(51)(52). It is generally accepted that, in the initial stage, CO2 is adsorbed on the catalyst surface to form CO2*, where * indicates a binding site at the catalyst surface.
The adsorption and desorption of CO2 on the catalyst surface can be precisely regulated by introducing defects and tuning the catalyst structures to improve formic acid production. Duan et al. reported that Bi/CeOx has excellent CO2 adsorption capacity due to the intrinsically enhanced ability to absorb CO2 with Bi and the inherent heterogeneity of amorphous CeOx (53).
The formation of CO2* with a reduced energy barrier in Bi/CeOx facilitated the subsequent 11 conversion into formic acid products.
In the following step, the formed CO2* obtains one electron to produce *CO2· − , which is the rate-determining step of the electrocatalytic conversion of CO2 into HCOOH/HCOO − .
The corresponding process faces the problem of a kinetically sluggish and excessive energy barrier. The formed *CO2· − is further protonated to generate *OCHO, which can finally be reduced to generate HCOO − by obtaining one electron. Different reaction pathways may occur due to multiple electron and proton transfer processes. Variable intermediate species, such as *COOH, *CO, *COH, and *CHO, could exist in the reaction system, leading to undesired products, such as CO, CH3OH, CH4, and multi-carbon products ( Figure 2F) (54). Therefore, promoting the formation of intermediates to obtain formic acid and avoiding the production of C-C bonds are necessary to eradicate the barrier from efficient conversion in practical applications. For instance, pure Sn and Bi usually have an unfavorable binding energy of the intermediate for producing formic acid, which can be solved to form Bi-Sn aerogel(55). The synthesized Bi-Sn can inhibit the formation of *COOH, which tends to transform into CO, and suppress the hydrogen evolution reaction with less H2 than pure Bi.
Thus far, great achievements have been made in electrocatalytic CO2 reduction into formic acid (56). The selectivity of formic acid production can reach more than 90% through variable heterogeneous catalysts, demonstrating considerable potential for commercialization.
Although great progress has been achieved, some challenges still exist and should be overcome to meet the criteria for practical applications. For example, the efficient preparation of formic acid is often achieved under alkaline conditions, which helps to form intermediates that tend to generate formic acid and accelerate electron transfers. In contrast, the produced formic acid often exists in the form of formate in an alkaline environment, leading to the need for further acidification treatment with more energy consumption. Therefore, exploring the efficient synthesis of formic acid under acidic conditions is meaningful, paving the way to an efficient one-step synthesis. Furthermore, the hydrogen evolution reaction should be suppressed due to water as the solvent competing with CO2 to participate in the reduction reaction.
Electrocatalytic CO2 reduction and hydrogen evolution reactions involve hydrogen transfer, adsorption, and desorption. By regulating the active sites of the catalyst to inhibit the occurrence of the hydrogen evolution reaction, the yield and selectivity of formic acid can be effectively improved. In addition, the reports on the formation of other carboxylic acids, such as acetic and aromatic carboxylic acid, are still rare due to the difficulty of controlling multiple electron transfers (that tend to have side reactions) and the inertness of C-H in organic reactants.
Fortunately, these challenges can be overcome by developing high-efficiency catalysts to accelerate the hydrogen transfer, reducing the activation energy required to form intermediates, *OCHO, and strengthening the ability to activate CO2 and promote the reaction with inert organic reactants.

Thermocatalytic CO2 Conversion Process
Recently, thermocatalytic CO2 conversion into carboxylic acids has been fully developed as an alternative strategy for using the CO2 molecule ( Figure 2G) (57,58). It is a relatively practical route to convert inert CO2 compared to the photocatalytic or electrocatalytic path. It has apparent advantages and feasibility in preparing carboxylic acids, such as benzoic, acetylenic, amino, and lactic acids, with higher commercial values and wider application prospects. However, the participation of CO2 in the reaction should usually be carried out under 13 high temperature and pressure due to the nature of its remarkable kinetic and thermodynamic stability (59). Thus, higher energy consumption is often required in thermocatalytic CO2 conversion into carboxylic acids, and the harsh reaction conditions could introduce great challenges in ensuring the purity of the products. As a result of recent extensive and in-depth research on the thermocatalytic conversion of CO2 to carboxylic acids, powerful strategies, such as improved atomic utilization and a regulated coordination environment, electronic structure, and space chemical environment of active sites, have been developed to drive the inert CO2 to participate in the reaction under relatively mild conditions. According to different reactants reacting with CO2, the reaction types of thermocatalytic CO2 into carboxylic acids primarily include the hydrogenation of CO2, Friedel-Crafts -acylation reaction, and carboxylation of terminal alkynes. Hydrogenation of CO2 to produce formic acid has been regarded as a mature and promising route to ensure the effective utilization of CO2 and economic viability (60). However, it is not thermodynamically favorable with the free energy change (ΔG 0 ) of 33.0 kJ mol −1 (CO2(g) + H2(g)/HCOOH(l)). The inherent thermodynamic obstacles could be overcome by reacting in an aqueous solution to make the conversion of CO2 into formate more likely (61). When the hydrogenation of CO2 occurs in the liquid phase, the reaction (CO2(aq) + H2(aq)/HCOOH(aq)) is more prone to occur with the decline in free energy (ΔG θ = −4 kJ mol −1 ). However, the conversion efficiency is still far from satisfactory if no catalyst is involved. The utilization of heterogeneous catalysts has been emphasized to accelerate CO2 conversion by enabling CO2 capture and facilitating the formation of specific intermediates, such as carbonate and bicarbonate, according to the possible mechanism process. In a typical CO2 hydrogenation process, it is widely accepted that H2 is first dissociated into a hydride species and proton through a heterolytic process ( Figure 2H) (62,63). The formed hydride species becomes an intermediate of metal-bound formate by undergoing a nucleophilic attack on the carbon atom of CO2 via metal-hydride bonds, which is usually regarded as the rate-determining step in the CO2 hydrogenation process. Then, formate is formed by reducing the produced intermediate using hydride, instantly recovering the surface of active sites.
Although CO2 hydrogenation can produce formic acid, some other challenging issues still exist and should be solved. For instance, organic amine bases, such as ethanolamine, are added to the reaction system to overcome unfavorable free energy, leading to increased costs and difficulty in separation and purification. Furthermore, the reaction of CO2 hydrogenation involves a gas-liquid-solid multi-phase, and this complex reaction system causes obstacles in exploring the mechanisms for the activation and transformation of H2 or CO2 on the surface of selected catalysts. Fortunately, abundant heterogeneous catalysts with different chemical environments and clear structures have been developed to obtain satisfactory performance and deeply investigate the inner origin of catalytic activity. In future studies, more research should explore efficient non-noble heterogeneous catalysts, understand the factors affecting H2 adsorption/activation on non-noble metal sites, and search reasonable methods to tune the microenvironments of the active sites to activate H2 and CO2.
The preparation of multi-carbon carboxylic acid products via carboxylation of terminal alkynes and Friedel-Crafts acylation reactions exhibit outstanding advantages that include a satisfactory yield and green reagents over other synthetic routes, such as the acid-mediated hydrolysis of nitriles (64), carbonylation of organic halides with toxic and odorless CO(65), and 15 oxidation of primary alcohols ( Figure 2I) (66). In the carboxylation of terminal alkynes with CO2, unsaturated terminal alkynes are deprotonated using an inorganic base and are coordinated onto the surface of metal sites. Then, the captured electrophilic CO2 is further inserted into the C≡C-metal bond to provide the intermediate, regarded as the rate-determining step for the entire process. Finally, the product, acetylenic acid anion, is released from the catalyst surface. However, heterogeneous catalysts containing Ag, Au, or Cu have been reported to catalyze the conversion of terminal alkynes into carboxylic acids with relatively good activity (67). Some difficulties include the need for an additional strong base and the inertness of weakly nucleophilic terminal alkynes that have difficulty attacking CO2. These problems lead to the need for acidification to obtain carboxylic acids, posing a serious challenge to the catalyst stability and poor conversion efficiency. Based on years of exploration, some practical strategies, such as enhancing the ability to capture CO2, controlling the electronic state of metal sites, and optimizing the utilization of active metal centers, have been adopted to overcome existing challenges. It is expected to develop novel heterogeneous catalysts that can activate alkynes and CO2 under acidic conditions and reduce the activation energy of forming acetylenic acid, achieving enhanced efficiency in carboxylic acid production.
The Friedel-Crafts acylation reaction to prepare aromatic carboxylic acids dates to the Al2Cl6/Al catalytic system created by Olah et al (68). The accepted pathway involves an initial complexation between CO2 and metal sites to form a CO2-metal complex (69). The produced electrophilic complex reacts with arene, turning into carboxylate coordinated on the catalyst surface and is identified as the rate-determination step. Subsequently, the final product, aromatic carboxylic acids, can be obtained via deprotonation and proton transfer. However, 16 various homogeneous systems, such as TiCl4, FeCl3, Ga(OTf)3, and CF3SO3H, and heterogeneous systems, including metal nanoparticles (NPs), metal-organic frameworks (MOFs), and others, have been explored to boost the reaction occurrence. They still have limited ability to activate C-H bonds in the benzene ring and inert CO2 and exhibit poor regioselectivity or stereoselectivity for aromatic substrates with the ortho or meta position to participate in the carboxylation reaction. Benefiting from the increased improvement in material characterization methods and increased maturity of preparation methods, the design of heterogeneous catalysts with a controllable spatial environment or surface confinement effect could effectively determine product selectivity by affecting the kinetic behavior, activation energy, and intermediate types.

Engineering Active Sites to Accelerate CO2 Conversion
Highly active heterogeneous catalysts play a decisive role in converting CO2 to carboxylic acids. Practical strategies can improve the activity, such as maximizing exposed active sites by constructing composite catalysts or tuning the catalyst morphology and increasing the intrinsic activity of the active site via doping heteroatoms and surface functionalization to enhance the activity of traditional catalysts (70). These strategies can sometimes work together in the same catalytic system to realize great improvements in activity and selectivity. In the following section, these strategies have been well explained, whereas the corresponding relationships between the structure and catalytic property for each strategy are briefly discussed.

Composite Structure
The composite catalysts primarily refer to the combination of different materials to form supported heterogeneous catalysts, enriching the structure types and delicately enhancing catalytic properties compared with single-component catalysts ( Figure 3A) (71)(72)(73)(74)(75).
In the early stages of the development of composite catalysts for accelerating CO2 conversion, traditional carriers, such as metal oxide, are often used to support noble metals or immobilize homogeneous metal complexes (84,85). The catalytic activity of this type of composite catalyst can be further improved by introducing defects, increasing the loading of active species, and other methods. As a result of the weak interaction between the active constituent and carrier, the stability of synthesized composite catalysts should be guaranteed by introducing composites with strong interactions. Recently, based on an elaborate design, a multiple-composite system composed of Cu2O-Pt/SiC/IrOx proved to be an efficient artificial photosynthetic catalyst for converting CO2 into formic acid due to the greatly prolonged lifetime of photogenerated electrons and holes ( Figure 3B-D) (86). This study realizes the separate operation of CO2 reduction and H2O oxidation, facilitating the suppression of the backward reaction of products and promoting whole conversion efficiency.
Another promising composite catalyst to enhance the intrinsic activity of active centers is selecting carbon-based materials as functional carriers. Among these carbon-based materials, graphene, carbon nanotubes, and carbon foam are regarded as appealing carriers due to their good electrical conductivity and nanostructure, conducive to enhancing the material transfer rate (87,88). The inherent activity of the active center is improved by ameliorating the poor electron conductivity and inefficient electron transfer, and the turnover number and turnover frequency of the composite catalysts increase greatly by fully exposing and dispersing the active center. Lou et al. reported that an emerging composite heterogeneous catalyst composed of Bi2O3 NSs and a conductive multi-channel carbon matrix could catalyze the CO2 reduction into formic acid with high activity ( Figure 3E-G) (89). This superior performance can be attributed to the synergistic effects of the composite matrix, facilitating electron transfer and enhancing CO2 by producing pyrrolic-N and pyridinic-N. Furthermore, another carbon-based material, carbon nitride, has a good ability to absorb light, and the abundant nitrogen sites are beneficial to capture CO2 and improve the turnover frequency of the active centers (90).
Compared with carbon-based composite catalysts, porous material-based composite catalysts have unparalleled advantages due to their ultrahigh specific surface area, adjustable pore environment, and abundant acid or basic sites (91). The porous structure can enhance the catalytic efficiency of the active site by capturing CO2 around the active site and increasing the substance transfer rate. Furthermore, the introduced basic sites and Lewis acid sites in the support are beneficial to activate the CO2 in a nonlinear configuration, which promotes the declination of the activation energy required for the rate-determining step of the reaction. In addition, some conjugated porous materials, such as MOFs, with the ability to capture and activate CO2 molecules are used to construct sandwich-shaped composite catalysts to produce alkynoic acid with CO2 ( Figure 3H-J) (92). Another type of porous material, COFs, can combine active centers to realize the efficient photocatalytic reduction of CO2 to formic acid upon visible-light irradiation due to the excellent visible-light harvesting ability and extended life of photogenerated charge carriers via accelerating the electron transfer(93).

Heteroatom Doping
Heteroatom doping is another effective strategy to enhance the activity of heterogeneous catalysts in converting CO2 into carboxylic acids, which can be widely used to form metaldoped oxides, metal alloys, intermetallic compounds, multiple metals with porous materials, and multi-metal complexes ( Figure 4A) (94)(95)(96)(97)(98)(99)(100). In addition, some nonmetal-doped heterogeneous catalysts, including doping nonmetal elements, such as N, P, S, and B, into carbon-based materials or metal-containing materials are designed to boost the catalytic conversion of CO2 (101)(102)(103). Benefiting from the introduction of doped elements, the electronic structure, coordination environment, and geometric configuration of pristine active sites change significantly, leading to enhanced catalytic activity compared with the original state.
Based on the low costs of preparation, nonmetal-doped heterogeneous catalysts have aroused enthusiasm in promoting the preparation of carboxylic acids from CO2. Doping heteroatoms can endow pristine catalysts with improved electrical conductivity and abundant catalytic active centers(104). For instance, nonmetal-doped heterogeneous catalysts, such as boron-doped graphene, can catalyze CO2 into formic acid with higher Faradaic efficiency than that of pure graphene (105). The enhanced catalytic performance can be attributed to the stronger adsorption of activated CO2 resulting from the high spin density of the introduced boron element. As nonmetal atoms have a weaker activation effect on CO2 than metal atoms, the increase in the inherent activity of the material itself is limited.
Given the limited activity of nonmetal catalytic systems, more attempts have been made 20 to dope heteroatoms on heterogeneous catalysts containing metal atoms (106)(107)(108)(109)(110). Doping sulfur into indium catalysts has been reported to catalyze the electrocatalytic CO2 reduction to formic acid with excellent catalytic performance due to the strong ability of sulfur to activate water to form hydrogen species, which can further react with CO2 to produce formate ( Figure   4B-D) (111). Theoretical calculations reveal that doping sulfur into indium significantly decreases the Gibbs free energies (ΔG) for forming HCOO* and HCOOH* in the pathway of The multi-metal-doped catalytic system is usually regarded as a more effective strategy to tune the inherent activity of the active center than the mentioned strategies due to the significant change in the electronic state and coordination environment, which can have a decisive influence on activating the CO2 and stabilizing intermediates(55, [112][113][114][115][116][117][118].

Morphology Control
Tuning the morphology of the heterogeneous catalysts effectively improves the insufficient utilization of catalytic sites of inherent catalysts (122)(123)(124). In general, the categories of morphology for the designed heterogeneous catalysts can be divided into hollow structures, one-dimensional nanowires, two-dimensional (2D) NSs, quantum dots, and hierarchical porous structures ( Figure 5A) (125)(126)(127)(128). The changes in catalyst morphology have positive effects on the specific surface area, exposed active site, dispersibility in solution, and transfer rate of reactants and can thus endow additional defects, grain boundaries, unsaturated sites, and improved conductivity, which are beneficial for enhancing activity compared with the unprocessed morphology(129-132).

22
The solid bulk nature of conventional heterogeneous catalysts leads to only a small amount of metal sites on the surface participating in the reaction with a low turnover and poor mass transfer rate. In contrast, the catalytic activity of these heterogeneous catalysts can be enhanced via elaborate morphology control. For instance, hollow heterogeneous catalysts, such as CuInS2, have been approved as excellent electrocatalysts to produce formic acid by facilitating the electron transfer and interaction between the reactant and active site and enriching the CO2 local concentration in the void of heterogeneous catalysts (133). Furthermore, a hollow photocatalyst can exhibit higher activity in photocatalytic CO2 reduction than that of its bulk counterpart due to the promoted light absorption ability, abundant active sites, and mass transfer channels. In addition, SnO2 quantum wires with grain boundaries have proved to be efficient catalysts to drive the conversion of CO2 into formic acid and exhibit better catalytic activity than SnO2 NPs ( Figure 5B-D) (134). The enhanced activity could be attributed to more

Surface Functionalization
Surface functionalization can effectively combine the merits of organic and inorganic components and greatly improve the catalytic activity of the inherent catalytic centers for converting CO2 into carboxylic acids ( Figure 6A) (137)(138)(139). The common organic units, including amide, Schiff base, amine, hydroxyl, pyridine, azazole, and light-adsorbing organic ligands, can be flexibly modified using chemical bonds on the surface of typical heterogeneous catalysts to meet the need to improve catalytic activity (140)(141)(142). Surface functionalization is suitable for a wide range of traditional heterogeneous catalysts, such as metal oxide, metal clusters, metal NPs, porous materials, and others (143)(144)(145). Moreover, some metal-containing organic molecules, such as porphyrin, phthalocyanine, and the Schiff base complex, can be grafted onto the surface of the original catalysts to boost catalytic performance (146,147).
Functionalization of organic groups on the surface of the active constituent allows many positive facilitators, such as an improved charge transfer rate, strong light-responsive ability, enhanced capture of CO2, and stabilization of intermediates (148). For instance, the semiconductor functionalized with an organic linker can obtain higher selectivity in photocatalytic CO2 reduction to formic acid than an untreated one (149). The introduced organic constituents could smoothly allow efficient hole and electron migration on the catalyst surface, 24 leading to the enhanced conversion efficiency of CO2. Furthermore, some organic groups with enhanced CO2 capture capacity, such as amide linkages and tris-N-heterocyclic carbene, can be introduced in heterogeneous catalysts to realize the selective electrocatalytic CO2 reduction to formic acid by enhancing the concentration of CO2 around the active site and suppressing the formation of intermediates to produce CO ( Figure 6B-D) (150). The theoretical calculations have found that the tris-N-heterocyclic carbene group on the surface of Pd favored the formation of COOH* and the hydrogenation of COOH* into HCOOH with a lower energy barrier than that of the pure Pd catalyst.
Recent studies have noted the importance of surface functionalization in enhancing the stability of the original heterogeneous catalyst and auxiliarily activating the reaction substrate.
For instance, a bipyridine-based covalent triazine framework with an Ru active center has been reported in catalyzing CO2 hydrogenation with high activity. However, the bond between the metal and bipyridine is not strong enough, and the active center could dissociate into the reaction solvent, leading to poor stability of the designed catalyst ( Figure 6E-G) (151,152). An oxyanionic ligand is a well-established approach in stabilizing the active species by avoiding the dissociation of metal sites from bipyridine. Moreover, the introduced oxyanionic ligand could assist in the heterolysis of H2, which is the rate-determining step for this conversion, leading to highly efficient conversion.
Organic functional groups can form specific intermediates to promote the reaction rate due to the reduced activation energy of the rate-determining step. For instance, different organic amines can be flexibly grafted onto the surface of noble metals, such as Au NPs, to catalyze CO2 hydrogenation, which can stabilize noble metals to avoid aggregation and enhance CO2 concentration (153). More importantly, these grafted organic amines can help activate CO2 through a nonbicarbonate route with negative free energies for adsorption of CO2 and can benefit from forming the zwitterion intermediate, which is regarded as beneficial for producing formic acid ( Figure 6H-J). The density functional theory (DFT) calculation has revealed that the added Schiff base plays a vital role in the activated CO2, which can be further hydrogenated by the activated H to form an HCO2 intermediate and become HCOOH with the assistance of another H atom.

Advanced Characterization Techniques and Theoretical Calculations
Although great progress has been achieved in the catalytic conversion of CO2 into carboxylic acids, previous studies have suffered from a lack of clarity in revealing the dynamic process of the CO2 transformation. Considering that the catalytic reaction and product selectivity are dominated by the evolution of the catalyst structure in the reaction process and the interaction between the active sites and reactants or intermediates, it is crucial to distinguish the true active centers and their dynamic evolution in the reaction process (154). However, identifying real active sites and monitoring the evolution of the catalysts or involved reactants has been vigorously challenged by dynamic changes in the catalyst structure during the reaction (155). In addition, difficulties exist in distinguishing complex intermediates due to multi-step reaction paths in converting CO2 into carboxylic acids (156). Thus, these obstacles preclude the recognition of real active sites and reveal the reaction mechanisms of CO2 conversion. As a result of developing numerous emerging in situ characterization instruments, we can approach observing the true state of the catalysts in the reaction process via operando techniques at the molecular level.
Parallel to the characterization technique development, the theoretical calculation should improve the fundamental understanding of the mechanical process in converting CO2 to carboxylic acids. The ongoing development of the DFT has led to significant progress in preparing the catalyst from a blind attempt to accurately predict using computer-aided prediction. Importantly, the reactivity origin of the designed heterogeneous catalysts could be uncovered by analyzing the effect of changes in the electronic state in metal sites, exposed crystal faces, and coordination environment based on DFT calculations. The following section introduces advanced in situ techniques and significant progress in theoretical calculations in converting CO2 into carboxylic acids.

In situ Characterization Techniques
Moreover, X-ray absorption spectroscopy (XAS) has become an essential characterization technique to detect the structural evolution of catalysts during CO2-involved reactions ( Figure 7A) (157). The specific elements, catalyst coordination environment, and chemical valence changes that affect the conversion efficiency of the CO2 and product selectivity can be determined using XAS ( Figure 7B). In addition, XAS can be divided into the extended x-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra based on different origins. The XANES spectrum provides fingerprint information on the geometric structure, oxidation state, and electronic structure of the detected metal sites. Furthermore, the EXAFS spectrum identifies the atomistic structure/configuration and coordination geometry of reactive centers combined with a fitting analysis. Recently, Li et al.
have analyzed the EXAFS results by building reasonable models, revealing abundant defects in reduced Bi2O3 in the CO2 reduction process through a model-based quantitative analysis (158). In addition, the authors have testified that the changes in chemical valences in bismuth could affect the electrocatalytic reduction of CO2 into formate with in situ XAS ( Figure   7C). to the symmetric stretching adsorption of CO3 2is more obvious over the S-doped Cu-based catalyst than the undoped one, manifesting that S doping can enhance the adsorption ability of carbonate intermediates ( Figure 7I)(160).

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Compared with the above-mentioned techniques, in situ IR spectroscopy has advantages in detecting the intermediates formed on active sites during the reaction, benefiting from its high sensitivity to the vibration mode of the adsorbed molecules ( Figure 7J). The IR absorption peak position is the characteristic signal of the adsorbed molecule, reflecting the structural composition and chemical groups in the reaction process ( Figure 7K). For example, Gong et al. have investigated the active species in CO2 reduction over SnOX as a catalyst using in situ attenuated total reflection surface-enhanced IR absorption spectroscopy (161). The signals of the HCOO* can be identified by analyzing the characteristic peaks when the potential is applied to the surface of Sn-OH branches. In addition, the intensity of HCOO* can be quantitatively analyzed to evaluate the content of produced intermediates, assessing the influence of different hydroxyl content on catalytic activity ( Figure 7L).

Theoretical Calculations
Extraordinary progress in theoretical calculation has created enormous opportunities in evaluating the energy required for the adsorption, activation, and conversion of reactants in the catalytic reaction process, allowing the design of highly efficient heterogeneous catalysts (162)(163)(164)(165).  (Figure 8D-F). The latter step is regarded as the rate-determining step for the overall reaction. By comparing the energy of the rate-determining step of different spin-state COF-367-Co catalysts, the spin-state transition of Co can be disclosed as the decisive factor in forming formic acid with CO2. Furthermore, product selectivity in converting CO2 can be explained by evaluating the energy superiority of various products for the designed catalysts. For example, the process of electrocatalytic conversion of CO2 into formic acid is often accompanied by producing CO. By comparing the Gibbs free energies of electrochemical CO2 reduction into the corresponding products, the generation of CO via a CO* intermediate or production of HCOOH(g) via the COOH* intermediate can be predicted over synthesized catalysts (169). In addition to explaining the fundamental reasons for product selectivity, the occurrence of competing reactions on the surface of catalysts can be inhibited by optimizing the catalyst structure.
With the development of machine learning and theoretical calculation methods, more efficient heterogeneous catalysts are predicted and synthesized to boost the conversion of CO2involved reactions (170). For instance, Guo et al. have successfully used machine learning to electro-catalyze CO2 into formic acid with high yield by discovering and optimizing additives in Cu-based catalysts ( Figure 8G) (171). In the initial step, they select combinations from the collected additive library, including different metal salts and water-soluble organic molecules, to tune the morphology and surface structure of Cu catalysts ( Figure 8H). Then, through the screening of machine learning, the experiment is guided using Sn containing metal salts and organic ligands containing aliphatic amino and carboxyl groups to obtain high selectivity of formic acid (FEHCOOH = 65%) ( Figure 8I). In the future, machine learning is expected to play a more important role in achieving efficient conversion and preparation of a variety of high-value carboxylic acids.

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The synthesis of multi-carbon carboxylic acids with a high conversion rate is challenging due to the transfer of more electrons involved in the photoelectric conversion process. Some breakthroughs have been made in preparing multi-carbon carboxylic acids, such as acetic acid, through photocatalytic CO2 reduction (172). Wang Figure 9F).
In addition, a great breakthrough has been made in promoting CO2 to participate in the photocatalytic organic reaction to produce carboxylic acids ( Figure 9G). Some organic carboxylic acids with important application values, such as trans-cinnamic acid and diaryl αamino acids, can be obtained under blue-light excitation or sunlight. For instance, Schmalzbauer et al. have reported that a redox-neutral C-H carboxylation of arenes and styrenes can be realized by reacting with CO2 to afford carboxylic acids using an anthrolate anion photocatalyst under the blue-light excitation ( Figure 9H) (175). Then, they have proposed a possible reaction mechanism for the photocatalytic C-H carboxylation of the heterocycle. In the initial step, the catalyst, 2,3,6,7-tetramethoxyanthracen-9(10H)-one (TMAH), is deprotonated to form an anionic species with the aid of a base. Next, the excited anion TMA* is formed under visible light (455 nm) and is quenched by the organic substrate (arene) to produce the radical TMA and radical arene anion. Afterward, CO2 attacks the radical arene anion to generate a radical carboxylate intermediate that is deprotonated immediately to produce a carboxylic anion in the presence of the base ( Figure 9I). The final products are 33 obtained by further treatment with hydrochloric acid. This work presents new opportunities for an atom-economic and energy-efficient utilization of CO2 in producing aromatic carboxylic acids and drug intermediates.

Electroreduction of CO2 into Carboxylic Acid
Selective production of formic acid from CO2 electroreduction over abundant catalysts has achieved ideal activity of over 90% Faradaic efficiency. However, the neglected challenge concerning mixed products in an alkaline aqueous system should be overcome to satisfy the practical applications of electrocatalytic preparation of renewable formic acid. The generated formic acid in the conventional H-cell or flow-cell reactors is formate ions mixed with other ion impurities, such as K + and HCO3 − . This problem can be overcome by improving the reaction device. For instance, Xia et al. have developed an efficient strategy to resolve this challenge by decoupling the ionic conduction and allowing product collection (176).
Specifically, a porous solid electrolyte (PSE) layer is used to replace the traditional liquid electrolytes, promoting fast ionic transportation and recombining the generated formate and proton to form pure formic acid ( Figure 10A). The selected insoluble solid electrolyte provides a proper platform to collect the formic acid efficiently by flowing the deionized water through the PSE layer. Furthermore, they have synthesized an ultrathin 2D Bi catalyst to catalyze CO2 into formic acid using the new H + -conducting solid electrolytes. A high FEHCOOH of 93.1% is achieved, and negligible amounts of other ion impurities have been detected using inductively coupled plasma atomic emission spectroscopy ( Figure 10B).
Although it has been demonstrated that pure formic acid can be obtained with solid electrolytes, the formic acid concentration is limited due to the need for a significant amount 34 of water. Recently, Fan et al. have realized continuous generation of high-purity and highconcentration formic acid using an all-solid-state electrochemical device in which the generated formic acid can be efficiently collected via an N2 stream flowing through the PSE layer ( Figure 10C) (177). High activity (maximal Faradaic efficiency ~97%) and ultrahigh concentrations of pure formic acid solutions (up to nearly 100%) can be obtained over the Bi catalyst with an abundant grain boundary ( Figure 10D). This emerging system is expected to be suitable for modular and high-pressure systems, which have exciting potential in future large-scale production of formic acid. Future research should develop highly stable and active catalysts and solid electrolytes with excellent performance and efficient ion-exchange membranes to realize low energy consumption to convert CO2 into formic acid.
In addition to the preparation of high-concentration pure formic acid, there are numerous attempts for further progress in producing multi-carbon carboxylic acids via the electrocatalysis of CO2 (178,179). For instance, Genovese et al. have reported that the selectivity to acetic acid (61%) could be reached in the three-electrode cell over nanostructured ferrihydrite-like (Fh-FeOOH)/nitrogen-doped carbon ( Figure 10E) (180). This high activity for acetic acid production can be attributed to the formed nitrogen-coordinated iron (II) sites at the interface between iron oxyhydroxide and the support, demonstrated by operando x-ray spectroscopy techniques ( Figure 10F). Compared to producing formic acid from CO2, the selectivity is far from satisfactory due to possible competitive reactions. More effort should be made to design effective catalysts to enable C-C coupling.

Thermocatalytic Conversion of CO2 into Carboxylic Acid
In the thermocatalytic conversion of CO2 into carboxylic acids, the carboxylic acid yield should be improved, and the product selectivity, including regioselectivity, stereoselectivity, and chemoselectivity, should also be controlled by the desired catalysts. Great achievements have been made in recent years, profiting from long-term efforts and exploration (181). For instance, Martin et al. have reported that a host of unactivated alkyl chlorides can react with CO2 to produce carboxylic acids with a high conversion rate over catalysts containing Ni ( Figure 11A) (182). Moreover, an anti-carbometalation product has obtained with high selectivity based on this designed catalytic system. The high selectivity of the anti-product in converting secondary alkyl bromides may be attributed to forming rapid isomerization of vinyl radical species, which would undergo recombination to produce a different configuration of the Ni(I)BrLn species ( Figure 11B). This work realizes exquisite chemoselectivity and a high conversion rate for carboxylic acid production with unactivated alkyl chlorides and CO2.
Ling et al. have exploited the interface of Ag@MOF to provide a pseudo microenvironment with high CO2 pressure to enhance the direct C-H carboxylation at ambient conditions ( Figure 11C) (183). Specifically, the interface of Ag@MOF comprised an array of Ag nanocubes has been grafted with mercaptophenol as reaction substrates. XPS has been used to characterize the interaction between the substrate and MOFs, confirming the existence of Zn-O bonds between phenoxide and MOF, which cannot be found in methylbenzenethiol functionalized Ag nanocubes without MOF ( Figure 11D). As a result of the activation and confinement effects of MOF on reaction substrates, the direct C-H carboxylation of arene occurs at the NP@MOF interface under ambient pressure at 25°C. More importantly, an unprecedented meta-carboxylated arene is generated rather than a traditional orthocarboxylated product ( Figure 11E). This work introduces new opportunities to synthesize high-value products that cannot be obtained under common conditions and provides new inspiration for improving the selectivity of carboxylic acids.
Recently, Xiong et al. have reported that the hydrocarboxylation of alkynes with CO2 can be catalyzed to form a wide array of α-acrylic acids with high regioselectivity through the combination of Pd(PPh3)4 and 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (binap) under mild conditions (184). For example, for phenylacetylene, the product of α-acrylic acids can be obtained with a 70% isolated yield in high selectivity by adding 0.5 mol% Pd(PPh3)4 and 0.5 mol% binap ( Figure 11F). DFT studies have further revealed that alkynes react with CO2 via the cyclopalladation process, generating a five-membered palladalactone intermediate, and undergo a σ bond metathesis step regarded as the rate-determining step of the entire process.
The calculated results have demonstrated that the Markovnikov adducts, α-acrylic acids, are the main products with a more favorable free-energy barrier than the anti-Markovnikov product ( Figure 11G). This work reveals that the designed catalyst is a powerful tool to use CO2 to produce complex molecules with high selectivity.

Conclusions and Prospects
In the past few decades, the exhaustion of fossil fuels and the deterioration of the environment have necessitated carbon neutrality, and prior studies have noted the importance of consuming the emission of CO2 via a stainable and green approach. Accordingly, the direct conversion of CO2 into chemical compounds accomplishes both goals. Climate warming can be slowed, and fine chemicals can be produced in an economically and environmentally friendly way. Considering the wide application prospects and social needs concerning carboxylic acids, the direct conversion of CO2 into carboxylic acids has attracted considerable attention in fundamental research and industrial applications. Given the importance and social needs regarding this field, a systematic and in-depth review of the selective conversion of CO2 into carboxylic acids is urgently needed.
The process of CO2 conversion requires a controllable catalytic system to accelerate the conversion of reactants; thus, recent developments and feasible tuning strategies on designing heterogeneous catalysts are systematically summarized in this review. The past few years have witnessed numerous breakthroughs involving the conversion of CO2 into carboxylic acids, benefiting from the advancements in characterization techniques and the understanding of the mechanical process. However, many issues should still be addressed to achieve large-scale production of carboxylic acid compounds with a high conversion rate and satisfactory selectivity. Second, although remarkable progress has been made in photo-catalyzing CO2 into carboxylic acids, several challenging issues should be considered and solved. The use of near IR light and visible light is far from satisfactory, and low quantum efficiency hinders the widespread practical application of photocatalysis. Increasing the efficiency of photocatalytic CO2 conversion and making the most of natural light by constructing stable heterogeneous catalysts with sufficient activity and a strong ability to harvest broad-spectrum light are highly desired. In addition, the introduction of sacrificial agents or cocatalysts to guarantee conversion efficiency may create additional problems, such as high production costs and poor catalyst stability. The appealing solution is to develop heterogeneous catalysts with multiple functions to improve light utilization and produce synergistic effects between the active sites and cocatalysts and thus to get rid of the usage of sacrificial agents.
Third, electrocatalytic conversion of CO2 into high-value-added products, such as formic acid, is expected to reach the goal of actual production based on the continued development of efficient catalysts and facile reactors. However, the generation of multi-carbon carboxylic acids with high selectivity still has a long way to go due to the competition of multiple side reaction paths. Regulating the active catalytic sites via the electronic state, coordination environment, and chemical components is feasible to inhibit undesired products. Furthermore, the research on disclosing the reconstruction of the catalyst in the electrolyte process is still in its infancy, leading to a poor understanding of the relationship between structure and activity. More attention should be focused on revealing the changes in composition and structure that have occurred in the reaction process, reducing the over-potential required for CO2 reduction and inhibiting the occurrence of competitive reactions under the premise of ensuring the efficiency of carboxylic acid products.
Fourth, the conversion efficiency of the thermo-catalytic conversion of CO2 can be improved under harsh reaction conditions, such as high temperature and high pressure, but introduces substantial challenges to the selectivity of products due to the byproducts caused by the high energy. Especially for carboxylic acid compounds involving organic substrates, the regioselectivity, stereoselectivity, and chemoselectivity of products determine the purity and produce specific chemicals with different values and applications. Highly selective carboxylic acids can be synthesized under relatively mild conditions only by accurately constructing a steric hindrance environment, using the pore restriction effect and regulating the strength of the Lewis acidity and alkalinity.
Fifth, relatively high energy consumption with high temperature and pressure is usually required to obtain satisfactory yields of carboxylic acids, especially for preparing multi-carbon carboxylic acids. The accompanying process poses great challenges to the reaction devices and the stability of catalysts. More efficient strategies are urgently needed to promote the conversion of CO2 under green, environmentally friendly, and mild conditions. It has been demonstrated that seeking strategies to realize photoelectric conversion or electrothermal conversion could improve the efficiency of the input energy and create new opportunities to reduce carbon emissions and significantly decrease production costs.
Sixth, theoretical algorithms play a vital role in the deeper understanding of the reaction mechanisms and correlations between catalytic performance and structure. However, the 40 inherent limitations of theoretical models and the narrow applicability of theoretical methods greatly compromise the guiding role of theoretical calculation. Machine learning and artificial intelligence are highly desirable for the design of experiments, optimization of catalysts, understanding of the mechanism. By analyzing the available fundamental data and basic theory with machine learning, the discovery of efficient catalysts can be accelerated with an accurate guide rather than tedious trial-and-error investigations.