Next Article in Journal
Synthesis of Iron Oxide Nanoparticles via Atmospheric Pressure Microplasma for High-Performance Energy Storage and Environmental Applications
Previous Article in Journal
Solvent-Free Selective Catalytic Oxidation of Benzyl Alcohol over Pd/g-C3N4: Exploring the Structural Impact of g-C3N4
Previous Article in Special Issue
Carbon-Coated Magnetic Catalysts for Enhanced Degradation of Nitrophenols: Stability and Efficiency in Catalytic Wet Peroxide Oxidation
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 5
10.3390/catal15050443
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Active Sites in Carbocatalysis: Tuning Their Activity

by
Raquel Pinto Rocha
1,2,* and
José Luís Figueiredo
1,2
1
LSRE-LCM—Laboratory of Separation and Reaction Engineering-Laboratory of Catalysis and Materials, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
2
ALiCE—Associate Laboratory in Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(5), 443; https://doi.org/10.3390/catal15050443
Submission received: 28 March 2025 / Revised: 23 April 2025 / Accepted: 27 April 2025 / Published: 1 May 2025
(This article belongs to the Special Issue Carbon-Based Catalysts to Address Environmental Challenges)
Browse Figures
Versions Notes

Abstract

:
Carbocatalysis, i.e., catalysis by metal-free carbon-based materials, has recently emerged as a “green” alternative for several thermal-, electric-, and solar-driven chemical processes in the gas phase and in the liquid phase. Indeed, and in addition to their more common role as a catalyst support, carbon materials can promote a large variety of reactions, replacing metal or metal oxide catalysts. However, the active sites are seldom identified and properly quantified, making it impossible to calculate turnover frequencies to benchmark the novel metal-free catalysts with those traditionally used. In order to advance the field, it is essential to correlate the catalytic properties of the carbon materials with their surface chemistry. In this work, we review a small selection of reactions catalyzed by metal-free carbon materials, emphasizing synthesis methodologies and characterization techniques used to identify the active sites, where relevant structure-activity relationships could be established.

1. Introduction

Carbon materials offer unique properties that can be exploited in the field of catalysis, such as their inertness (stability in acidic and basic media) and easily tuned texture (surface area, porosity, pore size distribution) and surface chemistry. They are often employed as carriers for metallic phases, in particular for precious metal catalysts in the pharmaceutical and fine chemicals industries. On the other hand, the use of carbon catalysts without supported phases is very unusual in the chemical industry; one rare example is the production of phosgene with activated carbon catalysts [1,2].
However, the potential of carbon as a catalyst on its own had already been recognized long ago [3]. Indeed, carbon materials can replace metals or metal oxides as catalysts in many reactions [4,5]. The versatility of carbon as a catalyst derives from the graphitic structure, which allows functionalization via the unsaturated carbon atoms at the edges of the graphene layers or at basal plane defects. The grafted functional groups can act as active sites for catalysis. In addition, heteroatom doping offers control of the electronic properties by introducing electron acceptors or donors. Thus, it is possible to design catalysts for specific applications by adequately tuning the surface chemistry of the carbon material.
The development of metal-free carbon catalysts is an area of intensive research nowadays [6,7,8], with a dedicated congress series initiated in 2004 (International Symposium on Carbon for Catalysis, “CarboCat”). In this feature article, we will briefly review the field by describing a few case studies, addressing oxidation as well as acid catalysis.

2. Overview of Carbon-Catalyzed Reactions

Table 1 presents a non-exhaustive compilation of reactions, both in the gas and liquid phases, that have been reported to be promoted by carbon catalysts [2,9,10]. The right column lists the nature of the active sites, or the type of surface chemistry, as identified by the authors in each reference. In some cases, the active sites have been properly quantified, allowing for the establishment of correlations of catalytic activity and determining the turnover frequency (TOF), which is an essential requirement for the correct benchmarking of the novel carbon catalysts. The highlighted entries in Table 1 correspond to the case studies that will be described in subsequent sections.
Inspection of Table 1 shows that the active sites for the reactions listed involve oxygen, nitrogen, and sulfur groups. Surface oxides are the most common groups on the surface of carbon materials, and range from very acidic (carboxylic groups) to basic (such as pyrone-type structures), as shown in Figure 1. Moreover, the π electrons on the basal planes confer basicity to the carbon surface [30,31,32,33]. The most important nitrogen structures are pyrrolic (N-5), pyridinic (N-6), quaternary (N-Q), and oxidized N (N-X) [34]. Among the sulfur groups, sulfonic acids (-SO3H) are relevant in the context of acid catalysis [35].
The quantitative determination of the active sites may be a major challenge. Temperature-programmed desorption (TPD) is the technique of choice for surface oxygen functional groups, which decompose by releasing CO and CO2 (and also H2O) in different temperature ranges, as indicated in Figure 1. Deconvolution of the evolved CO and CO2 profiles provides reliable estimates for the amounts of individual oxygen groups [36]. This technique has been recently reviewed, highlighting pitfalls and establishing best practices for the correct assessment of the surface groups [37]. Some S-containing groups decompose by releasing SO and SO2 species, and can also be determined by TPD [35,38,39].
TPD is not an option for the assessment of nitrogen groups, because there is no significant release of gaseous nitrogen species upon heating. N-6 starts to be gradually converted into N-Q above 450 °C. N-5 is stable up to about 600 °C, and then it is converted into N-6 and N-Q. Thus, most of the nitrogen will remain in the graphene lattice as N-6 and N-Q [40].
The alternative analytical method is X-ray photoelectron spectroscopy (XPS). The nitrogen functionalities can be determined by deconvolution of the N1s XPS spectrum [41,42,43]. XPS can also be used for the analysis of S- and O-surface groups, as indicated in Figure 1 [39]. It should be noted that misleading results may be obtained by XPS in the case of porous carbons, since the concentration of the functional groups will usually be higher at the external surface layers, which are those probed by the photoelectrons; therefore, the surface concentrations determined by XPS may not be representative of the material as a whole, especially in the case of microporous materials such as activated carbons [9]. The total amounts of the elements can be conveniently determined by elemental analysis (EA).
Catalysts 15 00443 g001
Figure 1. Oxygen, nitrogen, and sulfur surface groups, and data for their identification/quantification by TPD and XPS. Reprinted from reference [39].
Figure 1. Oxygen, nitrogen, and sulfur surface groups, and data for their identification/quantification by TPD and XPS. Reprinted from reference [39].
Catalysts 15 00443 g001
In aqueous solution, carbon materials exhibit an amphoteric behavior, due to the presence of acidic and basic surface groups. Dissociation of the acidic groups leaves the surface negatively charged; on the other hand, protons from the solution are attracted by the basic groups, imparting a positive charge to the surface. At some value of the pH, the surface charge will be zero; this is the point of zero charge (PZC), which can be determined by mass titration [32,44,45]. Thus, the surface will be negatively charged when pH > pHPZC, and positively charged when pH < pHPZC. The PZC is a convenient parameter for an overall description of the carbon surface chemistry.

3. Case Studies

3.1. Oxidative Dehydrogenation of Hydrocarbons

Alkenes can be obtained from alkanes by dehydrogenation; however, these reactions are reversible, thus, the conversions that can be attained are limited by equilibrium. An interesting alternative is oxidative dehydrogenation (ODH). Early studies on the ODH of ethylbenzene to styrene have shown that carbonaceous materials were catalysts for this reaction, and surface carbonyl (ketonic) groups were identified as active sites. Hydrogen is abstracted from the C-C bond of the ethyl chain by a pair of electron-rich carbonyl groups, which are thus converted into a pair of phenolic groups. These are subsequently re-oxidized, restoring the original sites and closing the catalytic cycle [46,47,48,49]. So, the reaction mechanism is explained in terms of the quinone/hydroquinone cycle shown in Figure 2.
In our pioneer study on the ODH of ethylbenzene catalyzed by activated carbons [11], we were able to quantitatively assess the active sites and determined that TOF = 3.87 × 10−4 s−1 for the reaction at 350 °C. Similar trends were observed in the ODH of isobutane, but the TOF was not reported [50].
Catalysts 15 00443 g002
Figure 2. Catalytic cycle proposed for the oxidative dehydrogenation of hydrocarbons on carbon materials. Reprinted from [51]. Copyright 2025 with permission from Elsevier.
Figure 2. Catalytic cycle proposed for the oxidative dehydrogenation of hydrocarbons on carbon materials. Reprinted from [51]. Copyright 2025 with permission from Elsevier.
Catalysts 15 00443 g002
More recently, we revisited the ODH of isobutane, but instead of activated carbons, carbon xerogels were used as catalysts [51], taking advantage of their mesoporous nature and negligible ash content. We used the same methodology as before [11], in order to prepare a set of catalysts with different amounts of active sites but with similar physical properties. First, we prepared a strongly oxidized carbon xerogel (CXO); other samples were obtained from CXO by thermal treatment under an inert atmosphere at distinct temperatures. This set of catalysts was then used in the ODH of isobutane, and their activity was correlated with the active site concentration (determined by TPD), as shown in Figure 3a. The TOF was obtained from the slope of this linear correlation: TOF = 3.17 × 10−4 s−1 at 375 °C. Turnover frequencies had not been previously reported for the ODH of lower alkanes with carbon catalysts. In addition, we could demonstrate the negative effect of carboxylic groups, as shown in Figure 3b, where we compare the performance of the original oxidized catalyst (CXO) with samples treated at 600 °C. At this temperature, the concentration of the active sites is not affected, but all carboxylic groups are removed from the surface. The presence of these electrophilic groups decreases the electron density at the active sites, therefore lowering their activity. Thus, CXO is less active than the thermally treated samples, in spite of having the same concentration of active sites. Catalyst deactivation by coke formation was found to be responsible for the yield decrease with time-on-stream observed in Figure 3b [33].
The effect of nitrogen doping on the ODH performance was also assessed in the same work. Carbon xerogels with N contents ranging from 0–5.3% were prepared. Although these samples were less active than CXO, they were more stable due to lower carbon formation; therefore, higher isobutene selectivities were obtained. Moreover, the isobutene yields were found to increase linearly with the total nitrogen content [51].

3.2. Advanced Oxidation Processes (AOPs)

Advanced Oxidation Processes (AOPs) are designed to remove pollutants from water and wastewater by oxidation, mainly via highly reactive radicals such as the hydroxyl (HO•) and the hydroperoxyl (HOO•) radicals [52,53]. Air (or oxygen), hydrogen peroxide, and ozone are the most frequently used oxidizing agents, the corresponding processes being called wet air oxidation (WAO), wet peroxide oxidation (WPO), and ozonation (OZ), respectively [54]. A wide variety of catalysts (both homogeneous and heterogeneous) have been used in order to reduce the severity of the AOPs operating conditions. Among them, carbon catalysts stand out as one of the best options, as they can be highly active for the complete mineralization of the organic pollutants (or their intermediate oxidation products) into CO2 and inorganic species, and there are no metal phases that might lixiviate to the liquid phase [55].
The reaction mechanisms are complex and may involve a combination of homogeneous and heterogeneous steps. Indeed, similar results were observed upon the addition of a free-radical scavenger to the liquid phase; thus, oxidation of the organic pollutant can be promoted by active oxygen species on the surface of the carbon catalyst, in addition to the process that occurs in the liquid phase promoted by free-radicals, as schematically shown in Figure 4 [39].
Nevertheless, it has been shown that the catalytic activity of carbon materials in AOPs increases with the concentration of basic sites, as shown in Figure 5 for the case of wet air oxidation [56]. Here, the conversion of oxalic acid was studied on a series of carbon nanotube catalysts obtained from a nitric acid oxidized sample (CNT-N), subsequently heat-treated at increasing temperatures (200, 400, 600 °C), in order to selectively remove the oxygenated surface groups, thereby increasing their pHPZC. Similar correlations were reported for wet peroxide oxidation [21] and ozonation [22].
The basic character of the carbon materials is due to the high density of π electrons on the basal planes. Therefore, the basicity could be enhanced by removing oxygenated electron-withdrawing groups [56] and/or by introducing nitrogen groups [57,58], which increase the electronic density and the activity of carbon materials as catalysts for AOPs [59,60,61,62]. The N-groups with delocalized extra electrons have been identified as the possible species responsible for the enhanced chemisorption [63] and activation of oxygen molecules [14].
For instance, N-doped carbon nanotubes exhibited excellent performance in the catalytic wet oxidation of organic compounds (phenol and oxalic acid), both in batch and continuous operation: 100% phenol degradation and 50% of total organic carbon (TOC) removal after 2 h in batch operation at 160 °C and 6 bar of oxygen partial pressure, while 80% and 50% of phenol and TOC removals were achieved at steady state in a fixed bed reactor at the same temperature and 12 bar of oxygen partial pressure. Under similar conditions, a cerium oxide catalyst showed very poor performance, while a 1% Pt catalyst supported on the N-doped carbon nanotubes showed only a slightly higher phenol removal, but better TOC removal. However, the Pt catalyst originated at much higher levels of the intermediates 1,4-benzoquinone and hydroquinone, which are highly toxic. Thus, the metal-free carbon catalyst seems to be a much better option for this application [64].

3.3. Acid Catalysis: Esterification and Etherification

Carbon materials functionalized with sulfonic acid groups can be used as solid acid catalysts for many reactions [38,65,66,67], such as those indicated in Table 1. In this section, we will focus on the results obtained in the esterification and etherification of different substrates.
The esterification of acetic acid with ethanol to produce ethyl acetate is a convenient model reaction to assess acid catalysts. In an earlier study, we prepared carbon xerogels with different concentrations of sulfonic acid groups and obtained a good correlation between the rate of formation of ethyl acetate and the concentration of the acid groups, as shown in Figure 6. The calculated TOF (3.17 × 10−2 s−1 at 70 °C) [68] was higher than that of sulfuric acid as a homogeneous catalyst (10−2 s−1), and other types of solid acid catalysts, such as the protonated Nafion resin NR50 (TOF = 1.8 × 10−2 s−1 at 70 °C) [69]. However, the activity was significantly reduced when the catalyst was reused in consecutive runs [68]. This is due to the leaching of the sulfonic acid groups, motivating the search for functionalization procedures capable of providing a stable bonding to the carbon surface. Thus, instead of a post-treatment with sulfuric acid, the functionalization can be performed by reaction of the carbon material with in-situ generated diazonium salt (4-benzenediazonium sulfonate). Promising results were obtained in the acetylation of glycerol to produce acetins, the catalyst being reused in four successive runs without any significant changes in the activity and selectivity to the desired products [70].
Biomass-derived carbons functionalized with sulfonic acid groups were used as catalysts for glycerol transesterification with ethyl acetate to produce acetins. A linear correlation between the initial rate of glycerol conversion and the concentration of the sulfonic acid groups was established, as shown in Figure 7a. The calculated TOF at 77 °C was 1.55 × 10−3 s−1 [71]. Nevertheless, this process was found to be less effective than the esterification of glycerol with acetic acid using the same catalysts [72].
Interesting results were also reported for the etherification of glycerol with tert-butyl alcohol in the presence of sulfonic acid-functionalized carbon nanotubes (CNTs), yielding glycerol tert-butyl ethers, namely the mono-, di-, and tri-substituted ethers (MTBGE, DTBGE, and TTBGE). Since the higher-substituted ethers are more valuable than MTBGE, the selectivity of the catalyst should be tuned to the production of DTBGE and TTBGE. Different functionalization protocols were used; in particular, the reaction of fuming sulfuric acid with ball-milled CNTs in the presence of glucose was found to be quite effective. A conversion of 52% was reached in 6 h at 110 °C, with a 7% combined yield of DTBGE and TTBGE. The initial rates of glycerol conversion were correlated with the concentration of the sulfonic acid groups, as shown in Figure 7b, and the TOF was determined: TOF = 7.26 × 10−3 s−1. Although there was a significant deactivation in subsequent runs, it was found that the catalyst activity could be restored by a simple regeneration treatment with HCl. The deactivation was mostly ascribed to the blocking of the active sites, rather than active site leaching [73]. Better results were obtained by functionalizing the CNTs with 4-benzenediazonium sulfonate generated in situ; in this case, the catalyst could be reused in three consecutive runs without significant loss of activity [74].
Although the reaction is mostly promoted by the sulfonic acid groups, the presence of acidic oxygen functional groups (carboxylic acid and phenolic groups) was found to improve the selectivity to the desired products. It was proposed that such groups facilitate the adsorption of the reactants by increasing the hydrophilicity of the catalyst, thereby promoting the conversion of the primary products (MTBGE) to the higher-substituted ethers [73]. These results show that carbon materials functionalized with sulfonic acid groups can replace mineral acids used in catalysis, with performances equal to or better than other solid acid catalysts such as Amberlyst [73,74]. They also highlight the main issues that must be addressed in the development of efficient carbon catalysts: the functionalization method should provide a stable bond of the functional groups to the carbon surface, in order to prevent leaching, thus allowing for the reutilization of the catalyst; on the other hand, the active sites can be adequately tuned in order to obtain high selectivity of the target products.

3.4. Electrocatalysis: The Oxygen Reduction Reaction (ORR)

The main barrier to the commercialization of fuel cell technologies is the current state-of-the-art catalyst for the oxygen reduction reaction (ORR) that consists of carbon-supported Pt, with metal loadings higher than 20%. These Pt-based catalysts are expensive and unstable, motivating the search for metal-free carbon-based alternatives. The incorporation of heteroatoms into the sp2 structure of carbon materials through functionalization or doping strategies enhances the local electronic density of the electrocatalyst, creating more active sites and enhancing both stability and catalytic performance towards ORR [75]. Heteroatom-doped carbons (e.g., N, B, S, P-doped graphene or carbon nanotubes) have shown high catalytic activity, even outperforming commercial Pt/C catalysts in alkaline conditions [76]. It has also been observed that N co-doping with other heteroatoms typically enhances the electrocatalytic activity of the resulting co-doped carbon materials for ORR compared to their single-doped counterparts. Co-doping with multiple elements (e.g., N + S, N + B, or N + P) creates stronger catalytic sites than single-element doping, enhancing the performance [76]. In addition to heteroatom doping, intrinsic defects (such as edges) can also influence the surface properties and electronic structure of carbon materials [77]. The introduction of defect regions in a sp2-C matrix may disrupt the integrity of π conjugation, similarly to the effect of heteroatom doping; it induces the charge polarization of carbon atoms to create strong adsorption of the O-containing species during the ORR [78]. Despite the large volume of work published on metal-free carbon-based ORR catalysts, the identification and quantification of the active sites responsible for the ORR mechanism remain ambiguous.
The role of the acidic/basic character of oxygen functional groups in carbon nanotube structures towards ORR was investigated by our group [75]. Oxidized CNTs were subjected to thermal treatment under nitrogen at different temperatures to selectively modify their oxygen surface groups. By removing acidic oxygen groups (carboxylic acids, anhydrides, phenols), improved ORR performance was observed, shifting the onset potential from −0.230 V to −0.194 V (vs. Ag/AgCl). Density Functional Theory (DFT) simulations showed that quinone groups favor O2 adsorption via an associative mechanism, while pyrone groups enable both dissociative and associative O2 adsorption, reducing the energy barrier for ORR. The presence of both groups created multiple active sites with different energy levels, extending the ORR activity range.
In another work, N-doped CNTs with nitrogen content varied between 0.2% and 4.8%, and combining pyridinic (N6), pyrrolic (N5), and quaternary nitrogen (NQ) were tested as electrocatalysts for ORR in alkaline medium [79]. The electrocatalysts exhibited onset potentials ranging from −0.159 to −0.128 V, approaching that of Pt/C (−0.124 V) but with higher stability and methanol tolerance than commercial Pt/C. The study confirmed that nitrogen doping enhances ORR activity and stability, but more than that, it was shown that ORR performance strongly correlated with the ratio of pyridinic to quaternary nitrogen (N6/NQ) rather than the total nitrogen content (Figure 8).
A combination of electrochemical measurements and DFT calculations was used to uncover the origin of the activity of various heteroatom-doped (N, B, O, S, P) graphene-based electrocatalysts for ORR by Jiao et al. [80]. N-doped graphene showed the highest ORR activity among the tested materials, the DFT calculations suggesting that catalytic activity is strongly linked to the molecular orbital energy levels of doped atoms, influencing the interaction of intermediates with the graphene surface. Although the N-doped carbon materials have shown considerable promise as metal-free catalysts for ORR, other factors affect their activity, such as the carbon defects and the nature and location of the doped heteroatoms. Despite decades of extensive research and significant advances, the slow kinetics and complex mechanisms of the ORR remain poorly understood. This is largely due to the difficulty in identifying active sites, the complicated transfer process from protons/electrons to the oxygen molecule, and the challenge of breaking O=O bonds. Additionally, precise control of the location and amounts of heteroatoms in the carbon basal plane and establishing a clear relationship between doped configurations and electrocatalytic activity remain ongoing challenges.
Zhang et al. introduced a novel potentiometric titration to quantify pyridinic nitrogen groups based on their Lewis basicity, allowing one to conclude that only surface-accessible pyridinic N-groups with appropriate Lewis basicity are catalytically active in the ORR. Indeed, X-ray photoelectron spectroscopy (XPS) and elemental analysis confirmed that many pyridinic N groups were buried within the bulk structure and not accessible to reactants [81]. The authors reported a direct proportionality between the ORR kinetic current density and the density of titratable pyridinic nitrogen groups. The turnover frequency of ORR was found to increase with the pKb of pyridinic N-groups (Figure 9), suggesting that stronger Lewis basicity of the pyridinic N-groups contributes to higher ORR activity by stabilizing key reaction intermediates. Supported by DFT calculations, pyridinic N-groups with larger pKb can stabilize the H atom moiety and induce a stronger electronic modification to the adjacent C atom to stabilize the O2 moiety. These two interactions (pyridinic N and H atom; adjacent C atom and the O2 moiety) stabilize the intermediates as the pKb of the pyridinic N-groups increases, thus decreasing the activation energy. This supports the conclusion that carbon atoms adjacent to pyridinic nitrogen act as Lewis base catalytic sites, facilitating oxygen adsorption, as also proposed by Guo et al. [29] in acidic media.
An alternative approach was proposed to calculate the TOF of N-doped carbon black by Chakraborty et al. [82]. They used catechol as a molecular probe to develop a new in-situ method based on the adsorption of catechol to estimate the active site density of N-doped carbons. Catechol undergoes a well-defined redox reaction (oxidation and reduction) at carbon surfaces, and it binds specifically to active sites on nitrogen-doped carbon via nucleophilic interactions. Two redox peaks are observed in the catechol adsorption voltammogram of nitrogen-doped carbon samples, which are assigned to two-electron and two-proton processes. Therefore, the electrochemical response of adsorbed catechol can be quantified and correlated with the number of active sites, calculating the active site density. This active site density is later correlated with the ORR activity, and the TOF can be estimated (values ranging from 0.116 to 0.568 e− site−1 s−1 at 0.8 V vs. RHE are obtained). TOF was calculated as e− site−1 s−1 by dividing the mass-specific activity in the ORR (A g−1) by the site density (sites g−1) and the electronic charge (1.6 × 10−19 C), as given by [83].
Multi-heteroatom doping has also been investigated as a strategy to enhance the electrocatalytic activity of carbon materials [84]. A combination of different atomic functionalities (e.g., electron donors and acceptors) generates synergistic effects that are not present in single-element doping, which is why some materials, such as N,B-doped graphene and N,S-doped graphene [85], outperform single-doped counterparts and even commercial Pt/C in alkaline media. While pyridinic-N-adjacent carbon atoms are usually assumed to be the true active sites, the second dopant can lower energy barriers and tune adsorption free energies by polarization of neighboring C atoms, creating more favorable adsorption environments and altering electron density at the active site. For instance, S atoms seem to introduce spin density at edges, aiding in catalytic activity, while P doping creates edge defects and boosts hydrophilicity, improving reactant access and product desorption, improving overall ORR efficiency [76]. Among various available co-doped carbon materials (N/S, N/B, N/P, N/F), N/S seems particularly interesting since the S heteroatom could induce structural defects, facilitating charge delocalization. In addition, the two lone pairs of electrons in the S atom can enhance the interaction between carbon and reaction intermediates, improving the reaction kinetics [86]. However, while synergistic effects are experimentally observed, the mechanistic understanding is still incomplete, which makes it difficult to predict TOF values in these cases. As an alternative, some authors are using different activity descriptors to clarify the mechanism [87].

4. Summary and Outlook

Considerable advances have been achieved in the design of carbon-based catalysts for a considerable number of chemical reactions, encompassing organic redox, substitution, addition, condensation/dehydration/hydration, hydrogenation/dehydrogenation, hydrolysis, and polymerization reactions [8]. Some impressive reaction rates/yields and TOFs have been reported using metal-free carbon catalysts, despite the actual mechanisms and the active sites remaining a constant point of discussion. Besides the significant progress being observed in the fine engineering/decoration of carbon surfaces, as well as the availability of advanced characterization techniques, further development and fundamental understanding in the field of carbocatalysis are limited by a lack of strategies that can allow the quantitative analysis of their intrinsic catalytic properties. The precise identification and quantification of active sites remains challenging due to limitations in current characterization techniques, hindering the accurate calculation of turnover frequencies necessary for benchmarking these novel catalysts against traditional ones. Future research should prioritize the development of more refined characterization techniques to accurately identify and quantify active sites on carbon catalysts. This is crucial for establishing clear structure-activity relationships, for the rational design of more efficient catalysts, and for fully realizing the potential of metal-free carbon-based catalysts in various industrial applications.
Alternative approaches based on the use of model reactions to characterize carbon materials, combined with other conventional techniques (XPS, TPD, etc.), are emerging as efficient tools for the assessment of active centers and their surroundings. It should be recalled that for different reaction media (liquid/gas phase reaction) and operating conditions (thermochemical, electrochemical reactions), different strategies will have to be adopted to find the interactions between the active centers and the molecules involved, and that a methodology used in one reaction may not be the most suitable for identifying the species involved in another. For example, the number of active centers measured in gas-phase model reactions can overestimate the active centers actually available in a liquid-phase reaction, especially with porous catalysts whose active centers may be located in narrow pores that are not completely wettable and are therefore inaccessible in a liquid-phase reaction. On the other hand, it should be borne in mind that the properties of materials can be changed by contact with other gases/reactants, exposure to temperature, combination with other ligands, or applied electrochemical potential. Consequently, the active sites identified by a given technique may be different or unavailable in the reaction environment.
The unique specificity of carbon-based catalysts derives from the graphitic structure of these materials, the role played by edge sites and structural defects, and their synergy. These aspects are especially relevant in the field of electrocatalysis. In situ and operando spectroscopic methods might be useful in this context, in order to clarify the dynamics of the active sites under reaction conditions.
Further progress in the next-generation carbon catalysts will be driven by the development of new methodologies for characterizing carbon materials based on the use of model reactions and in situ advanced characterization, validated by computational modelling. The integration of new data with the use of machine learning tools may also efficiently generate valuable insights on the use of carbon materials in catalytic processes [88].
Metal-free carbon catalysis is still a young field of research, but it has great potential for applications. For instance, the advantages of metal-free materials as catalysts for the treatment of contaminated waters by AOPs, as discussed in Section 3.2, are obvious; indeed, metal and metal oxide systems may suffer lixiviation of the active phases, introducing an additional type of pollution into the environment. The current industrial drive to replace fossil fuels with renewable raw materials offers a vast field of application for carbon-based catalysts, such as in the conversion of biomass to chemicals and fuels. For instance, glycerol, an abundant by-product from the production of biodiesel, can be converted into useful products by esterification and etherification reactions, as discussed in Section 3.3. In addition, the carbon catalysts can also be synthesized from biomass-derived precursors. Another industrial area where carbon catalysts might be useful is the Fine Chemicals sector. However, electrocatalysis is, perhaps, the most promising field for the application of metal-free carbon catalysts, as it replaces the current state-of-the-art platinum-based catalysts and promotes the energy transition.

Author Contributions

Conceptualization, R.P.R. and J.L.F.; Writing—original draft, review and editing, R.P.R. and J.L.F.; Funding acquisition, R.P.R. and J.L.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by national funds through FCT/MCTES, under project CATALYSE-CO2, Reference 2023.13478.PEX (DOI: 10.54499/2023.13478.PEX). This research was also supported by: UID/50020 of LSRE-LCM—Laboratory of Separation and Reaction Processes—Laboratory of Catalysis and Materials—funded by Fundação para a Ciência e a Tecnologia, I.P./MCTES through national funds; and ALiCE—LA/P/0045/2020 (DOI: 10.54499/LA/P/0045/2020).

Data Availability Statement

No new data were used for the research described in the article. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Rodríguez-Reinoso, F. The role of carbon materials in heterogeneous catalysis. Carbon 1998, 36, 159–175. [Google Scholar] [CrossRef]
  2. Figueiredo, J.L.; Pereira, M.F.R. Carbon as Catalyst. In Carbon Materials for Catalysis; Serp, P., Figueiredo, J.L., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2009; pp. 177–217. [Google Scholar] [CrossRef]
  3. Coughlin, R.W. Carbon as Adsorbent and Catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1969, 8, 12–23. [Google Scholar] [CrossRef]
  4. Su, D.S.; Perathoner, S.; Centi, G. Nanocarbons for the Development of Advanced Catalysts. Chem. Rev. 2013, 113, 5782–5816. [Google Scholar] [CrossRef] [PubMed]
  5. Yu, D.; Nagelli, E.; Du, F.; Dai, L. Metal-Free Carbon Nanomaterials Become More Active than Metal Catalysts and Last Longer. J. Phys. Chem. Lett. 2010, 1, 2165–2173. [Google Scholar] [CrossRef]
  6. Su, D.S.; Wen, G.; Wu, S.; Peng, F.; Schlögl, R. Carbocatalysis in Liquid-Phase Reactions. Angew. Chem. Int. Ed. 2017, 56, 936–964. [Google Scholar] [CrossRef]
  7. Duan, X.; Sun, H.; Wang, S. Metal-Free Carbocatalysis in Advanced Oxidation Reactions. Acc. Chem. Res. 2018, 51, 678–687. [Google Scholar] [CrossRef]
  8. Chua, C.K.; Pumera, M. Carbocatalysis: The State of “Metal-Free” Catalysis. Chem.—A Eur. J. 2015, 21, 12550–12562. [Google Scholar] [CrossRef]
  9. Figueiredo, J.L.; Pereira, M.F.R. The role of surface chemistry in catalysis with carbons. Catal. Today 2010, 150, 2–7. [Google Scholar] [CrossRef]
  10. Figueiredo, J.L. Functionalization of porous carbons for catalytic applications. J. Mater. Chem. A 2013, 1, 9351–9364. [Google Scholar] [CrossRef]
  11. Pereira, M.F.R.; Órfão, J.J.M.; Figueiredo, J.L. Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups. Appl. Catal. A Gen. 1999, 184, 153–160. [Google Scholar] [CrossRef]
  12. Moreno-Castilla, C.; Carrasco-Marín, F.; Parejo-Pérez, C.; López Ramón, M.V. Dehydration of methanol to dimethyl ether catalyzed by oxidized activated carbons with varying surface acidic character. Carbon 2001, 39, 869–875. [Google Scholar] [CrossRef]
  13. Szymański, G.S.; Rychlicki, G. Catalytic conversion of propan-2-ol on carbon catalysts. Carbon 1993, 31, 247–257. [Google Scholar] [CrossRef]
  14. Szymański, G.S.; Grzybek, T.; Papp, H. Influence of nitrogen surface functionalities on the catalytic activity of activated carbon in low temperature SCR of NOx with NH3. Catal. Today 2004, 90, 51–59. [Google Scholar] [CrossRef]
  15. Mochida, I.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. High catalytic activity of pitch-based activated carbon fibres of moderate surface area for oxidation of NO to NO2 at room temperature. Fuel 1997, 76, 543–548. [Google Scholar] [CrossRef]
  16. Raymundo-Piñero, E.; Cazorla-Amorós, D.; Linares-Solano, A. The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres. Carbon 2003, 41, 1925–1932. [Google Scholar] [CrossRef]
  17. Adib, F.; Bagreev, A.; Bandosz, T.J. Adsorption/Oxidation of Hydrogen Sulfide on Nitrogen-Containing Activated Carbons. Langmuir 2000, 16, 1980–1986. [Google Scholar] [CrossRef]
  18. Sotowa, C.; Watanabe, Y.; Yatsunami, S.; Korai, Y.; Mochida, I. Catalytic dehydrochlorination of 1,2-dichloroethane into vinyl chloride over polyacrylonitrile-based active carbon fiber. Appl. Catal. A Gen. 1999, 180, 317–323. [Google Scholar] [CrossRef]
  19. Zhou, K.; Li, B.; Zhang, Q.; Huang, J.-Q.; Tian, G.-L.; Jia, J.-C.; Zhao, M.-Q.; Luo, G.-H.; Su, D.S.; Wei, F. The Catalytic Pathways of Hydrohalogenation over Metal-Free Nitrogen-Doped Carbon Nanotubes. ChemSusChem 2014, 7, 723–728. [Google Scholar] [CrossRef]
  20. Rocha, R.P.; Sousa, J.P.S.; Silva, A.M.T.; Pereira, M.F.R.; Figueiredo, J.L. Catalytic activity and stability of multiwalled carbon nanotubes in catalytic wet air oxidation of oxalic acid: The role of the basic nature induced by the surface chemistry. Appl. Catal. B Environ. 2011, 104, 330–336. [Google Scholar] [CrossRef]
  21. Gomes, H.T.; Miranda, S.M.; Sampaio, M.J.; Silva, A.M.T.; Faria, J.L. Activated carbons treated with sulphuric acid: Catalysts for catalytic wet peroxide oxidation. Catal. Today 2010, 151, 153–158. [Google Scholar] [CrossRef]
  22. Gonçalves, A.G.; Figueiredo, J.L.; Órfão, J.J.M.; Pereira, M.F.R. Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts. Carbon 2010, 48, 4369–4381. [Google Scholar] [CrossRef]
  23. Budarin, V.L.; Clark, J.H.; Luque, R.; Macquarrie, D.J. Versatile mesoporous carbonaceous materials for acid catalysis. Chem. Commun. 2007, 634–636. [Google Scholar] [CrossRef]
  24. Zhao, Y.; Wang, H.; Zhao, Y.; Shen, J. Preparation of a novel sulfonated carbon catalyst for the etherification of isopentene with methanol to produce tert-amyl methyl ether. Catal. Commun. 2010, 11, 824–828. [Google Scholar] [CrossRef]
  25. Liang, X.; Li, C.; Qi, C. Novel carbon-based strong acid catalyst from starch and its catalytic activities for acetalization. J. Mater. Sci. 2011, 46, 5345–5349. [Google Scholar] [CrossRef]
  26. Onda, A.; Ochi, T.; Yanagisawa, K. Hydrolysis of Cellulose Selectively into Glucose Over Sulfonated Activated-Carbon Catalyst Under Hydrothermal Conditions. Top. Catal. 2009, 52, 801–807. [Google Scholar] [CrossRef]
  27. Matos, I.; Neves, P.D.; Castanheiro, J.E.; Perez-Mayoral, E.; Martin-Aranda, R.; Duran-Valle, C.; Vital, J.; Botelho do Rego, A.M.; Fonseca, I.M. Mesoporous carbon as an efficient catalyst for alcoholysis and aminolysis of epoxides. Appl. Catal. A Gen. 2012, 439-440, 24–30. [Google Scholar] [CrossRef]
  28. van Dommele, S.; de Jong, K.P.; Bitter, J.H. Nitrogen-containing carbon nanotubes as solid base catalysts. Chem. Commun. 2006, 46, 4859–4861. [Google Scholar] [CrossRef]
  29. Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351, 361–365. [Google Scholar] [CrossRef]
  30. Boehm, H.P. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994, 32, 759–769. [Google Scholar] [CrossRef]
  31. Boehm, H.P. Surface oxides on carbon and their analysis: A critical assessment. Carbon 2002, 40, 145–149. [Google Scholar] [CrossRef]
  32. Menéndez, J.A.; Phillips, J.; Xia, B.; Radovic, L.R. On the Modification and Characterization of Chemical Surface Properties of Activated Carbon:  In the Search of Carbons with Stable Basic Properties. Langmuir 1996, 12, 4404–4410. [Google Scholar] [CrossRef]
  33. Montes-Morán, M.A.; Suárez, D.; Menéndez, J.A.; Fuente, E. On the nature of basic sites on carbon surfaces: An overview. Carbon 2004, 42, 1219–1225. [Google Scholar] [CrossRef]
  34. Boehm, H.-P. Catalytic Properties of Nitrogen-Containing Carbons. In Carbon Materials for Catalysis; Serp, P., Figueiredo, J.L., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2009; pp. 219–265. [Google Scholar] [CrossRef]
  35. Gupta, P.; Paul, S. Solid acids: Green alternatives for acid catalysis. Catal. Today 2014, 236, 153–170. [Google Scholar] [CrossRef]
  36. Figueiredo, J.L.; Pereira, M.F.R.; Freitas, M.M.A.; Órfão, J.J.M. Modification of the surface chemistry of activated carbons. Carbon 1999, 37, 1379–1389. [Google Scholar] [CrossRef]
  37. Rocha, R.P.; Pereira, M.F.R.; Figueiredo, J.L. Characterisation of the surface chemistry of carbon materials by temperature-programmed desorption: An assessment. Catal. Today 2023, 418, 114136. [Google Scholar] [CrossRef]
  38. Kiciński, W.; Szala, M.; Bystrzejewski, M. Sulfur-doped porous carbons: Synthesis and applications. Carbon 2014, 68, 1–32. [Google Scholar] [CrossRef]
  39. Rocha, R.P.; Soares, O.S.G.P.; Figueiredo, J.L.; Pereira, M.F.R. Tuning CNT Properties for Metal-Free Environmental Catalytic Applications. C 2016, 2, 17. [Google Scholar] [CrossRef]
  40. Pels, J.R.; Kapteijn, F.; Moulijn, J.A.; Zhu, Q.; Thomas, K.M. Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis. Carbon 1995, 33, 1641–1653. [Google Scholar] [CrossRef]
  41. Jansen, R.J.J.; van Bekkum, H. XPS of nitrogen-containing functional groups on activated carbon. Carbon 1995, 33, 1021–1027. [Google Scholar] [CrossRef]
  42. Zhou, J.-H.; Sui, Z.-J.; Zhu, J.; Li, P.; Chen, D.; Dai, Y.-C.; Yuan, W.-K. Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR. Carbon 2007, 45, 785–796. [Google Scholar] [CrossRef]
  43. Kundu, S.; Xia, W.; Busser, W.; Becker, M.; Schmidt, D.A.; Havenith, M.; Muhler, M. The formation of nitrogen-containing functional groups on carbon nanotube surfaces: A quantitative XPS and TPD study. Phys. Chem. Chem. Phys. 2010, 12, 4351–4359. [Google Scholar] [CrossRef] [PubMed]
  44. Moreno-Castilla, C.; Ferro-Garcia, M.A.; Joly, J.P.; Bautista-Toledo, I.; Carrasco-Marin, F.; Rivera-Utrilla, J. Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments. Langmuir 1995, 11, 4386–4392. [Google Scholar] [CrossRef]
  45. Newcombe, G.; Hayes, R.; Drikas, M. Granular activated carbon: Importance of surface properties in the adsorption of naturally occurring organics. Colloids Surf. A Physicochem. Eng. Asp. 1993, 78, 65–71. [Google Scholar] [CrossRef]
  46. Iwasawa, Y.; Nobe, H.; Ogasawara, S. Reaction mechanism for styrene synthesis over polynaphthoquinone. J. Catal. 1973, 31, 444–449. [Google Scholar] [CrossRef]
  47. Emig, G.; Hofmann, H. Action of zirconium phosphate as a catalyst for the oxydehydrogenation of ethylbenzene to styrene. J. Catal. 1983, 84, 15–26. [Google Scholar] [CrossRef]
  48. Schraut, A.; Emig, G.; Sockel, H.G. Composition and structure of active coke in the oxydehydrogenation of ethylbenzene. Appl. Catal. 1987, 29, 311–326. [Google Scholar] [CrossRef]
  49. Schraut, A.; Emig, G.; Hofmann, H. Kinetic investigations of the oxydehydrogenation of ethylbenzene. J. Catal. 1988, 112, 221–228. [Google Scholar] [CrossRef]
  50. de Jesús Díaz Velásquez, J.; Suárez, L.M.C.; Figueiredo, J.L. Oxidative dehydrogenation of isobutane over activated carbon catalysts. Appl. Catal. A Gen. 2006, 311, 51–57. [Google Scholar] [CrossRef]
  51. Pelech, I.; Soares, O.S.G.P.; Pereira, M.F.R.; Figueiredo, J.L. Oxidative dehydrogenation of isobutane on carbon xerogel catalysts. Catal. Today 2015, 249, 176–183. [Google Scholar] [CrossRef]
  52. Oturan, M.A.; and Aaron, J.-J. Advanced Oxidation Processes in Water/Wastewater Treatment: Principles and Applications. A Review. Crit. Rev. Environ. Sci. Technol. 2014, 44, 2577–2641. [Google Scholar] [CrossRef]
  53. Gao, J.; Qin, T.; Wacławek, S.; Duan, X.; Huang, Y.; Liu, H.; Dionysiou, D.D. The application of advanced oxidation processes (AOPs) to treat unconventional water for fit-for-purpose reuse. Curr. Opin. Chem. Eng. 2023, 42, 100974. [Google Scholar] [CrossRef]
  54. Esplugas, S.; Giménez, J.; Contreras, S.; Pascual, E.; Rodríguez, M. Comparison of different advanced oxidation processes for phenol degradation. Water Res. 2002, 36, 1034–1042. [Google Scholar] [CrossRef] [PubMed]
  55. Stüber, F.; Font, J.; Fortuny, A.; Bengoa, C.; Eftaxias, A.; Fabregat, A. Carbon materials and catalytic wet air oxidation of organic pollutants in wastewater. Top. Catal. 2005, 33, 3–50. [Google Scholar] [CrossRef]
  56. Rocha, R.P.; Silva, A.M.T.; Romero, S.M.M.; Pereira, M.F.R.; Figueiredo, J.L. The role of O- and S-containing surface groups on carbon nanotubes for the elimination of organic pollutants by catalytic wet air oxidation. Appl. Catal. B Environ. 2014, 147, 314–321. [Google Scholar] [CrossRef]
  57. Chen, H.; Yang, G.; Feng, Y.; Shi, C.; Xu, S.; Cao, W.; Zhang, X. Biodegradability enhancement of coking wastewater by catalytic wet air oxidation using aminated activated carbon as catalyst. Chem. Eng. J. 2012, 198–199, 45–51. [Google Scholar] [CrossRef]
  58. Yang, G.; Chen, H.; Qin, H.; Feng, Y. Amination of activated carbon for enhancing phenol adsorption: Effect of nitrogen-containing functional groups. Appl. Surf. Sci. 2014, 293, 299–305. [Google Scholar] [CrossRef]
  59. Restivo, J.; Rocha, R.P.; Silva, A.M.T.; Órfão, J.J.M.; Pereira, M.F.R.; Figueiredo, J.L. Catalytic performance of heteroatom-modified carbon nanotubes in advanced oxidation processes. Cuihua Xuebao/Chin. J. Catal. 2014, 35, 896–905. [Google Scholar] [CrossRef]
  60. Rocha, R.P.; Restivo, J.; Sousa, J.P.S.; Órfão, J.J.M.; Pereira, M.F.R.; Figueiredo, J.L. Nitrogen-doped carbon xerogels as catalysts for advanced oxidation processes. Catal. Today 2015, 241, 73–79. [Google Scholar] [CrossRef]
  61. Rocha, R.P.; Gonçalves, A.G.; Pastrana-Martínez, L.M.; Bordoni, B.C.; Soares, O.S.G.P.; Órfão, J.J.M.; Faria, J.L.; Figueiredo, J.L.; Silva, A.M.T.; Pereira, M.F.R. Nitrogen-doped graphene-based materials for advanced oxidation processes. Catal. Today 2015, 249, 192–198. [Google Scholar] [CrossRef]
  62. Santiago, M.; Stüber, F.; Fortuny, A.; Fabregat, A.; Font, J. Modified activated carbons for catalytic wet air oxidation of phenol. Carbon 2005, 43, 2134–2145. [Google Scholar] [CrossRef]
  63. Pollak, E.; Salitra, G.; Soffer, A.; Aurbach, D. On the reaction of oxygen with nitrogen-containing and nitrogen-free carbons. Carbon 2006, 44, 3302–3307. [Google Scholar] [CrossRef]
  64. Santos, D.F.M.; Soares, O.S.G.P.; Silva, A.M.T.; Figueiredo, J.L.; Pereira, M.F.R. Catalytic wet oxidation of organic compounds over N-doped carbon nanotubes in batch and continuous operation. Appl. Catal. B Environ. 2016, 199, 361–371. [Google Scholar] [CrossRef]
  65. Peng, F.; Zhang, L.; Wang, H.; Lv, P.; Yu, H. Sulfonated carbon nanotubes as a strong protonic acid catalyst. Carbon 2005, 43, 2405–2408. [Google Scholar] [CrossRef]
  66. Yu, H.; Jin, Y.; Li, Z.; Peng, F.; Wang, H. Synthesis and characterization of sulfonated single-walled carbon nanotubes and their performance as solid acid catalyst. J. Solid State Chem. 2008, 181, 432–438. [Google Scholar] [CrossRef]
  67. Giménez, J.; Costa, J.; Cervera-March, S. Catalysis by ion-exchange sulfonated resins: Comparative study of gel and macroporous types and influence of divinylbenzene concentration. Appl. Catal. 1987, 31, 221–234. [Google Scholar] [CrossRef]
  68. Rocha, R.P.; Pereira, M.F.R.; Figueiredo, J.L. Carbon as a catalyst: Esterification of acetic acid with ethanol. Catal. Today 2013, 218-219, 51–56. [Google Scholar] [CrossRef]
  69. Hara, M.; Yoshida, T.; Takagaki, A.; Takata, T.; Kondo, J.N.; Hayashi, S.; Domen, K. A Carbon Material as a Strong Protonic Acid. Angew. Chem. Int. Ed. 2004, 43, 2955–2958. [Google Scholar] [CrossRef]
  70. Goscianska, J.; Malaika, A. A facile post-synthetic modification of ordered mesoporous carbon to get efficient catalysts for the formation of acetins. Catal. Today 2020, 357, 84–93. [Google Scholar] [CrossRef]
  71. Malaika, A.; Ptaszyńska, K.; Morawa Eblagon, K.; Pereira, M.F.R.; Figueiredo, J.L.; Kozłowski, M. Solid acid carbon catalysts for sustainable production of biofuel enhancers via transesterification of glycerol with ethyl acetate. Fuel 2021, 304, 121381. [Google Scholar] [CrossRef]
  72. Malaika, A.; Ptaszyńska, K.; Kozłowski, M. Conversion of renewable feedstock to bio-carbons dedicated for the production of green fuel additives from glycerol. Fuel 2021, 288, 119609. [Google Scholar] [CrossRef]
  73. Ptaszyńska, K.; Eblagon, K.M.; Malaika, A.; Figueiredo, J.L.; Kozłowski, M. The role of mechanochemical treatment of carbon nanotubes in promoting glycerol etherification. Catal. Sci. Technol. 2024, 14, 3184–3200. [Google Scholar] [CrossRef]
  74. Ptaszyńska, K.; Malaika, A.; Morawa Eblagon, K.; Figueiredo, J.L.; Kozłowski, M. Promoting Effect of Ball Milling on the Functionalization and Catalytic Performance of Carbon Nanotubes in Glycerol Etherification. Molecules 2024, 29, 1623. [Google Scholar] [CrossRef] [PubMed]
  75. Rocha, I.M.; Soares, O.S.G.P.; Figueiredo, J.L.; Freire, C.; Pereira, M.F.R. Bifunctionality of the pyrone functional group in oxidized carbon nanotubes towards oxygen reduction reaction. Catal. Sci. Technol. 2017, 7, 1868–1879. [Google Scholar] [CrossRef]
  76. Ma, R.; Lin, G.; Zhou, Y.; Liu, Q.; Zhang, T.; Shan, G.; Yang, M.; Wang, J. A review of oxygen reduction mechanisms for metal-free carbon-based electrocatalysts. npj Comput. Mater. 2019, 5, 78. [Google Scholar] [CrossRef]
  77. Tao, L.; Wang, Q.; Dou, S.; Ma, Z.; Huo, J.; Wang, S.; Dai, L. Edge-rich and dopant-free graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction. Chem. Commun. 2016, 52, 2764–2767. [Google Scholar] [CrossRef]
  78. Jiang, Y.; Yang, L.; Sun, T.; Zhao, J.; Lyu, Z.; Zhuo, O.; Wang, X.; Wu, Q.; Ma, J.; Hu, Z. Significant Contribution of Intrinsic Carbon Defects to Oxygen Reduction Activity. ACS Catal. 2015, 5, 6707–6712. [Google Scholar] [CrossRef]
  79. Rocha, I.M.; Soares, O.S.G.P.; Fernandes, D.M.; Freire, C.; Figueiredo, J.L.; Pereira, M.F.R. N-doped Carbon Nanotubes for the Oxygen Reduction Reaction in Alkaline Medium: Synergistic Relationship between Pyridinic and Quaternary Nitrogen. ChemistrySelect 2016, 1, 2522–2530. [Google Scholar] [CrossRef]
  80. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S.Z. Origin of the Electrocatalytic Oxygen Reduction Activity of Graphene-Based Catalysts: A Roadmap to Achieve the Best Performance. J. Am. Chem. Soc. 2014, 136, 4394–4403. [Google Scholar] [CrossRef]
  81. Zhang, Z.; Zhang, F.; Song, Z.; Zhang, L. Oxygen Reduction Reaction on Pyridinic Nitrogen-Functionalized Carbon: Active Site Quantification and Effects of Lewis Basicity. ACS Catal. 2025, 15, 296–309. [Google Scholar] [CrossRef]
  82. Chakraborty, A.; Devivaraprasad, R.; Bera, B.; Neergat, M. Electrochemical estimation of the active site density on metal-free nitrogen-doped carbon using catechol as an adsorbate. Phys. Chem. Chem. Phys. 2017, 19, 25414–25422. [Google Scholar] [CrossRef]
  83. Jaouen, F.; Dodelet, J.-P. Average turn-over frequency of O2 electro-reduction for Fe/N/C and Co/N/C catalysts in PEFCs. Electrochim. Acta 2007, 52, 5975–5984. [Google Scholar] [CrossRef]
  84. Wang, J.; Kong, H.; Zhang, J.; Hao, Y.; Shao, Z.; Ciucci, F. Carbon-based electrocatalysts for sustainable energy applications. Prog. Mater. Sci. 2021, 116, 100717. [Google Scholar] [CrossRef]
  85. Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D.W.; Baek, J.-B.; Dai, L. BCN Graphene as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2012, 51, 4209–4212. [Google Scholar] [CrossRef] [PubMed]
  86. Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S.Z. Sulfur and Nitrogen Dual-Doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance. Angew. Chem. Int. Ed. 2012, 51, 11496–11500. [Google Scholar] [CrossRef]
  87. Zhao, Z.; Xia, Z. Design Principles for Dual-Element-Doped Carbon Nanomaterials as Efficient Bifunctional Catalysts for Oxygen Reduction and Evolution Reactions. ACS Catal. 2016, 6, 1553–1558. [Google Scholar] [CrossRef]
  88. Goldsmith, B.R.; Esterhuizen, J.; Liu, J.-X.; Bartel, C.J.; Sutton, C. Machine learning for heterogeneous catalyst design and discovery. AIChE J. 2018, 64, 2311–2323. [Google Scholar] [CrossRef]
Catalysts 15 00443 g003
Figure 3. ODH of isobutane at 375 °C with carbon xerogel catalysts: (a) Initial isobutene yield vs. the initial concentration of carbonyl groups; (b) Isobutene yield as a function of reaction time: comparison of the original oxidized catalyst (CXO) with samples treated at 600 °C for different times (1, 2, and 5 h). Reprinted from [51] Copyright 2025 with permission from Elsevier.
Figure 3. ODH of isobutane at 375 °C with carbon xerogel catalysts: (a) Initial isobutene yield vs. the initial concentration of carbonyl groups; (b) Isobutene yield as a function of reaction time: comparison of the original oxidized catalyst (CXO) with samples treated at 600 °C for different times (1, 2, and 5 h). Reprinted from [51] Copyright 2025 with permission from Elsevier.
Catalysts 15 00443 g003
Catalysts 15 00443 g004
Figure 4. Reactions that might occur in the liquid phase and on the catalyst surface during wet oxidation and ozonation (Adapted from [39]).
Figure 4. Reactions that might occur in the liquid phase and on the catalyst surface during wet oxidation and ozonation (Adapted from [39]).
Catalysts 15 00443 g004
Catalysts 15 00443 g005
Figure 5. Correlations of catalytic activity with carbon surface basicity in wet oxidation (data from [56]).
Figure 5. Correlations of catalytic activity with carbon surface basicity in wet oxidation (data from [56]).
Catalysts 15 00443 g005
Catalysts 15 00443 g006
Figure 6. Ethyl acetate formation rate after 1 h using treated carbon xerogel samples as a function of the respective surface concentration of sulphonic acid groups. Data from [68].
Figure 6. Ethyl acetate formation rate after 1 h using treated carbon xerogel samples as a function of the respective surface concentration of sulphonic acid groups. Data from [68].
Catalysts 15 00443 g006
Catalysts 15 00443 g007
Figure 7. Correlation of catalytic activity in the conversion of glycerol in the presence of sulfonic acid functionalized carbon catalysts: (a) transesterification with ethyl acetate at 77 °C, reprinted from [71], with permission from Elsevier; (b) etherification with tert-butyl alcohol at 110 °C, reproduced from [73] with permission from the Royal Society of Chemistry.
Figure 7. Correlation of catalytic activity in the conversion of glycerol in the presence of sulfonic acid functionalized carbon catalysts: (a) transesterification with ethyl acetate at 77 °C, reprinted from [71], with permission from Elsevier; (b) etherification with tert-butyl alcohol at 110 °C, reproduced from [73] with permission from the Royal Society of Chemistry.
Catalysts 15 00443 g007
Catalysts 15 00443 g008
Figure 8. Variation of the onset potential (Eon) for the ORR in 0.1 molar KOH, determined by Linear Square Voltammetry (LSV), with the ratio of Npyridinic/Nquaternary. Reprinted with permission from [79].
Figure 8. Variation of the onset potential (Eon) for the ORR in 0.1 molar KOH, determined by Linear Square Voltammetry (LSV), with the ratio of Npyridinic/Nquaternary. Reprinted with permission from [79].
Catalysts 15 00443 g008
Catalysts 15 00443 g009
Figure 9. Turnover frequency versus pKb of metal-free carbon materials investigated as catalysts for the ORR in 0.1 M KOH (KB—Ketjenblack; VC—Vulcan carbon; CNT—carbon nanotubes; GN—graphene; ZIF-8—zeolitic imidazolate framework-8). Reprinted with permission from [81] Copyright 2025 American Chemical Society.
Figure 9. Turnover frequency versus pKb of metal-free carbon materials investigated as catalysts for the ORR in 0.1 M KOH (KB—Ketjenblack; VC—Vulcan carbon; CNT—carbon nanotubes; GN—graphene; ZIF-8—zeolitic imidazolate framework-8). Reprinted with permission from [81] Copyright 2025 American Chemical Society.
Catalysts 15 00443 g009
Table 1. A selection of reactions catalyzed by carbon (updated from previous reviews [2,9,10]).
Table 1. A selection of reactions catalyzed by carbon (updated from previous reviews [2,9,10]).
ReactionsSurface Chemistry/Active Sites [Reference]
Gas phase
Oxidative dehydrogenationCarbonyl/Quinone groups [11]
Dehydration of alcoholsCarboxylic acids [12]
Dehydrogenation of alcoholsLewis acids and basic sites [13]
NOx reduction (SCR with NH3)Acidic surface oxides (carboxylic and lactonic) +
basic sites (N6) [14]
NO oxidationBasic sites [15]
SO2 oxidationBasic sites, N6 [16]
H2S oxidationBasic N groups [17]
DehydrohalogenationPyridinic nitrogen (N6) [18]
HydrohalogenationQuaternary nitrogen (NQ) [19]
Liquid phase
Advanced Oxidation Processes
(CWAO; WPO; Ozonation)		Basic sites [20,21,22]
EsterificationSulfonic acid groups [23]
EtherificationSulfonic acid groups [24]
AlkylationSulfonic acid groups [23]
AcylationSulfonic acid groups [23]
AcetalizationSulfonic acid groups [25]
Hydrolysis of celluloseSulfonic acid groups [26]
Alcoholysis of epoxidesSulfonic acid groups [27]
Knoevenagel condensationPyridinic nitrogen (N6) [28]
Oxygen reduction (ORR)C atoms next to pyridinic N [29]
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Rocha, R.P.; Figueiredo, J.L. Active Sites in Carbocatalysis: Tuning Their Activity. Catalysts 2025, 15, 443. https://doi.org/10.3390/catal15050443

AMA Style

Rocha RP, Figueiredo JL. Active Sites in Carbocatalysis: Tuning Their Activity. Catalysts. 2025; 15(5):443. https://doi.org/10.3390/catal15050443

Chicago/Turabian Style

Rocha, Raquel Pinto, and José Luís Figueiredo. 2025. "Active Sites in Carbocatalysis: Tuning Their Activity" Catalysts 15, no. 5: 443. https://doi.org/10.3390/catal15050443

APA Style

Rocha, R. P., & Figueiredo, J. L. (2025). Active Sites in Carbocatalysis: Tuning Their Activity. Catalysts, 15(5), 443. https://doi.org/10.3390/catal15050443

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop