Defect Density Around Curvature‐Induced Tip‐Active Sites Dictates Volcano‐Type Catalytic Performance of Metal‑Free Carbon Catalysts for Acetylene Hydrochlorination

ABSTRACT

Metal‑free carbon catalysts have emerged as promising sustainable alternatives for acetylene hydrochlorination, yet the influence of defect density around curvature‑induced tip‑active sites on catalytic performance remains poorly understood. Here, we combine density functional theory calculations with experimental synthesis to investigate this structure–activity relationship. Theoretical calculations reveal that increasing defect density progressively strengthens acetylene adsorption but yields a volcano‑shaped trend in the reaction energy barrier. Experimentally, by employing an oxygen‑controlled carbonization strategy, we precisely tune the defect density in high‑curvature defective carbon catalysts while preserving curvature architecture, nitrogen doping level, and textural properties. The performance of HCDC catalysts exhibits a corresponding volcano‑shaped dependence on defect density, and the optimally defective HCDC‑O6 catalyst achieving 90.2% acetylene conversion. Mechanistic studies show that an optimal defect density balances adsorption strength and surface reaction kinetics, whereas excessive defects cause over binding and site poisoning. This work establishes defect density around tip‑active sites as a critical tunable parameter, providing rational design principles for high‑performance metal‑free carbon catalysts.

This work reveals that defect density around curvature‐induced tip‐active sites governs the catalytic performance of metal‐free carbon catalysts for acetylene hydrochlorination via a volcano‐shaped relationship. An optimal defect density balances acetylene adsorption strength and surface reaction kinetics, maximizing catalytic efficiency.

1. Introduction

Metal‐free carbon catalysts (MFCCs) have garnered significant attention as a promising class of environmentally friendly catalytic materials, owing to their environmental compatibility, cost‐effectiveness, and synthetic flexibility for tuning their surface chemistry at the molecular level [1, 2]. MFCCs have demonstrated remarkable catalytic performance across a diverse range of reactions, including heterogeneous catalysis [3], electrochemical transformations [4], and energy conversion and storage [5]. Within this class, MFCCs have attracted particular interest for their application in acetylene hydrochlorination, a pivotal industrial process for vinyl chloride monomer (VCM) production, where carbon catalysts are actively pursued as sustainable alternatives to the incumbent environmentally noxious mercuric chloride catalysts [6, 7].

The catalytic activity of MFCCs stems from an asymmetric electronic distribution within the carbon framework, a property that can be tuned via heteroatom doping (e.g., N, S, P, B), or the introduction of topological defects (e.g., pentagons or pentagon‐octagon‐pentagon sequences) [8, 9]. Early research primarily focused on heteroatom doping, recent studies have revealed that topological defects can exhibit superior catalytic efficiency in acetylene hydrochlorination, giving rise to the concept of defect‐mediated catalysis [10]. Despite the challenge of decoupling the individual contributions of heteroatoms and defects, substantial evidence points to a synergistic effect that enhances catalytic performance [11, 12]. Although the activity of MFCCs has been substantially improved through doping and defect engineering, their activity and stability still lag behind those of their metal counterparts.

A core strategy for enhancing the intrinsic activity of metal‐free carbon catalysts is to precisely modulate the electronic structure of catalytic active sites, thereby optimizing reactant adsorption/activation and lowering the reaction energy barrier. Unlike traditional defect and doping engineering, curvature engineering also disrupts the uniform charge distribution, leading to electron enrichment at the tip regions of curvature. Taking carbon nanotubes as an example, their curvature induces a significant difference in charge distribution between the inner and outer surfaces. The greater the curvature, the more pronounced this difference becomes, resulting in higher charge localization on the curved surface, particularly on the outer side [13, 14]. Moreover, if the reactant adsorption site happens to be located at a curved, coordinatively unsaturated atom or a defect site, such charge localization becomes even more pronounced [15], thereby significantly affecting its adsorption/activation capability.

Interestingly, Wei et al. discovered a strong synergy between curvature and defect structures. The introduction of curvature generates a tip‐enhanced local electric field around the active sites, which significantly strengthens acetylene adsorption and facilitates hydrogen chloride activation. Consequently, this effect substantially lowers the energy barrier for the acetylene hydrochlorination reaction and promotes its progression. C₂H₂‐TPD demonstrated that the acetylene adsorption capacity of a high‐curvature defective carbon (HCDC) catalyst is 98 times greater than that of a conventional defective carbon catalyst (DC), thereby corroborating the theoretical predictions. The HCDC catalyst exhibits the highest activity among reported metal‐free carbon catalysts, achieving an acetylene conversion rate 1.8 times that of a 2.5% Au/Carbon catalyst under identical conditions. Our previous research has confirmed the catalytic enhancement effect of curvature structures in defective carbon. However, the structure‐activity relationship linking the defect density around tip‐active sites to catalytic performance remains largely unexplored. Modulating the defect density in the vicinity of these tip‐active sites is expected to alter their electronic structure and modulate the adsorption‐desorption processes of reaction intermediates, thereby influencing the overall catalytic efficiency. Therefore, elucidating the impact of defect density on the catalytic behavior of tip‐active sites in acetylene hydrochlorination is of critical importance.

In this work, theoretical calculations initially revealed that increasing the density of point defects around tip‐active sites leads to a continuous enhancement in acetylene adsorption. However, the energy barrier for the acetylene hydrochlorination reaction exhibits a non‐monotonic trend, initially increasing and then decreasing. This suggests that an optimal increase in defect density can exert a beneficial enhancing effect on the catalytic properties of tip‐active sites. Guided by this insight, we employed the HCDC synthesis strategy [7] and achieved precise control over the defect density of the HCDC catalysts through an oxygen‐controlled carbonization method. A volcanic‐shaped correlation between acetylene conversion and defect density was observed in the HCDC catalysts, a finding that is entirely consistent with our theoretical calculations. This work demonstrates, for the first time, the catalytic enhancement effect of defect density on tip‐active sites and elucidates the critical balance required to achieve optimal performance in metal‐free catalytic systems.

2. Results and Discussion

2.1. Theoretical Investigation

Although defect and curvature engineering of carbon materials have garnered widespread attention, how the catalytic response of tip‐active sites is modulated by the abundance of surrounding topological defects remains an unresolved question. To elucidate this mechanism at the molecular level, specifically, how the defect concentration in the vicinity of tip‐active sites affects the intrinsic catalytic activity for acetylene hydrochlorination, we performed density functional theory (DFT) calculations. We selected a nitrogen‐doped defective C240 model with a convex configuration as the reference structure [7], based on the rationale that the synergistic effect of nitrogen doping, structural defects, and curvature induces an asymmetric charge distribution in the carbon lattice, which has been shown to be beneficial for acetylene hydrochlorination [6]. To systematically investigate the role of defect concentration, we constructed a series of model systems with progressively increasing defect density by sequentially removing one to three carbon atoms from sites adjacent to the nitrogen dopant in the curvature model, yielding structures denoted as curvature‐1, curvature‐2, and curvature‐3 (Figure 1a–d). All constructed models were subjected to full geometric optimization without any symmetry constraints. As shown in Figure 1e, after introducing a defect structure around the tip catalytic site, the adsorption energy of hydrogen chloride initially increased. However, upon further increasing the number of defects structure, the change in HCl adsorption energy became minimal, suggesting that excess defect concentration has little effect on the affinity for HCl. In contrast, as the defect density increased, the adsorption energy of C₂H₂ shifted significantly negatively, indicating a progressively stronger interaction between acetylene and the catalyst surface (Figure 1f). Specifically, the C₂H₂ adsorption energies on the curvature, curvature‐1, curvature‐2, and curvature‐3 surfaces were −1.33, −1.89, −2.07, and −2.21 eV, respectively. This progressive strengthening of acetylene adsorption underscores the pivotal role of atomic‐scale defects topological perturbations in modulating the interaction between the active site and the reactant.

FIGURE 1.

DFT calculations of acetylene hydrochlorination on curved and defective carbon models. Calculation models of (a) curvature, and (b) curvature‐1, (c) curvature‐2, and (d) curvature‐3 surfaces. Calculated adsorption energies of (e) HCl and (f) C₂H₂ on the curvature, curvature‐1, curvature‐2, and curvature‐3 surfaces. (g) Calculated reaction pathways and corresponding energy barriers for acetylene hydrochlorination on the four model surfaces. (h) The reaction energy barriers of acetylene hydrochlorination reaction on different curvature surface. (i) Reaction pathways for acetylene hydrochlorination on the curvature‐2 surfaces. Each panel illustrates the optimized geometries of the key intermediates and transition state along the reaction coordinate. The reaction proceeds via: (A) C₂H₂ adsorption, (B) co‐adsorption of HCl and C₂H₂, (C) the transition state for C─Cl bond formation coupled with HCl activation, and (D) formation of the vinyl chloride monomer (VCM). Note: In this work, the adsorbates are located on the convex (outer) side of the surfaces. Color code: blue, white, and gray spheres represent nitrogen, hydrogen, and carbon atoms, respectively.

To elucidate the reaction mechanism, we performed DFT calculations to investigate the pathways for acetylene hydrochlorination over the constructed model catalysts. The computed reaction profiles are presented in Figure 1h,i and Figure S1. The reaction is initiated by the strongly exothermic adsorption of C₂H₂ onto the curved surfaces (State A), in agreement with the adsorption energies discussed above. Subsequently, HCl interacts with the pre‐adsorbed C₂H₂ (State B), triggering its activation and the subsequent formation of a transition state (State C). In this transition state, the Cl atom bonds to one of the carbon atoms of the C₂H₂ molecule, whereas the H atom migrates to a nearby nitrogen site on the carbon surface. Finally, the H atom migrates from the nitrogen site to the C₂H₂ fragment, enabling C─Cl bond formation and the ensuing release of the vinyl chloride monomer (VCM, State D).

The calculated energy barriers for the rate‐determining step amount to 1.06, 1.01, 0.99, and 1.05 eV for the curvature, curvature‐1, curvature‐2, and curvature‐3 models, respectively (Figure 1g). With increasing defect density, the energy barrier follows a non‑monotonic trend, initially decreasing before increasing. These results indicate that an appropriate increase in defect density on the curvature surface both enhances acetylene adsorption and lowers the reaction energy barrier, thereby enhancing the overall catalytic kinetics. However, beyond this optimal defect density (as observed for the curvature‐3 model), the energy barrier rises sharply. This behavior is likely due to overly strong adsorption of C₂H₂, which hinders the approach and activation of HCl, thereby suppressing the overall reaction [16]. Consequently, a balanced ability to adsorb C₂H₂ emerges as a critical requirement for achieving high catalytic performance in acetylene hydrochlorination. This DFT study demonstrates that an optimal defect density exerts a pronounced enhancement effect on the catalytic activity of tip‐active sites. This key finding establishes a foundation for the rational design of advanced metal‐free catalysts through controlled defect engineering.

2.2. Synthesis and Characterization of HCDC Catalyst With Different Defect Density

To experimentally validate the conclusions drawn from our DFT calculations, it is essential to design carbon catalysts that possess both tunable curvature defects and a well‐defined defect density. In our previous work, we developed a high‑curvature defective carbon (HCDC) catalyst by using polyacrylonitrile (PAN) as a carbon precursor and nano‑silica as a hard template to replicate the curved structure [7]. This synthetic strategy provides an ideal model system for systematically investigating the influence of carbon curvature on catalytic performance. Building upon this HCDC platform, elucidating the enhancement effect of defect density on tip‑active sites hinges on achieving precise control over defect density while preserving the underlying curvature structure. However, conventional methods for generating defects in N‑doped carbon materials typically rely on high‑temperature treatments, which often induce heteroatom removal and inevitably cause the loss of active nitrogen species [17, 18]. An alternative and promising strategy involves the controlled introduction of oxygen‑containing functional groups, which are subsequently removed during the carbonization process. This approach is based on the premise that precisely controlling the initial oxygen content, and its subsequent removal during carbonization, can effectively tailor the density of defects generated in the final carbon material. In principle, a greater extent of oxygen removal should correlate with a higher density of defects within the carbon framework.

To establish a well‐defined platform for investigating the structure–activity relationship between defect density and catalytic performance, we regulated the defect density in the carbon catalysts by varying the pre‐oxidation time of the PAN precursor using an oxygen‐controlled carbonization process (Figure 2a). To ensure that any observed changes in catalytic activity could be attributed solely to variations in defect density, the nitrogen content of the catalysts was maintained at a constant level by fixing the carbonization temperature. The structural evolution of the PAN precursor during the pre‐oxidation step was monitored using Fourier transform infrared (FT‐IR) spectroscopy. As shown in Figure 2b, upon heating the PAN precursor to 270°C, the intensities of the peaks corresponding to ν(C─H) (2938 cm⁻¹), ν(C≡N) (2243 cm⁻¹), and δ(C−H) (1450 cm⁻¹) decreased significantly, indicating the onset of dehydrogenation and cyclization reactions [19]. Concurrently, a broad band emerged near 1600 cm⁻¹, attributed to the combined vibrations of newly formed C═C and C═N bonds, along with a peak at 794 cm⁻¹ corresponding to C═C─H bonds. These spectral features provide clear evidence for the occurrence of cyclization and dehydrogenation reactions during the pre‐oxidation process [20]. With prolonged pre‐oxidation time, a new peak appeared at 1659 cm^{− 1}, which is associated with conjugated and unconjugated C═O groups [21]. This observation confirms the incorporation of oxygen‐containing functional groups into the carbon framework during the extended oxidative treatment. Importantly, this oxidative treatment introduces oxygen species into the aromatic structure of the PAN precursor. The subsequent thermal removal of these oxygen‐containing groups during carbonization is expected to generate structural defects, thereby offering a versatile route for defect engineering.

FIGURE 2.

Preparation and characterization of high‐curvature defective carbon (HCDC). (a) Schematic illustration of the structural evolution of HCDC catalysts during the two‐step oxygen‐controlled carbonization process. (b) Fourier transform infrared (FT‐IR) spectra, (c) oxygen elemental analysis (EA) results, and (d) in situ thermogravimetry‐mass spectrometry (TG‐MS) profiles recorded under an argon atmosphere for PAN precursors subjected to different pre‐oxidation times. (e–h) Transmission electron microscopy (TEM) images of the resulting HCDC‐Ot catalysts prepared with pre‐oxidation times of (e) 2 h, (f and h) 6 h, and (g) 10 h, respectively.

The oxygen content in the pre‐oxidized PAN precursors was quantified by elemental analysis (EA). The results revealed that the oxygen content progressively increased from 8.0 to 17.6 wt.% as the pre‐oxidation time was extended from 2 to 10 h (Figure 2c and Table 1). This monotonic increase in oxygen incorporation is consistent with the FT‐IR observations, which showed the progressive emergence of C═O‐related peaks. Thermogravimetric analysis (TGA) revealed that pre‐oxidation significantly increased the carbonization yield of the PAN precursor (Figure 2d and Figure S2). During pyrolysis, the carbon skeleton of the PAN precursor undergoes rearrangement, leading to the formation of more stable aromatic structures. To gain deeper insight into the pyrolysis behavior of the pre‐oxidized precursors, we performed thermogravimetric analysis coupled with mass spectrometry (TG‐MS) under an argon atmosphere. During pyrolysis, carbon‐ and oxygen‐containing species were released primarily in the form of CH₃•, CH₄, H₂O, CO, and CO₂, as identified by online mass spectrometry (Figure 2d and Figures S3–S6). TG‐MS of the evolved gases revealed that the amounts of CO and CO₂ released during pyrolysis increased progressively with the pre‐oxidation time. The thermal removal of oxygen species introduced during pre‐oxidation is anticipated to generate vacancies within the carbon skeleton. This process represents an effective strategy for creating well‐defined defective structures in the final carbon material. This correlation between pre‐oxidation time (and thus oxygen content) and the evolution of CO/CO₂ provides strong experimental support for the theoretical expectation that a greater extent of oxygen removal results in a higher density of defect sites.

TABLE 1.

Textural properties and element composition of various carbon catalysts.

Catalysts	S.A. (m2 g−1)	P.V. (cm3g−1)	P.D. (nm) a	Elements composition (at %) b			O Content (wt %) c	ID1: IG	Conv. (%)
				C	N	O
HCDC‐O2	94	0.17	9.1	83.5	11.7	4.8	2.3	1.72	74.5
HCDC‐O4	95	0.17	9.4	83.0	12.1	4.9	2.6	1.80	82.1
HCDC‐O6	108	0.19	8.6	82.2	11.8	6.0	2.6	1.93	90.2
HCDC‐O8	84	0.17	9.2	82.6	11.4	6.2	2.4	2.05	83.7
HCDC‐O10	106	0.19	8.9	81.8	11.7	4.5	2.5	2.12	74.7

^a Pore size distributions via Barret‐Joyner‐Halenda (BJH) method for HCDC catalyst.

^b Elemental composition derived by XPS.

^c Elemental composition derived by EA.

To experimentally validate the relationship between pre‐oxidation time and defect density, a series of high‐curvature defective carbon catalysts, denoted as HCDC‐Ot, where t represents the pre‐oxidation time, with varying defect densities was synthesized. All samples were prepared by carbonization at 800°C for 2 h under an argon atmosphere. The morphologies and microstructures of the as‐synthesized samples were comprehensively characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images (Figure S7) revealed that the carbonaceous framework is constructed by the dense aggregation of spherical nanoparticles, resulting in a granular morphology. TEM results (Figure 2e–‐h) revealed that all HCDC‐Ot samples exhibit a three‐dimensional disordered pore network comprising spherical voids and abundant curved graphitic regions. This morphology closely reflects the geometric characteristics. Specifically, the curved structure of the close‐packed silica spheres used as the hard template. The well‐defined mesoporous structure and curved morphological features of the HCDC‐Ot catalysts were well‐preserved after the high‐temperature carbonization process, demonstrating the replicated carbon framework (Figure S8). Elemental distributions were subsequently examined by TEM‐based energy‐dispersive x‐ray spectroscopy (EDS) mapping. The results reveal a homogeneous distribution of C, N, and O throughout the HCDC‐O6 framework, confirming the successful incorporation of nitrogen into the carbon skeleton (Figure S9).

N₂ adsorption–desorption isotherms were conducted to determine the textural properties of the HCDC catalysts, including specific surface area, pore size distribution, and total pore volume, all of which are critical parameters for catalytic performance. As shown in Figure 3a, all HCDC‐Ot catalysts display a Type IV isotherm with a distinct H4‐type hysteresis loop in the relative pressure (P/P₀) range of 0.6–1.0, which is characteristic of mesoporous materials with slit‐shaped pores. The corresponding pore size distributions, calculated using the Barrett‐Joyner‐Halenda (BJH) method from the adsorption branches, are centered at 9.0 nm (Figure 3b). The specific surface areas, total pore volumes, and average pore diameters derived from these isotherms are summarized in Table 1. Notably, all HCDC‐Ot catalysts exhibit comparable specific surface areas, total pore volumes and pore size distributions, regardless of the pre‐oxidation time. These results collectively indicate that the high‐curvature carbon framework, templated by silica spheres, is well‐preserved across all HCDC‐Ot samples.

FIGURE 3.

Characterization and defect density correlation of HCDC catalysts. (a) N₂ adsorption–desorption isotherms, (b) corresponding pore size distributions calculated using the BJH method. (c) Raman spectra, and (d) high‐resolution C 1s XPS spectra and (e) electron paramagnetic resonance (EPR) spectra of HCDC‐Ot samples prepared with different pre‐oxidation times. (f) Correlation between pre‐oxidation time and O content, and the relative defect density of the HCDC catalysts (as quantified by the I_D1: I_G ratio from Raman spectroscopy). (g) Correlation between the amount of oxygen removed during carbonization and the resulting defect density of the HCDC catalysts, Elemental composition derived by EA.

For nitrogen‐doped defective carbon systems, the creation of defect structures is often accompanied by the loss of heteroatoms, complicating the deconvolution of their individual contributions. Therefore, maintaining a consistent nitrogen doping level across all catalysts is essential to isolate and systematically investigate the specific influence of defect structure on catalytic performance. To assess the nitrogen speciation and content, the surface chemical composition of the HCDC catalysts was analyzed by x‐ray photoelectron spectroscopy (XPS). The high‐resolution N 1s XPS spectra (Figure S10) were deconvoluted into four distinct peaks, corresponding to pyridinic N (398.1 eV), pyrrolic N (400.0 eV), graphitic (or quaternary) N (401.3 eV), and oxidized N (403.0 eV) [11]. The relative contents of these nitrogen species, determined from the peak areas, are summarized in Table 1 and Table S1. Notably, all HCDC‐Ot catalysts exhibit very similar distributions and relative contents of nitrogen species, regardless of pre‐oxidation time. This consistency effectively rules out any significant influence of nitrogen doping level or nitrogen speciation on the catalytic performance. The consistent properties of catalysts confirm that variations in pre‐oxidation time do not significantly alter the underlying curved architecture and doped level, thereby enabling a systematic investigation of defect density effects without confounding structural variables.

Raman spectroscopy was employed to further probe the defect density in the HCDC catalysts. As shown in Figure 3c, all HCDC‐Ot samples exhibit two characteristic Raman bands: the D1 band at approximately 1348 cm⁻¹, associated with structural defects and disordered carbon, and the G band at around 1587 cm⁻¹, originating from the E_2g phonon mode of sp²‐hybridized carbon in graphitic lattices. The degree of structural disorder is commonly quantified by the intensity ratio of the D1 to the G band (I_D1: I_G), which serves as a reliable indicator of defect density in carbon materials [22]. Spectral deconvolution revealed that the catalyst with the shortest pre‐oxidation time (HCDC‐O2) exhibited the lowest I_D1/I_G ratio of 1.72. As the pre‐oxidation time increased, this ratio increased progressively, reaching values of 1.80, 1.93, 2.05, and 2.12 for HCDC‐O4, HCDC‐O6, HCDC‐O8, and HCDC‐O10, respectively. This monotonic increase in I_D1: I_G indicates a corresponding rise in structural disorder and defect density with prolonged pre‐oxidation.

High‐resolution XPS C 1s spectra were deconvoluted into six component peaks, as shown in Figure 3d. The two dominant peaks at binding energies of approximately 284.5 and 285.3 eV were assigned to sp²‐hybridized and sp³‐hybridized carbon, respectively. The remaining four peaks were attributed to carbon‐oxygen functionalities: C─O (286.8 eV), C═O (287.8 eV), O─C═O (289.3 eV), and the π–π* satellite transition (291.2 eV) [23]. The presence of sp³‐hybridized carbon disrupts the long‐range conjugated network of sp²‐carbon and is therefore considered a form of topological defect. Consequently, the relative defect density can be quantified by the area ratio of sp³‐hybridized carbon to total carbon (sp³/(sp³+sp²)), as derived from the deconvoluted C 1s spectra [24]. As summarized in Table S2, this sp³/(sp³+sp²) ratio increases progressively with pre‐oxidation time, corroborating the trend observed by Raman spectroscopy and confirming a continuous increase in defect density. Electron paramagnetic resonance (EPR) spectroscopy provided complementary evidence for the evolution of defect density (Figure 3e). All samples display a symmetric EPR signal at g ≈ 2.003, which is characteristic of carbon‐centered unpaired electrons associated with structural defects [25]. The intensity of this signal progressively increases from HCDC‐O2 to HCDC‐O10, indicating a higher concentration of defects in catalysts subjected to longer pre‐oxidation times. The degree of graphitization was further examined by x‐ray diffraction (XRD), as shown in Figure S11. All HCDC‐Ot samples exhibit two broad and weak diffraction peaks corresponding to the (002) and (100) planes of graphitic carbon, indicating a predominantly amorphous carbon structure with limited long‐range order.

The curved architecture of the nano‐silica template was successfully replicated in the carbon framework using a hard‐templating method, yielding HCDC catalysts with well‐defined curvature. Subsequently, an oxygen‐controlled carbonization strategy was employed to effectively tailor the defect density of HCDC catalysts. As demonstrated by Raman spectroscopy, XPS, and EPR analyses, the O content and defect density of the HCDC catalysts increases progressively with prolonged pre‐oxidation time (Figure 3f). This progressive increase in defect density is ultimately governed by the amount of oxygen atoms removed from the pre‐oxidized precursors during the carbonization process. A direct correlation was observed between the amount of oxygen removed during carbonization and the resulting defect density in the final HCDC catalysts, as shown in Figure 3g. This finding underscores that tailoring the oxygen content in the carbon precursor is an effective and versatile strategy for modulating the defect density of the resulting nitrogen‐doped carbon catalysts. Notably, this modulation of defect density is achieved without significantly altering other key physicochemical parameters, such as the curvature structure, specific surface area, pore architecture, degree of graphitization, and nitrogen configuration. Consequently, this work establishes an ideal platform for systematically investigating the structure‐activity relationship between the defect density surrounding the tip‐active sites and their catalytic performance in acetylene hydrochlorination.

2.3. Catalytic Performance and Discussion of the Hcdc Catalysts

The catalytic performance of the HCDC‑Ot catalysts for acetylene hydrochlorination was evaluated at 220°C, with a gas hourly space velocity (GHSV) of C₂H₂ of 300 h⁻¹, and a feed volume ratio of V(HCl)/V(C₂H₂) = 1.2. As shown in Figure 4a, the HCDC‑O2 catalyst exhibited the lowest acetylene conversion, at approximately 74.5%. Increasing the pre‑oxidation time to 4 h (HCDC‑O4) resulted in a marked increase in acetylene conversion to approximately 82.1%. The conversion reached a maximum of approximately 90.2% with the HCDC‑O6 catalyst, indicating that this pre‑oxidation time (6 h) yields the optimal catalytic performance. However, further extending the pre‑oxidation time to 8 and 10 h led to a decline in acetylene conversion. Specifically, the conversion decreased to 84% for HCDC‑O8 and further to 74.7% for HCDC‑O10. Notably, all HCDC‑Ot catalysts maintained high selectivity toward vinyl chloride monomer (VCM), exceeding 99% in all cases. These results suggest that excessively long pre‑oxidation time is detrimental to catalytic performance. An initial increase followed by a decrease in activity with increasing pre‑oxidation time, reveals a volcano‑shaped relationship between defect density and catalytic activity (Figure 4b). Additionally, we have compared the space‐time yield (STY) of HCDC‑O6 with those of recently reported metal‐free carbon catalysts under similar reaction conditions. As shown in the Table S3, HCDC‑O6 exhibits superior acetylene conversion and vinyl chloride productivity compared to most literature‐reported catalysts. Furthermore, it is noteworthy that as the pre‑oxidation time increased, the deactivation rate of the catalysts exhibited a trend of first decreasing and then increasing. Specifically, the deactivation rates for HCDC‑O2, HCDC‑O4, HCDC‑O6, HCDC‑O8, and HCDC‑O10 were –1.69, 1.11, 0.82, 1.47, and 1.01% h⁻¹, respectively. We subsequently conducted a long‐term stability test on the HCDC‑O6 catalyst (Figure S13). After an initial period of stable performance, the catalyst began to deactivate gradually, with its activity declining from 99% to 75% after 500 h. Despite its excellent activity, the stability of this catalyst still requires further improvement.

FIGURE 4.

Catalytic performance and kinetic analysis of HCDC catalysts. (a) Acetylene conversion over HCDC‐Ot catalysts prepared with different pre‐oxidation times. Reaction conditions: T = 220°C, GHSV (C₂H₂) = 300 h⁻¹, and V(HCl)/V(C₂H₂) = 1.2. (b) Correlation between the I_D1/I_G ratio (a measure of defect density) and the acetylene conversion for the HCDC‐Ot catalysts. (c) C₂H₂‐TPD‐MS and (d) HCl‐TPD‐MS profiles of HCDC‐O2, HCDC‐O6, and HCDC‐O10 catalysts. (e) Determination of the reaction order with respect to C₂H₂ for HCDC‐O2, HCDC‐O6, and HCDC‐O10. Reaction conditions: 0.1 g catalyst, 220°C, ambient pressure. The concentrations of C₂H₂ and HCl were varied from 10% to 25% (balanced with N₂). The reaction order is given by the slope of the linear fit to the data. (f) Arrhenius plots for the determination of apparent activation energies (E_a) for HCDC‐O2, HCDC‐O6, and HCDC‐O10. The E_a values, calculated from the slopes of the linear fits. (g) Radar plot charting the relation of the conversion of C₂H₂ with key experimental indicators for the activity. (h) The relationship among defect density, acetylene adsorption capacity and catalytic activity.

As demonstrated in the preceding sections, all HCDC‑Ot catalysts possess similar curvature structures, nitrogen doping levels and configurations, specific surface areas, and pore architectures, which are key physicochemical properties closely associated with catalytic activity. This similarity confirms that all catalysts possess fundamentally similar tip‑active site architectures. Therefore, the systematic variation in defect density around these tip‑active sites emerges as the primary factor governing the observed differences in catalytic performance across the HCDC‑Ot series. This observation is consistent with the theoretical prediction. To validate the theoretical predictions and gain a deeper mechanistic understanding of how defect density around tip‑active sites governs catalytic performance, we investigated the adsorption behavior and reaction kinetics of representative HCDC catalysts. Temperature‑programmed desorption (TPD) was employed to probe the adsorption behavior of the reactants, as the adsorption strength at nitrogen functionalities and defect sites is known to directly influence catalytic activity. The C₂H₂‐TPD profiles for HCDC‐O2, HCDC‐O6, and HCDC‐O10 are presented in Figure 4c. All three catalysts exhibit two distinct desorption peaks, suggesting the presence of at least two types of acetylene adsorption sites. The stronger adsorption sites are likely attributable to π–π interactions between acetylene molecules and the electron‐rich aromatic systems (e.g., C = C, C–N bonds, and aromatic rings) within the carbon framework. Both the desorption temperature and the integrated peak area (corresponding to the quantity of adsorbed acetylene) increase in the order HCDC‐O2< HCDC‐O6< HCDC‐O10. This trend indicates that both the strength and capacity of acetylene adsorption are enhanced on catalysts with higher defect density, which is consistent with the DFT results. However, as suggested by the DFT results, an excessively high defect density, while enhancing C₂H₂ adsorption, hinders the approach and activation of HCl, ultimately suppressing the overall reaction. This over‑binding effect ultimately leads to the observed decline in catalytic activity for the excessively high defect density catalyst, HCDC‐O8 and HCDC‑O10. The HCl‑TPD profiles for the same three catalysts are shown in Figure 4d. All three catalysts exhibit two distinct desorption peaks, indicating the presence of two types of HCl adsorption sites. The adsorption of HCl is attributed to the basicity of the catalyst surface, arising from electron‑rich nitrogen species (pyridinic N and pyrrolic N) and from the local electronic perturbations associated with structural defects. The amount of HCl adsorbed on the HCDC catalysts progressively increases with increasing defect density.

To further elucidate the influence of defect density on reactant adsorption and the subsequent reaction kinetics, we performed kinetic studies on the representative HCDC catalysts. As shown in Figure 4e, the reaction order with respect to C₂H₂ is negative for all three HCDC catalysts, indicating strong adsorption of acetylene on the catalyst surface under the reaction conditions. The calculated reaction orders for C₂H₂ are −0.15, −0.17, and −0.24 for HCDC‐O2, HCDC‐O6, and HCDC‐O10, respectively. The increasingly negative reaction order with higher defect density indicates a progressive strengthening of acetylene adsorption. This trend is fully consistent with the C₂H₂‐TPD and DFT results. The apparent activation energies (E_a) for acetylene hydrochlorination were determined from variable‐temperature kinetic experiments, as shown in Figure 4f. The calculated activation energies values are 44.2, 37.2, and 16.8 kJ mol⁻¹ for HCDC‐O2, HCDC‐O6, and HCDC‐O10, respectively. Interestingly, while a lower apparent activation energy typically correlates with higher catalytic activity, the HCDC‐O10 catalyst presents an exception. Despite having the lowest activation energies (16.8 kJ mol⁻¹), its catalytic activity is inferior to that of HCDC‐O6, which has a higher activation energy of 37.2 kJ mol⁻¹.

It is well established that the apparent activation energy measured in a catalytic reaction is not a fundamental parameter but rather a composite value that reflects the activation energies and thermodynamic contributions of its individual elementary steps. Furthermore, the Sabatier principle dictates that an optimal catalyst should bind reactants with an intermediate strength; interactions that are either too weak or too strong are detrimental to the overall reaction rate. As the defect density increases to an optimal level (as in HCDC‐O6), the reaction may proceed via a more kinetically favorable pathway, leading to a decreased apparent activation energy and enhanced catalytic activity. In this regime, the reaction is likely controlled by the surface reaction step (kinetic‐controlled regime). However, beyond this optimum (as in HCDC‐O10), the adsorption of C₂H₂ becomes excessively strong. This over‐binding leads to excessively high surface coverage by acetylene, which can over‐stabilize reaction intermediates or physically hinder the subsequent activation of HCl and the surface reaction steps. Consequently, although the measured apparent activation energy may be low (as in the case of HCDC‐O10), the overall reaction rate decreases because the active sites become poisoned by the strongly adsorbed acetylene and are unable to efficiently facilitate the subsequent reaction steps. This phenomenon provides a compelling explanation for the low activity observed for HCDC‐O10 despite its low apparent activation energy, and is fully consistent with the DFT results and the volcano‐shaped relationship between defect density and catalytic performance established in this work.

3. Conclusions

In this work, we have elucidated the critical role of defect density surrounding curvature‐induced tip‐active sites in governing the catalytic performance of metal‐free carbon catalysts for acetylene hydrochlorination. Through theoretical calculations, we predicted the volcano‑shaped trend between defect density and reaction energy barrier. By employing an oxygen‐controlled carbonization strategy, we achieved independent modulation of defect density while preserving the curvature architecture, nitrogen doping level, and textural properties of the catalysts. Our results reveal a volcano‐shaped dependence of catalytic activity on defect density, demonstrating that an optimal level of defects balances acetylene adsorption strength and surface reaction kinetics, thereby maximizing catalytic efficiency. This study establishes defect density as a key tunable parameter in the design of high‐performance metal‐free carbon catalysts and provides fundamental insights into the structure‐activity relationship at tip‐active sites. The findings offer a rational design principle for advancing carbon‐based catalytic systems beyond traditional heteroatom doping strategies.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74699‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.