2025, Vol. 36 
Polyvinyl chloride (PVC) is produced from vinyl chloride (VCM) monomer and plays a pivotal role in the plastics industry, with global demand for PVC continuing to grow [1]. Mercury, previously a prevalent catalyst for the industrial synthesis of VCM via the calcium carbide process [2, 3], is being progressively eliminated from use due to its significant detrimental impact on the environment and human health. Nevertheless, an understanding of its catalytic principles and performance offers a foundation for the development of new catalysts. Metal-based catalysts are one of the main focuses of research. Various noble metals such as gold [4-6], palladium [7, 8] and ruthenium [9-11] have been widely studied. These catalysts exhibited high catalytic activity and selectivity. However, the high cost of these materials represents a significant barrier to large-scale industrial applications.
Copper (Cu) is a relatively inexpensive and resource-rich metal in comparison to precious metal catalysts, which renders Cu-based catalysts more cost-effective for large-scale industrial applications. In order to overcome the low dispersion of active components and insufficient active species of catalyst, researchers have prepared Cu-based catalysts by a variety of methods, such as the use of different preparation processes [12, 13], the addition of certain metal additives or ligands [14-16], and the modification of the carriers [17-19]. Zhang et al. [20] constructed Cu sites in both electron-rich and electron-deficient states by controlling the impregnation solution. Their findings revealed a volcano-shaped scalar relationship between catalytic activity and Cu particle size. The tribasic CuCl2 species facilitated the activation of the H—Cl bond in the HCl molecule, while the electron-deficient Cu sites promoted the high activation of the C≡C bond, thereby accelerating the surface reaction in VCM production. Zhang et al. [21] employed the robust interaction between methyldiphenyloxophosphine and CuCl2 to effectively modulate the electronic properties of the active metal centres, constructing highly dispersed Cu-P/Cl local active centres, thereby enhancing the anti-coking performance. Xu et al. [22] prepared N, P co-doped carriers using melamine pyrophosphate to dope activated carbon in order to maintain the valence of Cun+ sites. The interaction between Cu and heteroatoms on the doped carriers not only provided additional adsorption sites for reactive gases, but also significantly reduced the adsorption strength of Cu sites on C2H2 gas and inhibited the reduction of Cu species. These methods impact the catalytic performance by modulating the Cu dispersion and particle size. However, Cu-based catalysts frequently exhibited difficulty in maintaining high activity over extended periods of time, and continue to experience deactivation. The active sites of the catalysts are susceptible to destruction or coverage [14, 19, 23], which results in a reduction in activity. This phenomenon requires further investigation to elucidate the underlying mechanisms and develop effective solutions.
In recent years, there has been a proliferation of studies on the enhancement of catalyst stability through the construction of two-site structures. In a previous study, Wei et al. [24] identified a well-defined tunable Cu0-Cu+ interfacial site that modulated the adsorption strength of oxygenated intermediates. The reconfiguration of the catalyst interface and electron transfer resulted in a reduction in the energy barrier for C—H bond breaking in the intermediates, thereby enhancing the catalytic performance. Meanwhile, Wang et al. [25] also demonstrated that the asymmetric electronic structure between Co0 and Coδ+ sites at the interface can provide a controlled activation barrier to promote olefin generation. Wang et al. [26] used Cu2O nanoparticles as a model catalyst to construct a Cu0-Cu+-NH2 composite interface with the help of SiO2 to promote CO2 adsorption and activation on the catalyst surface. Subsequently, highly dispersed and stable Cu-Cu2O—CeO2 interfaces were constructed on reduced graphene oxide using strong electrostatic interactions for the electroreduction of CO2 to C2+ products [27], both demonstrating high activity and stability. Li et al. [28] prepared La2O3 modified Cu0-Cu+ dual-site catalysts to achieve Cu species stabilization and ensure high catalytic stability. The strong electron donor-acceptor interaction between La and Cu served to prevent the aggregation of Cu and to avoid the over-reduction of Cu+ species to Cu0. The construction of a two-site structure enabled the modulation of substrate adsorption selectivity, which in turn inhibited metal reduction and agglomeration.
The distinctive valence characteristics of Cu, in conjunction with its nanoscale structure, coordination environment, and the influence of chemical and electronic states, collectively regulated the activity and stability of carbon-loaded Cu catalysts in acetylene hydrochlorination [29]. The high reactivity of Cu2+ enabled its active participation in the breaking and formation of chemical bonds during the reaction, thereby accelerating the overall reaction process. In contrast, the stabilized Cu0 provided structural stability to the catalyst, preventing overreaction and the rapid deactivation of the catalyst. To rationally design an effective optimization strategy, P-doped Cu0-Cuδ+ two-site catalysts were prepared by high-temperature heat treatment. The construction of Cu0/Cuδ+ sites enabled the coordination of the adsorption properties of different valence Cu sites, thereby achieving an improvement in catalytic stability. In conjunction with characterization tests, an investigation was conducted to ascertain the activation evolution process and the source of activity of Cu species in the catalyst.
Details of the catalyst preparation process are given in Supporting information. Prior to performance testing, the catalysts were subjected to activation with HCl. The performance of a series of catalysts was evaluated to verify the effect of the Cu0/Cuδ+ structure in the 3Cu/5CuP/AC-800 catalyst on the hydrochlorination of acetylene. As shown in Fig. 1a, both the 3CuCl2/5CuCl2/AC and 3CuCl2/5CuP/AC catalysts exhibited high initial C2H2 conversion, yet displayed inferior stability. The 3Cu/5Cu/AC-800 and 3Cu/5CuP/AC-800 catalysts, prepared at high temperatures, exhibited diminished initial C2H2 conversion. This phenomenon may be attributed to the aggregation of Cu species into nanoparticles at elevated temperatures and the reduction of Cu valence state due to carbothermal reduction reaction at Cu sites [30], leading to a significant increase in the stability of the catalyst. In addition, the introduction of P species in the 3Cu/5CuP/AC-800 catalyst significantly enhanced the C2H2 conversion compared to the 3Cu/5Cu/AC-800 catalyst, close to about 30%. As demonstrated in Fig. S1a (Supporting information), in order to eliminate the impact of P introduction and heat treatment on the carrier AC, control experiments revealed that P exerts a minimal effect on the activity of the carrier, while heat treatment significantly impacts the stability of the carrier. By examining the effect of heat treatment on the catalyst, it was unequivocally determined that Cu plays a predominant role in the stability of the catalyst. As shown in Fig. S1b (Supporting information), the VCM selectivity of all catalysts was found to exceed 99%.
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| Fig. 1. The performance evaluation of catalysts. (a) The C2H2 conversion. (b) The deactivation rate vs. C2H2 conversion. Reaction conditions: T = 180 ℃, V(HCl)/V(C2H2) = 1.15, GHSV(C2H2) = 180 h-1. (c) The comparison of the deactivation rate with previous literature results. | |
The effects of P addition, Cu loading, introduction order and pyrolysis temperature on the P-doped Cu0/Cuδ+ two-site catalysts catalytic performance were investigated in greater detail. Fig. S2a (Supporting information) illustrated the catalytic performance of the 5CuP/AC, 3Cu/5Cu/AC and 3Cu/5CuP/AC-800 catalysts with varying Cu/P ratios, which exhibited a volcano-type trend in C2H2 conversion with the addition of TPPB, which exhibited optimal catalytic activity when the Cu/P ratio is 4. The Cu species acted as the active centre of the catalyst, and modulating the amount of Cu species had a significant impact on the catalytic activity. As illustrated in Fig. S2b (Supporting information), the 3Cu/5CuP/AC-800 catalyst exhibited the most favorable catalytic activity. As illustrated in Fig. S2c (Supporting information), the preparation conditions of the catalysts were further optimized. The 3CuCl2/5Cu-PAC and 3Cu/5CuP/AC-400 catalysts exhibited high initial conversions. As the temperature increased, the C2H2 conversion of the catalysts decreased significantly; however, their stability exhibited a gradual improvement. As illustrated in Fig. S2d (Supporting information), the doping sequence of Cu and P species was further refined, and the 3Cu/5CuP/AC-800 catalyst exhibited the most optimal catalytic activity. By optimizing the reaction temperature, it was confirmed that a reaction temperature of 180 ℃ is most conducive to reaction stability in Fig. S2e (Supporting information). Too low a temperature will result in lower catalytic activity. Higher temperatures resulted in higher catalytic activity but faster catalyst deactivation.
By evaluating the average deactivation rate of the catalysts, it could be clearly demonstrated in Fig. 1b that the deactivation rate of the CuCl2-impregnated catalysts was approximately 0.54%/h. The average deactivation rate of the catalysts following the high-temperature treatment was markedly diminished, reaching a value of < 0.05%/h, and the stability of the catalysts was considerably enhanced. As illustrated in Fig. 1c and Table S1 (Supporting information), the 3Cu/5CuP/AC-800 catalyst exhibited a distinct competitive advantage in terms of stability when compared with other reported superior Cu-based catalysts [31].
To gain greater insight into the form of Cu species present in the catalyst, the 3Cu/5CuP/AC-800 catalyst was characterized by transmission electron microscope (TEM). As illustrated in Fig. 2a, Cu nanoparticles were observed on the catalyst surface with a lattice spacing of d = 0.206 nm, which was attributed to the Cu(111) crystal surface [32, 33]. The EDS mapping revealed the presence of a considerable number of Cu particles. As illustrated in Fig. 2b, the diffraction peaks were evident at 43.2°, 50.4°, and 74.1°, which could be identified as the distinctive diffraction peaks of Cu metal in comparison to the standard card PDF #04–0836. Which indicated the presence of Cu0 in the catalyst. The catalyst was further analyzed by XPS as shown in Fig. 2c. The catalyst exhibited a single characteristic peak at 932.8 eV, which was attributed to Cu0/Cu+. This finding suggested that the Cu species present in the 3Cu/5CuP/AC-800 catalyst was predominantly Cu0/Cu+. In conclusion, the combination of X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and TEM data indicated the presence of a Cu0/Cu+ site structure in the 3Cu/5CuP/AC-800 catalyst.
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| Fig. 2. Morphology and chemical structural characterization over 3Cu/5CuP/AC-800 catalyst. (a) TEM images and the corresponding EDS mapping for Cu, Cl and P, respectively. (b) Powder XRD patterns. (c) Cu 2p XPS spectrum. (d) Cu K-edge normalized XANES spectra. (e) k3-weighted Fourier transform spectra. (f) P 2p XPS spectrum. (g) Cu 2p XPS spectra and (h) Cl 2p XPS spectra of 3CuCl2/5CuCl2/AC, 3CuCl2/5CuP/AC, 3Cu/5Cu/AC and 3Cu/5CuP/AC-800 catalysts. | |
To provide further clarification regarding the structure of the catalyst sites, the fine structure of the 3Cu/5CuP/AC-800 catalyst was analyzed by X-ray absorption fine structure (XAFS) [34, 35], with Cu, CuCl and CuCl2 employed as reference samples. As shown in Fig. 2d, the absorption edge of the 3Cu/5CuP/AC-800 catalyst was found to be in close alignment with that of the Cu foil, which suggested that the average valence state of the Cu atoms within the catalyst is 0. This finding was in accordance with the Cu 2p XPS result. As shown in Fig. 2e, the K-edge Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of Cu for the 3Cu/5CuP/AC-800 catalyst showed a distinct characteristic peak Cu-Cu signal peak at approximately 2.24 Å, which was similar to that of the reference sample of Cu foil, indicating the structure of Cu-Cu. In conjunction with the XPS data, this indicated the existence of the Cu0 structure. As illustrated in Fig. 2f, the chemical structure of the P species in the 3Cu/5CuP/AC-800 catalyst was analyzed. The presence of P-C, P-O and P = O species in the 3Cu/5CuP/AC-800 catalyst indicated that the P species were doped into the carbon material with different forms [36].
As illustrated in Fig. S3 (Supporting information), the XRD spectra of the 3CuCl2/5CuCl2/AC catalyst did not exhibit any discernible characteristic peaks of Cu, indicating that the Cu was uniformly dispersed on the carbon carriers or below the detection limit (< 4 nm). The 3Cu/5Cu/AC-800 and 3Cu/5CuP/AC-800 catalysts obtained from high-temperature treatment showed characteristic signal peaks of Cu at 43.2°, 50.4°, and 74.1°, indicating the appearance of Cu monomers during high-temperature treatment. In multiphase catalytic reactions, the form in which the active species are present (dispersion, particle size, etc.) exerts a significant influence on the catalytic performance [37]. To ascertain the state of Cu presence in the catalysts, direct observation was conducted using TEM. As illustrated in Fig. S4a (Supporting information), the TEM images of the 3CuCl2/5CuCl₂/AC catalysts did not reveal the presence of Cu particles, indicating that the Cu species were effectively dispersed. This finding was consistent with the XRD data. In Fig. S4b (Supporting information), the TEM image of the 3Cu/5Cu/AC catalyst could be observed that the Cu is violently agglomerated, with large particle sizes and distinctive Cu (111) crystalline lattice striations, which were responsible for its low catalytic activity. As illustrated in Fig. S4c (Supporting information), the TEM image of the 3CuCl2/5CuP/AC catalyst revealed the presence of distinguishable particles, indicative of the agglomeration of Cu species within the catalyst and the prevalence of Cuδ+ species resulting from the secondary impregnation of CuCl2 species. This phenomenon is the underlying cause of the 3CuCl2/5CuP/AC catalyst's exceptional initial activity. As shown in Fig. S4d (Supporting information), nanoparticles were present in the 3Cu/5CuP/AC-800 catalyst and the size of the nanoparticles was significantly smaller than that of the 3Cu/5Cu/AC and 3CuCl2/5CuP/AC catalysts, which indicated that more Cu sites were exposed in the 3Cu/5CuP/AC-800 catalyst. It was further confirmed that the introduction of P effectively inhibited the agglomeration of Cu species, which was also responsible for the higher catalytic activity of 3Cu/5CuP/AC-800.
The physical structure parameters of the catalysts were further investigated by the N2 adsorption and desorption isotherms of the Cu-based catalysts as shown in Fig. S5a (Supporting information). At relative pressure P/P0 < 0.1, the adsorption/desorption isotherms of the catalysts all showed a sharp increase, which was a type Ⅰ isotherm, indicating the presence of microporous structure. The adsorption profiles of the 3CuCl2/5CuCl2/AC and 3CuCl2/5CuP/AC catalysts exhibited a notable enhancement following high-temperature treatment when the relative pressure P/P0 was < 0.1. This suggested that the 3Cu/5Cu/AC-800 and 3Cu/5CuP/AC-800 catalysts possessed a greater abundance of micropores. The detailed structural parameters were shown in Table S2 (Supporting information). The catalyst 3Cu/5Cu/AC-800 exhibited an increase in the specific surface area of the catalyst from 504.9 m2/g to 954.0 m2/g, and an increase in the pore volume from 0.27 cm3/g to 0.47 cm3/g. This phenomenon may be attributed to ionic migration and carbothermal reduction of CuCl2 species within the pores of the 3CuCl2/5CuCl2/AC catalyst. During high-temperature calcination, ionic migration and carbothermal reduction occur within the catalyst pores, leading to agglomeration of CuCl2 species and exposure of the occupied catalyst pores [38]. This resulted in an increase in the specific surface area and pore volume of the catalyst. The enhanced specific surface area provided a greater number of reaction sites for the catalyst. Similarly, the specific surface area and pore volume of the 3Cu/5CuP/AC-800 catalyst exhibited a marked increase following high-temperature treatment, when compared with that of the 3CuCl2/5CuP/AC catalyst. In conjunction with the XRD data, it was discerned that the reduction process and migration of Cu ions in the catalyst subsequent to high-temperature treatment resulted in the agglomeration of metals, thereby re-exposing the originally occupied pores. The specific surface area and pore volume of the 3Cu/5Cu/AC-800 catalyst were larger than those of the 3Cu/5CuP/AC-800 catalyst, which could be attributed to the clogging of the catalyst pores due to the introduction of P, resulting in a decrease in specific surface area and pore volume.
The incorporation of heteroatoms into carbon materials had been demonstrated to effectively modulate the electronic structure and physical and chemical properties of these materials [1]. Furthermore, it had been shown to enhance the degree of defects in carbon materials [39]. To further investigate the effect of P in the catalyst on the catalyst structure, as shown in Fig. S5b (Supporting information), the Raman spectra of the catalysts showed two characteristic peaks at approximately 1350 and 1590 cm-1, which were ascribed to the absorption peaks in the D and G bands in carbon materials. The ID/IG values of the 3Cu/5CuP/AC-800 catalyst were markedly higher than those of the 3Cu/5Cu/AC-800 catalyst, indicating that the incorporation of P elevated the extent of defects in the catalyst [40].
The elemental composition of the catalyst was examined through XPS to ascertain the impact of varying preparation conditions on the elemental content. As illustrated in Table S3 (Supporting information), the 3Cu/5CuP/AC-800 catalyst exhibited a notable increase in the content of C elemental, rising from 84.52% to 91.07% in comparison to the 3CuCl2/5CuP/AC catalyst. Conversely, the content of O elemental was significantly reduced, while that of elemental Cl was diminished to 0.33% following the heat treatment. The same trend was observed for the elemental content of the heat-treated 3Cu/5Cu/AC-800 catalyst compared to the 3CuCl2/5CuCl2/AC catalyst. A reduction of the elements O and Cl is observed, which suggested that the catalyst has undergone a change in Cu species during calcination. 3CuCl2/5CuCl2/AC Furthermore, the combination of the XRD and TEM data indicated that the CuCl2 undergoes a transformation to Cu0. To gain further insight into the impact of P on the Cu form, a comprehensive analysis was conducted on the catalyst elements of 3Cu/5Cu/AC-800 and 3Cu/5CuP/AC-800. Compared to the 3Cu/5Cu/AC-800 catalyst, the 3Cu/5CuP/AC-800 catalyst showed a decrease in C element content and an increase in Cl and O element content, indicating that the valence states of the Cu species in the catalyst may be different.
As illustrated in Fig. 2g, the Cu 2p XPS spectra demonstrated the impact of elevated temperature treatment on the valence state of Cu species in the catalyst. The Cu 2p XPS spectrum of the 3CuCl2/5CuP/AC catalyst could be fitted to two characteristic peaks, which were attributed to Cu+/Cu0 and Cu2+. In contrast, the 3Cu/5CuP/AC-800 catalyst had only one characteristic peak, which was attributed to Cu+ and Cu0. In combination with XRD and TEM, this indicated that Cu0 was predominantly present in the 3Cu/5CuP/AC catalyst. Furthermore, a shift in the binding energy at Cu+/Cu0 to a lower binding energy was observed in the 3CuCl2/5CuP/AC catalyst, which provided additional evidence in support of a lower valence state for Cu in the 3Cu/5CuP/AC-800 catalyst, which was in agreement with the XRD and TEM results. In conjunction with the catalyst performance, it was found that the Cu species in the 3Cu/5Cu/AC-800 catalyst were highly agglomerated and had a low valence, resulting in a low C2H2 conversion. The presence of Cu0/Cu+ in the 3Cu/5CuP/AC-800 catalyst, in combination with TEM, demonstrated that the introduction of P atoms retained some of the Cu species in the ionic state to a certain extent and effectively suppressed the excessive agglomeration of Cu species, thereby exposing more active sites and consequently increasing the catalytic activity [41, 42]. As shown in Fig. S6 (Supporting information), the catalytic performance of the 5CuCl2/PAC catalyst was enhanced by about 10% after the introduction of P, indicating a positive effect on the catalyst performance, which is consistent with previous reports [36]. The inductively coupled plasma emission spectroscopy (ICP) data presented in Table S4 (Supporting information) demonstrated that the Cu content in the 3CuCl2/5CuP/AC catalyst was 6.3 wt%, which was higher than that observed in the 3Cu/5CuP/AC-800 catalyst, which exhibited a Cu content of 3.9 wt%. The 3CuCl2/5CuP/AC catalyst exhibited superior initial activity due to the presence of Cu2+ species. As illustrated in Fig. 2h, the catalysts were examined for the Cl element, and Cu-Cl species were distinctly discernible in the Cl 2p XPS spectra [43, 44]. Conversely, no distinctive signal peaks of the pertinent Cl species were identified in the 3Cu/5CuP/AC-800 catalysts. This indicated that the elevated temperature treatment resulted in the carbothermal reduction of the Cu-Cl species present within the catalyst [45], thereby facilitating the transformation of Cu-Cl to Cu0. The detailed data were shown in Table S5 (Supporting information), and the elemental Cl content in the 3CuCl2/5CuP/AC catalyst was 1.95%. The 3CuCl2/5CuP/AC catalyst was treated at high temperatures, which resulted in the breakage of the Cu-Cl bond, the transformation of Cu-Cl species to Cu0, and the decrease of the initial catalytic activity.
As shown in Fig. 3a, it could be clearly found that the initial C2H2 conversion of the catalyst after HCl activation was enhanced. It had been demonstrated that the active species of the catalyst exhibited a stronger correlation with the structure of the sites that were induced during the HCl activation [46]. The elemental states of the 3Cu/5CuP/AC-800 catalyst were analyzed before and after HCl activation, as illustrated in Fig. 3b. Following the HCl activation process, a notable alteration was observed in the valence state of the Cu species within the catalyst, manifesting as a distinctive peak at 935.05 eV, which could be attributed to the Cu2+ species. The Cu species of the 3Cu/5CuP/AC-800 catalyst underwent oxidation in the presence of HCl, while the characteristic peak of the Cu+/Cu0 species at 932.78 eV was shifted to a high binding energy to 933.14 eV, indicating an increase in the valence of the Cu species. The Cl elemental of the 3Cu/5CuP/AC-800 catalyst was subjected to further analysis before and after HCl activation, as illustrated in Fig. S7 (Supporting information). The elevated signals of Cu-Cl and C—Cl species in the Cl 2p XPS spectra of the 3Cu/5CuP/AC-800 catalyst following HCl activation indicated that the Cu species on the catalyst surface underwent activation to Cu-Cl species by HCl during the activation process, resulting in an elevated valence. This was conducive to the enhancement of catalytic activity.
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| Fig. 3. Intrinsic active sites and reaction mechanism characterization with/without HCl preactivation 3Cu/5CuP/AC-800. (a) Acetylene conversion (Reaction conditions: T = 180 ℃, V(HCl)/V(C2H2) = 1.15, GHSV(C2H2) = 240 h-1). (b) Cu 2p XPS spectra. (c) Cu K-edge normalized XANES spectra. (d) k3-weighted Fourier transform spectra. (e) HCl-TPD and (f) C2H2-TPD curves. | |
The chemical states of Cu in the 3Cu/5CuP/AC-800 and 3Cu/5CuP/AC-800-HCl catalysts were subjected to further analysis using XAFS, as illustrated in Fig. 3c. Following the HCl activation process, the absorption edge of the 3Cu/5CuP/AC-800-HCl catalyst exhibited an intermediate position between that of CuCl2 and Cu foil, thereby indicating that the average valence state of the Cu atoms with a positive charge in the catalyst was 0 < δ < 2 [47]. The absorption edge of the 3Cu/5CuP/AC-800-HCl catalyst was observed to shift towards higher energies in comparison to 3Cu/5CuP/AC-800, which provided evidence for a higher oxidation state of the activated Cu atoms [48, 49]. This was in alignment with the XPS data. As illustrated in Fig. 3d, a discernible Cu-Cu characteristic peak was evident in the 3Cu/5CuP/AC-800 catalyst. However, following HCl activation, a distinct Cu-Cu characteristic peak was not observed in the Cu K-edge FT-EXAFS spectra of the 3Cu/5CuP/AC-800-HCl catalyst. Instead, a Cu-Cl characteristic peak emerged at a distance of approximately 1.47 Å. This suggested an increase in the coordination number of Cu atoms in the 3Cu/5CuP/AC-800 catalyst [50], which was in accordance with the data obtained from Cl 2p XPS.
In conclusion, the observed enhancement in catalyst activity could be attributed to the HCl activation process. In the acetylene hydrochlorination reaction, previous studies had demonstrated that the Cl atoms in vinyl chloride were derived directly from the metal-chlorine coordination structure, rather than from the Cl atoms in HCl directly [51, 52]. In this regard, the formation of Cu-Cl species by the adsorption and activation of HCl at the catalytic site contributed to the enhancement of the activity of Cu-based catalysts in acetylene hydrochlorination.
The adsorption characteristics of the catalysts on the reaction gases were further analyzed by temperature-programmed desorption (TPD). As illustrated in Fig. 3e, the 3CuCl2/5CuCl2/AC, 3CuCl2/5CuP/AC and 3Cu/5CuP/AC-800 catalysts exhibited comparable adsorption strengths for HCl, whereas the 3Cu/5Cu/AC-800 catalyst demonstrated the lowest adsorption strength for HCl. The relative desorption area of the catalyst to the gas indicated that the adsorption strength of HCl was almost unchanged after heat treatment, while the adsorption amount was significantly reduced. This phenomenon may be attributed to the high-temperature-induced agglomeration of Cu sites, which had the effect of reducing the number of available adsorption sites. The introduction of P enhanced the adsorption strength of HCl on the catalyst, and also effectively inhibited the agglomeration of the catalyst Cu sites, thereby improving the adsorption of HCl. As illustrated in Fig. 3f, the adsorption intensity and amount of C2H2 over the catalysts following high-temperature treatment were markedly diminished, which was attributed to the reduction and aggregation of Cu sites resulting from the elevated temperature. This was the reason why the 3CuCl2/5CuCl2/AC and 3CuCl2/5CuP/AC catalysts exhibited a higher initial C2H2 conversion. The excessive adsorption strength of acetylene had been demonstrated to precipitate catalyst reduction. Consequently, the heat-treated catalyst displayed markedly enhanced stability.
To ascertain the reason for the enhanced stability of the 3Cu/5CuP/AC-800 catalyst, the elemental states of the two catalysts prior to and following the reaction were analyzed by XPS, with the 3CuCl2/5CuCl2/AC catalyst serving as the reference sample. As illustrated in Fig. 4a, the Cu/5CuP/AC-800 catalyst exhibited a distinctive peak for Cu+/Cu0 only at 932.78 eV. The 3Cu/5CuP/AC-800-Used catalyst exhibited a distinctive Cu2+ peak at 935.27 eV, accompanied by a notable shift in the Cu+/Cu0 peak towards higher binding energy in comparison to the 3Cu/5CuP/AC-800 catalysts. This observation suggested that the Cu species undergo continuous activation during the reaction process, leading to the formation of the high valence Cuδ+ active species. As illustrated in Fig. 4b, the Cu species in the 3CuCl2/5CuCl2/AC catalysts underwent reduction during the reaction. This resulted in a notable decline in the Cu2+ content and a shift in the characteristic peak of Cu2+ to a lower binding energy. These observations indicated that the catalysts exhibited a lower valence after the reaction. As illustrated in Fig. 4c, the Cu LMM spectra demonstrated that the Cu2+ content of the catalyst diminished and the zero-valent Cu content augmented subsequent to the reaction. The detailed contents were presented in Table S4, which showed that the Cu2+ content of the 3CuCl2/5CuCl2/AC catalyst decreased from 70.1% to 59.0% after the reaction, while the Cu0 content increased from 3.9% to 12.3%. This suggested that the 3CuCl2/5CuCl2/AC catalyst underwent reduction during the reaction, which resulted in catalyst deactivation. In order to analyze the valence state of Cu, the catalyst was subjected to a programmed warming reduction, as illustrated in Fig. 4d The reduction peak of the 3Cu/5CuP/AC-800 catalyst was observed to occur at 495.7 ℃, which was significantly higher than that of the 3CuCl2/5CuCl2/AC and 3CuCl2/5CuP/AC catalysts. This suggested that the 3Cu/5CuP/AC-800 catalyst impeded the reduction of Cu species, which was the underlying cause of the enhanced catalyst stability.
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| Fig. 4. (a) Cu 2p XPS spectra of 3Cu/5CuP/AC-800 and 3Cu/5CuP/AC-800-Used catalysts. (b) Cu 2p XPS spectra and (c) Cu LMM spectra of 3CuCl2/5CuCl2/AC and 3CuCl2/5CuCl2/AC-Used catalysts. (d) The H2-TPR curves of 3CuCl2/5CuCl2/AC, 3CuCl2/5CuP/AC, and 3Cu/5CuP/AC-800 catalysts. | |
In this study, P-doped Cu0/Cuδ+ two-site catalysts were prepared by modulating the structure of Cu0/Cuδ+ two-site through two-step heat treatment and HCl activation. The Cu0/Cuδ+ two-site catalysts demonstrated notable stability in acetylene hydrochlorination. The 3Cu/5CuP/AC-800 catalysts were operated for 50 h without significant deactivation under the reaction conditions of T = 180 ℃, V(HCl)/V(C2H2) = 1.15, GHSV(C2H2) = 180 h-1. It was confirmed by TEM that the introduction of P enhanced the stabilization of Cu species during heat treatment and inhibited the excessive agglomeration of Cu. The enhancement of the catalyst's activity could be attributed to the HCl activation process, whereby HCl facilitated the transformation of Cu0 species to Cu-Cl species, thereby increasing the valence state of Cu and consequently enhancing the catalytic activity. Concurrently, the valence modulation of Cu sites by HCl throughout the course of the reaction prevented the reduction of Cu sites by C2H2 and enhanced the stability of the catalyst. This preparation strategy provides a new reference for the stability study of Cu-based catalysts for acetylene hydrochlorination.
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statementJunchen Peng: Writing – original draft, Validation, Data curation. Zhongyuan Guo: Writing – original draft, Validation, Formal analysis, Data curation. Dandan Dong: Writing – review & editing, Investigation, Formal analysis, Conceptualization. Yusheng Lu: Writing – review & editing. Bao Wang: Writing – review & editing. Fangjie Lu: Writing – review & editing, Funding acquisition. Chaofeng Huang: Writing – review & editing, Supervision, Funding acquisition, Data curation, Conceptualization. Bin Dai: Writing – review & editing, Funding acquisition.
AcknowledgmentsThis work was supported by the National Natural Science Foundation of China (No. 22062021), the Science and Technology Project of Xinjiang supported by Central Government (No. 2022BC001), Science and Technology Planning Project (No. 2024AB048), Tianshan Talents Training Program of Xinjiang (Science and Technology Innovation Team, No. CZ002701), the Start-Up Foundation for high-level professionals of Shihezi University (No. RCZK201932), 2024 Talent Development Fund-Tianchi Young Doctor of Excellence (No. CZ002744).
Supplementary materialsSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111208.
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