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Review

The Advancement of Supported Bimetallic Catalysts for the Elimination of Chlorinated Volatile Organic Compounds

by
Hongxia Lin
1,2,
Yuxi Liu
1,2,
Jiguang Deng
1,2,*,
Lin Jing
1,2,
Zhiwei Wang
1,2,
Lu Wei
1,2,
Zhen Wei
1,2,
Zhiquan Hou
1,2,
Jinxiong Tao
1,2 and
Hongxing Dai
1,2,*
1
Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemical Engineering and Technology, College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
2
Key Laboratory of Beijing on Regional Air Pollution Control, College of Environmental Science and Engineering, Beijing University of Technology, Beijing 100124, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(8), 531; https://doi.org/10.3390/catal14080531
Submission received: 29 July 2024 / Revised: 11 August 2024 / Accepted: 13 August 2024 / Published: 16 August 2024
(This article belongs to the Special Issue Heterogeneous Catalysis: From Nano- and Cluster-Catalysts to Single-Atom Catalysts)
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Abstract

:
Chlorinated volatile organic compounds (CVOCs) are persistent pollutants and harmful to the atmosphere, environment, and human health. The catalytic elimination of CVOCs has become a hotspot of interest due to their self-toxicity, the secondary generation of chlorinated by-products, and the Cl poisoning of catalysts. The development of high-performance, highly selective, and anti-poisoning catalysts is a critical issue. Bimetallic catalysts exhibit an improved dechlorination performance, poisoning resistance, and product selectivity through the modulation of geometrical and electronic structures. The present review article gives a brief overview of the recent advancements in the preparation of bimetallic catalysts and their catalytic CVOC elimination activities. In addition, representative case studies are provided to investigate the physicochemical properties, CVOC conversion, COx and inorganic chlorine species selectivities, and by-product control so that the structure–performance relationships of bimetallic catalysts can be established. Furthermore, this review article provides a fundamental understanding of designing bimetallic catalysts with specific active components and the desired physicochemical properties for target reactions. In the end, related perspectives for future work are proposed.

Graphical Abstract

1. Introduction

Chlorinated volatile organic compounds (CVOCs) are widely used in chemical industries, agriculture, pesticides, pharmaceuticals, organic synthesis, and other fields, which cause ozone layer destruction, photochemical smog, and global warming through volatilization, leakage, or emissions. CVOCs attract significant attention due to their high toxicity, structural stability, difficulty to degrade, and toxic by-products. The catalytic hydrodechlorination of CVOCs is the most efficient treatment method without the generation of the chlorine by-products. The key issue is to design and synthesize highly active and stable catalysts with a good chlorine anti-poisoning performance. Bimetallic catalysts with unique physicochemical properties are different from their monometallic counterparts due to the generation of new active sites for activity and selectivity. New active sites in bimetallic catalysts are generated due to the following reasons: changes in the electronic structure due to strain effects, synergistic interactions between metals and/or substrates, the formation of alloys, changes in the surface structure due to spatial site resistance, and so on [1,2,3,4,5].
The catalytic combustion of chlorinated hydrocarbons usually proceeds as follows [6]: (i) the dechlorination of chlorinated hydrocarbons adsorbed at the acid centers (Lewis acid (L-acid) and Brönsted acid (B-acid)) happens due to the breakage of C–Cl bonds; (ii) dissociated chlorinated hydrocarbon molecules react with surface reactive oxygen species to produce carbon-containing products (CO and CO2); and (iii) dissociated Cl exists in the form of HCl or Cl2 (the Deacon’s reaction product). Synergistic interactions between the acidic centers and redox properties of the catalyst are the key to improving catalytic activity and stability. The acid centers determine the carbon product selectivity, while the generation of chlorinated by-products is influenced by the redox property of a catalyst [7]. The dissociation energy of the C–Cl bond in a chlorinated hydrocarbon molecule is related to the molecular structure. For example, 1,2-dichloroethane [8], chlorobenzene [9], and monochloromethane [10] exhibit different C–Cl bond dissociation energies of 82, 96, and 84 kcal/mol, respectively.
Generally speaking, Ru catalysts with strong redox properties display the best catalytic performance for CVOC removal among the noble metal (Au, Pd, Pt, and Ru) catalysts at low temperatures [11]. Chlorine species were efficiently removed from a catalyst surface via a Deacon reaction over Ru catalysts, which led to less polychlorinated by-product generation than that of the Pd- or Pt-based catalysts [12]. However, catalyst deactivation, which is caused by the strong adsorption of chlorine species on a catalyst surface, and high costs remain big challenges for noble metal catalysts [13]. The redox property of a catalyst plays a more important role than enhancing the reactivity and acidity for the catalytic elimination of chlorinated unsaturated hydrocarbons [14]. Transition metal oxides show excellent redox properties due to the abundant oxygen vacancies and multivalent transition metal ions, which have been widely used for CVOC elimination [15,16,17]. Transition metals are partially or completely chlorinated via oxygen–chlorine exchange at low temperatures to form transition metal chlorides, resulting in the loss of active components and catalytic performance. Zeolites catalysts with abundant Brønsted acid sites facilitate the adsorption and activation of CVOCs [18]. The unique pore structures of zeolites are beneficial for the high selectivities toward the intermediates or the final products [19]. However, carbon deposition and strong HCl adsorption at the active sites are the main reason for the deactivation of zeolitic catalysts [20]. In addition, the lack of a deep oxidation center results in a high level of dechlorination by-products. Perovskite (ABO3) catalysts are usually prepared after high-temperature calcination, which exhibit good thermal stability and resistance to sintering [21]. The poor redox potential at low temperatures and the weak acidity of perovskites can be modified by A-site substitution [22]. Ternary metallic PtSnM (M = Mn, W, Nb) catalysts have been reported to show a better catalytic performance and chlorine resistance for toluene and chlorobenzene co-oxidation [23]. The doping of the third metal altered the electronic structure around Pt and acted as another major adsorption site for chlorobenzene, which resulted in decreased chlorobenzene adsorption at the Pt site. In addition, chlorobenzene-induced deactivation over the ternary metal catalysts was reversible, and the generation of chlorine-containing by-products was inhibited compared with that of the monometallic Pt catalyst.
There are many reports on the dechlorination of CVOCs by hydrogenation in zero-valent monometallic iron systems [24,25,26]. The dechlorination of CVOCs to olefins could take place over monometallic iron effectively at a very slow reaction rate, but chlorinated olefins are generated in some cases. Depositing another transition metal (i.e., Cu [27], Ni [25], or Co [28]) on the surface of iron to form a bimetallic system could accelerate the reaction. Bimetallic reduction and hydrogenation dechlorination show the synergistic effects of metal reduction and catalytic hydrogenation. The second metal acts as a hydrogenation promoter to effectively encourage the transfer of reducing hydrogen atoms, while the active H2 reinforces the reduction of iron [29]. The empty orbitals of transition metals can form complexes with the p-electron pairs of chlorine or the π-electrons of double-bonded organics, resulting in the reduced activation energy of the dechlorination reaction.
In this review, we summarize the preparation of bimetallic catalysts and their catalytic CVOC elimination performance. It can be concluded from the catalytic application of bimetallic nanoparticles that the addition of a second metal can (i) generate new reactive oxygen species or acidic sites, which significantly enhances the catalytic performance; (ii) modify the redox properties of the catalyst surface, which promotes the selectivity to generate the target species and inhibit the formation of chlorine-containing by-products; and (iii) reduce the activation energy of the dechlorination reaction and promote the desorption of the Cl species to avoid the toxic deactivation of the catalyst.
This review covers the recent advances in the preparation methods and the CVOC elimination of bimetallic nanomaterials. Bimetallic catalysts are discussed as a separate topic to achieve a specific behavior in CVOC degradation by modulating the geometrical and electronic structures through the introduction of a second metal. In the catalytic oxidation process, CVOCs are not always completely converted to the final products (H2O, CO2, and Cl2) as expected; instead, they generate CO, HCl, and other toxic chlorinated by-products. Based on the key points of catalyst chlorine poisoning and by-product generation, bimetallic catalysts with multifunctional active sites are proposed to realize the inhibition of by-product generation, deep oxidation, and well stability by inducing a second metal. This review also provides fundamental insights and strategies to design specific active components to achieve high degradation activity, dechlorination product selectivity, and excellent resistance to Cl poisoning.

2. Synthesis of Bimetallic Nanoparticles

Bimetallic catalysts show a better catalytic performance than the single metal counterparts due to the synergistic effect between the two metal elements. The preparation method of a bimetallic catalyst determines its structural property, and ultimately affects its catalytic activity, selectivity, and stability. The following six factors which influence the bimetallic structure have been proposed by Ferrando et al. [3]: (i) Bond energy: When the bonding strength between two metals is stronger than that of monometallic self-bonding, bimetals tend to generate a tighter and homogeneous mixture to easily form alloys. On the contrary, two separated metallic phases are generated. (ii) Surface energy: A metal with a lower surface energy tends to form a monometallic shell on the surface of a bimetal. (iii) Relative atomic size: A second metal with a smaller atomic size tends to act as a bimetallic core. (iv) Charge transfer: A charge transfer between the two metals facilitates the efficient and homogeneous mixing of bimetals. (v) Electron effect: The presence of the special electronic effect drives the formation of a specific structure in the bimetallic compound. (vi) Stabilizing agent: When bimetallic nanoparticles are promoted by a stabilizing agent, the metal which shows a strong interaction with the stabilizing agent tends to form a shell in the alloy. In order to achieve the desired catalytic effect, it is necessary to develop a suitable and controllable synthesis method to prepare bimetallic catalysts with ideal structures. Suitable preparation approaches allow us to obtain good control of the size, geometrical configuration, and electronic structure of the bimetallic catalysts.
The impregnation method is one of the most widely used approaches for the preparation of bimetallic catalysts [30]. Metal salt ions are impregnated onto the support surface or within the pores via a strong interaction (van der Waals forces, electrostatic gravitational forces, or capillary pressure in the support pores) between the active metal and the support. The simultaneous introduction of the precursor solutions of both the metals favors the generation of bimetallic alloy catalysts, while the stepwise impregnation of the second metal after the conversion of the first soaked metal into an insoluble substance is beneficial for the formation of a core–shell-structured bimetallic catalyst. The structures of bimetallic particles prepared by the impregnation method are strongly non-homogeneous (i.e., which exhibit different compositions and structures). For instance, there are significant differences in Ni particle size, metal–support interactions, and the catalytic activities of the Ni/Al2O3 catalysts prepared via the impregnation route with different Ni precursors (nickel chloride, nickel nitrate, or nickel sulfate) [31].
The precipitation–deposition method converts a metal precursor to a hydroxide by adjusting the pH of the aqueous solution to precipitate on the surface of the support, and then the bimetallic particles are obtained after calcination [32]. The advantage of the precipitation–deposition method over the impregnation approach is that the obtained metal particles are smaller with a narrower particle size distribution. The conversion of metal precursors to hydroxides facilitates stronger metal–support interactions, hence resulting in higher catalytic activities and stability for the specific reactions.
The galvanic replacement method can be used to prepare bimetallic particles through direct redox reactions between two metals due to the difference in their electrode potentials [33,34]. The sequence of metallic loading is determined by the electrode potentials of the two target metals. Bimetallic catalysts prepared via the galvanic replacement route can effectively avoid the formation of two separate metal particles. The galvanic replacement method achieves the preparation of bimetallic catalysts with a variety of structures based on the different properties of the two metals (e.g., alloy and core–shell structures). The galvanic replacement reaction of Ag with PdCl42− resulted in the formation of a PdAg alloy structure [35]. However, the bond energy (307 kJ/mol) of Pt–Pt is higher than that (218 kJ/mol) of Pt–Ag; Pt tends to be self-deposited on Pt instead of Ag to obtain separated Pt and Ag rather than their alloy.
The principle of the photocatalytic deposition method is that the photogenerated holes on the support (semiconductor) surface react with the sacrificial agent (reducing agent, e.g., methanol, ethanol, etc.) in the solution under the UV light irradiation, while the photoelectrons migrate to the substrate surface to reduce the adsorbed metal ions [36,37,38]. The two metals are photocatalytically deposited in the reverse order of the replacement procedure to avoid the simultaneous occurrence of galvanic replacement reactions during the photocatalytic deposition process. Since the electrode potential (0.99 eV) of Pd/Pd2+ is higher than that (0.80 eV) of Ag/Ag+, bimetallic PdAg catalysts should be designed by photocatalytically depositing Pd first before Ag deposition [36,38].
The surface reduction approach is an effective strategy for the controlled preparation of bimetallic particles with core–shell structures [39]. The preparation principle is to take advantage of the successful activation of the reducing agent by the first metal to achieve the selective deposition of the second metal. The commonly used reducing agents include hydrogen, sodium borohydride, and formic acid. For instance, by using hydrogen as a reducing agent, the noble metal Pd that can dissociate hydrogen at room temperature was chosen as the first metal to be preloaded on the support. And the second metal (i.e., Cu) that is inert to hydrogen at room temperature was selectively deposited on Pd to form a bimetallic core–shell structure [40]. Li et al. used octadecylamine (a weak reducing agent) to co-reduce the transition metal and the noble metal [41]. In the co-reduction process, the noble metal was first reduced by octadecylamine, whereas the transition metal cations were adsorbed on the electron-rich surface of the reduced noble metal. The reduction of transition metal ions by octadecylamine was achieved through the electron transfer from the noble metal to the transition metal. The reduction process was induced by a noble metal in the octadecylamine system, which is called noble-metal-induced reduction [42].
The chemical growth of bimetallic nanoparticles involves the precipitation of the solid phase from a solution. Bimetallic nanoparticles are not thermodynamically stable in terms of crystal growth kinetics due to their small particle sizes. The stabilization of nanoparticles and the inhibition of growth can be realized by adding surface protection reagents or locating them in inert environments to provide an energy barrier to counteract the van der Waals or magnetic attractions between the nanoparticles. As mentioned above, the impregnation method is simple, but as-prepared bimetallic nanoparticles are highly inhomogeneous. The bimetallic particles obtained by the deposition precipitation method are smaller with a narrower size distribution, while the deposition constants of the two metal precursors should be similar to ensure simultaneous precipitation at the same pH. The galvanic replacement method achieves the preparation of alloy and core–shell structures, while similar electrode potentials are required for the formation of two separate metal particles. Photochemical deposition has the advantage of yielding highly dispersed bimetallic nanoparticles, but it is only suitable when the substrate is a semiconductor. Surface reduction achieves the co-reduction of noble metals and transition metals at a specific electronegativity, otherwise the transition metals will convert into oxides.

3. Catalytic Applications for CVOC Elimination

In recent years, there have been a number of works on the application of bimetallic catalysts for the catalytic oxidation of VOCs, especially the removal of CVOCs. Table 1 gives an activity overview of various bimetallic nanoparticles used for CVOC removal reported in the literature. It can be realized from the above investigations that the modification of a second metal can improve the redox property of the catalyst, induce the generation of abundant reactive oxygen species, alter the adsorption behavior of chlorinated intermediates, and enhance oxygen mobility, thus improving the catalytic performance of CVOC removal. It was observed that different components of bimetallic catalysts exhibited specific behaviors towards CVOC elimination through the modulation of geometrical and electronic structures. Such specific active components exhibit the desired performance to realize high catalytic activity, dechlorination product selectivity, and excellent resistance to Cl poisoning.
For example, Agnieszka et al. [43] investigated the oxidation properties and HCl selectivity of chlorobenzene, dichloroethane, and trichloroethylene over γ-Al2O3-supported bimetallic Pt–Pd catalysts. The results indicated that the catalytic activity increased with the increase in chlorine atoms in the reactant molecule and the decrease in C–H bonds in the reactant molecule. The bond dissociation energies of the C–Cl bonds were lower than those of the C–H bonds and decreased with the increased number of Cl atoms in the chloromethane molecule. Zhao et al. [44] prepared a series of the CeNb catalysts with different Ce/Nb molar ratios and studied their catalytic performance for o-dichlorobenzene oxidation. It was found that the weak acid center was mainly the hydroxyl group on the surface of the SiO2 support, whereas Nb mainly provided a strong acidic center. The oxidation of o-dichlorobenzene over the CeNb catalysts obeyed the MvK reaction mechanism, with surface lattice oxygen and adsorbed oxygen species being the active sites. The CeMn bimetallic catalysts were fabricated by the impregnation method to investigate the efficient catalytic performance of 1,2-dichlorobenzene combustion [45]. Mn ions were doped into the CeO2 fluorite lattice to form more reactive oxygen species, thus improving the redox property of the catalyst. The adsorption of Cl species on the surface of the CeMn catalyst led to its deactivation, and the deactivated catalyst could be restored via a regeneration step.
VNi bimetallic catalysts for dichloromethane oxidation (Figure 1) were prepared by the sol–gel method [46]. The NiV catalyst exhibited excellent catalytic activity and stability and inhibited the formation of the by-product monochloromethane. No coke deposition or chlorine species were formed during a 100 h on-stream reaction test.
It was reported that the good activity and selectivity in the chemoselective hydrogenation of halonitrobenzenes over the Pt-based catalysts were ascribable to the mutual coupling between the geometrical effect (the change in the ratio of high-to-low coordination atoms) and the electronic effect (the change in the Fermi energy level) in the Pt particles [47]. The theoretical calculations results confirmed that the geometrical effect played a major role in the large-sized Pt particles, whereas hydrogenation occurred in the more-coordinated atoms, and dechlorination side reactions took place more readily in the less-coordinated atoms. However, the dominance of the electronic effect reversed the above changing trends in activity and selectivity over the Pt clusters. Lu’s group generated monolayer Pt-coated Au@Pt catalysts using the accurate atomic layer deposition method for the chemoselective hydrogenation of p-chloronitrobenzene [48]. The lattice spacings of the outermost Pt atoms were stretched, and the geometrical and electronic structures of Pt in the bimetal were modified. The as-prepared Au@Pt catalyst exhibited a p-CAN intermediate selectivity of 99%, with a specific activity of 1.75 × 104 molp-CNB/(molPt h). Kinetic experiments also demonstrated that the Au@Pt catalyst showed a lower reaction barrier (24.2 kJ/mol) than the Pt catalyst (34.9 kJ/mol).
Mn–Ce/Al2O3 catalysts with various Me/Ce ratios were prepared via the once-through impregnation of an equal-volume aqueous solution mixture of manganese nitrate and cerium nitrate [49]. The characterization results revealed that the interaction between Ce and Mn reduced the crystal size of MnOx and increased its dispersion. The Mn–Ce/Al2O3 catalyst exhibited good activity (100% CB conversion below 400 °C). Among these catalysts, Mn8Ce2/Al2O3 performed the best for the oxidation of 1000 ppm chlorobenzene at 338 °C and a space velocity (SV) = 15,000 mL/(g h). The main oxidation products were CO2, H2O, and HCl, while a small amounts of Cl2 and polychlorinated benzenes were produced at higher temperatures. The introduction of an excessive amount of CeO2 led to a decrease in the content of the active component Mn, which deteriorated the redox property of the catalyst and gave rise to a loss in catalytic activity. In addition, the strong adsorption of the Cl species by-product on the catalyst surface inhibited the mobility of oxygen, which caused the catalyst to deactivate below 350 °C. When the reaction temperature was higher than 350 °C, the adsorbed Cl species on the catalyst surface reacted with O2 to form Cl2 or HCl during the semi-Deacon process and were smoothly desorbed from the surface, hence rendering the catalyst to exhibit good long-term stability.
PtxRuy/m-HZ catalysts were obtained by the ultrasonic impregnation method with the assistance of oleic acid [50]. There was an interface between the Pt and Ru nanoparticles, and the formed Pt–O–Ru structure promoted the redox performance and surface acidity of the catalyst. The Pt0.5Ru0.5/m-HZ catalyst showed the best catalytic activity for 1000-ppm CB oxidation (T50 = 234 °C and T90 = 270 °C) at a SV = 40,000 mL/(g h), good stability, and the lowest by-product yield. Such a result was attributed to the well-dispersed Pt–O–Ru structure on the zeolite framework, which promoted the highly synergistic effect of the catalytic redox performance and surface acidity of the catalyst. Combined with the catalytic process and in situ infrared spectroscopic characterization, it could be inferred that the nucleophilic substitution reaction of the physically adsorbed CB on the Pt0.5Ru0.5/m-HZ surface took place at the acidic site, rapidly forming HCl and phenolic compounds, which were successively converted into maleic anhydride, aldehyde, and carboxylic acid, and finally oxidized into CO2 and H2O.
Ce–Zr/UiO-66 bimetallic catalysts were generated by the solvothermal method and used for the catalytic degradation of hexachlorobenzene [51]. It indicated that cerium was successfully doped into the lattice of Zr/UiO-66, without destroying the structure of the metal–organic skeleton. Among the catalysts with different Ce/Zr ratios, Ce0.2Zr0.8/UiO-66 with a maximal surface area of 1502 m2/g showed the best low-temperature (100–150 °C) activity for the highest hexachlorobenzene removal efficiency (>85%) and the least dioxin production. The high activity was mainly attributed to the unique redox property of cerium and the superior C–Cl dissociation ability. When CB molecules were adsorbed at the Lewis acid sites on the catalyst surface, the nucleophilic lattice oxygen broke down the C–Cl bonds in CB molecules. The benzene ring was opened or oxidized to produce acyclic intermediates, phenols, or quinones. The conversion of Ce4+ to Ce3+ led to the generation of new oxygen vacancies that favored the adsorption of CB molecules, which reacted to produce phenyl oxygen radicals accompanied by the release of oxygen vacancies from desorption, thus reaching a cyclic adsorption–desorption process. Tungsten-modified Pt/CeO2 catalysts were prepared using the co-impregnation method and utilized for the catalytic oxidation of benzene, CB, and 1,2-dichlorobenzene [52]. Pt was highly dispersed on W/CeO2 in the form of the Pt–O–W and Pt–O–Ce species. The W–O–Ce species increased the oxygen vacancies and acidity, whereas the Pt–O–W species promoted the reduction and availability of the surface oxygen species. The high resistance of W–O–Ce and W–O–Pt toward the Cl species maintains the availability of the reactive O2 and O species, whereas the presence of Pt efficiently facilitates the conversion of O2 (g) to the surface oxygen species. The reaction occurs mainly at the sites with the W–O–Pt–O–Ce or W–O–Ce structure. The Pt–O–Ce species was accountable for the chlorination at low temperatures, while CeO2 acted as a site for chlorine generation at high temperatures. The Pt–4W/CeO2 catalyst exhibited the best performance with the almost undetectable chlorination products. T90 was 327, 363, 302, 335, and 400 for 1000-ppm CB, 1,2-dichlorobenzene, 1,2-dichloroethane, dichloromethane, and trichloroethylene oxidation at SV = 60,000 h−1, respectively, due to the synergistic effect between the availability of oxygen on the active surface and the removal of the Cl species. The Ru–O–Ce-constructed Ru–Ce/TiO2 catalysts for CB oxidation were obtained using the incipient wetness impregnation method [53]. It was found that the 0.4Ru–1.0Ce/TiO2 catalyst presented the highest activity and selectivity toward inorganic chlorine instead of the toxic chlorinated by-products; T10 and T90 were 130 and 180 °C, respectively. The excellent oxygen storage capacity of CeO2 was beneficial for the dispersion of Ru and the formation of higher-valence RuOx. Meanwhile, the strong interaction between Ru and CeO2 was accountable for the easier electron transfer and reduced reaction activation energy. The introduction of Ce favored the increases in COx yield and selectivity toward inorganic chlorine instead of the toxic chlorination by-products and dioxin.
The preferential growth of metallic Ag on the surface of Pt could take place using the sequential photo-deposition method, thus leading to the formation of core–shell-like Pt@Ag catalysts [54]. Additionally, surface Ag enrichment in bimetallic catalysts was accompanied by the decrease in exposed Pt atoms, which could be identified by the chemical adsorption of CO. The catalyst prepared by the photo-deposition method showed stable catalytic activity and higher ethylene selectivity in the hydrodechlorination of 1,2-dichloroethane with the rise in Ag loading. The surface reduction method is a suitable way to obtain the supported surface modified catalysts. Using the impregnation and surface reduction methods, the Pd–Ag/A12O3 and Pd–Cu/A12O3 catalysts were prepared for the hydrodechlorination of 1,2-dichloroethane to ethylene [55]. The selective deposition of promoters on the Pd-based catalysts through the reaction occurs between the active H provided by Pd and the promoter, and the promoter only modifies the surface of Pd to obtain higher ethylene selectivity than that derived from the impregnation method. Given the similar loading, the Ag-modified catalyst showed more stable catalytic activity and higher ethylene selectivity than the Cu-modified counterpart, which was attributed to following three factors: (i) Copper is a bivalent cation, whose deposition requires two hydrogen atoms provided by the adjacent Pd active sites, whereas the deposition of silver does not pose any constraint for the Pd sites since Ag is a monovalent cation; (ii) Compared with the hydrated ions of Cu and Ag, Ag is smaller than Cu, and is easier to obtain access to the surface of Pd. At an identical Ag content, the Pd catalysts were prepared by the impregnation method in the presence or absence of Ag. Different catalytic activities and ethylene selectivities were observed during the process of hydrodechlorination of 1,2-dechloethane, which was ascribed to the different active sites in the two catalysts. For the Pd/ZrO2 catalysts, Pd is the only active site for dechlorination and provides active hydrogen [56]. On the contrary, Ag is the active site for dechlorination, and Pd is responsible for providing H to regenerate deactivated Ag. In contrast to the monometallic catalysts, Ag–Pd/ZrO2 with a trace loading of Pd exhibited substantially enhanced ethylene selectivity, and the bimetallic catalyst with a Pd loading of 0.099 wt% (denoted as Ag1.99Pd0.099/ZrO2) showed a nearly 100% ethylene selectivity. The Pd–Au alloy nanoparticles (ANs) were synthesized using the aqueous phase co-reduction method and used for the efficient electrocatalysts for the hydrodechlorination of 4-chlorophenol [57]. The Pd atoms captured electrons from Au to realize the successive electronic structure modulation by inducing the Au promoter as an electron donor. The obtained electron-rich Pd atoms were favored for the desorption of the dechlorination product phenol as well as accelerating the release of the active site (Figure 2). The Pd7Au3 alloy nanoparticles exhibited the highest electrocatalytic activity with 98.35% conversion within 4 h of 4-chlorophenol hydrodechlorination and a mass activity of 0.47 h−1 mgPd−1, which was 7.83 times higher than that of the monometallic Pd nanoparticles.
The AgPd bimetallic nanoparticles were fabricated by the solvothermal method and used for the selective electrocatalytic hydrodechlorination of 2,4-dichlorophenol (2,4-DCP) to phenol [58]. The density functional theory calculations indicated that the balance between 2,4-DCP adsorption and phenol (P) desorption was dominant in the electrocatalytic 2,4-DCP conversion efficiency. The rate-determining step (Figure 3) in the electrocatalytic hydrodechlorination process was the desorption of phenol (target intermediate) from the catalyst surface rather than the generation of Hads*. The competitive adsorption of the active sites between phenol and 2,4-DCP caused the toxic deactivation of the catalyst. The alloying of Pd and Ag with a suitable content effectively weakened the over-adsorption of the product phenol, as well as facilitated its desorption and released the active sites to improve electrocatalytic efficiency.
Nickel/zero-valent iron catalysts (Figure 4) were generated by the aqueous phase co-reduction method to probe electron transfer in the dechlorination of polychlorinated biphenyls (PCBs) [59]. It was found that an appropriate temperature was one of the dominant factors to determine the reduction efficiency of dichlorination. The NiFe bimetallic catalyst reached a 75.9% conversion at 25 °C, in which enhanced electron transfer could be attributed to the thermodynamic evolution of Fe species to generate sufficient Fe (II) and oxygen vacancies. The NiFe bimetallic surface endured severe oxidation and particle aggregation at 35 °C, which led to the reduced electron density of Fe (II) and suppressed electron transfer. Theoretical calculations and mass spectrometry detection showed that the electrons tended to attack the C–Cl bond in the order of interstitial > para > neighbor.
The electrocatalytic hydrodechlorination performance of the catalyst was severely constrained by the hydrogen precipitation process, which consumed the typical active hydrogen (H*) species for hydrodechlorination and reduction. Ni–Fe bimetallic catalysts (Figure 5) with abundant reductive H atoms were prepared by the co-precipitation method and used for the hydrodechlorination of trichloroethylene to ethylene and ethane [25]. Ni–Fe (Ni/Fe molar ratio = 3.0) hydroxide showed extremely strong reducing activity at a neutral pH, and 80.9% of trichloroethylene was removed after 110 min of the reaction. The co-precipitation of Ni2+ and Fe2+ in water to form Ni–Fe hydroxides was accompanied by the production of H* species at a neutral pH. Fe2+ was the only electron donor and active adsorbed H* species in the Ni–Fe hydroxide lattice with a Ni/Fe molar ratio of 3.0, which showed the best hydrodechlorination performance. The defect modulation strategy of the cation vacancy-rich CoFe bimetallic catalyst coupled with ultrafine Pd nanoparticles (4–6 nm in particle size) was designed to induce the electron distribution optimization of Pd for the dechlorination of 2,4-dichlorophenol [60]. The Pd-modified CoFe catalyst exhibited 95.46% conversion of 2,4-dichlorophenol in a broad range (4–11) of pH, with outstanding resistance to toxic Cl−, NO3−, NO2−, and CO32−. The support–metal strong interaction (SMSI) effect between Pd and CoFe induced electron transfer from the support to the loaded Pd active site to obtain the electron-rich Pd, which is favorable for driving the adsorption of halogenated phenols with a high water activation capacity and H* selectivity.
The classical system of using iron as an electron donor and nickel as a catalyst produces a synergistic effect to promote the hydrodechlorination of chlorinated organics. Ni/Fe nanoparticles modified with a surfactant of cetyltrimethylammonium bromide (CTAB) were fabricated to investigate the removal mechanism of pentachlorophenol [60]. In pentachlorophenol-contaminated soil solution, the Ni/Fe-CTAB nanoparticles could remove nearly 100% pentachlorophenol within 5 min via multiple dechlorination routes. The excellent performance of NiFe bimetallic nanoparticles was mainly attributed to the rapid adsorption of pentachlorophenol by CTAB and multiple dechlorination on the Ni/Fe surface. Lower toxicity and highly biodegradable intermediates (e.g., trichlorophenols, dichlorophenols, monchlorophenols, and phenol) were detected, which indicated that the dechlorination process of pentachlorophenol was limited, and furthermore it was multistep. The modification of CTAB induced electrostatic attraction between the electronegative phenolic acid groups of pentachlorophenol and positively charged Ni/Fe-CTAB, which was beneficial for the adsorption of pentachlorophenol.
Hydrogen atom-rich Pd/CuOx electrocatalysts (Figure 6) were obtained using the atomic reconstruction method and investigated for the degradation of 2,4-dichlorophenoxyacetic acid [62]. The density functional theory calculations indicated that the coordination structure between the CuPd bimetals accelerated the dissociation of H2O and effectively reduced the free adsorption energy of hydrogen. Hydrogen atoms on the Pd/CuOx cathode increased by 0.009 μmol/cm2, as compared with those on pure Cu. The Pd/CuOx bimetallic catalyst showed an excellent dechlorination performance, with the acid removal efficiencies (the rate constant) of 4-chloroaniline, 4-chlorophenol, and 2,4-dichlorophenoxyacetic at a current density of 4 mA/cm2 being 99% (0.08 min−1), 85% (0.03 min−1), and 94% (0.04 min−1), respectively.
Bimetallic Pd–Cu catalysts were synthesized by the chemical reduction method and used for the electrocatalytic hydrodechlorination of diclofenac [27]. It was found that the formation of bimetallic Pd–Cu alloys decreased the Pd0/Pd2+ molar ratio. The introduction of Cu to the Pd catalyst effectively optimized the kinetics of highly reactive hydrogen species generation. The density functional theory calculations demonstrated that the modulation of the electronic valence of the Pd–Cu bimetals weakened the bonding energy of H* to facilitate the generation of Hads*. The number of Hads* and the reduction potential of the Pd–Cu catalyst was 1.5 times higher and 0.37 eV lower than those of the mono-Pd electrode, respectively. The Pd5Cu5 alloy electrode exhibited almost the same hydrodechlorination activity as that of the mono-Pd catalyst with a halved content (superior diclofenac degradation efficiency = 93.3 ± 0.1%). The dual monatomic FeNi cathode catalysts with multiple active sites were prepared using the thermal decomposition method and studied for the selective dechlorination of chlorinated aromatic hydrocarbons [63]. The Fe-N4 and Ni-N4 active sites enhanced the generation and adsorption of hydrogen and hydroxyl active species. The coupling of the Fe–O and Ni–O active sites with Fe-N4 or Ni-N4 weakened the breaking energy barrier of the C–Cl bonds for higher activity and stability. The FeNi bimetallic electrode was insensitive to the molecular structure and effectively reduced the high energy barrier of chlorobenzene molecules. The electrophilic nature of the H* species favored the formation of hyperconjugated bonds that could weaken the possibility of the Cl atom bonding to the benzene ring, hence resulting in the facilitation of the single-electron transfer dechlorination process. The removal of chlorinated aromatic hydrocarbons over the FeNi cathodes followed the principle of the direct single-electron transfer stepwise dechlorination, and 98.9, 90.4, and 85.7% conversions of monochlorobenzene, dichlorobenzene, and trichlorobenzene were achieved at low voltages.
Chlorine-containing exhaust gasses always coexist with complex components, such as H2O, NOx, CO2, and other VOCs [64]. Generally speaking, the presence of other components brings about competitive adsorption, which weakens the performance of Cl-VOC removal. Interestingly, the presence of water at a low concentration may show a facilitating effect on catalytic activity of Cl-VOC elimination, while the existence of water with a high concentration exerts a negative effect [65]. The adsorbed water molecules can be dissociated into H+ and OH− at the active site to facilitate protonation and further the oxidation of by-products, thus accelerating the removal of surface Cl species. Furthermore, the presence of water is favorable for desorption of the accumulated Cl species through the washing effect, thus reducing the chlorination of the active components and intermediates. Therefore, a suitable water content can retard the poisoning of the catalyst through hydrolysis promotion to reach a good stability. Wang et al. systematically investigated the effect of water content and found that the dichloromethane conversion rate decreased with an increased water content [66]. It can be concluded that the competitive adsorption plays a dominant role at a high water content. Meanwhile, the hydroxyl groups dissociated from water suppress the formation of surface oxygen, which inhibits the elimination of CVOCs.
The coexistence of other VOCs always brings about competitive adsorption to occupy a part of the active sites and accelerate the consumption of the surface reactive oxygen species, which lead to a loss in activity [67]. However, H-rich hydrocarbons can provide a H source for HCl generation, thus facilitating Cl desorption and promoting CVOC removal [68]. NO2 generated from NO oxidation shows a strong oxidative ability and can be used to assist O2 to restock lattice oxygen and accelerate the oxidation cycle process, which can be ascribable to the enhanced removal efficiency in the presence of NO and CVOCs [69]. Competitive adsorption induced by the addition of CO2 occupies the oxygen vacancies and inhibits the adsorption of CVOCs at the active sites, thus preventing an effective reaction with the active oxygen species [70]. As mentioned above, the presence of non-chlorinated VOCs exhibits the potential of an inhibition or promotion effect. The future work should focus on achieving low-chlorination by-products as well as a high CVOC oxidation performance in the presence of water, NOx, CO2, or other VOCs.
As mentioned above, bimetallic catalysts have better activity and stability than those of monometallic catalysts. However, the rapid separation and strong adsorption of inorganic chlorine species lead to deactivation, which hinders their practical industrial application. The synergistic interaction between the bimetallic sites effectively enhances the redox properties and adsorption/desorption behaviors, which is contributable to the inhibited generation of polychlorinated by-products and better Cl-resistant durability. Catalysts used for efficient CVOC degradation should have excellent redox capacity to enhance CO2 selectivity, provide abundant surface acid sites to inhibit the formation of polychlorinated by-products, thus improving HCl selectivity and Cl resistance. The complete oxidation of CVOCs to H2O, CO2, and HCl does not have toxic effects on the environment, but at the same time, results in a waste of resources. The catalytic selective oxidation of CVOCs for the resource utilization of target products should be prospected.
Table 1. Catalytic performance for CVOCs elimination over bimetallic nanoparticles.
Table 1. Catalytic performance for CVOCs elimination over bimetallic nanoparticles.
CatalystMethodParticle Size (nm)Reaction ConditionPollutantElimination EfficiencyRef.
PdPtCo-impregnation method–267 ppm, SV = 10,000 h−1CH2Cl2T50 = 360 °C, T90 = 500 °C[43]
PdPtCo-impregnation method–188 ppm, SV = 10,000 h−1CHCl3T50 = 315 °C, T90 = 409 °C[43]
PdPtCo-impregnation method–145 ppm, SV = 10,000 h−1CCl4T50 = 275 °C, T90 = 410 °C[43]
CeNbImpregnation method4.6–8.71000 ppm, SV = 15,000 h−11,2-DichlorobenzeneT50 = 297 °C, T90 = 335 °C[44]
CeMnImpregnation method19.9–29.8500 ppm, SV = 15,000 h−11,2-DichlorobenzeneT50 = 300 °C, T90 = 360 °C[44]
NiVCitrate sol–gel method132–1481000 ppm, SV = 15,000 h−1DichloromethaneT50 = 168 °C, T90 = 203 °C[46]
PtRuCo-impregnation method8.7–10.11000 ppm, SV = 40,000 mL/(g h)ChlorobenzeneT50 = 234 °C, T90 = 270 °C[48]
PtWCo-impregnation method3.6–4.01000 ppm, SV = 60,000 h−1Chlorobenzene T50 = 263 °C, T90 = 327 °C[52]
PtWCo-impregnation method3.6–4.01000 ppm, SV = 60,000 h−11,2-DichlorobenzeneT50 = 293 °C, T90 = 363 °C[52]
PtWCo-impregnation method3.6–4.01000 ppm, SV = 60,000 h−11,2-DichloroethaneT50 = 235 °C, T90 = 302 °C[52]
PtWCo-impregnation method3.6–4.01000 ppm, SV = 60,000 h−1DichloromethaneT50 = 235 °C, T90 = 335 °C[52]
PtWCo-impregnation method3.6–4.01000 ppm, SV = 60,000 h−1TrichloroethyleneT50 = 282 °C, T90 = 400 °C[52]
RuCeEqual-volume impregnation method20.23000 mg/m3, SV = 30,000 h−1ChlorobenzeneT50 = 165 °C, T90 = 180 °C[53]
AuPtAtomic layer deposition method5.3H2 pressure, 0.3 MPa; T = 65 °C; p-CNb, 8 mmolp-Chloronitrobenzenep-CAN selectivity of 99% with a specific activity of 1.75 × 104 molp-CNB/(molPt h)[48]
AgPtPhoto-deposition method2.2–3.57300 ppm 1,2-dichloroethane, 36,800 ppm H2, He (balance)Hydrodechlorination of 1,2-dichloroethane to ethylenePt1.03Ag0.93/TiO2 exhibited 68.1% ethylene selectivity[54]
AgPdGalvanic replacement method4.37300 ppm 1,2-dichloroethane, 36,800 ppm H2, He (balance)Hydrodechlorination of 1,2-dichloroethane to ethylenePt0.13Ag0.84/Al2O3 exhibited 94.6% ethylene selectivity[55]
AgPdCo-impregnation method–7300 ppm 1,2-dichloroethane, 36,800 ppm H2, He (balance)Hydrodechlorination of 1,2-dichloroethane to ethyleneAg1.99Pd0.099/ZrO2 exhibited 100% ethylene selectivity[56]
AuPdAqueous phase co-reduction method8.1Initial concentration = 50 mg/L 4-Chlorophenol hydrodechlorination98.35% 4-chlorophenol hydrodechlorination efficiency in 4 h of reaction and mass activity = 0.47 h−1 mgPd−1[57]
CeZrSolvothermal method1000.1 μL of HCB solution, SV = 12,000 mL/(g h)HexachlorobenzeneThe degradation efficiency of HCB over Ce0.2Zr0.8/UiO-66 was the highest (86.5%) at 100 °C[51]
AgPdSolvothermal method–Initial concentration = 50 mg/L 2,4-DichlorophenolMass activity of Ag32Pd68 reached 2.58 min−1 gPd−1[58]
FeNiAqueous phase co-reduction method6.550 mL of pentachlorobiphenyl (100 μg/L)2,2′,4,4′,5-PentachlorobiphenylThe dechlorination efficiency at 25 °C reached 75.9% within 2 h of reaction[59]
NiFeCo-precipitation method600pH = 7.5, initial concentration = 42 μmol/LTrichloroethyleneThe removal efficiency reached 80.9% after 110 min of reaction[25]
CoFeSolvothermal method–pH = 6.0, initial concentration = 10 mg/L 2,4-Dichlorophenol95.46% dechlorination activity[61]
NiFeAqueous phase co-reduction method50–100Initial concentration = 7.5 mg/LPentachlorophenolThe dechlorination efficiency was close to 100% in 5 min of reaction[60]
PdCuAtomic reconstruction method–Initial concentration = 40 μmol/L2,4-Dichlorophenoxyacetic acidDegradation efficiency = 94% in 60 min of reaction[62]
PdCuAtomic reconstruction method–Initial concentration = 40 μmol/L4-Chlorophenol Degradation efficiency = 85% in 60 min of reaction[62]
PdCuAtomic reconstruction method–Initial concentration = 40 μmol/L4-ChloroanilineDegradation efficiency = 99% in 60 min of reaction[62]
PdCuChemical reduction method6.16 ± 0.31Initial concentration = 5 mg/LDiclofenacDegradation efficiency = 93.3 ± 0.1% in 10 h of reaction[27]
NiFeThermal decomposition method–pH = 5, initial concentration = 20 mg/LMonochlorobenzeneDegradation efficiency = 98.9% in 30 min of reaction[63]
NiFeThermal decomposition method–pH = 5, initial concentration = 20 mg/LDichlorobenzeneDegradation efficiency = 90.4% in 30 min of reaction[63]
NiFeThermal decomposition method–pH = 5, initial concentration = 20 mg/LTrichlorobenzenedegradation efficiency of 85.7% in 30 min of reaction[63]

4. Conclusions and Perspectives

The rational design and development of bimetallic catalysts are an important research direction for catalytic dechlorination applications. The inorganic chlorine species generated by the dechlorination reaction are easily separated and strongly adsorbed on a catalyst surface, which lead to the toxic deactivation of catalysts. This review article presents the research advancements on the synthesis approaches and activities, selectivities and stability in the removal of CVOCs during removals over the bimetallic catalysts. The optimization scheme of bimetallic sites on the selectivity of polychlorinated by-products is briefly discussed to provide a useful reference for the design of efficient catalysts. The advantages of bimetallic catalysts are summarized as follows: (i) Bimetallic catalysts can induce the generation of abundant reactive oxygen species, which are ascribed to the enhanced redox ability, oxygen mobility, and surface acidity for an improved catalytic performance. (ii) The strong interaction between the bimetallic nanoparticles reduces the reaction activation energy and the surface adsorption energy to promote the desorption of the chlorine-containing by-products and release the active sites. (iii) The introduction of a second metal promotes to enhance the COx yield and inorganic chlorine generation, which inhibits the production of the chlorinated by-products so as to reduce the toxic dioxin emission. (iv) The introduction of a second metal can modify the bimetallic surface into an electron-rich state, which promotes the generation of the reduced H atom to facilitate the electrocatalytic hydrogenation dechlorination process.
Chlorine-containing exhaust gasses always coexist with complex components, such as H2O, NOx, CO2, and/or other VOCs. Generally speaking, the presence of other components brings about competitive adsorption, which weakens the performance of Cl-VOC removal. The presence of other components exhibits the potential of the inhibition or promotion effect, which is dependent on the desired catalyst. The future work should focus on designing target catalytic materials to achieve low-chlorination by-products, as well as a good CVOC oxidation performance in the presence of water, NOx, CO2, and/or other VOCs.
The controlled synthesis of bimetallic catalysts allows for the definite and intensive study of intrinsic reactivity, which provides a fundamental guidance for the rational designing of efficient CVOC elimination catalysts. However, the selective generation of polychlorinated products and the stabilization of the active components need to be fully considered for the practical application of the catalysts. The dynamics of redox kinetics on the surface of bimetallic catalysts can be tracked by in situ Raman and modulated ambient pressure XPS to clarify the direct correlation between by-product formation and the altered catalytic properties, which is beneficial for investigating the chemical stabilization of the active components. The systematic analysis of the mechanisms of dechlorination intermediate species allows us to explore possible inhibition pathways or the secondary elimination of by-products. CVOCs present diverse components and low concentrations in practical industrial applications. Thus, an investigation of enhanced resistance to chlorine poisoning and the efficient CVOC elimination of bimetallic catalysts is the most important in achieving industrial applications. Furthermore, good thermal stability is also critical in CVOC elimination. Improved resistance to thermal shock without the loss of activity is also one of the crucial issues to be overcome. Hence, the rational design of bimetallic catalysts with the desired activities, selectivities, and stability presents preferential consideration for practical industrial applications.

Author Contributions

This manuscript was written through the contributions of all the authors. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (2022YFB3504101 and 2022YFB3506200), the National Natural Science Foundation of China (22306008, 21876006, 21976009, and 21961160743), the Foundation on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions (IDHT20190503), the Natural Science Foundation of Beijing Municipal Commission of Education (KM201710005004), and the Development Program for the Youth Outstanding—Notch Talent of Beijing Municipal Commission of Education (CIT&TCD201904019).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Possible oxidation mechanism of dichloromethane over the Ni–V/TiO2 catalyst [47]. Reprinted with permission from Ref. [46]. Copyright 2019, Elsevier.
Figure 1. Possible oxidation mechanism of dichloromethane over the Ni–V/TiO2 catalyst [47]. Reprinted with permission from Ref. [46]. Copyright 2019, Elsevier.
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Figure 2. Adsorption model and adsorption energy of 4-chlorophenol and phenol on the (111) crystal plane of pure Pd, Pd3Au1, Pd2Au2 and pure Au, a Pd atom (gray), a Au atom (yellow), a C atom (brown), a H atom (white), an O atom (red), and a Cl atom (green) [58]. Reprinted with permission from Ref. [57]. Copyright 2022, Elsevier.
Figure 2. Adsorption model and adsorption energy of 4-chlorophenol and phenol on the (111) crystal plane of pure Pd, Pd3Au1, Pd2Au2 and pure Au, a Pd atom (gray), a Au atom (yellow), a C atom (brown), a H atom (white), an O atom (red), and a Cl atom (green) [58]. Reprinted with permission from Ref. [57]. Copyright 2022, Elsevier.
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Figure 3. (a) The reaction pathway for electrocatalytic hydrodechlorination of 2,4-DCP on the AgPd bimetallic nanoparticle (NP) catalysts, and (b) a schematic illustration of the role of Ag for electrocatalytic hydrodechlorination in AgPd NP catalysts [58]. Reprinted with permission from Ref. [58]. Copyright 2019, American Chemical Society.
Figure 3. (a) The reaction pathway for electrocatalytic hydrodechlorination of 2,4-DCP on the AgPd bimetallic nanoparticle (NP) catalysts, and (b) a schematic illustration of the role of Ag for electrocatalytic hydrodechlorination in AgPd NP catalysts [58]. Reprinted with permission from Ref. [58]. Copyright 2019, American Chemical Society.
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Figure 4. The possible dechlorination pathways of PCBs based on (a) the bond dissociation energy (kcal/mol), (b) the C–Cl bond length, and (c) the lowest unoccupied molecular orbital (LUMO) of PCB anions [59]. Reprinted with permission from Ref. [59]. Copyright 2023, Elsevier.
Figure 4. The possible dechlorination pathways of PCBs based on (a) the bond dissociation energy (kcal/mol), (b) the C–Cl bond length, and (c) the lowest unoccupied molecular orbital (LUMO) of PCB anions [59]. Reprinted with permission from Ref. [59]. Copyright 2023, Elsevier.
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Figure 5. DFT and schematic diagram of Pd@CoFeV-LDH. (a) Optimized adsorption configurations and adsorption energies of 2,4-dichlorophenol; (b) schematic profiles for energy reaction barriers of water activation pathway (Volmer Step) on Pd@CoFeV-LDH, CoFeV-LDH, and Pd NPs; (c) free-energy diagram for HER; and (d) schematic diagram of facilitation for EHDC reaction of 2,4-dichlorophenol over Pd@CoFeV-LDH/NF catalyst [61]. Reprinted with permission from Ref. [61]. Copyright 2024, Elsevier.
Figure 5. DFT and schematic diagram of Pd@CoFeV-LDH. (a) Optimized adsorption configurations and adsorption energies of 2,4-dichlorophenol; (b) schematic profiles for energy reaction barriers of water activation pathway (Volmer Step) on Pd@CoFeV-LDH, CoFeV-LDH, and Pd NPs; (c) free-energy diagram for HER; and (d) schematic diagram of facilitation for EHDC reaction of 2,4-dichlorophenol over Pd@CoFeV-LDH/NF catalyst [61]. Reprinted with permission from Ref. [61]. Copyright 2024, Elsevier.
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Figure 6. (a) The optimal H* adsorption configuration for Pd, Pd/Cu, Pd/CuO, and Pd/Cu2O; (b) the corresponding free energy diagram for the HER over Pd, Pd/Cu, Pd/CuO, and Pd/Cu2O; and (c) a schematic diagram of synergistic interaction over Pd/CuOx [62]. Reprinted with permission from Ref. [62]. Copyright 2024, Elsevier.
Figure 6. (a) The optimal H* adsorption configuration for Pd, Pd/Cu, Pd/CuO, and Pd/Cu2O; (b) the corresponding free energy diagram for the HER over Pd, Pd/Cu, Pd/CuO, and Pd/Cu2O; and (c) a schematic diagram of synergistic interaction over Pd/CuOx [62]. Reprinted with permission from Ref. [62]. Copyright 2024, Elsevier.
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MDPI and ACS Style

Lin, H.; Liu, Y.; Deng, J.; Jing, L.; Wang, Z.; Wei, L.; Wei, Z.; Hou, Z.; Tao, J.; Dai, H. The Advancement of Supported Bimetallic Catalysts for the Elimination of Chlorinated Volatile Organic Compounds. Catalysts 2024, 14, 531. https://doi.org/10.3390/catal14080531

AMA Style

Lin H, Liu Y, Deng J, Jing L, Wang Z, Wei L, Wei Z, Hou Z, Tao J, Dai H. The Advancement of Supported Bimetallic Catalysts for the Elimination of Chlorinated Volatile Organic Compounds. Catalysts. 2024; 14(8):531. https://doi.org/10.3390/catal14080531

Chicago/Turabian Style

Lin, Hongxia, Yuxi Liu, Jiguang Deng, Lin Jing, Zhiwei Wang, Lu Wei, Zhen Wei, Zhiquan Hou, Jinxiong Tao, and Hongxing Dai. 2024. "The Advancement of Supported Bimetallic Catalysts for the Elimination of Chlorinated Volatile Organic Compounds" Catalysts 14, no. 8: 531. https://doi.org/10.3390/catal14080531

APA Style

Lin, H., Liu, Y., Deng, J., Jing, L., Wang, Z., Wei, L., Wei, Z., Hou, Z., Tao, J., & Dai, H. (2024). The Advancement of Supported Bimetallic Catalysts for the Elimination of Chlorinated Volatile Organic Compounds. Catalysts, 14(8), 531. https://doi.org/10.3390/catal14080531

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