A Systematical Comparison of Catalytic Behavior of NM/γ-Al₂O₃ (NM = Ru, Rh, Pt, Pd, Au, Ir) on 1,2-Dichloroethane Oxidation: Distributions of By-Products and Reaction Mechanism

Featured Application

This work supplies a systematical comparison study on various typical noble metal catalysts with an exploration of the potential for CVOC destruction, which would supply an overall view of universal applicability for the practical operation of noble metals.

Abstract

Understanding the reaction path and mechanism of chlorinated volatile organic compound (CVOC) destruction is important for designing efficient catalysts, especially for the application of noble metal-based materials. Herein, several typical noble metals, Ru, Rh, Pt, Pd, Au, and Ir, supported on γ-Al₂O₃ catalysts were synthesized by the hydrazine hydrate reduction method for 1,2-dichloroethane (1,2-DCE) elimination. Various character measurements were conducted, and the results suggest that the high-valence state of noble metals is beneficial for the 1,2-DCE reaction as it enables the enhancement of the mobility of the surficial active oxygen species of catalysts. Among the noble metals, Ru/γ-Al₂O₃ expresses superior catalytic reactivity, with a 90% pollutant conversion rate at 337 °C, and competitive CO₂ selectivity, 99.15% at the temperature of total oxidation. The distribution of by-products and the degradation routes were analyzed online by GC-ECD and in situ diffuse reflectance infrared spectroscopy, which may provide helpful insight for the future application of noble metal-based catalysts for CVOC elimination in industrial fields.

1. Introduction

As one of the most toxic and complicated kinds of volatile organic compounds (VOCs), chlorinated volatile organic compounds (CVOCs) are usually difficult to eliminate [1,2] due to their high volatility, strong recalcitrance, and their side effects for the operation system [3,4,5]. CVOCs includes chlorinated alkanes [6,7], chlorinated olefins [8], chlorinated aromatic hydrocarbons [9], etc., which are regarded as vital compounds of common contaminants, which have been widely investigated for years [10,11]. Adsorption [12,13,14], absorption, condensation and thermal incineration [15], biodegradation [16], photocatalytic degradation [17], plasma technology, and catalytic combustion [4] are applied for elimination of these pollutants. Of which, catalytic oxidation is widely considered as one of the most promising method due to ultra-purification efficiency, few secondary pollution, and lower reaction temperature; therefore it is thought to be one of the most impressive methods for CVOCs elimination.

Currently, the catalysts for CVOC catalytic oxidation can be mainly applied to supported noble metals, molecular sieves, transition metal oxides, perovskite catalysts, etc. Among these different materials, noble metal-based catalysts have, in recent years, received the attention of many researchers due to their superior low-temperature activity, high CO₂ selectivity, and competitive stability. More importantly, noble metal-based catalysts have been successfully applied practically in various heterogeneous catalytic areas. Few other kinds of materials can be substituted for noble metals in practical operation nowadays [2]. Supported noble metal catalysts are generally catalysts with ruthenium (Ru) [18], platinum (Pt) [19], palladium (Pd) [20], or rhodium (Rh) [21] as active components, and inactive oxides (such as Al₂O₃ [22,23], SiO₂, ZrO₂ [24], TiO₂ [25,26], etc.), molecular sieves [27], perovskite composite oxide LaBO₃ [28,29], and pillared clay as carriers. Maupin et al. [23] proposed a bifunctional reaction mechanism of dichloromethane catalyzed by Pt/Al₂O₃, that is, CH₂Cl₂ disproportionated on Al₂O₃ to form CH₃Cl and CO, and then further oxidized on the active site of the Pt metal species. Kaluza et al. [30] believe that the catalytic activity of the catalyst increases with the noble metal loading amount, and the results showed that a Pt/CeO₂-ZrO₂ catalyst with 1% Pt loading is the best. López Fonseca et al. [31] synthesized an H-Beta-supported Pd material for TCE elimination, and the results showed that the 50% conversion of the TCE reaction temperature shifted forward by 150 °C for the molecular sieve supported by precious metal, and PdO was the main active phase, and its activity and stability were better than that of precious metal Pd. At the same time, PdO/H-Beta improves the selectivity of CO₂, but also increases the production of chlorine. Giraudon et al. [32] studied the catalytic decomposition of chlorobenzene over Pd/ZrO₂ and Pd/TiO₂ materials. It was found that, because TiO₂ has higher reducibility than ZrO₂, the Pd/TiO₂ catalyst has better activity, and both catalysts produce a large amount of polychlorinated benzene. In the same year, they found that [33] there was a synergistic effect between the metal Pd and perovskite, which made the catalytic efficiency of Pd/LaBO₃ better than that of pure perovskite. Dai et al. [34] prepared the supported Ru/CeO₂ catalyst and found that the dichlorination effect from catalyst surface was promoted due to superior deacon reaction activity of RuO₂, which is beneficial for active sites protection and catalyst stability. Based on the results, the author prepared a series of Ru/Ti-CeO₂ [35] catalysts by doping Ti, and found that the superior combination of 5% Ti doping showed the best reaction performance. Miranda et al. [36] compared the catalytic behavior over trichloroethene of Al₂O₃ supported Pd, Pt, Ru, and Rh materials, the results indicate that the byproduct distribution of Ru-based material was vitally different from other ones. Liu et al. [37] evaluated the detailed polychlorinated byproduct distributions of some noble metals (Pd, Pt, Ru, Rh) loaded on TiO₂ catalysts by an impregnation method for the catalytic destruction of chlorobenzene. As a widely used support material, γ-Al₂O₃ is low-cost and environmentally friendly, which is beneficial for universal application and commercial operation. In fact, many reports have recently pointed out that the catalytic performance of noble metal-based catalysts in the combustion of CVOCs can be improved by introducing different supports, as summarized in Table S1 of Supplementary Information. As a result, noble metals supported on γ-Al₂O₃ are more practical than other types of catalysts, and warrant a systematical discussion of various noble metal-based catalysts. For universal application, the synthesis routine should be simple, economic, and environmentally friendly. On the other hand, at present, only the reaction mechanism of specific pollutants on specific catalysts has been explored, but a universal reaction mechanism on various noble metal catalysts should be investigated to find out whether there is a common route for CVOC oxidation on noble metal materials.

Hence, in this work, we proposed several exemplary noble metals (Ru, Rh, Pt, Pd, Ir, and Au)-based catalysts supported on a common γ-Al₂O₃ material and then systematically investigated their performance on 1,2-dichloroethane (1,2-DCE) oxidation. After characteristic analysis and activity experiments via various measurements, the by-product distribution and detailed oxidation routes were presented to further evaluate the universal reaction mechanism for the purpose of designing superior catalysts.

2. Materials and Methods

2.1. Synthesis of NM (Ru, Rh, Pt, Pd, Au, Ir)/γ-Al₂O₃

The hydrazine hydrate reduction method was used to synthesize the noble metal-based catalysts. Respectively, RuCl₃, (NH₄)₃RhCl₆, H₂PtCl₆∙6H₂O, PdCl₂, AuCl₃, and H₂IrCl₆∙xH₂O were used for each noble metal precursor. During preparation, the loading amount of noble metal was set to 1.0 wt.%. In detail, 1 g of γ-Al₂O₃ (a commercial sample that was purchased from Sigma-Aldrich Co., St. Louis, MO, USA) was dispersed in 60 mL deionized water, 60 mL absolute ethanol, and 20 mL precious metal precursor solution by ultrasonic for about 30 min, followed by stirring for another 20 min to mix evenly. Afterward, the mixture was transferred into a three-neck flask. The mixture was then saturated by N₂, 0.01 g NaBH₄ in 20 mL 0.5 M NaOH solution as an alkaline regulator and 6 mL hydrazine hydrate solution, while a reducing agent was added dropwise into the mixture under stirring by a peristaltic pump. After 4 h, the suspension blend was filtered, washed with deionized water and ethanol several times, and then the resultant solid was dried at 100 °C for 10 h. The final products were calcined at 450 °C for 4 h at a heating rate of 2 °C/min. The catalysts were denoted as Ru/γ-Al₂O₃, Rh/γ-Al₂O₃, Pt/γ-Al₂O₃, Pd/γ-Al₂O₃, Ir/γ-Al₂O₃, and Au/γ-Al₂O₃.

2.2. Reactivity Evaluation

The catalytic activity was assessed in a fixed-bed reactor (I.D. = 10 mm). With each batch tested, 0.3 g of the prepared catalysts (40–60 mesh) was put at the center of a quartz reactor. Moreover, 1000 ppm of 1,2-DCE diluted in air with a flow rate of 300 mL/min was used as the inlet reaction stream, recalculated as a gas hourly space velocity (GHSV) of approximately 36,000 mL∙g⁻¹∙h⁻¹. The concentration of 1,2-DCE and the organic by-products was measured using an online gas chromatograph (GC9890, China) equipped with an electron capture detector (ECD). The content of CO and CO₂ from the outlet gas flow was analyzed by an online gas chromatograph (GC9890A, China) equipped with a flame ionization detector (FID) and nickel converting equipment, similar to that reported by our group previously [38,39,40]. The 1,2-DCE conversion efficiency and the yield of CO and CO₂ were determined using the following equations:

$$X_{1,2-\mathrm{DCE}}=\frac{C_{\mathrm{in}}-C_{\mathrm{out}}}{C_{\mathrm{in}}}\times 100\%$$

(1)

$$Y_{\mathrm{CO}}=\frac{C_{\mathrm{CO}}}{2C_{\mathrm{in}}\times X_{1,2-\mathrm{DCE}}}\times 100\%$$

(2)

$$Y_{{\mathrm{CO}}_2}=\frac{C_{{\mathrm{CO}}_2}}{2C_{\mathrm{in}}\times X_{1,2-\mathrm{DCE}}}\times 100\%$$

(3)

where C_in, C_out are the 1,2-DCE concentration before and after the reaction, respectively, and $C_{\mathrm{CO}}$ and $C_{{\mathrm{CO}}_2}$ are the outlet consistent of CO and CO₂, respectively.

The normalized reaction rate (r_1,2-DCE, mmol∙g⁻¹∙s⁻¹) was derived using Equation (4):

$$r_{1,2-\mathrm{DCE}}=\frac{X_{1,2-\mathrm{DCE}}\times V_{1,2-\mathrm{DCE}}}{W_{\mathrm{cat}}(wt\%\mathrm{NM})}$$

(4)

where W_cat is the amount of catalyst (g), wt% NM is the weight ratio of noble metal in the catalyst (%), while V_1,2-DCE is the 1,2-DCE gas flow rate (mol s⁻¹).

When the reaction efficiency of 1,2-DCE is below 15%, the reaction rate would be described as an alternative formula, as follows:

$$r_{1,2-\mathrm{DCE}}=A\exp (\frac{E_a}{RT})P_{1,2-\mathrm{DCE}}^{\alpha }{P}_{{\mathrm{O}}_2}^{\beta }$$

(5)

where E_a represents the apparent activation energy (J mol⁻¹), R delegates the ideal gas constant (J mol⁻¹ K⁻¹), T is the test temperature (K), P^α represents the fractional pressure of 1,2-DCE (Pa), and P^β is fractional pressure of O₂ (Pa).

Activation energies were calculated at low temperatures for 1,2-DCE conversion efficiencies lower than 15%, which is determined by the Arrhenius relationship via Equation (5) simplified into Equation (6):

$$\ln r=\frac{-E_a}{RT}+C$$

(6)

The turnover frequency was calculated using the following equation:

$$TOF=\frac{C_{\mathrm{DCE}}\times X_i\times \nu }{\frac{m_{NM}}{M_{NM}}\times D_i}$$

(7)

where X_i represents the 1,2-DCE conversion efficiencies of different catalysts at certain temperatures; C_DCE is the feed concentration of 1,2-DCE (mol L⁻¹), ν is the volumetric flow rate (L s⁻¹), M_NM is the molecular weight of NM (g mol-1), m_NM is the NM loading on the catalyst (g), and D_i is the NM dispersion (%) on the catalyst surface obtained from the CO-chemisorption results.

2.3. Catalyst Characterizations

Various measurements were conducted to characterize the physiochemical properties of the prepared samples, including inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), N₂ adsorption-desorption measurements, field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM) images, H₂ temperature-programmed reduction (H₂-TPR), O₂ temperature-programmed desorption (O₂-TPD), NH₃ temperature-programmed desorption (NH₃-TPD) experiments, pulsed CO chemisorption measurements, X-ray photoelectron spectroscopy (XPS) analysis, etc. The detailed descriptions are listed in Supporting Information.

2.4. In Situ DRIFTS Experiments

To deeply explore the kinetics of 1,2-DCE oxidation over the noble metal catalysts, in situ diffuse reflectance infrared spectroscopy (in situ DRIFTS) experiments were implemented and the details are illustrated in Supporting Information.

3. Results and Discussion

3.1. Catalyst Activity Performance

The reaction activity and apparent activation energies of 1,2-DCE over the (Ru, Rh, Pt, Pd, Ir, and Au)/γ-Al₂O₃ materials were tested and calculated, and are exhibited in Figure 1a. As shown, the catalytic performance of the prepared catalysts can destruct 1,2-DCE below 400 °C. By comparing the prepared materials at T₉₀ (the reaction temperature of 90% of 1,2-DCE conversion), the conversion efficiencies of all prepared catalysts can be ordered as follows: of Ru/γ-Al₂O₃ (337 °C) > Rh/γ-Al₂O₃ (349 °C) > Pt/γ-Al₂O₃ (361 °C) = Pd/γ-Al₂O₃ (361 °C) > Au/γ-Al₂O₃ (373 °C) > Ir/γ-Al₂O₃ (380 °C). Overall, the Ru-based catalyst showed the best 1,2-DCE degradation performance among the noble metal catalysts, which might be associated with its excellent dechlorination effect from Deacon reaction process [9], and which has the advantage of decreasing the toxic impact of chlorine species compared to other noble metal materials. On the other hand, the simple synthesis method of NaBH₄ and hydrazine hydrate reduction could create superior NM-supported catalysts compared with most of the other samples, as shown in Table S1. What is more, the catalytic ability of γ-Al₂O₃ was tested and illustrated in Figure S2, where a 90% 1,2-DCE conversion occurred at 419 °C, the efficiency is far lower than that of noble metal-supported catalysts, indicating that the enhancement of the oxidation ability is mainly ascribed to the participation of noble metals.

The CO and CO₂ yield from effluent streams of the NM/γ-Al₂O₃ catalysts was tested and is displayed in Figure 1c. As shown, the CO₂ yield of NM/γ-Al₂O₃ catalysts enhances with the increase in reaction temperature. Along with the 1,2-DCE complete oxidation condition, the CO₂ yield efficiency of the prepared catalysts can be ordered as follows: Ru/γ-Al₂O₃ (99.15%) > Rh/γ-Al₂O₃ (96.79%) > Pt/γ-Al₂O₃ (94.31%) > Pd/γ-Al₂O₃ (92.20%) > Ir/γ-Al₂O₃ (54.52%) > Au/γ-Al₂O₃ (40.76%). As shown, the CO yield increases along with the climbing temperature and decreases rapidly when 1,2-DCE conversion is more than 50% for Ru/γ-Al₂O₃, Rh/γ-Al₂O₃, Pt/γ-Al₂O_3, and Pd/γ-Al₂O₃ catalysts. Moreover, CO concentration increases with the increase in temperature over Ir/γ-Al₂O₃ and Au/γ-Al₂O₃. In summary, the CO yield of those catalysts at the temperature of 1,2-DCE complete oxidation can be ordered as follows: Ir/γ-Al₂O₃ (33.14%) > Au/γ-Al₂O₃ (28.33%) > Pd/γ-Al₂O₃ (3.81%) > Pt/γ-Al₂O₃ (1.00%) > Rh/γ-Al₂O₃ (0.07%) > Ru/γ-Al₂O₃ (0%). CO_x generation is usually represented as an indicator of mineralization of VOC oxidation; in this aspect, Ru/γ-Al₂O₃ displayed the highest mineralization efficiency compared with other catalysts.

Figure 1b and

Table 1. Catalytic activity of synthesized catalysts.

Samples	Conversion		Ea a	TOF b
	T50 (°C)	T90 (°C)	kJ mol−1	10−3 s−1
Ru/γ-Al2O3	289	337	30.90	3.19
Rh/γ-Al2O3	220	349	36.11	2.89
Pt/γ-Al2O3	228	361	35.89	0.84
Pd/γ-Al2O3	227	361	27.45	4.75
Au/γ-Al2O3	267	373	40.89	2.12
Ir/γ-Al2O3	230	380	31.29	7.16

^a Apparent activation energy: calculated by Equation (6); ^b Turnover frequency: calculated by Equation (7) at 200 °C.

show the E_a tendency of the prepared catalysts, which was calculated to further evaluate the catalytic behavior of the prepared materials. The E_a indexes of the synthesized catalysts can be ordered as follows: Pt/γ-Al₂O₃ (47.89 KJ·mol⁻¹) > Rh/γ-Al₂O₃ (36.11 KJ·mol⁻¹)> Ru/γ-Al₂O₃ (35.90 KJ·mol⁻¹) > Au/γ-Al₂O₃ (28.89 KJ·mol⁻¹) > Pd/γ-Al₂O₃ (27.45 KJ·mol⁻¹) > Ir/γ-Al₂O₃ (21.29 KJ·mol⁻¹), suggesting that Ir/γ-Al₂O₃ boasts the lowest light-off temperature and expresses the highest 1,2-DCE conversion activity at low temperatures. At the same time, the TOFs were also estimated at a low conversion efficiency range (200 °C was used in this case) and listed in Table 1. Of course, the TOF value of NM/Al₂O₃ catalysts for 1,2-DCE oxidation at 200 °C can be ordered as follows: Ir/Al₂O₃ > Pd/Al₂O₃ > Ru/Al₂O₃ > Rh/Al₂O₃ > Au/Al₂O₃ > Pt/Al₂O₃, indicating the superior low-temperature activity of the Ir- and Pd-based catalysts and poorest performance of the Pt-based sample. However, a slightly different trend is found with that of E_a, where the Ru- and Rh-based catalysts exhibited higher TOF values compared to that of Au, which is the opposite of the E_a tendency, suggesting an intrinsic potential activity of Ru/Al₂O₃ and Rh/Al₂O₃ in the 1,2-DCE oxidation reaction [20].

The organic by-products were further evaluated during 1,2-DCE destruction. As shown in Figure 1d, all by-products in the process of 1,2-DCE destruction over the NM/γ-Al₂O₃ catalysts were acetaldehyde (C₂H₄O), dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃), tetrachloromethane (CCl₄), trichloroethylene (C₂HCl₃), 1,1,2-trichloroethane (1,1,2-C₂H₃Cl₃), and tetrachloroethylene (C₂Cl₄). The amounts of by-products increased gradually with the elevating temperature first, and then decreased slightly with the temperature at 50% conversion of 1,2-DCE over the Ru/γ-Al₂O₃, Rh/γ-Al₂O₃, and Pt/γ-Al₂O₃ catalysts. For the other catalysts, the amounts of by-products increased gradually with the increase in temperature over the Pd/γ-Al₂O₃, Ir/γ-Al₂O₃, and Au/γ-Al₂O₃ catalysts until the highest test temperature of 400 °C. Summarily, the amounts of organic by-products of the prepared catalysts can be ordered as follows: Au/γ-Al₂O₃ > Ir/γ-Al₂O₃ > Pt/γ-Al₂O₃ > Pd/γ-Al₂O₃ > Rh/γ-Al₂O₃ > Ru/γ-Al₂O₃. Along with the analysis of the CO_x yield tendency, it was observed that although the Au- and Ir-based catalysts obtained a higher 1,2-DEC conversion efficiency at lower temperatures, the conversion just stops at the point of “degradation” at higher temperatures, with higher organic compound generation and lower mineralization efficiency. In contrast, a superior “destruction” ability was found for Ru/γ-Al₂O₃, which exhibited the maximum emergence level of organic by-products at 300 °C and then decreased significantly with the increasing temperature, which shows the lowest organic generation under its T₉₀ reaction conditions compared with that of other catalysts.

The durability ability of the NM/Al₂O₃ catalysts was also tested for 1,2-DCE catalytic oxidation at each T₉₀ condition to evaluate their stability performance. As shown in Figure 2, the Pt, Rh, and Ru catalysts could sustain 90% conversion efficiency in 50 h of operation, while Au, Ir, and Pd could only sustain this level of conversion efficiency for 44, 47, and 40 h, respectively. In summary, Ru/Al₂O₃ exhibited a better long-term operation stability at a lower temperature of 337 °C, with higher mineralization efficiency and lower organic by-product generation, making it more suitable for practical applications compared with other NM/Al₂O₃ catalysts.

3.2. Crystalline and Morphology

Figure S1 shows the XRD pattern of supporting material γ-Al₂O₃, in which the typical diffraction peaks of 2θ at 32.0°, 37.7°, 45.7°, and 66.6° are well agreed with PDF card # 97-003-9014. Figure 3 showed the XRD patterns of all prepared materials. Of course, all NM/γ-Al₂O₃ catalysts have similar diffraction peaks at 2θ = 45.7° and 67.1°, which can be attributed to the presence of γ-Al₂O₃. The Ru/γ-Al₂O₃ catalyst exhibits three obvious diffraction peaks at 2θ of 27.9°, 34.8°, and 54.2°, suggesting the formation of RuO₂ phase [36]. No crystalline phase related to rhodium oxide can be identified with Rh/γ-Al₂O₃, suggesting that the rhodium species exhibits a high level of dispersion on the surficial framework of the γ-Al₂O₃ support [41]. A diffraction peak can be identified at 2θ = 39.7° over Pt/γ-Al₂O₃ catalyst, attributed to the Pt (111) plane of metallic Pt [42]. There are three small diffraction peaks for the Pd/γ-Al₂O₃ catalyst at 2θ = 34.3°, 54.9°, and 61.2°, which can be attributed to the PdO phase [43]. The Ir/γ-Al₂O₃ catalyst exhibits two small diffraction peaks at approximately 35.1° and 53.1°, suggesting the appearance of IrO₂ phase [44]. No crystalline phase can be found for Au/γ-Al₂O₃, which suggests the Au species was dispersed well on the surface of the γ-Al₂O₃ support [45].

Figure 4 and Figure 5 show the typical SEM images and the EDS elemental mapping of the prepared samples. As displayed, the FE-SEM photos in Figure 4a,g,m suggest that precious metal species are well-distributed, especially for Rh/γ-Al₂O₃. Meanwhile, Pt/γ-Al₂O₃ and Pd/γ-Al₂O₃ exhibit a slight reunion, which is also reflected by the sharp peak in the XRD patterns. Figure 4d–f,j–l,p–r and Figure 5d–f,j–l,p–r depict the EDS images of all prepared materials. It also can be recognized that Al, O, Ru, Rh, Pt, Pd, Ir, and Au were dispersed homogeneously over each catalyst.

The textural features of the synthesized samples were evaluated by N₂ physisorption measurements. As expressed in Figure 6a, the adsorption-desorption isotherm curves of all prepared catalysts are type IV [46], which forms a closed loop with the previous adsorption curve at medium pressure. After analyzing the BET surface areas (

Table 2. Physicochemical properties of synthesized catalysts.

Samples	SBET a m2·g−1	V b cm3·g−1	Dp c nm	Loading d %	D (NM Dispersion) e %
γ-Al2O3	207.87	0.436	4.20	/	/
Ru/γ-Al2O3	229.29	0.468	4.09	0.97	31.1
Rh/γ-Al2O3	219.97	0.595	5.41	0.98	27.4
Pt/γ-Al2O3	227.60	0.813	7.14	1.01	25.6
Pd/γ-Al2O3	226.79	0.502	4.43	1.02	29.7
Au/γ-Al2O3	266.62	0.589	4.42	0.96	40.2
Ir/γ-Al2O3	230.35	0.510	4.43	0.96	33.5

^a Specific surface area based on the calculation of P/P₀ = 0.05–0.30; ^b Total pore volume tested at P/P₀ = 0.99; ^c BJH pore size calculated from the N₂ desorption branch; ^d Weight loading of noble metals of prepared materials tested by ICP-OES; ^e Calculated from CO chemisorption measurements using average CO:NM stoichiometry of 1:1; D (%) = N_surface of NM/N_total of NM.

) of the prepared catalysts, the values could be ordered as follows: Au/γ-Al₂O₃ (266.79 m²∙g⁻¹) > Ir/γ-Al₂O₃ (230.35 m²∙g⁻¹) > Ru/γ-Al₂O₃ (229.27 m²∙g⁻¹) > Pt/γ-Al₂O₃ (227.60 m²∙g⁻¹) > Pd/γ-Al₂O₃ (226.79 m²∙g⁻¹) > Rh/γ-Al₂O₃ (219.97 m²∙g⁻¹) > γ-Al₂O₃ (207.87 m²∙g⁻¹). Moreover, in terms of pore volume, the catalysts can be ordered as follows: Pt/γ-Al₂O₃ (0.813 cm³∙g⁻¹) > Rh/γ-Al₂O₃ (0.595 cm³∙g⁻¹) > Au/γ-Al₂O₃ (0.589 cm³∙g⁻¹) > Ir/γ-Al₂O₃ (0.510 cm³∙g⁻¹) > Pd/γ-Al₂O₃ (0.502 cm³∙g⁻¹) > Ru/γ-Al₂O₃ (0.468 cm³∙g⁻¹) > γ-Al₂O₃ (0.436 cm³∙g⁻¹). Generally, a large pore volume is conducive to exposing more active sites, which would be profitable for reactant capture and for improving the anti-carbon deposition ability of the catalyst. As depicted in Figure 6b, the most probable pore sizes of all prepared materials were distributed between 2 and 4 nm, combining the existence hysteresis loop between a P/P₀ of 0.6 and 0.8, suggesting the occurrence of a mesoporous structure.

3.3. Catalyst Reducibility, Acid Properties, and Oxygen Species

The redox characteristics of the as-prepared catalysts were determined by H₂-TPR experiments. First of all, an H₂-TPR pattern of the supporting material γ-Al₂O₃ is shown in Figure 7a, where no obvious H₂ consumption appeared for γ-Al₂O₃, with the exception of a small peak centered at 735 °C, demonstrating the poor reduction properties of the supporting material γ-Al₂O₃. For all supported noble metal-based catalysts in this work, the low-temperature range peaks could be recognized as a noble metal reduction action. For instance, a sharp peak located at about 153 °C in the Ru/γ-Al₂O₃ profile resulted from the reduction of Ru⁴⁺ to Ru⁰. As for the Rh/γ-Al₂O₃ sample, the reduction peak at the temperature range 78 °C was related to the reduction of Rh³⁺ to Rh⁺, but then the second emergence peak centered at 286 °C was caused by the reduction of Rh⁺ to Rh⁰. The hydride consumption spike at the temperature of 473 °C for the Pt/γ-Al₂O₃ catalyst can be attributed to the transformation of Pt²⁺ to Pt⁰. Interestingly, a negative peak occurred at about 44 °C and can be observed for the Pd/γ-Al₂O₃ material; this represents the decomposition of palladium hydrides [47] due to the superior hydrogen storage capacity of Pd, which might generate partial palladium hydrides during the hydrazine hydrate treatment process. Moreover, the peak centered at 428 °C resulted from the reduction of Pd²⁺ to Pd⁰. For the Ir/γ-Al₂O₃ material, the reduction peak at 199 °C corresponded to the reduction of Ir³⁺ to Ir⁺, and the following peak located at 417 °C can be attributed to the reduction of Ir⁺ to Ir⁰. There is no obvious reduction peak in the profile of the Au/γ-Al₂O₃ catalyst, which corresponded to the redox properties of the Au metal. The reduction procedures of the prepared materials result from electron transfer among different metal compositions, which would promote the reaction rate of 1,2-DCE oxidation.

In a typical oxidation process, the acid property of the catalysts is another important influencing factor for the 1,2-DCE reaction [48,49]. The surface acidity of the prepared samples was investigated by NH₃-TPD measurement, and the resulting profiles are exhibited in Figure 7c. Across all catalysts, the acidic sites identified from the NH₃-TPD patterns can be classified into three categories, namely, those containing weak acidity (<200 °C), those with moderate acidity (200–350 °C), and those with strong acidity (>350 °C) [38,50,51,52]. For the supporting material γ-Al₂O₃, only one significant peak at 506 °C was observed, indicating that γ-Al₂O₃ belongs to the strong acidity category. When supporting noble metals, the desorption of the strong acidity range of Pt/γ-Al₂O₃ was strengthened significantly, and the Au-, Ir-, and Pd-based materials were also increased extensively. Interestingly, the desorption peaks in the moderate and weak acid ranges emerged in all NM/γ-Al₂O₃ catalysts, especially in those with moderate acidity, suggesting the possible existence of Brønsted acid sites, where Ru/γ-Al₂O₃ was observed to exhibit more moderate acid emergence than other samples. Strong acid sites generally refer to Lewis acid sites [41]; 1,2-DCE is usually easily trapped on Lewis acid sites, causing the dehydrochlorination effect and inducing the formation of the main by-product (vinyl chloride). On the other hand, the Brønsted acid sites can efficiently promote the desorption of chlorine species from the catalyst surface, further relieving the chlorination effect during 1,2-DCE degradation, as well as hindering the formation of chlorine by-products [25]. As a result, the intensive nature of Brønsted acid sites and the suitable amount of Lewis acid sites may contribute to the excellent performance of Ru/γ-Al₂O₃ in 1,2-DCE destruction.

Moreover, O₂-TPD was carried out for the determination of the oxygen species (which can be generally divided into three types [53,54]), as displayed in Figure 7b: the desorption of oxygen species at low temperature < 300 °C can be associated with surface physically adsorbed oxygen (O_2(ad)) and surface chemically adsorbed oxygen (O₂⁻_(ad), O⁻_(ad)). On the other hand, the middle-temperature emergence of desorption peaks in the range of 300 and 700 °C can be attributed to surface lattice oxygen (O²⁻_(ad)). Finally, the adsorption peak overcrossing higher temperature (> 700 °C) regions may be related to the lattice oxygen (O²⁻) of the bulk phase. It is shown that only a weak desorption peak at 503 °C emerged for γ-Al₂O₃, which is likely associated with the surface lattice oxygen species. When supporting noble metals, for all the prepared materials, there is no adsorption peak in the low- and high-temperature ranges, but only at the temperature range 300–700 °C, which indicates that surface lattice oxygen represents the main oxygen species in noble metal-based catalysts. In addition, the evolution processes of oxygen species usually convert following the procedures O_2(ad) → O₂⁻_(ad) → O⁻_(ad) → O²⁻_(lattice) [55], while the adsorbed oxygen changes for all prepared catalysts in the following manner O_2(ad) → O₂⁻_(ad) → O⁻_(ad), which is attributed to the influence of precious metal oxide species. Moreover, as compared with the support γ-Al₂O₃, all noble metal catalysts showed an obvious increasing desorption amount in the temperature region of 300–700 °C, as well as a significant low-temperature shift in oxygen desorption, suggesting a promotion of active oxygen and oxygen mobility. By combining other characterization methods, the presence of noble metals in all prepared catalysts strengthens significantly the mobility of oxygen adsorbed on the surface of the materials.

This section may be divided into subheadings. It aims to provide a concise and precise description of the experimental results and interpretations of them, as well as the experimental conclusions that can be drawn.

3.4. Catalyst Surface Status

The information on the element composition, the metallic valence status, and oxygen species on the surface of samples can be analyzed by XPS measurement, where O 1s, Ru 3p, Rh 3d, Pt 4f, Pd 3d, Ir 4f, and Au 4f were analyzed and are shown in Figure 8 and Figure 9. For all supported noble metal catalysts, as illustrated in Figure 8, O 1s patterns could be categorized into two types: the one with a binding energy (BE) of ~529.8 eV may be related to lattice oxygen species (O^α), while the other one centered at 531.5 eV belongs to the surface adsorbed oxygen species (O^β), respectively [56]. The O^α/(O^α + O^β) ratio by Gauss fitting is listed in

Table 3. XPS results of O 1s of prepared catalysts.

Samples	O 1s
	Oα a	Oβ b	Oα/(Oα + Oβ)
Ru/γ-Al2O3	53,918.9	7038.0	0.88
Rh/γ-Al2O3	48,460.7	11,390.2	0.81
Pt/γ-Al2O3	38,974.4	8706.5	0.82
Pd/γ-Al2O3	25,771.9	11,941.7	0.68
Ir/γ-Al2O3	41,634.6	7145.6	0.85
Au/γ-Al2O3	46,326.8	7463.6	0.86

^a Lattice oxygen species; ^b Surface adsorbed oxygen species.

, and allows for the ordering of the catalysts as follows: Ru/γ-Al₂O₃ (0.88) > Au/γ-Al₂O₃ (0.86) > Ir/γ-Al₂O₃ (0.85) > Pt/γ-Al₂O₃ (0.82) > Rh/γ-Al₂O₃ (0.81) > Pd/γ-Al₂O₃ (0.68).

For the noble metal XPS patterns, the Ru 3p spectra of Ru/γ-Al₂O₃ could be fitted into two obvious peaks, where the BEs of 496.6 and 463.6 eV are attributed to the surface Ru⁴⁺ species [57]. The Rh 3d spectrum of Rh/γ-Al₂O₃ includes two BE combinations of 308.6 and 313.4 eV, which means the Rh³⁺ species are present on the surface [58]. The Pt 4f spectrum of the Pt/γ-Al₂O₃ catalyst showed doublet peaks with BEs of 75.1 and 71.5 eV; these belong to the surface Pt²⁺ species [25]. The Pd 3d spectra included two peaks at 336.6 and 342.6 eV both associated with Pd²⁺ species [37]. In the Ir 4f spectrum, the peaks can be identified as 4f_5/2 (BEs ≈ 64.5 eV) and Ir 4f_7/2 (BEs ≈ 62.0 eV), respectively, confirming the existence of Ir³⁺ species on the surface [44]. Two peaks assigned to Au 4f at BEs of 88.1 and 84.2 eV were associated with the surface Au⁺ species [45]. Abundant adsorbed oxygen concentration on the catalyst surface as well as a higher noble metal valence are conducive to improving oxidation behavior, which plays a key role in the catalytic oxidation process of noble metal catalysts [54].

3.5. Catalyst Intermediate Species and Oxidation Mechanisms

Two typical kinetic models of the oxidation mechanisms of 1,2-DCE over noble metal catalysts as displayed in Scheme 1. In the low-temperature region, the oxidation mechanism of 1,2-DCE on noble metal catalysts obeys the Langmuir-Hinshelwood model: (1) 1,2-DCE and the oxygen species are simultaneously adsorbed onto the surficial active sites; (2) 1,2-DCE molecules react with the adsorbed oxygen and then produce Cl₂, H₂O, HCl, and CO₂; (3) intermediates continuously react with the adsorbed oxygen and generate H₂O, CO₂, HCl, and Cl₂. On the other hand, at high temperatures, the 1,2-DCE oxidation routes of the NM/γ-Al₂O₃ catalysts follow the Eley-Rideal model [29]: (1) oxygen species are adsorbed; (2) the adsorbed oxygen species further react with the gas-phase 1,2-DCE to produce H₂O, HCl, Cl₂, and CO₂. In summary, the surface-adsorbed oxygen species are the main source of 1,2-DCE oxidation.

By combining other characterization methods and the by-product distribution, four reaction paths can be inferred, as displayed in Scheme 2 [40,59]. Path 1: 1,2-DCE induces the dehydrochlorination reaction to produce vinyl chloride; vinyl chloride reacts with H· to generate carbon dioxide positive ions; carbon dioxide positive ions react with O²⁻ to create acetate. Path 2: vinyl chloride reacts with chlorine to produce trichloroethane; trichloroethylene can be generated by trichloroethane reacting with chlorine and the dehydrochlorination reaction. Path 3: 1,2-DCE induces C-C bond cleavage to create chloromethane; chloromethane reacts with H⁺ and O²⁻ to produce formate. Path 4: chloromethane reacts with chlorine in the dehydrochlorination reaction to generate dichloromethane; dichloromethane reacts with chlorine in the dehydrochlorination reaction to create trichloromethane; trichloromethane reacts with chlorine in the dehydrochlorination reaction to produce carbon tetrachloride. Finally, these intermediates are then deeply oxidized into H₂O, HCl, Cl_2, and CO₂.

To further explore the oxidation mechanisms of the 1,2-DCE catalyst over noble metals, in situ DRIFTS measurements were applied to discover the reaction intermediates of all catalyst materials when 1,2-DCE degrades from 90 to 260 °C (Figure 10). All of the catalysts could be observed in the bands in the region of 3550–3060 cm⁻¹, which are determined by the -OH vibration with surficial adsorbed water. Of course, with the elevated operation temperature, the strength of these bands is reduced contrarily. Following the stretching vibration of the C-H band observed in the bands of 2930 and 2850 cm⁻¹, the vibration of the -CH₂ group can be distinguished. At the same time, the C-H vibration appeared at 2820 and 2720 cm⁻¹, indicating the emergence of the -CH₃ group. Moreover, the bands in the region of 1544–1371cm⁻¹ are also associated with C-H vibration, which is attributed to the oscillation of -CH/-CH₂/-CH₃ radicals. An overcrossing band at 2360–2340 cm⁻¹, growing in intensity alongside the reaction temperature, is ascribed to the stretching vibration of C=O. Meanwhile, the bands at 1680–1640 cm⁻¹ are associated with the stretching vibration of the C=C bond, which indicates the appearance of the banding vibrations of alkene. The band located at 1378 cm⁻¹ belongs to the COO- of acetate species, while the one located at 1338 cm⁻¹ is associated with the COO- of formate species. The bands at 1250–1000 cm⁻¹ can be ascribed to the stretching oscillation of the C-C of 1,2-DCE.

In conclusion, the reaction path of 1,2-DCE over the prepared noble metal-based catalysts could be conducted in several of the main reaction routes of these typical noble metal materials. First, the vibration of C-H (-CH/-CH₂/-CH₃ groups), COO- (acetate), and C=C (alkene) can be found in the Ru/γ-Al₂O₃ sample, and thus the main reaction paths of the Ru/γ-Al₂O₃ catalyst are reaction paths 1 and 4. The vibration of COO- (acetate), C-H (-CH/-CH₂/-CH₃ groups), and C=C (alkene) can be observed for the Rh/γ-Al₂O₃ material, and thus the main reaction paths of the Rh/γ-Al₂O₃ material are reaction paths 1 and 3. In the Pt/γ-Al₂O₃ catalyst spectrum, the vibrations of C-H (-CH₂ group), C-H (-CH/-CH₂/-CH₃ groups), COO- (acetate), and C=C (alkene) are obvious, and thus the reaction paths 1, 2, and 4 are the main reaction paths of Pt/γ-Al₂O₃ material. The vibration of C-H (-CH/-CH₂/-CH₃ groups), COO- (acetate), and C=C (alkene) can be found with the Pd/γ-Al₂O₃ catalyst, so reaction paths 1 and 2 can be observed from the process of Pd/γ-Al₂O₃ material. The vibration of C-H (-CH₂ group), C-H (-CH/-CH₂/-CH₃ group), COO- (acetate), and C=C (alkene) can be observed on the Ir/γ-Al₂O₃ material surface. The vibration of C-H (-CH₂ group), C-H (-CH/-CH₂/-CH₃ groups), COO- (acetate), and C=C (alkene) can be observed on the Au/γ-Al₂O₃ material surface. Finally, the catalyst of Ir/γ-Al₂O₃ follows reaction paths 1, 2, and 4, as does Au/γ-Al₂O₃.

4. Conclusions

In this work, Al₂O₃ supported a series of precious metals; Ru, Rh, Pt, Pd, Ir, and Au catalysts were prepared with mesopores (pore sizes distributed at 2–4 nm) and all were proven to be able to totally destruct 1,2-DCE below 400 °C. The results suggest that noble metals with high-valence states and superior mobility of surface-adsorbed oxygen species are favorable for 1,2-DCE degradation. To summarize the oxidation procedure, 1,2-DCE is first captured by the surficial active sites and then activated, and oxidized into intermediate products (such as vinyl chloride, acetate, trichloroethane, trichloroethylene, dichloromethane, trichloromethane, tetrachloromethane, and trichloroethylene), before the products are deeply oxidized to form H₂O, CO₂, HCl, and Cl₂, which suggests that the prepared materials had a high-valence state and abundant active oxygen species, making them promising candidates for CVOC destruction. This work supplies a systematical comparison study of various typical noble metal catalysts with an exploration of the potential for CVOC destruction.