Effect of Au nano-particle aggregation on the deactivation of the AuCl₃/AC catalyst for acetylene hydrochlorination

Abstract

A detailed study of the valence state and distribution of the AuCl₃/AC catalyst during the acetylene hydrochlorination deactivation process is described and discussed. Temperature-programmed reduction and X-ray photoelectron spectral analysis indicate that the active Au³⁺ reduction to metallic Au⁰ is one reason for the deactivation of AuCl₃/AC catalyst. Transmission electron microscopy characterization demonstrated that the particle size of Au nano-particles increases with increasing reaction time. The results indicated that metallic Au⁰ exhibits considerable catalytic activity and that Au nano-particle aggregation may be another reason for the AuCl₃/AC catalytic activity in acetylene hydrochlorination.

Introduction

Acetylene hydrochlorination is an important process in the industrial production of vinyl chloride monomer, which is the primary raw material used to synthesize polyvinyl chloride, especially in China, because of the vast domestic coal resources and the increasing cost of petroleum¹. HgCl₂ supported on activated carbon is a common catalyst for industrial acetylene hydrochlorination. However, the HgCl₂ catalyst is toxic and harmful to human health. Therefore, the exploitation of non-mercury catalysts to replace HgCl₂ is of significant importance for the survival and development of the PVC industry through the acetylene route.

Numerous metal complexes, including Bi³⁺, Pt⁶⁺, Pt⁴⁺ and Pd²⁺, have recently been applied as candidate catalysts for acetylene hydrochlorination^2,3,4,5. Hutchings^6,7 reported that Au-based catalysts are the optimal metal chloride catalysts for acetylene hydrochlorination. However, the Au-based catalyst is easily deactivated over the course of the reaction.

Many investigations have attempted to elucidate the deactivation mechanism of AuCl₃/AC catalyst in acetylene hydrochlorination. Hutchings proposed that the reduction of Au³⁺ to Au⁰ is the primary deactivation pathway under the operating condition of acetylene hydrochlorination^8,9. Hutchings further disclosed the reason for active Au³⁺ reduction in acetylene hydrochlorination and proposed that acetylene was more easily adsorbed by AuCl₃ catalyst compared with the hydrogen chloride species and that the strong adsorption of acetylene caused the reduction of Au^{3+ 10}. Shen et al. reported similar results. These researchers determined that the initial coordination of hydrogen chloride with AuCl₃ produced a calculated energy of -23.90 kJ mol⁻¹, whereas acetylene in the vacant coordination site of AuCl₃ generates a relative energy of -66.65 kJ mol⁻¹. Therefore, the adsorption of acetylene on AuCl₃ is relatively stronger than that of hydrogen chloride¹¹. Zhang et al. suggested a detailed deactivation mechanism of AuCl₃ catalyst for acetylene hydrochlorination. These researchers determined that the electron in the p orbital of acetylene transfers to the unoccupied molecular orbital of Au³⁺, thereby causing the reduction of Au³⁺ to its low-valence state via the loss of its Cl atoms¹².

From the above-described literature, the reduction of active Au³⁺ ions to metallic Au⁰ is the primary reason for the AuCl₃/AC catalyst deactivation in acetylene hydrochlorination. However, all of these investigators ignored an obvious fact. Au nano-particles are present in the deactivated catalyst. Thus, Au nano-particle aggregation may play a role in the AuCl₃ catalyst deactivation because the particle size of Au nano-particles significantly affects catalytic performance, as described in many reports on heterogeneous catalysis^13,14,15. In our previous work, we observed that the AuCl₃ particle size after acetylene hydrochlorination is notably large¹⁶ and metallic Au⁰ (3-10) clusters may exhibit considerable catalytic activity based on the results of simulations using density functional theory¹⁷. Therefore, Au nano-particle agglomeration may also influence the catalytic performance of AuCl₃ catalyst in acetylene hydrochlorination.

In this paper, we investigate the relationship between the metallic Au⁰ particle size and its catalytic performance to elucidate the deactivation mechanism of AuCl₃/AC for acetylene hydrochlorination. A series of characterizations was performed to determine the valence state and distribution of the AuCl₃/AC catalysts in various stages of catalyst life.

Experimental Section

Materials

Activated carbon (marked as AC, neutral, 40-60 mesh), HAuCl₄·4H₂O (with 47.8% Au content), C₂H₂ (gas, 98%) and HCl (gas, 99%) were used in this study.

Catalyst preparation

The AuCl₃/AC catalysts were prepared using an incipient wetness impregnation technique that used aqua regia as a solvent, as described in the literature¹⁸. The gold precursor, HAuCl·4H₂O, was dissolved in aqua regia [3:1 HCl (Fisher, 32%): HNO₃ (Fisher, 70%) by volume, 6.4 mL] and the solution was added dropwise with stirring to the activated carbon support (3.00 g) to obtain a catalyst with a final metal loading of 1 wt.%. The mixture was maintained for 24 h at room temperature and then the product was dried for 18 h at 140 °C before being used as a catalyst.

The Au/AC catalyst (1 wt.% Au) was prepared using a colloidal deposition technique similar to that in the literature¹⁹, with a few modifications. First, 100 mL of distilled water was mixed with 3.2 mL of 2.43 × 10⁻² M AuCl₄^- solution and then 5 mL of 0.1 M fresh NaBH₄ solution was added dropwise. The solution was stirred for 2 h. Next, activated carbon (1.50 g) was added and then the pH value was adjusted to 1.0 using 1.0 M HCl. Subsequently, the solution was vigorously stirred overnight. Finally, the mixture was washed with distilled water and then dried in a vacuum oven at 80 °C to yield the Au/AC catalyst. Two samples were calcined for 4 h at 300 °C and at 800 °C (5 °C /min), the obtained samples were labeled as Au/AC-300 and Au/AC-800, respectively.

Catalyst characterization

X-ray diffractometer data were collected using a Bruker D8 advanced X-ray diffractometer equipped with a Cu-K_α irradiation source (λ = 1.5406 Å) operating at 40 kV and 40 mA, with data collected over the 2θ scanning range between 20° and 90°. The morphologies of the samples were examined via transmission electron microscopy using a JEM 2010 electron microscope operating at an accelerating voltage of 200 kV, with a line resolution of 0.14 nm and a point-to-point resolution of 0.23 nm. Temperature-programmed reduction was performed using a similar Micromeritic ASAP 2720 apparatus equipped with a TCD detector. The reducing gas was 10% H₂ in Ar, with a flow rate of 40 mL min⁻¹. The temperature range was from 50 °C to 400 °C, with a heating rate of 10 °C min⁻¹. X-ray photoelectron spectrum data were recorded using an Axis Ultra spectrometer with a monochromatized Al-K_α X-ray source (225 W), a minimum energy resolution of 0.48 eV (Ag 3d_5/2) and a minimum X-ray photoelectron spectrum analysis area of 15 μm.

Catalytic performance evaluation

The catalytic performance during acetylene hydrochlorination was evaluated in a fixed-bed glass microreactor (i.d. of 10 mm) operating just above atmospheric pressure. A CKW 1100 temperature controller (Chaoyang Automation Instrument Factory, Beijing, China) regulated the reactor temperature. The reactor was purged with nitrogen to remove water and air in the reaction system before the reaction process. Hydrogen chloride gas was passed through the reactor at a flow rate of 20 mL min⁻¹ to activate the catalyst. After the reactor was heated to 180 °C, acetylene (24.3 mL min⁻¹) and hydrogen chloride (29.4 mL min⁻¹) were fed through the heated reactor containing 2 mL of catalyst, with a gas hourly space velocity of 870 h⁻¹. The reaction products were analyzed using a gas chromatograph (GC-2014C) produced by Shimadzu International Trading Company Ltd. (Shanghai). Analysis conditions: chromatographic column type of 2 m × Φ 4 mm, stuffing of GDX-301, column temperature of 120 °C, FID detector, detector and vaporizer temperature of 150 °C and injection volume of 60 μL. The conversion of acetylene (X_A) and the selectivity to vinyl chloride monomer (S_VC) as the criteria of catalytic performance²⁰ were defined as the equations (1) and (2), respectively.

In the equations, is defined as the volume fraction of acetylene in the raw gas, is defined as the volume fraction of remaining acetylene in the product gas and is the volume fraction of vinyl chloride in the product gas.

Results and Discussion

To accelerate the deactivation process of Au-based catalysts, we applied both a gas hourly space velocity and a reaction temperature that were higher than those reported in the literature²¹. The catalysts were tested under fixed reaction conditions (C₂H₂/HCl = 1.15:1, gas hourly space velocity = 870 h⁻¹, reaction temperature = 180 °C). The result is illustrated in Fig. 1A. The catalyst initially exhibited low conversion (ca. 16.71%), followed by an increase in activity after approximately 2 h and then reached ca. 84.61% conversion. From the initial stage of the reaction (~ 2 h), there exists an activation period for the fresh AuCl₃/AC catalyst during the acetylene hydrochlorination reaction. This result indicated that the most active sites for acetylene hydrochlorination are formed during this period. We characterized the AuCl₃/AC catalyst during the initial stages of the reaction (~ 0.5 h, 1 h and 1.5 h) by X-ray photoelectron spectrum experiments and the results are listed in Fig. 2 and Table 1. From the results of Table 1, the compositions of Au³⁺ species in AuCl₃/AC-0.5, AuCl₃/AC-1 and AuCl₃/AC-1.5 are observed to be 42.39%, 38.58% and 36.82%, respectively. Note that the content of the Au³⁺ species decreased, while the catalytic activity increased. This result indicated that Au³⁺ was not the only active site for acetylene hydrochlorination. The acetylene conversion decreased to ca. 68.70% after reacting for 4 h and then the rate of acetylene conversion of the AuCl₃/AC catalyst decreased to ca. 9.96% after 8 h and the selectivity to vinyl chloride monomer of all AuCl₃/AC catalysts was maintained at 99.99%, as shown in Fig. 1B. The results indicated that the AuCl₃/AC catalyst was easily deactivated with increasing reaction time on the stream under these reaction conditions. Thus, we investigated the valence state and distribution of the catalyst at various stages of catalyst life. The catalyst was denoted as AuCl₃/AC-x (where x represents the reaction time of 0, 2, 4, 6 and 8 h). The actual Au amount in AuCl₃/AC-0, AuCl₃/AC-2, AuCl₃/AC-4, AuCl₃/AC-6 and AuCl₃/AC-8 catalysts was 1.09 wt.%, 0.98 wt.%, 1.02 wt.%, 1.04 wt.% and 1.01 wt.%, respectively, as determined by inductively coupled plasma-atomic emission spectrometry.

Table 1 Composition of the Au species obtained by deconvolution of the Au 4f peaks.

Figure 1

Acetylene conversion (A) and vinyl chloride monomer selectivity (B) during acetylene hydrochlorination catalyzed by (a) AuCl₃/AC-0, (b) AuCl₃/AC-2, (c) AuCl₃/AC-4, (d) AuCl₃/AC-6 and (e) AuCl₃/AC-8. Reaction conditions: temperature = 180 °C, C₂H₂ gas hourly space velocity = 870 h⁻¹, feed volume ratio V_HCl/V_C2H2 = 1/1.15.

Figure 2

High-resolution X-ray photoelectron spectrum for Au 4f of (a) AuCl₃/AC-0.5, (b) AuCl₃/AC-1 and (c) AuCl₃/AC-1.5.

X-ray diffractometer patterns of the representative AuCl₃/AC-x catalysts are shown in Fig. 3. Two diffraction peaks in the pattern of AuCl₃/AC-0 catalyst appear at 2θ = 24.4° and 43.7°, corresponding to the planes of (0 0 2) and (1 0 1) , respectively, of the AC support²². No obvious diffraction of Au³⁺ or metallic Au⁰ is observed in the fresh AuCl₃/AC-0 catalyst, indicating that active Au³⁺ components are highly dispersed on the surface of the AC support. For AuCl₃/AC-2, AuCl₃/AC-4, AuCl₃/AC-6 and AuCl₃/AC-8 catalysts, obvious diffraction peaks appear at 38.5°, 44.7°, 64.8° and 77.9° (2θ), respectively, corresponding to the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), i.e., the diffraction peaks of metallic Au⁰ (JCPDS, No. 4-0784), for 38.5°, 44.7°, 64.8° and 77.9° (2θ), respectively²³. The X-ray diffractometer results show that Au³⁺ is reduced to metallic Au⁰ in acetylene hydrochlorination. Therefore, the reduction of an active Au³⁺ component is one reason for the deactivation of the AuCl₃/AC catalyst in the acetylene hydrochlorination, which is consistent with the literature results^8,10.

Figure 3

X-ray diffractometer patterns of (a) AuCl₃/AC-0, (b) AuCl₃/AC-2, (c) AuCl₃/AC-4, (d) AuCl₃/AC-6 and (e) AuCl₃/AC-8.

In view of the above results, there still must be detailed investigations on the deactivation mechanism to make it clear whether a change of surface gold oxidation state was responsible for the deactivation for acetylene hydrochlorination. To correlate the bulk changes of the surface Au³⁺ in catalysts with the surface of the gold oxidation state, X-ray photoelectron spectroscopy was systematically performed for all AuCl₃/AC-x catalysts. The deconvolution results in Fig. 4 indicate that each gold species shows two peaks, the gold species with the binding energy at 84.1 eV and 87.7 eV, due to Au 4f_7/2 and the Au 4f_5/2 spectrum of metallic Au^{0 24}; the spectral peaks at binding energies of 85.2 eV and 88.6 eV are assigned with Au³⁺. As shown in Fig. 4, the gold species on the surface of the catalysts exists as metallic Au and Au³⁺ states. The relative content of Au species in the fresh air and in the used catalysts, based on the deconvolution of X-ray photoelectron spectroscopy are tabulated in Table 2. As listed in Table 2, the amounts of gold species in the used catalysts differ greatly from those in the fresh one. Compared with the AuCl₃/AC-0 catalyst, the amounts of Au⁰ increase from 50.55% to 65.47%, while the contents of Au³⁺ decrease after undergoing the reaction for 2 h. With increasing reaction time on the stream, the surface composition of approximately 89.38% of the Au⁰ species and 10.62% of the Au³⁺ species after 8 h, indicating that a great amount of Au³⁺ can be reduced to Au⁰ in the acetylene atmosphere. Therefore, the gold species on the catalysts surface exists as metallic Au⁰ and Au³⁺ states in acetylene hydrochlorination, while, approximately 54.65% of the Au³⁺ component is reduced to Au⁰ after reaction, which is one reason for the deactivation of AuCl₃/AC catalyst in acetylene hydrochlorination.

Table 2 Composition of the Au species obtained by deconvolution of the Au 4f peaks.

Figure 4

High-resolution X-ray photoelectron spectrum for Au 4f of (a) AuCl₃/AC-0, (b) AuCl₃/AC-2, (c) AuCl₃/AC-4, (d) AuCl₃/AC-6 and (e) AuCl₃/AC-8.

To further quantify of the bulk Au³⁺ amount and the gold oxidation state during acetylene hydrochlorination reaction, temperature-programmed reduction was performed to monitor the amount of active Au³⁺ by integrating the reduction peak area of these AuCl₃/AC-x catalysts at different reaction times during acetylene hydrochlorination. As shown in Fig. 5, all AuCl₃/AC-x catalysts exhibit one characteristic reduction band between 250 °C and 350 °C with the peak centered at 277 °C, which is attributed to Au³⁺ reduction²⁵. Determined from the reduction peak of temperature-programmed reduction, the Au³⁺ content was found to follow the same general trend obtained from X-ray photoelectron spectroscopy. Hence, the relative amounts of Au³⁺ decrease with increasing reaction time on stream, which is consistent with the order of the catalytic performance of the catalysts (shown in Fig. 1A). Moreover, the Au³⁺ content in these catalysts is well correlated with the acetylene conversion approximated by a single line, as shown in Fig. 6.

Figure 5

Temperature-programmed reduction profiles of (a) AuCl₃/AC-0, (b) AuCl₃/AC-2, (c) AuCl₃/AC-4, (d) AuCl₃/AC-6 and (e) AuCl₃/AC-8.

Figure 6

Correlation between the acetylene conversion and the amount of Au³⁺ species in the AuCl₃/AC catalyst (Au³⁺ species were determined from Table 1 and Table 2).

The above results indicate that approximately 13.59% of the Au³⁺ component is reduced to metallic Au⁰ when the reaction time is 2 h. If the Au³⁺ component is the only active site for acetylene hydrochlorination and the acetylene conversion of AuCl₃/AC-0 should be higher than that of the AuCl₃/AC-2 catalyst. However, we observed a contradictory phenomenon in Fig. 1. The acetylene conversion of AuCl₃/AC-2 (84.61%) is significantly higher than that of AuCl₃/AC-0 (16.71%), as shown in Fig. 1. Therefore, we hypothesize that Au³⁺ is not the only active site for acetylene hydrochlorination and metallic Au⁰ may also be active for this reaction.

To prove this hypothesis, we synthesized carbon support Au nano-particle with NaBH₄ as a reducing agent and compared its catalytic performance for acetylene hydrochlorination with that of the AuCl₃/AC catalyst. NaBH₄ is a very strong reducing agent; thus, only metallic Au⁰ is present in the prepared Au/AC catalyst, as reported in the literature²⁶. The catalytic performance of Au/AC catalyst for acetylene hydrochlorination was evaluated under a fixed reaction condition (C₂H₂/HCl = 1:1.15, gas hourly space velocity = 870 h⁻¹, reaction temperature = 180 °C) compared with the AuCl₃/AC catalyst. Notably, the acetylene conversion of AuCl₃/AC is relatively higher than that in Fig. 1 because of the different reaction conditions. The acetylene conversion of the Au/AC catalyst is 57.1%, which is considerably lower than that of AuCl₃/AC catalyst in Fig. 7. This result indicates that Au³⁺ is more active than metallic Au⁰, consistent with literature^25,27. However, the catalytic activity of Au/AC is still higher than that of the AuCl₃/AC-8 catalyst. If the reduction of Au³⁺ to Au⁰ is the only reason for the deactivation of AuCl₃/AC catalyst, then the AuCl₃/AC-8 catalytic activity should be close to that of Au/AC. Therefore, the possibility exists for another cause of the deactivation of AuCl₃/AC catalyst for acetylene hydrochlorination, apart from the reduction of the active Au³⁺ component.

Figure 7

Acetylene conversion during acetylene hydrochlorination catalyzed by (a) AuCl₃/AC and (b) Au/AC. Reaction conditions: temperature = 180 °C, C₂H₂ gas hourly space velocity = 870 h⁻¹ and feed volume ratio V_HCl/V_C2H2 = 1.15.

Representative transmission electron microscopy images of the AuCl₃/AC-x catalysts are presented in Fig. 8 to determine the distribution of Au nano-particles in the acetylene hydrochlorination process. Fig. 8(a) shows no obvious presence of Au⁰ particles in the AuCl₃/AC-0 catalyst, which demonstrates that the Au element is found in the form of Au³⁺ ion, which is invisible in the transmission electron microscopy micrographs. The X-ray diffractometer results confirm the transmission electron microscopy measurements and indicate that the AuCl₃/AC-0 catalyst contains Au³⁺ only. While the X-ray photoelectron spectroscopy experiment exhibits an obvious Au⁰ peak in the sample, because the quantitative assessment of the gold oxidation state by X-ray photoelectron spectroscopy characterization alone is limited, the final state effects associated with particle size could significantly disturb the X-ray photoelectron spectroscopy features of Au²⁸. For the AuCl₃/AC-2, AuCl₃/AC-4 and AuCl₃/AC-6 catalysts, the average particle sizes of the Au⁰ species are 22.1, 34.2, 66.1 and 81.2 nm, respectively. The particle size of Au nano-particle increases with increasing reaction time on stream, which indicates that the Au nano-particles form aggregates during acetylene hydrochlorination. As previously discussed, metallic Au⁰ exhibits considerable catalytic activity for acetylene hydrochlorination and the catalytic activity of the AuCl₃/AC-x catalyst decreases with the reaction time increment during the acetylene hydrochlorination process. Therefore, Au nano-particle aggregation is another reason for the deactivation of AuCl₃/AC catalyst in acetylene hydrochlorination.

Figure 8

Transmission electron microscopy images of the catalysts: (a) AuCl₃/AC-0, (b) AuCl₃/AC-2, (c) AuCl₃/AC-4, (d) AuCl₃/AC-6 and (e) AuCl₃/AC-8.

We prepared Au/AC catalysts with different particle sizes by subsequent heat-treatment under a nitrogen atmosphere at 300 °C and 800 °C for 4 h to support this viewpoint further. The particle sizes of the Au/AC, Au/AC-300 and Au/AC-800 catalysts are 21.3, 40.6 and 83.6 nm, respectively, as shown in Fig. 9. The catalytic activity of the Au catalysts decreases in the order of Au/AC > Au/AC-300 > Au/AC-800 (as shown in Fig. 10). The result determined that the Au/AC catalyst with larger particle size exhibited lower catalytic activity for acetylene hydrochlorination. This result also provides evidence that Au nano-particle aggregation is one of the reasons for the deactivation of the AuCl₃/AC catalyst in acetylene hydrochlorination.

Figure 9

Transmission electron microscopy images of (a) Au/AC, (b) Au/AC-300 and (c) Au/AC-800.

Figure 10

Acetylene conversion during acetylene hydrochlorination catalyzed by (a) Au/AC, (b) Au/AC-300 and (c) Au/AC-800. Reaction conditions: temperature = 180 °C, C₂H₂ gas hourly space velocity = 870 h⁻¹ and feed volume ratio V_HCl/V_C2H2 = 1.15.

This paper reported the valence state and distribution of the AuCl₃/AC catalyst at various stages of catalyst life for acetylene hydrochlorination. Metallic Au⁰ was found to exhibit considerable catalytic activity for acetylene hydrochlorination; as a result, Au nano-particle aggregation is another reason for the deactivation of AuCl₃/AC catalyst, apart from the reduction of Au³⁺ component. Au nano-particle inhibition may be a development strategy for the exploration of a stable Au-based catalyst for acetylene hydrochlorination.