Influence of Surface Chemistry of Activated Carbon ... - ACS Publications

Article pubs.acs.org/IECR

Influence of Surface Chemistry of Activated Carbon on the Activity of Gold/Activated Carbon Catalyst in Acetylene Hydrochlorination Jinhui Xu, Jia Zhao, Jiangtao Xu, Tongtong Zhang, Xiaonian Li,* Xiaoxia Di, Jun Ni, Jianguo Wang,* and Jie Cen Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Hangzhou, 310014 P.R. China ABSTRACT: The main goal of this work is to study the relationship between the surface chemistry of activated carbon (AC) and the performance of respective gold-supported catalysts in the acetylene hydrochlorination. For this purpose, a set of modified activated carbons with different levels of oxygenated groups on the surface, but with no major differences in their textural parameters, was prepared. A strong effect of the surface chemistry of activated carbon on the Au/AC catalytic activity was observed. Comparison of characterizations, catalytic results, and DFT calculations suggests that phenol, ether, and carbonyl groups on activated carbon surface are the key members governing the unique catalytic activity and stability of Au3+ catalysts. The comprehensive experimental and theoretical study of the surface chemistry of Au3+ supported on activated carbon support is believed to be of great benefit for the rational design of gold−carbon composite catalysts for acetylene hydrochlorination.

1. INTRODUCTION Over the past decade, the manufacture of vinyl chloride monomer (VCM) based on acetylene, mainly used for producing polyvinyl chloride (PVC),1,2 has become increasingly competitive due to the lower cost of the coal-based process than the petroleum-based one.3−5 Gas phase catalytic hydrochlorination of acetylene is a promising route to the manufacture of VCM, provided that the catalysts used are sufficiently active and stable.6 However, the industrial catalysts based on mercuric chloride have serious pollution problems due to its easy sublimation, which makes their use particularly difficult.6,7 Recently, there have been many studies dealing with the hydrochlorination of acetylene using supported gold as catalysts.8−11 One advantage of using this metal is its higher activity, compared with the mercuric-based catalysts, allowing the use for industrial application.12 Although gold can be considered as the best catalyst in terms of initial activity, severe deactivation is still the main problem of the supported gold catalyst system employed for selective hydrochlorination of acetylene. For example, M. Conte et al. reported that Au catalyst lost about 10% of its initial activity within 2 h of the reaction.9 Generally, the reasons behind the deactivation of catalysts were identified as Au3+ reduction and oligomer formation.14 High cost and difficulty of purification together have largely limited the application and development of goldbased catalysts in industrial hydrochlorination of acetylene.13,14 Carbon materials such as activated carbons (AC), carbon blacks, and graphitic materials are widely used as support for metal nanoparticles (NPs) in many reactions, including acetylene hydrochlorination,15,16 because of their high surface area, stability and relative inertness, and potentially high electronic conductivity. As we all know, the surface chemistry of support is an extremely important factor that influences both preparation of catalyst and catalytic performance.17−20 The relatively in-depth investigation of the surface chemistry of © XXXX American Chemical Society

conventional oxide supports (silica, alumina, zeolites) has already allowed the design of supported NPs and single-site catalysts at molecular level.21 For carbon materials, the complex surface chemistry often impedes further examination. However, we cannot ignore the important role of surface functional groups which can dramatically affect the catalytic performance of carbon-based catalysts, as they induce an acid−base and/or hydrophilic character on carbon surface.22 For instance, it has been shown that carbonyl/quinone groups were the active sites for the oxidative dehydrogenation of ethylbenzene to styrene, and a linear correlation between the activity of carbon catalysts and the concentration of such sites was established.23 Similarly, the catalytic activity of carbon materials for the dehydration of methanol to dimethyl ether was correlated with the concentration of strong acid sites.24 Recently, we have intensively studied surface oxygenated groups (SOGs) of carbon materials, whose alteration could induce geometric and electronic modifications to Pd catalysts simultaneously and thereby improve metal dispersion as well as hydrophilic properties.25,26 Interestingly, it has been pointed out that, for carbonaceous materials, depending on different amounts and different types of SOGs, the results and even the mechanism of reaction could be completely different,27 while there is no available literature that describes the effect of SOGs on acetylene hydrochlorination. In this work, we studied the impact of SOGs and found that both the activity and stability of Au catalysts were greatly enhanced when abundant SOGs were introduced by treating activated carbon with a concentrated nitric acid solution. Activated carbon supports were modified by chemical and thermal treatments to change their density of SOGs, with no Received: July 7, 2014 Revised: August 25, 2014 Accepted: August 27, 2014

A

dx.doi.org/10.1021/ie502683r | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

2.4. Catalytic Test. Catalysts were tested for acetylene hydrochlorination in a fixed-bed glass microreactor (i.d. 10 mm). Acetylene (5 mL min−1, 1 bar) and hydrogen chloride (6 mL min−1, 1 bar) were fed through a mixing vessel via calibrated mass flow controllers into a heated glass reactor containing catalyst (200 mg), with a total C2H2 GHSV of 740 h−1. A reaction temperature of 180 °C was chosen. Blank test using a reactor filled with quartz wool only did not reveal any catalytic activity. Quartz sand was used to extend the bed length, above and below the catalyst itself, separated by quartz wool. The gas phase products were first passed through an absorption bottle containing NaOH solution and then analyzed online by GC equipped with a flame ionization detector (FID). Chromatographic separation and identification of the products was carried out using a Porapak N packed column (6 ft × 1/ 800 stainless steel). 2.5. Computational Section. All of calculations were performed using a Vienna ab initio simulation package (VASP),28−30 a periodic density functional theory (DFT) code with projector augmented wave (PAW) potentials. The (6, 6) graphene with or without preadsorbed oxygen was used in this study. The adsorption of small Au cluster and Au2Cl6 on the two kinds of graphene was investigated. In order to better describe the weak interactions, the vdw-DF functional with Perdew−Burke−Ernzehof (PBE) exchange was used.31,32 The Brillouin zone integration was performed using the Monkhorst−Pack scheme with 4 × 4 × 1. All structures were optimized with a convergence criterion of 10 meV/ Å for the forces and 0.01 meV for the energy.

major differences in their textural parameters. The reaction activity was correlated with the amount of SOGs which played an important role in the acetylene hydrochlorination for Au catalysts. Then, the possible role of the surface chemistry of support in the reaction mechanism was discussed.

2. EXPERIMENTAL SECTION 2.1. Support Preparation. A commercial starting activated carbon NORIT ROX 0.8 (pellets of 0.8 mm diameter and 5 mm length) was selected for the preparation of supports. The activated carbon was first pretreated with HNO3 (65 wt %) at 90 °C for 6 h to introduce SOGs. The pretreated activated carbon was then filtered, rinsed by deionized water until pH = 7 and eventually dried at 110 °C for 12 h (AC-n). The AC-n sample was used as starting material for thermal and chemical treatments. For thermal treatments, the as-made AC-n was thermally treated at 400, 600, and 900 °C under nitrogen flow for 1 h, with a heating rate of 10 °C min−1, to obtain the AC-n-N400, AC-n-N600, and AC-n-N900, respectively. The same procedure was repeated but under a flow of hydrogen at 900 °C (AC-n-H900). For comparison, the samples mentioned above were also impregnated with aqua regia after the thermal and chemical treatments to obtain AC-nN400-aq, AC-n-N600-aq, AC-n-N900-aq, and AC-n-H900-aq, respectively. 2.2. Preparation of Catalysts. Au/AC catalysts were prepared using an incipient wetness impregnation technique. A HAuCl4·4H2O (assay: 48%) solution in neat water (H2O), hydrochloric acid (HCl), or aqua regia (aq) was added dropwise to the pretreated activated carbon with agitated stirring (dropwise added solution to the support with stirring and then added a second drop of solution after the first liquid completely dispersed into the support) to obtain a catalyst with a final Au loading of 1 wt %, respectively. After the solution was homogeneously mixed with the support, the system was aged at 40 °C for 4 h and then dried at 110 °C for 12 h. The samples were designated as Au(x)/AC-n-Ny and Au(x)/AC-n-Hy, respectively, where x represents H2O, HCl, or aqua regia, and y represents the temperature of thermal treatment. For comparison, Au(aq)/AC-n catalyst was also prepared using the same procedures. This catalyst was used as a reference. 2.3. Catalyst Characterization. BET surface area analysis was performed by obtaining nitrogen adsorption isotherms at 77 K with a Micromeritics ASAP 2020 instrument. The qualitative and quantitative determination of SOGs was performed by temperature-programmed desorption-mass spectrometry (TPD-MS). CO and CO2 TPD profiles were obtained using automated AMI-200 equipment (Altamira Instruments). The samples (100 mg) were placed in a quartz tube and subjected to a 5 °C min−1 linear temperature increase up to 900 °C from 30 °C under helium flow (25 mL min−1). A quadrupole mass spectrometer (Dymaxion 200, Ametek) was used to monitor CO and CO2 signals. For quantification of the CO and CO2 released, calibration of these gases was carried out. Temperature-programmed reduction (TPR) experiments were carried out on a microflow reactor fed with a gas mixture of 10% H2 in Ar with a rate of 45 mL min−1. The weight of the tested samples was about 100 mg. The temperature increased from 30 to 900 °C at a rate of 10 °C min−1. The hydrogen consumption was measured using a thermal conductivity detector (TCD).

3. RESULTS AND DISCUSSION 3.1. Support Characterization. This work deals with the relationship between the surface chemistry (mainly SOGs) of activated carbon and the catalytic performance of Au supported on activated carbon for acetylene hydrochlorination. To clarify the surface chemistry, the specific textural parameters were calculated according to the Brunauer−Emmett−Teller (BET) method. In addition, TPD experiments were carried out to characterize the surface chemistry of activated carbon. 3.1.1. Textural Properties. The textural parameters of pristine activated carbon (AC), concentrated HNO3 treated carbon (AC-n), and different thermally treated carbons (AC-nN400, AC-n-N600, AC-n-N900, AC-n-H900) calculated from the nitrogen physisorption experiments were summarized in Table 1. It could be seen that nitric acid treatment only slightly Table 1. Textural Properties of Different Activated Carbons sample

SBET (m2/g)

Dpore (nm)

Vpore (cm3/g)

AC AC-n AC-n-N400 AC-n-N600 AC-n-N900 AC-n-H900

1116.86 946.51 1031.09 1107.13 1055.79 1141.34

2.06 2.04 2.04 2.04 2.04 2.07

0.22 0.15 0.19 0.21 0.18 0.33

decreased the surface area and pore volume, probably caused by sharp increase of SOGs. The carbons which were submitted to thermal treatments under inert/reductive atmosphere using different treating temperatures did not result in significant textural differences. It has been reported in many works that the liquid phase oxidation by nitric acid might not significantly change the B



Article

textural properties of activated carbon,33−36 and the thermal treatments under inert/reductive atmosphere also did not engender drastic changes to textural properties,34,37 which perfectly confirms the results obtained in Table 1. It is worth noting that, after treatment at 900 °C under H2 atmosphere, the SOGs were removed completely, and the surface area of corresponding support, compared with the pristine carbon, slightly increased, suggesting that the collapse of the pores did not occur. 3.1.2. Surface Chemistry. Different techniques are available to characterize activated carbon, such as TPD, XPS, FTIR, and chemical or electrochemical titration methods.10 Nevertheless, XPS is a surface technique that fails to give an estimate of the chemical composition of inner surfaces of holes. Elsewhere, FTIR is only used as a qualitative technique for the evaluation of the chemical structure of carbon materials, which cannot allow a straightforward quantitative analysis of the surface functional groups.12 Chemical analysis, such as the traditional Boehm titration method, can only determine about 50% of the total SOGs approachable on activated carbon, coupled with the problem of reproducibility when dealing with small amounts of sample, severely impairing its practical application.13,14 Temperature-programmed desorption (TPD) is a suitable method for the analysis of the SOGs of activated carbon. In this technique, all of the SOGs are thermally decomposed releasing CO and/or CO2 and in some cases H2O, at different temperatures. The nature of SOGs can be assessed by the decomposing temperature and the type of gas released, and their respective amount by the area of corresponding peak, as confirmed by comparing the oxygen content obtained by TPD with that by elemental analysis.15 Indeed, there have been some works reporting that released CO2 resulted from the decomposition of carboxylic acid at low temperature (150− 450 °C,38 280 °C,39 510−720 K33) and peroxide (520 °C39) and lactone at high temperatures (600−800 °C,38 620−720 °C,39 630−730 °C33); carboxylic anhydride gave both CO2 and CO (400−650 °C,38 460 °C,39 530−630 °C33). CO was generated from phenol (600−800 °C,38 720−770 °C33), carbonyl/quinone (750−1000 °C,38 790 °C,39 820−880 °C33), and ether (660 °C39). Figure 1 shows the CO2 and CO TPD evolution profiles of AC, AC-n, AC-n-N400, AC-n-N600, AC-n-N900, and AC-nH900. In order to quantify the SOGs, the deconvolution method proposed in the literature was applied.33,38,39 In this study, the CO2 curves were deconvoluted into five peaks which are carboxyl, carboxylic anhydride, peroxide, and two kinds of lactone in different positions. Similarly, the CO curves were also deconvoluted into five peaks corresponding to adsorptive CO, carboxylic anhydride, phenol, ether, and carbonyl. It should be noted that, as the TPD curves of samples like AC, AC-n-H900, and AC-n-N900 which have very limited SOGs are difficult to be deconvoluted successfully, the deconvolution of these curves would not be carried out. The amount and decomposing temperatures of different SOGs and the total amount of SOGs, obtained by the deconvolution of CO2 and CO TPD curves, were summarized in Tables 2 and 3, respectively. The pristine activated carbon, as shown in Figure 1, presents very limited SOGs, especially those decomposing into CO, which are almost negligible. After treating AC with concentrated HNO3 solution, the SOGs drastically increased (Figure 1), and in particular, a large amount of CO2 was released by carboxyl and carboxylic anhydride (Table 2) and the rest by peroxide and lactones. The liquid phase oxidation

Figure 1. TPD evolution profiles of different carbon samples: (a) CO and (b) CO2.

also significantly increased all types of SOGs decomposing into CO, particularly phenol and carbonyl. Thermal treatment at 900 °C under reductive (H2) or inert (N2) atmosphere led to removing almost all of the SOGs, and only 4.62 μmol/g groups remained, releasing CO2 for sample AC-n-H900, as can be seen in Figure 1 and Tables 2 and 3. Thermal treatment at 400 °C under N2 atmosphere removed mostly carboxyl and carboxylic anhydride whose decomposing temperatures are most likely below or close to 400 °C, while, interestingly, the amount of SOGs like lactones, phenol, ether, and carbonyl with high decomposing temperatures increased, instead of decreasing. Thermal treatment at 600 °C under N2 atmosphere further eliminated a small proportion of carboxyl, almost completely the carboxylic anhydride, low energetic lactone (lactone 1) and part of phenol, combined with the increase of high energetic lactone (lactone 2), ether, and carbonyl. The same phenomenon by which the quantities of SOGs with high decomposing temperatures increase during thermal treatments at 400 and 600 °C may be explained by that observation that under these conditions SOGs with low decomposing temperatures decompose into CO2, which then is adsorbed on pore walls and transforms into groups with high decomposing temperatures.34,35,40 The total amount of SOGs releasing CO2 decreases with increasing treating temperature monotonously until thorough elimination (Table 2). The thermal treatment at 400 °C under N2 atmosphere results in the most total amount of SOGs releasing CO which then reduce with the climbing C



Article

Table 2. Amount (A) and Decomposing Temperatures (T) of Different SOGs and the Total Amount of SOGs (Obtained by the Deconvolution of CO2-TPD Curves) peak 1 carboxyl

peak 2 carboxylic anhydride

peak 3 peroxide

peak 4 lactone 1

peak 5 lactone 2

sample

Aa

Tb

A

T

A

T

A

T

A

T

total A

AC AC-n AC-n-H900 AC-n-N400 AC-n-N600 AC-n-N900

4.2 80.8 2.1 34.7 25.2 \

259 272 321 304 315 \

5.2 200.4 \ 50.2 2.2 \

476 409 \ 515 464 \

\c 2.9 0.9 \ 2.7 \

\ 616 527 \ 538 \

8.8 11.9 1.0 62.2 0.7 \

678 687 656 643 590 \

0.5 5.3 0.6 2.3 26.2 \

865 816 834 848 712 \

18.7 301.2 4.6 149.4 57.2 \

The amount of SOGs with the unit of μmol/g. bThe decomposing temperature with the unit of °C. (It is similarly hereinafter if there is no additional description.) cThere is no such surface oxygenated group present on activated carbon according to the result of deconvolution. a

Table 3. Amount and Decomposed Temperatures of Different SOGs and the Total Amount of SOGs (Obtained by the Deconvolution of CO-TPD Curves) peak 1 adsorptive CO

a

peak 2 carboxylic anhydride

peak 3 phenol

peak 4 ether

peak 5 carbonyl

sample

A

T

A

T

A

T

A

T

A

T

total A

AC AC-n AC-n-H900 AC-n-N400 AC-n-N600 AC-n-N900

\a 12.8 \ \ \ \

\ 280 \ \ \ \

\ 125.7 \ 94.2 \ \

\ 518 \ 540 \ \

\ 191.8 \ 264.1 82.4 \

\ 673 \ 659 655 \

\ 63.8 \ 101.0 136.8 \

\ 745 \ 731 720 \

\ 199.2 \ 327.0 367.5 \

\ 850 \ 838 841 \

\ 593.3 \ 786.3 586.7 \

There is no such surface oxygenated group present on activated carbon according to the result of deconvolution.

2, the first peaks of catalysts in group 2 and group 3, representing the H2 consumption by Au3+, are more distinct than the counterparts in group 1, which means that the catalysts impregnated with HCl solution and aqua regia solution present more Au3+, which is known as the active site for acetylene hydrochlorination,15,41 compared with the catalysts impregnated with H2O solution. The catalysts in group 1 present negligible Au3+ reduction peaks. The catalysts in group 2 have the approximate magnitude of Au3+ reduction peaks compared with those in group 3. As for the samples of Au(HCl)/AC-nN900, Au(HCl)/AC-n-H900, Au(aq)/AC-n-N900, and Au(aq)/AC-n-H900, the peaks attributed to the reduction of Au3+ present higher reduction temperatures than the other samples. This may be due to the fact that the thermal treatment at 900 °C under N2 or H2 atmosphere removed almost completely the SOGs (Figure 1), leading to enhanced surface basic property42,43 and promoted electron mobility34,44,45 of the activated carbon support, which may subsequently enhance the interaction between Au3+, whose precursor is a strong acid (HAuCl4), and carbon support. It should be noted here that the reduction temperature of Au3+ obtained in this work is higher than that published in the existing article,15 considering that the TPR result could be affected by factors like gas flow rate, gas constitution, heating rate, and instrument employed, which could be taken as a reasonable situation. In addition, the TPR experiment of standard sample, pure AgO, was carried out and proved to be the same situation: the reduction temperature obtained is higher than that reported in literature. The second peak stands for the H2 consumption caused by carbon support which may consist of two contributions. For one thing, H2 may be consumed by the reaction with some SOGs during thermal treatment. As shown in Figure 2, there is a common phenomenon present on the three groups that the catalysts using AC-n-N400 and AC-n-N600 as supports have the almost

treating temperature (Table 3). It is worth mentioning that thermal treatment, under N2 or H2 atmosphere, with different treating temperatures, can selectively remove the SOGs, leaving the specific SOGs still on activated carbon. This is a doubtlessly powerful technology to perfect fundamental study of carbon, identify active sites of carbon catalysts, and modify surface chemistry of carbon to improve performance of carbonsupported catalysts. 3.2. Catalysts Characterization. Figure 2 displays the TPR profiles of catalysts, gold supported on different pretreated activated carbons, obtained using different impregnating solutions (H2O, HCl, aqua regia). As can be seen from Figure

Figure 2. TPR profiles of catalysts using different impregnating solutions and supports submitted to different pretreatments: group 1, impregnated with H2O solution; group 2, impregnated with HCl solution; group 3, impregnated with aqua regia solution. D



Article

no discernible Au reflection by XRPD and the Au particle sizes of them were both 3 ± 2 nm as determined by TEM. 3.3. Catalytic Performance. Figure 4a shows the performance of Au(aq)/AC-n catalyst in acetylene hydrochlorination. The fresh catalyst displayed a relatively good initial performance, with almost 70% C2H2 conversion and more than 99.9% VCM selectivity with trace amounts (0.1%) of 1,2-dichloroethane and chlorinated oligomers only.9,13 In addition, it still had 65% C2H2 conversion after 24 h, indicating its good catalytic stability in acetylene hydrochlorination. The results of acetylene hydrochlorination by catalysts obtained using different impregnating solutions and different supports are shown in Figure 4. The catalysts obtained using H2O as solvent exhibited a good initial performance, but a sharp decline in its activity occurred after 2 h from the highest conversion to a steady conversion about 10% (Figure 4b). The controlled test of activated carbon without Au showed that the steady 10% conversion was identical to that of the support. This residual activity of the activated carbon could have arisen from the presence of trace amounts of K+ and Al3+ in carbon matrix, as these metals could display some activity to the hydrochlorination of acetylene.6 We speculated that the high conversion in preliminary stage might be caused by coke deposition, as was the subsequent rapid deactivation. The catalysts impregnated using HCl solution have distinctly different performance with each other. Au(HCl)/AC-n-N900 and Au(HCl)/AC-n-H900 both displayed low activity with the highest acetylene conversion of 35% and 26%, respectively, as shown in Figure 4c. Au(HCl)/AC-n-N400 and Au(HCl)/ACn-N600 showed relatively higher activity with the highest acetylene conversion of 54% and 62%, respectively. As also can be observed, these catalysts all showed low stability. Au(aq)/AC-n-N400 and Au(aq)/AC-n-N600 showed relative high activity and stability among the catalysts impregnated with aqua regia, with the highest acetylene conversion of 73% and 72% which then dropped to 68% and 64% within 16 h, respectively. Au(aq)/AC-n-H900 had the lowest activity and stability with the highest acetylene conversion of 50% which was lower than the 54% obtained by Au(aq)/AC-n-N900 (Figure 4d). It should be noticed that the comparison between the results obtained in this work and those reported in the literature is particularly difficult since the reaction conditions are different between each other. For example, some of Hutchings’ works15,41 obtained results under flow conditions with 5 mL min−1 C2H2, 6 mL min−1 HCl, and 10 mL min−1 N2. Bin Dai et al.46 obtained results under flow conditions with 3.3 mL min−1 C2H2 and 3.8 mL min−1 HCl. Moreover, the results are often presented as conversion obtained after a certain reaction time. Hutchings et al.9 presented the results starting from the highest acetylene conversion, without providing rising stage of conversion. 3.4. Discussion. The catalysts obtained using different impregnating solutions and different supports show distinctly different performance between each other, as shown in Figure 4. The different catalytic performance may stem from a different amount of Au3+ or Au dispersion or surface chemistry of support. On the basis of this, these factors were taken into discussion. 3.4.1. Effect of Au Dispersion and Au3+ Quantity. In a comparison of the catalysts obtained using H2O as solvent with those impregnated using HCl, the former show evidently different performance with the latter, as shown in Figure 4. The

overlapped second peaks, while they have bigger second peaks than the catalysts using AC-n as supports. Since there is not any peak below 400 °C attributed to carbon support (Figure 2), the SOGs with low decomposing temperatures like carboxyl and carboxylic anhydride may not consume H2. Taking this into account, one can say that only the decomposition of SOGs with high decomposing temperatures such as phenol, ether, and carbonyl may consume H2. For another, the removal of SOGs left a surface containing very reactive carbon sites which could subsequently react with H2 to form stable configuration.34,35,37 As anticipated, the second peaks of catalysts in group 3 have higher intensity than corresponding catalysts in group 1, especially those of Au(aq)/AC-n-N900 and Au(aq)/AC-nH900. Indeed, impregnating thermally treated carbon with aqua regia could effectively recover the SOGs. There is only slight difference between the second peaks of catalysts in group 2 and those in group 1. Since impregnating catalyst with H2O solution would not affect the surface chemistry of support significantly, it could be concluded indirectly that applying HCl solution to impregnate catalyst impacts the surface chemistry of support slightly.15 Figure 3 shows the XRD patterns of different catalysts. It can be seen obviously in Figure 3 that the catalysts in group 1

Figure 3. XRD patterns of catalysts using different impregnating solutions and supports submitted to different pretreatments: (a) Au(H2O)/AC-n-N400; (b) Au(H2O)/AC-n-N600; (c) Au(H2O)/ AC-n-N900; (d) Au(H2O)/AC-n-H900; (e) Au(HCl)/AC-n-N400; (f) Au(HCl)/AC-n-N600; (g) Au(HCl)/AC-n-N900; (h) Au(HCl)/ AC-n-H900; (i) Au(aq)/AC-n-N400; (j) Au(aq)/AC-n-N600; (k) Au(aq)/AC-n-N900; (l) Au(aq)/AC-n-H900;.

present four distinct peaks for Au(111), Au(200), Au(220), and Au(311), respectively, indicating that the catalysts impregnated with H2O solution contain large Au particles due to the Au3+ reducing to Au0. In addition, no discernible Au reflection was detected for those catalysts in groups 2 and 3, indicating particles below 4 nm or materials with a large amount of Au3+ centers were obtained, which is consistent with the results of TPR experiments as shown in Figure 2. Elsewhere, the catalysts in group 2 have no significant difference of gold dispersion with the catalysts in group 3. The XRD patterns highlight that the solvent has an effect on the nucleation process of Au particles. The results obtained in this work can also be confirmed by the work finished by Hutchings15 where the catalysts dried at 110 °C using HCl or aqua regia as solvent were proven to present E



Article

Figure 4. Acetylene hydrochlorination by different gold catalysts under reaction conditions: 180 °C, HCl/C2H2 mol ratio = 1.2:1, reactant flow rate 11 mL/min. (a) Au(aq)/AC-n; catalysts impregnated with (b) H2O solution; (c) HCl solution; (d) aqua regia solution.

carried out, and the results are shown in Figure 5. The amounts of SOGs present on these aqua regia treated carbons were obtained by the deconvolution of the curves in Figure 5 and summarized in Table 4. It should be mentioned here that, for samples AC-n-N900 and AC-n-H900, the amount of phenol, ether, and carbonyl were obtained by dividing total amount of SOGs releasing CO by 1:4:9, which may not affect the final result significantly since these two samples possess obviously lower amounts of SOGs than the other samples. Without a doubt, treating thermal treated carbons with aqua reiga recovered large amounts of SOGs, especially in the cases of AC-n-N900-aq and AC-n-H900-aq. As the performance of catalysts impregnated with aqua regia is better than those impregnated with HCl, the modification of surface chemistry generated by aqua regia treatment seems to be beneficial for the performance of catalyst. On the other hand, the catalytic activity and the total amount of SOGs of the four catalysts impregnated with aqua regia share the same sequence as the following, as shown in Figures 4d and 5: Au(aq)/AC-n-N400 > Au(aq)/AC-n-N600 > Au(aq)/AC-n-N900 > Au(aq)/AC-nH900. In addition, when correlating the oxygen content of support with catalytic performance as shown in Figure 6, it could be seen clearly that the more oxygen content the support contains, the better catalytic performance the catalyst has. On the basis of these three points, it could be concluded that the performance of catalyst improves with the increasing amount of SOGs.

catalysts impregnated with H2O, on whose surfaces large Au particles are predominant (Figure 3), present negligible Au3+ (Figure 2). On the contrary, the catalysts impregnated with HCl have high dispersion and a large amount of Au3+ (Figures 2 and 3). As a result, the apparent difference of performance between them is most likely caused by the difference of Au dispersion and Au3+ quantity. The catalysts impregnated with aqua regia show relatively higher activity and stability than those impregnated with HCl. As the catalysts impregnated with aqua regia have similar Au dispersion and Au3+ quantity with those impregnated with HCl, these two factors have a negligible contribution to the difference of catalytic performance between them. 3.4.2. Effect of Surface Chemistry. Indeed, the Au dispersion and the amount of Au3+ present on catalysts could affect the activity and stability of catalysts. But these seem not to be the main reasons for the different performance between catalysts impregnated with aqua regia and those impregnated with HCl. It is more likely the surface chemistry playing the principal role of affecting the performance of catalysts in this case. Since aqua regia, with stronger oxidizability than concentrated HNO3 solution, can recover SOGs, in order to figure out how the surface chemistry will influence the catalytic performance, it still needs to be known how the aqua regia treatment will affect the surface chemistry of the thermal treated activated carbons. Taking this into account, the carbonTPD experiments for the aqua regia treated carbons were F



Article

Figure 6. Correlation between the oxygen content of support (obtained by the amount of releasing CO and CO2) and catalytic performance: (a) the highest acetylene conversion in 24 h and (b) deactivation rate.

Figure 5. Carbon-TPD evolution profiles of aqua regia treated carbons: (a) CO and (b) CO2.

3.4.3. Correlation between the Catalytic Activity and Specific SOGs for Au Catalysts. As can be seen from Figure 4c,d, the catalysts impregnated with HCl and aqua regia have a similar phenomenon in that the difference of catalytic performance between Au/AC-n-N400 and Au/AC-n-N600 is obviously smaller than that between Au/AC-n-N600 and Au/ AC-n-N900, which means that the SOGs changed between ACn-N600 and AC-n-N900 are most probably the key tuning catalytic performance which are phenol, ether, and carbonyl (see Table 4). Figure 7 displays the correlation between the total amount of phenol, ether, and carbonyl and the catalytic performance. Interestingly, when comparing Figure 6a with Figure 7a and Figure 6b with Figure 7b, it could be observed

that Figure 6a and Figure 6b are almost the same as Figure 7a and Figure 7b in profile, respectively. This could be strong evidence confirming that phenol, ether, and carbonyl are the groups affecting catalytic performance the most. As a conclusion, the activity and stability of Au catalyst are both enhanced with the increase amount of phenol, ether, and carbonyl. 3.4.4. DFT Calculation. It was proven that SOGs on carbon materials, especially phenol, ether, and carbonyl, might have obvious effect on the catalytic performance of Au catalysts. In order to further investigate the interaction between Au

Table 4. Summary of the Amount of SOGs Present on Activated Carbons (Obtained by the Deconvolutoin of CO-TPD and CO2-TPD Curves) sample

carboxyl (μmol/g)

carboxylic anhydride (μmol/g)

peroxide (μmol/g)

lactone (μmol/g)

phenol (μmol/g)

ether (μmol/g)

carbonyl (μmol/g)

AC-n-N400 AC-n-N400-aq AC-n-N600 AC-n-N600-aq AC-n-N900 AC-n-N900-aq AC-n-H900 AC-n-H900-aq

34.7 67.0 25.2 91.6 \a 78.2 2.1 56.4

50.2 24.5 2.2 74.9 \ 21.7 \ 21.3

48.9 64.6 2.7 30.1 \ 34.3 0.9 29.5

64.6 35.1 27.0 70.8 \ 33.6 1.6 17.2

264.1 245.5 82.4 574.0 9.8 341.5 3.5 39.4

101.0 270.8 136.8 480.8 39.4 194.4 14.0 424.4

327.0 148.7 367.5 130.3 88.6 18.8 31.6 13.5

a

There is no such surface oxygenated group present on activated carbon according to the result of deconvolution. G



Article

nanoparticles. As phenol, ether, and carbonyl would result in forming the same configuration, which was Au−O−C, after anchoring gold nanoparticles, only the aforementioned configuration was taken into consideration in this study. Figure 8 shows the optimized bonding geometries of Au3 and Au1/ Au2Cl6 on graphene with oxygen. By means of DFT calculations, it was identified that small Au clusters could not be adsorbed on pristine graphene.26 However, on the graphene with oxygen, the adsorption energy of Au3 is −1.26 eV. Furthermore, the adsorption of Au2Cl6 complex on Au1/ graphene with oxygen was also investigated. The adsorption energies of Au2Cl6 range from −1.47 to −2.17 eV, depending on different geometries. In the most stable structure, it seems that Au1 and Au2Cl6 are combined together. According to experimental observations and DFT results, we supposed that the active sites consisted of AuCl3 or Au2Cl6 covering small Au particles. The configuration of Au−O−C, derived from anchoring of gold by phenol, ether, and carbonyl, could stabilize these Au particles and Au2Cl6 species.

4. CONCLUSIONS In this work, liquid phase oxidation processes using concentrated HNO3 solution and thermal treatments under different temperatures were applied to bring in and selectively get rid of SOGs, respectively. Carbon-TPD was carried out to characterize surface chemistry, and a deconvolution method using multiple Gaussian functions was applied to estimate the amount of each type of SOG. These activated carbons with surface chemistry modified were used to support AuCl3 as catalysts for acetylene hydrochlorination. The DFT calculations, combined with reaction results, confirmed that SOGs could stabilize Au particles and Au3+ species. The correlation between SOGs and catalytic performance was obtained: the catalytic performance including activity and stability of Au/AC catalyst is improved with increasing amounts of SOGs, especially phenol, ether, and carbonyl.

Figure 7. Correlation between the amount of specific SOGs and catalytic performance of catalyst: (a) the highest acetylene conversion in 24 h and (b) deactivation rate.

■

nanoparticle and SOGs, the DFT calculation was carried out. According to our previous study,25 the interaction between Pd and carbon materials with oxygen, which can be characterized as Pd−O−C, was stronger than those with carboxyl groups. D.A. Bulushev et al.47 reported that phenolic groups present on the surface of activated carbon fibers were able to attach Au3+ to form Au−O−C, leading to the formation of small Au

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 571 88320002. E-mail: [email protected]. *Tel.: +86 571 88871037. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 8. Optimized geometries and adsorption energies of Au3 and Au1/Au2Cl6 on graphene with adsorbed oxygen. H



■

Article

(21) Basset, J. M.; Psaro, R.; Roberto, D.; Ugo, R. Modern Surface Organometallic Chemistry; John Wiley & Sons: New York, 2009. (22) Netskina, O.; Komova, O.; Tayban, E.; Oderova, G.; Mukha, S.; Kuvshinov, G.; Simagina, V. The influence of acid treatment of carbon nanofibers on the activity of palladium catalysts in the liquid-phase hydrodechlorination of dichlorobenzene. Appl. Catal., A 2013, 467, 386−393. (23) Pereira, M.; Orfao, J.; Figueiredo, J. Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups. Appl. Catal., A 1999, 184, 153−160. (24) Moreno-Castilla, C.; Carrasco-Marın, F.; Parejo-Pérez, C.; López Ramón, M. Dehydration of methanol to dimethyl ether catalyzed by oxidized activated carbons with varying surface acidic character. Carbon 2001, 39, 869−875. (25) Xu, T. Y.; Zhang, Q. F.; Yang, H. F.; Wang, J. G.; Li, X. N. Role of phenolic groups in the stabilization of palladium nanoparticles. Ind. Eng. Chem. Res. 2013, 52, 9783−9789. (26) Xie, P. Y.; Zhuang, G. L.; Lu, Y. A.; Wang, J. G.; Li, X. N. Enhanced bonding between noble metal adatoms and graphene with point defects. Acta Phys.-Chim. Sin. 2012, 28, 331−337. (27) Machado, B. F.; Oubenali, M.; Rosa Axet, M.; Trang NGuyen, T.; Tunckol, M.; Girleanu, M.; Ersen, O.; Gerber, I. C.; Serp, P. Understanding the surface chemistry of carbon nanotubes: Toward a rational design of Ru nanocatalysts. J. Catal. 2014, 309, 185−198. (28) Kresse, G.; Hafner, J. Ab initio molecular dynamics for openshell transition metals. Phys. Rev. B 1993, 48, 13115−13118. (29) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169−11186. (30) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758−1775. (31) Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals density functional for general geometries. Phys. Rev. Lett. 2004, 92, 246401. (32) Gulans, A.; Puska, M. J.; Nieminen, R. M. Linear-scaling selfconsistent implementation of the van der Waals density functional. Phys. Rev. B 2009, 79, 201105. (33) Figueiredo, J. L.; Pereira, M. F.; Freitas, M. M.; Ó rfão, J. J. Characterization of active sites on carbon catalysts. Ind. Eng. Chem. Res. 2007, 46, 4110−4115. (34) Rodrigues, E. G.; Pereira, M. F.; Chen, X.; Delgado, J. J.; Ó rfão, J. J. Influence of activated carbon surface chemistry on the activity of Au/AC catalysts in glycerol oxidation. J. Catal. 2011, 281, 119−127. (35) Figueiredo, J.; Pereira, M.; Freitas, M.; Orfao, J. Modification of the surface chemistry of activated carbons. Carbon 1999, 37, 1379− 1389. (36) Alegre, C.; Gálvez, M. E.; Baquedano, E.; Moliner, R.; Pastor, E.; Lázaro, M. J. S. Oxygen-functionalized highly mesoporous carbon xerogel based catalysts for direct methanol fuel cell anodes. J. Phys. Chem. C 2013, 117, 13045−13058. (37) Menéndez, J. A.; Phillips, J.; Xia, B.; Radovic, L. R. On the modification and characterization of chemical surface properties of activated carbon: in the search of carbons with stable basic properties. Langmuir 1996, 12, 4404−4410. (38) Rodrigues, E. G.; Delgado, J. J.; Chen, X.; Pereira, M. F.; Ó rfão, J. J. Selective oxidation of glycerol catalyzed by gold supported on multiwalled carbon nanotubes with different surface chemistries. Ind. Eng. Chem. Res. 2012, 51, 15884−15894. (39) Zhou, J. H.; Sui, Z. J.; Zhu, J.; Li, P.; Chen, D.; Dai, Y. C.; Yuan, W. K. Characterization of surface oxygen complexes on carbon nanofibers by TPD, XPS and FT-IR. Carbon 2007, 45, 785−796. (40) Vivo-Vilches, J. F.; Bailón-García, E.; Pérez-Cadenas, A. F.; Carrasco-Marín, F.; Maldonado-Hódar, F. J. Tailoring the surface chemistry and porosity of activated carbons: Evidence of reorganization and mobility of oxygenated surface groups. Carbon 2014, 68, 520−530. (41) Conte, M.; Davies, C. J.; Morgan, D. J.; Davies, T. E.; Carley, A. F.; Johnston, P.; Hutchings, G. J. Modifications of the metal and support during the deactivation and regeneration of Au/C catalysts for

ACKNOWLEDGMENTS The financial support from Natural Science Foundation of China (NSFC Grants 20976164 and 21303163), National Basic Research Program of China (973 Program) (No.2011CB710800), Qianjiang Talent Project in Zhejiang Province (QJD1302011), and Scientific Research Fund of Zhejiang Provincial Education Department (Y201328681) are gratefully acknowledged.

■

REFERENCES

(1) Clegg, I. M.; Hardman, R. U.S. Patent 5,763,710, 1998. (2) Speight, J. G.; Speight, J. Chemical and Process Design Handbook; McGraw-Hill: New York, 2002; p 2542. (3) Hutchings, G. J. Vapor phase hydrochlorination of acetylene: Correlation of catalytic activity of supported metal chloride catalysts. J. Catal. 1985, 96, 292−295. (4) Hutchings, G. J. Catalysis: A golden future. Gold Bull. 1996, 29, 123−130. (5) Nkosi, B.; Coville, N. J.; Hutchings, G. J. Reactivation of a supported gold catalyst for acetylene hydrochlorination. J. Chem. Soc., Chem. Commun. 1988, 1, 71−72. (6) Shinoda, K. The vapor-phase hydrochloride of acetylene over metal chlorides supported on actived carbon. Chem. Lett. 1975, 4, 219−220. (7) Matar, S.; Hatch, L. F. Chemistry of Petrochemical Processes; Gulf Professional Publishing: Houston, TX, 2001; p 202. (8) Hashmi, A. S. K.; Hutchings, G. J. Gold catalysis. Angew. Chem., Int. Ed. 2006, 45, 7896−7936. (9) Conte, M.; Carley, A. F.; Heirene, C.; Willock, D. J.; Johnston, P.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Hydrochlorination of acetylene using a supported gold catalyst: A study of the reaction mechanism. J. Catal. 2007, 250, 231−239. (10) Conte, M.; Davies, C. J.; Morgan, D. J.; Carley, A. F.; Johnston, P.; Hutchings, G. J. Characterization of Au3+ species in Au/C catalysts for the hydrochlorination reaction of acetylene. Catal. Lett. 2014, 144, 1−8. (11) Conte, M.; Carley, A. F.; Hutchings, G. J. Reactivation of a carbon-supported gold catalyst for the hydrochlorination of acetylene. Catal. Lett. 2008, 124, 165−167. (12) Nkosi, B.; Coville, N. J.; Hutchings, G. J. Vapour phase hydrochlorination of acetylene with group VIII and IB metal chloride catalysts. Appl. Catal. 1988, 43, 33−39. (13) Nkosi, B.; Adams, M. D.; Coville, N. J.; Hutchings, G. J. Hydrochlorination of acetylene using carbon-supported gold catalysts: A study of catalyst reactivation. J. Catal. 1991, 128, 378−386. (14) Nkosi, B.; Coville, N. J.; Hutchings, G. J.; Adams, M. D.; Friedl, J.; Wagner, F. E. Hydrochlorination of acetylene using gold catalysts: A study of catalyst deactivation. J. Catal. 1991, 128, 366−377. (15) Conte, M.; Davies, C. J.; Morgan, D. J.; Davies, T. E.; Elias, D. J.; Carley, A. F.; Johnston, P.; Hutchings, G. J. Aqua regia activated Au/C catalysts for the hydrochlorination of acetylene. J. Catal. 2013, 297, 128−136. (16) Conte, M.; Carley, A. F.; Attard, G.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Hydrochlorination of acetylene using supported bimetallic Au-based catalysts. J. Catal. 2008, 257, 190−198. (17) Serp, P.; Corrias, M.; Kalck, P. Carbon nanotubes and nanofibers in catalysis. Appl. Catal., A 2003, 253, 337−358. (18) Prado-Burguete, C.; Linares-Solano, A.; Rodriguez-Reinoso, F.; de Lecea, C. The effect of oxygen surface groups of the support on platinum dispersion in Pt/carbon catalysts. J. Catal. 1989, 115, 98− 106. (19) Derbyshire, F.; De Beer, V.; Abotsi, G.; Scaroni, A.; Solar, J.; Skrovanek, D. The influence of surface functionality on the activity of carbon-supported catalysts. Appl. Catal. 1986, 27, 117−131. (20) Sepúlveda-Escribano, A.; Coloma, F.; Rodrıguez-Reinoso, F. Platinum catalysts supported on carbon blacks with different surface chemical properties. Appl. Catal., A 1998, 173, 247−257. I



Article

the hydrochlorination of acetylene. Catal. Sci. Technol. 2013, 3, 128− 134. (42) Papirer, E.; Li, S.; Donnet, J. B. Contribution to the study of basic surface groups on carbons. Carbon 1987, 25, 243−247. (43) Leon, C.; Solar, J.; Calemma, V.; Radovic, L. R. Evidence for the protonation of basal plane sites on carbon. Carbon 1992, 30, 797−811. (44) Sebastián, D.; Suelves, I.; Moliner, R.; Lázaro, M. The effect of the functionalization of carbon nanofibers on their electronic conductivity. Carbon 2010, 48, 4421−4431. (45) Hashisho, Z.; Rood, M. J.; Barot, S.; Bernhard, J. Role of functional groups on the microwave attenuation and electric resistivity of activated carbon fiber cloth. Carbon 2009, 47, 1814−1823. (46) Li, X.; Zhu, M.; Dai, B. AuCl3 on polypyrrole-modified carbon nanotubes as acetylene hydrochlorination catalysts. Appl. Catal., B 2013, 142−143, 234−240. (47) Bulushev, D. A.; Yuranov, I.; Suvorova, E. I.; Buffat, P. A.; KiwiMinsker, L. Highly dispersed gold on activated carbon fibers for lowtemperature CO oxidation. J. Catal. 2004, 224, 8−17.

J


Influence of Surface Chemistry of Activated Carbon ... - ACS Publications

Recommend Documents