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Catalytic oxidation of toluene and p-xylene using gold supported on Co3O4 catalyst prepared by colloidal precipitation method
Journal of Molecular Catalysis A: Chemical 351 (2011) 188–195
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Catalytic oxidation of toluene and p-xylene using gold supported on Co3 O4 catalyst prepared by colloidal precipitation method Hongjing Wu ∗ , Liuding Wang, Zhongyuan Shen, Jinghui Zhao Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 15 June 2011 Received in revised form 24 August 2011 Accepted 1 October 2011 Available online 8 October 2011 Keywords: Au/Co3 O4 Toluene and p-xylene oxidation Electrophilic oxygen species Oxygen vacancies Strong interaction
a b s t r a c t Gold supported on cobalt oxide has been successfully synthesized through a colloidal precipitation method and tested in toluene and p-xylene total oxidation. It has been demonstrated that the catalytic activity of Au/Co3 O4 for toluene and p-xylene oxidation is much higher than that of Au/Al2 O3 and Au/MgO in spite of its lower BET surface area and larger gold crystalline size. The enhanced catalytic activity in toluene and p-xylene oxidation has been linked to a high concentration of superficial electrophilic oxygen species and oxygen vacancies, which may be originated from a strong interaction in the colloidal Au–Co3 O4 system. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Benzene, toluene and xylene (BTX) are major pollutants in indoor and outdoor air, emitted from many industrial processes and transportation activities [1]. They often act as three major categories of greenhouse gases and suspected carcinogens [2]. Therefore, it is an extremely urgent task to reduce the emission of these pollutants. Catalytic oxidation is one of the most effective ways for the elimination of BTX, which operates at relatively low temperatures with respect to the conversional thermal combustion [1]. Furthermore, it provides the potential to destroy BTX totally to carbon dioxide and water [3]. Supported noble metal (Pt and Pd) catalysts [4–9] have been used as catalysts for this reaction due to their high activity at relatively low temperatures and the tolerance to the moisture. Since Haruta’s discovery [10] of the remarkably high activity of supported gold catalysts for low-temperature CO oxidation, reports on many practical applications, such as the reduction of NOx [11], hydrogenation, water–gas shift reaction and complete oxidation of hydrocarbons [12,13], have increased dramatically. The use of supported gold catalysts offers the advantages of both high reactivity and a relatively low price compared with earlier reported supported Pt or Pd catalysts. Among metal oxides cobalt oxide, in the form of Co3 O4 , has been shown to be one of the most efficient non-noble metal
∗ Corresponding author. Tel.: +86 29 88431664; fax: +86 29 88431664. E-mail address: [email protected] (H. Wu). 1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2011.10.005
oxides for VOCs elimination, and it seems to be the most active for propane combustion [14] due to its excellent reducibility, high concentration of electrophilic O-species (Oads , O− or O2 − ) [15] and oxygen vacancies [16]. Furthermore, improved catalytic performance has been associated with the high surface areas and 3D ordered mesoporous structure of the Co3 O4 [17–19]. In the last years, the complete oxidation of VOCs over gold on cobalt oxide has been widely studied, especially that of alkanes. Gold on cobalt oxide was shown to be active for the catalytic combustion of methane and propane [20,21]. The results suggested that active catalysts were related to the chemical state of gold phase. Recently gold on cobalt oxide has been turned to be highly active for low-temperature oxidation of trace ethylene [22] and it achieves a 76% conversion at 0 ◦ C when mesoporous Co3 O4 used as support [23]. Furthermore, they found that support morphology has a significant effect on catalytic activity, which is related to the exposed planes of different morphological Co3 O4 [24]. Moreover, catalytic oxidation of toluene, propane and CO has been investigated on a non-fully ordered nanocast Co3 O4 deposited by gold, and it exhibits high activity for the pollutants total oxidation [25]. It was proved that the enhanced catalytic activity has been related to both the improved reducibility of Co3 O4 when gold is added and the simultaneous presence of Auı+ and Au0 . In previous paper we reported a highly efficient Au/ZnO catalyst for low-temperature BTX oxidation [26]. Our results confirmed that strong metal-oxide interaction between Au {1 1 1} and ZnO {1 0 1} planes occurs due to the very small lattice mismatch. In present paper we investigated the properties of Au/Co3 O4 catalysts prepared by colloidal precipitation method and tested in
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Table 1 structural properties and surface atom ratios of supported gold catalysts. Catalysta
Au/Co3 O4 Au/Al2 O3 Au/MgO
BET area
1.6 205 44
Co3 O4 size average (nm)b
122 – –
Au size average (nm)c
10.6 ± 4.18 4.2 ± 1.79 3.3 ± 1.02
Au loading (wt.%)d
1.58 1.40 1.69
Au dispersion (%)e
9.43 27.55 35.07
Au/M atom ratio (%)
0.95 0.46 0.79
XPSf (m2 g−1 ) Au3+
Auı+
Au0
O2 −
Oads
O2−
Co2+ /Co3+
0.40 0 0
0.39 1.00 1.00
0.21 0 0
0.65 0 0
0.23 0.88 0.63
0.12 0.12 0.37
1.80 – –
Calcined at 350 ◦ C. Calculated by TEM method. c Calculated by HRTEM method. d Calculated from semi-quantitative EDX data. e Calculated by formula: DAu = 6ns MAu /Au NA dAu , where ns is the number of Au atoms at the surface per unit area (1.15 × 1019 m−2 ), MAu is the molar mass of gold (196.97 g·mol−1 ), Au is the density of gold (19.5 g·cm−3 ), NA is Avogadro’s number (6.023 × 1023 mol−1 ) and dAu is the average Au particle size (determined by HRTEM). f Catalysts before reaction. a
b
the total oxidation of toluene and p-xylene, chosen as probe molecules of alkyl aromatic compounds. The results were compared with the analogous series of gold supported on Al2 O3 (or MgO) with the same preparation procedure and suggested that the amount of chemisorbed electrophilic O-species and oxygen vacancies are the determining factors leading to high activity and stability of the Au/Co3 O4 , which may be originated from a strong metal-oxide interaction between Au and Co3 O4 .
X-ray source. Gold catalysts (calcined at 350 ◦ C in air for 4 h) before and after use in catalytic oxidation were pressed into a pellet and transferred to a test chamber with the vacuum below 5 × 10−5 Torr. The binding energies were calibrated by referencing the C1s at 284.6 eV.
2. Experimental
The catalytic activity of supported gold catalysts in toluene and p-xylene oxidation was measured in a fixed tubular quartz reactor under atmospheric pressure. The reactant and product mixtures were analyzed by an on-line gas chromatograph equipped with a hydrogen flame ionization detector and a HP-5 column. The following conditions were chosen: catalyst volume 0.082 cm3 , inlet toluene concentration 0.6 g m−3 (146 ppmv) and p-xylene concentration 0.40 g m−3 (85 ppmv) in synthetic gas (O2 , 10 vol.%; N2 , balance), volume hour space velocity 14,690 h−1 , the temperature range 50–300 ◦ C. The primary products were CO2 and H2 O only analyzed by GC–MS and FTIR. Thus, the catalytic activity was expressed as a degree of the conversion of hydrocarbon. The conversion of toluene (or p-xylene) was calculated as following:
2.1. Catalyst preparation A chemical precipitation method was used to synthesize supports of Co3 O4 , Al2 O3 and MgO. 5 ml of ammonia was gradually added into 10 ml of an aqueous 0.01 M solution of nitrate with continuous stirring. After 2 h, the percolated precipitation was filtered and washed with deionized water. The solids were dried at 80 ◦ C and finally calcined at 500 ◦ C for 2 h for crystallization. The supported gold catalysts were prepared by the colloidal deposition method by using PVA (Mw = 10,000) as a protecting agent and HAuCl4 ·3H2 O as the gold precursor [27]. For more details of the preparation procedure, refer to our previous work [26]. The nominal gold loading amount of catalyst is 1.50 wt.%. The resulting catalysts were denoted as Au/Co3 O4 , Au/Al2 O3 and Au/MgO, respectively.
2.3. Activity measurement for toluene and p-xylene oxidation
Conversion (%) =
[Reactant]in − [Reactant]out × 100% [Reactant]in
2.2. Catalysts characterization
3. Results and discussion
The specific surface area was determined by the BET method with N2 adsorption–desorption measurements at 77 K (Micromeritics ASAP 2020). The crystalline structure of the catalyst was analyzed by powder X-ray diffraction (XRD, Rigaku D/max-RB) with Cu K␣ radiation ˚ operated at 40 kV and 30 mA. Phases were identified ( = 1.54016 A) by matching experimental patterns to the JCPDS powder diffraction file. The morphology and microstructure of the catalysts were observed by the scanning electron microscopy (SEM, JEOL JSM-5610LV) and field emission transmission electron microscopy (FETEM, JEOL JEM-2100F). TEM samples were treated by sonicating in absolute ethanol for several minutes and then a few drops of the resulting suspension were deposited onto a holey-carbon film supported in a copper grid, which was dried naturally. The chemical composition and metal content of the catalysts were analyzed by the energy-dispersive X-ray spectroscopy analysis (EDX, OXFORD INCA). The atomic percentage and chemical state of surface element in the catalysts were measured by X-ray photoelectron spectroscopy (Perkin-Elmer, ESCA PHI 5400) using a monochromatic Mg K␣
3.1. Structural and textural properties The N2 adsorption–desorption isotherm and pore size distribution of Au/Co3 O4 are shown in Fig. S1. The Au/Co3 O4 gives an isotherm characteristic of macroporous material with a hysteresis loop of type III (Fig. S1a). The BET surface area and single-point total pore volume of Au/Co3 O4 are 1.6 m2 g−1 and 0.0043 cm3 g−1 , respectively. Besides, the BJH desorption average pore diameter is determined to be 59.7 nm (Fig. S1b). In contrast, the BET surface area measured for Au/Al2 O3 and Au/MgO are significantly higher than that of Au/Co3 O4 . Table 1 summarizes the textural parameters of supported gold catalysts, alongside with the Au loading, average Au particle size, Au dispersion and surface atom ratio. The phase composition and structural characterization of supported gold catalysts were examined by powder X-ray diffraction (PXRD) as shown in Fig. S2. It can be observed that the Au/Co3 O4 ˚ in crystal structure [space group: Fd3m is cubic Co3 O4 (a = 8.056 A) (2 2 7)] by comparing the XRD pattern of the standard cobalt oxide sample (JCPDS PDF 65-3103). Moreover, a new peak centered at 38.2◦ related to metallic Au0 can be roughly identified in the sample containing Au. The result indicates that, regardless of the low
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Fig. 1. TEM/HRTEM images of (a and b) gold nanoparticles, (c and d) Co3 O4 and (e) Au/Co3 O4 samples.
loading amount of gold, the Au element can be distinguished by XRD, which is probably due to the congregation of Au nanocrystals [25]. However, this diffraction feature is not distinguishable in Au/Al2 O3 and Au/MgO, indicating that the metallic Au0 is so small that it cannot be facilely identified by XRD [26].
3.2. Chemical composition and morphology Fig. S3 shows scanning electron microscopy (SEM) and energydispersive X-ray spectra (EDS) of supported gold catalysts. The SEM images indicate that the products consist of bulk particles. These
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Fig. 2. HRTEM images of (a) Au/Al2 O3 and (b) Au/MgO samples.
bulk particles are comprised of large numbers of Co3 O4 , Al2 O3 or MgO crystallites, which are covered by nanocrystalline gold. The EDS confirms the presence of Au element. It can be also confirmed by EDS that there are not any impurities except for Na+ , Ca2+ , indicating that Cl− ion is completely removed during washing. The Au loading amount of Au/MOx (M = Co, Al or Mg) are 1.58 wt.%, 1.40 wt.% and 1.69 wt.%, respectively. Transmission electron microscopy (TEM) images of the asprepared Au colloids are shown in Fig. 1. The Au colloids are highly crystalline in the gold solution (Fig. 1a and b). As one can see, the ultra-fine and homogeneously dispersed Au colloids are observed in the colloidal solution. Fig. 1a displays the TEM image of the gold dispersion to ascertain the mean particle size of Au colloids. The gold particle size is well fit to the Gaussian distribution and the average particle size and standard deviation are 3.9 nm and ±0.74 nm, respectively. Besides, the interplanar spacing of the Au colloids in the HRTEM image (Fig. 1b) is ca. 0.24 nm, corresponding to the interplanar distance of the {1 1 1} plane of Au nanocrystals. TEM images of pure Co3 O4 support calcined at 500 ◦ C are presented in Fig. 1c and d. The Co3 O4 particles are uniform hexagons and their average size is ca. 122 nm. Fig. 1d shows that the dominant exposed planes of Co3 O4 nanocrystals are {3 1 1} with a lattice space of ca. 0.24 nm. This is perfectly in agreement with the XRD results, indicating that the Co3 O4 particles are well crystallized after calcination at 500 ◦ C. Fig. 1e shows the Au colloids are dispersed in the pores of Co3 O4 support after calcination at 350 ◦ C for 4 h. The Au nanocrystals have a crystallite size of ca. 10 nm. The Co3 O4 particles have no changes in the shape and size when used as support, demonstrating a good crystallinity and a high stability of Co3 O4 particles after the loading of Au colloids. However, the particle size of Au colloids obviously increases during calcining at 350 ◦ C. Fig. 2 shows the HRTEM images of Au/Al2 O3 and Au/MgO. The Au nanoparticles are uniformly dispersed on Al2 O3 and MgO supports, with very small average particle sizes (4.2 and 3.3 nm, respectively), indicating that the Au nanoparticles on Al2 O3 or MgO is far more stable than that on Co3 O4 during calcining due to their higher specific surface area.
than the theoretic Au/M atom ratios of them. It indicates that Au colloids could be effectively dispersed on the surface of the supports, especially for Co3 O4 support. Fig. 4a and Table 1 show the XPS spectra of Au4f and surface species composition of Au for Au/MOx before oxidation reaction. The XPS spectrum of Au4f for Au/Co3 O4 shows strongly broadened peak, with the FWHMs around 3.11 eV, suggesting a contribution of Au species with varing electronic states. XPS Au4f spectrum for Au/Co3 O4 displays a pair of distinct peaks at 84.28 and 87.98 eV, which is typical of metallic Au0 species [28]. The additional peak at 91.79 eV, closes to that reported for Au3+ species [28]. This is in accordance with the XRD results, which shows an Au0 diffraction peak (2 = 38.2◦ ) of low intensity for the Au/Co3 O4 . In the case of Au/Al2 O3 and Au/MgO the presence of peaks at 85.8 and 89.5 eV indicates the presence of cationic Au+ or Auı+ . No peaks corresponding to metallic Au0 (around 83.7 and 87.4 eV) have been observed in the XPS Au4f spectra. However, it cannot be discarded that Au0 exists in these catalysts. In fact, these catalysts may be composed of a major amount of Auı+ and a minor amount of Au0 . Fig. 4b shows the O1s core-level spectra for fresh Au/MOx . For the Au/Co3 O4 , the O1s XPS spectrum can be decomposed into two components at BE = 529.68 and 531.49 eV; the former is due to the
3.3. Surface chemical state The full-range XPS spectra of the nominal 1.50 wt.% Au/MOx samples are shown in Fig. 3. The XPS spectra reveal that the elements on the surface of all samples are Au, M (M = Co, Al or Mg), O and C. The surface Au/M atom ratios in XPS spectra are ca. 0.95%, 0.46% and 0.79%, respectively (Table 1), which is relatively higher
Fig. 3. Full-range XPS spectra of the 1.5 wt.% supported gold catalysts.
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Fig. 4. XPS spectra of (a) Au4f region and (b) O1s region for 1.5 wt.% gold supported on various oxides catalysts.
surface lattice oxygen (Olatt ) species, whereas the latter is due to the surface adsorbed oxygen (Oads ) species [19,29]. There is another larger signal at 534.83 eV, which could be assigned to the electrophilic O-species (O2 − or O− ). Nevertheless, there are only two peaks at around 528.4 and 531.5 eV for Au/Al2 O3 and Au/MgO, which is similar to those reported in literature [30]. The quantitative results (Table 1) reveal the Oads + O2 − (O− )/O2− atomic ratio of Au/Co3 O4 is 7.32, which is very approximate to the Oads /O2− atomic ratio (7.57) of Au/Al2 O3 . Additionally, the Oads /O2− atomic ratio (1.72) of Au/MgO is markedly lower than that of Au/Co3 O4 and Au/Al2 O3 , indicating that the Au/Co3 O4 possesses various oxygen adspecies and may facilitate the electrophilic oxidation reaction. 3.4. Catalytic results Fig. 5a and b shows the catalytic performance of Au/MOx in toluene and p-xylene oxidation at a VHSV of 14,690 h−1 . Apparently, the conversion increases with the rise in temperature. Au/Co3 O4 shows a higher activity by comparing its performance against the other two gold catalysts at 300 ◦ C, in good agreement with the Oads + O2 − /O2− atomic ratio sequences of these materials (Table 1). In Table 2, the temperatures at 50% conversion (T50 ) over various catalysts are listed. The temperature corresponding to 50% conversion of Au/Co3 O4 is observed to be much lower than those of the reference samples, i.e., T50 as low as 200 ◦ C can be achieved over the Au/Co3 O4 catalyst. The excellent activity of the Au/Co3 O4
sample can be further seen from Table 2, where a comparison of the reaction rate per cobalt oxide mass unit, the areal specific activity based on the specific surface area of the Au/Co3 O4 , and TOF for toluene and p-xylene oxidation with Au/Al2 O3 and Au/MgO has been made. It can be seen that Au/Co3 O4 exhibits the highest performance in terms of toluene (or p-xylene) oxidation as compared with other samples. It is also interesting to make a further rough comparison of the activity of Au/Co3 O4 with other superior catalysts. For example, regarding the toluene oxidation over Au/Co3 O4 the T50 is only 200 ◦ C, slightly higher than that required by the gold on a high surface area Co3 O4 (180 ◦ C) [25]. However, if Au/Co3 O4 is compared with Au/Fe2 O3 and Au/CeO2 in spite of working at different reaction conditions, it can be obtained that the former is much more active than the others [31,32], as the value for T50 in Au/Fe2 O3 and Au/CeO2 is about 280 ◦ C. Interestingly, it can be seen that toluene is oxidised somewhat more readily than p-xylene under fairly similar conditions. This result is in agreement with the earlier report that the catalytic activity of aromatic compounds is highly dependent on the relative strength of adsorption of the model compounds, the ionization potential of the methyl derivatives, and the strength of the weakest C–H bond in the structure [26]. When a reactant is very strongly adsorbed, forming a layer that blocks the adsorption and decomposition of oxygen, oxygen can be adsorbed on the limited defect sites. So, in our study, the oxygen surface concentration is the limiting factor, the inhibition effect of p-xylene is stronger than that of toluene, and hence the reaction rate of toluene is higher than
Fig. 5. (a) Toluene and (b) p-xylene conversions as a function of temperature on 1.5 wt.% supported gold catalysts. Reaction conditions: toluene and p-xylene concentration = 0.60 and 0.40 g m−3 , total flow rate = 20 ml min−1 .
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Table 2 Activities and T50 calculated for supported gold catalysts. Catalyst
Au/Co3 O4 Au/Al2 O3 Au/MgO
Reaction rate (10−5 mol gcat −1 h−1 )
Specific activity (10−5 mol m−2 h−1 )
TOF (h−1 )
T50 (◦ C)
Toluene
p-Xylene
Toluene
Toluene
p-Xylene
Toluene
1.07 0.92 0.86
0.34 0.21 0.13
0.67 0.038 0.077
1.41 0.47 0.29
0.44 0.11 0.042
200 >300 >300
Reaction temperature is 300 ◦ C. M −1 TOF (h ) = XTX FTX M mTX XAu D , TX
cat Au Au
where XTX is the toluene and xylene (TX) conversion at 150 ◦ C, FTX is the flow rate of TX (20 ml min−1 ), TX is the concentration of TX, MTX is the molar weight of TX, mcat is the amount of catalyst, XAu is the Au loading in the catalyst, DAu is the dispersion of Au, and MAu is the molar weight of Au (196.97 g mol−1 ).
that of p-xylene. Furthermore, we can infer that the reaction rate of benzene is higher than that of toluene and xylene because of its higher ionization potential (9.3 eV) and smaller relative adsorption affinity [4,26]. The supported gold catalysts exhibit extremely stable catalytic performance within 5 h of on-stream reaction shown in Fig. 6a. The conversion remains stable for at least 5 h at 300 ◦ C. The result obtained demonstrates the Au/Co3 O4 catalytically durable (Fig. 6a), which is very well consistent with the previous report on gold deposited on high surface area Co3 O4 catalysts [25]. Fig. 6b shows that aggregation of Au nanocrystals is observed after toluene oxidation reaction for 5 h at 300 ◦ C. The Au colloids appear to be hemispherical after being used under usual operating conditions as shown in Fig. 6b. Moreover, a well-defined perimeter between Au and Co3 O4 is formed, which is claimed by Haruta and co-workers to be responsible for the improved activity over Au/TiO2 [33]. Therefore, our results are in agreement with the report that the support interacts with gold clusters, leading to a change in the shape of the Au nanoparticles, which contributes to the enhanced activity of the Au/Co3 O4 catalyst [27]. We consider that this enhanced interaction is induced by the distinct perimeter between Au and Co3 O4 under the catalytic conditions (Fig. 6b). Due to this effect, lots of eletrophilic O-species and oxygen vacancies are formed on the perimeter. Thus the good stability of the Au/Co3 O4 catalyst may be attributed to the increase of the perimeter between Au and Co3 O4 despite of the decrease of gold dispersion. Fig. 7a shows XPS spectrum of Au4f for Au/Co3 O4 after toluene oxidation reaction for 5 h at 300 ◦ C. The Au/Co3 O4 shows the peaks at 87.7 and 83.7 eV for Au4f7/2 and Au4f5/2 lines, respectively. These binding energies are close to Au4f binding energies of metallic Au0 [25,28]. As for the O1s XPS spectrum (Fig. 7b), the signals at BE = 528.1 and 531.1 eV could be attributed to the surface lattice oxygen (Olatt ) and adsorbed O-species (Oads ), respectively. Fig. 7c
shows the Co2p XPS spectrum of the Au/Co3 O4 after reaction. Two broad peaks appear at 778.1 and 793.0 eV, which are close to Co2p binding energies of Co3+ . Therefore, the gold phase is found to be changed from Au3+ and metallic Au0 coexisting to metallic Au0 alone during the reaction. As compared with the fresh Au/Co3 O4 , the electrophilic O-species (BE = 534.83 eV) disappear and the Co2+ ions are oxidised almost completely to Co3+ ions during the reaction. The results indicate that the Au/Co3 O4 after the reaction would lose most of highly active O-species and oxygen vacancies.
3.5. Discussion Gold supported on Co3 O4 has been shown to be much more reactive than Au/Al2 O3 and Au/MgO in toluene and p-xylene total oxidation in spite of its very low BET surface area and large gold particle size (Table 1). Furthermore, Au/Co3 O4 presented a good stability in the total oxidation of toluene and p-xylene at 300 ◦ C (Fig. 6a). A sintering of Au nanoparticles was observed for the used Au/Co3 O4 (Fig. 6b). Although the Au/Co3 O4 lost most of highly active O-species and oxygen vacancies after use in catalytic oxidation (see Fig. 7), it could still restore such surface species when being treated in air at 300 ◦ C, which was confirmed by XPS O1s spectrum as shown in Fig. S4. So, it is inferred that the increased perimeter between Au and Co3 O4 may be the origin of this good stability, which provides more active sites for oxygen adsorption and dissociation [26]. In order to rationalize the above catalytic behavior it must be taken into consideration that the low-temperature toluene (or pxylene) oxidation on reducible metal oxides supported Au catalysts has been reported to occur via a Mars–van Krevelen mechanism, which involves reaction of VOC and oxygen molecules on different redox sites [4,6,31,32].
Fig. 6. (a) Catalytic activity versus on-stream reaction time over 1.5 wt.% Au/Co3 O4 at 300 ◦ C, and (b) TEM image for the Au/Co3 O4 catalyst after toluene oxidation reaction.
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Fig. 7. Decomposed (a) Au4f, (b) O1s and (c) Co2p XPS spectra for Au/Co3 O4 catalyst after toluene oxidation reaction for at least 5 h at 300 ◦ C.
In the present case the main factors responsible for the high oxidation activity of cobalt oxide-based catalysts are considered to be: (i) bulk oxygen mobility or reducibility – that can be enhanced, on one side, by oxygen vacancies in a close contact with small Au nanoparticles and, on the other side, by an incorporation of Co2+ into the lattice – and (ii) electron transfer from reactants to the oxygen molecule via the catalytic surface, by activation of the reactant on Au sites able to enhance the redox transfer [34]. In the case of the Au/Co3 O4 system, several synergistic effects could take place and must be taken into account. In fact, on the Au/Co3 O4 system, the structural defects of the spinel lattice play a key role in creating oxygen vacancies on the surface [35], which might play an important favorable role in accelerating the adsorption and dissociation of oxygen molecules resulting in the formation of highly active electrophilic O-species [34]. It must be also considered that oxygen vacancies are preferential sites for Au anchoring on Co3 O4 surface and, conversely, the presence of Au nanoparticles leads to a strong interaction with cobalt oxide, which would result in facile oxygen vacancies formation on the interface and an enhanced metal-oxide interaction between Au and Co3 O4 [26,34]. Furthermore, it is known that BTX molecules strongly adsorb on the metal sites, probably with formation of oxygenate species [36,37]. In this study, the high catalytic activity of Au/Co3 O4 is not likely related to the specific surface area since the catalyst presents a BET surface area appreciably lower than Au/Al2 O3 and Au/MgO. Neither the crystalline size of Au particles seems to be determining since the Au/Co3 O4 with larger gold particle size displays a much higher activity than the other gold-based catalysts. This high activity in
the elimination of the aromatic hydrocarbons seems to be reasonably related to the low-temperature reducibility of Co3 O4 [19]. However, the reducibility of the support and its ability to activate oxygen does not seem to be the decisive factor [27]. For instance, Lopez found that differences in activity between reducible or nonreducible supports for the identical crystalline size of gold were negligible, suggesting that the reducibility is not a key factor [38]. In fact, Garcia has recently found that the shift of the reduction bands towards lower temperatures must be due to the improved reducibility of Co3 O4 when Au nanoparticles are present [25]. It can be suggested that there is a strong interaction between Au and Co3 O4 [26], which leads to the existence of abundant superficial electrophilic O-species (O2 − or O− ) and oxygen vacancies (induced by the increased proportion of Co2+ ) on the surface, assumed to be responsible for the high catalytic activity of Au/Co3 O4 [15]. In fact, electrophilic O-species on the surface of Au/Co3 O4 was observed in XPS spectra (Fig. 4b). Thus, the Au/Co3 O4 with various active O-species could exhibit higher catalytic activity than the other gold-based catalysts. Meanwhile, these electrophilic Ospecies did not seem to exist alone on the surface of Au/Co3 O4 , there should be rich oxygen vacancies co-existed with the electrophilic O-species according to the MVK mechanism [39]. Table 1 showed that the Co2+ /Co3+ ratio on the surface of Au/Co3 O4 is as high as 1.80, much higher compared with that on the surface of pure Co3 O4 (0.59) according to Ref. [25]. This result indicates that the Au/Co3 O4 presents a high concentration of oxygen vacancies in the vicinity of the surface [18,25]. These structural defects could facilitate the oxidation step in the redox cycle of the toluene and p-xylene
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starting seed fund of Northwestern Polytechnical University (no. Z2011011) for financial support. The helpful suggestions of anonymous reviewer are greatly appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molcata.2011.10.005. References
Fig. 8. Descomposed Co2p XPS spectra for Au/Co3 O4 catalyst.
oxidation [16]. Therefore, the higher activity of Au/Co3 O4 for toluene and p-xylene oxidation can be attributed to the higher concentration of surface electrophilic O-species and oxygen vacancies on the surface of the catalyst. Finally, a strong interaction between Au and Co3 O4 could occur through electron transfer from Au colloids to Co3 O4 particles. The surface chemical valence of Co3+ decreased when gold was present as validated by XPS (Fig. 8). Furthermore, the presence of peak at 91.79 eV for Au4f (Fig. 4a) indicated that the cationic Auı± species existed in the colloidal system, which was very likely to be Au3+ [28] because of their higher in energy than that of the main Au4f spin-orbit doublet (85.8 eV and 89.5 eV) assigned to the Auı+ [25]. Therefore, it can be suggested that there may be a strong interaction in this colloidal Au–Co3 O4 system, which can well account for the improved catalytic performance of Au/Co3 O4 as compared with other gold-based catalysts. 4. Conclusions Supported gold catalysts have been successfully prepared by a colloidal precipitation method. The Au/Co3 O4 displays a considerably higher catalytic activity in toluene and p-xylene total oxidation compared with Au/Al2 O3 and Au/MgO in spite of its lower BET surface area and larger gold crystalline size. This result of the gold on cobalt oxide catalyst seems to be seasonably related to a strong interaction between gold and the surface oxide species, which might play a key role in creating active sites in a close contact with ultra-fine Au nanocrystals, leading to the formation of abundant chemisorbed electrophilic O-species and oxygen vacancies, and finally being responsible for the high activity and stability of the colloidal Au–Co3 O4 system. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (grant nos. 50771082 and 60776822) and graduate
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Catalytic oxidation of toluene and p-xylene using gold supported on Co3 O4 catalyst prepared by colloidal precipitation method Hongjing Wu ∗ , Liuding Wang, Zhongyuan Shen, Jinghui Zhao Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 15 June 2011 Received in revised form 24 August 2011 Accepted 1 October 2011 Available online 8 October 2011 Keywords: Au/Co3 O4 Toluene and p-xylene oxidation Electrophilic oxygen species Oxygen vacancies Strong interaction
a b s t r a c t Gold supported on cobalt oxide has been successfully synthesized through a colloidal precipitation method and tested in toluene and p-xylene total oxidation. It has been demonstrated that the catalytic activity of Au/Co3 O4 for toluene and p-xylene oxidation is much higher than that of Au/Al2 O3 and Au/MgO in spite of its lower BET surface area and larger gold crystalline size. The enhanced catalytic activity in toluene and p-xylene oxidation has been linked to a high concentration of superficial electrophilic oxygen species and oxygen vacancies, which may be originated from a strong interaction in the colloidal Au–Co3 O4 system. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Benzene, toluene and xylene (BTX) are major pollutants in indoor and outdoor air, emitted from many industrial processes and transportation activities [1]. They often act as three major categories of greenhouse gases and suspected carcinogens [2]. Therefore, it is an extremely urgent task to reduce the emission of these pollutants. Catalytic oxidation is one of the most effective ways for the elimination of BTX, which operates at relatively low temperatures with respect to the conversional thermal combustion [1]. Furthermore, it provides the potential to destroy BTX totally to carbon dioxide and water [3]. Supported noble metal (Pt and Pd) catalysts [4–9] have been used as catalysts for this reaction due to their high activity at relatively low temperatures and the tolerance to the moisture. Since Haruta’s discovery [10] of the remarkably high activity of supported gold catalysts for low-temperature CO oxidation, reports on many practical applications, such as the reduction of NOx [11], hydrogenation, water–gas shift reaction and complete oxidation of hydrocarbons [12,13], have increased dramatically. The use of supported gold catalysts offers the advantages of both high reactivity and a relatively low price compared with earlier reported supported Pt or Pd catalysts. Among metal oxides cobalt oxide, in the form of Co3 O4 , has been shown to be one of the most efficient non-noble metal
∗ Corresponding author. Tel.: +86 29 88431664; fax: +86 29 88431664. E-mail address: [email protected] (H. Wu). 1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2011.10.005
oxides for VOCs elimination, and it seems to be the most active for propane combustion [14] due to its excellent reducibility, high concentration of electrophilic O-species (Oads , O− or O2 − ) [15] and oxygen vacancies [16]. Furthermore, improved catalytic performance has been associated with the high surface areas and 3D ordered mesoporous structure of the Co3 O4 [17–19]. In the last years, the complete oxidation of VOCs over gold on cobalt oxide has been widely studied, especially that of alkanes. Gold on cobalt oxide was shown to be active for the catalytic combustion of methane and propane [20,21]. The results suggested that active catalysts were related to the chemical state of gold phase. Recently gold on cobalt oxide has been turned to be highly active for low-temperature oxidation of trace ethylene [22] and it achieves a 76% conversion at 0 ◦ C when mesoporous Co3 O4 used as support [23]. Furthermore, they found that support morphology has a significant effect on catalytic activity, which is related to the exposed planes of different morphological Co3 O4 [24]. Moreover, catalytic oxidation of toluene, propane and CO has been investigated on a non-fully ordered nanocast Co3 O4 deposited by gold, and it exhibits high activity for the pollutants total oxidation [25]. It was proved that the enhanced catalytic activity has been related to both the improved reducibility of Co3 O4 when gold is added and the simultaneous presence of Auı+ and Au0 . In previous paper we reported a highly efficient Au/ZnO catalyst for low-temperature BTX oxidation [26]. Our results confirmed that strong metal-oxide interaction between Au {1 1 1} and ZnO {1 0 1} planes occurs due to the very small lattice mismatch. In present paper we investigated the properties of Au/Co3 O4 catalysts prepared by colloidal precipitation method and tested in
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Table 1 structural properties and surface atom ratios of supported gold catalysts. Catalysta
Au/Co3 O4 Au/Al2 O3 Au/MgO
BET area
1.6 205 44
Co3 O4 size average (nm)b
122 – –
Au size average (nm)c
10.6 ± 4.18 4.2 ± 1.79 3.3 ± 1.02
Au loading (wt.%)d
1.58 1.40 1.69
Au dispersion (%)e
9.43 27.55 35.07
Au/M atom ratio (%)
0.95 0.46 0.79
XPSf (m2 g−1 ) Au3+
Auı+
Au0
O2 −
Oads
O2−
Co2+ /Co3+
0.40 0 0
0.39 1.00 1.00
0.21 0 0
0.65 0 0
0.23 0.88 0.63
0.12 0.12 0.37
1.80 – –
Calcined at 350 ◦ C. Calculated by TEM method. c Calculated by HRTEM method. d Calculated from semi-quantitative EDX data. e Calculated by formula: DAu = 6ns MAu /Au NA dAu , where ns is the number of Au atoms at the surface per unit area (1.15 × 1019 m−2 ), MAu is the molar mass of gold (196.97 g·mol−1 ), Au is the density of gold (19.5 g·cm−3 ), NA is Avogadro’s number (6.023 × 1023 mol−1 ) and dAu is the average Au particle size (determined by HRTEM). f Catalysts before reaction. a
b
the total oxidation of toluene and p-xylene, chosen as probe molecules of alkyl aromatic compounds. The results were compared with the analogous series of gold supported on Al2 O3 (or MgO) with the same preparation procedure and suggested that the amount of chemisorbed electrophilic O-species and oxygen vacancies are the determining factors leading to high activity and stability of the Au/Co3 O4 , which may be originated from a strong metal-oxide interaction between Au and Co3 O4 .
X-ray source. Gold catalysts (calcined at 350 ◦ C in air for 4 h) before and after use in catalytic oxidation were pressed into a pellet and transferred to a test chamber with the vacuum below 5 × 10−5 Torr. The binding energies were calibrated by referencing the C1s at 284.6 eV.
2. Experimental
The catalytic activity of supported gold catalysts in toluene and p-xylene oxidation was measured in a fixed tubular quartz reactor under atmospheric pressure. The reactant and product mixtures were analyzed by an on-line gas chromatograph equipped with a hydrogen flame ionization detector and a HP-5 column. The following conditions were chosen: catalyst volume 0.082 cm3 , inlet toluene concentration 0.6 g m−3 (146 ppmv) and p-xylene concentration 0.40 g m−3 (85 ppmv) in synthetic gas (O2 , 10 vol.%; N2 , balance), volume hour space velocity 14,690 h−1 , the temperature range 50–300 ◦ C. The primary products were CO2 and H2 O only analyzed by GC–MS and FTIR. Thus, the catalytic activity was expressed as a degree of the conversion of hydrocarbon. The conversion of toluene (or p-xylene) was calculated as following:
2.1. Catalyst preparation A chemical precipitation method was used to synthesize supports of Co3 O4 , Al2 O3 and MgO. 5 ml of ammonia was gradually added into 10 ml of an aqueous 0.01 M solution of nitrate with continuous stirring. After 2 h, the percolated precipitation was filtered and washed with deionized water. The solids were dried at 80 ◦ C and finally calcined at 500 ◦ C for 2 h for crystallization. The supported gold catalysts were prepared by the colloidal deposition method by using PVA (Mw = 10,000) as a protecting agent and HAuCl4 ·3H2 O as the gold precursor [27]. For more details of the preparation procedure, refer to our previous work [26]. The nominal gold loading amount of catalyst is 1.50 wt.%. The resulting catalysts were denoted as Au/Co3 O4 , Au/Al2 O3 and Au/MgO, respectively.
2.3. Activity measurement for toluene and p-xylene oxidation
Conversion (%) =
[Reactant]in − [Reactant]out × 100% [Reactant]in
2.2. Catalysts characterization
3. Results and discussion
The specific surface area was determined by the BET method with N2 adsorption–desorption measurements at 77 K (Micromeritics ASAP 2020). The crystalline structure of the catalyst was analyzed by powder X-ray diffraction (XRD, Rigaku D/max-RB) with Cu K␣ radiation ˚ operated at 40 kV and 30 mA. Phases were identified ( = 1.54016 A) by matching experimental patterns to the JCPDS powder diffraction file. The morphology and microstructure of the catalysts were observed by the scanning electron microscopy (SEM, JEOL JSM-5610LV) and field emission transmission electron microscopy (FETEM, JEOL JEM-2100F). TEM samples were treated by sonicating in absolute ethanol for several minutes and then a few drops of the resulting suspension were deposited onto a holey-carbon film supported in a copper grid, which was dried naturally. The chemical composition and metal content of the catalysts were analyzed by the energy-dispersive X-ray spectroscopy analysis (EDX, OXFORD INCA). The atomic percentage and chemical state of surface element in the catalysts were measured by X-ray photoelectron spectroscopy (Perkin-Elmer, ESCA PHI 5400) using a monochromatic Mg K␣
3.1. Structural and textural properties The N2 adsorption–desorption isotherm and pore size distribution of Au/Co3 O4 are shown in Fig. S1. The Au/Co3 O4 gives an isotherm characteristic of macroporous material with a hysteresis loop of type III (Fig. S1a). The BET surface area and single-point total pore volume of Au/Co3 O4 are 1.6 m2 g−1 and 0.0043 cm3 g−1 , respectively. Besides, the BJH desorption average pore diameter is determined to be 59.7 nm (Fig. S1b). In contrast, the BET surface area measured for Au/Al2 O3 and Au/MgO are significantly higher than that of Au/Co3 O4 . Table 1 summarizes the textural parameters of supported gold catalysts, alongside with the Au loading, average Au particle size, Au dispersion and surface atom ratio. The phase composition and structural characterization of supported gold catalysts were examined by powder X-ray diffraction (PXRD) as shown in Fig. S2. It can be observed that the Au/Co3 O4 ˚ in crystal structure [space group: Fd3m is cubic Co3 O4 (a = 8.056 A) (2 2 7)] by comparing the XRD pattern of the standard cobalt oxide sample (JCPDS PDF 65-3103). Moreover, a new peak centered at 38.2◦ related to metallic Au0 can be roughly identified in the sample containing Au. The result indicates that, regardless of the low
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Fig. 1. TEM/HRTEM images of (a and b) gold nanoparticles, (c and d) Co3 O4 and (e) Au/Co3 O4 samples.
loading amount of gold, the Au element can be distinguished by XRD, which is probably due to the congregation of Au nanocrystals [25]. However, this diffraction feature is not distinguishable in Au/Al2 O3 and Au/MgO, indicating that the metallic Au0 is so small that it cannot be facilely identified by XRD [26].
3.2. Chemical composition and morphology Fig. S3 shows scanning electron microscopy (SEM) and energydispersive X-ray spectra (EDS) of supported gold catalysts. The SEM images indicate that the products consist of bulk particles. These
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Fig. 2. HRTEM images of (a) Au/Al2 O3 and (b) Au/MgO samples.
bulk particles are comprised of large numbers of Co3 O4 , Al2 O3 or MgO crystallites, which are covered by nanocrystalline gold. The EDS confirms the presence of Au element. It can be also confirmed by EDS that there are not any impurities except for Na+ , Ca2+ , indicating that Cl− ion is completely removed during washing. The Au loading amount of Au/MOx (M = Co, Al or Mg) are 1.58 wt.%, 1.40 wt.% and 1.69 wt.%, respectively. Transmission electron microscopy (TEM) images of the asprepared Au colloids are shown in Fig. 1. The Au colloids are highly crystalline in the gold solution (Fig. 1a and b). As one can see, the ultra-fine and homogeneously dispersed Au colloids are observed in the colloidal solution. Fig. 1a displays the TEM image of the gold dispersion to ascertain the mean particle size of Au colloids. The gold particle size is well fit to the Gaussian distribution and the average particle size and standard deviation are 3.9 nm and ±0.74 nm, respectively. Besides, the interplanar spacing of the Au colloids in the HRTEM image (Fig. 1b) is ca. 0.24 nm, corresponding to the interplanar distance of the {1 1 1} plane of Au nanocrystals. TEM images of pure Co3 O4 support calcined at 500 ◦ C are presented in Fig. 1c and d. The Co3 O4 particles are uniform hexagons and their average size is ca. 122 nm. Fig. 1d shows that the dominant exposed planes of Co3 O4 nanocrystals are {3 1 1} with a lattice space of ca. 0.24 nm. This is perfectly in agreement with the XRD results, indicating that the Co3 O4 particles are well crystallized after calcination at 500 ◦ C. Fig. 1e shows the Au colloids are dispersed in the pores of Co3 O4 support after calcination at 350 ◦ C for 4 h. The Au nanocrystals have a crystallite size of ca. 10 nm. The Co3 O4 particles have no changes in the shape and size when used as support, demonstrating a good crystallinity and a high stability of Co3 O4 particles after the loading of Au colloids. However, the particle size of Au colloids obviously increases during calcining at 350 ◦ C. Fig. 2 shows the HRTEM images of Au/Al2 O3 and Au/MgO. The Au nanoparticles are uniformly dispersed on Al2 O3 and MgO supports, with very small average particle sizes (4.2 and 3.3 nm, respectively), indicating that the Au nanoparticles on Al2 O3 or MgO is far more stable than that on Co3 O4 during calcining due to their higher specific surface area.
than the theoretic Au/M atom ratios of them. It indicates that Au colloids could be effectively dispersed on the surface of the supports, especially for Co3 O4 support. Fig. 4a and Table 1 show the XPS spectra of Au4f and surface species composition of Au for Au/MOx before oxidation reaction. The XPS spectrum of Au4f for Au/Co3 O4 shows strongly broadened peak, with the FWHMs around 3.11 eV, suggesting a contribution of Au species with varing electronic states. XPS Au4f spectrum for Au/Co3 O4 displays a pair of distinct peaks at 84.28 and 87.98 eV, which is typical of metallic Au0 species [28]. The additional peak at 91.79 eV, closes to that reported for Au3+ species [28]. This is in accordance with the XRD results, which shows an Au0 diffraction peak (2 = 38.2◦ ) of low intensity for the Au/Co3 O4 . In the case of Au/Al2 O3 and Au/MgO the presence of peaks at 85.8 and 89.5 eV indicates the presence of cationic Au+ or Auı+ . No peaks corresponding to metallic Au0 (around 83.7 and 87.4 eV) have been observed in the XPS Au4f spectra. However, it cannot be discarded that Au0 exists in these catalysts. In fact, these catalysts may be composed of a major amount of Auı+ and a minor amount of Au0 . Fig. 4b shows the O1s core-level spectra for fresh Au/MOx . For the Au/Co3 O4 , the O1s XPS spectrum can be decomposed into two components at BE = 529.68 and 531.49 eV; the former is due to the
3.3. Surface chemical state The full-range XPS spectra of the nominal 1.50 wt.% Au/MOx samples are shown in Fig. 3. The XPS spectra reveal that the elements on the surface of all samples are Au, M (M = Co, Al or Mg), O and C. The surface Au/M atom ratios in XPS spectra are ca. 0.95%, 0.46% and 0.79%, respectively (Table 1), which is relatively higher
Fig. 3. Full-range XPS spectra of the 1.5 wt.% supported gold catalysts.
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Fig. 4. XPS spectra of (a) Au4f region and (b) O1s region for 1.5 wt.% gold supported on various oxides catalysts.
surface lattice oxygen (Olatt ) species, whereas the latter is due to the surface adsorbed oxygen (Oads ) species [19,29]. There is another larger signal at 534.83 eV, which could be assigned to the electrophilic O-species (O2 − or O− ). Nevertheless, there are only two peaks at around 528.4 and 531.5 eV for Au/Al2 O3 and Au/MgO, which is similar to those reported in literature [30]. The quantitative results (Table 1) reveal the Oads + O2 − (O− )/O2− atomic ratio of Au/Co3 O4 is 7.32, which is very approximate to the Oads /O2− atomic ratio (7.57) of Au/Al2 O3 . Additionally, the Oads /O2− atomic ratio (1.72) of Au/MgO is markedly lower than that of Au/Co3 O4 and Au/Al2 O3 , indicating that the Au/Co3 O4 possesses various oxygen adspecies and may facilitate the electrophilic oxidation reaction. 3.4. Catalytic results Fig. 5a and b shows the catalytic performance of Au/MOx in toluene and p-xylene oxidation at a VHSV of 14,690 h−1 . Apparently, the conversion increases with the rise in temperature. Au/Co3 O4 shows a higher activity by comparing its performance against the other two gold catalysts at 300 ◦ C, in good agreement with the Oads + O2 − /O2− atomic ratio sequences of these materials (Table 1). In Table 2, the temperatures at 50% conversion (T50 ) over various catalysts are listed. The temperature corresponding to 50% conversion of Au/Co3 O4 is observed to be much lower than those of the reference samples, i.e., T50 as low as 200 ◦ C can be achieved over the Au/Co3 O4 catalyst. The excellent activity of the Au/Co3 O4
sample can be further seen from Table 2, where a comparison of the reaction rate per cobalt oxide mass unit, the areal specific activity based on the specific surface area of the Au/Co3 O4 , and TOF for toluene and p-xylene oxidation with Au/Al2 O3 and Au/MgO has been made. It can be seen that Au/Co3 O4 exhibits the highest performance in terms of toluene (or p-xylene) oxidation as compared with other samples. It is also interesting to make a further rough comparison of the activity of Au/Co3 O4 with other superior catalysts. For example, regarding the toluene oxidation over Au/Co3 O4 the T50 is only 200 ◦ C, slightly higher than that required by the gold on a high surface area Co3 O4 (180 ◦ C) [25]. However, if Au/Co3 O4 is compared with Au/Fe2 O3 and Au/CeO2 in spite of working at different reaction conditions, it can be obtained that the former is much more active than the others [31,32], as the value for T50 in Au/Fe2 O3 and Au/CeO2 is about 280 ◦ C. Interestingly, it can be seen that toluene is oxidised somewhat more readily than p-xylene under fairly similar conditions. This result is in agreement with the earlier report that the catalytic activity of aromatic compounds is highly dependent on the relative strength of adsorption of the model compounds, the ionization potential of the methyl derivatives, and the strength of the weakest C–H bond in the structure [26]. When a reactant is very strongly adsorbed, forming a layer that blocks the adsorption and decomposition of oxygen, oxygen can be adsorbed on the limited defect sites. So, in our study, the oxygen surface concentration is the limiting factor, the inhibition effect of p-xylene is stronger than that of toluene, and hence the reaction rate of toluene is higher than
Fig. 5. (a) Toluene and (b) p-xylene conversions as a function of temperature on 1.5 wt.% supported gold catalysts. Reaction conditions: toluene and p-xylene concentration = 0.60 and 0.40 g m−3 , total flow rate = 20 ml min−1 .
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Table 2 Activities and T50 calculated for supported gold catalysts. Catalyst
Au/Co3 O4 Au/Al2 O3 Au/MgO
Reaction rate (10−5 mol gcat −1 h−1 )
Specific activity (10−5 mol m−2 h−1 )
TOF (h−1 )
T50 (◦ C)
Toluene
p-Xylene
Toluene
Toluene
p-Xylene
Toluene
1.07 0.92 0.86
0.34 0.21 0.13
0.67 0.038 0.077
1.41 0.47 0.29
0.44 0.11 0.042
200 >300 >300
Reaction temperature is 300 ◦ C. M −1 TOF (h ) = XTX FTX M mTX XAu D , TX
cat Au Au
where XTX is the toluene and xylene (TX) conversion at 150 ◦ C, FTX is the flow rate of TX (20 ml min−1 ), TX is the concentration of TX, MTX is the molar weight of TX, mcat is the amount of catalyst, XAu is the Au loading in the catalyst, DAu is the dispersion of Au, and MAu is the molar weight of Au (196.97 g mol−1 ).
that of p-xylene. Furthermore, we can infer that the reaction rate of benzene is higher than that of toluene and xylene because of its higher ionization potential (9.3 eV) and smaller relative adsorption affinity [4,26]. The supported gold catalysts exhibit extremely stable catalytic performance within 5 h of on-stream reaction shown in Fig. 6a. The conversion remains stable for at least 5 h at 300 ◦ C. The result obtained demonstrates the Au/Co3 O4 catalytically durable (Fig. 6a), which is very well consistent with the previous report on gold deposited on high surface area Co3 O4 catalysts [25]. Fig. 6b shows that aggregation of Au nanocrystals is observed after toluene oxidation reaction for 5 h at 300 ◦ C. The Au colloids appear to be hemispherical after being used under usual operating conditions as shown in Fig. 6b. Moreover, a well-defined perimeter between Au and Co3 O4 is formed, which is claimed by Haruta and co-workers to be responsible for the improved activity over Au/TiO2 [33]. Therefore, our results are in agreement with the report that the support interacts with gold clusters, leading to a change in the shape of the Au nanoparticles, which contributes to the enhanced activity of the Au/Co3 O4 catalyst [27]. We consider that this enhanced interaction is induced by the distinct perimeter between Au and Co3 O4 under the catalytic conditions (Fig. 6b). Due to this effect, lots of eletrophilic O-species and oxygen vacancies are formed on the perimeter. Thus the good stability of the Au/Co3 O4 catalyst may be attributed to the increase of the perimeter between Au and Co3 O4 despite of the decrease of gold dispersion. Fig. 7a shows XPS spectrum of Au4f for Au/Co3 O4 after toluene oxidation reaction for 5 h at 300 ◦ C. The Au/Co3 O4 shows the peaks at 87.7 and 83.7 eV for Au4f7/2 and Au4f5/2 lines, respectively. These binding energies are close to Au4f binding energies of metallic Au0 [25,28]. As for the O1s XPS spectrum (Fig. 7b), the signals at BE = 528.1 and 531.1 eV could be attributed to the surface lattice oxygen (Olatt ) and adsorbed O-species (Oads ), respectively. Fig. 7c
shows the Co2p XPS spectrum of the Au/Co3 O4 after reaction. Two broad peaks appear at 778.1 and 793.0 eV, which are close to Co2p binding energies of Co3+ . Therefore, the gold phase is found to be changed from Au3+ and metallic Au0 coexisting to metallic Au0 alone during the reaction. As compared with the fresh Au/Co3 O4 , the electrophilic O-species (BE = 534.83 eV) disappear and the Co2+ ions are oxidised almost completely to Co3+ ions during the reaction. The results indicate that the Au/Co3 O4 after the reaction would lose most of highly active O-species and oxygen vacancies.
3.5. Discussion Gold supported on Co3 O4 has been shown to be much more reactive than Au/Al2 O3 and Au/MgO in toluene and p-xylene total oxidation in spite of its very low BET surface area and large gold particle size (Table 1). Furthermore, Au/Co3 O4 presented a good stability in the total oxidation of toluene and p-xylene at 300 ◦ C (Fig. 6a). A sintering of Au nanoparticles was observed for the used Au/Co3 O4 (Fig. 6b). Although the Au/Co3 O4 lost most of highly active O-species and oxygen vacancies after use in catalytic oxidation (see Fig. 7), it could still restore such surface species when being treated in air at 300 ◦ C, which was confirmed by XPS O1s spectrum as shown in Fig. S4. So, it is inferred that the increased perimeter between Au and Co3 O4 may be the origin of this good stability, which provides more active sites for oxygen adsorption and dissociation [26]. In order to rationalize the above catalytic behavior it must be taken into consideration that the low-temperature toluene (or pxylene) oxidation on reducible metal oxides supported Au catalysts has been reported to occur via a Mars–van Krevelen mechanism, which involves reaction of VOC and oxygen molecules on different redox sites [4,6,31,32].
Fig. 6. (a) Catalytic activity versus on-stream reaction time over 1.5 wt.% Au/Co3 O4 at 300 ◦ C, and (b) TEM image for the Au/Co3 O4 catalyst after toluene oxidation reaction.
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Fig. 7. Decomposed (a) Au4f, (b) O1s and (c) Co2p XPS spectra for Au/Co3 O4 catalyst after toluene oxidation reaction for at least 5 h at 300 ◦ C.
In the present case the main factors responsible for the high oxidation activity of cobalt oxide-based catalysts are considered to be: (i) bulk oxygen mobility or reducibility – that can be enhanced, on one side, by oxygen vacancies in a close contact with small Au nanoparticles and, on the other side, by an incorporation of Co2+ into the lattice – and (ii) electron transfer from reactants to the oxygen molecule via the catalytic surface, by activation of the reactant on Au sites able to enhance the redox transfer [34]. In the case of the Au/Co3 O4 system, several synergistic effects could take place and must be taken into account. In fact, on the Au/Co3 O4 system, the structural defects of the spinel lattice play a key role in creating oxygen vacancies on the surface [35], which might play an important favorable role in accelerating the adsorption and dissociation of oxygen molecules resulting in the formation of highly active electrophilic O-species [34]. It must be also considered that oxygen vacancies are preferential sites for Au anchoring on Co3 O4 surface and, conversely, the presence of Au nanoparticles leads to a strong interaction with cobalt oxide, which would result in facile oxygen vacancies formation on the interface and an enhanced metal-oxide interaction between Au and Co3 O4 [26,34]. Furthermore, it is known that BTX molecules strongly adsorb on the metal sites, probably with formation of oxygenate species [36,37]. In this study, the high catalytic activity of Au/Co3 O4 is not likely related to the specific surface area since the catalyst presents a BET surface area appreciably lower than Au/Al2 O3 and Au/MgO. Neither the crystalline size of Au particles seems to be determining since the Au/Co3 O4 with larger gold particle size displays a much higher activity than the other gold-based catalysts. This high activity in
the elimination of the aromatic hydrocarbons seems to be reasonably related to the low-temperature reducibility of Co3 O4 [19]. However, the reducibility of the support and its ability to activate oxygen does not seem to be the decisive factor [27]. For instance, Lopez found that differences in activity between reducible or nonreducible supports for the identical crystalline size of gold were negligible, suggesting that the reducibility is not a key factor [38]. In fact, Garcia has recently found that the shift of the reduction bands towards lower temperatures must be due to the improved reducibility of Co3 O4 when Au nanoparticles are present [25]. It can be suggested that there is a strong interaction between Au and Co3 O4 [26], which leads to the existence of abundant superficial electrophilic O-species (O2 − or O− ) and oxygen vacancies (induced by the increased proportion of Co2+ ) on the surface, assumed to be responsible for the high catalytic activity of Au/Co3 O4 [15]. In fact, electrophilic O-species on the surface of Au/Co3 O4 was observed in XPS spectra (Fig. 4b). Thus, the Au/Co3 O4 with various active O-species could exhibit higher catalytic activity than the other gold-based catalysts. Meanwhile, these electrophilic Ospecies did not seem to exist alone on the surface of Au/Co3 O4 , there should be rich oxygen vacancies co-existed with the electrophilic O-species according to the MVK mechanism [39]. Table 1 showed that the Co2+ /Co3+ ratio on the surface of Au/Co3 O4 is as high as 1.80, much higher compared with that on the surface of pure Co3 O4 (0.59) according to Ref. [25]. This result indicates that the Au/Co3 O4 presents a high concentration of oxygen vacancies in the vicinity of the surface [18,25]. These structural defects could facilitate the oxidation step in the redox cycle of the toluene and p-xylene
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starting seed fund of Northwestern Polytechnical University (no. Z2011011) for financial support. The helpful suggestions of anonymous reviewer are greatly appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molcata.2011.10.005. References
Fig. 8. Descomposed Co2p XPS spectra for Au/Co3 O4 catalyst.
oxidation [16]. Therefore, the higher activity of Au/Co3 O4 for toluene and p-xylene oxidation can be attributed to the higher concentration of surface electrophilic O-species and oxygen vacancies on the surface of the catalyst. Finally, a strong interaction between Au and Co3 O4 could occur through electron transfer from Au colloids to Co3 O4 particles. The surface chemical valence of Co3+ decreased when gold was present as validated by XPS (Fig. 8). Furthermore, the presence of peak at 91.79 eV for Au4f (Fig. 4a) indicated that the cationic Auı± species existed in the colloidal system, which was very likely to be Au3+ [28] because of their higher in energy than that of the main Au4f spin-orbit doublet (85.8 eV and 89.5 eV) assigned to the Auı+ [25]. Therefore, it can be suggested that there may be a strong interaction in this colloidal Au–Co3 O4 system, which can well account for the improved catalytic performance of Au/Co3 O4 as compared with other gold-based catalysts. 4. Conclusions Supported gold catalysts have been successfully prepared by a colloidal precipitation method. The Au/Co3 O4 displays a considerably higher catalytic activity in toluene and p-xylene total oxidation compared with Au/Al2 O3 and Au/MgO in spite of its lower BET surface area and larger gold crystalline size. This result of the gold on cobalt oxide catalyst seems to be seasonably related to a strong interaction between gold and the surface oxide species, which might play a key role in creating active sites in a close contact with ultra-fine Au nanocrystals, leading to the formation of abundant chemisorbed electrophilic O-species and oxygen vacancies, and finally being responsible for the high activity and stability of the colloidal Au–Co3 O4 system. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (grant nos. 50771082 and 60776822) and graduate
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