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Fe2O3–TiO2 catalyst for preferential CO oxidation
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Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation Apanee Luengnaruemitchai a, Kunanya Srihamat a, Chinchanop Pojanavaraphan a, Ratchaneekorn Wanchanthuek b,* a
The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Patumwan, Bangkok 10330, Thailand b Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahasarakham University, Kantarawichai District, Mahasarakham 44150, Thailand
article info
abstract
Article history:
A series of Au supported on Fe2O3eTiO2 catalysts were prepared, characterized and
Received 2 June 2015
investigated the catalytic activity in the preferential CO oxidation (PROX) under a H2-rich
Received in revised form
stream. The effects of the different metal support ratio, the calcination temperature, the
22 July 2015
gold loading, and the pre-treatment condition were studied on both the characteristics and
Accepted 30 July 2015
catalytic activity of the catalysts. The results obviously showed that 1% Au/Fe2O3eTiO2 (1:4)
Available online xxx
exhibited higher CO conversion than the 1% Au/TiO2 and 1% Au/Fe2O3 catalysts, especially temperature lower than 100 C, which due to the formation of reducible gold species at
Keywords:
lower temperature. TPR results suggested the improvement of AueAu and metalemetal (in
CO conversion
support) interaction. Additionally, XPS spectra suggested the Au0 could be the predominant
CO selectivity
active site. The effect of O2 pretreatment showed a negative effect in both activity and
Au catalyst
selectivity which suggested the formation of adsorbed oxygen species on the surface of the
Titania
catalyst. Moreover, the deactivation was affected by injection H2O, CO2 or the combination
Iron oxide
of CO2eH2O into the typical feed.
PROX
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the desire to replace fossil fuels with sustainable energy sources is increasing. Many alternative types of sustainable energy sources have been considered, such as electricity from renewable resources (wind, solar light, water power, etc.) as well as hydrogen which is much favoured because it is clean, potentially renewable and it could be generated electricity in a fuel cell. Many types of fuel cell have been developed and the proton exchange membrane fuel cell
(PEMFC) has been proposed to be an ideal for electricity production from hydrogen. PEMFCs have been widely studied for use in residential applications and for transportation systems, especially for automobiles [1]. The advantages of PEMFC against other fuel cells are the low temperature of operation, fast cold start, perfect CO2 tolerance and high energy conversion density [2]. However, the Pt electrodes in the PEM fuel cell are very sensitive to CO. Therefore, the hydrogen gas stream needs to be almost CO-free, preferably to be less than 10 ppm [3e5].
* Corresponding author. Tel.: þ66 43 754 246. E-mail address: [email protected] (R. Wanchanthuek). http://dx.doi.org/10.1016/j.ijhydene.2015.07.148 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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In principle, the hydrogen stream for a PEMFC should be produced on-broad from various hydrocarbon feed stocks (such as methane, ethanol etc.). During the hydrogen production, CO is produced and so there are several steps used to remove the CO, such as CO oxidation, water-gas shift and PROX reactions [5e9]. The advantage of PROX is a low cost of operation. PROX is the CO conversion to CO2 by using O2 with a suitable catalyst. There are three main catalysts that have been applied; platinum, copper and gold. Recently, gold catalysts have shown the higher activity in CO oxidation and made it possible to develop better catalysts than platinum and copper [9e11]. The gold catalyst is active even at subambient temperature, i.e. 73 C and also showed more economic competitiveness than platinum catalysts [12,13]. The activity of the gold catalyst is controlled by 3 main factors; type of support, particle size of gold and the preparation method. In terms of the support, the nature of the support including the reducibility, oxygen storage capacity, the thermal tolerant etc. were the key factors in determining the catalytic activity. From previous researches, the preferred gold is prepared in the form of supported gold by loading gold onto a metal oxide support such as Al2O3 [14], MnOx [15], Fe2O3 [16], TiO2 [17] as well as mixed oxides, such as MgOeAl2O3 [18]. Such supported gold catalysts were found to be very active for CO oxidation at low temperatures [10,11]. Among the different metal oxide supports, TiO2 has been shown to be a suitable support for gold because of its high activity [17,19e21]. The TiO2 support stabilizes the small metastable gold nanoparticles against agglomeration and sintering, and thereby stabilizes the active form of gold. Moreover, Fe2O3 was also used as a support for Au in the PROX reaction at ambient conditions and resulted in high catalytic performance with long life time [16,22]. It has been proposed that the high activity of Au/Fe2O3 was due to the high O2 adsorption over the Fe2O3 which acted as the oxygen supply during the reaction [22]. We previously found that a Au/Fe2O3 catalyst was active for PROX, and it gave 98% CO conversion and 53% selectivity at 50 C. In addition, it showed stable catalytic activity during the testing time [23]. In addition, the importance of Auþ and Au3þ species was reported and it was claimed that these were more active than Au0 species for this reaction. Therefore, it is very interesting to study the catalytic activity of mixed metal oxides as supports for gold catalysts (Au/Fe2O3eTiO2) in the PROX reaction. The purpose of this present study is to develop a new catalyst (Au/Fe2O3eTiO2) for the PROX reaction in the presence of H2 rich gas for fuel cell applications. The catalysts are prepared by the deposition-precipitation method. In order to understand the relationship between the catalyst properties and activity involving selectivity, several kinds of characterization techniques have been employed.
Experimental Catalyst preparation The pure Fe2O3 support was prepared by a precipitation method. The aqueous solution of 0.1 M iron (ІІІ) nitrate nonahydrate, Fe(NO3)3.9H2O, was heated to 80 C and the pH
adjusted to 8e9 by using 0.1 M Na2CO3 to obtain the suspension. Then the mixture was washed by warm deionized water, dried at 110 C overnight and calcined in air at 400 C for 4 h. The mixed support, Fe2O3eTiO2, was prepared by incipient-wetness impregnation method. The atomic ratios of metals (Fe:Ti) were chosen as follows: 0:1, 4:1, 1:1, 1:4, and 1:0. The desired amount of iron (ІІІ) nitrate nonahydrate, Fe(NO3)3.9H2O solution was dropped into titanium (IV) oxide powder (Degussa P25) under grinding. The support slurry was dried at 110 C overnight and calcined in air at 400 C for 4 h. For the preparation of all the series of gold catalysts, gold was loaded to the support using the deposition-precipitation (DP) method using hydrogen tetrachloroaurate (III), HAuCl4.3H2O as the gold precursor. The dried support was added to the specific gold solution and then pH of the mixture was adjusted to 8e9 by adding 0.1 M Na2CO3. This resulted in the deposition of the gold on the support. The suspension was washed by warm deionized water, dried at 110 C overnight and calcined in air at specific temperatures for 4 h.
Characterization The BET surface areas were measured using an Autosorb-1 Gas Sorption system (Quantachrome Corporation). Before each measurement, the samples were degassed at 150 C for 2 h. The specific surface area, the average pore diameter and the pore volume were calculated. XRD patterns were obtained by using a Rigaku X-Ray Diffractometer system (RINT-2200) with a Cu tube for gener˚ ) and a nickel filter. The samples ating CuKa radiation (1.5406 A of catalyst and support powder were prepared as a discshaped pressed powder. The XRD patterns were analyzed. Atomic Absorption Spectroscopy (AAS) was used to determine the content of gold in the catalysts. The catalysts were dissolved in aqua regia solution, which is composed of hydrochloric acid and nitric acid with a ratio of 82:18, and then heated to 100 C for an hour. Temperature-programmed reduction (TPR) measurements were carried out using a conventional flow apparatus with TCD detector, by placing a sample in a quartz reactor. The catalyst was pretreated under argon flow rate 50 ml/min at 400 C for an hour to eliminate the adsorbed gas species over the surface. After the catalyst was cooled down to room temperature, the catalyst was subjected to TPR analysis using 10% H2 in Ar at a total flow of 30 ml/min. The reduction temperature was raised from 30 to 850 C with a ramp rate of 10 C/min. The average crystallite size of the gold particles was measured by Transmission electron microscopy (TEM) using a JEM 2010 microscope operating at 200 kV in bright and dark field modes. The powdered samples were prepared by crushing and grinding in a mortar. Then, the fine powder was ultrasonically dispersed in ethanol, and a drop of the suspension was deposited on a thin carbon film supported on a standard electron microscope grid. The average Au diameter (dTEM) was calculated from the following formula: dTEM ¼ S(nidi)/ni where ni is the number of Au particles of diameter di. X-ray photoelectron spectroscopy, XPS (Kratos Axis Ultra DLD) was used to determine the oxidation states of Au, Fe and Ti in the obtained catalysts. A monochromatic Al Ka was used
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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as an X-ray source (anode HT ¼ 15 kV). The residual pressure in the ion-pumped analysis chamber was lower than 5 107 torr. Fourier transform infrared spectrometry (FTIR) was used to examine the functional groups over the surface of the prepared catalyst. The catalyst and potassium bromide (KBr) powder were milled to form a very fine powder. The obtained powder was then compressed as a thin pellet and the spectra were collected on a PerkineElmer (Spectrum one) spectrometer equipped with a mercury-cadmium-telluride (MCT) detector.
Activity test The catalytic performances were tested for the preferential CO oxidation (PROX) using a fixed-bed catalytic micro-reactor at atmospheric pressure. Typically, a 100 mg of dried catalyst were loaded into the quartz reactor with an internal diameter of 6 mm, and fixed in place by a sandwich of quartz wool, and the reactor located in a ceramic furnace. The feed gas consisted of 1% CO, 1% O2 and 40% H2 in He balance which was introduced through the catalyst bed with a total flow rate of 50 ml/min (GHSV ¼ 30000 mlg1h1) and the reactivity was observed at various temperatures over the range of 40e180 C. The gas effluents were analyzed by auto-sampling in an online gas chromatograph equipped with a packed carbon sphere column, 80/100 mesh, 10 ft 1/8 in. and a thermal conductivity detector (TCD).
Results and discussion Catalyst characterization Surface properties Table 1 summarizes the mean crystallite size of TiO2, the average size of the gold particles as determined by TEM, and the BET surface area of the prepared catalysts. The BET surface areas of Au/TiO2 and the series of Au/Fe2O3eTiO2 (with both different atomic ratios and different calcination temperatures) are very close to the surface area of the TiO2 support which has a specific surface area of 58.5 m2/g. This was suggested that the surface morphology of mixed oxide support was dominant by TiO2. However, an increase in the calcination temperature (200e400 C) for the Au/Fe2O3eTiO2 catalysts could not affect the surface area of the resulted catalysts. The more addition of Fe2O3 to the mixed oxide support (from 1:4e4:1 in Fe2O3:TiO2) showed the slightly decrease in BET surface area is possibly due to the blockage of some of the TiO2 pores by Fe2O3 [24]. The crystallite size of the TiO2 in the Au/Fe2O3eTiO2 catalyst was clearly increased by increasing the calcination temperature (up to 400 C), whereas the Au mean size was scarcely changed. The Au loadings of the prepared catalysts were analyzed by AAS and it was found that the loadings were lower than the expected values.
XRD patterns The microstructure of supported Au catalysts was investigated by XRD, as illustrated in Fig. 1(A). There was no peaks of
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metallic gold (2q ¼ 38.2 , 44.4 , and 64.5 ) or gold oxide (2q ¼ 25.5 and 49.8 ) were observed, suggesting that the fine dispersion of gold particles on the support and the small amount of gold species puts this outside the detection limit of the XRD [25]. However, a very strong peak of crystalline TiO2 was detected at 2q ¼ 25.3 [26]. The peaks of Au/TiO2 suggested that the TiO2 is in the anatase phase without the characteristic of rutile phase [19] whereas the XRD profile of Au/Fe2O3 showed the support existed in both Fe2O3 (hematite) and Fe3O4 (magnetite) or in the a-Fe2O3 phase within the hexagonal corundum-type structure [27]. In the case of 1% Au/Fe2O3eTiO2 catalysts, there were reflections similar to that of 1% Au/TiO2 without any characteristics of Fe2O3. This could be suggested a very small quantity of Fe2O3 particles was attached to TiO2 to form the heterogenous structure, which was either the Fe2O3 particles were highly dispersion or they were very small for be detectable by XRD. Moreover, there is no peak of TieFeeO (mixed TieFe metal oxide), suggesting within the detection limits that there is no corporation of Fe3þ into the lattice of TiO2 or Ti4þ in the lattice of Fe2O3. Therefore, only the crystallite sizes of TiO2 were calculated, using the Scherrer equation and the results are summarized in Table 1. From Table 1, the higher addition of Fe2O3 in Fe2O3eTiO2 resulted in the lower surface area (54e50 m2/g) and the XRD pattern of the Au/Fe2O3eTiO2 catalysts showed only the characteristic of TiO2 crystallite phase (Fig. 1(A)). Therefore, both surface area data and XRD supported that the lower surface area was related to the loss of surface area of porous TiO2. The mechanism seemed that the small Fe2O3 particle was inserted inside the porous of TiO2 crystallite. This situation could be occurred only in the event of the present of the very tiny Fe2O3 particles in the Fe2O3eTiO2 which was under the detection of XRD. Therefore, when adding more Fe2O3 component to Fe2O3eTiO2 support, the obtained Fe2O3eTiO2 support demonstrated the lower surface area. The proposed structure of Fe2O3eTiO2 support was in agreement of the support preparation (impregnation). This proposed structure of the support could explain the lower actual gold loading in 1% Au/Fe2O3eTiO2 (4:1) catalyst (only 0.58%), whereas the actual gold loading in 1%Au/ Fe2O3eTiO2 (1:4) catalyst was about 1%, the less surface area in the 1% Au/Fe2O3eTiO2 (4:1) sample might be one of the possible reason to consider about Au deposition. The XRD patterns of 1% Au/Fe2O3eTiO2 (1:4) catalysts with different calcination temperatures are illustrated in Fig. 1(B). The changes induced by thermal treatment were observed. The XRD patterns of 1% Au/Fe2O3eTiO2 (1:4) calcined at 200 C showed the lowest crystallinity followed by the samples calcined at 300 and 400 C, respectively. The results clearly suggested the crystallinity of TiO2 increased with increasing calcination temperature.
TPR measurement TPR was used to characterize the metal oxides, mixed metal oxides, metal oxides dispersed on a support including metalemetal and metal-support interaction of the prepared catalysts. The reduction peak of Fe2O3, TiO2, and 1% Au catalysts are shown in Fig. 2. The reduction peak of pure TiO2 sample showed a poorly identified peak at 640 C which matches with
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Table 1 e Physicochemical properties of the catalysts. Catalyst 1% 1% 1% 1% 1% 1% 3% 5% 1% a b c
Au/TiO2 Au/Fe2O3eTiO2(1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:1) Au/Fe2O3eTiO2 (4:1) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3
Calcination temperature (ºC)
Au loading (wt.%)a
Crystallite size (nm)b
Au mean size (nm)c
BET surface area (m2/g)
400 200 300 400 400 400 400 400 400
0.42 0.93 0.76 1.00 0.98 0.58 1.81 3.72 0.94
14 10 11 15 14 15 21 21 e
e 4 5 5 e e 6 5 e
56.7 56.2 54.2 54.2 50.3 50.5 49.9 49.6 81.2
Gold content were determined by AAS. Mean crystallite sizes were calculated from the average values of TiO2 plane (101), (110), (200), (105), (211). Au crystallite sizes were evaluated by TEM.
the reduction of the titanium oxide surface. However, 1% Au/ TiO2 was observed to give two small broad peaks at low temperature (200 C), associated to partial reduction by H2 of AuxOy [29], and at high temperature (500 C) associated to reduction of bulk TiO2 which interacted with Au and resulted in the lower reduction temperature when compared to the reduction of bulk TiO2 in pure TiO2. In addition, the reduction of pure Fe2O3 consisted of one low temperature peak between 300 C and 430 C, corresponding to the reduction peak of Fe2O3 to Fe3O4 (hematite-tomagnetite), and a broad peak at between 510 C and 730 C,
Fig. 1 e XRD patterns of (A) supported Au catalysts, (B) 1% Au/Fe2O3eTiO2 calcined at different temperatures: (a) calcined at 200 C; (b) calcined at 300 C; (c) calcined at 400 C.
corresponding to the reduction peak of Fe3O4 to Fe (Fe3O4 to FeO and finally to Fe) [28,30]. The reduction peaks of 1% Au/ Fe2O3 catalyst showed smaller peaks than pure Fe2O3 and also shifted toward lower temperature at between 280 C and 360 C and between 500 C and 680 C that can be ascribed to the reduction peak of Fe2O3 to its corresponding metallic phase. These two reduction peaks could be assigned to Fe2O3 and Fe3O4 interact with Au species, respectively. However, the very weak broadened peak at 180 C was related to the reduction of AuxOy. According to the TPR profiles of 1% Au/ Fe2O3 and 1% Au/TiO2 catalysts, it can be implied that the addition of Au provided the higher reducibility of the support by shifting the peak toward a lower temperature. This was suggested the development of Au-support interaction (AueFe or AueTi interaction).
Fig. 2 e H2-TPR profiles of all supports and supported Au catalysts (calcined at 400 C).
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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In the series of 1% Au/Fe2O3eTiO2 samples, the large broad reduction peak between 250 and 650 C was assigned to the reduction of the metal oxide in the support which could be Fe2O3 to Fe and/or the reduction of bulk TiO2. It was clearly that they were moved to the reduction temperature between the reduction of bulk TiO2 in Au/TiO2 and iron oxide in Au/ Fe2O3, thus suggesting some of interaction between Ti and Fe in mixed oxide support (supportesupport interaction). This was supported our proposed model of Fe2O3eTiO2 (Fe2O3 inserted in the porous of TiO2). In this proposed model, there was the clear interfacial interaction between TiO2 and Fe2O3, therefore, the particles of TiO2 and Fe2O3 could not be presented in the individual particle. The apparent sharp reduction peak at 100 C was assigned to the reduction of gold oxide (AuxOy) to metallic gold (Au0) and the peak position was lower than that found in the 1% Au/ Fe2O3 or 1% Au/TiO2 samples, suggesting the gold oxide was in the easier reduced form since the nature of the obtained Fe2O3eTiO2 support had been modified. In other hand, the lower reduction temperature of gold oxide peak was indicated the increasing AueAu interaction. This mean the reduction property of Au over Fe2O3eTiO2 support has been developed. In addition, at higher amounts of Fe in mixed oxide support, these peaks were shifted to slightly higher temperatures, indicating a slightly more interaction of Au-support occurred. The quantity analysis of reducibility gold particles in catalyst was investigated from the peak area of the gold reduction peak. The much larger gold reduction peak area found in 1% Au/Fe2O3eTiO2 catalysts than 1%Au/Fe2O3 and 1% Au/TiO2, indicating the improvement of the gold reducibility. Similarly, the peak area of metal oxide (in the support) reduction peak was very small (found in all the 1% Au/ Fe2O3eTiO2 samples), suggesting the low support reducibility. The TPR profiles of the 1% Au/Fe2O3eTiO2 catalysts at different calcination temperatures are presented in Fig. 3(a). The profiles indicated two reduction peaks along with a broad peak at high temperature. These peaks are attributed to the reduction peak of Au and Fe oxides and/or bulk TiO2 to the corresponding metallic species. The sample calcined at 200 C showed a reduction peak at 200 C with a small shoulder peak at 240 C, which could be assigned to the reduction peak of AuxOy with the weakly and strongly interaction with the support, respectively. When increasing the calcination temperature, the result showed a decrease in the reduction peak and they also shifted to lower temperature. It can be explained that increasing the calcination temperature can improve the AueAu interaction or depressed the Au-support interaction but the gold catalyst was active at lower reaction temperature. However, the area of gold reduction peak was decreased at high calcination temperature, suggesting less reducible gold particles since the thermal could reduce some of the acidic gold to metallic gold. In the range of reduction peaks of metal oxides in the support, the peak area was increased at higher calcination temperature. This indicated the thermal treatment could obviously affect the interaction of both AueAu interaction and supportesupport interaction. The TPR profiles of different gold loading catalysts showed in Fig. 3(b), it showed that the increasing of Au loading does not seem to improve the AueAu interaction or Au-support interaction or supportesupport interaction since there is no
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significant difference in reduction peak between 70 C to 180 C, (reduction peak of AuxOy). Similarly to previous section, the peak between 260 C to 650 C is assigned to the metal oxides reduction (in the support) to its corresponding metallic species and those peaks were shown no shifting, indicating the increasing amount of gold loading could not affect the reducibility of the metal oxides in the support of the obtained catalysts.
TEM results The TEM images of 1% Au/Fe2O3eTiO2 (1:4) catalysts with different calcination temperatures are shown in Fig. 4(a)e(c)). As can be seen, the particle size exhibited no different as increasing calcination temperature. Fig. 4(c)e(e) illustrates the TEM images of Au/Fe2O3eTiO2 (1:4) catalysts calcined at 400 C with different Au loadings. It is not clearly seen that the mean Au particle sizes changed with increasing the gold loading. Therefore, in these two experiments suggested that the Au loading and calcinations temperature seem not have an obvious effect to the Au particle size on the Au/Fe2O3eTiO2 catalysts. Fig. 4(f) shows the TEM image of 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C, with O2 pretreatment for 2 h. The mean Au particle sizes increased to 6.37 nm compared to 1% Au/ Fe2O3eTiO2 (1:4) calcined at 400 C without O2 pretreatment was only 4.58 nm. This was clearly that the O2 pretreatment enhanced the growth of Au particle over the catalyst. Similar results were reported over that a Au/TiO2eSiO2 catalyst that the O2 pretreatment enhances the growth of gold particles on [31].
Fourier transform infrared spectroscopy (FTeIR) FTeIR measurement was utilized to examine the catalyst surface before and after the 16 h deactivation test. The results from the FTeIR spectroscopy were shown in Fig. 5. A broad transmission band was observed in the range of 3200e3600 cm1 for the fresh catalyst, owing to the OeH stretching mode of H2O molecules [32]. Treatment of the sample with CO and with CO2 led to the marked changes in the IR spectra in the OeH stretching region, including a decrease in intensity to give a negative band in this range (OeH stretching mode, already assigned to bicarbonates).
X-ray photoelectron spectroscopy The surface structures of the Au/Fe2O3eTiO2, Au/TiO2, and Fe2O3eTiO2 catalysts were investigated by XPS, as shown in Fig. 6. According to the support spectra, the Fe 2p spectra were found into two binding energies; the strong bands at 725 eV and 712 eV which represents the 2p1/2 and 2p3/2 spectra [33], respectively. Unlike our previous work [34], it is hardly detectable to the weak band of 718.5 eV which represents the Fe(III) species in the hematite (aeFe2O3) phase. In addition, the appearance of another Fe 2p3/2 at 711.4 eV in the catalyst calcined at 300 C represented the Fe3þ species of the magnetite phase [35]. It could be implied that the 2p spectra could be corresponded to Fe3þ species. The deposition of active Au metal and the change of support concentration seem not to affect the peaks of Fe 2p spectra. For the Ti 2p spectra, two main peaks were detected in the range of 456e468 eV, as shown in Fig. 7(a)e(c). The peak at
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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~464.3 eV was attributed to the 2p1/2 spectra, and the strong peak at ~458.6 eV was assigned to the 2p3/2 spectra, which are the characteristic of TiO2 [36]. The Ti 2p3/2 peaks indicated the presences Ti3þ and Ti4þ oxidation states at 457.6 and 458.5e459 eV, respectively [36]. Meanwhile, the Ti 2p1/2 peaks showed the presences of Ti2þ, Ti3þ, and Ti4þ oxidation states at 459.5e460.1, 461.6e461.9, 463.4e463.5, and 464.3e464.6 eV, respectively [36]. Focusing on the Ti intensity, it was found that the intensities of Ti4þ species were the highest in all samples, indicating that the species composition in the Au/ Fe2O3eTiO2 catalyst was mainly Ti4þ; however, the exact amount (or percentage) of each species was not determined and reported in this paper. The peak of Ti3þ species was only observable in Au/TiO2, and 1% and 5% Au/Fe2O3eTiO2 (1:4) calcined 400 C, whereas the Ti0 peak was not found in the Au/ TiO2 and 1% Au/Fe2O3eTiO2 (1:4) calcined 400 C. The Au/TiO2 catalyst mainly presented Ti4þ and small portion of Ti3þ, while a variety of Ti oxidation states were found in the Fe2O3eTiO2 catalysts. It was postulated that the appearance of those various oxidation states might be the consequences of the interaction between Fe and Ti in the mixed oxide support, where the change in electronic effect was possible. Nonetheless, the electronic effect on the surface structure was not discussed in this study. The binding energies of Au 4f spectra show various oxidation states of Au, which were affected by the support composition, catalyst calcination, and amount of Au deposition (Fig. 8 (a)e(b)). For the 4f7/2 spectra, the Au0, Auþ, and Au3þ species were assigned to the binding energies at 83.5e83.9, 84.1e84.4, and 85.3e85.7 eV, respectively [34]. For the 4f5/2 spectra, the Au0, Auþ, and Au3þ species were observed at 87.2e87.8, 88.8, and 89.9e90.3 eV, respectively [34]. The metallic gold species were predominant in the Au catalysts, even though the overlapping of the cationic (Auþ and Au3þ) and metallic gold spectra was found. Interestingly, among the Au/Fe2O3eTiO2 (1:4) catalysts, the intensity of Au3þ became the highest at the calcination temperature of 300 C. When increasing the calcination temperature to 400 C, an increase of Au0 intensity was clearly observed with the decreasing of Au3þ intensity. This was due to the thermal treatment effect that partially reduced the Au3þ into Au0 form [34]. When increasing the Fe concentration in the support site from Fe/Ti
of 1/4 to 1/1, the intensity of Au3þ seemed to be lower, while that of Auþ was higher. When relating the amount of Au species with the intensity, it was inferred that the major Au interaction sites were most likely from Au0/Au3þ for the lowcalcination-temperature catalyst and Au0/Auþ for the highcalcination-temperature catalyst. After increasing the amount of Au deposit from 1% to 5%, the significant increase of all Au species was apparently observed, while the Au0 intensity was still the highest. This revealed that the variation of Au loading strongly affected the amount of Audþ (d ¼ 0, 1 and 3) species on the catalyst surface.
Activity measurement The catalytic activity of the catalysts was carried out under atmospheric pressure in a fixed-bed catalytic micro-reactor. The reactant flow consisted of 1% CO, 1% O2, and 40% H2 balanced in He.
Effect of support atomic ratio Fig. 9(a) presents the effect of support composition on the gold catalysts in terms of catalytic activity and selectivity. It was found that CO conversion of Au/Fe2O3 increased with increasing temperature (between 40 and 80 C) and then rapidly decayed at higher temperatures. The activity of Au/ Fe2O3eTiO2 catalysts followed the similar trends except the 1% Au/TiO2 catalyst. This could be because two reasons: firstly, the competition of H2 oxidation occurs at higher temperature which leads to the formation of water. It is well known that both CO and H2 oxidation reactions involving molecular oxygen over metal oxide catalysts proceed by a redox process [37]. This reason could also explain the selectivity trend as increased temperature showed lower selectivity for all gold catalysts. Secondly, completion of adsorbed oxygen and some amount of adsorbed CO (Langmuir adsorption mechanism). The catalytic activity of 1% Au/Fe2O3eTiO2 (1:4) catalyst at 60 C reached the highest activity about 95% CO conversion, which is higher than that of the Au catalyst over pure oxide support (87% and 4% for 1% Au/Fe2O3 and 1% Au/TiO2 at 60 C, respectively). Therefore, this implied that there was the support improvement by preparing mixed oxide between FeeTi
Fig. 3 e H2-TPR profiles for 1% Au/Fe2O3eTiO2 (1:4) catalysts with (a) different calcination temperatures and (b) with different loading. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Fig. 4 e TEM images of Au/Fe2O3eTiO2 (1:4) catalysts (a) 1% Au calcined at 200 C, (b) 1% Au calcined at 300 C, (c) 1% Au calcined at 400 C, (d) 3% Au calcined at 400 C, (e) 5% Au calcined at 400 C and (f) 1% Au calcined at 400 C with O2 pretreatment. which is supported by the TPR and XPS results, showing the nature of Fe2O3eTiO2 was different from both Fe2O3 and TiO2. For the effect of Fe/Ti atomic ratio, it was found that the support atomic ratio had a significant effect on the CO
conversion in the order of 1% Au/Fe2O3eTiO2 (1:4) > 1% Au/ Fe2O3eTiO2 (4:1) > 1% Au/Fe2O3 > 1% Au/Fe2O3eTiO2 (1:1). Consequently, the highest activity for 1% Au/Fe2O3eTiO2 (1:4) catalyst could be related to the TPR results which exhibited
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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(4:1) and 1% Au/Fe2O3eTiO2 (1:1) catalysts. The XPS results in Fig. 7(B) and 8(A) were also confirmed that the different support atomic ratio showed the different nature of 1% Au/ Fe2O3eTiO2 system and these was responded to its different activities. Therefore, it is clearly confirmed that the nature of
Fig. 5 e FTIR spectra of 1% Au/Fe2O3eTiO2 (1:4) catalysts: (a) fresh catalyst, (b) spent catalyst.
the gold reduction peak at lower temperature than other samples, suggesting the highest AueAu interaction and resulted in the formation of the gold species which were easier reduced or active at lower temperature. However, the activity of 1% Au/Fe2O3eTiO2 (4:1) and 1% Au/Fe2O3eTiO2 (1:1) was only slightly lower than 1% Au/Fe2O3eTiO2 (1:4) which in agreement of the slightly higher reduction temperature of the gold reduction temperature found in both 1% Au/Fe2O3eTiO2
Fig. 6 e XPS spectra of Fe 2P over Fe2O3eTiO2 support and 1% Au/Fe2O3eTiO2 catalysts.
Fig. 7 e XPS spectra of Ti 2P over (a) Au/TiO2, Fe2O3eTiO2 support, and 1% Au/Fe2O3eTiO2 catalyst, (b) 1% Au/ Fe2O3eTiO2 catalysts with different metal support ratios and calcined temperatures and (c) 1% Au/Fe2O3eTiO2 catalysts with different gold loadings.
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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the support was a key role in directing the PROX reaction over Au/Fe2O3eTiO2 system.
Effect of calcination temperature Fig. 9(b) displayed the CO conversion of 1% Au/Fe2O3eTiO2 (1:4) catalysts calcined at different temperatures as a function of temperature. It was found that the calcination temperature played an important role in providing good catalytic performance for CO oxidation. The catalyst calcined at 400 C showed the best performance at the lower temperature required for CO conversion (in the range of 40e100 C), this is possibly because of the enhancement of reducibility as suggested from the TPR profiles. The XRD patterns of the sample calcined at 400 C, Fig. 1(B), also supported the high CO PROX activity of this sample by exhibiting the highest crystalline structure. Additionally, the XPS results in Fig. 8(A) showed that the Au3þ was reduced to Auþ and Au0 by thermal in calcinations. The main gold species for 1% Au/Fe2O3eTiO2 (1:4) calcined at 300 C and 1% Au/Fe2O3eTiO2 (1:4) calcined 400 C are Au3þ and Au0; indicating that Au0 is the higher effective
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species to respond the exhibited catalyst activity (active site). The Au/Fe2O3eTiO2 (1:4) calcined at 300 C catalyst also shows an additional peak at the lower BE value of 82.1 eV, which is due to small gold particles with an excess of electrons. With increasing the reaction temperature, significant decreases in CO conversion could be observed for all calcined catalysts. The low catalytic activity of the catalysts calcined at 200 C and 300 C could be due to both the poor interaction between AueAu interaction and too strong interaction of Aumetal (in the support) interaction as indicated by the TPR. However, when the reaction temperature reached 100 C the catalytic activity of these 3 catalysts was similar, suggesting that the thermal annealing from the reaction could improve the reducibility of gold particle.
Effect of gold loading The effect of different gold loadings on CO conversion and the PROX selectivity versus temperature curves obtained over Au/ Fe2O3eTiO2 (1:4) catalysts is shown in Fig 10 (a). With increasing Au loading, the catalytic activity for PROX was slightly unchanged. The little higher catalytic activity could be obtained from 5% Au loading with related to the higher gold actual content in Table 1 and the higher Au0 portion were found in Fig. 8 (B). Moreover, the gold mean size was slightly similar (4 and 5 nm for 1%Au and 5% Au/Fe2O3eTiO2 (1:4) catalysts, respectively). According to the TPR profiles of the samples with different Au loading, it obviously showed that the pattern of reduction peaks were all identical. Thus, the higher gold loading could not affect any interaction in the catalyst. Similarly, the XRD profiles of gold supported catalysts with different Au loadings (Fig. 3(b)) still showed no peaks of metallic gold species. It can be concluded that Au loading has not much significant effect on both CO conversion and PROX selectivity. A preferred Au loading over FeeTi oxide support was proposed to be at 1 wt% when both catalytic activity and cost of catalyst were taken into consideration. Therefore, from the obtained results regarded the effect of Fe/Ti ratio, calcination temperature and gold loading; it suggested that the thermal treatment is the key factor, affecting the crystallinity rather than the Fe/Ti ratio and gold loading content.
Effect of O2 pretreatment
Fig. 8 e XPS spectra of Au 4f over (a) different gold catalysts and (b) 1% Au/Fe2O3eTiO2 catalysts with different gold loadings.
The oxygen pretreatment of the catalyst was one of the parameters to influence the activity of the catalyst. Therefore, the 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C was pretreated with oxygen at the temperature of 200 C for 2 h. After that, the catalyst was tested (PROX reaction) with 1% CO, 1% O2, 40% H2 and balanced in He in the temperature range of 60 Ce180 C. The catalyst activity, in terms of CO conversion and PROX selectivity, of oxygen pretreated catalyst is shown in Fig. 10(b). The CO conversion, after oxygen pretreatment of the catalyst, decreased from 95% to 44% (at 60 C). The reason behind this phenomenon was that surface oxygen species were formed on the gold surface after the catalyst was treated with oxygen at low temperatures [38]. From the above experiments, it could be concluded that the activity of the gold catalysts for PROX reaction was related to the surface oxygen species on gold, and the interaction of oxygen
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Fig. 9 e CO conversion and selectivity as a function of reaction temperature for PROX reaction over (a) 1% Au/Fe2O3eTiO2 catalysts with various atomic ratios of Fe/Ti and (b) 1% Au/Fe2O3eTiO2 catalysts with different calcination temperatures.
Fig. 10 e (a) Effect of gold loading and (b) pretreatment on CO conversion and PROX selectivity over Au/Fe2O3eTiO2 (1:4) catalysts. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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species with gold. As we know, the reaction of CO with O2 requires both CO and O2 to be adsorbed on the catalyst surface. However, the reaction temperature for CO oxidation was quite low (40 Ce100 C) so the surface oxygen species were not desorbed at these temperatures, and the oxygen species at surface were occupying reactive sites. Thus the strongly adsorbed surface oxygen species blocked the adsorption and diffusion of weakly adsorbed surface oxygen resulting in a decrease in the catalytic activity. Moreover, the TPR profile of O2 pretreated catalyst had a higher gold metal reduction temperature than unpretreated catalyst (data not shown). The reduction peak area of the pretreated catalyst was smaller than that of the untreated catalyst and also combined with the support, which suggested that oxygen pretreatment led to an increase in the Au-support interaction and might cause the gold particles to aggregate to be larger prticles (related with TEM result) and decreased the metal dispersion of the catalyst [28], resulting in low catalytic activity for the PROX reaction. The present work showed that the PROX selectivity was decreased with increased temperature. Moreover, at the same reaction temperature the selectivity was roughly independent of the conversion. It was apparently clear that the selectivity of the different catalysts was unaffected by the support system (Fig. 9(a)) and the gold loading (Fig. 10(a)). In term of the effect of calcination temperature and pretreatment, the selectivities were different in the low temperature range (40e80 C).
Deactivation test Catalyst deactivation is one of the major problems for practical PROX catalysts. It is mainly due to some modification in the surface structure and the chemical composition of the catalyst, which is believed to take place at some stages in the reaction process. For gold-based catalyst in the PROX reaction, the catalyst deactivation may be caused by phase transitions such as the formation of carbonate species and/or blocking the access of CO2 and H2O at the catalytic active sites [39]. Thus, the catalyst deactivation evaluation is vital to enable practical catalyst designs and also to develop better understanding of the catalyst activity. In the present work, catalyst deactivation evaluation was undertaken at a temperature of 60 C. The stability of the prepared catalyst was tested in simulated reformed gas mixtures containing 1% CO, 1% O2, 40% H2 and balanced in He, under atmospheric pressure. The stability test of 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C under various conditions is presented in Fig. 11. It was clearly seen that the prepared catalyst was stable and its activity kept constant for 16 h and then slightly decreased. The FTeIR results in Fig. 5 suggested the interactions between adsorbed carbonate-like species and surface hydroxyl groups through hydrogen bonding. The appearance of IR bands in the 1800e900 cm1 region was due to carbonate-like species formed on the support and on gold. Y. Hao and co-workers also studied the relationship between the intensity of carbonate-like species band with the catalytic activity and they found that the intensities of these bands were correlated inversely with the conversion of CO to CO2, consistent with the inference that carbonatelike species formed on gold clusters and resulting in the
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blockage the active sites and, finally, caused the deactivation [40]. This phenomenon could explain the slightly increasing deactivation over the gold catalyst at longer reaction period. In general, it is believed that the catalytic activity of the catalyst is liable to be suppressed in the presence of water vapor [8]. On the contrary, the presence of water on the reaction stream was found to be favorable to the catalytic activity for CO oxidation reaction [41]. In the present work, the prepared catalyst showed a stable catalytic activity, even though the H2O concentration was as high as 10% in the feed and coworkers [42] suggested stream, as shown in Fig. 11. Date that the presence of OH groups extend the lifetime of the catalyst and prevents the deactivation by carbonate-like species accumulation. The changes in the activity of the catalyst when CO2 was added into the feed stream are illustrated in Fig. 11. It can be clearly seen that the presence of CO2 showed a depressing effect on the catalytic activity owing to both the build-up of carbonate species on the surface of the catalyst [22] and the CO2 adsorption on the catalyst where surface carbonate-like species reduce the supply of oxygen required for the CO oxidation reactions. The CO conversion decreased significantly from 95% to around 43%. The simulated practical fuel gas mixture containing 1% CO, 1% O2, 10% CO2, 10% H2O, 40% H2 and balanced in He was used to examine the stability of the prepared catalyst. The effect of CO2 and H2O on the catalytic performance of the prepared 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C was investigated at a constant temperature (60 C). Fig. 11 shows that the presence of CO2 and H2O in the feed stream slightly decreased the CO conversion and the PROX selectivity. This result was similar to the result of Naknam et al. (2009) [43] who studied the influence of CO2 and H2O on the PROX activities over a Au/ a-Fe2O3 catalyst. They explained that the accumulation of carbonate-like species on the catalyst surface blocks the active sites for the PROX reaction. In addition, adding CO2 and H2O into feed might cause some blocking of the active sites, as mentioned above [39].
Fig. 11 e Deactivation test of 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C.
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Conclusion In this research, a series of Au-based catalysts (Au/ Fe2O3eTiO2) have been used to catalyze the preferential CO oxidation reaction (PROX) in the presence of H2 over a range of operating temperatures of 40e180 C. Base on the XRD, TPR and the results in Table 1, the proposed structure model of bimetallic support might be the insertion of very small F2O3 particle into the porous of TiO2 crystallite. The activity results had been demonstrated that the Au/Fe2O3eTiO2 catalyst was more active than Au/Fe2O3 and Au/TiO2 catalysts. The TPR and XPS profiles confirmed that there was some of interaction between Fe and Ti in support. This related to the changing activity of Au/Fe2O3eTiO2 catalysts (compared to gold over pure support). The higher activity of the Fe-containing catalyst was also attributed to present of the gold particles which both active at lower temperature and more reducible gold species formed in Au/Fe2O3eTiO2 compared to Au/Fe2O3 and Au/TiO2 catalysts. The calcination step changes the gold particle in the Au/Fe2O3eTiO2 catalyst to be active at lower reaction temperature and also slightly improved the crystallinity of the catalyst. Thus, at low reaction temperature (<100 C), the catalyst with higher calcination treatment could perform the higher CO conversion. The XPS results suggested the active gold species for Au/Fe2O3eTiO2 system in PROX is metallic gold. Therefore, the 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C which exhibited the highest CO conversion and selectivity at low temperature, could be applied in the optimum condition for PEMFC application. The CO conversion and PROX selectivity of the O2 pretreatment catalyst were all decreased which might be due to the surface oxygen species were formed on gold with strong interaction, and then they were occupying reactive sites, resulting in a decrease in catalytic activity of the catalyst. From the deactivation study, the 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C showed a good stability and the catalyst was able to withstand the presence of water up to 10% in the feed stream without any significant decrease in catalytic activity. However, when CO2 (10%) was injected into the feed stream, the catalyst activity was negatively affected.
Acknowledgments This research has been supported by the Energy Policy and Planning office (EPPO), The National Research University Project of CHE, the Ratchadaphiseksomphot Endowment Fund (EN276B). The authors also acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education.
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Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation Apanee Luengnaruemitchai a, Kunanya Srihamat a, Chinchanop Pojanavaraphan a, Ratchaneekorn Wanchanthuek b,* a
The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Patumwan, Bangkok 10330, Thailand b Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahasarakham University, Kantarawichai District, Mahasarakham 44150, Thailand
article info
abstract
Article history:
A series of Au supported on Fe2O3eTiO2 catalysts were prepared, characterized and
Received 2 June 2015
investigated the catalytic activity in the preferential CO oxidation (PROX) under a H2-rich
Received in revised form
stream. The effects of the different metal support ratio, the calcination temperature, the
22 July 2015
gold loading, and the pre-treatment condition were studied on both the characteristics and
Accepted 30 July 2015
catalytic activity of the catalysts. The results obviously showed that 1% Au/Fe2O3eTiO2 (1:4)
Available online xxx
exhibited higher CO conversion than the 1% Au/TiO2 and 1% Au/Fe2O3 catalysts, especially temperature lower than 100 C, which due to the formation of reducible gold species at
Keywords:
lower temperature. TPR results suggested the improvement of AueAu and metalemetal (in
CO conversion
support) interaction. Additionally, XPS spectra suggested the Au0 could be the predominant
CO selectivity
active site. The effect of O2 pretreatment showed a negative effect in both activity and
Au catalyst
selectivity which suggested the formation of adsorbed oxygen species on the surface of the
Titania
catalyst. Moreover, the deactivation was affected by injection H2O, CO2 or the combination
Iron oxide
of CO2eH2O into the typical feed.
PROX
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the desire to replace fossil fuels with sustainable energy sources is increasing. Many alternative types of sustainable energy sources have been considered, such as electricity from renewable resources (wind, solar light, water power, etc.) as well as hydrogen which is much favoured because it is clean, potentially renewable and it could be generated electricity in a fuel cell. Many types of fuel cell have been developed and the proton exchange membrane fuel cell
(PEMFC) has been proposed to be an ideal for electricity production from hydrogen. PEMFCs have been widely studied for use in residential applications and for transportation systems, especially for automobiles [1]. The advantages of PEMFC against other fuel cells are the low temperature of operation, fast cold start, perfect CO2 tolerance and high energy conversion density [2]. However, the Pt electrodes in the PEM fuel cell are very sensitive to CO. Therefore, the hydrogen gas stream needs to be almost CO-free, preferably to be less than 10 ppm [3e5].
* Corresponding author. Tel.: þ66 43 754 246. E-mail address: [email protected] (R. Wanchanthuek). http://dx.doi.org/10.1016/j.ijhydene.2015.07.148 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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In principle, the hydrogen stream for a PEMFC should be produced on-broad from various hydrocarbon feed stocks (such as methane, ethanol etc.). During the hydrogen production, CO is produced and so there are several steps used to remove the CO, such as CO oxidation, water-gas shift and PROX reactions [5e9]. The advantage of PROX is a low cost of operation. PROX is the CO conversion to CO2 by using O2 with a suitable catalyst. There are three main catalysts that have been applied; platinum, copper and gold. Recently, gold catalysts have shown the higher activity in CO oxidation and made it possible to develop better catalysts than platinum and copper [9e11]. The gold catalyst is active even at subambient temperature, i.e. 73 C and also showed more economic competitiveness than platinum catalysts [12,13]. The activity of the gold catalyst is controlled by 3 main factors; type of support, particle size of gold and the preparation method. In terms of the support, the nature of the support including the reducibility, oxygen storage capacity, the thermal tolerant etc. were the key factors in determining the catalytic activity. From previous researches, the preferred gold is prepared in the form of supported gold by loading gold onto a metal oxide support such as Al2O3 [14], MnOx [15], Fe2O3 [16], TiO2 [17] as well as mixed oxides, such as MgOeAl2O3 [18]. Such supported gold catalysts were found to be very active for CO oxidation at low temperatures [10,11]. Among the different metal oxide supports, TiO2 has been shown to be a suitable support for gold because of its high activity [17,19e21]. The TiO2 support stabilizes the small metastable gold nanoparticles against agglomeration and sintering, and thereby stabilizes the active form of gold. Moreover, Fe2O3 was also used as a support for Au in the PROX reaction at ambient conditions and resulted in high catalytic performance with long life time [16,22]. It has been proposed that the high activity of Au/Fe2O3 was due to the high O2 adsorption over the Fe2O3 which acted as the oxygen supply during the reaction [22]. We previously found that a Au/Fe2O3 catalyst was active for PROX, and it gave 98% CO conversion and 53% selectivity at 50 C. In addition, it showed stable catalytic activity during the testing time [23]. In addition, the importance of Auþ and Au3þ species was reported and it was claimed that these were more active than Au0 species for this reaction. Therefore, it is very interesting to study the catalytic activity of mixed metal oxides as supports for gold catalysts (Au/Fe2O3eTiO2) in the PROX reaction. The purpose of this present study is to develop a new catalyst (Au/Fe2O3eTiO2) for the PROX reaction in the presence of H2 rich gas for fuel cell applications. The catalysts are prepared by the deposition-precipitation method. In order to understand the relationship between the catalyst properties and activity involving selectivity, several kinds of characterization techniques have been employed.
Experimental Catalyst preparation The pure Fe2O3 support was prepared by a precipitation method. The aqueous solution of 0.1 M iron (ІІІ) nitrate nonahydrate, Fe(NO3)3.9H2O, was heated to 80 C and the pH
adjusted to 8e9 by using 0.1 M Na2CO3 to obtain the suspension. Then the mixture was washed by warm deionized water, dried at 110 C overnight and calcined in air at 400 C for 4 h. The mixed support, Fe2O3eTiO2, was prepared by incipient-wetness impregnation method. The atomic ratios of metals (Fe:Ti) were chosen as follows: 0:1, 4:1, 1:1, 1:4, and 1:0. The desired amount of iron (ІІІ) nitrate nonahydrate, Fe(NO3)3.9H2O solution was dropped into titanium (IV) oxide powder (Degussa P25) under grinding. The support slurry was dried at 110 C overnight and calcined in air at 400 C for 4 h. For the preparation of all the series of gold catalysts, gold was loaded to the support using the deposition-precipitation (DP) method using hydrogen tetrachloroaurate (III), HAuCl4.3H2O as the gold precursor. The dried support was added to the specific gold solution and then pH of the mixture was adjusted to 8e9 by adding 0.1 M Na2CO3. This resulted in the deposition of the gold on the support. The suspension was washed by warm deionized water, dried at 110 C overnight and calcined in air at specific temperatures for 4 h.
Characterization The BET surface areas were measured using an Autosorb-1 Gas Sorption system (Quantachrome Corporation). Before each measurement, the samples were degassed at 150 C for 2 h. The specific surface area, the average pore diameter and the pore volume were calculated. XRD patterns were obtained by using a Rigaku X-Ray Diffractometer system (RINT-2200) with a Cu tube for gener˚ ) and a nickel filter. The samples ating CuKa radiation (1.5406 A of catalyst and support powder were prepared as a discshaped pressed powder. The XRD patterns were analyzed. Atomic Absorption Spectroscopy (AAS) was used to determine the content of gold in the catalysts. The catalysts were dissolved in aqua regia solution, which is composed of hydrochloric acid and nitric acid with a ratio of 82:18, and then heated to 100 C for an hour. Temperature-programmed reduction (TPR) measurements were carried out using a conventional flow apparatus with TCD detector, by placing a sample in a quartz reactor. The catalyst was pretreated under argon flow rate 50 ml/min at 400 C for an hour to eliminate the adsorbed gas species over the surface. After the catalyst was cooled down to room temperature, the catalyst was subjected to TPR analysis using 10% H2 in Ar at a total flow of 30 ml/min. The reduction temperature was raised from 30 to 850 C with a ramp rate of 10 C/min. The average crystallite size of the gold particles was measured by Transmission electron microscopy (TEM) using a JEM 2010 microscope operating at 200 kV in bright and dark field modes. The powdered samples were prepared by crushing and grinding in a mortar. Then, the fine powder was ultrasonically dispersed in ethanol, and a drop of the suspension was deposited on a thin carbon film supported on a standard electron microscope grid. The average Au diameter (dTEM) was calculated from the following formula: dTEM ¼ S(nidi)/ni where ni is the number of Au particles of diameter di. X-ray photoelectron spectroscopy, XPS (Kratos Axis Ultra DLD) was used to determine the oxidation states of Au, Fe and Ti in the obtained catalysts. A monochromatic Al Ka was used
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as an X-ray source (anode HT ¼ 15 kV). The residual pressure in the ion-pumped analysis chamber was lower than 5 107 torr. Fourier transform infrared spectrometry (FTIR) was used to examine the functional groups over the surface of the prepared catalyst. The catalyst and potassium bromide (KBr) powder were milled to form a very fine powder. The obtained powder was then compressed as a thin pellet and the spectra were collected on a PerkineElmer (Spectrum one) spectrometer equipped with a mercury-cadmium-telluride (MCT) detector.
Activity test The catalytic performances were tested for the preferential CO oxidation (PROX) using a fixed-bed catalytic micro-reactor at atmospheric pressure. Typically, a 100 mg of dried catalyst were loaded into the quartz reactor with an internal diameter of 6 mm, and fixed in place by a sandwich of quartz wool, and the reactor located in a ceramic furnace. The feed gas consisted of 1% CO, 1% O2 and 40% H2 in He balance which was introduced through the catalyst bed with a total flow rate of 50 ml/min (GHSV ¼ 30000 mlg1h1) and the reactivity was observed at various temperatures over the range of 40e180 C. The gas effluents were analyzed by auto-sampling in an online gas chromatograph equipped with a packed carbon sphere column, 80/100 mesh, 10 ft 1/8 in. and a thermal conductivity detector (TCD).
Results and discussion Catalyst characterization Surface properties Table 1 summarizes the mean crystallite size of TiO2, the average size of the gold particles as determined by TEM, and the BET surface area of the prepared catalysts. The BET surface areas of Au/TiO2 and the series of Au/Fe2O3eTiO2 (with both different atomic ratios and different calcination temperatures) are very close to the surface area of the TiO2 support which has a specific surface area of 58.5 m2/g. This was suggested that the surface morphology of mixed oxide support was dominant by TiO2. However, an increase in the calcination temperature (200e400 C) for the Au/Fe2O3eTiO2 catalysts could not affect the surface area of the resulted catalysts. The more addition of Fe2O3 to the mixed oxide support (from 1:4e4:1 in Fe2O3:TiO2) showed the slightly decrease in BET surface area is possibly due to the blockage of some of the TiO2 pores by Fe2O3 [24]. The crystallite size of the TiO2 in the Au/Fe2O3eTiO2 catalyst was clearly increased by increasing the calcination temperature (up to 400 C), whereas the Au mean size was scarcely changed. The Au loadings of the prepared catalysts were analyzed by AAS and it was found that the loadings were lower than the expected values.
XRD patterns The microstructure of supported Au catalysts was investigated by XRD, as illustrated in Fig. 1(A). There was no peaks of
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metallic gold (2q ¼ 38.2 , 44.4 , and 64.5 ) or gold oxide (2q ¼ 25.5 and 49.8 ) were observed, suggesting that the fine dispersion of gold particles on the support and the small amount of gold species puts this outside the detection limit of the XRD [25]. However, a very strong peak of crystalline TiO2 was detected at 2q ¼ 25.3 [26]. The peaks of Au/TiO2 suggested that the TiO2 is in the anatase phase without the characteristic of rutile phase [19] whereas the XRD profile of Au/Fe2O3 showed the support existed in both Fe2O3 (hematite) and Fe3O4 (magnetite) or in the a-Fe2O3 phase within the hexagonal corundum-type structure [27]. In the case of 1% Au/Fe2O3eTiO2 catalysts, there were reflections similar to that of 1% Au/TiO2 without any characteristics of Fe2O3. This could be suggested a very small quantity of Fe2O3 particles was attached to TiO2 to form the heterogenous structure, which was either the Fe2O3 particles were highly dispersion or they were very small for be detectable by XRD. Moreover, there is no peak of TieFeeO (mixed TieFe metal oxide), suggesting within the detection limits that there is no corporation of Fe3þ into the lattice of TiO2 or Ti4þ in the lattice of Fe2O3. Therefore, only the crystallite sizes of TiO2 were calculated, using the Scherrer equation and the results are summarized in Table 1. From Table 1, the higher addition of Fe2O3 in Fe2O3eTiO2 resulted in the lower surface area (54e50 m2/g) and the XRD pattern of the Au/Fe2O3eTiO2 catalysts showed only the characteristic of TiO2 crystallite phase (Fig. 1(A)). Therefore, both surface area data and XRD supported that the lower surface area was related to the loss of surface area of porous TiO2. The mechanism seemed that the small Fe2O3 particle was inserted inside the porous of TiO2 crystallite. This situation could be occurred only in the event of the present of the very tiny Fe2O3 particles in the Fe2O3eTiO2 which was under the detection of XRD. Therefore, when adding more Fe2O3 component to Fe2O3eTiO2 support, the obtained Fe2O3eTiO2 support demonstrated the lower surface area. The proposed structure of Fe2O3eTiO2 support was in agreement of the support preparation (impregnation). This proposed structure of the support could explain the lower actual gold loading in 1% Au/Fe2O3eTiO2 (4:1) catalyst (only 0.58%), whereas the actual gold loading in 1%Au/ Fe2O3eTiO2 (1:4) catalyst was about 1%, the less surface area in the 1% Au/Fe2O3eTiO2 (4:1) sample might be one of the possible reason to consider about Au deposition. The XRD patterns of 1% Au/Fe2O3eTiO2 (1:4) catalysts with different calcination temperatures are illustrated in Fig. 1(B). The changes induced by thermal treatment were observed. The XRD patterns of 1% Au/Fe2O3eTiO2 (1:4) calcined at 200 C showed the lowest crystallinity followed by the samples calcined at 300 and 400 C, respectively. The results clearly suggested the crystallinity of TiO2 increased with increasing calcination temperature.
TPR measurement TPR was used to characterize the metal oxides, mixed metal oxides, metal oxides dispersed on a support including metalemetal and metal-support interaction of the prepared catalysts. The reduction peak of Fe2O3, TiO2, and 1% Au catalysts are shown in Fig. 2. The reduction peak of pure TiO2 sample showed a poorly identified peak at 640 C which matches with
Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Table 1 e Physicochemical properties of the catalysts. Catalyst 1% 1% 1% 1% 1% 1% 3% 5% 1% a b c
Au/TiO2 Au/Fe2O3eTiO2(1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:1) Au/Fe2O3eTiO2 (4:1) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3eTiO2 (1:4) Au/Fe2O3
Calcination temperature (ºC)
Au loading (wt.%)a
Crystallite size (nm)b
Au mean size (nm)c
BET surface area (m2/g)
400 200 300 400 400 400 400 400 400
0.42 0.93 0.76 1.00 0.98 0.58 1.81 3.72 0.94
14 10 11 15 14 15 21 21 e
e 4 5 5 e e 6 5 e
56.7 56.2 54.2 54.2 50.3 50.5 49.9 49.6 81.2
Gold content were determined by AAS. Mean crystallite sizes were calculated from the average values of TiO2 plane (101), (110), (200), (105), (211). Au crystallite sizes were evaluated by TEM.
the reduction of the titanium oxide surface. However, 1% Au/ TiO2 was observed to give two small broad peaks at low temperature (200 C), associated to partial reduction by H2 of AuxOy [29], and at high temperature (500 C) associated to reduction of bulk TiO2 which interacted with Au and resulted in the lower reduction temperature when compared to the reduction of bulk TiO2 in pure TiO2. In addition, the reduction of pure Fe2O3 consisted of one low temperature peak between 300 C and 430 C, corresponding to the reduction peak of Fe2O3 to Fe3O4 (hematite-tomagnetite), and a broad peak at between 510 C and 730 C,
Fig. 1 e XRD patterns of (A) supported Au catalysts, (B) 1% Au/Fe2O3eTiO2 calcined at different temperatures: (a) calcined at 200 C; (b) calcined at 300 C; (c) calcined at 400 C.
corresponding to the reduction peak of Fe3O4 to Fe (Fe3O4 to FeO and finally to Fe) [28,30]. The reduction peaks of 1% Au/ Fe2O3 catalyst showed smaller peaks than pure Fe2O3 and also shifted toward lower temperature at between 280 C and 360 C and between 500 C and 680 C that can be ascribed to the reduction peak of Fe2O3 to its corresponding metallic phase. These two reduction peaks could be assigned to Fe2O3 and Fe3O4 interact with Au species, respectively. However, the very weak broadened peak at 180 C was related to the reduction of AuxOy. According to the TPR profiles of 1% Au/ Fe2O3 and 1% Au/TiO2 catalysts, it can be implied that the addition of Au provided the higher reducibility of the support by shifting the peak toward a lower temperature. This was suggested the development of Au-support interaction (AueFe or AueTi interaction).
Fig. 2 e H2-TPR profiles of all supports and supported Au catalysts (calcined at 400 C).
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In the series of 1% Au/Fe2O3eTiO2 samples, the large broad reduction peak between 250 and 650 C was assigned to the reduction of the metal oxide in the support which could be Fe2O3 to Fe and/or the reduction of bulk TiO2. It was clearly that they were moved to the reduction temperature between the reduction of bulk TiO2 in Au/TiO2 and iron oxide in Au/ Fe2O3, thus suggesting some of interaction between Ti and Fe in mixed oxide support (supportesupport interaction). This was supported our proposed model of Fe2O3eTiO2 (Fe2O3 inserted in the porous of TiO2). In this proposed model, there was the clear interfacial interaction between TiO2 and Fe2O3, therefore, the particles of TiO2 and Fe2O3 could not be presented in the individual particle. The apparent sharp reduction peak at 100 C was assigned to the reduction of gold oxide (AuxOy) to metallic gold (Au0) and the peak position was lower than that found in the 1% Au/ Fe2O3 or 1% Au/TiO2 samples, suggesting the gold oxide was in the easier reduced form since the nature of the obtained Fe2O3eTiO2 support had been modified. In other hand, the lower reduction temperature of gold oxide peak was indicated the increasing AueAu interaction. This mean the reduction property of Au over Fe2O3eTiO2 support has been developed. In addition, at higher amounts of Fe in mixed oxide support, these peaks were shifted to slightly higher temperatures, indicating a slightly more interaction of Au-support occurred. The quantity analysis of reducibility gold particles in catalyst was investigated from the peak area of the gold reduction peak. The much larger gold reduction peak area found in 1% Au/Fe2O3eTiO2 catalysts than 1%Au/Fe2O3 and 1% Au/TiO2, indicating the improvement of the gold reducibility. Similarly, the peak area of metal oxide (in the support) reduction peak was very small (found in all the 1% Au/ Fe2O3eTiO2 samples), suggesting the low support reducibility. The TPR profiles of the 1% Au/Fe2O3eTiO2 catalysts at different calcination temperatures are presented in Fig. 3(a). The profiles indicated two reduction peaks along with a broad peak at high temperature. These peaks are attributed to the reduction peak of Au and Fe oxides and/or bulk TiO2 to the corresponding metallic species. The sample calcined at 200 C showed a reduction peak at 200 C with a small shoulder peak at 240 C, which could be assigned to the reduction peak of AuxOy with the weakly and strongly interaction with the support, respectively. When increasing the calcination temperature, the result showed a decrease in the reduction peak and they also shifted to lower temperature. It can be explained that increasing the calcination temperature can improve the AueAu interaction or depressed the Au-support interaction but the gold catalyst was active at lower reaction temperature. However, the area of gold reduction peak was decreased at high calcination temperature, suggesting less reducible gold particles since the thermal could reduce some of the acidic gold to metallic gold. In the range of reduction peaks of metal oxides in the support, the peak area was increased at higher calcination temperature. This indicated the thermal treatment could obviously affect the interaction of both AueAu interaction and supportesupport interaction. The TPR profiles of different gold loading catalysts showed in Fig. 3(b), it showed that the increasing of Au loading does not seem to improve the AueAu interaction or Au-support interaction or supportesupport interaction since there is no
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significant difference in reduction peak between 70 C to 180 C, (reduction peak of AuxOy). Similarly to previous section, the peak between 260 C to 650 C is assigned to the metal oxides reduction (in the support) to its corresponding metallic species and those peaks were shown no shifting, indicating the increasing amount of gold loading could not affect the reducibility of the metal oxides in the support of the obtained catalysts.
TEM results The TEM images of 1% Au/Fe2O3eTiO2 (1:4) catalysts with different calcination temperatures are shown in Fig. 4(a)e(c)). As can be seen, the particle size exhibited no different as increasing calcination temperature. Fig. 4(c)e(e) illustrates the TEM images of Au/Fe2O3eTiO2 (1:4) catalysts calcined at 400 C with different Au loadings. It is not clearly seen that the mean Au particle sizes changed with increasing the gold loading. Therefore, in these two experiments suggested that the Au loading and calcinations temperature seem not have an obvious effect to the Au particle size on the Au/Fe2O3eTiO2 catalysts. Fig. 4(f) shows the TEM image of 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C, with O2 pretreatment for 2 h. The mean Au particle sizes increased to 6.37 nm compared to 1% Au/ Fe2O3eTiO2 (1:4) calcined at 400 C without O2 pretreatment was only 4.58 nm. This was clearly that the O2 pretreatment enhanced the growth of Au particle over the catalyst. Similar results were reported over that a Au/TiO2eSiO2 catalyst that the O2 pretreatment enhances the growth of gold particles on [31].
Fourier transform infrared spectroscopy (FTeIR) FTeIR measurement was utilized to examine the catalyst surface before and after the 16 h deactivation test. The results from the FTeIR spectroscopy were shown in Fig. 5. A broad transmission band was observed in the range of 3200e3600 cm1 for the fresh catalyst, owing to the OeH stretching mode of H2O molecules [32]. Treatment of the sample with CO and with CO2 led to the marked changes in the IR spectra in the OeH stretching region, including a decrease in intensity to give a negative band in this range (OeH stretching mode, already assigned to bicarbonates).
X-ray photoelectron spectroscopy The surface structures of the Au/Fe2O3eTiO2, Au/TiO2, and Fe2O3eTiO2 catalysts were investigated by XPS, as shown in Fig. 6. According to the support spectra, the Fe 2p spectra were found into two binding energies; the strong bands at 725 eV and 712 eV which represents the 2p1/2 and 2p3/2 spectra [33], respectively. Unlike our previous work [34], it is hardly detectable to the weak band of 718.5 eV which represents the Fe(III) species in the hematite (aeFe2O3) phase. In addition, the appearance of another Fe 2p3/2 at 711.4 eV in the catalyst calcined at 300 C represented the Fe3þ species of the magnetite phase [35]. It could be implied that the 2p spectra could be corresponded to Fe3þ species. The deposition of active Au metal and the change of support concentration seem not to affect the peaks of Fe 2p spectra. For the Ti 2p spectra, two main peaks were detected in the range of 456e468 eV, as shown in Fig. 7(a)e(c). The peak at
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~464.3 eV was attributed to the 2p1/2 spectra, and the strong peak at ~458.6 eV was assigned to the 2p3/2 spectra, which are the characteristic of TiO2 [36]. The Ti 2p3/2 peaks indicated the presences Ti3þ and Ti4þ oxidation states at 457.6 and 458.5e459 eV, respectively [36]. Meanwhile, the Ti 2p1/2 peaks showed the presences of Ti2þ, Ti3þ, and Ti4þ oxidation states at 459.5e460.1, 461.6e461.9, 463.4e463.5, and 464.3e464.6 eV, respectively [36]. Focusing on the Ti intensity, it was found that the intensities of Ti4þ species were the highest in all samples, indicating that the species composition in the Au/ Fe2O3eTiO2 catalyst was mainly Ti4þ; however, the exact amount (or percentage) of each species was not determined and reported in this paper. The peak of Ti3þ species was only observable in Au/TiO2, and 1% and 5% Au/Fe2O3eTiO2 (1:4) calcined 400 C, whereas the Ti0 peak was not found in the Au/ TiO2 and 1% Au/Fe2O3eTiO2 (1:4) calcined 400 C. The Au/TiO2 catalyst mainly presented Ti4þ and small portion of Ti3þ, while a variety of Ti oxidation states were found in the Fe2O3eTiO2 catalysts. It was postulated that the appearance of those various oxidation states might be the consequences of the interaction between Fe and Ti in the mixed oxide support, where the change in electronic effect was possible. Nonetheless, the electronic effect on the surface structure was not discussed in this study. The binding energies of Au 4f spectra show various oxidation states of Au, which were affected by the support composition, catalyst calcination, and amount of Au deposition (Fig. 8 (a)e(b)). For the 4f7/2 spectra, the Au0, Auþ, and Au3þ species were assigned to the binding energies at 83.5e83.9, 84.1e84.4, and 85.3e85.7 eV, respectively [34]. For the 4f5/2 spectra, the Au0, Auþ, and Au3þ species were observed at 87.2e87.8, 88.8, and 89.9e90.3 eV, respectively [34]. The metallic gold species were predominant in the Au catalysts, even though the overlapping of the cationic (Auþ and Au3þ) and metallic gold spectra was found. Interestingly, among the Au/Fe2O3eTiO2 (1:4) catalysts, the intensity of Au3þ became the highest at the calcination temperature of 300 C. When increasing the calcination temperature to 400 C, an increase of Au0 intensity was clearly observed with the decreasing of Au3þ intensity. This was due to the thermal treatment effect that partially reduced the Au3þ into Au0 form [34]. When increasing the Fe concentration in the support site from Fe/Ti
of 1/4 to 1/1, the intensity of Au3þ seemed to be lower, while that of Auþ was higher. When relating the amount of Au species with the intensity, it was inferred that the major Au interaction sites were most likely from Au0/Au3þ for the lowcalcination-temperature catalyst and Au0/Auþ for the highcalcination-temperature catalyst. After increasing the amount of Au deposit from 1% to 5%, the significant increase of all Au species was apparently observed, while the Au0 intensity was still the highest. This revealed that the variation of Au loading strongly affected the amount of Audþ (d ¼ 0, 1 and 3) species on the catalyst surface.
Activity measurement The catalytic activity of the catalysts was carried out under atmospheric pressure in a fixed-bed catalytic micro-reactor. The reactant flow consisted of 1% CO, 1% O2, and 40% H2 balanced in He.
Effect of support atomic ratio Fig. 9(a) presents the effect of support composition on the gold catalysts in terms of catalytic activity and selectivity. It was found that CO conversion of Au/Fe2O3 increased with increasing temperature (between 40 and 80 C) and then rapidly decayed at higher temperatures. The activity of Au/ Fe2O3eTiO2 catalysts followed the similar trends except the 1% Au/TiO2 catalyst. This could be because two reasons: firstly, the competition of H2 oxidation occurs at higher temperature which leads to the formation of water. It is well known that both CO and H2 oxidation reactions involving molecular oxygen over metal oxide catalysts proceed by a redox process [37]. This reason could also explain the selectivity trend as increased temperature showed lower selectivity for all gold catalysts. Secondly, completion of adsorbed oxygen and some amount of adsorbed CO (Langmuir adsorption mechanism). The catalytic activity of 1% Au/Fe2O3eTiO2 (1:4) catalyst at 60 C reached the highest activity about 95% CO conversion, which is higher than that of the Au catalyst over pure oxide support (87% and 4% for 1% Au/Fe2O3 and 1% Au/TiO2 at 60 C, respectively). Therefore, this implied that there was the support improvement by preparing mixed oxide between FeeTi
Fig. 3 e H2-TPR profiles for 1% Au/Fe2O3eTiO2 (1:4) catalysts with (a) different calcination temperatures and (b) with different loading. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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Fig. 4 e TEM images of Au/Fe2O3eTiO2 (1:4) catalysts (a) 1% Au calcined at 200 C, (b) 1% Au calcined at 300 C, (c) 1% Au calcined at 400 C, (d) 3% Au calcined at 400 C, (e) 5% Au calcined at 400 C and (f) 1% Au calcined at 400 C with O2 pretreatment. which is supported by the TPR and XPS results, showing the nature of Fe2O3eTiO2 was different from both Fe2O3 and TiO2. For the effect of Fe/Ti atomic ratio, it was found that the support atomic ratio had a significant effect on the CO
conversion in the order of 1% Au/Fe2O3eTiO2 (1:4) > 1% Au/ Fe2O3eTiO2 (4:1) > 1% Au/Fe2O3 > 1% Au/Fe2O3eTiO2 (1:1). Consequently, the highest activity for 1% Au/Fe2O3eTiO2 (1:4) catalyst could be related to the TPR results which exhibited
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(4:1) and 1% Au/Fe2O3eTiO2 (1:1) catalysts. The XPS results in Fig. 7(B) and 8(A) were also confirmed that the different support atomic ratio showed the different nature of 1% Au/ Fe2O3eTiO2 system and these was responded to its different activities. Therefore, it is clearly confirmed that the nature of
Fig. 5 e FTIR spectra of 1% Au/Fe2O3eTiO2 (1:4) catalysts: (a) fresh catalyst, (b) spent catalyst.
the gold reduction peak at lower temperature than other samples, suggesting the highest AueAu interaction and resulted in the formation of the gold species which were easier reduced or active at lower temperature. However, the activity of 1% Au/Fe2O3eTiO2 (4:1) and 1% Au/Fe2O3eTiO2 (1:1) was only slightly lower than 1% Au/Fe2O3eTiO2 (1:4) which in agreement of the slightly higher reduction temperature of the gold reduction temperature found in both 1% Au/Fe2O3eTiO2
Fig. 6 e XPS spectra of Fe 2P over Fe2O3eTiO2 support and 1% Au/Fe2O3eTiO2 catalysts.
Fig. 7 e XPS spectra of Ti 2P over (a) Au/TiO2, Fe2O3eTiO2 support, and 1% Au/Fe2O3eTiO2 catalyst, (b) 1% Au/ Fe2O3eTiO2 catalysts with different metal support ratios and calcined temperatures and (c) 1% Au/Fe2O3eTiO2 catalysts with different gold loadings.
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the support was a key role in directing the PROX reaction over Au/Fe2O3eTiO2 system.
Effect of calcination temperature Fig. 9(b) displayed the CO conversion of 1% Au/Fe2O3eTiO2 (1:4) catalysts calcined at different temperatures as a function of temperature. It was found that the calcination temperature played an important role in providing good catalytic performance for CO oxidation. The catalyst calcined at 400 C showed the best performance at the lower temperature required for CO conversion (in the range of 40e100 C), this is possibly because of the enhancement of reducibility as suggested from the TPR profiles. The XRD patterns of the sample calcined at 400 C, Fig. 1(B), also supported the high CO PROX activity of this sample by exhibiting the highest crystalline structure. Additionally, the XPS results in Fig. 8(A) showed that the Au3þ was reduced to Auþ and Au0 by thermal in calcinations. The main gold species for 1% Au/Fe2O3eTiO2 (1:4) calcined at 300 C and 1% Au/Fe2O3eTiO2 (1:4) calcined 400 C are Au3þ and Au0; indicating that Au0 is the higher effective
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species to respond the exhibited catalyst activity (active site). The Au/Fe2O3eTiO2 (1:4) calcined at 300 C catalyst also shows an additional peak at the lower BE value of 82.1 eV, which is due to small gold particles with an excess of electrons. With increasing the reaction temperature, significant decreases in CO conversion could be observed for all calcined catalysts. The low catalytic activity of the catalysts calcined at 200 C and 300 C could be due to both the poor interaction between AueAu interaction and too strong interaction of Aumetal (in the support) interaction as indicated by the TPR. However, when the reaction temperature reached 100 C the catalytic activity of these 3 catalysts was similar, suggesting that the thermal annealing from the reaction could improve the reducibility of gold particle.
Effect of gold loading The effect of different gold loadings on CO conversion and the PROX selectivity versus temperature curves obtained over Au/ Fe2O3eTiO2 (1:4) catalysts is shown in Fig 10 (a). With increasing Au loading, the catalytic activity for PROX was slightly unchanged. The little higher catalytic activity could be obtained from 5% Au loading with related to the higher gold actual content in Table 1 and the higher Au0 portion were found in Fig. 8 (B). Moreover, the gold mean size was slightly similar (4 and 5 nm for 1%Au and 5% Au/Fe2O3eTiO2 (1:4) catalysts, respectively). According to the TPR profiles of the samples with different Au loading, it obviously showed that the pattern of reduction peaks were all identical. Thus, the higher gold loading could not affect any interaction in the catalyst. Similarly, the XRD profiles of gold supported catalysts with different Au loadings (Fig. 3(b)) still showed no peaks of metallic gold species. It can be concluded that Au loading has not much significant effect on both CO conversion and PROX selectivity. A preferred Au loading over FeeTi oxide support was proposed to be at 1 wt% when both catalytic activity and cost of catalyst were taken into consideration. Therefore, from the obtained results regarded the effect of Fe/Ti ratio, calcination temperature and gold loading; it suggested that the thermal treatment is the key factor, affecting the crystallinity rather than the Fe/Ti ratio and gold loading content.
Effect of O2 pretreatment
Fig. 8 e XPS spectra of Au 4f over (a) different gold catalysts and (b) 1% Au/Fe2O3eTiO2 catalysts with different gold loadings.
The oxygen pretreatment of the catalyst was one of the parameters to influence the activity of the catalyst. Therefore, the 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C was pretreated with oxygen at the temperature of 200 C for 2 h. After that, the catalyst was tested (PROX reaction) with 1% CO, 1% O2, 40% H2 and balanced in He in the temperature range of 60 Ce180 C. The catalyst activity, in terms of CO conversion and PROX selectivity, of oxygen pretreated catalyst is shown in Fig. 10(b). The CO conversion, after oxygen pretreatment of the catalyst, decreased from 95% to 44% (at 60 C). The reason behind this phenomenon was that surface oxygen species were formed on the gold surface after the catalyst was treated with oxygen at low temperatures [38]. From the above experiments, it could be concluded that the activity of the gold catalysts for PROX reaction was related to the surface oxygen species on gold, and the interaction of oxygen
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Fig. 9 e CO conversion and selectivity as a function of reaction temperature for PROX reaction over (a) 1% Au/Fe2O3eTiO2 catalysts with various atomic ratios of Fe/Ti and (b) 1% Au/Fe2O3eTiO2 catalysts with different calcination temperatures.
Fig. 10 e (a) Effect of gold loading and (b) pretreatment on CO conversion and PROX selectivity over Au/Fe2O3eTiO2 (1:4) catalysts. Please cite this article in press as: Luengnaruemitchai A, et al., Activity of Au/Fe2O3eTiO2 catalyst for preferential CO oxidation, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.07.148
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species with gold. As we know, the reaction of CO with O2 requires both CO and O2 to be adsorbed on the catalyst surface. However, the reaction temperature for CO oxidation was quite low (40 Ce100 C) so the surface oxygen species were not desorbed at these temperatures, and the oxygen species at surface were occupying reactive sites. Thus the strongly adsorbed surface oxygen species blocked the adsorption and diffusion of weakly adsorbed surface oxygen resulting in a decrease in the catalytic activity. Moreover, the TPR profile of O2 pretreated catalyst had a higher gold metal reduction temperature than unpretreated catalyst (data not shown). The reduction peak area of the pretreated catalyst was smaller than that of the untreated catalyst and also combined with the support, which suggested that oxygen pretreatment led to an increase in the Au-support interaction and might cause the gold particles to aggregate to be larger prticles (related with TEM result) and decreased the metal dispersion of the catalyst [28], resulting in low catalytic activity for the PROX reaction. The present work showed that the PROX selectivity was decreased with increased temperature. Moreover, at the same reaction temperature the selectivity was roughly independent of the conversion. It was apparently clear that the selectivity of the different catalysts was unaffected by the support system (Fig. 9(a)) and the gold loading (Fig. 10(a)). In term of the effect of calcination temperature and pretreatment, the selectivities were different in the low temperature range (40e80 C).
Deactivation test Catalyst deactivation is one of the major problems for practical PROX catalysts. It is mainly due to some modification in the surface structure and the chemical composition of the catalyst, which is believed to take place at some stages in the reaction process. For gold-based catalyst in the PROX reaction, the catalyst deactivation may be caused by phase transitions such as the formation of carbonate species and/or blocking the access of CO2 and H2O at the catalytic active sites [39]. Thus, the catalyst deactivation evaluation is vital to enable practical catalyst designs and also to develop better understanding of the catalyst activity. In the present work, catalyst deactivation evaluation was undertaken at a temperature of 60 C. The stability of the prepared catalyst was tested in simulated reformed gas mixtures containing 1% CO, 1% O2, 40% H2 and balanced in He, under atmospheric pressure. The stability test of 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C under various conditions is presented in Fig. 11. It was clearly seen that the prepared catalyst was stable and its activity kept constant for 16 h and then slightly decreased. The FTeIR results in Fig. 5 suggested the interactions between adsorbed carbonate-like species and surface hydroxyl groups through hydrogen bonding. The appearance of IR bands in the 1800e900 cm1 region was due to carbonate-like species formed on the support and on gold. Y. Hao and co-workers also studied the relationship between the intensity of carbonate-like species band with the catalytic activity and they found that the intensities of these bands were correlated inversely with the conversion of CO to CO2, consistent with the inference that carbonatelike species formed on gold clusters and resulting in the
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blockage the active sites and, finally, caused the deactivation [40]. This phenomenon could explain the slightly increasing deactivation over the gold catalyst at longer reaction period. In general, it is believed that the catalytic activity of the catalyst is liable to be suppressed in the presence of water vapor [8]. On the contrary, the presence of water on the reaction stream was found to be favorable to the catalytic activity for CO oxidation reaction [41]. In the present work, the prepared catalyst showed a stable catalytic activity, even though the H2O concentration was as high as 10% in the feed and coworkers [42] suggested stream, as shown in Fig. 11. Date that the presence of OH groups extend the lifetime of the catalyst and prevents the deactivation by carbonate-like species accumulation. The changes in the activity of the catalyst when CO2 was added into the feed stream are illustrated in Fig. 11. It can be clearly seen that the presence of CO2 showed a depressing effect on the catalytic activity owing to both the build-up of carbonate species on the surface of the catalyst [22] and the CO2 adsorption on the catalyst where surface carbonate-like species reduce the supply of oxygen required for the CO oxidation reactions. The CO conversion decreased significantly from 95% to around 43%. The simulated practical fuel gas mixture containing 1% CO, 1% O2, 10% CO2, 10% H2O, 40% H2 and balanced in He was used to examine the stability of the prepared catalyst. The effect of CO2 and H2O on the catalytic performance of the prepared 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C was investigated at a constant temperature (60 C). Fig. 11 shows that the presence of CO2 and H2O in the feed stream slightly decreased the CO conversion and the PROX selectivity. This result was similar to the result of Naknam et al. (2009) [43] who studied the influence of CO2 and H2O on the PROX activities over a Au/ a-Fe2O3 catalyst. They explained that the accumulation of carbonate-like species on the catalyst surface blocks the active sites for the PROX reaction. In addition, adding CO2 and H2O into feed might cause some blocking of the active sites, as mentioned above [39].
Fig. 11 e Deactivation test of 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C.
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Conclusion In this research, a series of Au-based catalysts (Au/ Fe2O3eTiO2) have been used to catalyze the preferential CO oxidation reaction (PROX) in the presence of H2 over a range of operating temperatures of 40e180 C. Base on the XRD, TPR and the results in Table 1, the proposed structure model of bimetallic support might be the insertion of very small F2O3 particle into the porous of TiO2 crystallite. The activity results had been demonstrated that the Au/Fe2O3eTiO2 catalyst was more active than Au/Fe2O3 and Au/TiO2 catalysts. The TPR and XPS profiles confirmed that there was some of interaction between Fe and Ti in support. This related to the changing activity of Au/Fe2O3eTiO2 catalysts (compared to gold over pure support). The higher activity of the Fe-containing catalyst was also attributed to present of the gold particles which both active at lower temperature and more reducible gold species formed in Au/Fe2O3eTiO2 compared to Au/Fe2O3 and Au/TiO2 catalysts. The calcination step changes the gold particle in the Au/Fe2O3eTiO2 catalyst to be active at lower reaction temperature and also slightly improved the crystallinity of the catalyst. Thus, at low reaction temperature (<100 C), the catalyst with higher calcination treatment could perform the higher CO conversion. The XPS results suggested the active gold species for Au/Fe2O3eTiO2 system in PROX is metallic gold. Therefore, the 1% Au/Fe2O3eTiO2 (1:4) calcined at 400 C which exhibited the highest CO conversion and selectivity at low temperature, could be applied in the optimum condition for PEMFC application. The CO conversion and PROX selectivity of the O2 pretreatment catalyst were all decreased which might be due to the surface oxygen species were formed on gold with strong interaction, and then they were occupying reactive sites, resulting in a decrease in catalytic activity of the catalyst. From the deactivation study, the 1% Au/Fe2O3eTiO2 (1:4) catalyst calcined at 400 C showed a good stability and the catalyst was able to withstand the presence of water up to 10% in the feed stream without any significant decrease in catalytic activity. However, when CO2 (10%) was injected into the feed stream, the catalyst activity was negatively affected.
Acknowledgments This research has been supported by the Energy Policy and Planning office (EPPO), The National Research University Project of CHE, the Ratchadaphiseksomphot Endowment Fund (EN276B). The authors also acknowledge the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education.
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