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CeO2-ZrO2 catalysts
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Preferential oxidation of CO in excess of hydrogen over Au/CeO2-ZrO2 catalysts rdoba a, Angel Martı´nez-Herna ndez b,* Luis Fernando Co Universidad Nacional de Colombia, Departamento de Ingenierı´a Quı´mica y Ambiental, Grupo de Investigacion en , Colombia Procesos Quı´micos y Bioquı´micos, Cra. 30 calle 45, Bogota b s de Universidad Aut onoma de Nuevo Leon, UANL, Facultad de Ciencias Quı´micas, Av. Universidad S/N, San Nicola los Garza, Ciudad Universitaria, C.P. 66451, N.L, Mexico a
article info
abstract
Article history:
The preferential oxidation of CO to CO2 in a simulated reformed gas using Au/-CeO2-ZrO2
Received 25 July 2015
catalysts was examined. The studied catalyst showed high catalytic activity and stability
Received in revised form
during long-term tests even in presence of CO2 and water vapour, which inhibited the
15 September 2015
conversion of CO to a small extent. The catalyst preparation at controlled pH (6) favours the
Accepted 18 September 2015
high dispersion of deposited gold particles, while the catalytic activity and selectivity was
Available online 24 October 2015
correlated with the size and oxidation state of gold particles. Furthermore, it was observed that oxidizing treatment resulted in the formation of gold particles having different shapes
Keywords:
and/or side length, which was evidenced by a broadening of the plasmon resonance band
Hydrogen purification
(From DRS UVevis).
Gold catalyst
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
PROX-CO Catalytic stability Metallic Au nanoparticles
Introduction Supply, transformation and energy consumption [1] affect the development of global societies as the world population and its energy demands are increasing. These factors should be managed so that the use and production of energy are sustainable, without compromising the natural resources that should be available for future generations as well as to prevent pollution. In this way, it is necessary to find sustainable energy sources that can achieve the goals of environmentally friendly technologies. Hydrogen is considered to be an important alternative that contributes to the diminution of the environmental problems resulting from electrical power production because it has
great potential as an environmentally clean fuel and also has high energetic efficiency [2]. In this sense, the most promising technologies for obtaining energy from H2 are based on devices, such as proton exchange membrane (PEM) fuel cells. These fuel cells have attracted much attention due their good fuel efficiency and cleanliness when they are used in automobiles [3], along with their low operating temperature, fast cold start, good tolerance by the electrolyte to the CO2 adsorption and combination of high power density and high energy conversion efficiency. Pure hydrogen is the ideal fuel for PEM fuel cells because it maximizes their efficiency, lead to zero emissions. However, until now, there has not been a safe method for storing an adequate amount of hydrogen in automobiles. To overcome this problem, a potential alternative is the implementation of
* Corresponding author. Tel.: þ52 81 8329 4000x3454. ndez). E-mail address: [email protected] (A. Martı´nez-Herna http://dx.doi.org/10.1016/j.ijhydene.2015.09.133 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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an on-board hydrogen generation system that can convert methanol steam or partially oxidize liquid hydrocarbons followed by a water gas shift reaction [4]. These processes performed in series could supply sufficient H2 to the fuel cell to power an on-board electric motor in a vehicle, increasing the autonomy of the transport and leading to the commercial implementation of this technology. Typically, the obtained hydrogen-rich gas in the steam reforming reaction even after the water-gas shift reactor, contain 1e2% v/v of CO [5e7]. The former compound downgrades the energy efficiency of the PEM fuel cell because its electrodes are typically made of platinum, which is very sensitive to CO because it is strongly adsorbed and poisons the electrode surface. There are several methods reported for carbon monoxide removal from the steam reforming stream, such as the preferential oxidation of CO (CO-PROX), selective methanation of CO, and selective diffusion through membranes [8e10]. Among these, CO-PROX is the method with the highest efficiency and economy [11]. CeO2 has been broadly reported to be an active support for the synthesis of oxidation catalysts due to its structural properties [3]. This oxide has redox behaviour that allows high oxygen mobility, enhancing oxygen exchange with the reaction environment [12]. Additionally, it has been found that the addition of ZrO2 improves the redox properties of CeO2 and that the oxygen mobility can be improved by the formation of a mixed system with a strong interaction, preferably as a solid solution to promote electronic distortions in the oxide framework. These oxides contribute to the homogeneous dispersion of the deposited metal and long-term stability of the catalyst and increase the resistance to high temperatures by decreasing the sintering problems on the catalyst surface [13e16]. On the other hand, gold-supported catalysts are more active and selective with regard to the conversion of CO at low temperatures among the catalysts that use noble metals [18] showed that gold nanoparticles [12,17]. Haruta and Date with diameters between 2 and 4 nm were very effective for CO oxidation. Several factors, such as the size of the gold particles, their oxidation state [19,20] and the nature of the support [21,22], affect the catalytic activity of gold-based catalysts for the CO-PROX reaction in rich H2 streams. The latter study noted the relevance of the choice of the appropriate method for the synthesis of catalysts because it is crucial to control all of these factors. Park et al. [23] Carry out a broad screening of gold-based catalytic systems used in selective CO removal in a H2-rich stream, where they demonstrate the exceptional catalytic activity of these catalysts in the CO-PROX at low temperature. Nevertheless, they report that selectivity toward CO oxidation decreased rapidly with increasing reaction temperature, because in such conditions the H2 oxidation is activated. Then, it is important to study the selectivity of CO-PROX reaction over gold-based catalysts with special attention in to minimize the H2 oxidation. The aim of this work was to study the purification of a H2 rich stream from CO by performing the preferential CO oxidation reaction over synthesize gold catalysts using CeO2ZrO2 as support. The PROX reaction results have been discussed in function of catalyst properties.
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Experimental procedure Catalyst preparation The fluorite (CeO2-ZrO2) support was synthesized by a pseudo solegel method based on the thermal decomposition of propionate precursors [24]. The starting materials were cerium (III) acetate (SigmaeAldrich) and zirconium (IV) acetylacetonate (SigmaeAldrich), and these starting salts were individually dissolved in boiling propionic acid at a concentration of 0.12 mol L1 at 100 C for 1 h. This procedure exclusively formed the propionate precursors of the metals. The precursors were mixed while still boiling, and the resulting solution was kept boiling for 2 h under reflux. The solvent was then evaporated, eventually producing a resin. Finally, the resin was calcined under static air at 600 C for 6 h (a heating ramp of 2 C min1 was used). To load gold into the support, we used the procedure reported by Lin and Wan [25]. Briefly, it consisted of the following: a suitable amount of chloroauric acid (HAuCl4$3H2O, SigmaeAldrich) was dissolved in 250 ml of deionized water while being stirred, and the solution pH was adjusted to a value of 6 by adding a sodium hydroxide solution (0.1 M). Then, 2 g of the CeO2-ZrO2 support was added, and the resultant slurry was heated under stirring for up to 80 C, maintaining this condition for 2 h. After that time, the prepared catalysts were filtered and washed with enough deionized water to ensure chloride removal from the solid. The resulting gold sample was dried at 60 C for 12 h, and the catalyst was labelled as Au/CeO2-ZrO2(c). For comparison purposes, a second catalyst was prepared using the same procedure but with uncontrolled pH; this catalyst was labelled Au/CeO2-ZrO2(u).
Catalyst characterisation The gold loading of the fresh catalysts was determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer Optima 3300DV). Powder X-ray diffraction (XRD, Siemens D-5000) patterns were used to obtain information about the structure and composition of crystalline materials. A specific surface area measurement was made by N2 adsorption isotherms (Micromeritics, Asap 2020). UVevis diffuse reflectance spectra (Varian, Cary 5000) of the catalysts were recorded using a Praying Mantis accessory equipped with a high temperature reaction chamber (Harrick Scientific) attached to the spectrophotometer. Temperature programed reduction with hydrogen (H2-TPR) experiments were performed with homemade equipment. Briefly, a sample of 50 mg of catalyst was reduced by flowing 25 cm3/min of a mixture consisting of 5% H2 in an Ar balance through the reactor; the temperature of the reactor was raised from room temperature to 900 C at a heating rate of 10 C/min. Hydrogen consumption was measured by a thermal conductivity detector (TCD).
Activity measurement The CO-PROX reaction was carried out in a fixed-bed quartz micro-reactor using a total flow of 100 cm3/min and 100 mg of
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catalyst. The reactant gas typically consisted of 2% CO, 1e2% O2, 2% CO2, 2% H2O, and 50% H2 in a balance of helium. The content of CO2 and H2O in the reaction stream was limited to 2% instead of 10e20% v/v (real content) due to experimental restrictions of our reaction equipment. However, this amount was enough for to observe the kinetic inhibition effect (competitive adsorption) of these compounds upon the COPROX reaction. The individual gas streams were controlled by electronic mass flow controllers to achieve the desired composition. The reactants and products were analysed online by gas chromatography (Agilent 6820 equipped with a thermal conductivity detector and a HP Molesieve capillary column). The reactor temperature was measured by a K-type thermocouple inside of a quartz thermo-well placed in the middle of the catalyst bed. Before the reaction, the catalysts were pre-treated under an oxidative (air) environment at 300 C for 1 h or a reducing atmosphere (10% of H2 in balance He) at 100 C for 1 h to evaluate the influence of pretreatments on the catalytic activity of Au/CeO2-ZrO2 catalysts. The carbon monoxide conversion and oxidation selectivity to CO2 were calculated using the relationships expressed in Eqs. (1) and (2), respectively. XCO ¼
SCO ¼
out Fin CO FCO 100 Fin CO
(1)
Fin Fout CO 100 CO in 2 FO2 Fout O2
(2)
where X in Eq. (1) represents the percentage of CO conversion and FCO is the molar flow of CO at the inlet or outlet of the reactor. In Eq. (2), SCO represents the selectivity of the percentage of CO that reacts with O2, exclusively forming CO2. There was no methane formation under the reaction conditions used in this study.
Fig. 1 e CO conversion versus temperature. Effect of the controlled pH on gold deposition for Au/CeO2ZrO2(c) and uncontrolled pH for Au/CeO2ZrO2(u) catalysts. Reaction mixture composition: 2% CO, 2% O2, and 50% H2, with helium as a balance.
production are obtained. Hereafter, only the catalyst with the controlled pH will be discussed because of its high activity for CO oxidation. Fig. 2 shows the effect of the pre-treatments on the activity and selectivity of the Au/CeO2-ZrO2(c) catalyst. It is noteworthy that both oxidative and reducing treatments led to a similar catalytic behaviour; the light-off curves in both cases began at 50 C, and the conversion increased with the temperature until a maximum was obtained at 130 C. Thereafter, a slight decrease was observed as the temperature of the reactor increased. Additionally, it can be seen from the figure that the oxidative treatment led to a slightly better selectivity
Results and discussion Catalytic activity Fig. 1 shows the effect of the preparation method on the catalytic activity for Au/CeO2-ZrO2 catalysts with an O2/CO ratio equal to one. The catalyst prepared with controlled pH showed the best catalytic performance, whereas the Au/CeO2ZrO2(u) catalysts was practically inactive. The conversion obtained with the Au/CeO2-ZrO2(c) catalyst was more than 95% between 100 C and 150 C. This temperature range is the most suitable temperature for operating fuel cells. At higher temperatures, a continuous decrease in activity and selectivity was observed, indicating the activation of the H2 oxidation reaction, which competes with the oxidation of CO [4], because the dissociated hydrogen atoms are oxidized (OH formation), which lessens the CO conversion [26]. On the other hand, the results of Fig. 1 are in agreement with literature reports (e.g., [27]), with regard that the preparation method significantly influences the activity of deposited gold particles, as more dispersed are the gold particles (as seen in characterisation section) the higher reducibility of ceria surface [28] and therefore an enhanced catalytic activity towards CO2
Fig. 2 e Effect of pre-treatment in either an oxidizing or reducing atmosphere on the CO conversion (filled symbols) and selectivity (empty symbols) for the Au/CeO2-ZrO2(c) catalyst. Reaction mixture composition: 2% CO, 2% O2, and 50% H2, with helium as a balance.
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of the catalyst. This result contrasted with some reports in which the reducing treatment led to a better activity of gold supported on Fe-HMS (e.g., [29]), but was in agreement with other reports in which Au/TiO2 [18] and Au/La-CeOx [22] were used, likely due to the differences in the support. For Au/MnOx catalysts, the enrichment of the Au/MnOx surface with oxygen after treatment in the presence of He was observed, but the oxygen was decreased in the unsupported manganese oxide [30]. This enrichment was attributed to the Au-MnOx interaction and could explain the enhanced activity of some of the catalysts; however, it is expected that there is a fast deactivation due to the oxygen depletion in the catalysts with the time on the stream [29]. In the present case, the ability of CeO2 to exchange oxygen with the reaction environment could eliminate the need for a reservoir of oxygen to efficiently carry out CO oxidation.
Effects of O2, CO2 and water vapour upon activity and selectivity Fig. 3 shows the effect on CO conversion and the selectivity toward CO oxidation as a function of the O2 concentration in the feed for the Au/CeO2-ZrO2(c) catalyst. As expected, the CO conversion and selectivity strongly depend on the O2 concentration, and the catalytic activity increases with an increase of the amount of oxygen in the reaction stream, but at the same time, this reduces the selectivity toward the oxidation of CO. The same behaviour with different types of catalysts has been observed (e.g., [31]). From the results of the reaction at 100 C, we observed that when using an O2/CO ratio of 0.45, 0.65 and 0.9, the maximum CO conversion (and selectivity) was ~ 72% (88%), 92% (72%), and 98% (60%), respectively. From the analysis of the reaction data, it seems difficult to choose the best O2/CO ratio because there is a high degree of compromise between the elimination of CO from the stream and maintaining the H2 untouched. However, in our
Fig. 3 e Effect of the amount of O2 on CO conversion (solid line, filled symbols) and selectivity (dashed line, empty symbols) versus temperature for the Au-CeO2-ZrO2(c) catalyst. Reactant composition: 2% CO, 0.5e2% O2, 50% H2 and helium.
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case, the O2/CO ratio of 0.65 seems to be the best ration for accomplishing both goals. The effect of CO2 and the presence of water vapour in the reaction feed stream on the activity and selectivity of the Au/ CeO2-ZrO2(c) catalyst are shown in Fig. 4. The reaction feed mixture was humidified by bubbling it through a container of deionized water at room temperature, yielding 2% water vapour in the gas fed to the reactor. As seen in the figure, at lower temperatures, the effect on the conversion of CO was marked due to the competitive adsorption of the compounds by the active sites. Furthermore, at 100 C (the temperature of the highest activity), the addition of 2% CO2 to the reaction feed stream slightly diminished the conversion of CO, from ~ 98% to ~ 95%, whereas the presence of both 2% CO2 and 2% water vapour further decreased the conversion from 98% to 91%, which suggests a cumulative inhibition effect promoted by the competitive adsorption on the active sites of CO2, H2O and CO (or H2) during the reaction [32]. From the results, it was observed that the presence of CO2 and water did not significantly change the performance of the catalyst at T > 100 C because the inhibition of the reaction by the adsorption of these compounds on the active sites is minimized by thermal effects (the desorption of these non-reactive compounds is favoured), although the long-term effects could not be discarded because there is the possibility forming carbonate groups on the catalytic surface [31].
Long-term activity With this in mind, the Au/CeO2-ZrO2(c) catalyst was subjected to oxidative pre-treatment before the catalytic stability test because it was shown to promote the best catalytic performance. The catalytic stability at 100 C was tested over time using the same gas compositions as those shown in Fig. 4. As seen in Fig. 5, the CO conversion and selectivity were
Fig. 4 e Effect of CO2 and water vapour on the CO conversion (solid line, filled symbols) and selectivity (dashed line, empty symbols) for the Au-CeO2-ZrO2(c) catalyst. Reactant composition: 2% CO, O2/CO ratio of 0.65, 0e2% H2O, 0e2% CO2, 50% H2 and helium.
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Table 1 e Specific surface area of the catalyst and percentage of gold loaded. Catalyst
BET surface area (m2/g)
Au loading (wt %)
Au loadeda (%)a
CeO2-ZrO2 Au/CeO2ZrO2(c)b Au/CeO2ZrO2(u)c
55 31
e 2.1
e 100
19
1.2
60
a
Au loaded (%): percentage of gold in the solution deposited on Au/ CeO2-ZrO2. Catalyst prepared via bcontrolled pH and cuncontrolled pH.
Fig. 5 e CO conversion (solid line) and selectivity (dashed line) versus time on stream for the Au-CeO2-ZrO2(c) catalyst as a function of the reactor feed composition. The reactor feed contained 2% CO, O2/CO ratio of 0.65, 0e2% CO2, 0e2% H2O, 50% H2, and helium. maintained for approximately 2 days, even in the presence of CO2 and water. The presence of these compounds only slightly decreased the activity and selectivity because of the competitive adsorption of water and CO on the active sites, but the catalyst showed good stability with regard to the time on stream. Luengnaruemitchai et al. [22] observed a decrease in the CO conversion and a slightly increase in the PROX selectivity when the reaction mixture contained 10% v/v of both CO and H2O. In our case only a decrease of conversion and selectivity was observed, likely by the difference in the concentration of these compounds which was not enough to promote the competition by the reaction sites between the H2 and H2O. However, our results demonstrate that this catalyst has good resistance toward the formation of carbonate compounds compared with other catalysts (e.g. [29], [33]). The ability of ceria to store oxygen and exchange it with the reaction atmosphere could be the main cause of the high stability of this type of catalyst as well its capacity to efficiently perform CO oxidation independently of the treatment applied to activate the catalyst.
gold loaded in these catalysts was only 60%. These results are explained in part because gold is deposited in small nanoparticles, preferentially at a pH of 6e7 [35], and these small particles do not completely occlude the pores of the support. When the pH is uncontrolled during gold deposition, it is expected that the size of the generated gold particles will be greater, blocking the pores. The low efficiency of the loaded gold may be due to the dependence on the pH of the solution, of which the equilibrium does not favour the deposition of the hydroxide species [25]. It is known that deposition of Au nanoparticles on the support is function of the interaction between the charge of the support surface and the gold species in the solution. It has been reported that CeO2-ZrO2 has a PZC of 4.04e7.20, which depends on the synthesis method employed and the Zr content in the material [36]. Furthermore, the speciation of gold in solution depends of the pH; at pH values above 6 the predominating species are ½AuCl2 ðOHÞ2 and ½AuCl3 ðOHÞ , whereas below of pH 6 the ½AuCl4 species predominate [36e38]. When the catalyst was synthesized under controlled pH (6), the deposition of gold species on the support surface was total (100% of Au loading, see Table 1) indicating that the electrostatic attraction between ½AuCln ðOHÞ4n (n ¼ 1e3) species and the support surface was favoured. In the case of the gold deposition at uncontrolled pH, only 60% of the intended gold was loaded, since the acid gold solution causes a low hydrolysis of ½AuCl4 , which have a low adsorption constant [37]. These reaction conditions led to a stronger electrostatic repulsion between the support surface and gold species that caused the low gold deposition observed [36].
Chemical and textural analysis
XRD
Table 1 shows the measured BET surface areas for the support and catalysts as well as the gold percentage loaded for both the pH-controlled and uncontrolled catalysts. The BET surface area for CeO2-ZrO2 was in accordance with those reported in the literature when the solegel method is employed for the synthesis of this support [34]. The gold loaded percentage was defined as the percentage ratio of the gold deposited on the catalyst and contained in the chloroauric acid solution. In the Table 1 is shown that the deposition of gold significantly decreased the specific surface area of the catalysts. However, the Au/CeO2-ZrO2(u) catalysts (synthesized by the uncontrolled pH method) showed the lowest surface area, and the
Fig. 6 shows the XRD patterns for the only-calcined Au/CeO2ZrO2 catalysts. It is not expected that the size of Au nanoparticles shown significant changes with treatments such as suggests the results of the reaction tests (see Fig. 2), and in agreement with the reported for Au-Zeolite catalysts [39]. The samples clearly showed the reflections corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) planes of the Ce0.75Zr0.25O2 fluorite cubic structure (JCPDS Card No. 28-0271). This structure is known to be preferred with respect to the tetragonal structure at the Ce/Zr ratio used in this work. In addition, no segregation phases of CeO2 or ZrO2 were detected. This suggests that ZrO2 is completely incorporated into the
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Fig. 6 e XRD Patterns of fresh solids: (a) CeO2-ZrO2 support, (b) Au/CeO2-ZrO2(u) and (c) Au/CeO2-ZrO2(c).
Fig. 7 e H2-TPR profiles of (a) CeO2-ZrO2 support, (b) Au/ CeO2-ZrO2(u) and (c) Au/CeO2-ZrO2(c).
CeO2 lattice to form a homogeneous solid solution [9]. Additionally, it can be seen from the XRD patterns that the Au/ CeO2-ZrO2(u) catalyst showed peaks for metallic gold at 2q ~ 38, 45 and 65 , whereas for Au/CeO2-ZrO2(c), only the pattern of the support was observed. Although the observed peaks are very small, it can be presumed that the particle size of gold was bigger when the pH in the gold deposition was not controlled, supporting the data found for the specific surface area measurements.
peaks were observed in the temperature range of 70e250 C, whereas the main peak became small and shifted to low temperature, even more than in Au/CeO2-ZrO2(u) catalysts. Additionally, a broad reduction peak at high temperature was observed. As reported by Fonseca et al. [3], the peaks at low temperature could be assigned, in part, to the reduction of oxygen species from the small gold particles and the ceria at the interface of some of the gold nanoparticles. Giordano et al. [41] performed an analysis of ceria reduction and found that the diffusion of oxygen from the bulk is faster compared with the kinetics of the reduction, as the process is controlled or limited by the kinetic and thermodynamic constraints. Therefore, the observation of low temperature peaks in the H2-TPR pattern for Au/CeO2-ZrO2(c) catalysts could correspond to a kinetic effect, in which the gold particles accelerates the reduction of the surface oxygen. We believe that the multiplicity of the peaks could be explained by gold nanoparticles of different sizes, which enhanced in dissimilar manner the reduction of oxygen of ceria either on the available surface or in bulk.
H2-TPR The reduction profile of pure CeO2 is characterized by two widely reported peaks (not shown) [7,13,31], one at low temperature centred at 503 C, which is generally attributed to the reduction of the surface layer of CeO2, and another small peak at a higher temperature centred at approximately 752 C, which is attributed to the reduction of the material in the bulk. In the case of pure ZrO2, the H2-TPR pattern did not show any evident reduction peak. Furthermore, for the studied support CeO2-ZrO2, the reduction peak at a higher temperature was not present (Fig. 7), indicating that in the presence of ZrO2, the bulk reduction of CeO2 does not occur easily, most likely because there is a strong interaction between CeO2 and ZrO2, which formed a solid solution, as suggested by Biswas and Kunzru [13]. Fig. 7 shows that when gold was deposited in the CeO2ZrO2 support, evident changes in the H2-TPR patterns arose. It can be seen that for Au/CeO2-ZrO2(u) catalysts, the presence of gold enhances the reduction of CeO2, the main peak showed a shift to a lower temperature, and there was an increase in the peak area, indicating an enhanced reduction of the surface oxygen of ceria [40]. Additionally, the peak at high temperature (732 C) was present, which suggests that gold particles promoted oxygen diffusion from the oxide framework (bulk), and another peak arose at 256 C, which was attributed to the withdrawal of oxygen from the interface of gold nanoparticles and ceria. For Au/CeO2-ZrO2(c), four hydrogen consumption
DRS characterisation The diffuse reflectance UVevis spectra of the synthesized Au/ CeO2-ZrO2(c), Au/CeO2-ZrO2(u) and CeO2-ZrO2 support are shown in Fig. 8. As seen in this figure, the support show two bands centred at approximately 269 and 389 nm, respectively, which are frequently associated with the presence of the Ce4þ ions in an 8-fold coordination of the fluorite structure [42,43]. However, the width of this band suggests that the ceria ions could have more than a single oxidation state, possibly because of the presence of ZrO2. On the other hand, with regard the absorption bands of the gold species, there are differences from the literature regarding the location of the maximum values of the bands [29,43], which are likely due to the different supports used or to the presence of catalytic promoters; however, there is a broad consensus regarding the range in which the bands for the gold species are found. The
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Fig. 8 e UVeVis spectra of the calcined-only solids.
common assignation for the gold bands is as follows: (i) the plasmon resonance of metallic gold particles at 520e560 nm; (ii) partially charged gold clusters (Audþ) at 300e390 nm; and (iii) gold cations (Auþ, Au3þ) at 260e295 nm [29]. The spectra of both Au/CeO2-ZrO2 catalysts presented a broad band between 220 and 420 nm that features partially charged gold clusters and the gold cations as well as the band associated with the plasmon resonance between 500 and 700 nm. As observed, the Au/CeO2-ZrO2(u) catalyst showed the best defined plasmon resonance band, suggesting that this catalyst has the largest gold particles compared with the Au/ CeO2-ZrO2(c), whereas for the last catalyst, the broad plasmon resonance band suggests a multiplicity of particle sizes and its low intensity suggests a small particle size [44]. To observe the effect upon the gold species in the active catalyst Au/CeO2-ZrO2(c), it was heated in situ under a reducing or oxidizing atmosphere. Fig. 9a shows the UVevis spectra as a function of the temperature under oxidizing conditions. A marked change in the absorption bands at temperatures above 100 C was observed, with the intensity of the plasmon band corresponding to metallic species increased, and all of the bands for the gold species overlapped. This behaviour suggests that the gold particles rearrange under the surface of the support by forming metallic particles of a larger size because the light can no longer polarize the nanoparticles homogeneously, resulting in shifts to and broadening of the surface plasmon in the spectrum; frequently, a red shift is observed as the gold particles increase [45]. The other possibility to explain the broadening of the plasmon resonance band is that the gold particles acquire a different shape [46] or a different side length [47] while they grow, which could be promoted by the movement of oxygen atoms in the ceria lattice during the oxidizing treatment. Additionally, at the same time that metallic particles increase in size, the amount of partially charged gold clusters seems to increase, most likely due to the re-arrangement of gold atoms over the ceria vacancies, which stimulates the reintegration of Au atoms to the nearest Au clusters [48], leaving some particles partially deficient in charge during these movements.
Fig. 9 e UVeVis spectra of Au/CeO2-ZrO2(c) as function of in-situ treatments, (a) Oxidising atmosphere; (b) Reducing atmosphere.
After the oxidation treatment was finished, the reaction chamber was cooled to room temperature, and then, the gas stream was changed to the reducing atmosphere (5% H2 balance Ar). Then, the reaction chamber was heated gradually. The spectra in Fig. 9b show that the main change was because of the best resolution of the bands assigned to partially charged gold clusters (392 nm) and the plasmon resonance of metallic gold particles (563 nm), where the latter was sharpened compared with the oxidation conditions. This suggests that the dispersion of gold particles under a reducing atmosphere is not promoted, and this causes the particle size to become more defined. Moreover, the bands showed no obvious changes in intensity, suggesting that the narrowness of the bands (compared with the oxidizing treatment) could be attributed to a partial sintering of gold, which appears to be promoted by gold atoms in the neighbourhood of the metal particles, likely at the expense of partial charge clusters (Audþ), as seen from the blue shift of the maximum of this band. Furthermore, the temperature range at which the main changes in the spectra were observed correspond to the range at which the catalyst displays a higher CO conversion, and beyond such a temperature, the conversion decreases continuously. There is a broad consensus that the high activity of gold catalysts depends strongly on the choice of an appropriate support [49] and that the gold-support interaction could thus determine the activity of gold species in the catalysts or whether they contribute to the catalyst activity. For example, it was claimed in some studies that for Au/TiO2 catalysts, the gold species responsible for achieving a high CO conversion in oxidation reactions are metallic gold nanoparticles [18,20], whereas other studies suggests that the superior CO oxidation activity could be ascribed to the presence of nano-sized metallic gold particles plus Au3þ ions, and the titania provides the active sites for CO and oxygen dissociation [50]. In the case of other catalysts, such as Au/Fe-HMS, Au/CeO2/Al2O3 and Au/CeO2, the cationic gold species are proposed to be responsible for the high activity [19,29,40,51].
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The results obtained in this work suggest that metallic particles and cationic species in a specific ratio could be responsible for the high activity shown by this type of catalyst and that an imbalance could change the activity and therein the selectivity of the reaction. In the case of Au/CeO2-ZrO2(u) catalysts, the gold metallic nanoparticles were present, but poor activity of CO oxidation was shown.
Conclusions The catalytic activity and selectivity of Au/CeO2-ZrO2 catalysts prepared using a controlled and uncontrolled pH for gold incorporation were examined. It was found that the catalyst prepared with strict pH control was the most active, exhibiting high activity and good selectivity at 100 C even in the presence of CO2 and water vapour in the reaction feed stream. Additionally, this catalyst showed good long-term stability in presence of CO2 and moisture. From the characterisation of the catalysts, it was found that controlling the pH to a value of 6 promoted the activity of the gold particles for CO oxidation, whereas when the pH was not controlled, less gold was deposited and the gold species were almost inactive. The active and inactive gold species were clearly noted in the H2TPR patterns because only the active gold particles improved oxygen removal from the support at low temperature. The UVevis analysis showed a broadening of the bands when the active catalysts were subjected to an oxidizing treatment, suggesting a diversity of the sizes of the metallic gold particles, with a larger size or different shape of particles, whereas with the reducing treatments, the size and shape of the gold particles became more defined as the temperature of the controlled reaction chamber increased. This seems to occur at the expense of partially charged gold clusters (Audþ).
Acknowledgements The authors are grateful to the Universidad Nacional de and the UANL (PAICYT project IT-641Colombia sede Bogota 11) for the financial support for this work. The authors are also grateful to Professor Gustavo Fuentes from the UAM-I (Mexico) for their collaboration in the UVevis characterisation.
references
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Preferential oxidation of CO in excess of hydrogen over Au/CeO2-ZrO2 catalysts rdoba a, Angel Martı´nez-Herna ndez b,* Luis Fernando Co Universidad Nacional de Colombia, Departamento de Ingenierı´a Quı´mica y Ambiental, Grupo de Investigacion en , Colombia Procesos Quı´micos y Bioquı´micos, Cra. 30 calle 45, Bogota b s de Universidad Aut onoma de Nuevo Leon, UANL, Facultad de Ciencias Quı´micas, Av. Universidad S/N, San Nicola los Garza, Ciudad Universitaria, C.P. 66451, N.L, Mexico a
article info
abstract
Article history:
The preferential oxidation of CO to CO2 in a simulated reformed gas using Au/-CeO2-ZrO2
Received 25 July 2015
catalysts was examined. The studied catalyst showed high catalytic activity and stability
Received in revised form
during long-term tests even in presence of CO2 and water vapour, which inhibited the
15 September 2015
conversion of CO to a small extent. The catalyst preparation at controlled pH (6) favours the
Accepted 18 September 2015
high dispersion of deposited gold particles, while the catalytic activity and selectivity was
Available online 24 October 2015
correlated with the size and oxidation state of gold particles. Furthermore, it was observed that oxidizing treatment resulted in the formation of gold particles having different shapes
Keywords:
and/or side length, which was evidenced by a broadening of the plasmon resonance band
Hydrogen purification
(From DRS UVevis).
Gold catalyst
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
PROX-CO Catalytic stability Metallic Au nanoparticles
Introduction Supply, transformation and energy consumption [1] affect the development of global societies as the world population and its energy demands are increasing. These factors should be managed so that the use and production of energy are sustainable, without compromising the natural resources that should be available for future generations as well as to prevent pollution. In this way, it is necessary to find sustainable energy sources that can achieve the goals of environmentally friendly technologies. Hydrogen is considered to be an important alternative that contributes to the diminution of the environmental problems resulting from electrical power production because it has
great potential as an environmentally clean fuel and also has high energetic efficiency [2]. In this sense, the most promising technologies for obtaining energy from H2 are based on devices, such as proton exchange membrane (PEM) fuel cells. These fuel cells have attracted much attention due their good fuel efficiency and cleanliness when they are used in automobiles [3], along with their low operating temperature, fast cold start, good tolerance by the electrolyte to the CO2 adsorption and combination of high power density and high energy conversion efficiency. Pure hydrogen is the ideal fuel for PEM fuel cells because it maximizes their efficiency, lead to zero emissions. However, until now, there has not been a safe method for storing an adequate amount of hydrogen in automobiles. To overcome this problem, a potential alternative is the implementation of
* Corresponding author. Tel.: þ52 81 8329 4000x3454. ndez). E-mail address: [email protected] (A. Martı´nez-Herna http://dx.doi.org/10.1016/j.ijhydene.2015.09.133 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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an on-board hydrogen generation system that can convert methanol steam or partially oxidize liquid hydrocarbons followed by a water gas shift reaction [4]. These processes performed in series could supply sufficient H2 to the fuel cell to power an on-board electric motor in a vehicle, increasing the autonomy of the transport and leading to the commercial implementation of this technology. Typically, the obtained hydrogen-rich gas in the steam reforming reaction even after the water-gas shift reactor, contain 1e2% v/v of CO [5e7]. The former compound downgrades the energy efficiency of the PEM fuel cell because its electrodes are typically made of platinum, which is very sensitive to CO because it is strongly adsorbed and poisons the electrode surface. There are several methods reported for carbon monoxide removal from the steam reforming stream, such as the preferential oxidation of CO (CO-PROX), selective methanation of CO, and selective diffusion through membranes [8e10]. Among these, CO-PROX is the method with the highest efficiency and economy [11]. CeO2 has been broadly reported to be an active support for the synthesis of oxidation catalysts due to its structural properties [3]. This oxide has redox behaviour that allows high oxygen mobility, enhancing oxygen exchange with the reaction environment [12]. Additionally, it has been found that the addition of ZrO2 improves the redox properties of CeO2 and that the oxygen mobility can be improved by the formation of a mixed system with a strong interaction, preferably as a solid solution to promote electronic distortions in the oxide framework. These oxides contribute to the homogeneous dispersion of the deposited metal and long-term stability of the catalyst and increase the resistance to high temperatures by decreasing the sintering problems on the catalyst surface [13e16]. On the other hand, gold-supported catalysts are more active and selective with regard to the conversion of CO at low temperatures among the catalysts that use noble metals [18] showed that gold nanoparticles [12,17]. Haruta and Date with diameters between 2 and 4 nm were very effective for CO oxidation. Several factors, such as the size of the gold particles, their oxidation state [19,20] and the nature of the support [21,22], affect the catalytic activity of gold-based catalysts for the CO-PROX reaction in rich H2 streams. The latter study noted the relevance of the choice of the appropriate method for the synthesis of catalysts because it is crucial to control all of these factors. Park et al. [23] Carry out a broad screening of gold-based catalytic systems used in selective CO removal in a H2-rich stream, where they demonstrate the exceptional catalytic activity of these catalysts in the CO-PROX at low temperature. Nevertheless, they report that selectivity toward CO oxidation decreased rapidly with increasing reaction temperature, because in such conditions the H2 oxidation is activated. Then, it is important to study the selectivity of CO-PROX reaction over gold-based catalysts with special attention in to minimize the H2 oxidation. The aim of this work was to study the purification of a H2 rich stream from CO by performing the preferential CO oxidation reaction over synthesize gold catalysts using CeO2ZrO2 as support. The PROX reaction results have been discussed in function of catalyst properties.
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Experimental procedure Catalyst preparation The fluorite (CeO2-ZrO2) support was synthesized by a pseudo solegel method based on the thermal decomposition of propionate precursors [24]. The starting materials were cerium (III) acetate (SigmaeAldrich) and zirconium (IV) acetylacetonate (SigmaeAldrich), and these starting salts were individually dissolved in boiling propionic acid at a concentration of 0.12 mol L1 at 100 C for 1 h. This procedure exclusively formed the propionate precursors of the metals. The precursors were mixed while still boiling, and the resulting solution was kept boiling for 2 h under reflux. The solvent was then evaporated, eventually producing a resin. Finally, the resin was calcined under static air at 600 C for 6 h (a heating ramp of 2 C min1 was used). To load gold into the support, we used the procedure reported by Lin and Wan [25]. Briefly, it consisted of the following: a suitable amount of chloroauric acid (HAuCl4$3H2O, SigmaeAldrich) was dissolved in 250 ml of deionized water while being stirred, and the solution pH was adjusted to a value of 6 by adding a sodium hydroxide solution (0.1 M). Then, 2 g of the CeO2-ZrO2 support was added, and the resultant slurry was heated under stirring for up to 80 C, maintaining this condition for 2 h. After that time, the prepared catalysts were filtered and washed with enough deionized water to ensure chloride removal from the solid. The resulting gold sample was dried at 60 C for 12 h, and the catalyst was labelled as Au/CeO2-ZrO2(c). For comparison purposes, a second catalyst was prepared using the same procedure but with uncontrolled pH; this catalyst was labelled Au/CeO2-ZrO2(u).
Catalyst characterisation The gold loading of the fresh catalysts was determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, Perkin Elmer Optima 3300DV). Powder X-ray diffraction (XRD, Siemens D-5000) patterns were used to obtain information about the structure and composition of crystalline materials. A specific surface area measurement was made by N2 adsorption isotherms (Micromeritics, Asap 2020). UVevis diffuse reflectance spectra (Varian, Cary 5000) of the catalysts were recorded using a Praying Mantis accessory equipped with a high temperature reaction chamber (Harrick Scientific) attached to the spectrophotometer. Temperature programed reduction with hydrogen (H2-TPR) experiments were performed with homemade equipment. Briefly, a sample of 50 mg of catalyst was reduced by flowing 25 cm3/min of a mixture consisting of 5% H2 in an Ar balance through the reactor; the temperature of the reactor was raised from room temperature to 900 C at a heating rate of 10 C/min. Hydrogen consumption was measured by a thermal conductivity detector (TCD).
Activity measurement The CO-PROX reaction was carried out in a fixed-bed quartz micro-reactor using a total flow of 100 cm3/min and 100 mg of
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catalyst. The reactant gas typically consisted of 2% CO, 1e2% O2, 2% CO2, 2% H2O, and 50% H2 in a balance of helium. The content of CO2 and H2O in the reaction stream was limited to 2% instead of 10e20% v/v (real content) due to experimental restrictions of our reaction equipment. However, this amount was enough for to observe the kinetic inhibition effect (competitive adsorption) of these compounds upon the COPROX reaction. The individual gas streams were controlled by electronic mass flow controllers to achieve the desired composition. The reactants and products were analysed online by gas chromatography (Agilent 6820 equipped with a thermal conductivity detector and a HP Molesieve capillary column). The reactor temperature was measured by a K-type thermocouple inside of a quartz thermo-well placed in the middle of the catalyst bed. Before the reaction, the catalysts were pre-treated under an oxidative (air) environment at 300 C for 1 h or a reducing atmosphere (10% of H2 in balance He) at 100 C for 1 h to evaluate the influence of pretreatments on the catalytic activity of Au/CeO2-ZrO2 catalysts. The carbon monoxide conversion and oxidation selectivity to CO2 were calculated using the relationships expressed in Eqs. (1) and (2), respectively. XCO ¼
SCO ¼
out Fin CO FCO 100 Fin CO
(1)
Fin Fout CO 100 CO in 2 FO2 Fout O2
(2)
where X in Eq. (1) represents the percentage of CO conversion and FCO is the molar flow of CO at the inlet or outlet of the reactor. In Eq. (2), SCO represents the selectivity of the percentage of CO that reacts with O2, exclusively forming CO2. There was no methane formation under the reaction conditions used in this study.
Fig. 1 e CO conversion versus temperature. Effect of the controlled pH on gold deposition for Au/CeO2ZrO2(c) and uncontrolled pH for Au/CeO2ZrO2(u) catalysts. Reaction mixture composition: 2% CO, 2% O2, and 50% H2, with helium as a balance.
production are obtained. Hereafter, only the catalyst with the controlled pH will be discussed because of its high activity for CO oxidation. Fig. 2 shows the effect of the pre-treatments on the activity and selectivity of the Au/CeO2-ZrO2(c) catalyst. It is noteworthy that both oxidative and reducing treatments led to a similar catalytic behaviour; the light-off curves in both cases began at 50 C, and the conversion increased with the temperature until a maximum was obtained at 130 C. Thereafter, a slight decrease was observed as the temperature of the reactor increased. Additionally, it can be seen from the figure that the oxidative treatment led to a slightly better selectivity
Results and discussion Catalytic activity Fig. 1 shows the effect of the preparation method on the catalytic activity for Au/CeO2-ZrO2 catalysts with an O2/CO ratio equal to one. The catalyst prepared with controlled pH showed the best catalytic performance, whereas the Au/CeO2ZrO2(u) catalysts was practically inactive. The conversion obtained with the Au/CeO2-ZrO2(c) catalyst was more than 95% between 100 C and 150 C. This temperature range is the most suitable temperature for operating fuel cells. At higher temperatures, a continuous decrease in activity and selectivity was observed, indicating the activation of the H2 oxidation reaction, which competes with the oxidation of CO [4], because the dissociated hydrogen atoms are oxidized (OH formation), which lessens the CO conversion [26]. On the other hand, the results of Fig. 1 are in agreement with literature reports (e.g., [27]), with regard that the preparation method significantly influences the activity of deposited gold particles, as more dispersed are the gold particles (as seen in characterisation section) the higher reducibility of ceria surface [28] and therefore an enhanced catalytic activity towards CO2
Fig. 2 e Effect of pre-treatment in either an oxidizing or reducing atmosphere on the CO conversion (filled symbols) and selectivity (empty symbols) for the Au/CeO2-ZrO2(c) catalyst. Reaction mixture composition: 2% CO, 2% O2, and 50% H2, with helium as a balance.
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of the catalyst. This result contrasted with some reports in which the reducing treatment led to a better activity of gold supported on Fe-HMS (e.g., [29]), but was in agreement with other reports in which Au/TiO2 [18] and Au/La-CeOx [22] were used, likely due to the differences in the support. For Au/MnOx catalysts, the enrichment of the Au/MnOx surface with oxygen after treatment in the presence of He was observed, but the oxygen was decreased in the unsupported manganese oxide [30]. This enrichment was attributed to the Au-MnOx interaction and could explain the enhanced activity of some of the catalysts; however, it is expected that there is a fast deactivation due to the oxygen depletion in the catalysts with the time on the stream [29]. In the present case, the ability of CeO2 to exchange oxygen with the reaction environment could eliminate the need for a reservoir of oxygen to efficiently carry out CO oxidation.
Effects of O2, CO2 and water vapour upon activity and selectivity Fig. 3 shows the effect on CO conversion and the selectivity toward CO oxidation as a function of the O2 concentration in the feed for the Au/CeO2-ZrO2(c) catalyst. As expected, the CO conversion and selectivity strongly depend on the O2 concentration, and the catalytic activity increases with an increase of the amount of oxygen in the reaction stream, but at the same time, this reduces the selectivity toward the oxidation of CO. The same behaviour with different types of catalysts has been observed (e.g., [31]). From the results of the reaction at 100 C, we observed that when using an O2/CO ratio of 0.45, 0.65 and 0.9, the maximum CO conversion (and selectivity) was ~ 72% (88%), 92% (72%), and 98% (60%), respectively. From the analysis of the reaction data, it seems difficult to choose the best O2/CO ratio because there is a high degree of compromise between the elimination of CO from the stream and maintaining the H2 untouched. However, in our
Fig. 3 e Effect of the amount of O2 on CO conversion (solid line, filled symbols) and selectivity (dashed line, empty symbols) versus temperature for the Au-CeO2-ZrO2(c) catalyst. Reactant composition: 2% CO, 0.5e2% O2, 50% H2 and helium.
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case, the O2/CO ratio of 0.65 seems to be the best ration for accomplishing both goals. The effect of CO2 and the presence of water vapour in the reaction feed stream on the activity and selectivity of the Au/ CeO2-ZrO2(c) catalyst are shown in Fig. 4. The reaction feed mixture was humidified by bubbling it through a container of deionized water at room temperature, yielding 2% water vapour in the gas fed to the reactor. As seen in the figure, at lower temperatures, the effect on the conversion of CO was marked due to the competitive adsorption of the compounds by the active sites. Furthermore, at 100 C (the temperature of the highest activity), the addition of 2% CO2 to the reaction feed stream slightly diminished the conversion of CO, from ~ 98% to ~ 95%, whereas the presence of both 2% CO2 and 2% water vapour further decreased the conversion from 98% to 91%, which suggests a cumulative inhibition effect promoted by the competitive adsorption on the active sites of CO2, H2O and CO (or H2) during the reaction [32]. From the results, it was observed that the presence of CO2 and water did not significantly change the performance of the catalyst at T > 100 C because the inhibition of the reaction by the adsorption of these compounds on the active sites is minimized by thermal effects (the desorption of these non-reactive compounds is favoured), although the long-term effects could not be discarded because there is the possibility forming carbonate groups on the catalytic surface [31].
Long-term activity With this in mind, the Au/CeO2-ZrO2(c) catalyst was subjected to oxidative pre-treatment before the catalytic stability test because it was shown to promote the best catalytic performance. The catalytic stability at 100 C was tested over time using the same gas compositions as those shown in Fig. 4. As seen in Fig. 5, the CO conversion and selectivity were
Fig. 4 e Effect of CO2 and water vapour on the CO conversion (solid line, filled symbols) and selectivity (dashed line, empty symbols) for the Au-CeO2-ZrO2(c) catalyst. Reactant composition: 2% CO, O2/CO ratio of 0.65, 0e2% H2O, 0e2% CO2, 50% H2 and helium.
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Table 1 e Specific surface area of the catalyst and percentage of gold loaded. Catalyst
BET surface area (m2/g)
Au loading (wt %)
Au loadeda (%)a
CeO2-ZrO2 Au/CeO2ZrO2(c)b Au/CeO2ZrO2(u)c
55 31
e 2.1
e 100
19
1.2
60
a
Au loaded (%): percentage of gold in the solution deposited on Au/ CeO2-ZrO2. Catalyst prepared via bcontrolled pH and cuncontrolled pH.
Fig. 5 e CO conversion (solid line) and selectivity (dashed line) versus time on stream for the Au-CeO2-ZrO2(c) catalyst as a function of the reactor feed composition. The reactor feed contained 2% CO, O2/CO ratio of 0.65, 0e2% CO2, 0e2% H2O, 50% H2, and helium. maintained for approximately 2 days, even in the presence of CO2 and water. The presence of these compounds only slightly decreased the activity and selectivity because of the competitive adsorption of water and CO on the active sites, but the catalyst showed good stability with regard to the time on stream. Luengnaruemitchai et al. [22] observed a decrease in the CO conversion and a slightly increase in the PROX selectivity when the reaction mixture contained 10% v/v of both CO and H2O. In our case only a decrease of conversion and selectivity was observed, likely by the difference in the concentration of these compounds which was not enough to promote the competition by the reaction sites between the H2 and H2O. However, our results demonstrate that this catalyst has good resistance toward the formation of carbonate compounds compared with other catalysts (e.g. [29], [33]). The ability of ceria to store oxygen and exchange it with the reaction atmosphere could be the main cause of the high stability of this type of catalyst as well its capacity to efficiently perform CO oxidation independently of the treatment applied to activate the catalyst.
gold loaded in these catalysts was only 60%. These results are explained in part because gold is deposited in small nanoparticles, preferentially at a pH of 6e7 [35], and these small particles do not completely occlude the pores of the support. When the pH is uncontrolled during gold deposition, it is expected that the size of the generated gold particles will be greater, blocking the pores. The low efficiency of the loaded gold may be due to the dependence on the pH of the solution, of which the equilibrium does not favour the deposition of the hydroxide species [25]. It is known that deposition of Au nanoparticles on the support is function of the interaction between the charge of the support surface and the gold species in the solution. It has been reported that CeO2-ZrO2 has a PZC of 4.04e7.20, which depends on the synthesis method employed and the Zr content in the material [36]. Furthermore, the speciation of gold in solution depends of the pH; at pH values above 6 the predominating species are ½AuCl2 ðOHÞ2 and ½AuCl3 ðOHÞ , whereas below of pH 6 the ½AuCl4 species predominate [36e38]. When the catalyst was synthesized under controlled pH (6), the deposition of gold species on the support surface was total (100% of Au loading, see Table 1) indicating that the electrostatic attraction between ½AuCln ðOHÞ4n (n ¼ 1e3) species and the support surface was favoured. In the case of the gold deposition at uncontrolled pH, only 60% of the intended gold was loaded, since the acid gold solution causes a low hydrolysis of ½AuCl4 , which have a low adsorption constant [37]. These reaction conditions led to a stronger electrostatic repulsion between the support surface and gold species that caused the low gold deposition observed [36].
Chemical and textural analysis
XRD
Table 1 shows the measured BET surface areas for the support and catalysts as well as the gold percentage loaded for both the pH-controlled and uncontrolled catalysts. The BET surface area for CeO2-ZrO2 was in accordance with those reported in the literature when the solegel method is employed for the synthesis of this support [34]. The gold loaded percentage was defined as the percentage ratio of the gold deposited on the catalyst and contained in the chloroauric acid solution. In the Table 1 is shown that the deposition of gold significantly decreased the specific surface area of the catalysts. However, the Au/CeO2-ZrO2(u) catalysts (synthesized by the uncontrolled pH method) showed the lowest surface area, and the
Fig. 6 shows the XRD patterns for the only-calcined Au/CeO2ZrO2 catalysts. It is not expected that the size of Au nanoparticles shown significant changes with treatments such as suggests the results of the reaction tests (see Fig. 2), and in agreement with the reported for Au-Zeolite catalysts [39]. The samples clearly showed the reflections corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) planes of the Ce0.75Zr0.25O2 fluorite cubic structure (JCPDS Card No. 28-0271). This structure is known to be preferred with respect to the tetragonal structure at the Ce/Zr ratio used in this work. In addition, no segregation phases of CeO2 or ZrO2 were detected. This suggests that ZrO2 is completely incorporated into the
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Fig. 6 e XRD Patterns of fresh solids: (a) CeO2-ZrO2 support, (b) Au/CeO2-ZrO2(u) and (c) Au/CeO2-ZrO2(c).
Fig. 7 e H2-TPR profiles of (a) CeO2-ZrO2 support, (b) Au/ CeO2-ZrO2(u) and (c) Au/CeO2-ZrO2(c).
CeO2 lattice to form a homogeneous solid solution [9]. Additionally, it can be seen from the XRD patterns that the Au/ CeO2-ZrO2(u) catalyst showed peaks for metallic gold at 2q ~ 38, 45 and 65 , whereas for Au/CeO2-ZrO2(c), only the pattern of the support was observed. Although the observed peaks are very small, it can be presumed that the particle size of gold was bigger when the pH in the gold deposition was not controlled, supporting the data found for the specific surface area measurements.
peaks were observed in the temperature range of 70e250 C, whereas the main peak became small and shifted to low temperature, even more than in Au/CeO2-ZrO2(u) catalysts. Additionally, a broad reduction peak at high temperature was observed. As reported by Fonseca et al. [3], the peaks at low temperature could be assigned, in part, to the reduction of oxygen species from the small gold particles and the ceria at the interface of some of the gold nanoparticles. Giordano et al. [41] performed an analysis of ceria reduction and found that the diffusion of oxygen from the bulk is faster compared with the kinetics of the reduction, as the process is controlled or limited by the kinetic and thermodynamic constraints. Therefore, the observation of low temperature peaks in the H2-TPR pattern for Au/CeO2-ZrO2(c) catalysts could correspond to a kinetic effect, in which the gold particles accelerates the reduction of the surface oxygen. We believe that the multiplicity of the peaks could be explained by gold nanoparticles of different sizes, which enhanced in dissimilar manner the reduction of oxygen of ceria either on the available surface or in bulk.
H2-TPR The reduction profile of pure CeO2 is characterized by two widely reported peaks (not shown) [7,13,31], one at low temperature centred at 503 C, which is generally attributed to the reduction of the surface layer of CeO2, and another small peak at a higher temperature centred at approximately 752 C, which is attributed to the reduction of the material in the bulk. In the case of pure ZrO2, the H2-TPR pattern did not show any evident reduction peak. Furthermore, for the studied support CeO2-ZrO2, the reduction peak at a higher temperature was not present (Fig. 7), indicating that in the presence of ZrO2, the bulk reduction of CeO2 does not occur easily, most likely because there is a strong interaction between CeO2 and ZrO2, which formed a solid solution, as suggested by Biswas and Kunzru [13]. Fig. 7 shows that when gold was deposited in the CeO2ZrO2 support, evident changes in the H2-TPR patterns arose. It can be seen that for Au/CeO2-ZrO2(u) catalysts, the presence of gold enhances the reduction of CeO2, the main peak showed a shift to a lower temperature, and there was an increase in the peak area, indicating an enhanced reduction of the surface oxygen of ceria [40]. Additionally, the peak at high temperature (732 C) was present, which suggests that gold particles promoted oxygen diffusion from the oxide framework (bulk), and another peak arose at 256 C, which was attributed to the withdrawal of oxygen from the interface of gold nanoparticles and ceria. For Au/CeO2-ZrO2(c), four hydrogen consumption
DRS characterisation The diffuse reflectance UVevis spectra of the synthesized Au/ CeO2-ZrO2(c), Au/CeO2-ZrO2(u) and CeO2-ZrO2 support are shown in Fig. 8. As seen in this figure, the support show two bands centred at approximately 269 and 389 nm, respectively, which are frequently associated with the presence of the Ce4þ ions in an 8-fold coordination of the fluorite structure [42,43]. However, the width of this band suggests that the ceria ions could have more than a single oxidation state, possibly because of the presence of ZrO2. On the other hand, with regard the absorption bands of the gold species, there are differences from the literature regarding the location of the maximum values of the bands [29,43], which are likely due to the different supports used or to the presence of catalytic promoters; however, there is a broad consensus regarding the range in which the bands for the gold species are found. The
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Fig. 8 e UVeVis spectra of the calcined-only solids.
common assignation for the gold bands is as follows: (i) the plasmon resonance of metallic gold particles at 520e560 nm; (ii) partially charged gold clusters (Audþ) at 300e390 nm; and (iii) gold cations (Auþ, Au3þ) at 260e295 nm [29]. The spectra of both Au/CeO2-ZrO2 catalysts presented a broad band between 220 and 420 nm that features partially charged gold clusters and the gold cations as well as the band associated with the plasmon resonance between 500 and 700 nm. As observed, the Au/CeO2-ZrO2(u) catalyst showed the best defined plasmon resonance band, suggesting that this catalyst has the largest gold particles compared with the Au/ CeO2-ZrO2(c), whereas for the last catalyst, the broad plasmon resonance band suggests a multiplicity of particle sizes and its low intensity suggests a small particle size [44]. To observe the effect upon the gold species in the active catalyst Au/CeO2-ZrO2(c), it was heated in situ under a reducing or oxidizing atmosphere. Fig. 9a shows the UVevis spectra as a function of the temperature under oxidizing conditions. A marked change in the absorption bands at temperatures above 100 C was observed, with the intensity of the plasmon band corresponding to metallic species increased, and all of the bands for the gold species overlapped. This behaviour suggests that the gold particles rearrange under the surface of the support by forming metallic particles of a larger size because the light can no longer polarize the nanoparticles homogeneously, resulting in shifts to and broadening of the surface plasmon in the spectrum; frequently, a red shift is observed as the gold particles increase [45]. The other possibility to explain the broadening of the plasmon resonance band is that the gold particles acquire a different shape [46] or a different side length [47] while they grow, which could be promoted by the movement of oxygen atoms in the ceria lattice during the oxidizing treatment. Additionally, at the same time that metallic particles increase in size, the amount of partially charged gold clusters seems to increase, most likely due to the re-arrangement of gold atoms over the ceria vacancies, which stimulates the reintegration of Au atoms to the nearest Au clusters [48], leaving some particles partially deficient in charge during these movements.
Fig. 9 e UVeVis spectra of Au/CeO2-ZrO2(c) as function of in-situ treatments, (a) Oxidising atmosphere; (b) Reducing atmosphere.
After the oxidation treatment was finished, the reaction chamber was cooled to room temperature, and then, the gas stream was changed to the reducing atmosphere (5% H2 balance Ar). Then, the reaction chamber was heated gradually. The spectra in Fig. 9b show that the main change was because of the best resolution of the bands assigned to partially charged gold clusters (392 nm) and the plasmon resonance of metallic gold particles (563 nm), where the latter was sharpened compared with the oxidation conditions. This suggests that the dispersion of gold particles under a reducing atmosphere is not promoted, and this causes the particle size to become more defined. Moreover, the bands showed no obvious changes in intensity, suggesting that the narrowness of the bands (compared with the oxidizing treatment) could be attributed to a partial sintering of gold, which appears to be promoted by gold atoms in the neighbourhood of the metal particles, likely at the expense of partial charge clusters (Audþ), as seen from the blue shift of the maximum of this band. Furthermore, the temperature range at which the main changes in the spectra were observed correspond to the range at which the catalyst displays a higher CO conversion, and beyond such a temperature, the conversion decreases continuously. There is a broad consensus that the high activity of gold catalysts depends strongly on the choice of an appropriate support [49] and that the gold-support interaction could thus determine the activity of gold species in the catalysts or whether they contribute to the catalyst activity. For example, it was claimed in some studies that for Au/TiO2 catalysts, the gold species responsible for achieving a high CO conversion in oxidation reactions are metallic gold nanoparticles [18,20], whereas other studies suggests that the superior CO oxidation activity could be ascribed to the presence of nano-sized metallic gold particles plus Au3þ ions, and the titania provides the active sites for CO and oxygen dissociation [50]. In the case of other catalysts, such as Au/Fe-HMS, Au/CeO2/Al2O3 and Au/CeO2, the cationic gold species are proposed to be responsible for the high activity [19,29,40,51].
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The results obtained in this work suggest that metallic particles and cationic species in a specific ratio could be responsible for the high activity shown by this type of catalyst and that an imbalance could change the activity and therein the selectivity of the reaction. In the case of Au/CeO2-ZrO2(u) catalysts, the gold metallic nanoparticles were present, but poor activity of CO oxidation was shown.
Conclusions The catalytic activity and selectivity of Au/CeO2-ZrO2 catalysts prepared using a controlled and uncontrolled pH for gold incorporation were examined. It was found that the catalyst prepared with strict pH control was the most active, exhibiting high activity and good selectivity at 100 C even in the presence of CO2 and water vapour in the reaction feed stream. Additionally, this catalyst showed good long-term stability in presence of CO2 and moisture. From the characterisation of the catalysts, it was found that controlling the pH to a value of 6 promoted the activity of the gold particles for CO oxidation, whereas when the pH was not controlled, less gold was deposited and the gold species were almost inactive. The active and inactive gold species were clearly noted in the H2TPR patterns because only the active gold particles improved oxygen removal from the support at low temperature. The UVevis analysis showed a broadening of the bands when the active catalysts were subjected to an oxidizing treatment, suggesting a diversity of the sizes of the metallic gold particles, with a larger size or different shape of particles, whereas with the reducing treatments, the size and shape of the gold particles became more defined as the temperature of the controlled reaction chamber increased. This seems to occur at the expense of partially charged gold clusters (Audþ).
Acknowledgements The authors are grateful to the Universidad Nacional de and the UANL (PAICYT project IT-641Colombia sede Bogota 11) for the financial support for this work. The authors are also grateful to Professor Gustavo Fuentes from the UAM-I (Mexico) for their collaboration in the UVevis characterisation.
references
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