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Kinetic Study of H2 Oxidation in the Preferential Oxidation of CO on a Nanosized Au/CeO2 Catalyst Jing Xu, Ping Li, Xingfu Song, Zhiwen Qi, Jianguo Yu, Weikang Yuan, and Yi-Fan Han* State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, People’s Republic of China
Kinetic study of H2 oxidation in the preferential oxidation (PROX) of CO was implemented over a nanosize Au/CeO2 catalyst in a temperature range 313-353 K. The Langmuir-Hinshelwood mechanism was proposed to be mainly responsible for H2 oxidation, and CO oxidation can be accelerated by coadsorbed H at low temperatures. On the other hand, the water in the system has proved to suppress both CO and H2 oxidation by increasing the energy bars. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption on the Au surface indicates that linear CO-Au bond can be weakened in the presence of H2; meanwhile, water can be a poison taking effects via the bonding of water and the lattice oxygen at the interface of Au/CeO2. The irreversible loss of activity during reaction may be caused by the reconstruction of Au particles, at least in part. 1. Introduction Au catalysts have proven highly reactive and selective for CO oxidation applied in the purification of H2 gas from steam reforming methanol or other materials,1–7 which is a crucial reaction for the production of pure H2 supplying to the polymer electrolyte membrane (PEM) fuel cells. Generally, two reactions, 2CO + O2 f 2CO2 and 2H2 + O2 f 2H2O, compete in the preferential oxidation system (PROX). Until now, in the study of this process, most of out efforts have been devoted to CO oxidation, but less attention to H2 oxidation which also plays an essential role in the evaluation of the fuel efficiency and the designation of the PROX reactor. As defined in eq 1, the lower selectivity (S) toward CO2 means a decrease in the amount of H2 supplied into fuel cells, being equivalent to the loss of fuel efficiency. For instance, a selectivity of 50% in the PROX of CO, equal reaction rates of CO and H2 oxidation, correspond to a loss of approximately 2.7% in fuel efficiency for typical 2% vol CO in the methanol reformate. S)
∆O(CO) 2 2) ∆O(CO) + ∆O(H 2 2
× 100%
(1)
H2 oxidation has been proposed to occur via a path different from CO oxidation on Au catalysts.1,8 The kinetics of the PROX of CO on Au/Fe2O3 has revealed that two reactions may proceed through the Langmuir-Hinshelwood mechanism, by which adsorbed oxygen is reacted with adatom H and adsorbed CO at the perimeter of the interface.3 However, mechanistic understanding of H2 oxidation over Au-based catalysts especially in the PROX of CO is still very limited. On the other hand, the water from H2 oxidation, e.g., 1.5% H2O is produced at a selectivity of 50% for the PROX of CO if given 1.5% CO in a methanol reformate, and from reforming methanol (5-10% H2O) generally deteriorates the catalyst performance significantly. Moisture has proven to enhance the rate of CO oxidation greatly on Au/R-Fe2O3,9 but a negative effect was found for Au/TiO2.10 Interestingly, Date and Haruta11 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +86-21-64251928. Fax: +86-2164251928.
have reported that the rate of CO oxidation could increase by a factor of 10 with raising water level from 0.1 to 200 ppm in a well-defined system on Au/TiO2, but decreased with a further increase in water concentration. More recently, Yi et al has studied the kinetics of the PROX of CO on different type of CeO2 supported Au catalysts; especially, the promotional effects of H2 and water on CO oxidation was found to be dependent on the structure of CeO2.12 Obviously, the kinetic behavior of CO oxidation can be affected by water concentration, catalyst properties and other factors.1 In this study, insight into the kinetic behavior of H2 oxidation in the PROX of CO is obtained by measuring the kinetics of H2 oxidation in both simulated and ideal reformates on a Au/ CeO2 catalyst, one of the most promising catalysts for the PROX of CO and the water-gas shift reaction aimed for H2 purification.13–15 In addition, CO adsorption on the Au surface is studied under the reaction conditions using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The plausible mechanism of H2 oxidation is proposed. 2. Experimental Section 2.1. Catalyst Preparation, Reactant, and Activity Measurement. A Au(2.62 wt %)/CeO2 catalyst was prepared by deposition-precipitation. The pure support precursor (CeO2) was produced from a solution of Ce(NH3)2(NO3)6 and Na2CO3. After washing out NH4+, the reaction mixture was kept at 333 K with maintaining pH 6.5-7.0. Then a HAuCl4 solution was added under vigorously stirring, while buffered with Na2CO3. After filtration, the precipitation was washed with hot water until chloride-free, then dried at 353 K in air for 12 h. Prior to the measurement, the catalyst was pretreated at 673 K in O2 (20 mL/min) for 30 min. The reaction was carried out in both ideal and simulated reformates. The former usually consists of 0.03-1.0 kPa O2, 75 kPa H2, rest N2, and the latter represents a mixture of 0.03-1.0 kPa CO/O2, 75 kPa H2, rest N2, without any emphasis. In order to obtain differential reaction rates, CO and H2 conversions were controlled below 10% by diluting the catalyst with R-Al2O3 powders in ratios ranging from 1 to 1000. Analysis was performed with an online gas chromatograph (Shimadzu GC-2010) equipped with a CPcarbonBOND column.
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Absolute reaction rate of CO (rCO) and H2 (rH2) oxidation in the simulated reformate is expressed as eqs 2 and 3, respectively. In the ideal reformate, rH2 is calculated for the average concentration of oxygen, c˙O2, at the inlet and outlet of the reactor (eq 4): rCO )
c˙CO,inXCOV˙gas [mol s-1 gMe-1] mMe
( S1 - 1)
rH2 ) rCO rH2 )
c˙O2,inXO2V˙gas mMe
(2)
(3)
[mol s-1 gMe-1]
(4)
mMe mass of Au in the reactor bed, V˙gas total molar flow rate, XCO CO conversion based on CO2 formation in the simulated reformate, XO2 O2 conversion, S selectivity for CO oxidation, c˙CO, c˙O2 concentration of CO and O2 in the gas mixture, equal to pi/p0, pi partial pressure of reactants, p0 total pressure in the system. Consequently, the selectivity in the simulated reformate can then be calculated as S)
out 0.5cCO 2
cOin2 - cOout2
)
out 0.5cCO 2
∆cO2
(5)
In the present study, the rH2 vs pO2 was measured in both the ideal and simulated reformate with pO2 ranging from 0.03 to 1 kPa. In case of the modification of mechanism caused by a high excess of pO2 (aspect of CO oxidation), the ratio of pO2/pCO was kept constant in the whole kinetic measurements. The advantages of this method have been introduced in the previous studies.3,16 To calculate the reaction order in pO2 for H2 oxidation, a simple power-law functionality is assumed as eq 6 log(rH2) ) log(kH2) + RH2 log(pH2) + RO2 log(pO2) + RCO log(pCO) + RH2O log(pH2O)
(6)
with neglecting the potential interference from CO and water, eq 6 can be simply written as: log(rH2) ) log(kH′ 2) + RO2 log(pO2)
(7)
The slope RO2 of log(rH2) versus log(pO2) at a constant pH2 represents the reaction order with respect to O2
|
∂log(rH2) ∂log(PO2)
|
PH )75kPa 2
) RO2
(8)
as it is assumed that only the oxidation of CO and H2 exist in the reaction system, so that the selectivity toward CO2 in the simulated reformate can be in the form as: S)
rCO ) rCO + rH2
1 1+
rH2
(9)
rCO
2.2. Characterization. Transmission Electron Microscopy (TEM). The microstructure and morphology of the catalyst were determined by TEM (JEOL 2010). The catalyst was suspended in ethanol, and one drop of this slurry was
Figure 1. (a) Dependence of the H2 oxidation rate on the partial pressure of O2 in the simulated (353 (b), 333 (9), 313 K (2), CO/O2: 0.03-1 kPa, H2: 75 kPa, balance N2) and ideal (353 (O), 333 (0), 313 (∆), O2: 0.03-1 kPa, H2: 75 kPa, balance N2) reformate. (b) Selectivity toward CO2 in the simulated reformate. Dilution ratio 1:20-40 with R-Al2O3; flow rates 60-135 N mL/min.
deposited on a carbon-coated copper grid. The liquid phase was evaporated before the grid was loaded into the microscope. X-ray Diffraction (XRD). The X-ray powder diffraction patterns were obtained with a Bruker D8 diffractometer using CuK(R) radiation. By means of determining X-ray diffraction line broadening of the Au(111) diffraction peak and using the Scherer equation, the average gold crystallite on the calcined catalyst is estimated. X-ray Photoelectron Spectroscopy (XPS). XPS analysis was performed on a VG ESCALAB 250 spectrometer, using Al KR radiation (1486.6 eV, pass energy 20.0 eV). The base pressure of the instrument is 1 × 10-9 Torr. The background contribution B (E) (obtained by the shirley method) caused by the inelastic process was subtracted, while the curve-fitting was performed with Gaussian-Lorentzian profile by an standard software. The binding energies (BEs) over supported catalysts were calibrated using C1s peak at 285.0 eV. The instrument was also calibrated by using Au wire. XPS spectra were recorded at θ ) 90° of X-ray sources. In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). All DRIFTS of CO adsorption were performed in an in situ reaction cell (modified Harricks model HV-DR2) in order to allow gas flowing continuously through the catalyst bed (ca. 0.1 g) during spectra acquisition. The spectra were recorded on a Bio-Rad FT-IR 3000 MX (resolution: 4 cm-1) spectrometer, equipped with an MCT narrow band detector, by coadding 300 single-beam spectra, equivalent to an acquisition time of ∼5 min. All infrared data were evaluated in Kubelka-Munk units, which were linearly related to the absorber concentration in DRIFTS. Spectral contributions from gaseous CO were eliminated by subtracting the corresponding spectra from the pure support material. 3. Results 3.1. Kinetics of H2 Oxidation. As shown in Figure 1a (solid symbols), with fitting all the data to eq 7 rH2 vs pO2 yields the RO2 of 0.52 (353 K), 0.50 (333 K), and 0.48 (313 K) in the simulated reformate, while a RO2 of 0.08 was measured for Au/ R-Fe2O3.3 For comparison, in the ideal reformate, a weak dependence of rH2 vs pO2 in Figure 1a (hollow symbols) produces a RO2 of ∼0.31 at 353 and 333 K, and approximate zero-order at 313 K. The rate constants and RO2 were listed in Table 1. An increase in the selectivity toward CO2 with increased pressure pO2 ) pCO (Figure 1b) indicates a mild competition between H2 and CO oxidation.
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Table 1. Kinetic Parameters for H2 Oxidation in the Presence and Absence of CO on the Basis of the Power Law Equation: γH2 ) k′ pORO2 2 k′ × 10-4
RO2
a
T (K)
S
Ib
S
I
353 333 313
7.18 ( 1.2% 6.81 ( 1.6% 6.15 ( 1.3%
7.31 ( 0.4% 6.90 ( 0.5% 6.27 ( 0.7%
0.52 ( 7.5% 0.50 ( 11% 0.48 ( 12%
0.30 ( 4% 0.32 ( 6% 0.08 ( 3%
a In the simulated reformate, CO/O2: 0.03-1 kPa, H2:75 kPa, balance N2. b In the ideal reformate, O2: 0.03-1 kPa, H2: 75 kPa, balance N2.
The rH2 vs 1/T (Figure 2) yields the activation energies (E*a ) in the temperature range 303-373 K: 46.5 kJ/mol in the ideal reformate, 30.5 (>323 K) and 107.1 kJ/mol (1 nm) and the overlap of the both particles, the Au size determinated by XRD is adopted in the present study. Nevertheless, the results measured by two methods have demonstrated that Au is highly dispersed over ceria surface. It is noted that both Au and ceria particle size remained even after 10 h of reaction in a simulated reformate. 3.3.2. Chemical State of Au. The XPS Au4f spectra (Figure 6) were recorded over Au/CeO2 before and after treatment. In line with the previous studies,2 the binding energies (BE) at 84.0 and 85.5 eV can be assigned to Au4f7/2 of metallic Au and Au2O3, respectively. The BE of Au4f7/2 shifts down significantly after the treatment. The peaks with deconvolution reveal the existence of metallic and oxidative Au on as-treated sample. In contrast, oxide Au predominates on the precursor.
The variation of BE indicates the transformation from Au(OH)x to metallic Au during the calcination process. 3.3.3. In situ DRIFT of CO Adsorption. CO adsorption on Au sites in the presence of H2 and water was detected using in situ DRIFTS. The spectrum in Figure 7A recorded in CO 1 kPa/N2 shows two peaks at 2119 and 2171 cm-1, respectively. The first peak can be assigned to a linear CO-Au band;4,17–21 the weak band at ca. 2171 cm-1 may be caused by the terminal CO-Ce4+ species.22 With taking account of the possible synergism between Au and CeO2, we assume that the high vibrational frequency may be caused by CO molecule adsorbed on the unsaturated Ce4+ sites locating at the interface of Au/ CeO2. Reasonably, no such species is detectable on pure CeO2 under the same conditions, due to few unsaturated Ce4+ sites produced on ceria surface. However, a previous work23 suggested that the band at 2174 cm-1 may be owing to the gold surface not fully reduced. Nevertheless, the assignment of this band is still an opening question. In H2 atmosphere (CO 1 kPa/H2), as shown in Figure 7B, the linear CO-Au band shifted down to ca. 2019 cm-1 and the CO-Ce4+ band disappeared completely, while companying with a decrease in the peak intensity by 30%. We infer that the downshift of the linear Au-CO band in an H2 atmosphere indicates an increase in electron intensity in Au sites caused by H adsorption on the Au surface. The linear CO-Au band remained unchanged with adding water (CO 1 kPa, H2O 2.5 kPa/H2, Figure 7C) and putting into the reaction (CO/O2 1 kPa, H2O 2.5 kPa/H2, Figure 7D). 4. Discussion 4.1. Kinetics of H2 Oxidation on Au-Based Catalysts. The mass-specific H2 oxidation rates over various Au/support catalysts were summarized in Table 2. Noting that some parameters, including the E*a in Table 3, were calculated based on the published data. The promotional effect of water observed by Grisel and Nieuwenhuys1 is in good agreement with our results. However, the relatively low reaction rates reported by the Haruta group8,9,20,23 may be due to high excess of O2 in the feed stream (a mixture of H2 in air (pH2: 1 kPa, PO2: 23 kPa)), while a H2-rich atmosphere (pH2: 3.2 kPa, PO2: 0.8 kPa, balance He) was used for the former and the present study. Additionally, under the same reaction conditions the rates obtained for the Au/support catalysts are almost two magnitudes higher than that
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Table 2. Comparison of the Reaction Rate of H2 Oxidation on Various Au/Support Catalysts catalysts Au(10 atom %)/Co3O4 Au(10 atom %)/NiO Au(10 atom %)/R-Fe2O3 Au(13 wt %)/R-Fe2O3 Au(3.33 wt %)/R-Fe2O3 Au(10 wt %)/MnOx Au(4.8 wt %)/MnOx/Al2O3
temp (K)
rate (mol · gAu-1 · s-1)
9.8
339 346 346 313 323 323 375
0.93 × 10-5 0.93 × 10-5 0.93 × 10-5 0.86 × 10-5 3.10 × 10-5 1.25 × 10-6 0.93 × 10-3
8a 8a 8a 9 20 7 1
0.8
17.4
330
0.93 × 10-3
1
0.8
2.1
323 K, water-free) to 72.8 kJ/mol (1.5 kPa water). We assume that (i) the adsorption of water on the Au surface blocks the adsorption of CO and H2sif this is the case, there will be no water promotional effectss(ii) the adsorption of water occurs on the active site at the Au-substrate interface, blocking O2 from adsorbing on the CeO2 surface, or the lattice oxygen from interacting with the adsorbed CO and adatom H. The significant decrease in both reaction rates indicates that the CeO2 surface is poisoned by water, thus concurrently reduces the probability of the adatom oxygen to participate the reaction. The
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DRIFT spectra (Figure 7) show that the intensity of the linear CO band on the Au surface is reduced by ∼30% in the presence of H2 and water. That is to say, part of the active site could be lost permanently because the catalytic activity was not be fully recovered after switching off water. Considering the fact that no water can be adsorbed on the Au surface,31 the loss of active site may be ascribed to the reconstruction of Au particles caused by water adsorption on the substrate, thus explains the catalytic deactivation in the presence of water. The present study has demonstrated that H2 could inherently enhance CO oxidation over Au-based catalysts during purifying H2, especially when the reaction is carried out at lower temperatures. However, H2 can be competitive with CO on the adsorption and reaction on the Au surface with increased temperature. The suppression of H2 oxidation during the PROX of CO by finely tuning the surface structure of Au is an ongoing work in our group. 5. Conclusions For the first time, the kinetics of H2 oxidation affected by CO and water has been studied systematically on a nanosized Au/CeO2 catalyst. H2 oxidation can be strongly suppressed by CO, especially, at temperatures below the inflection point, thus leads to an increase in the selectivity of the PROX of CO. The water poisoning effect is due to the adsorption of water, perhaps, at the interface; while the loss of some active sites is perhaps due to the reconstruction of Au particles in the moisture atmosphere. In general, Au/CeO2 can be an excellent catalyst for the selective oxidation of CO, especially if the reaction temperature is controlled at below the inflection point, which depends on the property of the substrate of Au catalysts. Acknowledgment The authors are grateful to the support from the Chinese Education Ministry 111 project (B08021), International Cooperation Project of Chinese Ministry of Science and Technology (2007DFC61690), Non-Government International Cooperation Project of Shanghai Ministry of Science and Technology (2010/ 10230705900), Opening Project of State Key Laboratory of Chemical Engineering (SKL-ChE-08C05). Literature Cited (1) Grisel, R. J. H.; Nieuwenhuys, B. E. Selective oxidation of CO over supported Au catalysts. J. Catal. 2001, 199, 48–59. (2) Han, Y.-F.; Zhong, Z.; Ramesh, K.; Chen, F.; Chen, L. Effects of different types of gamma-Al2O3 on the activity of gold nanoparticles for CO oxidation at low-temperatures. J. Phys. Chem. C 2007, 111, 3163– 3170. (3) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. Kinetics of the selective low-temperature oxidation of CO in H2-rich gas over Au/alpha-Fe2O3. J. Catal. 1999, 182, 430–440. (4) Schubert, M. M.; Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/gamma-Al2O3 and Au/alpha-Fe2O3: an in-situ DRIFTS study. J. Power. Sources. 1999, 84, 175–182. (5) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. CO oxidation over supported gold catalysts-“inert” and “active” support materials and their role for the oxygen supply during reaction. J. Catal. 2001, 197, 113–122. (6) Schubert, M. M.; Plazk, V.; Garche, J.; Behm, R. J. Activity, selectivity, and long-term stability of different metal oxide supported gold catalysts for the preferential CO oxidation in H2-rich gas. Catal. Lett. 2001, 76, 143–150. (7) Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. Preparation of nanometer gold strongly interacted with TiO2 and the structure sensitivity in low-temperature oxidation of CO. Stud. Surf. Sci. Catal. 1995, 91, 227–235.
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ReceiVed for reView February 10, 2010 ReVised manuscript receiVed March 16, 2010 Accepted March 21, 2010 IE100325Y
