Article pubs.acs.org/JPCC
CO Desorption Ability from Pt Enhanced by Al2O3: An in Situ Real-Time Attenuated Total Reflection Infrared Investigation Gengshen Hu,*,†,‡ Hongsheng Gao,‡ and Christopher T. Williams*,‡ †
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, P.R. China ‡ Department of Chemical Engineering, University of South Carolina, 301 Main Street, Columbia, South Carolina 29208, United States ABSTRACT: The influence of Al2O3 on CO desorption from Pt film was studied by means of in situ real-time attenuated total reflection infrared (ATR-IR) spectroscopy. When CO adsorbs on pure Pt film, the interaction between Pt and CO, especially the linear adsorbed CO, is very strong. Thus, the desorption rate of CO from Pt is very slow. However, the CO desorption rate can be significantly increased when Pt is covered by some amount of Al2O3, which may be due to the CO spillover effect from Pt to Al2O3. The CO desorption rates from pure Pt film and Pt films covered by Al2O3 follow the zero-order reaction kinetics; i.e., the desorption rates are coverage-independent. This study also shows that ATR-IR is a very powerful tool to investigate the gas− solid interface.
1. INTRODUCTION In recent years, low-temperature CO oxidation was extensively studied due to the increasing importance of cleaning air and lowering automotive emissions. The support effect or the interaction between metals and supports has a significant influence on the CO oxidation rate.1,2 Meanwhile, CO is widely used as a probe molecule to explore the surface properties of catalysts and the interaction between supports and metals because its vibration is sensitive to the nature of the adsorption site. Metal catalysts are often supported on oxide materials, which makes it challenging to study the interaction between supports and metals and to determine the role of supports. If the supported model catalysts are compared with the unsupported model catalysts, it would be easier to determine the effect of supports. Transmission infrared and diffuse reflectance FTIR spectroscopies (DRIFTs) are most widely used to investigate the adsorptions of gas molecules on catalysts. However, the much stronger signal from bulk gases makes it hard to monitor the adsorption process in real time at elevated gas pressures. Attenuated total reflection infrared (ATR-IR) spectroscopy can overcome this problem because the internal total reflection of the infrared beam can minimize the bulk (gas or liquid) signal while maximizing the surface signal. Thus, adsorption kinetics of surface species can be monitored in real time without the interference from the bulk signal.3,4 In this paper, we present an ATR-IR study of the influence on CO desorption of Al2O3 particles deposited on a sputtered Pt thin film. The spectroscopic results show that Al2O3 on Pt film can significantly enhance the desorption rates of adsorbed CO from the Pt surface.
helium (99.99%) gases were obtained from Praxair. γ-Al2O3 powder was provided by Alfa Aesar with a mean particle size of 37 nm. Preparation of Pt Films and Al2O3-Coated Pt Films. Platinum thin films with thickness of 0.80 nm were deposited on the surface of a ZnSe element by plasma sputtering deposition. The thicknesses of Pt films were monitored using a quartz crystalline microbalance (QCM). The Pt films covered with different amounts of Al2O3 (Al2O3/Pt) were prepared by depositing various amounts of aqueous Al2O3 suspension on the Pt films. After water evaporated, a dry Al2O3 film remained on the Pt film. The coverage of Al2O3 was calculated based on the surface area of the ZnSe element, Al2O3 particle size, and the density of Al2O3. Atomic Force Microscopy (AFM). All AFM measurements were carried out using a PicoSPM AFM (Molecular Imaging) operated in the acoustically driven, intermittent contact mode. All AFM measurements were performed in air and at ambient temperature. Infrared Spectroscopy. All spectra were acquired using a Thermo 6700 FTIR spectrometer with a liquid-nitrogen-cooled MCT detector. A horizontal ATR accessory (Spectra Tech) was used in conjunction with a home-built aluminum flow cell. The design of the flow cell has been reported previously.3,5 The flow rate used for each of the gases (CO and He) through the cell was 20 cm3/min. For each experiment, the ATR accessory optics were aligned and optimized, and the sample was left under a flow of He until the system reached equilibrium (usually at least 2 h, as determined by achieving a consistent, unchanging
2. EXPERIMENTAL SECTION Chemicals. The Pt target of the plasma sputtering machine was purchased from Anatech. Carbon monoxide (1%) and
Received: November 14, 2011 Revised: February 9, 2012 Published: February 15, 2012
© 2012 American Chemical Society
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Figure 1. AFM images of (A) a bare ZnSe element and (B) a 0.8 nm Pt on ZnSe and Pt films covered by (C) 0.2, (D) 0.5, and (E) 1.0 layer of Al2O3.
surface, resulting from the final polish used at the factory when the optics were made. Figure 1B shows the AFM image of Pt film on ZnSe. Since the surface of ZnSe is not flat and the Pt film is very thin, the Pt film is not very obvious. However, from the figure, the Pt film did form, and some Pt particles were visible in some regions. Figure 1C, 1D, and 1E give the AFM images of Pt film coated by 0.2, 0.5, and 1.0 layer of Al2O3, respectively. It is clear that Al2O3 particles are dominant, and the coverage of Al2O3 is larger in order.
absorbance spectrum). Data collection consisted of 128 scans per spectrum with a resolution of 4 cm−1 and a collection time of 94 s. The interval between two spectra is 1 min. Therefore, the spectra were taken every 154 s. All the experiments were performed at room temperature (25 °C). The spectra were analyzed using a multivariate method which has been described elsewhere.3
3. RESULTS AND DISCUSSION The AFM image of the bare ZnSe element is shown in Figure 1A. It can be seen that there are small scratches throughout the 6248
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function of time. During flowing CO, the intensities of CO peaks gradually increased. The adsorption rate on pure Pt film is faster than that on Al2O3/Pt film, especially at the beginning step of adsorption. This may be due to the less mass transfer resistance on pure Pt film. After switching the flowing gas from CO to He, the intensity of linear CO on Pt film decreased by a small amount, with only 3% CO desorbing after 800 min. However, the intensities of linear CO on various Al2O3-covered Pt film gradually decreased. The peak intensity of linear CO on Pt film covered by one layer of Al2O3 is around 88% of the original intensity after flowing He for 200 min and just 54% of the original intensity after 800 min. This means that around 12% and 46% of linear CO have desorbed after flowing He for 200 and 800 min, respectively, indicating a much higher CO desorption rate on Al2O3-covered Pt film than on Pt film. This shows that the Al2O3 significantly enhances the CO desorption from the Pt film. Interestingly, the curves of CO desorption from both Pt film and Al2O3-covered Pt films are a straight line, indicating that the CO desorption is zero-order in adsorbed CO concentration, which can be mathematically described in the following equations
The ATR-IR spectra of CO on a Pt film and on Pt film covered with one layer of Al2O3 are shown in Figure 2. The two
Figure 2. ATR-IR spectra of CO on Pt film (top) and Pt film covered with one layer of Al2O3 (bottom) at different times: (a) the time point that flowing gas was switched from CO to He and (b) after flowing He for 257 min, (c) 513 min, and (d) 770 min at room temperature.
[CO] = [CO]0 − kt
r=−
−1
peaks at 2041 and 1805 cm on pure Pt film (Figure 2A) are assigned to linear and bridged CO on the Pt surface, respectively.6−8 After switching the flowing gas from CO to He purge gas, the peak intensity of linear CO remained essentially constant, while that of bridged CO gradually decreased. On the Pt film covered by one layer of Al2O3 (Figure 2B), the intensities of two peaks are weaker than those of pure Pt film as shown in Figure 2B. This is attributed to the fact that some of the Pt surface sites were covered by Al2O3. The peak positions of both linear and bridged CO are still at around 2040 and 1805 cm−1, respectively. However, an interesting phenomenon was observed; i.e., the peak intensity of linear CO on Al2O3covered Pt film gradually decreased over time. In other words, CO desorbed gradually during He purging. Figure 3 shows the normalized intensities of linear CO and bridged CO on both Pt film and Al2O3-covered Pt films as a
(1)
d[CO] = −k dt
(2)
where [CO] is the surface linear-adsorbed CO at time t; [CO]0 is the initial surface linear-adsorbed CO; k is the rate constant of CO desorption; t is time; and r is the desorption rate. From eq 2, the desorption rate of linear CO is a constant which is equal to the negative of the slope. In other words, the desorption rate of linear CO on Pt is independent of the concentration of surface adsorbed CO. The normalized desorption rates of CO from Pt film and Pt films covered by one layer of Al2O3 are 3.7 × 10−5 and 5.5 × 10−4 %/min, respectively. Apparently, the desorption rate on Pt film covered by 1.0 layer of Al2O3 is much faster than that on pure Pt film. As also shown in Figure 2, the desorption rate of linear CO from Pt will be faster if the Pt film is covered by a larger amount of Al2O3. Table 1 lists the normalized desorption rates Table 1. Normalized Desorption Rates of Linear and Bridged CO from Pt and Pt Films Covered by 0.2, 0.5, and 1.0 Layer of Al2O3 desorption rate (%/min) linear CO pure Pt 0.2 Al2O3/Pt 0.5 Al2O3/Pt 1.0 Al2O3/Pt
3.7 2.3 5.0 5.5
× × × ×
−5
10 10−4 10−4 10−4
bridged CO 2.2 2.4 3.6 6.2
× × × ×
enhanced times linear CO
bridged CO
6.2 13.5 14.9
1.1 1.7 2.8
−4
10 10−4 10−4 10−4
of CO from Pt film and Al2O3/Pt films. The desorption rates of CO from Pt films covered by 0.2, 0.5, and 1.0 layer of Al2O3 are 2.3 × 10−4, 5.0 × 10−4, and 5.5 × 10−4 %/min, respectively. This further reveals that Al2O3 has enhanced the CO desorption in He flow. As shown in Figures 1 and 2, the bridge-adsorbed CO on pure Pt film can desorb during He purging which is different from linear CO as discussed above. Similarly, the additional Al2O3 films on Pt film also enhance the desorption of bridged CO on Pt, and the desorption rates of CO on Pt film and
Figure 3. Normalized linear adsorbed CO (top) and bridged adsorbed CO (bottom) intensities as a function of time: CO on Pt film (black) and CO on Pt films covered with 0.2 layer (red), 0.5 layer (blue), and 1.0 layer of Al2O3. 6249
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Notes
Al2O3-covered Pt film are close to zero-order desorption. The desorption rates of CO on Pt film and Pt films covered with 0.2, 0.5, and 1.0 layer of Al2O3 are 2.2 × 10−4, 2.4 × 10−4, 3.6 × 10−4, and 6.2 × 10−4 %/min, respectively. Furthermore, from the desorption rates listed in Table 1, it can be concluded that the influence of Al2O3 on bridged CO is not as much as on linear CO. For example, the desorption rate of linear CO was enhanced 14.9 times by 1.0 layer of Al2O3, while the corresponding rate of bridge-adsorbed CO was enhanced only around a factor of 2.8. We also investigated the CO adsorption on Al2O3 film for comparison, but no peaks were observed (the spectrum is not shown here). In a previous study, Nasser et al.9 have investigated the CO adsorption on Al2O3 by using transmission FTIR, and they found that CO weakly adsorbed on Al2O3. Since ATRIR has a much weaker signal than that of transmission FTIR combined with the weak adsorption of CO on oxides,10 CO peaks on Al2O3 were not observed by using ATR-IR in this study. Green et al. have proved that the CO can spill over from Pt to TiO2 by using temperature-programmed desorption (TPD).11 They attributed the CO spillover to the strong metal−support interaction. Recently, Al-Shemmary et al.12 also ascribed the increase of CO desorption efficiency to the CO spillover from Pt film to Al2O3 support by Fourier transform infrared reflection absorption spectroscopy. These previous studies indicate surface species can spill over from metal to supports during the desorption process, even though metal has a stronger interaction with molecules than supports. Therefore, based on our results and the studies by other researchers, the possible mechanism of CO desorption on Pt enhanced by Al2O3 could be explained as the spillover effect of CO from Pt to Al2O3 as shown in Scheme 1.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of this work from Zhejiang Normal University (grant ZC304011016 and KYJ06Y11018), Department of Education of Zhejiang Province (Y201121744), and the USA National Science Foundation (CBET-0731074).
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REFERENCES
(1) Comotti, M.; Li, W. C.; Spliethoff, B.; Schuth, F. J. Am. Chem. Soc. 2006, 128, 917−924. (2) Fu, Q.; Li, W. X.; Yao, Y. X.; Liu, H. Y.; Su, H. Y.; Ma, D.; Gu, X. K.; Chen, L. M.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. H. Science 2010, 328, 1141−1144. (3) Ortiz-Hernandez, I.; Owens, D. J.; Strunk, M. R.; Williams, C. T. Langmuir 2006, 22, 2629−2639. (4) Ferri, D.; Behzadi, B.; Kappenberger, P.; Hauert, R.; Ernst, K. H.; Baiker, A. Langmuir 2007, 23, 1203−1208. (5) Ortiz-Hernandez, I.; Williams, C. T. Langmuir 2007, 23, 3172− 3178. (6) Kunimatsu, K.; Sato, T.; Uchida, H.; Watanabe, M. Electrochim. Acta 2008, 53, 6104−6110. (7) Kunimatsu, K.; Sato, T.; Uchida, H.; Watanabe, M. Langmuir 2008, 24, 3590−3601. (8) Watanabe, S.; Kimura, H.; Sato, T.; Shibata, H.; Sakamoto, F.; Azumi, R.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2008, 24, 8735−8741. (9) Nasser, H.; Redey, A.; Yuzhakova, T.; Toth, Z. N.; Ollar, T. React. Kinet. Catal. Lett. 2007, 92, 329−335. (10) Linsebigler, A.; Lu, G. Q.; Yates, J. T. J. Chem. Phys. 1995, 103, 9438−9443. (11) Green, R.; Morrall, P.; Bowker, M. Catal. Lett. 2004, 98, 129− 133. (12) Al-Shemmary, A.; Buchwald, R.; Al-Shamery, K. J. Phys.: Condens. Matter 2010, 22, 084011−084016.
Scheme 1. Proposed Mechanism of CO Desorption from Pt Enhanced by Al2O3
4. CONCLUSIONS In summary, the ATR-IR results clearly show that Al2O3 deposited on Pt film can enhance the desorption rates of CO surface species. Furthermore, the influence of Al2O3 on linear and bridged CO is different. The enhanced desorption ability can be explained as the spillover effect of CO from Pt to Al2O3. As we know, the CO oxidation occurs on the surface of catalysts. The desorption and desorption rate of reactants may influence the conversion rate. Therefore, it is very important to know the interaction between metal catalysts and supports. This study shows that in situ ATR-IR may be a very powerful tool to investigate these types of systems in the future.
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AUTHOR INFORMATION
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