Cataluminescence-Based Array Imaging for High-Throughput

Department of Chemistry, Key Laboratory for Atomic and Molecular Nanosciences of the Education Ministry, Tsinghua University, 100084, Beijing, P. R. C...
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Anal. Chem. 2009, 81, 2092–2097

Cataluminescence-Based Array Imaging for High-Throughput Screening of Heterogeneous Catalysts Na Na, Sichun Zhang,* Xin Wang, and Xinrong Zhang* Department of Chemistry, Key Laboratory for Atomic and Molecular Nanosciences of the Education Ministry, Tsinghua University, 100084, Beijing, P. R. China High-throughput screening of catalysts could dramatically improve performance and reduce costs in the discovery and study of various catalysts. Here we report a cataluminescence-based array imaging as a high-throughput screening technique in the combinatorial discovery of active catalysts for CO oxidation. This strategy is based on the fact that the CO oxidation generates cataluminescence emission on the surface of nanomaterials, whose intensity is correlated to the activity of the catalyst. To demonstrate the feasibility of the cataluminescence-based array imaging for high-throughput screening of catalysts, different nanosized metal catalysts supported on TiO2 nanoparticles were prepared. These catalysts include monometallic Au, Pt, and the bimetallic Au-Pt heteroaggregate catalysts, at total metal loadings of 0.5%, 1.0%, and 2.5%, and with atomic ratios of 1:1, 1:2, and 2:1 (Au/Pt). A 4 × 4 array was integrated by depositing these nanosized catalysts onto the ceramic chip, and the brightness of each spot in the image was recorded. The catalytic activities of those catalysts for the CO oxidation were evaluated parallelly by both the cataluminescence imaging and the gas chromatography method. The correlation coefficient is 0.914 for the two techniques, indicating that the cataluminescence imaging technique can be applied for the evaluation of the catalytic activities. Moreover, fast evaluation of multiple catalysts at a series of working temperature can be achieved by this cataluminescene-based array imaging. With the development of nanotechnology as well as the catalyst industry, the cataluminescence-based array imaging will address its importance in the highthroughput screening of catalysts. Heterogeneous catalysis plays important roles in industry, and many economic and environmental benefits may be achieved by the optimization of catalyst formulations.1,2 As a result of the complexity of heterogeneous catalysts, combinatorial synthetic techniques were introduced, which allow the parallel or combi* To whom correspondence should be addressed. Prof. Xinrong Zhang and Dr. Sichun Zhang, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China. E-mail: [email protected] (X.Z.); sczhang@ mail.tsinghua.edu.cn. (1) Pirkanniemi, K.; Sillanpaa, M. Chemosphere 2002, 48, 1047–1060. (2) Guibal, E. Prog. Polym. Sci. 2005, 30, 71–109.

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natorial preparation and processing of large number of chemically distinct catalysts by establishing an integrated workflow.3-6 However, the processes of discovery and optimization of heterogeneous catalysts are still lengthy and largely dependent on trialand-error procedures based on the gas chromatography (GC) method.7 Thus, finding a new method for high-throughput screening of multiple catalysts without product separation by a chromatography technique is still a challenge.8,9 High-throughput screening method can evaluate tens or hundreds of samples simultaneously. This method has the potentials for the rational design and time-savings in the development of new catalysts, considerably reducing the time-to-market of new industry developments.8-12 Spontaneously, in conjunction with combinatorial synthesizing to high-throughput screening techniques, significant advances for the discovery and screening of catalysts have been witnessed, which dramatically improve performance and reduce costs.9,13,14 A number of high-throughput screening approaches have been developed,8,15,16 such as IR thermography, a noncontact and nonintrusive technique, which offers the most general screening protocol to date for the catalytic activity.17-19 In addition, laser-induced fluorescence imaging,20,21 (3) Hagemeyer, A.; Jandeleit, B.; Liu, Y. M.; Poojary, D. M.; Turner, H. W.; Volpe, A. F.; Weinberg, W. H. Appl. Catal., A 2001, 221, 23–43. (4) Dahmen, S.; Brase, S. Synth.-Stuttgart 2001, 1431–1449. (5) Takeuchi, I.; Lauterbach, J.; Fasolka, M. J. Mater. Today 2005, 8, 18–26. (6) Senkan, S. M.; Ozturk, S. Angew. Chem., Int. Ed. 1999, 38, 791–795. (7) Trapp, O. J. Chromatogr., A 2008, 1184, 160–190. (8) Murphy, V.; Volpe, A. F.; Weinberg, W. H. Curr. Opin. Chem. Biol. 2003, 7, 427–433. (9) Cong, P. J.; Doolen, R. D.; Fan, Q.; Giaquinta, D. M.; Guan, S. H.; McFarland, E. W.; Poojary, D. M.; Self, K.; Turner, H. W.; Weinberg, W. H. Angew. Chem., Int. Ed. 1999, 38, 484–488. (10) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Angew. Chem., Int. Ed. 2006, 45, 3278–3283. (11) Senkan, S. M. Nature 1998, 394, 350–353. (12) Guram, A.; Hagemeyer, A.; Lugmair, C. G.; Turner, H. W.; Volpe, A. F.; Weinberg, W. H.; Yaccato, K. Adv. Synth. Catal. 2004, 346, 215–230. (13) Pescarmona, P. P.; van der Waal, J. C.; Maxwell, I. E.; Maschmeyer, T. Catal. Lett. 1999, 63, 1–11. (14) Potyrailo, R. A.; Mirsky, V. M. Chem. Rev. 2008, 108, 770–813. (15) Wennemers, H. Comb. Chem. High Throughput Screening 2001, 4, 273– 285. (16) Potyrailo, R. A. TrAC, Trends Anal. Chem. 2003, 22, 374–384. (17) Holzwarth, A.; Schmidt, P. W.; Maier, W. E. Angew. Chem., Int. Ed. 1998, 37, 2644–2647. (18) Cypes, S.; Hagemeyer, A.; Hogan, Z.; Lesik, A.; Streukens, G.; Volpe, A. F.; Weinberg, W. H.; Yaccato, K. Comb. Chem. High Throughput Screening 2007, 10, 25–35. 10.1021/ac802132c CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

resonance-enhanced multiphoton ionization,11 and microprobe sampling mass spectrometry22 have also been used in the highthroughput screening of catalysts. Although those techniques have their advantages, it is still essential to develop a simple and straightforward method for fast evaluation of the catalysts. Cataluminescence (CTL) is a kind of chemiluminescence emission during catalytic oxidation of combustible gases on the surface of solid catalysts in an atmosphere containing oxygen, which was found by Breysse and co-workers through a catalytic oxidation of carbon monoxide on thoria.23 The mechanism of CTL emission can be attributed to recombinant radiation and radiation from excited species.24 It has been reported by our and other groups that the CTL emission can be observed on various nanomaterials, such as TiO2, Y2O3, Al2O3, SrCO3, ZrO2, ZnO, MgO, Fe2O3, etc.24-32 In an early communication, we reported a proof of principle CTL array imaging screening method for evaluating the catalytic activity of gold catalysts supported on five kinds of nanomaterials supports (TiO2, MgO, SiO2, ZrO2, and ZnO).33 The brightness of the spots on the array indicates the catalytic activities of corresponding oxide-supported Au catalysts for CO oxidation. However, this study is limited to study only five gold catalysts, a wider adaptation for its application has not been demonstrated yet. For example, when different metal catalysts are loaded on the same oxide-support, there is a question of whether CTL responses can still be correlated to catalytic activities, therefore achieving a high-throughput screening of catalysts by this simple CTL-based array imaging method. In this article, an extended work on rapid screening of catalysts by CTL-based array imaging was carried out, which focuses on the evaluation of different metal catalysts loading on same kind of oxide-support. The heterogeneous catalysts included monometallic Au, Pt, and bimetallic Au-Pt heteroaggregate nanoparticles supported on nanosized TiO2 with different total metal loading and atomic ratio of Au/Pt. The well correlation between CTL intensities and catalytic activities of those catalysts was observed. A 4 × 4 array based on the CTL-imaging technique was (19) Moates, F. C.; Somani, M.; Annamalai, J.; Richardson, J. T.; Luss, D.; Willson, R. C. Ind. Eng. Chem. Res. 1996, 35, 4801–4803. (20) Su, H.; Yeung, E. S. J. Am. Chem. Soc. 2000, 122, 7422–7423. (21) Su, H.; Hou, Y. J.; Houk, R. S.; Schrader, G. L.; Yeung, E. S. Anal. Chem. 2001, 73, 4434–4440. (22) Service, R. F. Science 1997, 277, 474–475. (23) Breysse, M.; Claudel, B.; Faure, L.; Guenin, M.; Williams, R. J. J.; Wolkenstein, T. J. Catal. 1976, 45, 137–144. (24) Nakagawa, M.; Yamashita, N. In Frontiers in Chemical Sensors: Novel Principles and Techniques; Springer-Verlag Berlin: Berlin, Germany, 2005; Vol. 3, pp 93-132. (25) Zhang, Z. Y.; Xu, K.; Xing, Z.; Zhang, X. R. Talanta 2005, 65, 913–917. (26) Liu, G. H.; Zhu, Y. F.; Zhang, X. R.; Xu, B. Q. Anal. Chem. 2002, 74, 6279– 6284. (27) Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Analyst 2002, 127, 792–796. (28) Zhu, Y. F.; Shi, J. J.; Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Anal. Chem. 2002, 74, 120–124. (29) Na, N.; Zhang, S. C.; Wang, S. A.; Zhang, X. R. J. Am. Chem. Soc. 2006, 128, 14420–14421. (30) He, Y. H.; Lv, Y.; Li, Y. M.; Tang, H. R.; Tang, L.; Wu, X.; Hou, X. D. Anal. Chem. 2007, 79, 4674–4680. (31) Tang, H. R.; Li, Y. M.; Zheng, C. B.; Ye, J.; Hou, X. D.; Lv, Y. Talanta 2007, 72, 1593–1597. (32) Zhang, Z. Y.; Jiang, H. J.; Xing, Z.; Zhang, X. R. Sens. Actuators, B: Chem. 2004, 102, 155–161. (33) Wang, X.; Na, N.; Zhang, S. C.; Wu, Y. Y.; Zhang, X. R. J. Am. Chem. Soc. 2007, 129, 6062.

fabricated for the high-throughput screening of metal catalysts in CO oxidation. EXPERIMENTAL SECTION Materials and Methods. Reagents. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O), potassium platinum(IV) hexachloride (K2PtCl6), sodium borohydride (NaBH4), and trisodium citrate were obtained from Beijing Chemical Co. Ltd. Poly(vinyl alcohol) (PVA, Mw 10 000) was obtained from Beijing Yili Chemical Co. Ltd. Water was deionized and further purified with a Mili-Q water purification system (Millipore, Milford, MA). Carbon monoxide (99.99%) was purchased from Beijing Haipu-Gas Co. Ltd. The support of TiO2 (P25) was supplied by Nanjing Haitai. Nano. Co. Ltd. Catalysts Preparation. Monometallic Au and Pt nanoparticles were prepared by employing minor modifications on published procedures,34,35 and bimetallic Au-Pt heteroaggregate particles were obtained by modified standard procedures for sequential Aucore/Pt-shell structure growth.36 Briefly, the protecting agent was added (Au/PVA ) 1.5 mg/1 mg; Au/citrate ) 1.5 mmol/1 mmol) to a 0.30 mmol L-1 aqueous HAuCl4 solution at room temperature under vigorous stirring. The obtained solution was then left under stirring in an ice-water bath for 10 min. A slow injection of an ice-cold aqueous solution of NaBH4 (2.5 mL, Au/NaBH4) 1 mmol/10 mmol) made the color-change from pale yellow to a dark orange-brown solution, indicating the formation of the Au colloid. The similar process was adopted for the synthesis of Pt and Au-Pt colloid, which showed the gray color. The pH value of colloid solution was then adjusted to 6-7, and the TiO2 support was added to the colloid suspension under vigorous stirring. The solution was kept in the dark overnight for the certain adsorption (wt % 0.5%, 1.0%, or 2.5% of metal catalysts on the support), indicated by the decoloration of the solutions. Then, the wet solid catalysts were separated by the centrifugation and washed by deionized water. The collected precipitates were dried in a heating oven at 80 °C overnight. The average particle size of metal nanoparticles supported by TiO2 is in the range of 2-4 nm. The representative TEM images of the supported catalysts are shown in Figure 1, which demonstrate that the metal nanoparticles are well dispersed on the surface of TiO2, and no obvious aggregation is observed. CTL Apparatus and Imaging. The apparatus for recording CTL signals and the images has been described previously.29 A BPCL ultraweak chemiluminescence analyzer (made by the Biophysics Institute of the Chinese Academy of Science in China) equipped with a CR-105 photomultiplier tube (Hamamatsu, Tokkyo, Japan) was used forachieving the CTL spectra. Data acquisition and treatment were achieved by BPCL software running under Microsoft Windows 2000. To fabricate the catalysts array, catalysts were deposited individually on the surface of a ceramic chip, forming a 4 × 4 array with a thickness of 0.1 mm and diameter of 2.0 mm for each spot. The working temperature of the ceramic chip was controlled by (34) Zhou, S. G.; McIlwrath, K.; Jackson, G.; Eichhorn, B. J. Am. Chem. Soc. 2006, 128, 1780–1781. (35) Comotti, M.; Li, W. C.; Spliethoff, B.; Schuth, F. J. Am. Chem. Soc. 2006, 128, 917–924. (36) Wu, M. L.; Chen, D. H.; Huang, T. C. Chem. Mater. 2001, 13, 599–606.

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Figure 1. TEM images of 15 supported catalysts. (a) 0.5% Au/TiO2; (b) 1.0% Au/TiO2; (c) 2.5% Au/TiO2; (d) 0.5% Pt/TiO2; (e) 1.0% Pt/TiO2; (f) 2.5% Pt/TiO2; (g) 0.5% (1:1)AuPt/TiO2; (h) 1.0% (1:1)AuPt/TiO2; (i) 2.5% (1:1)AuPt/TiO2; (j) 0.5% (2:1)AuPt/TiO2; (k) 1.0% (2:1)AuPt/TiO2; (l) 2.5% (2:1)AuPt/TiO2; (m) 0.5% (1:2)AuPt/TiO2; (n) 1.0% (1:2)AuPt/TiO2; (o) 2.5% (1:2)AuPt/TiO2.

was kept at 15 mL min-1. The corresponding CO conversion was calculated according to the peak area synchronously.

RESULTS AND DISCUSSION

Figure 2. Schematic diagram of the catalyst screening system.

a digital temperature controller. A flowmeter (Beijing Keyi Laboratory Instrument Co. Ltd., Beijing, China) was used to measure the gas flow rate. CO gas was introduced by an air flow at atmospheric pressure, passing through the surfaces of the catalysts for the oxidation. The schematic diagram of the catalyst screening system is shown in Figure 2. The catalysts array was placed in a quartz chamber with a condensed water system. The monochromatic film was attached on the surface of the quartz chamber to record the CTL image. Calculation of CO Conversion. The gas products after the detection by BPCL were collected online and injected into a SP6800A gas chromatograph (Shandong Lunan Apparatus), which was equipped with a dual molecular sieve/porous polymer column and a thermal conductivity detector. The conditions for GC experiments were as follows: sampler temperature was set at 120 °C, column temperature was set at 50 °C, and flow rate of H2 gas 2094

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Correlation between Catalytic Activity and CTL Response of Catalysts. To evaluate the correlation between catalytic activity and CTL response, a group of catalysts with different activities for CO oxidation were designed. These catalysts included monometallic Au, Pt, and bimetallic Au-Pt nanoparticles supported on TiO2 nanoparticles, which exhibited different catalytic performances on the conversion of CO to CO2.33,37-41 CTL responses were recorded when CO gas was passed through the catalysts, and the corresponding products were collected for the measurement of CO conversion by the GC method. As shown in Figure 3A, three catalysts, Au, Pt, and Au-Pt nanoparticles supported on nanosized TiO2 (loading rate 2.5%), were studied in CO oxidation. On the basis of the data of CTL (37) Lang, H. G.; Maldonado, S.; Stevenson, K. J.; Chandler, B. D. J. Am. Chem. Soc. 2004, 126, 12949–12956. (38) Okumura, M.; Nakamura, S.; Tsubota, S.; Nakamura, T.; Azuma, M.; Haruta, M. Catal. Lett. 1998, 51, 53–58. (39) Yan, W. F.; Chen, B.; Mahurin, S. M.; Schwartz, V.; Mullins, D. R.; Lupini, A. R.; Pennycook, S. J.; Dai, S.; Overbury, S. H. J. Phys. Chem. B 2005, 109, 10676–10685. (40) Budroni, G.; Corma, A. Angew. Chem., Int. Ed. 2006, 45, 3328–3331. (41) Wolf, A.; Schuth, F. Appl. Catal., A 2002, 226, 1–13.

Figure 3. The comparisons of catalytic activities and CTL behaviors on different catalysts surfaces. (A) Different kinds of metal catalysts: (a) 2.5% Au/TiO2; (b) 2.5% Pt/TiO2; (c) 2.5% (1:1) AuPt/TiO2. (B) Different atomic ratio of AuPt/TiO2 catalysts: (a) 0.5% (1:1)AuPt/TiO2; (b) 0.5% (2:1)AuPt/TiO2; (c) 0.5% (1:2)AuPt /TiO2. (C) Different Au loadings of Au/TiO2 catalysts: (a) 0.5% Au; (b) 1.0% Au; (c) 2.5% Au. (D) Different total metal loadings of AuPt/TiO2 catalysts: (a) 0.5% (2:1)AuPt/TiO2; (b) 1.0% (2:1)AuPt/TiO2; (c) 2.5% (2:1)AuPt/TiO2. Gas flow rate, 200 mL min-1; working temperature, 126 °C.

Figure 4. Correlation of CTL intensities with CO conversions for 15 CO catalysts at 126 °C. The 95% prediction limit is labeled in the figure.

responses (Figure 3A, left side of the figure), an order of CTL intensity can be ranked as 2.5% Pt/TiO2 > 2.5% Au/TiO2 > 2.5% (1:1)AuPt/TiO2. The same order of CO conversion can be observed (Figure 3A, right side of the figure). These results indicate that CTL responses of different metal catalysts are correlated to their catalytic activities. With different atomic ratios of 2:1, 1:2, and 1:1 (Au/Pt), the catalysts of 0.5% bimetallic AuPt/TiO2 were prepared and the correlation between catalytic activities and CTL responses were studied. The same order of CO conversion and CTL intensities is as follows: (1:1)AuPt/TiO2 > (1:2)AuPt/TiO2 > (2:1)AuPt/ TiO2 (Figure 3B), indicating a good consistency between CO conversion and CTL intensity of the catalysts with a different atomic ratio of Au/Pt. Total metal loading rate of the catalyst on the support was also changed in the preparation of heterogeneous catalysis. Monome-

Figure 5. The image obtained from the CTL-based array after exposure to CO gas flow. (A) The arrangement of CO oxidation catalysts. The catalysts are arranged as follows (from left to right, top to bottom): 0.5% (1:1)AuPt/TiO2 (1,1); 2.5% Pt/TiO2 (1,2); 1.0% Pt/TiO2 (1,3); 0.5% Pt/TiO2 (1,4); 1.0% (2:1)AuPt/TiO2 (2,1); 0.5% (2:1)AuPt/TiO2 (2,2); 2.5% (1:1)AuPt/TiO2 (2,3); 1.0% (1:1)AuPt/TiO2 (2,4); 2.5% (1:2)AuPt/TiO2 (3,1); 1.0% (1:2)AuPt/TiO2 (3,2); 0.5% (1: 2)AuPt/TiO2 (3,3); 2.5% (2:1)AuPt/TiO2 (3,4); 2.5% Au/TiO2 (4,1); 1.0% Au/TiO2 (4,2); 0.5% Au/TiO2 (4,3); blank (4,4). (B) Image obtained upon exposure to CO gas flow at 126 °C. Gas flow rate: 200 mL min-1.

tallic catalysts, Au/TiO2 catalysts at different Au loadings (0.5%, 1.0%, and 2.5%) on TiO2 support, were selected as examples for the study. As shown in Figure 3C, the orders for both catalytic activity and CTL intensity can be ranked as 1.0% Au/ TiO2 > 2.5% Au/TiO2 > 0.5% Au/TiO2. In addition, Au-Pt bimetallic catalysts with 0.5%, 1.0%, and 2.5% loadings on TiO2 supports were also synthesized for the studying, whose atomic ratios were controlled at 2:1(Au/Pt). The same order of catalytic activity and CTL response can also result: 2.5% AuPt/TiO2 > Analytical Chemistry, Vol. 81, No. 6, March 15, 2009

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Figure 6. CTL intensity and CO conversion as a function of working temperature for the catalyst of 1.0% (1:1)AuPt/TiO2.

0.5% AuPt/TiO2 > 1.0% AuPt/TiO2 (Figure 3D). Thus, a good consistency between CO conversion and CTL intensity can also be obtained with changing total metal loadings on supports. In the experiments, we examined the correlations between our CTL-based technique and the reference technique based on GC42-44 at different temperatures for all the heterogeneous catalysts. The results indicate a good correlation between two techniques (see Supporting Information). Figure 4 shows the correlation between our CTL-based technique and the reference technique for evaluating 15 catalysts at 126 °C. The coefficient of correlation is 0.914. Fast Screening of Catalysts with CTL-Based Array Imaging. The major advantage of using CTL-based array imaging method for high throughput screening is the ability to collect the CTL emission from various catalysts simultaneously. A 4 × 4 array containing different catalyst spots on a ceramic chip with a temperature controller was designed (Figure 5A). These heterogeneous catalysts included monometallic Au, Pt, and bimetallic Au-Pt heteroaggregate catalysts loaded on TiO2 supports, which were prepared with different total metal loadings or different Au/Pt atomic ratios as described above. Upon the reaction of catalytic CO oxidation, CTL on each spot of the array was imaged simultaneously on the monochromatic film. As shown in Figure 5B, the different catalysts spots give different catalytic activities. The brightest spots correspond to (1,2), (1,3), (1,4), and (3,4), while the weaker spots correspond to (1,1), (2,2), (2,3), (3,3), (4,1), (4,2), and (4,3). For the spots of (3,1) and (3,2), no signal can be recorded. It is convenient to describe the relative CTL intensities of catalysts spots on the monochromatic film as grayscale (GS) values, which can be quantified using Adobe Photoshop software. The lower grayscale value represents the higher CTL intensity. From the image, CTL intensity varies with changing metal catalysts, total metal loadings on the support, and atomic ratio of Au/Pt for Au-Pt bimetallic catalysts. Taking the catalysts at a metal loading of 1.0% on TiO2 support as examples, the spot of 1.0% Pt/TiO2 (1,3) has the highest CTL intensity (GS value, 3%), the spot of 1.0% Au/TiO2 (4,2) gives the weaker intensity (GS value, 26%), whereas the weakest one is attribute to 1.0% (42) Han, Y. F.; Kahlich, M. J.; Kinne, M.; Behm, R. J. Appl. Catal., B 2004, 50, 209–218. (43) Suh, D. J.; Kwak, C.; Kim, J. H.; Kwon, S. M.; Park, T. J. J. Power Sources 2005, 142, 70–74. (44) Galletti, C.; Specchia, S.; Saracco, G.; Specchia, V. Int. J. Hydrogen Energy 2008, 33, 3045–3048.

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AuPt (1:1)/TiO2 (2,4) (GS value, 37%). The coefficient of correlation between CO conversions and grayscales for these three catalysts is 0.99. For the catalysts with different total metal loadings on supports, such as the bimetallic Au-Pt catalysts at the atomic ratio of 1:1 (Au/Pt), the spot of 0.5% AuPt/TiO2 (1,1) has the highest CTL intensity (GS value, 20%), the intensity of 2.5% AuPt/TiO2 (2,3) is much weaker (GS value: 34%), while the weakest one is attributed to 1.0% AuPt/TiO2 (2,4) (GS value, 37%). Moreover, at a total metal loading of 2.5% on TiO2, Au-Pt bimetallic catalyst with atomic ratio of 2:1 (Au/ Pt) (3,4) gives the strongest CTL signal (GS value, 4%), AuPt (1:1)/TiO2 gives the weaker signal at (2,3) (GS value, 34%), while AuPt (1:2)/TiO2 (3,1) gives the weakest signal (GS value, 41%). In conclusion, a good correlation between CO conversions and grayscales can be obtained. Thus, a qualitative overview of the catalytic activities of those catalysts for the CO oxidation can be achieved by the CTL-based array image method. Evaluation of Catalysts at Different Working Temperatures. A variety of catalytic reactions are temperature dependent, which makes fast screening multiple catalysts at a series of temperatures still a challenge to date.7,32 Because of the ability of collecting information from various catalysts simultaneously, the present CTL-based method might be used for fast screening of catalysts at different working temperature. In order to study the CTL response and catalytic activity with changing temperature, 1.0% (1:1)AuPt /TiO2 was selected to examine CO conversions and CTL responses as a function of working temperature. As shown in Figure 6, a good correlation between CO conversion and CTL response at different temperatures is obtained. This indicates that the CTL technique could be applied for evaluating catalytic activity at various working temperatures. The simultaneous evaluation of catalyst activities at different working temperatures was carried out by the CTL-based array imaging approach. The CTL images corresponding to CO oxidation obtained at four working temperatures are shown in Figure 7. At the temperature of 96 °C (Figure 7A), only the signals on (3,2) (GS value, 35%), (3,3) (GS value, 14%), (3,4) (GS value, 12%), (4,1) (GS value, 36%), (4,2) (GS value, 26%) and (4,3) (GS value, 23%) are observed. The higher brightness comes from the spots of (3,3) and (3,4), which demonstrates that 0.5% AuPt (1:2)/TiO2 (3,3) and 2.5% AuPt (2:1)/TiO2 (3,4) have the relatively higher catalytic activity at this condition. When the working temperature increased to 106 °C, other spots at (1,1) (GS value, 20%), (1,2) (GS value, 8%), and (1,3) (GS value, 13%) come to be brightened, indicating their increased catalytic activities for CO oxidation (Figure 7B). At this temperature, the catalyst spots of 2.5% Pt/TiO2 (1,2) (GS value, 8%), 0.5% AuPt (1:2)/TiO2 (3,3) (GS value, 10%), and 2.5% AuPt (2:1)/TiO2 (3,4) (GS value, 8%) have relatively higher catalytic activities. With a further increase of the working temperature to 126 °C (shown in Figure 7C), the CTL signals can be observed on more spots, such as (1,4) (GS value, 11%), (2,2) (GS value, 35%), and (2,3) (GS value, 34%), indicating that these catalysts have relatively higher catalytic activities for CO oxidation at relatively higher temperature. At 140 °C (Figure 7D), little difference on (1,2), (1,3), and (3,4) is observed by comparing with the image at 126 °C, which might be due to the signal saturation at high temperature. However, when increasing temperature from 126 to 140 °C, no significant

Figure 7. The CTL images obtained at working temperatures of 96 (A), 106 (B), 126 (C), and 140 °C (D). Gas flow rate: 200 mL min-1.

change is observed on the spots of (1,1) (GS value, 19% and 20%), (2,1) (GS value, 37% and 38%), (2,4) (GS value, 37% and 38%), and (4,3) (GS value, 32% and 30%), but the relative brightness of spot (3,3) becomes much weaker (GS value, 22-26%). This indicates different functions of temperature dependence for different catalysts. In comparison with the GC method, this is a very simple way for fast evaluation of multiple catalysts at a series of temperatures in a few minutes. CONCLUSION A high-throughput screening technique for catalyst evaluation has been established based on CTL-based array imaging. The catalytic activities of 15 TiO2 supported catalysts, including monometallic Au, Pt, and the bimetallic Au-Pt heteroaggregate nanoparticles have been evaluated. Different CTL responses as well as CO conversions on catalysts can be observed with changing metal catalysts, total metal loadings, and atomic ratios of Au/Pt for bimetallic Au-Pt catalysts. A good correlation between two results was obtained, which indicated the CTLbased technique can be used in the evaluation of catalytic activities. Furthermore, this CTL-based technique can be used

in the fast screening of multiple catalysts at a series of working temperatures. The present screening method is just a pilot study and further studies are still needed. However, with the development of the combinatorial synthesis of heterogeneous catalysts and sensor techniques, the CTL array imaging-based high-throughput screening method will exhibit its importance in the catalysis industry. ACKNOWLEDGMENT We gratefully acknowledged financial support of the work by the National Natural Science Foundation of China (Grant Nos. 20535020 and 20875053) and Chinese MOST Innovation Method Foundation (2008). SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review October 8, 2008. Accepted January 23, 2009. AC802132C

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