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Co-promoted Pt catalysts supported on silica aerogel for preferential oxidation of CO
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Catalysis Communications 9 (2008) 880–885 www.elsevier.com/locate/catcom
Co-promoted Pt catalysts supported on silica aerogel for preferential oxidation of CO Jinsoon Choi a, Chee Burm Shin a, Dong Jin Suh b
b,*
a Department of Chemical Engineering, Ajou University, Wonchon, Paldal, Suwon, Kyunggi 442-749, Republic of Korea Clean Technology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, Republic of Korea
Received 12 January 2007; received in revised form 11 September 2007; accepted 14 September 2007 Available online 6 October 2007
Abstract Pt and Co doped silica aerogels have been synthesized by sol–gel method and subsequent supercritical drying. Wet-gel state of silica was used in the adsorption process in order to accommodate Pt and Co. Catalytic activities for preferential oxidation of CO in H2-rich fuels were measured in the temperature range from 25 C to 300 C. At 2500 ppm of carbon monoxide, CoPt–SiO2 aerogels showed good activities in a wide range of temperatures maintaining less than 20 ppm at 125–250 C. 2007 Elsevier B.V. All rights reserved. Keywords: PROX; CO oxidation; Silica; Aerogel; Pt; Co
1. Introduction Silica aerogel is an excellent catalytic support due to its desirable characteristics, such as inertness, high specific surface area, pronounced mesoporosity, and good thermal stability. Since Teichner et al. [1] produced the alcogel in 1970s, there have been numerous reports on the synthetic methods of silica aerogel in catalytic applications [2–6], focused on controlling and optimizing its physical/chemical parameters, and in present days it is commercially available in various forms from powder types to monolithic bricks. Silica-supported platinum catalysts were used for methane/methanol reformation [7,8], CO oxidation [9,10], and hydrodechlorination [11,12] reactions. High specific surface area with little interactions to noble metal caused highly dispersed active sites, and the reducibility of metal oxides on silica was much greater than that on alumina. As platinum has great stability in oxidation processes, many researchers attempted to employ supported Pt catalysts with additional compounds for the preferential oxidation (PROX) from hydrogen-rich fuels [13,14]. Despite the *
Corresponding author. Tel.: +82 2 958 5192; fax: +82 2 958 5205. E-mail address: [email protected] (D.J. Suh).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.09.036
trend has been recently shifted to gold catalysts [15–17], where the reactions were mostly conducted around room temperatures due to the drastic decrease in activity at above 100 C, there have been increasing demands for improved platinum catalysts to satisfy various operational conditions in the range of 100–300 C. The usual method for the preparation of Pt/SiO2 catalysts involved electrostatic adsorption, where point of zero charge (PZC) of silica determined adsorption ability of Pt precursors according to the charge attraction [18]. In general, chloroplatinic acid (H2PtCl6) was not strongly adsorbed on silica but on alumina or activated carbon. Thus, Pt(NH3)4Cl and Pt(NH3)2(NO2)2 were alternatively used as cationic platinum precursors. In addition, for the PROX reaction, it was known that addition of base metals (Na, K, Cs) or transition metals (Co, Ni, Cr) significantly enhanced the activity of Pt catalysts, probably due to the changes in the adsorption characteristics of adsorbates either through electron transfer [19] or resulting bimetallic characteristics/effects [13]. In this study, Pt–SiO2 and CoPt–SiO2 aerogels were prepared by adsorption method with H2PtCl6, not with cationic Pt sources, in a wet-gel state by solvent exchanges. We considered that surface conditions of silica wet-gel
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
would be qualitatively different from silica oxides or silica dried-gels. As Livage et al. [20] suggested in titania polymerization, there would be an increasing portion of partially positive (or slightly negative) charged OR groups in the silica wet-gel networks as polymeric condensations proceed. The purpose of choosing H2PtCl6 as a Pt precursor is two fold: to facilitate the Pt adsorption on the positive charged surface and to attract the cationic metal ion such as Co2+ in order to derive Pt–Co bimetallic species.
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aerogels for an appreciation of the interaction between Pt and Co. Samples placed in a quartz reactor was reduced in a stream of 4% H2/Ar mixture with 30 ml/min flow rate in the temperature range between 27 C and 927 C, at the heating rate of 5 C/min. Inductive coupled plasma atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS) were used for estimating the Pt and Co contents, respectively. 2.3. Catalytic activity
2. Experimental 2.1. Catalyst preparation In the synthesis of silica aerogels, two sets of solutions were prepared. (i) (CH3OH + HCON(CH3)2 (DMF) + Si(OC2H5)4 (TEOS)), (ii) (NH4OH + NH4F + H2O). After an individual mixing for an hour, the solutions were mixed and stirred until gelation occurred. The overall mole ratio was TEOS:CH3OH:DMF:NH4OH:NH4F:H2O = 1:5.7:5.7: 0.005:0.0005:4 with 20.8 ml of CH3OH basis. Prior to the adsorption of Pt and Co, silica wet-gels were aged at room temperature for a couple of days and the solvents were exchanged to ethanol solution of H2PtCl6 Æ 6H2O until the yellowish color was optically homogeneous (Pt–SiO2). Platinum content is automatically determined by the adsorption ability of the silica wet-gels. Cobalt nitrate was added in a similar fashion as the adsorption of the platinum precursor (PtCo–SiO2) in series, and it was also used in impregnation on the prepared Pt–SiO2 aerogel (Co/Pt–SiO2). Comparisons were made by Al2O3 aerogels with the non-alkoxide method. The mole ratio of AlCl3 Æ 6H2O:C2H5OH:Propylene oxide:HNO3:H2O was 1:30:4.5: 0.01:3. Platinum and cobalt were imposed through cogelation and soaking methods. Supercritical drying was conducted with carbon dioxide and its final conditions were 24 MPa and 80 C. The dried aerogel was subjected to a standard calcination procedure in a tubular furnace, by heating at 300 C in helium and at 500 C in oxygen. The oxidized aerogels were reduced in flowing hydrogen at 400 C right before the catalytic reaction. 2.2. Characterization BET surface areas, pore volumes, and pore size distributions were measured by nitrogen adsorption–desorption at 196 C using a Micromeritics ASAP 2000 and 2001 apparatus. The pore size distributions were calculated by Baret– Joyner–Halenda (BJH) method. Prior to the measurement, all the samples were outgassed at 110 C overnight. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/MAX-IIA diffractometer using Cu Ka radiation. Transmission electron microscopy (TEM) images were taken with a Philips CM 30 microscope. Temperature programmed reduction (TPR) equipped with a thermal conductivity detector (TCD) was used on the oxidized
Prior to the reaction, all the prepared catalysts were reduced at 400 C for 2 h. The PROX reactions were carried out in a fixed-bed reactor system under atmospheric pressure with 0.1 g of catalysts. Total flow rate was 100 ml/min containing 1.1% of hydrogen, 2500 ppm of carbon monoxide, and 2500 ppm of oxygen. The reaction temperature was 25–300 C with 25 C increment and held for 1.5 h at each temperature. For analyzing the effluent gas, CO analyzer (Thermo Environmental Instruments Inc., 48 C CO analyzer) and a gas chromatograph (Younglin M600D), equipped with thermal conductivity detector (TCD) and flame ionization detector (FID) employing a packed column (6 0 · 1/8 0 0 · 0.085 0 0 SS, Carbosphere 80/ 100) with a methanator, were used. 3. Result and discussion 3.1. Synthesis Most of inorganic gels such as alumina, titania, zirconia, etc. are usually obtained by an acid catalyzed method in order not to have precipitate-like gels. Unlike these, in the process of silica gel synthesis, basic catalysts with fluoride ions were often used to reduce the gelation time. This makes it difficult to obtain silica-containing multi-component cogels due to easy formation of precipitates of other metal precursors. The soaking/adsorption method could be suggested as an alternative, though it may take a long time to complete the preparation. The benefits include adjustable acidity after gelation, which lead to less restriction of using various metal precursors, surface dominancy of active sites without encapsulation, and simplification in synthesis of multiple components with diverse solubility. When composite aerogels are synthesized by cogelation method, a certain amount of additives are entrapped in the structures, especially when two species are strongly interacted. This entrapping phenomenon is not rare even in other catalytic preparation method after thermal treatment, for example, atomic diffusion of vanadium into titania is very common [20]. N,N-Dimethylformamide (DMF) as a cosolvent improved not only transparency and bouncing in silica gels, but also Pt anion stability compared with using a single solvent such as methanol or ethanol. Though the reasons were obscure, it is presumed that aprotic characteristics of DMF affected the phenomena. It might
shield the partially positive Si from interaction with negative RO–M (or HO–M) groups in silica structure and derive more linear gel networks, and also encapsulate positively charged atom/group so that PtCl62 can act more freely as anions without Pt–Cl bond breakage or precipitation. DMF is an aprotic solvent that has non-bonded pair electrons on oxygen. These electrons can act as nucleophiles to positively charged metal atoms (here Si) as the same way of actylacetone, and protect metal atoms from the attack of other nucleophiles such as RO–M and HO– M. Precipitation of Pt is initiated by Pt–Cl breakage probably due to nucleophilic attacks. As long as there is negligible nucleophile in the solution (NH3OH mole ratio is too small to have freely movable OH anion, which mostly attached to Si atom), positively charged atom/group tends to attract the PtCl62 anion and stimulate it to react with other terminal groups in silica structure. Negatively charged OR groups mostly bond to positively charged Si species. Thus, encapsulation of positively charged species by ‘bulk DMF’ prohibit PtCl62 from the contact with terminal OR groups. This also resulted in very low content of Pt species in composite aerogels. Theoretically, the partial charge of OR (or OH) diminishes its absolute value of negativity and/or become positive occasionally in the middle of the polymeric networks as Livage et al. [21] suggested in the case of titania gelation. Thus, for the adsorption of additional metals in a wet-gel state, two kinds of probable adsorption would be considerable for Pt anions according to the surface charges of silica wet-gels. One could be the adsorption on partially positively charged OR groups, and the other be on the positive sites made by terminal hydrogen on non-dehydrated OH2 attached to Si atom as the same concept as in the acidic condition in accordance with the point of zero charge. And we also postulated that cobalt cation possibly attracted the formerly adsorbed Pt anion and repelled the positive outer surface of the gel networks. As often considered that heavy metal ions are more readily incorporated into the support, it would be reasonable that the process may proceed in the sequence of Pt and Co. Platinum anion and cobalt cation could be adsorbed (or linked) on different sites during the cogelation process. Terminal OR group changes its partial charge considerably as the gelation proceeds as Livage et al. suggested [21]. Thus, during the cogelation there would be no selective attraction for both anions and cations to silica networks. Partial charge of OR in the middle of linkage could have opposite negativity to that in the ends. This could separate Pt sites from Co sites. The availability of characterization tools for small amount of Pt (<0.5 wt.%) loaded samples were extremely limited for analyzing Pt state. Thus, an indirect method of temperature programmed reduction (TPR) was employed to predict the qualitative characteristics of Pt and Co. The results of TPR as shown in Figs. 1 and 2 provided the information of Pt/Co systems in two different methods of coge-
H2 consumption (a.u.)
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
(c)
(b)
(a)
200
400
600
Temperature
800
(oC)
Fig. 1. TPR patterns for aerogels of (a) Pt–Al2O3 by soaking, (b) Pt– Al2O3 by cogelation, and (c) Pt–Co–Al2O3 by cogelation; Pt (s) and Co (d) sites.
* H2 consumption (a.u.)
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(d)
(c)
*
(b) (a)
200
400
600
Temperature
800
(oC)
Fig. 2. TPR patterns for (a) Pt–SiO2, (b) Co–SiO2, (c) Co/Pt–SiO2, and (d) PtCo–SiO2 aerogels; Pt (s), Co (d) and Pt–Co () sites.
lation and soaking/adsorption. Inevitably, alumina cogels were compared to distinguish cogelation effects. Cogelation method showed relatively less homogeneous Pt dispersion than the soaking method. As presented in Fig. 1a and b, Pt sites (s) in alumina aerogels were formed in two kinds, which might be caused by the differences in the Pt particle size and the Pt–alumina interactions [22]. Moreover, the addition of cobalt restrained Pt from setting on different positions, but cobalt sites (d) were formed independently. During the chemical processes of hydrolysis and condensation, charges of the surface of sols were changeable, so that both anion and cation could have attraction to the support materials. Hence, components would be apart from each other. Silica aerogels containing a reducible component such as Pt or Co exhibited dissimilar TPR patterns. Pt has no
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
3.2. Textural properties Prepared aerogels exhibited the intrinsic characteristics, such as high specific surface area, large pore volume, mesoporosity, and narrow pore size distribution as given in Table 1 and Fig. 3. While Co impregnation had an effect on decreasing the original properties, PtCo–SiO2 aerogel catalyst maintained those properties even after Pt and Co soaking/adsorption because liquid exchange did not affect the gel network shrinkage, which is usually caused by surface tension during the process of solvent drying. The surface areas of silica and alumina were over 900 m2/g and 500 m2/g, respectively, with 10– 20 nm pore diameters after 500 C calcination. The addition of Pt, Co or Pt + Co in silica gel may exert a minor effect on the final textural properties of the composite aerogels. The differences in cobalt content between PtCo–SiO2 and Co/Pt–SiO2 implied that Co was randomly distributed on silica wet-gels in impregnation method. Cobalt cation may be selectively attracted to Pt anion sites during the soaking/adsorption procedure. This result was in good agreement with the TPR indicating that Pt and Co coexisting sites were dominant on PtCo–SiO2 catalyst. Distribution of cobalt species in impregnation method depends on the concentration of Co and support materials. Here, the maximum cobalt content is 1.32 wt.%, and sur-
Table 1 BET surface area (SBET), total pore volume (Vp), and average pore size (Dp) of the catalysts calcined at 773 K Sample name
Content (wt.%)
SBET (m2/ g)a
Vp (cc/ g)b
Dp (nm)c
SiO2 Pt–SiO2 Co–SiO2 PtCo–SiO2
– Pt (0.18) Co (0.60) Pt (0.12), Co (0.32) Pt (0.17), Co (1.32) Pt (0.85) Pt (0.97), Co (1.29) Pt (1.74)
1184 998 1020 1132
5.09 3.42 4.96 4.69
16.6 13.7 18.8 16.0
901
2.30
10.2
628 504
3.03 2.51
19.3 19.0
544
2.22
15.6
Co/Pt–SiO2 Pt–Al2O3 (cogel) Pt–Co–Al2O3 (cogel) Pt–Al2O3 (soaking) a b c
Specific surface area. Total pore volume. Average pore diameter.
30
Pore Volume (dV/dlog(D), cc/g)
discernable peaks while Co had peaks at ca. 300 C, 650 C and 850 C (Co–SiO2) as given in Fig. 2a and b. When cobalt was impregnated on Pt–SiO2, Co reduction peaks were also observed independently, which implied that Co was not selectively adsorbed on Pt sites but all over the silica surface as shown in Fig. 2c. However, Fig. 2d indicated that soaking method enabled cobalt to be attracted mostly to the Pt sites, resulting in the formation of easily reducible Pt–Co (*) sites.
883
Pt-SiO 2 aerogel PtCo-SiO 2 aerogel
25
Co/Pt-SiO 2 aerogel
20 15 10 5 0 1
10
100
Pore Diameter (nm) Fig. 3. Pore size distributions of the prepared aerogels.
face area of silica is higher than 900 m2/g, which means that Co should have a large scale of distribution. If cobalt has selective interaction with Pt sites even in impregnation method it should have much more Pt–Co interactions than in PtCo–SiO2 catalyst because Co/Pt–SiO2 has 3 times higher Co content. Soaking/adsorption method takes place when Co and Pt species are cation and anion. But, in impregnation, Pt is in metallic or oxides state, which are totally different state. We believe that the higher Co content with smaller Pt–Co interaction means that Co is randomly distributed. It is meaningful because the Co content is higher in Co/Pt–SiO2 than in PtCo–SiO2, which implies that preparation method determines the Pt–Co interactions. The Pt content on alumina was much higher than that on silica as given in Table 1, which presented that Pt anions were more preferentially adsorbed on alumina. It is not clear that acidity was responsible for more attraction of Pt anion to alumina wet-gels because the acidity conditions for gel formation were opposite, i.e. acidic for alumina and basic for silica gels. However, catalytic activity for PROX reaction was much more sensitive to the availability of interacted active sites than to the content of Pt, which can be observed from Figs. 5 and 7. Well-developed three-dimensional aerogel networks could be observed in TEM images as illustrated in Fig. 4. Pores surrounded by the primary particles revealed similar size (10–20 nm) to the N2-sorption results. Although it was difficult to clearly discriminate Pt particles, the aerogel was presumably composed of platinum particles of 5 nm or less and primary particles of silica having a diameter of ca. 10 nm. 3.3. Catalytic activities Catalytic evaluations were performed for CO oxidation from hydrogen-rich fuels in a temperature range of 25–
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J. Choi et al. / Catalysis Communications 9 (2008) 880–885
2500 PtCo-SiO2
CO concentration (ppm)
25
Co/Pt-SiO2
50
2000
1500
75
1000
500
275 300
100
250 125-175
200
225
0 0
3
6
9
12
15
18
Time (hr) Fig. 6. Results of the PROX reaction over Co/Pt–SiO2 and PtCo–SiO2 aerogel catalysts through the on-line CO analyzer.
Fig. 4. TEM images of Pt–SiO2 aerogel.
300 C. The variation of CO concentration as a function of temperature was presented in Figs. 5 and 6. The concentrations in Fig. 5 were numerical mean values for 1.5 h reac-
tion time for an approximate estimation. Fig. 6 showed the exact changes in CO concentration measured by an on-line CO analyzer. Overall catalytic activities were in order of Co/Pt–SiO2 PtCo–SiO2 Pt–SiO2 > commercial Pt/Al2O3 (Aldrich) > Pt–Co–Al2O3 Pt–Al2O3. A commercial 1wt.% Pt/Al2O3 (Aldrich) was taken as a reference. The temperature ranges where catalytic activities reached to the desired CO level of below 20 ppm were different for different catalytic systems, i.e. 100–175 C for Co/Pt–SiO2, 125–250 C for PtCo–SiO2, 200–225 C for Pt–SiO2, and 225 C for a commercial Pt/ Al2O3 catalyst. These results suggested that the onset and completion temperatures of the reaction were closely related to TPR profiles, particularly Pt reduction temperatures. Moreover, catalytic stability at high temperatures was dependent upon
1400
2500
Pt-Al2O3 cogel
Co/Pt-SiO2
CO concentration (ppm)
Pt-SiO2
2000
Aldrich Pt-Al2O3
1500
1000
500
CO Concentration (ppm)
1200
PtCo-SiO2
Pt-Co-Al2O3 cogel
1000 800 600 400 200 0
0 50
100
150
200
Temperature
250
300
(oC)
Fig. 5. Preferential oxidation of CO over Co/Pt–SiO2, PtCo–SiO2, Pt– SiO2, and a commercial Pt/Al2O3 (Aldrich).
50
100
150
200
Temperature
250
300
(oC)
Fig. 7. PROX activity over Pt–Co–Al2O3 and Pt–Al2O3 co-aerogels; initial concentration of CO was 1100 ppm.
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
the ability to keep Pt present in a reduced state during the PROX reaction proceeded in a redox environment. Easily reducible Pt sites may be more responsible for obtaining better CO oxidation activity. As Co rendered Pt reducible at lower temperature, the reaction started at much lower temperatures. And the activity for Pt–Co– Al2O3 and Pt–Al2O3 co-aerogels prepared by non-alkoxide method indicated that Pt and Co sites were formed in individual positions as TPR patterns suggested. 4. Conclusion Platinum and cobalt sites were found in their different states according to the preparation methods. Soaking/ adsorption procedure in a silica wet-gel state enabled the components to form co-existing sites. Solvent DMF was likely to stabilize Pt anions, and cobalt cations selectively attached to Pt sites. Compared with cogelation and impregnation, the resulting characteristics of homogeneity and textural properties were largely improved. The catalytic activity for preferential oxidation was significantly influenced by the low-temperature reducible Pt sites. The major roles of cobalt were not only to make Pt be easily reduced, but also to stabilize Pt in its reduced state, as inferred from the high catalytic activity in a wide range of temperature. Acknowledgements One of the authors (C.B. Shin) acknowledges the Korea Science and Engineering Foundation (KOSEF R01-2006000-10239-0) for the partial financial support of this work.
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References [1] S.J. Teichner, G.A. Nicolaon, M.A. Vicarini, G.E.E. Gardes, Adv. Colloid. Sci. 5 (1976) 245. [2] B.C. Dunn, P. Cole, D. Covington, M.C. Webster, R.J. Pugmire, R.D. Ernst, E.M. Eyring, N. Shah, G.P. Huffman, Appl. Catal. A: Gen. 278 (2005) 233. [3] M.D. Wildberger, T. Mallat, U. Gobel, A. Baiker, Appl. Catal. A: Gen. 168 (1998) 69. [4] F. Somma, P. Canton, G. Strukul, J. Catal. 229 (2005) 490. [5] A.M. Siouffi, J. Chromatogr. A 1000 (2003) 801. [6] A. Rahmani, P. Jund, C. Benoit, R. Jullien, J. Phys.: Condens. Matter 13 (2001) 5413. [7] Michael C.J. Bradford, M. Albert Vannice, Catal. Today 50 (1999) 87. [8] T. Miyao, M. Yamauchi, S. Naito, Catal. Today 87 (2003) 227. [9] C. Pedrero, T. Waku, E. Iglesia, J. Catal. 233 (2005) 242. [10] F.J. Gracia, L. Bollmann, E.E. Wolf, J.T. Miller, A.J. Kropf, J. Catal. 220 (2003) 382. [11] W.D. Rhodes, J.L. Margitfalvi, I. Borbath, K. Lazar, V.I. Kovalchuk, J.L. d’Itri, J. Catal. 230 (2005) 86. [12] T. Mori, D. Tomikawa, Y. Katano, J. Kubo, Y. Morikawa, Appl. Catal. A: Gen. 271 (2004) 77. [13] C. Kwak, T.-J. Park, D.J. Suh, Appl. Catal. A: Gen. 278 (2005) 181. [14] D.J. Suh, C. Kwak, J.-H. Kim, S.M. Kwon, T.-J. Park, J. Power Sources 142 (2005) 70. [15] M.J. Kahlich, H.A. Gasteiger, R.J. Behm, J. Catal. 182 (1999) 430. [16] T.V. Choudhary, C. Sivadinarayana, C.C. Chusuei, A.K. Datye, J.P. Fackler, D.W. Goodman, J. Catal. 207 (2002) 247. [17] G. Panzera, V. Modafferi, S. Candamano, A. Donato, F. Frusteri, P.L. Antonucci, J. Power Sources 135 (2004) 177. [18] M. Schreier, J.R. Regalbuto, J. Catal. 225 (2004) 190. [19] J. Gu¨nster, R. Souda, Chem. Phys. Lett. 371 (2003) 534. [20] J. Choi, C.B Shin, D.J. Suh, J. Mol. Catal A: Chem. submitted for publication. [21] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [22] L. Pino, A. Vita, F. Cipiti, M. Lagana, V. Recupero, Appl. Catal. A: Gen. 306 (2006) 68.
Catalysis Communications 9 (2008) 880–885 www.elsevier.com/locate/catcom
Co-promoted Pt catalysts supported on silica aerogel for preferential oxidation of CO Jinsoon Choi a, Chee Burm Shin a, Dong Jin Suh b
b,*
a Department of Chemical Engineering, Ajou University, Wonchon, Paldal, Suwon, Kyunggi 442-749, Republic of Korea Clean Technology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, Republic of Korea
Received 12 January 2007; received in revised form 11 September 2007; accepted 14 September 2007 Available online 6 October 2007
Abstract Pt and Co doped silica aerogels have been synthesized by sol–gel method and subsequent supercritical drying. Wet-gel state of silica was used in the adsorption process in order to accommodate Pt and Co. Catalytic activities for preferential oxidation of CO in H2-rich fuels were measured in the temperature range from 25 C to 300 C. At 2500 ppm of carbon monoxide, CoPt–SiO2 aerogels showed good activities in a wide range of temperatures maintaining less than 20 ppm at 125–250 C. 2007 Elsevier B.V. All rights reserved. Keywords: PROX; CO oxidation; Silica; Aerogel; Pt; Co
1. Introduction Silica aerogel is an excellent catalytic support due to its desirable characteristics, such as inertness, high specific surface area, pronounced mesoporosity, and good thermal stability. Since Teichner et al. [1] produced the alcogel in 1970s, there have been numerous reports on the synthetic methods of silica aerogel in catalytic applications [2–6], focused on controlling and optimizing its physical/chemical parameters, and in present days it is commercially available in various forms from powder types to monolithic bricks. Silica-supported platinum catalysts were used for methane/methanol reformation [7,8], CO oxidation [9,10], and hydrodechlorination [11,12] reactions. High specific surface area with little interactions to noble metal caused highly dispersed active sites, and the reducibility of metal oxides on silica was much greater than that on alumina. As platinum has great stability in oxidation processes, many researchers attempted to employ supported Pt catalysts with additional compounds for the preferential oxidation (PROX) from hydrogen-rich fuels [13,14]. Despite the *
Corresponding author. Tel.: +82 2 958 5192; fax: +82 2 958 5205. E-mail address: [email protected] (D.J. Suh).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.09.036
trend has been recently shifted to gold catalysts [15–17], where the reactions were mostly conducted around room temperatures due to the drastic decrease in activity at above 100 C, there have been increasing demands for improved platinum catalysts to satisfy various operational conditions in the range of 100–300 C. The usual method for the preparation of Pt/SiO2 catalysts involved electrostatic adsorption, where point of zero charge (PZC) of silica determined adsorption ability of Pt precursors according to the charge attraction [18]. In general, chloroplatinic acid (H2PtCl6) was not strongly adsorbed on silica but on alumina or activated carbon. Thus, Pt(NH3)4Cl and Pt(NH3)2(NO2)2 were alternatively used as cationic platinum precursors. In addition, for the PROX reaction, it was known that addition of base metals (Na, K, Cs) or transition metals (Co, Ni, Cr) significantly enhanced the activity of Pt catalysts, probably due to the changes in the adsorption characteristics of adsorbates either through electron transfer [19] or resulting bimetallic characteristics/effects [13]. In this study, Pt–SiO2 and CoPt–SiO2 aerogels were prepared by adsorption method with H2PtCl6, not with cationic Pt sources, in a wet-gel state by solvent exchanges. We considered that surface conditions of silica wet-gel
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
would be qualitatively different from silica oxides or silica dried-gels. As Livage et al. [20] suggested in titania polymerization, there would be an increasing portion of partially positive (or slightly negative) charged OR groups in the silica wet-gel networks as polymeric condensations proceed. The purpose of choosing H2PtCl6 as a Pt precursor is two fold: to facilitate the Pt adsorption on the positive charged surface and to attract the cationic metal ion such as Co2+ in order to derive Pt–Co bimetallic species.
881
aerogels for an appreciation of the interaction between Pt and Co. Samples placed in a quartz reactor was reduced in a stream of 4% H2/Ar mixture with 30 ml/min flow rate in the temperature range between 27 C and 927 C, at the heating rate of 5 C/min. Inductive coupled plasma atomic emission spectrometry (ICP-AES) and atomic absorption spectrometry (AAS) were used for estimating the Pt and Co contents, respectively. 2.3. Catalytic activity
2. Experimental 2.1. Catalyst preparation In the synthesis of silica aerogels, two sets of solutions were prepared. (i) (CH3OH + HCON(CH3)2 (DMF) + Si(OC2H5)4 (TEOS)), (ii) (NH4OH + NH4F + H2O). After an individual mixing for an hour, the solutions were mixed and stirred until gelation occurred. The overall mole ratio was TEOS:CH3OH:DMF:NH4OH:NH4F:H2O = 1:5.7:5.7: 0.005:0.0005:4 with 20.8 ml of CH3OH basis. Prior to the adsorption of Pt and Co, silica wet-gels were aged at room temperature for a couple of days and the solvents were exchanged to ethanol solution of H2PtCl6 Æ 6H2O until the yellowish color was optically homogeneous (Pt–SiO2). Platinum content is automatically determined by the adsorption ability of the silica wet-gels. Cobalt nitrate was added in a similar fashion as the adsorption of the platinum precursor (PtCo–SiO2) in series, and it was also used in impregnation on the prepared Pt–SiO2 aerogel (Co/Pt–SiO2). Comparisons were made by Al2O3 aerogels with the non-alkoxide method. The mole ratio of AlCl3 Æ 6H2O:C2H5OH:Propylene oxide:HNO3:H2O was 1:30:4.5: 0.01:3. Platinum and cobalt were imposed through cogelation and soaking methods. Supercritical drying was conducted with carbon dioxide and its final conditions were 24 MPa and 80 C. The dried aerogel was subjected to a standard calcination procedure in a tubular furnace, by heating at 300 C in helium and at 500 C in oxygen. The oxidized aerogels were reduced in flowing hydrogen at 400 C right before the catalytic reaction. 2.2. Characterization BET surface areas, pore volumes, and pore size distributions were measured by nitrogen adsorption–desorption at 196 C using a Micromeritics ASAP 2000 and 2001 apparatus. The pore size distributions were calculated by Baret– Joyner–Halenda (BJH) method. Prior to the measurement, all the samples were outgassed at 110 C overnight. Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/MAX-IIA diffractometer using Cu Ka radiation. Transmission electron microscopy (TEM) images were taken with a Philips CM 30 microscope. Temperature programmed reduction (TPR) equipped with a thermal conductivity detector (TCD) was used on the oxidized
Prior to the reaction, all the prepared catalysts were reduced at 400 C for 2 h. The PROX reactions were carried out in a fixed-bed reactor system under atmospheric pressure with 0.1 g of catalysts. Total flow rate was 100 ml/min containing 1.1% of hydrogen, 2500 ppm of carbon monoxide, and 2500 ppm of oxygen. The reaction temperature was 25–300 C with 25 C increment and held for 1.5 h at each temperature. For analyzing the effluent gas, CO analyzer (Thermo Environmental Instruments Inc., 48 C CO analyzer) and a gas chromatograph (Younglin M600D), equipped with thermal conductivity detector (TCD) and flame ionization detector (FID) employing a packed column (6 0 · 1/8 0 0 · 0.085 0 0 SS, Carbosphere 80/ 100) with a methanator, were used. 3. Result and discussion 3.1. Synthesis Most of inorganic gels such as alumina, titania, zirconia, etc. are usually obtained by an acid catalyzed method in order not to have precipitate-like gels. Unlike these, in the process of silica gel synthesis, basic catalysts with fluoride ions were often used to reduce the gelation time. This makes it difficult to obtain silica-containing multi-component cogels due to easy formation of precipitates of other metal precursors. The soaking/adsorption method could be suggested as an alternative, though it may take a long time to complete the preparation. The benefits include adjustable acidity after gelation, which lead to less restriction of using various metal precursors, surface dominancy of active sites without encapsulation, and simplification in synthesis of multiple components with diverse solubility. When composite aerogels are synthesized by cogelation method, a certain amount of additives are entrapped in the structures, especially when two species are strongly interacted. This entrapping phenomenon is not rare even in other catalytic preparation method after thermal treatment, for example, atomic diffusion of vanadium into titania is very common [20]. N,N-Dimethylformamide (DMF) as a cosolvent improved not only transparency and bouncing in silica gels, but also Pt anion stability compared with using a single solvent such as methanol or ethanol. Though the reasons were obscure, it is presumed that aprotic characteristics of DMF affected the phenomena. It might
shield the partially positive Si from interaction with negative RO–M (or HO–M) groups in silica structure and derive more linear gel networks, and also encapsulate positively charged atom/group so that PtCl62 can act more freely as anions without Pt–Cl bond breakage or precipitation. DMF is an aprotic solvent that has non-bonded pair electrons on oxygen. These electrons can act as nucleophiles to positively charged metal atoms (here Si) as the same way of actylacetone, and protect metal atoms from the attack of other nucleophiles such as RO–M and HO– M. Precipitation of Pt is initiated by Pt–Cl breakage probably due to nucleophilic attacks. As long as there is negligible nucleophile in the solution (NH3OH mole ratio is too small to have freely movable OH anion, which mostly attached to Si atom), positively charged atom/group tends to attract the PtCl62 anion and stimulate it to react with other terminal groups in silica structure. Negatively charged OR groups mostly bond to positively charged Si species. Thus, encapsulation of positively charged species by ‘bulk DMF’ prohibit PtCl62 from the contact with terminal OR groups. This also resulted in very low content of Pt species in composite aerogels. Theoretically, the partial charge of OR (or OH) diminishes its absolute value of negativity and/or become positive occasionally in the middle of the polymeric networks as Livage et al. [21] suggested in the case of titania gelation. Thus, for the adsorption of additional metals in a wet-gel state, two kinds of probable adsorption would be considerable for Pt anions according to the surface charges of silica wet-gels. One could be the adsorption on partially positively charged OR groups, and the other be on the positive sites made by terminal hydrogen on non-dehydrated OH2 attached to Si atom as the same concept as in the acidic condition in accordance with the point of zero charge. And we also postulated that cobalt cation possibly attracted the formerly adsorbed Pt anion and repelled the positive outer surface of the gel networks. As often considered that heavy metal ions are more readily incorporated into the support, it would be reasonable that the process may proceed in the sequence of Pt and Co. Platinum anion and cobalt cation could be adsorbed (or linked) on different sites during the cogelation process. Terminal OR group changes its partial charge considerably as the gelation proceeds as Livage et al. suggested [21]. Thus, during the cogelation there would be no selective attraction for both anions and cations to silica networks. Partial charge of OR in the middle of linkage could have opposite negativity to that in the ends. This could separate Pt sites from Co sites. The availability of characterization tools for small amount of Pt (<0.5 wt.%) loaded samples were extremely limited for analyzing Pt state. Thus, an indirect method of temperature programmed reduction (TPR) was employed to predict the qualitative characteristics of Pt and Co. The results of TPR as shown in Figs. 1 and 2 provided the information of Pt/Co systems in two different methods of coge-
H2 consumption (a.u.)
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
(c)
(b)
(a)
200
400
600
Temperature
800
(oC)
Fig. 1. TPR patterns for aerogels of (a) Pt–Al2O3 by soaking, (b) Pt– Al2O3 by cogelation, and (c) Pt–Co–Al2O3 by cogelation; Pt (s) and Co (d) sites.
* H2 consumption (a.u.)
882
(d)
(c)
*
(b) (a)
200
400
600
Temperature
800
(oC)
Fig. 2. TPR patterns for (a) Pt–SiO2, (b) Co–SiO2, (c) Co/Pt–SiO2, and (d) PtCo–SiO2 aerogels; Pt (s), Co (d) and Pt–Co () sites.
lation and soaking/adsorption. Inevitably, alumina cogels were compared to distinguish cogelation effects. Cogelation method showed relatively less homogeneous Pt dispersion than the soaking method. As presented in Fig. 1a and b, Pt sites (s) in alumina aerogels were formed in two kinds, which might be caused by the differences in the Pt particle size and the Pt–alumina interactions [22]. Moreover, the addition of cobalt restrained Pt from setting on different positions, but cobalt sites (d) were formed independently. During the chemical processes of hydrolysis and condensation, charges of the surface of sols were changeable, so that both anion and cation could have attraction to the support materials. Hence, components would be apart from each other. Silica aerogels containing a reducible component such as Pt or Co exhibited dissimilar TPR patterns. Pt has no
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
3.2. Textural properties Prepared aerogels exhibited the intrinsic characteristics, such as high specific surface area, large pore volume, mesoporosity, and narrow pore size distribution as given in Table 1 and Fig. 3. While Co impregnation had an effect on decreasing the original properties, PtCo–SiO2 aerogel catalyst maintained those properties even after Pt and Co soaking/adsorption because liquid exchange did not affect the gel network shrinkage, which is usually caused by surface tension during the process of solvent drying. The surface areas of silica and alumina were over 900 m2/g and 500 m2/g, respectively, with 10– 20 nm pore diameters after 500 C calcination. The addition of Pt, Co or Pt + Co in silica gel may exert a minor effect on the final textural properties of the composite aerogels. The differences in cobalt content between PtCo–SiO2 and Co/Pt–SiO2 implied that Co was randomly distributed on silica wet-gels in impregnation method. Cobalt cation may be selectively attracted to Pt anion sites during the soaking/adsorption procedure. This result was in good agreement with the TPR indicating that Pt and Co coexisting sites were dominant on PtCo–SiO2 catalyst. Distribution of cobalt species in impregnation method depends on the concentration of Co and support materials. Here, the maximum cobalt content is 1.32 wt.%, and sur-
Table 1 BET surface area (SBET), total pore volume (Vp), and average pore size (Dp) of the catalysts calcined at 773 K Sample name
Content (wt.%)
SBET (m2/ g)a
Vp (cc/ g)b
Dp (nm)c
SiO2 Pt–SiO2 Co–SiO2 PtCo–SiO2
– Pt (0.18) Co (0.60) Pt (0.12), Co (0.32) Pt (0.17), Co (1.32) Pt (0.85) Pt (0.97), Co (1.29) Pt (1.74)
1184 998 1020 1132
5.09 3.42 4.96 4.69
16.6 13.7 18.8 16.0
901
2.30
10.2
628 504
3.03 2.51
19.3 19.0
544
2.22
15.6
Co/Pt–SiO2 Pt–Al2O3 (cogel) Pt–Co–Al2O3 (cogel) Pt–Al2O3 (soaking) a b c
Specific surface area. Total pore volume. Average pore diameter.
30
Pore Volume (dV/dlog(D), cc/g)
discernable peaks while Co had peaks at ca. 300 C, 650 C and 850 C (Co–SiO2) as given in Fig. 2a and b. When cobalt was impregnated on Pt–SiO2, Co reduction peaks were also observed independently, which implied that Co was not selectively adsorbed on Pt sites but all over the silica surface as shown in Fig. 2c. However, Fig. 2d indicated that soaking method enabled cobalt to be attracted mostly to the Pt sites, resulting in the formation of easily reducible Pt–Co (*) sites.
883
Pt-SiO 2 aerogel PtCo-SiO 2 aerogel
25
Co/Pt-SiO 2 aerogel
20 15 10 5 0 1
10
100
Pore Diameter (nm) Fig. 3. Pore size distributions of the prepared aerogels.
face area of silica is higher than 900 m2/g, which means that Co should have a large scale of distribution. If cobalt has selective interaction with Pt sites even in impregnation method it should have much more Pt–Co interactions than in PtCo–SiO2 catalyst because Co/Pt–SiO2 has 3 times higher Co content. Soaking/adsorption method takes place when Co and Pt species are cation and anion. But, in impregnation, Pt is in metallic or oxides state, which are totally different state. We believe that the higher Co content with smaller Pt–Co interaction means that Co is randomly distributed. It is meaningful because the Co content is higher in Co/Pt–SiO2 than in PtCo–SiO2, which implies that preparation method determines the Pt–Co interactions. The Pt content on alumina was much higher than that on silica as given in Table 1, which presented that Pt anions were more preferentially adsorbed on alumina. It is not clear that acidity was responsible for more attraction of Pt anion to alumina wet-gels because the acidity conditions for gel formation were opposite, i.e. acidic for alumina and basic for silica gels. However, catalytic activity for PROX reaction was much more sensitive to the availability of interacted active sites than to the content of Pt, which can be observed from Figs. 5 and 7. Well-developed three-dimensional aerogel networks could be observed in TEM images as illustrated in Fig. 4. Pores surrounded by the primary particles revealed similar size (10–20 nm) to the N2-sorption results. Although it was difficult to clearly discriminate Pt particles, the aerogel was presumably composed of platinum particles of 5 nm or less and primary particles of silica having a diameter of ca. 10 nm. 3.3. Catalytic activities Catalytic evaluations were performed for CO oxidation from hydrogen-rich fuels in a temperature range of 25–
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J. Choi et al. / Catalysis Communications 9 (2008) 880–885
2500 PtCo-SiO2
CO concentration (ppm)
25
Co/Pt-SiO2
50
2000
1500
75
1000
500
275 300
100
250 125-175
200
225
0 0
3
6
9
12
15
18
Time (hr) Fig. 6. Results of the PROX reaction over Co/Pt–SiO2 and PtCo–SiO2 aerogel catalysts through the on-line CO analyzer.
Fig. 4. TEM images of Pt–SiO2 aerogel.
300 C. The variation of CO concentration as a function of temperature was presented in Figs. 5 and 6. The concentrations in Fig. 5 were numerical mean values for 1.5 h reac-
tion time for an approximate estimation. Fig. 6 showed the exact changes in CO concentration measured by an on-line CO analyzer. Overall catalytic activities were in order of Co/Pt–SiO2 PtCo–SiO2 Pt–SiO2 > commercial Pt/Al2O3 (Aldrich) > Pt–Co–Al2O3 Pt–Al2O3. A commercial 1wt.% Pt/Al2O3 (Aldrich) was taken as a reference. The temperature ranges where catalytic activities reached to the desired CO level of below 20 ppm were different for different catalytic systems, i.e. 100–175 C for Co/Pt–SiO2, 125–250 C for PtCo–SiO2, 200–225 C for Pt–SiO2, and 225 C for a commercial Pt/ Al2O3 catalyst. These results suggested that the onset and completion temperatures of the reaction were closely related to TPR profiles, particularly Pt reduction temperatures. Moreover, catalytic stability at high temperatures was dependent upon
1400
2500
Pt-Al2O3 cogel
Co/Pt-SiO2
CO concentration (ppm)
Pt-SiO2
2000
Aldrich Pt-Al2O3
1500
1000
500
CO Concentration (ppm)
1200
PtCo-SiO2
Pt-Co-Al2O3 cogel
1000 800 600 400 200 0
0 50
100
150
200
Temperature
250
300
(oC)
Fig. 5. Preferential oxidation of CO over Co/Pt–SiO2, PtCo–SiO2, Pt– SiO2, and a commercial Pt/Al2O3 (Aldrich).
50
100
150
200
Temperature
250
300
(oC)
Fig. 7. PROX activity over Pt–Co–Al2O3 and Pt–Al2O3 co-aerogels; initial concentration of CO was 1100 ppm.
J. Choi et al. / Catalysis Communications 9 (2008) 880–885
the ability to keep Pt present in a reduced state during the PROX reaction proceeded in a redox environment. Easily reducible Pt sites may be more responsible for obtaining better CO oxidation activity. As Co rendered Pt reducible at lower temperature, the reaction started at much lower temperatures. And the activity for Pt–Co– Al2O3 and Pt–Al2O3 co-aerogels prepared by non-alkoxide method indicated that Pt and Co sites were formed in individual positions as TPR patterns suggested. 4. Conclusion Platinum and cobalt sites were found in their different states according to the preparation methods. Soaking/ adsorption procedure in a silica wet-gel state enabled the components to form co-existing sites. Solvent DMF was likely to stabilize Pt anions, and cobalt cations selectively attached to Pt sites. Compared with cogelation and impregnation, the resulting characteristics of homogeneity and textural properties were largely improved. The catalytic activity for preferential oxidation was significantly influenced by the low-temperature reducible Pt sites. The major roles of cobalt were not only to make Pt be easily reduced, but also to stabilize Pt in its reduced state, as inferred from the high catalytic activity in a wide range of temperature. Acknowledgements One of the authors (C.B. Shin) acknowledges the Korea Science and Engineering Foundation (KOSEF R01-2006000-10239-0) for the partial financial support of this work.
885
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