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SiO2 bimetallic catalysts and its co oxidation activity
A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control 111 Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.
179
REDUCTION BEHAVIOR OF Rh-Sn/SiO2 BIMETALLIC CATALYSTS AND ITS CO OXIDATION ACTIVITY Satoru Nishiyama, Ikuo Yamamoto, Masahiro Akemoto, Shigeru Tsuruya, and Mitsuo Masai Department of Chemical Science and Engineermg, Faculty of Engineermg, Kobe University, Rokkodai, Nada, Kobe 657, Japan
ABSTRACT
Reduction behavior of Rh-Sn on SiO2 was studied by temperature programmed reduction methods. During the reduction of Rh-Sn/SiO2 preoxidized, Rh oxide was reducible in the range of 370-440 K first, then the reduction of Sn oxide proceeded at a higher temperature up to 470 K. X-ray diffraction powder patterns indicates that the higher temperature than 673 K was required in order to obtain well-mixed Rh-Sn bimetallic system. The catalyst reduced at 673 and 773 K indicated the high activity for CO oxidation reaction at low reaction temperatures. These results indicate the reduction behavior in the following; Rh oxide was reduced first and metallic Rh was formed, then Sn oxide was reduced by the spillover hydrogen which was formed on the metallic Rh particles. Finally, bimetallic system of Rh and Sn, e.g. Rh-Sn alloy, was formed at the higher temperature than 673 K. The obtained bimetallic surface was markedly active for CO oxidation reaction.
1. INTRODUCTION Supported Rh- and Ru-Sn catalysts have shown high activity for reduction of N O by CO [1 ]. Especially, the effective removal of N O was observed in 02rich atmosphere. The high removal of N O in the oxidative atmosphere was attributable to a cleaning effect of Sn in the bimetallic system by spillover of oxygen atoms adsorbed on Rh or Ru sites to Sn sites [2]. Recently, we have also reported that oxidation of CO over the silica supported Rh catalysts was enhanced by incorporation of Sn [3]. The temperature which CO oxidation
180 started was lowered by the addition of Sn [3]. These studies indicate that Sn is an effective additive in CO oxidation reaction itself as well as NO reduction. The XPS study [4] have indicated that the Sn in Rh-Sn/SiO2 was reduced in a lower oxidation state, may be in Sn 0 - Sn 2+. The reduced Sn plays a significant role in activation of 02 molecules [4]. Mixing state between Rh and Sn may be important for CO oxidation, because an oxidation state of Sn is strongly influenced by the adjacent Rh atoms. Preparation method and activation condition (temperature and atmosphere) seem to be very important to control the mixing state of the bimetallic system. Dautzenberg et al. [5] have reported the interaction between Pt and Sn over A1203. The TPR (temperature programmed reduction) of the oxidized Pt-Sn/A1203 indicated that the reduction of Pt and Sn was greatly affected in the presence of Sn. They have also reported that Sn was not completely reduced to metallic state over A1203. Burch has reported the details of TPR spectra of Pt-Sn/A1203 [6]. Srinivasan et al. [7, 8] have reported that the microelectron diffraction method in order to study micro crystals of Pt-Sn system on A1203 and SiO2. They have found the alloy formation on A1203 support by the micro diffraction method, whereas no peak was observed in X-ray diffraction powder patterns. These results suggest that the alloy formation between Pt and Sn is not so difficult even over A1203, which strongly interacts with Sn species and may stabilize the state of Sn2+. Even in those case, 70 % of Sn was not reduced to metallic Sn on A1203. The state of Sn was strongly affected by loading of Sn and Pt and condition of pretreatment, that is, atmosphere m~d temperature. Bacaud et al. have reported that the percent of reduction of Sn was strongly affected by the content of Pt and Sn [9]. The starting material of Sn and the preparation method were important for the reduction state of Sn as reported by Sexton et al. [10]. The silica is a suitable support for reduction of Sn species and alloy formation [10]. In our case, SiO2 support, the high content of Rh and Sn (5 wt% each), and 773 K of reduction temperature will bring about the reduction to metallic Sn and the alloy formation might be expected. In this paper, we report the detail of reduction behavior of Rh-Sn/SiO2 by temperature programmed reduction method. The reduction temperature was one of the important factors to control the oxidation state of Sn in Rh-Sn bimetallic system. The relationship between the reduction behavior of Rh-Sn system and the activity for CO oxidation is studied.
181 2. EXPERIMENTAL
2.1. Catalyst preparation The details of the preparation method of the catalysts was described in the previous paper [4]. Rh-Sn/SiO2 was prepared by a conventional impregnation method. The silica gel was co-impregnated with a mixed aqueous solution of Rh(NO3)3 (Nakarai Tesque Inc., Kyoto Japan) and SnC12 (Nakarai Tesque Inc., Kyoto Japan). Loading of Rh was at 5 wt% with respect to the support and Sn was introduced in unity of Sn/Rh atomic ratio. It was reported that the efficient Sn/Rh ratio was unity for activation of oxygen molecules [4, 11 ]. The catalyst precursor prepared above was activated by calcination in flowing air followed by reduction in flowing H2 at prescribed temperatures for 5 h, respectively for adsorption experiments and for CO oxidation reaction. For temperature programmed reduction, the precursor was used directly without any thermal treatment, that means as impregnated samples were used, in order to investigate effects of treatment temperature, given below.
2.2. Temperature programmed reduction (TPR) Temperature programmed reduction was carried out in a flowing H2 (5.14 vol%) diluted by N2 with a flow system equipped with a thermo balance (Model DGC-40, Shimadzu Co., Kyoto). A furnace in the thermo balance was controlled by the thermal analyzer DT-40 (Shimadzu Co., Kyoto). The catalyst precursor was calcined in a flowing air at a prescribed temperature for 0.5 h in the system, then the sample was cooled in the flowing air to room temperature. The carrier gas, N2 or H2/N2 mixture, was dehydrated by molecular sieve just before the TPR cell. The calcined precursor was heated in flowing H2/N2 mixture in a constant rate of 10 K/min. A typical heating pattern, the calcination was carried out at 773 K, is shown in Fig. 1. The obtained responses weight loss or gain (TG) and difference of temperatures between the sample cell in which the catalyst was placed and the reference cell in which the SiO2 support was placed (DTA) were accumulated in a personal computer, and were processed with a soft ware supplied by Shimadzu Co. (Kyoto, Japan). It should be noted that the weight of the sample was kept in ca. 25 mg in order to obtain precise and reproducible results.
182 calcination
873
TPR
purge I J
air
" ~
~ 5.14 % H2 in N2----~
N2
t
L_
673
Z~
e~
E 473 in
FI
0
30
..
I
;
,
I
90 60 Elapsed time / min
........
I
120
Figure 1. Typical heating pattern in the TPR experiment 2.3. Adsorption Adsorption capacity of the catalysts for H2 and CO was measured in a static vacuum system. The adsorption was carried out at room temperature in 10 kPa of each gas. The capacity was evaluated by the difference between total amount of adsorption and reversible adsorption. The absorption of H2 at 373 K over the calcined precursor at was also measured in the system.
2.4. X-ray diffraction powder pattern The mixing-state of supported Rh and Sn was investigated by X-ray diffraction powder patters. The X-ray powder pattern of the catalysts was obtained by using Ni filtered Cu-Ka.
2.5. CO oxidation Oxidation of CO was carried out in a closed circulation system. The catalyst was reduced again in a reactor in 13 kPa of H2 prior to the reaction. The reactant gas was 15.9 kPa of CO-O2 mixture, which molar ratio was two in CO/O2. The conversion of CO was calculated from decrease of total pressure in the system The reaction rate was evaluated by an initial rate method.
183
a\
pI~DTA i exO"
...J '~_ TG b
0.5 mg[
,~ DTA
/'X. DTA
0.5 mgI
TG c
273
l
I,
1
I
I
I
373
473
573
673
e
i DTA
I
773
Temperature
J
373
473
I
i
573
673
773
/ K
Figure 2. TPR spectra o f Rh/Si02 and Rh-Sn/Si02 catalysts a: Rh/Si02 calcined at 773 K, b: Rh-Si02 calcined at 573 K c: at 673 K, d: at 773 K, e: at 873 K
3. RESULTS AND DISCUSSION
3.1. Reduction behavior of Rh-SrdSiO2 Temperature programmed reduction spectra of Rh/SiO2 and Rh-Sn/SiO2 calcined at different temperatures were shown in Fig. 2. The spectnun a indicates a strong exothermic peak at 360 K over Rh/SiO2 (in DTA line) accompanying with the weight loss which began at 356 K (in TG line). This result indicates that
184 Rh oxide supported on SiO2 was reduced at c.a. 360 K. The spectra b to e show DTA-TG lines over Rh-Sn/SiO2. The DTA spectra indicate that two major peaks, one was sharp near 370 K, the other was a broad beyond 420 K. The first sharp peak was similar to reduction of Rh oxide over Rh-Sn/SiO2, which accompanied no weight loss (TG). The formed H20 was not desorbed from the catalyst below 400 K. The position of peaks in the DTA lines obtained over RhSn/SiO2 catalyst are summarized in Table 1.
Table 1 Temperature offirst DTA peak and weight loss stated Temperature first peak
Catalyst Rh/Si 02
773K,
Rh-Sn/SiO2
573K 673K 773K 873K 9 calcination temperature
360K
Temperature weight loss started 356K
395K 383K 400K 442K
469K 465K 442K 428K
The temperature of the first peak except the spectrum c, which was calcined at 673 K, tended to shift toward higher temperature with increasing calcination temperature as shown in Table 2 and Fig. 2. The reducibility of Rh oxide was lowered with addition of Sn. The H2-absorption capacity of the calcined catalysts at 773 K was measured at room temperature and 373 K in order to study the reducibility of the catalysts as shown in Table 2.
Table 2 H2-absorptionof Rh and Rh-Sn/Si02 Absorption temperature
room temp.
1st dose 2nd dose total 1st dose 2nd dose total
Amount of absorbed H2 rtmole/g-cat Rh/SiO2 Rh-Sn/SiO2 189 5.2 136 2.8 325 8.0 600 484 111 158 711 642
185 At room temperature, a significant amount of absorption was observed over Rh/SiO2, whereas no hydrogen was absorbed over Rh-Sn/SiO2 as shown in Table 2. At 373 K, 711 lamole/g of H2 was absorbed over Rh/SiO2, which was almost same as the theoretical value of H2 consumption to reduce Rh3+ to RhO, 694 ~tmole/g-H2 for 5 wt% Rh/SiO2. Over Rh-Sn/SiO2, the H2 consumption was 642 lamole/g, which was identical to the theoretical value to reduce only Rh3+ to RhO, 658 lamole/g. These results indicate that Rh oxide was more easily reduced to metallic Rh in Rh/SiO2 than in Rh-Sn/SiO2 and that the reduction peak in the range of 370-440 K in TPR spectra was ascribed to the reduction of Rh oxide in Rh-Sn/SiO2. No weight loss at 400 K in TPR spectra means that desorption of H20 from the catalyst required much higher temperature than the reduction of Rh oxide. The absorption experiment also indicated that H2 absorption was observed without H20 evolution at room temperature and 373 K over Rh/SiO2 and RhSn/SiO2. The calcination at a high temperature will result in a Sn-enriched surface of Rh-Sn/SiO2 because the aff'mity of Sn for oxygen is greater than that of Rh. The enrichment of Sn on the surface would suppress the reduction of Rh oxide. Butch [6] has also reported that the peak corresponding the reduction of Pt oxide in the calcined Pt-Sn/A1203 was shifted toward higher temperature with increasing Sn content. The second broad peak over 420 K was ascribed to the reduction of Sn oxide, which was accompanied by the weight loss. The weight loss corresponded not only to reduction of Sn oxide but also to the decrease of weight by the desorption of H20 which had been formed during the reduction of Rh oxide described above. Enough reduction of the calcined precursors were required reduction at 573 K as shown in Fig. 2. The weight loss was summarized in Table 3.
Table 3 Weight loss and corresponding oxygen eliminated
catalyst Rh/SiO2 Rh-Sn/SiO2
773K* 573K 673K 773K 873K 9 calcination temperature
weight loss (mg/g-cat) 15.8 39.9 38.9 23.2 14.8
corresponding 02 molecules ~tmole/g-cat 0.49 1.25 1.22 0.72 0.46
O/Rh (atomic ratio) 2.1 5.7 5.2 3.3 2.1
186 The corresponding eliminated-oxygen calculated from the weight loss was also shown in the table. The theoretical value of the O/Rh was estimated at 3.5 in order to reduce both Rh (III) and Sn (IV) to the corresponding metals. The larger values than 3.5 in Table 3 would be ascribed to the residual water which came from the cartier gas, although the cartier gas was dehydrated by molecular sieve just before the DTA-TG cell. The original adsorbed water on the catalysts would not affect the weight loss because the sample was calcined at 573 K before TPR experiment, which was higher than the desorption temperature (350-440 K) of H20 as shown in Fig. 2. The larger values observed over Rh-Sn/SiO2 than over Rh/SiO2 suggested that the significant amotmt of Sn in Rh-Sn/SiO2 was readily reduced to lower oxidation state, Sn (0). The previous paper [11] has also indicated that the reduced Sn/SiO2 at 773 K did not absorb oxygen during oxidation experiment at 773 K. Tin in Sn/SiO2 was hardly reduced at 773 K in H2 [11]. The TPR spectra of Sn/SiO2 (5 wt% of Sn) calcined at 773 K showed no reduction peak up to 773 K. These results indicate that the reduction of Sn oxide was catalyzed by the presence of an active component, Rh metal, that is, the spillover hydrogen which was formed on metallic Rh particles reduced the Sn oxide in Rh-Sn/SiO2 catalyst. Recently, Aranda et al. [12] have reported that the reduction of Sn was catalyzed by the presence of Pt over A1203. They have also found that ca. 28 % of Sn (IV) was reduced to Sn (0). The similar behavior was observed over Nb205 support [12]. In the case of A1203, the interaction between Sn and A1203 may be too strong to reduce Sn oxide to metallic Sn at 773 K completely. Our results indicate that a part of Sn in Rh-Sn/SiO2 is readily reduced to metallic Sn and some part of reduced Sn might be alloyed with Rh. Srinivasan and Davis have also reported the formation of Pt-Sn alloy on silica support by X-ray diffraction and electron micro diffraction [8].
3.2. Adsorption capacity The adsorption capacity of the catalysts were shown in Tables 4 and 5 for H2 and CO. Table 4 indicates the influence of reduction temperature on the capacity. The adsorption capacity for H2 and CO was drastically decreased by addition of Sn. Especially, the amount of H2 adsorbed decreased by 10 to 20, whereas CO decreased by 4. This result was ascribed that hydrogen molecules required at least two adjacent atoms of Rh to be adsorbed dissociatively. This is well known as an ensemble effect for bimetallic catalysts. The adsorption capacity of the Rh-Sn/SiO2 catalyst which was reduced at 873 K was larger than that reduced at 473 K twice for H2 and CO adsorption. Table 5 indicates the
187
Table 4 Effect of reduction temperature on adsorption capacity catalyst Rh/SiO2 Rh-Sn/SiO2
773K, 473K 573K 673K 773K 873K
H2 (~tmole/g-cat) 52.5 2.02 2.44 1.32 2.00 4.94
CO (~tmole/g-cat) 103.2 26.2 26.6 30.1 31.3 41.0
*reduction temperature
Table 5 Effect of calcination temperature on adsorption capacity catalyst Rh/SiO2 Rh-Sn/SiO2
773K, 573K 673K 773K 873K ,calcination temperature
H2 (~tmole/g-cat) 52.5 6.80 5.48 2.43 1.80
CO (lamole/g-cat) 103.2 56.8 42.6 26.6 18.3
influence of calcination temperature on the capacity. Both the capacity for H2 and CO was decreased with increasing calcination temperature. These results suggest that the higher reduction temperature brings about a Rh rich surface and that the higher calcination temperature brings about a Sn-rich surface. The Sn-enriched surface would suppress the reduction of Sn oxide itself as well as the reduction of Rh oxide as discussed above. The low value of oxygen consumption of RhSn/SiO2, calcined at 873 K, calculated from TPR spectra is ascribable to the Snenriched surface as shown in Table 3. It should be checked whether Sn affects the dispersion of Rh particles or not. We have already studied the effect of Sn addition on the dispersion of Rh by comparing the XPS intensity and the adsorption capacity of H2 and CO [ 11 ]. The relative XPS intensity of Rh3ds/2 to a monolayer catalyst was larger than the theoretical value calculated from the adsorption capacity of H2 and CO by the method of Kerkhof and Mouljin [13]. These results indicated that the decrease of the adsorption capacity of Rh-Sn/SiO2 catalysts was ascribed not to the increase of isolated Rh particle size, but to surface composition of Rh-Sn bimetallic
188 particles so called the ensemble effect and/or to chemical modification of Rh metals by adjacent Sn atoms so called a ligand effect.
3.3. X-ray diffraction powder pattern Figure 3 shows the X-ray diffraction powder patterns of Rh-Sn/SiO2 reduced at different temperatures. The catalyst which were reduced below 573 K showed no shift of Rh (111) diffraction peak as shown in Fig. 3-a. The reduction over 673 K brought about a shift toward lower diffraction angle, which indicates well-mixing between Rh and Sn [3, 4, 11 ]. The Sn oxide in the calcined precursor
Rh3S.n
Rh
5
. ...,.
(/I c . ,.....
c
cr l,.-,wl
I
38
I
I
40 /-,2 20 /deg.
I
44
I
I
38
I
I
I
40 42 44 20 1 deg.
Figure 3. (left) XRD patterns of Rh-Sn/Si02 reduced at various temperatures 1:873 K, 2:773 K, 3:673 K, 4:573 K, 5:Rh/Si02 at 773 K Figure 4. (righO XRD patterns of Rh-Sn/Si02 calcined at various temperatures 1:873 K, 2:773 K, 3:673 K, 4:Rh/Si02 at 773 K
189 was enough reduced around 573 K as shown in Fig. 2. The mixing between the reduced Rh and Sn is required much higher temperature. Figure 4 shows the XRD powder patterns of the Rh-Sn/SiO2 which was calcined at different temperatures followed by reduction at 773 K. The catalyst calcined at 673 K indicates both Rh and Rh-Sn structure. The calcination at a low temperature brought about the significant segregation of Rh and Rh-Sn. Although neither metallic Sn or Sn oxide was observed in the XRD patterns, a segregated Sn could not be excluded. 3.4. CO oxidation Figure 5 shows the CO oxidation activity over Rh-Sn/SiO2 catalysts which were reduced at different temperatures. The activity was evaluated with the apparent first order rate constant. The initial reaction rate for CO oxidation depended on partial pressure of 02 in first order over Rh and Rh-Sn/SiO2 described previously [3]. The dashed line indicates the activity over Rh/SiO2. The activity over the catalyst reduced at 573 K was identical to that over Rh/SiO2 as shown in Fig. 5.
~~' ,,"" 0"-0-0
1.0 T
cn
'I/i
0.5
0
'
D-ZS
'
373
'"' 0
~ '
'
'
423 Temperature / K
'
'
m
I
473
Figure 5. CO oxidation activity o f Rh-Sn/Si02 reduced at various temperatures O" reduced at 573 K, A: at 673 K, D: at 773 K The dashed line means the activity o f Rh/Si02 reduced at 773 K.
The catalysts reduced at 673 and 773 K indicated much higher activity at low reaction temperatures. The temperature which the activity appeared was lowered by 40-50 K. These results are well correlated to the peak shit~ in XRD powder
190 patterns as shown in Fig. 3. The well-mixing of Rh and Sn brings about the high activity at low temperatures of Rh-Sn/SiO2 for CO oxidation reaction. The surface composition and oxidation state would be changed from the state just after pretreatment because the catalysts were exposed to the oxygen containing atmosphere during the reaction, whereas only the metallic Rh phase (or Rh-Sn phase) was observed in the XRD patterns atter the reaction. The unusual temperature dependence of the CO oxidation as shown in Fig. 5 has been reported [14] and would be ascribed to the modified surface composition of adlayer, which was consisted of CO and O atoms, during the reaction discussed previously [3]. Although the change of the surface composition during the reaction seemed to be considered, the initiation temperature of the reaction would be affected directly by the original surface composition and electronic state. 3.5. State of Rh-Sn on SiO2 and CO oxidation activity The reduction behavior of Rh-Sn/SiO2 can be summarized below. 1)Rhodium oxide of the calcined catalyst was reduced first in the range of 360 to 420 K. 2)Tin oxide was reduced over 440 K with catalyzing by metallic Rh. 3)The alloying between Rh and Sn proceeded at a higher temperature than 673 K during the reduction. For CO oxidation reaction, the most important factor was the alloying between metallic Rh and reduced Sn, because the high activity of the catalyst required the reduction at higher temperature than 673 K as shown Fig. 5. The reduction up to 573 K may gave metallic Rh and the reduced low valent Sn (some part of Sn was reduced to Sn0), which was separated each other. The separated Rh and Sn indicated the same activity for CO oxidation as Rh/SiO2. The reduction at higher than 673 K would induce the mixing between Rh and Sn. The well-mixed Rh-Sn on SiO2 seems to be markedly active for CO oxidation.
191 REFERENCES
M. Masai, K. Murata and M. Yabashi, Chem. Lett., (1979) 989. M. Masai, K. Nakahara, K. Murata, S. Nishiyama, and S. Tsuruya, Stud. Surf. Sci. & Catal., 17 (1983) 89. S. Nishiyama, M. Akemoto, I, Yamamoto, S. Tsuruya, and M. Masai, J. Chem. Soc. Faraday Trans., 88 (1992) 3483. S. Nishiyama, H. Yanagi, H. Nakayama, S. Tsuruya, and M. Masai, Appl. Catal., 47 (1989) 25. F. M. Dautzenberg, J. N. Helle, P. Biloen, and W. M. H. Sachtler, J. Catal., 63 (1980) 119. 6 R. Butch and L. C. Garla, J. Catal., 71 (1981) 360. 7 R. Srinivasan, L.A. Rice, and B.H. Davis, J. Catal., 129 (1991) 257. 8 R. Srinivasan and B.H. Davis, Appl. Catal. A, 87 (1992) 45. R. Bacaud, P. Bussi6re, and F. Figueras, J. Catal., 69 (1981) 399. 9 10 B.A. Sexton, A.E. Hughes, and K. Foger, J. Catal., 88 (1984) 466. 11 S. Nishiyama, H. Yanagi, S. Tsuruya, and M. Masai, Proc. 2nd Intem. Conf. Spillover (Leipzig, 1989), (1989)pp 116-126. 12 D.A.G. Aranda, F.B. Noronha, M. Schmal, and F.B. Passos, Appl. Catal. A, 100 (1993) 77. 13 F.P.J.M. Kerkhof and J.M. Mouljin, J. Phys. Chem., 83 (1979) 1612. 14 G.K. Boreskov, in Catalysis, ed. J.R. Anderson and M. Bourdart, Springer-Verlag, Berlin, 1982, vol. 3, p.39.
179
REDUCTION BEHAVIOR OF Rh-Sn/SiO2 BIMETALLIC CATALYSTS AND ITS CO OXIDATION ACTIVITY Satoru Nishiyama, Ikuo Yamamoto, Masahiro Akemoto, Shigeru Tsuruya, and Mitsuo Masai Department of Chemical Science and Engineermg, Faculty of Engineermg, Kobe University, Rokkodai, Nada, Kobe 657, Japan
ABSTRACT
Reduction behavior of Rh-Sn on SiO2 was studied by temperature programmed reduction methods. During the reduction of Rh-Sn/SiO2 preoxidized, Rh oxide was reducible in the range of 370-440 K first, then the reduction of Sn oxide proceeded at a higher temperature up to 470 K. X-ray diffraction powder patterns indicates that the higher temperature than 673 K was required in order to obtain well-mixed Rh-Sn bimetallic system. The catalyst reduced at 673 and 773 K indicated the high activity for CO oxidation reaction at low reaction temperatures. These results indicate the reduction behavior in the following; Rh oxide was reduced first and metallic Rh was formed, then Sn oxide was reduced by the spillover hydrogen which was formed on the metallic Rh particles. Finally, bimetallic system of Rh and Sn, e.g. Rh-Sn alloy, was formed at the higher temperature than 673 K. The obtained bimetallic surface was markedly active for CO oxidation reaction.
1. INTRODUCTION Supported Rh- and Ru-Sn catalysts have shown high activity for reduction of N O by CO [1 ]. Especially, the effective removal of N O was observed in 02rich atmosphere. The high removal of N O in the oxidative atmosphere was attributable to a cleaning effect of Sn in the bimetallic system by spillover of oxygen atoms adsorbed on Rh or Ru sites to Sn sites [2]. Recently, we have also reported that oxidation of CO over the silica supported Rh catalysts was enhanced by incorporation of Sn [3]. The temperature which CO oxidation
180 started was lowered by the addition of Sn [3]. These studies indicate that Sn is an effective additive in CO oxidation reaction itself as well as NO reduction. The XPS study [4] have indicated that the Sn in Rh-Sn/SiO2 was reduced in a lower oxidation state, may be in Sn 0 - Sn 2+. The reduced Sn plays a significant role in activation of 02 molecules [4]. Mixing state between Rh and Sn may be important for CO oxidation, because an oxidation state of Sn is strongly influenced by the adjacent Rh atoms. Preparation method and activation condition (temperature and atmosphere) seem to be very important to control the mixing state of the bimetallic system. Dautzenberg et al. [5] have reported the interaction between Pt and Sn over A1203. The TPR (temperature programmed reduction) of the oxidized Pt-Sn/A1203 indicated that the reduction of Pt and Sn was greatly affected in the presence of Sn. They have also reported that Sn was not completely reduced to metallic state over A1203. Burch has reported the details of TPR spectra of Pt-Sn/A1203 [6]. Srinivasan et al. [7, 8] have reported that the microelectron diffraction method in order to study micro crystals of Pt-Sn system on A1203 and SiO2. They have found the alloy formation on A1203 support by the micro diffraction method, whereas no peak was observed in X-ray diffraction powder patterns. These results suggest that the alloy formation between Pt and Sn is not so difficult even over A1203, which strongly interacts with Sn species and may stabilize the state of Sn2+. Even in those case, 70 % of Sn was not reduced to metallic Sn on A1203. The state of Sn was strongly affected by loading of Sn and Pt and condition of pretreatment, that is, atmosphere m~d temperature. Bacaud et al. have reported that the percent of reduction of Sn was strongly affected by the content of Pt and Sn [9]. The starting material of Sn and the preparation method were important for the reduction state of Sn as reported by Sexton et al. [10]. The silica is a suitable support for reduction of Sn species and alloy formation [10]. In our case, SiO2 support, the high content of Rh and Sn (5 wt% each), and 773 K of reduction temperature will bring about the reduction to metallic Sn and the alloy formation might be expected. In this paper, we report the detail of reduction behavior of Rh-Sn/SiO2 by temperature programmed reduction method. The reduction temperature was one of the important factors to control the oxidation state of Sn in Rh-Sn bimetallic system. The relationship between the reduction behavior of Rh-Sn system and the activity for CO oxidation is studied.
181 2. EXPERIMENTAL
2.1. Catalyst preparation The details of the preparation method of the catalysts was described in the previous paper [4]. Rh-Sn/SiO2 was prepared by a conventional impregnation method. The silica gel was co-impregnated with a mixed aqueous solution of Rh(NO3)3 (Nakarai Tesque Inc., Kyoto Japan) and SnC12 (Nakarai Tesque Inc., Kyoto Japan). Loading of Rh was at 5 wt% with respect to the support and Sn was introduced in unity of Sn/Rh atomic ratio. It was reported that the efficient Sn/Rh ratio was unity for activation of oxygen molecules [4, 11 ]. The catalyst precursor prepared above was activated by calcination in flowing air followed by reduction in flowing H2 at prescribed temperatures for 5 h, respectively for adsorption experiments and for CO oxidation reaction. For temperature programmed reduction, the precursor was used directly without any thermal treatment, that means as impregnated samples were used, in order to investigate effects of treatment temperature, given below.
2.2. Temperature programmed reduction (TPR) Temperature programmed reduction was carried out in a flowing H2 (5.14 vol%) diluted by N2 with a flow system equipped with a thermo balance (Model DGC-40, Shimadzu Co., Kyoto). A furnace in the thermo balance was controlled by the thermal analyzer DT-40 (Shimadzu Co., Kyoto). The catalyst precursor was calcined in a flowing air at a prescribed temperature for 0.5 h in the system, then the sample was cooled in the flowing air to room temperature. The carrier gas, N2 or H2/N2 mixture, was dehydrated by molecular sieve just before the TPR cell. The calcined precursor was heated in flowing H2/N2 mixture in a constant rate of 10 K/min. A typical heating pattern, the calcination was carried out at 773 K, is shown in Fig. 1. The obtained responses weight loss or gain (TG) and difference of temperatures between the sample cell in which the catalyst was placed and the reference cell in which the SiO2 support was placed (DTA) were accumulated in a personal computer, and were processed with a soft ware supplied by Shimadzu Co. (Kyoto, Japan). It should be noted that the weight of the sample was kept in ca. 25 mg in order to obtain precise and reproducible results.
182 calcination
873
TPR
purge I J
air
" ~
~ 5.14 % H2 in N2----~
N2
t
L_
673
Z~
e~
E 473 in
FI
0
30
..
I
;
,
I
90 60 Elapsed time / min
........
I
120
Figure 1. Typical heating pattern in the TPR experiment 2.3. Adsorption Adsorption capacity of the catalysts for H2 and CO was measured in a static vacuum system. The adsorption was carried out at room temperature in 10 kPa of each gas. The capacity was evaluated by the difference between total amount of adsorption and reversible adsorption. The absorption of H2 at 373 K over the calcined precursor at was also measured in the system.
2.4. X-ray diffraction powder pattern The mixing-state of supported Rh and Sn was investigated by X-ray diffraction powder patters. The X-ray powder pattern of the catalysts was obtained by using Ni filtered Cu-Ka.
2.5. CO oxidation Oxidation of CO was carried out in a closed circulation system. The catalyst was reduced again in a reactor in 13 kPa of H2 prior to the reaction. The reactant gas was 15.9 kPa of CO-O2 mixture, which molar ratio was two in CO/O2. The conversion of CO was calculated from decrease of total pressure in the system The reaction rate was evaluated by an initial rate method.
183
a\
pI~DTA i exO"
...J '~_ TG b
0.5 mg[
,~ DTA
/'X. DTA
0.5 mgI
TG c
273
l
I,
1
I
I
I
373
473
573
673
e
i DTA
I
773
Temperature
J
373
473
I
i
573
673
773
/ K
Figure 2. TPR spectra o f Rh/Si02 and Rh-Sn/Si02 catalysts a: Rh/Si02 calcined at 773 K, b: Rh-Si02 calcined at 573 K c: at 673 K, d: at 773 K, e: at 873 K
3. RESULTS AND DISCUSSION
3.1. Reduction behavior of Rh-SrdSiO2 Temperature programmed reduction spectra of Rh/SiO2 and Rh-Sn/SiO2 calcined at different temperatures were shown in Fig. 2. The spectnun a indicates a strong exothermic peak at 360 K over Rh/SiO2 (in DTA line) accompanying with the weight loss which began at 356 K (in TG line). This result indicates that
184 Rh oxide supported on SiO2 was reduced at c.a. 360 K. The spectra b to e show DTA-TG lines over Rh-Sn/SiO2. The DTA spectra indicate that two major peaks, one was sharp near 370 K, the other was a broad beyond 420 K. The first sharp peak was similar to reduction of Rh oxide over Rh-Sn/SiO2, which accompanied no weight loss (TG). The formed H20 was not desorbed from the catalyst below 400 K. The position of peaks in the DTA lines obtained over RhSn/SiO2 catalyst are summarized in Table 1.
Table 1 Temperature offirst DTA peak and weight loss stated Temperature first peak
Catalyst Rh/Si 02
773K,
Rh-Sn/SiO2
573K 673K 773K 873K 9 calcination temperature
360K
Temperature weight loss started 356K
395K 383K 400K 442K
469K 465K 442K 428K
The temperature of the first peak except the spectrum c, which was calcined at 673 K, tended to shift toward higher temperature with increasing calcination temperature as shown in Table 2 and Fig. 2. The reducibility of Rh oxide was lowered with addition of Sn. The H2-absorption capacity of the calcined catalysts at 773 K was measured at room temperature and 373 K in order to study the reducibility of the catalysts as shown in Table 2.
Table 2 H2-absorptionof Rh and Rh-Sn/Si02 Absorption temperature
room temp.
1st dose 2nd dose total 1st dose 2nd dose total
Amount of absorbed H2 rtmole/g-cat Rh/SiO2 Rh-Sn/SiO2 189 5.2 136 2.8 325 8.0 600 484 111 158 711 642
185 At room temperature, a significant amount of absorption was observed over Rh/SiO2, whereas no hydrogen was absorbed over Rh-Sn/SiO2 as shown in Table 2. At 373 K, 711 lamole/g of H2 was absorbed over Rh/SiO2, which was almost same as the theoretical value of H2 consumption to reduce Rh3+ to RhO, 694 ~tmole/g-H2 for 5 wt% Rh/SiO2. Over Rh-Sn/SiO2, the H2 consumption was 642 lamole/g, which was identical to the theoretical value to reduce only Rh3+ to RhO, 658 lamole/g. These results indicate that Rh oxide was more easily reduced to metallic Rh in Rh/SiO2 than in Rh-Sn/SiO2 and that the reduction peak in the range of 370-440 K in TPR spectra was ascribed to the reduction of Rh oxide in Rh-Sn/SiO2. No weight loss at 400 K in TPR spectra means that desorption of H20 from the catalyst required much higher temperature than the reduction of Rh oxide. The absorption experiment also indicated that H2 absorption was observed without H20 evolution at room temperature and 373 K over Rh/SiO2 and RhSn/SiO2. The calcination at a high temperature will result in a Sn-enriched surface of Rh-Sn/SiO2 because the aff'mity of Sn for oxygen is greater than that of Rh. The enrichment of Sn on the surface would suppress the reduction of Rh oxide. Butch [6] has also reported that the peak corresponding the reduction of Pt oxide in the calcined Pt-Sn/A1203 was shifted toward higher temperature with increasing Sn content. The second broad peak over 420 K was ascribed to the reduction of Sn oxide, which was accompanied by the weight loss. The weight loss corresponded not only to reduction of Sn oxide but also to the decrease of weight by the desorption of H20 which had been formed during the reduction of Rh oxide described above. Enough reduction of the calcined precursors were required reduction at 573 K as shown in Fig. 2. The weight loss was summarized in Table 3.
Table 3 Weight loss and corresponding oxygen eliminated
catalyst Rh/SiO2 Rh-Sn/SiO2
773K* 573K 673K 773K 873K 9 calcination temperature
weight loss (mg/g-cat) 15.8 39.9 38.9 23.2 14.8
corresponding 02 molecules ~tmole/g-cat 0.49 1.25 1.22 0.72 0.46
O/Rh (atomic ratio) 2.1 5.7 5.2 3.3 2.1
186 The corresponding eliminated-oxygen calculated from the weight loss was also shown in the table. The theoretical value of the O/Rh was estimated at 3.5 in order to reduce both Rh (III) and Sn (IV) to the corresponding metals. The larger values than 3.5 in Table 3 would be ascribed to the residual water which came from the cartier gas, although the cartier gas was dehydrated by molecular sieve just before the DTA-TG cell. The original adsorbed water on the catalysts would not affect the weight loss because the sample was calcined at 573 K before TPR experiment, which was higher than the desorption temperature (350-440 K) of H20 as shown in Fig. 2. The larger values observed over Rh-Sn/SiO2 than over Rh/SiO2 suggested that the significant amotmt of Sn in Rh-Sn/SiO2 was readily reduced to lower oxidation state, Sn (0). The previous paper [11] has also indicated that the reduced Sn/SiO2 at 773 K did not absorb oxygen during oxidation experiment at 773 K. Tin in Sn/SiO2 was hardly reduced at 773 K in H2 [11]. The TPR spectra of Sn/SiO2 (5 wt% of Sn) calcined at 773 K showed no reduction peak up to 773 K. These results indicate that the reduction of Sn oxide was catalyzed by the presence of an active component, Rh metal, that is, the spillover hydrogen which was formed on metallic Rh particles reduced the Sn oxide in Rh-Sn/SiO2 catalyst. Recently, Aranda et al. [12] have reported that the reduction of Sn was catalyzed by the presence of Pt over A1203. They have also found that ca. 28 % of Sn (IV) was reduced to Sn (0). The similar behavior was observed over Nb205 support [12]. In the case of A1203, the interaction between Sn and A1203 may be too strong to reduce Sn oxide to metallic Sn at 773 K completely. Our results indicate that a part of Sn in Rh-Sn/SiO2 is readily reduced to metallic Sn and some part of reduced Sn might be alloyed with Rh. Srinivasan and Davis have also reported the formation of Pt-Sn alloy on silica support by X-ray diffraction and electron micro diffraction [8].
3.2. Adsorption capacity The adsorption capacity of the catalysts were shown in Tables 4 and 5 for H2 and CO. Table 4 indicates the influence of reduction temperature on the capacity. The adsorption capacity for H2 and CO was drastically decreased by addition of Sn. Especially, the amount of H2 adsorbed decreased by 10 to 20, whereas CO decreased by 4. This result was ascribed that hydrogen molecules required at least two adjacent atoms of Rh to be adsorbed dissociatively. This is well known as an ensemble effect for bimetallic catalysts. The adsorption capacity of the Rh-Sn/SiO2 catalyst which was reduced at 873 K was larger than that reduced at 473 K twice for H2 and CO adsorption. Table 5 indicates the
187
Table 4 Effect of reduction temperature on adsorption capacity catalyst Rh/SiO2 Rh-Sn/SiO2
773K, 473K 573K 673K 773K 873K
H2 (~tmole/g-cat) 52.5 2.02 2.44 1.32 2.00 4.94
CO (~tmole/g-cat) 103.2 26.2 26.6 30.1 31.3 41.0
*reduction temperature
Table 5 Effect of calcination temperature on adsorption capacity catalyst Rh/SiO2 Rh-Sn/SiO2
773K, 573K 673K 773K 873K ,calcination temperature
H2 (~tmole/g-cat) 52.5 6.80 5.48 2.43 1.80
CO (lamole/g-cat) 103.2 56.8 42.6 26.6 18.3
influence of calcination temperature on the capacity. Both the capacity for H2 and CO was decreased with increasing calcination temperature. These results suggest that the higher reduction temperature brings about a Rh rich surface and that the higher calcination temperature brings about a Sn-rich surface. The Sn-enriched surface would suppress the reduction of Sn oxide itself as well as the reduction of Rh oxide as discussed above. The low value of oxygen consumption of RhSn/SiO2, calcined at 873 K, calculated from TPR spectra is ascribable to the Snenriched surface as shown in Table 3. It should be checked whether Sn affects the dispersion of Rh particles or not. We have already studied the effect of Sn addition on the dispersion of Rh by comparing the XPS intensity and the adsorption capacity of H2 and CO [ 11 ]. The relative XPS intensity of Rh3ds/2 to a monolayer catalyst was larger than the theoretical value calculated from the adsorption capacity of H2 and CO by the method of Kerkhof and Mouljin [13]. These results indicated that the decrease of the adsorption capacity of Rh-Sn/SiO2 catalysts was ascribed not to the increase of isolated Rh particle size, but to surface composition of Rh-Sn bimetallic
188 particles so called the ensemble effect and/or to chemical modification of Rh metals by adjacent Sn atoms so called a ligand effect.
3.3. X-ray diffraction powder pattern Figure 3 shows the X-ray diffraction powder patterns of Rh-Sn/SiO2 reduced at different temperatures. The catalyst which were reduced below 573 K showed no shift of Rh (111) diffraction peak as shown in Fig. 3-a. The reduction over 673 K brought about a shift toward lower diffraction angle, which indicates well-mixing between Rh and Sn [3, 4, 11 ]. The Sn oxide in the calcined precursor
Rh3S.n
Rh
5
. ...,.
(/I c . ,.....
c
cr l,.-,wl
I
38
I
I
40 /-,2 20 /deg.
I
44
I
I
38
I
I
I
40 42 44 20 1 deg.
Figure 3. (left) XRD patterns of Rh-Sn/Si02 reduced at various temperatures 1:873 K, 2:773 K, 3:673 K, 4:573 K, 5:Rh/Si02 at 773 K Figure 4. (righO XRD patterns of Rh-Sn/Si02 calcined at various temperatures 1:873 K, 2:773 K, 3:673 K, 4:Rh/Si02 at 773 K
189 was enough reduced around 573 K as shown in Fig. 2. The mixing between the reduced Rh and Sn is required much higher temperature. Figure 4 shows the XRD powder patterns of the Rh-Sn/SiO2 which was calcined at different temperatures followed by reduction at 773 K. The catalyst calcined at 673 K indicates both Rh and Rh-Sn structure. The calcination at a low temperature brought about the significant segregation of Rh and Rh-Sn. Although neither metallic Sn or Sn oxide was observed in the XRD patterns, a segregated Sn could not be excluded. 3.4. CO oxidation Figure 5 shows the CO oxidation activity over Rh-Sn/SiO2 catalysts which were reduced at different temperatures. The activity was evaluated with the apparent first order rate constant. The initial reaction rate for CO oxidation depended on partial pressure of 02 in first order over Rh and Rh-Sn/SiO2 described previously [3]. The dashed line indicates the activity over Rh/SiO2. The activity over the catalyst reduced at 573 K was identical to that over Rh/SiO2 as shown in Fig. 5.
~~' ,,"" 0"-0-0
1.0 T
cn
'I/i
0.5
0
'
D-ZS
'
373
'"' 0
~ '
'
'
423 Temperature / K
'
'
m
I
473
Figure 5. CO oxidation activity o f Rh-Sn/Si02 reduced at various temperatures O" reduced at 573 K, A: at 673 K, D: at 773 K The dashed line means the activity o f Rh/Si02 reduced at 773 K.
The catalysts reduced at 673 and 773 K indicated much higher activity at low reaction temperatures. The temperature which the activity appeared was lowered by 40-50 K. These results are well correlated to the peak shit~ in XRD powder
190 patterns as shown in Fig. 3. The well-mixing of Rh and Sn brings about the high activity at low temperatures of Rh-Sn/SiO2 for CO oxidation reaction. The surface composition and oxidation state would be changed from the state just after pretreatment because the catalysts were exposed to the oxygen containing atmosphere during the reaction, whereas only the metallic Rh phase (or Rh-Sn phase) was observed in the XRD patterns atter the reaction. The unusual temperature dependence of the CO oxidation as shown in Fig. 5 has been reported [14] and would be ascribed to the modified surface composition of adlayer, which was consisted of CO and O atoms, during the reaction discussed previously [3]. Although the change of the surface composition during the reaction seemed to be considered, the initiation temperature of the reaction would be affected directly by the original surface composition and electronic state. 3.5. State of Rh-Sn on SiO2 and CO oxidation activity The reduction behavior of Rh-Sn/SiO2 can be summarized below. 1)Rhodium oxide of the calcined catalyst was reduced first in the range of 360 to 420 K. 2)Tin oxide was reduced over 440 K with catalyzing by metallic Rh. 3)The alloying between Rh and Sn proceeded at a higher temperature than 673 K during the reduction. For CO oxidation reaction, the most important factor was the alloying between metallic Rh and reduced Sn, because the high activity of the catalyst required the reduction at higher temperature than 673 K as shown Fig. 5. The reduction up to 573 K may gave metallic Rh and the reduced low valent Sn (some part of Sn was reduced to Sn0), which was separated each other. The separated Rh and Sn indicated the same activity for CO oxidation as Rh/SiO2. The reduction at higher than 673 K would induce the mixing between Rh and Sn. The well-mixed Rh-Sn on SiO2 seems to be markedly active for CO oxidation.
191 REFERENCES
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