- Email: [email protected]
Spillover of hydrogen over zirconium oxide promoted by sulfate ion and platinum
W A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 146 (1996) 157-164
Spillover of hydrogen over zirconium oxide promoted by sulfate ion and platinum Tetsuya Shishido, Hideshi Hattori * Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13, W8, Sapporo 060, Japan
Abstract
Spillover of hydrogen over zirconium oxide promoted by sulfate ion and a platinum catalyst (Pt/SO42--ZrO2) was studied by temperature-programmed desorption (TPD), in which hydrogen was adsorbed at different temperatures and a sequential adsorption of hydrogen and deuterium was undertaken. As the hydrogen adsorption temperature was raised, the hydrogen uptake increased and the desorption peak shifted to higher temperatures. A sequential adsorption of hydrogen and deuterium resulted in an incomplete isotope mixing in the desorbed gases. Without platinum in the catalyst, hydrogen was scarcely adsorbed. The results are discussed in terms of hydrogen spillover. It is suggested that dissociative adsorption of the hydrogen molecule takes place on the platinum site followed by hydrogen spillover onto the support of zirconium oxide modified with sulfate ion (SO2--ZrO2). Keywords: Spillover;Platinum supported on sulfated zirconia;Temperature-programmeddesorption
1. Introduction
Although the SO42--ZrO2 catalyst possesses strong acid sites to catalyze skeletal isomerization of alkanes, the activity rapidly decreased with reaction time. Addition of platinum to the SO2--ZrO2 results in the formation of the acidic catalyst (Pt/SO2--ZrO2) that retains a high activity for a long time in the presence of hydrogen [1]. The hydrogen effect on the catalytic activity of Pt/SO2--ZrO2 is proposed to be due to the formation of protonic acid sites from hydrogen molecules [2-4]. It was suggested that the hydrogen molecule is dissociatively adsorbed on the platinum species to form hydrogen atoms which * Corresponding author. Tel.: [email protected].
(+81-11)
706-7119;
fax: (+81-11)
0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PI1 S0926-860X(96)001 6 1-5
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158
T. Shishido, H. Hattori/Applied Catalysis A: General 146 (1996) 157-164
undergo spillover on the SO~ - Z r O 2. The hydrogen atom migrates over the surface of SO~--ZrO 2 to the Lewis acid site where the hydrogen atom loses an electron to form a proton. The Lewis acid site trapping an electron reacts with a second hydrogen atom to adsorb hydrogen in the form of a hydride. In the present work, hydrogen adsorption on P t / S O 4 - - Z r O 2 was studied by temperature-programmed desorption (TPD) to picture the behavior of hydrogen on the surface of P t / S O ~ - - Z r O 2.
2. Experimental
2.1. Catalyst preparation The sulfate ion-treated Z r ( O H ) 4 (SO2--Zr(OH)4) was prepared by impregnation of Z r ( O H ) 4 with 1 N H 2SO 4 aqueous solution, followed by filtration and drying at 383 K. The Z r ( O H ) 4 w a s obtained by the hydrolysis of ZrOC12 • 8 H 2 0 (Wako) with 25% NH4OH aqueous solution, followed by filtration. The resulting gel was washed with distilled water until no C1 ions could be detected. The SO42--ZrO2 sample was obtained by calcination o f SO2--Zr(OH)4 at 873 K. The P t / S O 2 - Z r O 2 sample (0.5 wt.-% Pt) was prepared by impregnation of SO42--Zr(OH)4 with H2PtC16 aqueous solution followed by drying at 383 K and calcination at 873 K in air. The physical mixture of Pt and SO 2 - - z r O 2 (0.5 wt.-% Pt) was prepared by mixing Pt black (Wako) with SO2--ZrOe in an agate mortar.
2.2. TPD for adsorption of H2 The sample was pretreated at 623 K in a hydrogen flow for 2 h, followed by evacuation at 673 K for 1 h. After cooling to the H 2 adsorption temperature, the sample was exposed to 53.2 kPa of H 2 for 1 h, and then cooled to room temperature in the presence of hydrogen. After cooling to room temperature, the sample was evacuated for 1 h, and TPD was run up to 623 K at the heating rate of 10 K min -~. The desorbed gases were analyzed by mass spectrometry (ANELVA AQA- 100R).
2.3. TPD for sequential adsorption of H2 and O 2 Two sets of sequential adsorption were carried out. For the first set, the pretreated sample was exposed to 26.6 kPa of H 2 at 423 K for 15 min, followed by evacuation for 15 min. Then, the sample was exposed to 26.6 kPa of D 2 for 15 min. The sample was cooled to room temperature in the presence of D 2 and evacuated for 15 min before TPD was run up to 623 K at the heating rate of 10 K min- 1.
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
159
For the second set, the adsorptions of H 2 and D 2 were carried out at different temperatures. The pretreated sample was exposed to 26.6 kPa of H 2 at 523 K for 15 min. After cooling to 373 K in the presence of H 2, the sample was evacuated for 15 min, and exposed to 26.2 kPa of D 2 for 15 min. The sample was cooled to room temperature in the presence of D 2, and evacuated for 15 rain before TPD was run.
3. Results Fig. 1 shows TPD plots for hydrogen adsorbed at 473 K on P t / s o Z - - Z r O 2 , the physical mixture of Pt black and SO 2 - Z r O 2 , and SO42--ZRO2. For P t / s o Z - - Z r O 2 , a large amount of H 2 w a s desorbed with a peak temperature of 495 K. For the physical mixture of Pt black and SO2--ZrO2, a small amount of H 2 w a s desorbed with a peak temperature of 475 K. Without Pt, the amount of H 2 was even much smaller than that of the physical mixture. TPD plots for H 2 adsorbed on P t / s o Z - - Z r O 2 at different temperatures are shown in Fig. 2. As the adsorption temperature increased, the amount of desorbed H 2 increased, and the desorption temperature shifted to higher temperatures. In particular, the increases in the amount and the peak temperature were marked as the adsorption temperature was changed from 423 K to 473 K. The adsorption and desorption involve the process requiring a high activation energy. Fig. 3 shows TPD plots for two sets of sequential adsorption of H 2 and D 2. In the first experiment, H 2 and D 2 were adsorbed at the same temperature, 423 K, and the TPD plots for H 2, HD, and D 2 are shown in Fig. 3(a). The desorption peaks for D 2 , HD, and H 2 appeared at different temperatures, 410 K, 440 K, and 465 K, respectively. The peak areas for De, HD, and H 2 were of the
d
.~
(b) (c) t
300
j I
I
400
t
I
500
I
J
600
Desorption temperature / K Fig. 1. TPD plots for hydrogen adsorbed at 473 K on (a) P t / S O 2 - Z r O 2, (b) physical mixture of Pt black and S042 - Z r O 2, and (c) S042- - Z r O 2.
160
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
(e)
(a) i
i
i
300
I
i
400
I
500
i
600
Desorption temperature / K Fig. 2. TPD plots for hydrogen adsorbed on Pt/SO42- -ZrO 2 at different temperatures: (a) room temperature, (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, and (f) 523 K.
I
1.0
#. . ;..:
f/
"--
~ . . - - ' : ~ J i
250
"'-
l
300
~
i
350
.'m,--
i
400
#'. i
450
i
500
i
550
600
650
Desorption temperature / K
-,, ,4
[0.1
.
/ .. ,, .' • i
250
/~p/~
: ,'
-~ ~ ~
(b)
/
/
/ // /
/ i
300 350
N i
i
i
i
400
450
500
550
I
600 650
Desorption temperature / K Fig. 3. TPD plots for sequential adsorption of H 2 and D 2 on P t / S O ~ - - Z r O 2 : (a) H 2 adsorbed at 23 K, evacuated at 423 K, D 2 adsorbed at 423 K (b) H 2 adsorbed at 523 K, evacuated at 373 K, D 2 adsorbed at 373 K: ( ) H2, ( - - - ) HD, ( . . . . . ) D 2.
T. Shishido, H. Hatto ri / Applied Catalysis A: General 146 (1996) 157-164
/
161
\
eL
250
300
350
400
450
500
550
Desorption temperature/
600
650
K
Fig. 4. TPD plots for (a) H 2 and (b) D 2 adsorbed at 473 K and 26.6 kPa on Pt/SO42- - Z r O 2 .
ratio 1.5:1:1.3. If D and H were completely mixed on the surface, binominal distribution of D 2, HD, and H 2 would have been expected, and the value [HD]2/[H2] × [D 2] should have been close to 4. The observed value was [HD]2/[H2] × [D 2] = 0.52. It was concluded that the mixing of H and D was incomplete. In the second experiment, H 2 and D 2 were adsorbed at different temperatures, 523 K for H 2 and 373 K for D 2. The desorption peaks for D 2, HD, and H 2 appeared at 390 K, 420 K, and 510 K, respectively. The peak areas for D 2, HD, and H 2 were of the ratio 0.7:1:8.9. The isotopic distribution was far from the binominal distribution, indicating incomplete mixing of H and D on the surface. In Fig. 4, the TPD plot for H 2 is compared with that of D 2 when H 2 and D 2 were adsorbed independently, but under the same conditions (473 K, 26.2 kPa). The peak area was larger for H 2 than for D 2, and the H 2 peak appeared at a slightly lower temperature.
4. Discussion All results indicate hydrogen spillover and the importance of the migration of hydrogen atoms away from active centers into the surrounding regions of the surface of SO2--ZrO2 . Since the adsorption of H 2 on SO2--ZrO2 was negligibly small as compared to the other samples, it is suggested that dissociative adsorption of hydrogen occurs only at the Pt centers. The hydrogen atoms dissociated on the Pt species spillover onto the surface of SO4z - - Z r O 2, and there is subsequent radial migration away from the Pt species into the surrounding SO42--ZrO2 . This model is essentially the same as that proposed by Beck and White for the adsorption of H 2 on P t / T i O 2 [5-7].
162
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164 < Hz adsorption > H2
< D2 adsorption > D2
~l
(a)
(b)
H ---~ H - - ~ H
< desorption >
D~, H D , H2
(e)
t Fig. 5. The models of adsorption and desorption of H2, HD, D2: (a) H 2 adsorption, (b) D 2 adsorption, (c) desorption.
The average migration distance increases with the adsorption temperature of H 2, and therefore, the amount of adsorbed hydrogen increases with the adsorption temperature. Assuming that desorption occurs by migration back to the Pt species, the desorption peak of He appears at a higher temperature with increasing the adsorption temperature because the average distance of the adsorbed H from the Pt species becomes larger. The model of the sequential adsorption is illustrated in Fig. 5. The hydrogen molecule is dissociatively adsorbed on the platinum species to form hydrogen atoms which migrate on the surface of SO42 - Z r O 2 by the concentration gradient as shown in Fig. 5(a). A brief evacuation removes hydrogen from the Pt species and nearby oxide region. Hydrogen located at a long distance from the Pt species does not easily migrate back to the Pt species because of a reduced concentration gradient. Subsequent exposure to deuterium saturates the Pt species and the oxide region nearby the Pt species. After these exposures, it is expected that the adsorbed H and D are spatially separated with deuterium locating at a small average distance from the Pt species as shown in Fig. 5(b). Isotopic mixing occurs at the interface between H adsorbing region and D
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
163
adsorbing region. Consequently, we expect that the order of desorption is D2, HD, and H 2 from lower temperatures as shown in Fig. 5(c). There may be an alternative explanation for the experimental results. For desorption may result from a direct recombination of H + and H - to give H 2. In this case, there should be several types of adsorption sites different in the strength of adsorption. The initial adsorption of H 2 occupies the stronger adsorption sites, and the second adsorption of D 2 occupies the weaker sites. Evacuation with increasing temperature resulted in the evolution of isotopic hydrogen in the order, D e from weaker sites and H 2 from stronger sites. However, it is not certain at present time as to whether desorption of hydrogen takes place via direct recombination of H + and H - , or reverse spillover. We are in favor of the latter view. The surface migration seems to be slower for D than for H. Therefore, average distance from the Pt species is smaller for D than for H, provided that the adsorption conditions are the same. Slower migration results in smaller desorption peaks for D 2 as can be seen in Fig. 4. If forward migration is slow, reverse migration should also be slow. Slower reverse migration for D results in the desorption peak for D 2 at a high temperature. As described earlier, the characteristic feature of P t / S O 4 - Z r O 2 is the formation of protonic acid sites upon heating in the presence of H 2 [2-4]. The formation of protonic acid sites is accompanied by elimination of Lewis acid sites. By evacuation of the catalyst adsorbing hydrogen, the protonic acid sites are eliminated and original Lewis acid sites are restored. It is suggested in the present study that hydrogen atoms spillover the surface o f SO42--ZRO2 . Therefore, it is plausible that the spilt over hydrogen forms protonic acid site over S O ~ - - Z r O 2, though all the spilt over hydrogen atoms are not necessarily converted to protons. The behavior of hydrogen on the Pt/SO42 - Z r O 2 may be summarized as follows. The hydrogen molecule is adsorbed on the platinum particle and dissociates into two H atoms. The H atom adsorbed on the platinum particle spills over the support and migrates to Lewis acid site where the H atom releases an electron and becomes an H +. The H + is stabilized at the oxygen atom nearby the Lewis acid site, and acts as an active site for acid-catalyzed reactions. The Lewis acid site is weakened by accepting an electron. A second H atom may react with the Lewis acid site accepting an electron to form a bond H--Lewis acid site. By evacuation of hydrogen in gas phase, the reverse processes take place to restore the original Lewis acid sites and to eliminate the protonic acid sites. In this way, the protonic acid sites are generated and eliminated in response to the gas phase hydrogen pressure. References [1] T. Hosoi, T. Shimadzu, S. Ito, S. Baba, H. Takaoka, T. Imai and N. Yokoyama, Prepr. Syrup. Div. Petr. Chem., Am. Chem. Soc., (1988) 562.
164 [2] [3] [4] [5] [6] [7]
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
K. Ebitani, K. Ebitani, H. Hattori, D.D. Beck D.D. Beck D.D. Beck
J. Konishi and H. Hattori, J. Catal., 130 (1991) 257. J. Tsuji, H. Hattori and H. Kita, J. Catal., 135 (1992) 609. Stud. Surf. Sci. Catal., 77 (1993) 69. and J.M. White, J. Phys. Chem., 88 (1984) 174. and J.M. White, J. Phys. Chem., 88 (1984) 2764. and J.M. White, J. Phys. Chem., 88 (1984) 2771.
ELSEVIER
Applied Catalysis A: General 146 (1996) 157-164
Spillover of hydrogen over zirconium oxide promoted by sulfate ion and platinum Tetsuya Shishido, Hideshi Hattori * Center for Advanced Research of Energy Technology (CARET), Hokkaido University, N13, W8, Sapporo 060, Japan
Abstract
Spillover of hydrogen over zirconium oxide promoted by sulfate ion and a platinum catalyst (Pt/SO42--ZrO2) was studied by temperature-programmed desorption (TPD), in which hydrogen was adsorbed at different temperatures and a sequential adsorption of hydrogen and deuterium was undertaken. As the hydrogen adsorption temperature was raised, the hydrogen uptake increased and the desorption peak shifted to higher temperatures. A sequential adsorption of hydrogen and deuterium resulted in an incomplete isotope mixing in the desorbed gases. Without platinum in the catalyst, hydrogen was scarcely adsorbed. The results are discussed in terms of hydrogen spillover. It is suggested that dissociative adsorption of the hydrogen molecule takes place on the platinum site followed by hydrogen spillover onto the support of zirconium oxide modified with sulfate ion (SO2--ZrO2). Keywords: Spillover;Platinum supported on sulfated zirconia;Temperature-programmeddesorption
1. Introduction
Although the SO42--ZrO2 catalyst possesses strong acid sites to catalyze skeletal isomerization of alkanes, the activity rapidly decreased with reaction time. Addition of platinum to the SO2--ZrO2 results in the formation of the acidic catalyst (Pt/SO2--ZrO2) that retains a high activity for a long time in the presence of hydrogen [1]. The hydrogen effect on the catalytic activity of Pt/SO2--ZrO2 is proposed to be due to the formation of protonic acid sites from hydrogen molecules [2-4]. It was suggested that the hydrogen molecule is dissociatively adsorbed on the platinum species to form hydrogen atoms which * Corresponding author. Tel.: [email protected].
(+81-11)
706-7119;
fax: (+81-11)
0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PI1 S0926-860X(96)001 6 1-5
726-0731;
e-mail:
158
T. Shishido, H. Hattori/Applied Catalysis A: General 146 (1996) 157-164
undergo spillover on the SO~ - Z r O 2. The hydrogen atom migrates over the surface of SO~--ZrO 2 to the Lewis acid site where the hydrogen atom loses an electron to form a proton. The Lewis acid site trapping an electron reacts with a second hydrogen atom to adsorb hydrogen in the form of a hydride. In the present work, hydrogen adsorption on P t / S O 4 - - Z r O 2 was studied by temperature-programmed desorption (TPD) to picture the behavior of hydrogen on the surface of P t / S O ~ - - Z r O 2.
2. Experimental
2.1. Catalyst preparation The sulfate ion-treated Z r ( O H ) 4 (SO2--Zr(OH)4) was prepared by impregnation of Z r ( O H ) 4 with 1 N H 2SO 4 aqueous solution, followed by filtration and drying at 383 K. The Z r ( O H ) 4 w a s obtained by the hydrolysis of ZrOC12 • 8 H 2 0 (Wako) with 25% NH4OH aqueous solution, followed by filtration. The resulting gel was washed with distilled water until no C1 ions could be detected. The SO42--ZrO2 sample was obtained by calcination o f SO2--Zr(OH)4 at 873 K. The P t / S O 2 - Z r O 2 sample (0.5 wt.-% Pt) was prepared by impregnation of SO42--Zr(OH)4 with H2PtC16 aqueous solution followed by drying at 383 K and calcination at 873 K in air. The physical mixture of Pt and SO 2 - - z r O 2 (0.5 wt.-% Pt) was prepared by mixing Pt black (Wako) with SO2--ZrOe in an agate mortar.
2.2. TPD for adsorption of H2 The sample was pretreated at 623 K in a hydrogen flow for 2 h, followed by evacuation at 673 K for 1 h. After cooling to the H 2 adsorption temperature, the sample was exposed to 53.2 kPa of H 2 for 1 h, and then cooled to room temperature in the presence of hydrogen. After cooling to room temperature, the sample was evacuated for 1 h, and TPD was run up to 623 K at the heating rate of 10 K min -~. The desorbed gases were analyzed by mass spectrometry (ANELVA AQA- 100R).
2.3. TPD for sequential adsorption of H2 and O 2 Two sets of sequential adsorption were carried out. For the first set, the pretreated sample was exposed to 26.6 kPa of H 2 at 423 K for 15 min, followed by evacuation for 15 min. Then, the sample was exposed to 26.6 kPa of D 2 for 15 min. The sample was cooled to room temperature in the presence of D 2 and evacuated for 15 min before TPD was run up to 623 K at the heating rate of 10 K min- 1.
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
159
For the second set, the adsorptions of H 2 and D 2 were carried out at different temperatures. The pretreated sample was exposed to 26.6 kPa of H 2 at 523 K for 15 min. After cooling to 373 K in the presence of H 2, the sample was evacuated for 15 min, and exposed to 26.2 kPa of D 2 for 15 min. The sample was cooled to room temperature in the presence of D 2, and evacuated for 15 rain before TPD was run.
3. Results Fig. 1 shows TPD plots for hydrogen adsorbed at 473 K on P t / s o Z - - Z r O 2 , the physical mixture of Pt black and SO 2 - Z r O 2 , and SO42--ZRO2. For P t / s o Z - - Z r O 2 , a large amount of H 2 w a s desorbed with a peak temperature of 495 K. For the physical mixture of Pt black and SO2--ZrO2, a small amount of H 2 w a s desorbed with a peak temperature of 475 K. Without Pt, the amount of H 2 was even much smaller than that of the physical mixture. TPD plots for H 2 adsorbed on P t / s o Z - - Z r O 2 at different temperatures are shown in Fig. 2. As the adsorption temperature increased, the amount of desorbed H 2 increased, and the desorption temperature shifted to higher temperatures. In particular, the increases in the amount and the peak temperature were marked as the adsorption temperature was changed from 423 K to 473 K. The adsorption and desorption involve the process requiring a high activation energy. Fig. 3 shows TPD plots for two sets of sequential adsorption of H 2 and D 2. In the first experiment, H 2 and D 2 were adsorbed at the same temperature, 423 K, and the TPD plots for H 2, HD, and D 2 are shown in Fig. 3(a). The desorption peaks for D 2 , HD, and H 2 appeared at different temperatures, 410 K, 440 K, and 465 K, respectively. The peak areas for De, HD, and H 2 were of the
d
.~
(b) (c) t
300
j I
I
400
t
I
500
I
J
600
Desorption temperature / K Fig. 1. TPD plots for hydrogen adsorbed at 473 K on (a) P t / S O 2 - Z r O 2, (b) physical mixture of Pt black and S042 - Z r O 2, and (c) S042- - Z r O 2.
160
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
(e)
(a) i
i
i
300
I
i
400
I
500
i
600
Desorption temperature / K Fig. 2. TPD plots for hydrogen adsorbed on Pt/SO42- -ZrO 2 at different temperatures: (a) room temperature, (b) 323 K, (c) 373 K, (d) 423 K, (e) 473 K, and (f) 523 K.
I
1.0
#. . ;..:
f/
"--
~ . . - - ' : ~ J i
250
"'-
l
300
~
i
350
.'m,--
i
400
#'. i
450
i
500
i
550
600
650
Desorption temperature / K
-,, ,4
[0.1
.
/ .. ,, .' • i
250
/~p/~
: ,'
-~ ~ ~
(b)
/
/
/ // /
/ i
300 350
N i
i
i
i
400
450
500
550
I
600 650
Desorption temperature / K Fig. 3. TPD plots for sequential adsorption of H 2 and D 2 on P t / S O ~ - - Z r O 2 : (a) H 2 adsorbed at 23 K, evacuated at 423 K, D 2 adsorbed at 423 K (b) H 2 adsorbed at 523 K, evacuated at 373 K, D 2 adsorbed at 373 K: ( ) H2, ( - - - ) HD, ( . . . . . ) D 2.
T. Shishido, H. Hatto ri / Applied Catalysis A: General 146 (1996) 157-164
/
161
\
eL
250
300
350
400
450
500
550
Desorption temperature/
600
650
K
Fig. 4. TPD plots for (a) H 2 and (b) D 2 adsorbed at 473 K and 26.6 kPa on Pt/SO42- - Z r O 2 .
ratio 1.5:1:1.3. If D and H were completely mixed on the surface, binominal distribution of D 2, HD, and H 2 would have been expected, and the value [HD]2/[H2] × [D 2] should have been close to 4. The observed value was [HD]2/[H2] × [D 2] = 0.52. It was concluded that the mixing of H and D was incomplete. In the second experiment, H 2 and D 2 were adsorbed at different temperatures, 523 K for H 2 and 373 K for D 2. The desorption peaks for D 2, HD, and H 2 appeared at 390 K, 420 K, and 510 K, respectively. The peak areas for D 2, HD, and H 2 were of the ratio 0.7:1:8.9. The isotopic distribution was far from the binominal distribution, indicating incomplete mixing of H and D on the surface. In Fig. 4, the TPD plot for H 2 is compared with that of D 2 when H 2 and D 2 were adsorbed independently, but under the same conditions (473 K, 26.2 kPa). The peak area was larger for H 2 than for D 2, and the H 2 peak appeared at a slightly lower temperature.
4. Discussion All results indicate hydrogen spillover and the importance of the migration of hydrogen atoms away from active centers into the surrounding regions of the surface of SO2--ZrO2 . Since the adsorption of H 2 on SO2--ZrO2 was negligibly small as compared to the other samples, it is suggested that dissociative adsorption of hydrogen occurs only at the Pt centers. The hydrogen atoms dissociated on the Pt species spillover onto the surface of SO4z - - Z r O 2, and there is subsequent radial migration away from the Pt species into the surrounding SO42--ZrO2 . This model is essentially the same as that proposed by Beck and White for the adsorption of H 2 on P t / T i O 2 [5-7].
162
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164 < Hz adsorption > H2
< D2 adsorption > D2
~l
(a)
(b)
H ---~ H - - ~ H
< desorption >
D~, H D , H2
(e)
t Fig. 5. The models of adsorption and desorption of H2, HD, D2: (a) H 2 adsorption, (b) D 2 adsorption, (c) desorption.
The average migration distance increases with the adsorption temperature of H 2, and therefore, the amount of adsorbed hydrogen increases with the adsorption temperature. Assuming that desorption occurs by migration back to the Pt species, the desorption peak of He appears at a higher temperature with increasing the adsorption temperature because the average distance of the adsorbed H from the Pt species becomes larger. The model of the sequential adsorption is illustrated in Fig. 5. The hydrogen molecule is dissociatively adsorbed on the platinum species to form hydrogen atoms which migrate on the surface of SO42 - Z r O 2 by the concentration gradient as shown in Fig. 5(a). A brief evacuation removes hydrogen from the Pt species and nearby oxide region. Hydrogen located at a long distance from the Pt species does not easily migrate back to the Pt species because of a reduced concentration gradient. Subsequent exposure to deuterium saturates the Pt species and the oxide region nearby the Pt species. After these exposures, it is expected that the adsorbed H and D are spatially separated with deuterium locating at a small average distance from the Pt species as shown in Fig. 5(b). Isotopic mixing occurs at the interface between H adsorbing region and D
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
163
adsorbing region. Consequently, we expect that the order of desorption is D2, HD, and H 2 from lower temperatures as shown in Fig. 5(c). There may be an alternative explanation for the experimental results. For desorption may result from a direct recombination of H + and H - to give H 2. In this case, there should be several types of adsorption sites different in the strength of adsorption. The initial adsorption of H 2 occupies the stronger adsorption sites, and the second adsorption of D 2 occupies the weaker sites. Evacuation with increasing temperature resulted in the evolution of isotopic hydrogen in the order, D e from weaker sites and H 2 from stronger sites. However, it is not certain at present time as to whether desorption of hydrogen takes place via direct recombination of H + and H - , or reverse spillover. We are in favor of the latter view. The surface migration seems to be slower for D than for H. Therefore, average distance from the Pt species is smaller for D than for H, provided that the adsorption conditions are the same. Slower migration results in smaller desorption peaks for D 2 as can be seen in Fig. 4. If forward migration is slow, reverse migration should also be slow. Slower reverse migration for D results in the desorption peak for D 2 at a high temperature. As described earlier, the characteristic feature of P t / S O 4 - Z r O 2 is the formation of protonic acid sites upon heating in the presence of H 2 [2-4]. The formation of protonic acid sites is accompanied by elimination of Lewis acid sites. By evacuation of the catalyst adsorbing hydrogen, the protonic acid sites are eliminated and original Lewis acid sites are restored. It is suggested in the present study that hydrogen atoms spillover the surface o f SO42--ZRO2 . Therefore, it is plausible that the spilt over hydrogen forms protonic acid site over S O ~ - - Z r O 2, though all the spilt over hydrogen atoms are not necessarily converted to protons. The behavior of hydrogen on the Pt/SO42 - Z r O 2 may be summarized as follows. The hydrogen molecule is adsorbed on the platinum particle and dissociates into two H atoms. The H atom adsorbed on the platinum particle spills over the support and migrates to Lewis acid site where the H atom releases an electron and becomes an H +. The H + is stabilized at the oxygen atom nearby the Lewis acid site, and acts as an active site for acid-catalyzed reactions. The Lewis acid site is weakened by accepting an electron. A second H atom may react with the Lewis acid site accepting an electron to form a bond H--Lewis acid site. By evacuation of hydrogen in gas phase, the reverse processes take place to restore the original Lewis acid sites and to eliminate the protonic acid sites. In this way, the protonic acid sites are generated and eliminated in response to the gas phase hydrogen pressure. References [1] T. Hosoi, T. Shimadzu, S. Ito, S. Baba, H. Takaoka, T. Imai and N. Yokoyama, Prepr. Syrup. Div. Petr. Chem., Am. Chem. Soc., (1988) 562.
164 [2] [3] [4] [5] [6] [7]
T. Shishido, H. Hattori / Applied Catalysis A: General 146 (1996) 157-164
K. Ebitani, K. Ebitani, H. Hattori, D.D. Beck D.D. Beck D.D. Beck
J. Konishi and H. Hattori, J. Catal., 130 (1991) 257. J. Tsuji, H. Hattori and H. Kita, J. Catal., 135 (1992) 609. Stud. Surf. Sci. Catal., 77 (1993) 69. and J.M. White, J. Phys. Chem., 88 (1984) 174. and J.M. White, J. Phys. Chem., 88 (1984) 2764. and J.M. White, J. Phys. Chem., 88 (1984) 2771.