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7 Time Dependence of H2 and O Chemisorption on Rh-TiO Catalysts 2

2

H. F. J. van't Blik1, P. H. A. Vriens, and R. Prins

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch007

Laboratory for Inorganic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

A fast as well as a slow component was observed in the room temperature adsorption of H2 and O2 on Rh on rutile and anatase TiO2 catalysts. Both components of the H2 adsorption decreased in magnitude with increasing catalyst reduction temperature, while the fast O2 adsorption increased in magnitude and the slow O2 adsorption stayed constant. The fast H2 adsorption proved to be due to chemisorption on the metal and the slow H2 adsorption to spillover from the metal to the support. The fast O2 adsorption was due to chemisorption on the metal as well as on the support, with the concurrent reoxidation of Ti3+ ions formed during Η-spillover. The slow O2 adsorption was caused by corrosive chemisorption of the metal particles. Some years ago Tauster et a l . reported that after a high tempera ture reduction of group VIII metals on supports l i k e T1O2, Μ>25 and V2O3 the adsorption of H2 and CO was suppressed, while O2 adsorption was unaffected (1-3). They believed that these effects were caused by a special type of metal-support interaction and therefore c a l l e d this interaction Strong Metal Support Interaction (SMSI). C a t a l y t i c studies showed that with the catalyst in the SMSI state many reactions were inhibited too. Thus hydrogenolysis of ethane (4, 5) and butane (5,, 6), hydrogénation of ethene, benzene and styrene and dehydrogenation of cyclohexane (5, 6) and reforming of hexane (]_) were much slower with catalysts reduced at 500°C than at 200 or 250 C. On the other hand CO hydrogénation was not i n h i b i t e d at a l l , on the contrary for some metals on T1O2 i t was even somewhat better after high temperature reduction (on the basis of t o t a l metal present) (8-10). e

Current address: Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands

1

0097-6156/86/0298-0060$06.00/0 © 1986 American Chemical Society

Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

7.

61

H2 and O2 Chemisorption on Rh-Ti02

VAN T BLIK ET AL.

The i n s e n s i t i v i t y of CO hydrogénation to reduction temperature may be connected to the fact that oxidants l i k e O2 and H2O are capable of bringing the catalyst back from the SMSI state to the normal adsorption state. Thus Tauster et a l . published that the H*2 adsorption capacity of a Pd/Ti02 c a t a l y s t , which had been brought into the SMSI state by reduction at S00°C, was completely restored after oxidation at 400°C for 1 h and rereduction at 175°C (1). Baker et a l . published that H 0 at 250°C for 1 h could restore the h*2 and CO adsorption capacities of a Pt/Ti02 catalyst, although to a less extent than oxidation at 600°C (11). On the other hand, Mériaudeau et a l . reported that H2 adsorption as well as c a t a l y t i c a c t i v i t i e s f o r hydrogenolysis and hydrogénation of Ti02~supported Pt, I r and Rh catalysts recovered after O2 admission at room temperature and subsequent reduction at low temperature (6). Explanations f o r SMSI have ranged from e l e c t r o n i c theories, such as a l l o y formation (1, 12), metal-semiconductor interaction (3, 6) and metal-support cation charge transfer (3, 13), to geometrical theories l i k e sintering, poisoning and covering (5, 2 , 14-19). Today the e l e c t r o n i c theory based on metal-support cation interaction and the geometrical covering theory are favoured most. In a l l theories the r e d u c i b i l i t y of the support plays an important r o l e . In the electronic theories i t i s responsible f o r the formation of reduced cations on the surface of the support, e.g. T i on T1O2. Χα scattered wave molecular o r b i t a l calculations on the interaction between a (T1O5) - c l u s t e r and a Pt atom indicated a strong electron transfer to the Pt atom, leading to Pt"* ' . As a consequence there was a strong ionic bonding between Pt and the Ti cluster (13). I t i s a p i t y that this calculation was done on such an unbalanced system, because the large difference i n charge between (T1O5) " and Pt w i l l always favour an electron flow towards Pt. A new calculation with compensating cationic charges around the (T1O5) " cluster would be of interest. R e d u c i b i l i t y of the support and the formation of surface defects by dehydration at high temperatures are important ingredients i n the model of Baker et a l . (11, 20) and Huizinga and Prins (21). They suggested that the interaction between metal p a r t i c l e s and domains of T14O7 under and around the metal p a r t i c l e s might be strong, because of the metallic properties of the Magnelli phase type T14O7 suboxide. In the geometrical, covering theory the r e d u c i b i l i t y of the support, as well as the formation of surface defects by dehydration at high temperatures, are e s s e n t i a l to explain the migration of reduced support species onto the metal p a r t i c l e s (5, 7., 14). The r e d u c i b i l i t y of T1O2 has i n recent years been studied with ESR and NMR techniques (21-23). In the course of our studies (21) we noticed that support reduction i s a r e l a t i v e l y slow process and that, as a consequence, hydrogen chemisorption on a metal on T1O2 catalyst has a fast component, due to adsorption on the metal surface, and a slow component, due to s p i l l o v e r of H atoms from metal to support and subsequent support reduction. We have studied the time dependence of H2 chemisorption, as well as that of O2 chemisorption, i n more d e t a i l and the results of this study are presented i n this paper.


2

3 +

7

0

6

3 +

7

7


62

STRONG METAL-SUPPORT INTERACTIONS

Experimental Two Rh/Ti(>2 catalysts were prepared by pore volume impregnation of the support with an aqueous solution of RI1CI3.X H2O (39 wt%, Drijfhout). One catalyst was made with 0.99 wt% Rh on the r u t i l e modification of T1O2 (Tioxide CLDD 1627/1, pore volume 0.57 ml g " and surface area 20 m g " ) , t h i s catalyst w i l l be further denoted as Rh/R-Ti02- The other catalyst with 1.00 wt% Rh was made with anatase as the support (Tioxide CLDD 1367, pore volume 0.64 ml g " and surface area 20 m g"l) and w i l l be denoted as Rh/A~Ti02. I t was checked that the main X-ray d i f f r a c t i o n lines of the support, as well as those of the catalysts prepared therefrom, were those of r u t i l e or anatase. Before use, both supports were washed twice with d i s t i l l e d water, dried at room temperature and calcined at 500°C f o r 1 h to s t a b i l i s e the surface area. After impregnation the catalysts were dried at room temperature f o r 24 h and subsequently for 10 h at 120 C. Hydrogen and oxygen chemisorption measurements were performed in a conventional glass system at 22°C. Before a H2 chemisorption measurement, the catalyst was reduced, or oxidized and reduced, at temperatures and during times to be specified under Results. These temperatures were reached with a heating rate of 5°C min"" . Reduction and oxidation took place i n flowing hydrogen and oxygen, respectively. After evacuation (10" Pa) at the same temperature the c e l l was cooled down to room temperature and the chemisorption of H2 was started. The H2 used was p u r i f i e d by passing through a Pd d i f f u s i o n c e l l . To check f o r activated chemisorption, i n some cases the chemisorption of H2 was already started at the same temperature as that of evacuation and the catalyst was then cooled down to room temperature under H2. Similar procedures were used for oxygen chemisorption. A l l chemisorption experiments were single point measurements at 8.10* Pa. By measuring the adsorption isotherms from 10 -10 Pa for a few catalysts, i t was checked that a r e l a t i v e comparison of the thus obtained chemisorption values was as j u s t i f i a b l e as any other method based on other measuring points, or on extrapolation of measuring points to zero pressure, as advocated by Benson and Boudart (24). No corrections were made f o r chemisorption on the bare supports as such, because this was found n e g l i g i b l e . 1

2

1

1


2

e

1

2

3

5

Results H? chemisorption. Hydrogen chemisorption of a Rh/R-Ti02 and a Rh/A-Ti02 catalyst were measured at 8.10* Pa as a function of time. In Figure 1 the results f o r the Rh/R-Ti02 catalyst are presented by p l o t t i n g the H/Rh r a t i o s , calculated from the H2 consumption and the t o t a l amount of Rh present, as a function of In t . This was done because i n that way nearly l i n e a r curves were obtained. Starting from the impregnated and dried catalyst the f i r s t measurement was carried out after reduction f o r 1 h and evacuation f o r 1 h at 215°C (Figure 1A). Further experiments were carried out after a subsequent prereduction at 520°C for 2 h, followed by a reoxidation at 140°C for 1 h to break the SMSI state which results from the high temperature reduction. In the following section i t w i l l be shown that


7.

VANT BLIK ET AL.

63

H2 and O2 Chemisorption on Rh-TiÛ2

140° C i s s u f f i c i e n t to re-establish normal chemisorption behaviour. After this treatment reductions and evacuations (for 1 h each) were c a r r i e d out subsequently at 200 300, 400 and 500 °C, followed by h*2 chemisorption measurements. The results of these measurements are presented in Figures IB, 1C, ID and IE, respectively. After the l a s t measurement (Figure IE), the catalyst was further reduced at 500°C for 6 h, again an oxidation was applied at 125°C for 0.5 h, the catalyst was reduced for 1 h and subsequently evacuated f o r 1 h at 220°C, and then i t s H2 chemisorption capacity was measured again (Figure I F ) . Further prolonged reduction at 220°C for 9 h, followed by evacuation at 205°C for 2 h d i d not change the chemisorption behaviour. The resulting H/Rh vs. In t curve was equal to curve F. A l l measurements show a l i n e a r relationship between H*2 chemisorption (expressed as H/Rh r a t i o ) and In t (especially in the f i r s t hours), a decrease in chemisorption with an increase in reduction and evacuation temperature, and a decrease in the slope of the H/Rh vs. In t curves with increasing treatment temperature. The results furthermore demonstrate that no equilibrium chemisorption was established within 18 h (In t = 7) on t h i s c a t a l y s t . The decrease in H/Rh values between curves A and Β demonstrates that the treatment at 520°C had caused s i n t e r i n g . Further treatment at intermediate temperatures followed by another treatment at 500°C did not induce further s i n t e r i n g , or only a l i t t l e b i t ( c f . curves F and B). Therefore sintering of the Rh p a r t i c l e s cannot be the reason f o r the decreased H2 chemisorption and decreased time dependence ( c f . curves Β to E). Note that within the uncertainty of the measurements there i s no difference in slope between curves A, B, and F, indicating that the slope i s independent of the dispersion of the metal. To see i f the time dependence of the chemisorption was caused by a slow establishment of H2 chemisorption at the Rh metal surface, the measurements were also c a r r i e d out for a 3.7 wtX Rh/Si02 catalyst ( s i l i c a Grace S.P. 2-324.382, pore volume 1.2 ml g " and surface area 290 m g" ) after reduction at 500°C. The H2 chemi sorption of t h i s catalyst measured after a day under H2 d i f f e r e d only 10% from that measured d i r e c t l y after reduction, evacuation and cooling (H/Rh= 0.46 and 0.42, respectively). Thus slowness of H2 chemisorption onto the metal cannot be the reason for the time dependence of the H2 chemisorption on the Rh/R-Ti02 c a t a l y s t . That the time dependence of the H2 chemisorption was anyway caused by a slow attainment of equilibrium, was established by performing the H2 chemisorption measurement in a modified way. Hydrogen was admitted to the evacuated catalyst at ca. 200°C and subsequently the reactor was slowly cooled down to room temperature and H2 chemisorption was measured. In this case equilibrium was quickly established and the chemisorption value was substantially higher (about 20%) than that reached during room temperature measurements f o r 18 h. Furthermore, since there was no difference between the H2 chemisorption measured after admission of H2 at 190°C during 10 min and that measured after admission at 205°C during 30 min, a temperature of about 200°C during 10 min seems s u f f i c i e n t to quickly reach equilibrium. Similar results as described above f o r the Rh/R-Ti02 ( r u t i l e )


9

1

2

1


64



catalyst were obtained for the Rh/A-Ti2 (anatase) catalyst, be i t that the slopes of the H/Rh versus In t curves were smaller for Rh on anatase than for Rh on r u t i l e . Breaking of SMSI by oxidation. In the l i t e r a t u r e i t has been stated that a metal-on-Ti(>2 catalyst which i s i n the SMSI state, can be brought back to the normal state by a treatment i n oxygen. Several c o n f l i c t i n g statements have been published, however, about the severeness of oxidation. Thus i t has been said that keeping an SMSI catalyst f o r some time i n a i r would be s u f f i c i e n t (6), but i n another publication i t has been claimed that high temperature oxidation i s absolutely necessary (1, 11). Furthermore, also water vapour has been claimed to be able to break the SMSI state (11). Taken a l l published information together, i t seems clear that an oxidative treatment i s necessary, either with oxygen, water or any other oxygen containing oxidant. The temperature and time needed to completely break the SMSI state and to f u l l y restore the normal metal-on-support state are s t i l l unsettled, however. To study this further we subjected a 0.99 wt% Rh/R-Ti02 catalyst to a series of successive reductίοη-oxidation-rereduction (200°C) treatments, with evacuation after each reduction step, and measured the H2 chemi sorption after each treatment. The catalyst had been pretreated at 500°C for several hours to s t a b i l i s e i t s rhodium dispersion. This catalyst was precalcined at 150°C, (0.5 h), rereduced (1 h) and evacuated (1 h) at 200°C; the subsequently measured H2 chemi sorption i s presented as curve A i n Figure 2. Subsequently the catalyst was reduced again at 500°C f o r 1 h, oxidized i n oxygen at room temperature f o r 15 min (passivated), rereduced and evacuated for 1 h at 200°C, and the H2 chemisorption was measured. The resulting curve Β i n Figure 2 demonstrates that the H2 chemi sorption had decreased, indicating that passivation for 15 min cannot completely break the SMSI state. An extended passivation f o r 17 h l e f t the chemisorption unaltered (curve C), proving that even prolonged passivation could not f u l l y restore the o r i g i n a l chemi sorption capacity. Oxidation at 150°C for 0.5 h (curve D) and oxidation at 260°C for 0.5 h (curve Β ) , however, could restore the o r i g i n a l chemisorption capacity and t h i s shows that the restoration of the catalyst from SMSI to normal state i s an activated process. Similar results were obtained f o r the Rh/A-Ti02 c a t a l y s t . O2 chemisorption. Whereas the chemisorption of hydrogen i s suppressed i n the SMSI state, chemisorption of oxygen s t i l l takes place after a metal-on-Ti02 catalyst has been reduced at high temperature. No information i s available, however, whether there i s a difference i n O2 chemisorption i n SMSI and normal state and whether the reduction temperature has any influence on t h i s . Therefore we looked into the O2 chemisorption capacity of Rh/R-Ti02 and Rh/A-Ti02 catalysts at room temperature and 8.10* Pa after successive reductions and evacuations (1 h each) at temperatures of 200, 245, 280, 350 and 500°C. The results are presented i n Figures 3A, B, C, D and B, respectively. To ensure that the starting condi tions were the same i n a l l cases, before each reduction-evacuation treatment and subsequent O2 chemisorption experiment the catalyst was oxidized at 150°C for 0.5 h because, as shown i n the foregoing,


VANT BLIK ET AU.

H and 0 2

2

Chemisorption on Rh- TiQ

2

1,2-,


*

I

Figure 1. Influence of reduction temperature on the time dependence of the H adsorption of 0.99 wt% Rh/Ti022



66


such an oxidation i s s u f f i c i e n t to bring the catalyst back to the normal state. Furthermore, to eliminate sintering effects the catalyst was prereduced at 500°C for 2 h and reoxidized at 140°C for 1 h. We have presented the chemisorption results r e l a t i v e to the t o t a l amount of rhodium present, that i s as O/Rh values. The results demonstrate that, j u s t as for H2 chemisorption, the O2 chemisorption i s dependent on time. In the f i r s t few hours there i s a more or less l i n e a r relationship with In t, thereafter the chemisorption levels o f f . The O2 chemisorption i s strongly dependent on the pretreatment too, increasing reduction-evacuation temperature leads to increasing O2 chemisorption (Figure 3 curves A to D). On the other hand the slope of the curves i s not much dependent on reduction temperature. The results f o r the Rh/A-Ti02 catalyst are q u a l i t a t i v e l y similar to those of the Rh/R-Ti0 c a t a l y s t , but quantitatively the O2 chemisorption i s always somewhat higher for the Rh/A-Ti02 catalyst and also the slope of the O/Rh vs. In t curves i s larger. Measurement of the O2 chemisorption of a reduced Rh/Si0 catalyst showed that this catalyst behaved s i m i l a r l y as the Rh/Ti0 catalysts, with a very fast O2 consumption, followed by a slow O2 uptake which had a l i n e a r behaviour with In t. In contrast to the Rh/Ti02 catalyst, however, the fast 0 consumption of the Rh/Si0 catalyst proved to be independent of reduction temperature. 2

2

2

2

2

Discussion H2 chemisorption. Both Rh/R-Ti02 and Rh/A-Ti02 show a decrease in H chemisorption when the reduction and evacuation temperature is increased, while at the same time the slope of the chemisorption vs. In t curve decreases. The decrease in H2 chemisorption i s of course due to the gradual t r a n s i t i o n of the Rh p a r t i c l e s into the SMSI state. Whatever the explanation f o r t h i s state, an electronic interaction between metal p a r t i c l e s and support or a covering of the metal p a r t i c l e s by the support, in t h i s SMSI state the metal p a r t i c l e s are unable to adsorb H2. The decreased slope of the H/Rh-ln t curve can be explained in several ways, such as slow H chemisorption on Rh because of an activated process, dependence on metal dispersion, or an e f f e c t related to the support. The experiments in which H2 chemisorption was started around 200°C proved that the time dependence i s indeed due to a slow adsorption at room temperature, but the experiment with Rh/Si02 showed that there i s no k i n e t i c l i m i t a t i o n in the h*2 chemisorption on the metal part of the c a t a l y s t . In accordance with this conclusion, no e f f e c t of rhodium dispersion on the time dependence of the h*2 chemisorption was observed for catalysts in the normal state (cf. Figure 1 curves A, Β and F ) . The conclusion must be that the support i s the cause of the slow uptake of H by the catalyst and this suggests that, in addition to H being bonded to the metal, H must be bonded to the support. I t i s of course well known that supports l i k e T1O2 and WO3 can be reduced by hydrogen atoms which s p i l l over from metal p a r t i c l e s to the support and i t i s not unlogic to presume that s p i l l o v e r i s an activated process, and thus i s slow at room 2

2

2

2

2


VANT BLIK ET AL.

H2 and O2 Chemisorption on Rh-TiC>2


1,2-,

Int (t in min.)

Figure 3. Influence of reduction temperature on the time dependence of the 0 adsorption of 0.99 wt% Rh/R-Ti022



68

temperature (25., 26). At f i r s t sight, however, one encounters a problem trying to explain our measurements with a s p i l l o v e r model. Since a l l our catalysts had been reduced at temperatures of 200°C and higher before the H chemisorption at room temperature, s p i l l over and support reduction should have been complete before the start of the H chemisorption measurements and no support reduction would be expected during subsequent chemisorption measure ments at room temperature. A closer look at the mechanism of support reduction by s p i l l e d over hydrogen and at the conditions under which the experiments were carried out, c l a r i f i e s this contradiction. The answer l i e s in the fate of the s p i l l e d over H atoms. Depending on the temperature of reduction two reduction processes may take place, one at low temperature 2


2

2 Ti

4 +

2

+ 2 Ο " + H

2

-> 2 T i

3 +

+ 2 OIT

(1)

and one at high temperature 2 Ti

4 +

2

+ 0 ~ + H

2

-* 2 T i

3 +

+ H0 t

(2)

2

Actually both processes can be composed of the following steps H 2 H + 2 Ti

2 H

2

4 +

-» 2 H

(3) +

+ 2 Ti

3 +

(4)

2

2 H+ + 2 0 ~ -> 2 OH"

(5)

2

2 OH*" -» O - + H 0 t

(6)

2

The hydrogen atoms which are s p i l l e d over from the metal to the support reduce the Ti*+ ions and the r e s u l t i n g protons are trapped as OH ions (Equations 3, 4 and 5). At elevated temperatures the protons are removed from the support surface by dehydration (Equation 6). Huizinga and Prins demonstrated by means of ESR measurements that Equations 3, 4 and 5 are reversible (21). Thus reduction of P t / T i 0 and Rh/Ti0 catalysts with H at 300°C induced a strong T i ESR signal, which disappeared after evacuation at 300°C, but stayed constant after evacuation at room temperature. The authors concluded that desorption of H by reversed s p i l l o v e r and desorption from the metal was an activated process. These phenomena were found reversible i n the sense that when a catalyst, which had been evacuated at 300°C, was exposed once more to H , the strong T i ESR signal reappeared. This r e v e r s i b i l i t y was not observed when the reduction had taken place at 500°C, because i n that case the protons had l e f t the T i 0 surface as H 0 molecules and Equations 5, 4 and 3 could no longer be reversed. At the same time the titanium ions formed during high temperature reduction were trapped i n the 3+ oxidation state. In view of these ESR r e s u l t s , the explanation of the time dependence of the H chemisorption must be as follows. After reduction i n H at 200°C indeed quite a b i t of T i 0 support w i l l be reduced, but during subsequent evacuation at 200°C a l l Ti ions w i l l be reoxidized. As a consequence i n the subsequent -

2

2

2

3 +

2

2

3 +

2

2

2

2

2

3 +


7.

VANT BLIK ET AL.


H2 chemisorption not only fast adsorption of h*2 on Rh may occur, but also a slow s p i l l o v e r and reduction of ΤΙ**. On the other hand, after reduction at high temperature T i w i l l be formed and the surface of the support w i l l be dehydrated. Therefore during the following evacuation no reoxidation of the T i ions can take place. At the same time the rhodium has been changed into the SMSI state and therefore no H chemisorption w i l l take place at a l l . At intermediate reduction and evacuation temperatures there w i l l be an intermediate behaviour and as a result of this the H chemisorption values w i l l decrease with increasing treatment temperature and so w i l l t h e i r time dependencies. The maximum amount of H which can be chemisorbed onto the support of the Rh/Ti0 catalyst (determined by admission of H to the catalyst at 200°C) i s of the same order of magnitude as the number of T i + ions formed on Pt/Ti02 (21) and Rh/Ti02 (27) and detected by ESR. The agreement between H chemisorption and ESR data means that a l l Ti*+ ions formed during reduction were indeed detected by the ESR technique and proves that these cations are not situated close together as nearest neighbours, because in that case antiferromagnetic coupling between adjacent T i ions would have l e d to a much decreased ESR signal. The fact that the reduction of the support by hydrogen s p i l l o v e r i s an activated process may be explained by hindered s p i l l o v e r from metal to support, by hindered reduction of T i * ions by Η atoms or by d i f f u s i o n of Η atoms over the support surface Our results demonstrate that H chemisorption equilibrium i s quickly reached at 200°C, while the ESR results have shown that the Ti ESR signal formed after reduction at 300°C could be decreased by two orders of magnitude by evacuation at 300°C, but not by evacuation at room temperature. Both the reduction and the reoxidation of the support therefore are activated processes, with similar activation energies. In the reduction step as well as in the reoxidation step, the oxidation state of the titanium cation changes. As a consequence also the cation radius and the Ti-0 interatomic distances of the reduced or oxidized s i t e w i l l change and this w i l l give r i s e to an activation energy b a r r i e r . After the i n i t i a l reduction of Ti*+ cations in the immediate neighbourhood of a metal p a r t i c l e , further reduction of Ti*+ ions can only occur i f the resulting T i ions diffuse away from the metal p a r t i c l e . The d i f f u s i o n of an Η atom over the very well dried, but not dehydroxylated, support surface can be described as hopping from one s i t e to another, or a l t e r n a t i v e l y as a synchronous hopping of a proton and an electron (25, 28). Each hopping consists of a simul taneous reduction of the receiving titanium cation and an oxidation of the leaving s i t e , which may explain the s i m i l a r i t y in activation energy for H chemisorption on and desorption from the support. Instead of the observed In t dependence, a t ^ dependence of the H chemisorption might have been expected. Such a relationship has indeed been observed up to t=5 min (29), but in our experiments i t proved d i f f i c u l t to obtain accurate data at short measuring times. At the moment we do not have a physical explanation for the In t dependence, but we note that a t ^ dependence i s only expected under special conditions and that at large t , when the observed limited value of the slow adsorption i s approached, the adsorption is neither described by a t ^ dependence, nor by a In t dependence. 3 +

3 +

2

2

2

2

2


3

2

3 +

+

2

3 +

3 +

2

2


70

S T R O N G METAL-SUPPORT INTERACTIONS

The proposed mechanism also explains why the slopes of the H chemisorption versus In t curve f o r Rh on anatase were smaller than those f o r Rh on r u t i l e . For the Ti*+ cations at the r u t i l e surface are predicted to be more e a s i l y reduced than those at the anatase surface (30). The question may be posed i f the slow reduction of the support by s p i l l o v e r of H atoms from the metal p a r t i c l e s takes place through the bulk of the T i 0 or on i t s surface only. The H consumption after exposure of H to the Rh/R-Ti0 catalyst at 200°C was found to be independent of exposure time. Apparently only a limited amount (0.3%) of the t o t a l available T i + ions can be reduced at 200°C and this suggests that this amount (which would constitute about 10% of the surface Ti*+ ions) might well be located at the surface. In this respect i t i s of interest to remark that Iyengar et a l . observed that the ESR signal of T i ions, formed after a 200°C reduction of T i 0 , was quenched after 0 admission at room temperature, while the ESR signal obtained after a 500°C reduction remained (31). Apparently T i ions formed at the surface by reduction only d i f fuse to the bulk at temperatures in excess of 200°C. In agreement with this our 0 chemisorption results also show that the major part of the reoxidation of support T i ions i s fast (vide i n f r a ) . In a recent publication Dumesic c.s. described adsorption and desorption measurements of H on N i / T i 0 and P t / T i 0 catalysts, which showed that a larger amount of H could be desorbed (after 15-20 h e q u i l i b r a t i o n of these catalysts under about 40 kPa H ) than could be d i r e c t l y adsorbed (32). In agreement with our conclusions t h e i r explanation was that, apart from a fast H adsorption on the metal, hydrogen apparently also adsorbed slowly on the T i 0 support v i a a s p i l l o v e r process from metal to support. These authors noticed that the amounts of H desorbed from the M/Ti0 catalysts in the SMSI state were in f a i r agreement with metal p a r t i c l e sizes determined by X-ray l i n e broadening and electron microscopy and suggested that H desorption could be used to estimate metal p a r t i c l e sizes in M/Ti0 catalysts in the SMSI state. In a former publication (21) we have presented ESR evidence which shows that indeed for P t / T i 0 the number of H atoms s p i l l e d over to the support is about as large as the number of H atoms which can be adsorbed on the Pt p a r t i c l e s , provided they are not in the SMSI state. Apparently hydrogen s p i l l o v e r can occur both in the SMSI and i n the normal metal-on-support state and i s related to metal p a r t i c l e size. Similar ESR results were obtained f o r Rh/Ti0 catalysts (27). Our present H adsorption results f o r Rh on r u t i l e are in conformance with the ESR r e s u l t s , but the results f o r Rh on anatase indicate a r a t i o of hydrogen adsorbed on the support to hydrogen adsorbed on the rhodium below one. Judged from this limited experience i t seems that the amount of s p i l l o v e r hydrogen might give a semiquantitative idea of the metal p a r t i c l e size in metal catalysts in the SMSI state. Be that as i t may, we would at the moment be reluctant to generalize the above findings without further study and would much rather support the method of breaking the SMSI state by reoxidation and subsequently measuring the metal dispersion by H adsorption. 2

2

2

2

2

4


3 +

2

2

3 +

2

3 +

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

0 chemisorption. Like the H chemisorption, also the 0 chemisorption had a slow component with a In t behaviour in addition to a fast uptake ( c f . Figure 3). The fast component increased with 2

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VAN T BLIK ET AL.

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temperature of reduction, while the slope of the O/Rh vs. In t curve did not depend on reduction temperature. The Rh/Si02 catalyst had a In t type 0 chemisorption too and thus the slow chemisorption cannot be related to the T i 0 support, but must be due to the oxidation process of the rhodium p a r t i c l e s . Temperature programmed oxidation (TPO) measurements on rhodium catalysts with dispersions around 0.5 have demonstrated that chemisorption and further oxidation of the metal p a r t i c l e s could be distinguished (33, 34). TPO p r o f i l e s of both Rh/Ti0 catalysts indeed showed a low temperature peak due to chemisorption of 0 and a second peak around 250°C. Vis et a l . attributed the occurrence of a second peak to a d i f f u s i o n l i m i t a t i o n of Rh and 0 ~ ions through the rhodium oxide layer formed during the i n i t i a l (corrosive) chemisorption (33, 34). For rhodium catalysts with a very low dispersion they even detected a t h i r d TPO peak at s t i l l higher temperature and attributed i t to the oxidation of the kernel of the very large metal p a r t i c l e s . But, for that assignment to be correct there should be a difference i n the d i f f u s i o n mechanisms responsible for the second and t h i r d TPO peaks, because otherwise only one peak would be present. In view of the metal dispersions of about 0.4 present in the Rh/Ti0 catalysts after prèsintering, the second TPO peak around 250°C must be correlated with the slow 0 chemisorption. The d i f f e r e n t d i f f u s i o n mechanisms then are a In t type mechanism for the second TPO peak and possibly a t ^ type mechanism for the t h i r d TPO peak. This seems a very reasonable proposal since in metallurgy a In t oxidation process has indeed been observed for metal layer thicknesses of a few tens of an Â, while a type oxidation i s the normal oxidation d i f f u s i o n mechanism for thick metal layers (35, 36). In accordance with the explanation given for the slow 0 chemisorption process, the slope of the O/Rh-ln t curves i s independent on reduction temperature. The catalysts had been pretreated at 500°C and any reduction at a lower temperature thus w i l l not change the rhodium p a r t i c l e s i z e , therefore the slopes stay constant. The fact that the slope for the anatase based catalyst was about 50% larger than that f o r the r u t i l e catalyst i s also in agreement with this explanation. Judged from the i n i t i a l O/Rh values f o r the anatase and r u t i l e c a t a l y s t , the Rh p a r t i c l e s in the Rh/A-Ti0 catalyst had a dispersion which i s 53% higher than that of the Rh p a r t i c l e s in the Rh/R-Ti0 c a t a l y s t . Since the slope of the 0 chemisorption curves i s caused by further oxidation of the metal p a r t i c l e s in a d i f f u s i o n limited process, the rate of this process w i l l be dependent on the metal-oxygen i n t e r f a c i a l area, thus on the dispersion of the metal. Adsorption of 0 on the Rh p a r t i c l e s w i l l contribute to the fast i n i t i a l 0 chemisorption, but there must also be a contribution from the T i 0 support to explain the increase in 0 chemisorption with increasing reduction temperature of the Rh/Ti0 catalysts. When increasing the reduction temperature T i cations w i l l be formed. Some of these cations w i l l be reoxidized during the subsequent evacuation at the same temperature (the reverse of Equations 5, 4 and 3), but at increasing temperature more and more of these cations w i l l stay in the reduced form because of Equation 6. As a consequence increasing numbers of T i ions can be oxidized during 0 chemisorption. The major part of this 2

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72

STRONG METAL-SUPPORT INTERACTIONS 3 +

reoxidation of T i i o n s by 0 must be r e l a t i v e l y f a s t , because otherwise the support reoxidation would have shown up in the slow 0 chemisorption process in the form of increasing slopes at increasing reduction temperatures. This fast reoxidation i s not in contradiction with the observed slow reduction of the corresponding Ti*+ ions, considering that 0 may d i r e c t l y chemisorb on Ti " " because both 0 and T i have r a d i c a l character, while H can only dissociâtively adsorb on the metal and must be transported to the T i * ions v i a an activated s p i l l o v e r process. 2

2

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SMSI. F i n a l l y we want to draw some conclusions which have a d i r e c t bearing on the SMSI state. The results presented in Figure 2 i l l u s trated that a catalyst which had been brought into the SMSI state by reduction at 500°C and had subsequently been passivated by 0 at room temperature and rereduced at 200°C, had the same slope of the H chemisorption vs. In t curve as a catalyst which had only been reduced at 200°C. In view of our discussion about the o r i g i n of the slope, t h i s suggests that the surface of the support i s back to normal after admission of 0 at room temperature. This i s in agreement with our discussion of the 0 chemisorption r e s u l t s , which disclosed that reoxidation of T i ions i s fast at room temperature. In view of these conclusions i t i s d i f f i c u l t to believe that suboxides l i k e T 1 4 O 7 are formed in the neighbourhood of the metal p a r t i c l e s to a great extent (20, 21), because such suboxides are known to be stable at room temperature in a i r . The results presented in Figure 2 demonstrate that after reduction at high temperature, passivation and rereduction at 200°C the i n i t i a l fast H chemisorption only amounts to about 60% of the value attained after low-temperature reduction only. Thus, although passivation seems to have made at least part of the metal available again f o r H chemisorption, i t i s not s u f f i c i e n t to completely restore the H chemisorption capacity. That passivation indeed brings at least part of the metal back to normal i s also apparent from the 0 chemisorption results and from temperature programmed reduction (TPR) measurements. Figure 3 i l l u s t r a t e s that even in the SMSI state the metal quickly adsorbs 0 at room temperature, while a subsequent TPR experiment shows a reduction peak at - 50°C (37), which i s attributed to the reduction of the outer layer of the rhodium oxide on rhodium metal p a r t i c l e s . The TPR p r o f i l e also shows a negative consumption of H between 50 and about 250°C. This must be due to H desorption from the rhodium, proving that metallic rhodium was present and attainable f o r H . Furthermore the fact that T i ions are quickly oxidized by 0 at room temperature, but that the SMSI state cannot be comp l e t e l y broken under the same conditions, suggests that the charge transfer model for the explanation of SMSI (3, 13) i s not very l i k e l y . The covering model (5, 7, 14-19). on the other hand, would not be in contradiction to the r e s u l t s . Because, as suggested by Mériaudeau et a l . (J), the reduced T i O species which have been formed and migrated over the metal surface during high temperature reduction may very well be two dimensional i n shape. During reoxidation they w i l l transform into three-dimensional T i 0 p a r t i c l e s on the metal surface. Thus even during room temperature admission of 0 part of the metal surface w i l l be uncovered (.7).


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H2 and O2 Chemisorption on Rh-TiÛ2

VANT BLIK ET AL.

7.

One may further presume that large metal p a r t i c l e s w i l l have a thinner T i O layer and w i l l be more e a s i l y uncovered than small metal p a r t i c l e s . During further 0 admission at elevated temperatures the T i 0 p a r t i c l e s w i l l migrate from the metal surface to the support and w i l l f u l l y uncover the metal surface ( c f . Figure 2). We want to end with a remark on the consequences of the existence of a fast and slow H adsorption on metal c a t a l y s t s . The existence of H s p i l l o v e r to the support forces the s c i e n t i s t to c a r e f u l l y design his experiments i f he wants to extract only the H adsorption on the metal from his data, in order to determine metal dispersion and metal p a r t i c l e s i z e . In the past the proven or suspected presence of s p i l l o v e r l e d to the method of measuring a H adsorption isotherm and extrapolating i t s high pressure part back to zero pressure (24, 38). In another method a second isotherm was measured after intermediate evacuation and the r e s u l t i n g , so-called "reversible**, H adsorption was attributed to s p i l l o v e r . The difference between f i r s t and second isotherm then was supposed to be equal to the H chemisorbed on the metal and was c a l l e d "irreversible** H adsorption (39, 40). Even disregarding the fact that there never exists such a thing as ··irreversible adsorption", the problem with both methods i s that they give results which are very dependent on the experimental conditions. Just because of the fact that H adsorption on the metal i s fast and reversible, the amount of H adsorbed w i l l depend on the f i n a l H pressure and, in the second method, on the pressure reached at the catalyst during evacuation (41). For that reason H adsorption measurements performed around 100 kPa always give higher results than measurements performed around 10 kPa or even 1 kPa. Of course one has to take into account also the p o s s i b i l i t y that at higher pressure more than one hydrogen atom w i l l be adsorbed per surface metal atom. Evidence that the surface M:H stoichiometry may exceed one f o r some metals has been growing l a t e l y (39, 40, 42, 43). A l l t h i s leads to the conclusion that our knowledge of f r a c t i o n a l coverage of hydrogen on a metal surface, of H:M stoichiometry and of hydrogen s p i l l o v e r in supported metal catalysts i s s t i l l l i mited and that c a r e f u l studies to disentangle these three factors should be encouraged. In t h i s respect we f e e l that our result of a slow H s p i l l o v e r onto T i 0 strongly suggests that a better method to separate H adsorption on the metal from that on the support would be to extrapolate H adsorption measurments to zero time, instead of to zero pressure. x

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Acknowledgment This study was supported by the Netherlands Foundation for Chemical Research (SON) with f i n a n c i a l aid from the Netherlands Organisation for the Advancement of Pure Research (ZWO).

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H and 0 Chemisorption on Rh- TiQ 2

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40. McVicker, G.B.; Collins, P.J.; Ziemiak, J.J. J. Catal. 1982, 74, 156. 41. Crucq, Α.; Degols, L.; Lienard, G.; Frennet, A. Acta Chim. Acad. Sci. Hung. 1982, 111. 42. Wanke, S.E.; Dougharty, N.A. J. Catal. 1972, 24, 367. 43. van 't Blik, H.F.J.; van Zon, J.B.A.D.; Huizinga, T.; Vis, J.C.; Koningsberger, D.C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139.


RECEIVED October 17, 1985


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Catal - ACS Publications - American Chemical Society

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