SMSI state temperature

The role of the support on hydrogen chemisorption on supported rhodium catalysts was studied using static and frequency response techniques. In all Instances, several klnetlcally distinct H2 cheml-sorptlve sites were observed. On the basis of the kinetics, at least one site appears to sorb H2 molecularly at temperatures below 150°C, regardless of the support. At higher temperatures, a dissociative mechanism may become dominant. Inducement of the SMSI state In Rh/T102 does not significantly alter Its equilibrium H2 chemisorption. 

To Illustrate the utility of the technique, we have addressed the question of the anomalous chemlsorptlve behavior of tltanla-supported group VIII metals reduced at high temperatures. The suppression of strong H2 chemisorption on these catalysts has been ascribed to a strong-metal-support Interaction (SMSI) ( ). It has also been found that the reaction activity and selectivity patterns of the catalysts are different In normal and SMSI states... 

An interesting explanation for Pt/TiOz catalysts in the SMSI state, which tends to suppress the chemisorption of H2, has been advanced by Chen and White (97). If Ti02 is reduced by H2 at 875°C before deposition of Pt, the SMSI state is also observed. If this reduced Ti02 is reoxidized before the deposition of Pt, the catalyst irreversibly adsorbs H2 at room temperature, since any Pt/Ti02 reduced at low temperatures and the SMSI state is not achieved. It may, therefore, be concluded that the nature of Ti ions should be correlated with the SMSI state. Further, Pt deposited on Ti203 or on TiO does not adsorb H2 (as in the SMSI state). High-temperature reduction of the... 

The activation energy E of the dissociation of H2 is the same in both cases, although Ex and E2, the heats of adsorption of H, differ, with , > E2. In other words, the SMSI state is observed with a support of high electronic conductivity. In this state the work function of the support is smaller than that of the supported metal. If the TiOz support is not initially a conductor, it becomes one by reduction with H2 in the presence of Pt at high temperature. This reduction may not be limited to the surface. For low reduction temperatures, surface OH" groups and Ti3+ ions are formed by hydrogen spillover, as shown previously (100). But this formation of Ti3+ and OH" is not the source of SMSI, since the electrons are not transferred into the conductivity bands to be trapped by Pt. 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

Fig. 8. Interactions in the Ni/titania system as the activation temperature is increased cn, NiO an, bulk Ni metal surface Ni (< 1 nm) in the SMSI state t , Ni in a partially ionized subsurface state c, oxygen anion vacancy (113).

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

In situ STM studies of the oxidation of a Pd film in the SMSI state at elevated temperature show a thickening of the encapsulating film (Fig. 8.7a-c). The film prior to oxidation had a hexagonal pin-wheel structure on the raised triangular island on the Pd film. After oxidation (Fig. 8.7c), the island was decorated heavily with a thickened, rough layer of Ti implying the formation of an oxide film of higher stoichiometry (possibly TP+), or mass transport of Ti to the Pd surface from the Ti surface. 

Figure 14 Schematic representation of the activation of the C=0 bond in a,)S-unsaturated carbonyls on Pt/Ti02 (SMSI state) and Pt/Ce02 (reduced at high temperature).

The Insensitivity of CO hydrogenation to reduction temperature may be connected to the fact that oxidants like O2 and H2O are capable of bringing the catalyst back from the SMSI state to the normal adsorption state. Thus Tauster et al. published that the H2 adsorption capacity of a Pd/TiOj catalyst, which had been brought into the SMSI state by reduction at SOO°C, was completely restored after oxidation at 400 C for 1 h and rereduction at 175 C (1). Baker et al. published that H2O at 250 C for 1 h could restore the H2 and CO adsorption capacities of a Pt/TiOj catalyst, although to a less extent than oxidation at 600 C (11). On the other hand, Mdriaudeau et al. reported that H2 adsorption as well as catalytic activities for hydrogenolysis and hydrogenation of Tl02 supported Pt, Ir and Rh catalysts recovered after O2 admission at room temperature and subsequent reduction at low temperature (6). 

O2 chemisorption. Whereas the chemisorption of hydrogen is suppressed in the SMSI state, chemisorption of oxygen still takes place after a metal-on-Ti<>2 catalyst has been reduced at high temperature. No information is available, however, whether there is a difference in O2 chemisorption in SMSI and normal state and whether the reduction temperature has any influence on this. Therefore we looked into the O2 chemisorption capacity of Rh/R-Ti<>2 and Rh/A-Ti(>2 catalysts at room temperature and 8.10 Pa after successive reductions and evacuations (1 h each) at temperatures of 200, 245, 280, 3S0 and 500 C. The results are presented in Figures 3A, B, C, 0 and E, respectively. To ensure that the starting conditions were the same in all cases, before each reduction-evacuation treatment and subsequent O2 chemisorption experiment the catalyst was oxidized at 150 C for O.S h because, as shown in the foregoing,... 

Figure 2. Influence of reoxidation temperature and time on the breaking of the SMSI state of 0.99 wt% Rh/R-TiOj.

H2 chemisorption. Both Rh/R-Ti02 and Rh/A-Ti02 show a decrease in H2 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 is of course due to the gradual transition of the Rh particles into the SMSI state. Whatever the explanation for this state, an electronic interaction between metal particles and support or a covering of the metal particles by the support, in this SMSI state the metal particles are unable to adsorb H2. The decreased slope of the H/Rh-ln t curve can be explained in several ways, such as slow H2 chemisorption on Rh because of an activated process, dependence on metal dispersion, or an effect related to the support. The experiments in which H2 chemisorption was started around 200°C proved that the time dependence is indeed due to a slow adsorption at room temperature, but the experiment with Rh/Si(>2 showed that there is no kinetic limitation in the H2 chemisorption on the metal part of the catalyst. In accordance with this conclusion, no effect of rhodium dispersion on the time dependence of the H2 chemisorption was observed for catalysts in the normal state (cf. Figure 1 curves A, B and F). 

Hj chemisorption not only fast adsorption of H2 on Rh may occur, but also a slow spillover and reduction of Ti +. On the other hand, after reduction at high temperature Ti + will be formed and the surface of the support will be dehydrated. Therefore during the following evacuation no reoxidation of the Ti ions can take place. At the same time the rhodium has been changed into the SMSI state and therefore no H2 chemisorption will take place at all. At Intermediate reduction and evacuation temperatures there will be an intermediate behaviour and as a result of this the H2 chemisorption values will decrease with increasing treatment temperature and so will their time dependencies. 

Furthermore the fact that Ti3+ ions are quickly oxidized by O2 at room temperature, but that the SMSI state cannot be completely broken under the same conditions, suggests that the charge transfer model for the explanation of SMSI (3, 13) is not very likely. The covering model (5., 14-19). on the other hand, would not... 

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