Supported catalysts SMSI state

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... 

Figure 9.7 Transmission electron microscopy of rhodium particles on a model titania support after reduction in H2 at 200 °C (top) and the same catalyst in the SMSI state after reduction at 500 °C (bottom). An amorphous overlayer on the surface of the SMSI catalyst is clearly discerned (from Logan etal. [25]).

Careful infrared study of CO chemisorbed by Pd/Si02 catalysts in an SMSI versus a non-SMSI state verified that after HTR, silicon species are distributed in the Pd surface layer. For the catalysts reduced at 300°C (LTR), the B/L intensity ratio (B = bridging CO L = linearly bound CO) is a monotonic function of Pd particle size (Fig. 17). On the other hand, the B/L ratios for Pd/Si02 catalysts that experienced HTR show considerable departure from this universal curve (Fig. 17) (208). Of course, a relatively higher proportion of linearly bound CO for Pd/Si02 catalysts in the SMSI state is believed to follow from the existence of silicon, rather uniformly interdispersed in the metal surface, resembling the case of CO adsorption on silica-supported Pd-Ag alloys (209). 

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... 

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. 

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). 

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). 

In a recent publication Dumeslc c.s. described adsorption and desorption measurements of H2 on Ni/Ti(>2 and Pt/Ti(>2 catalysts, which showed that a larger amount of H2 could be desorbed (after 15-20 h equilibration of these catalysts under about 40 kPa H2) than could be directly adsorbed (32). In agreement with our conclusions their explanation was that, apart from a fast H2 adsorption on the metal, hydrogen apparently also adsorbed slowly on the Ti02 support via a spillover process from metal to support. These authors noticed that the amounts of H2 desorbed from the M/Ti(>2 catalysts in the SMSI state were in fair agreement with metal particle sizes determined by X-ray line broadening and electron microscopy and suggested that H2 desorption could be used to estimate metal... 

In the present paper, the metal-support electronic interactions in various metal catalysts-mainly Pt - were followed by measuring in situ the electrical conductivity of the solids either in the "normal" or the "SMSI" state, when in contact with various atmospheres (vacuum, H2> 02, (C0+H2). The manifestation of the electronic factor... 

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