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... 

M. Current evidence Indicates that the effect may be ascribed to the formation of a TIO film on the surface of the group VIII metal particles (6,7) Whatever the cause of the SMSI state. It Is certainly the case that It has altered site energetics and possibly their density on the catalysts. The extraction of site... 

In Its H2 chemisorption behavior, Rh/T102 In the "normal" state Is more like Rh/T102 In the SMSI state than Rh/S102>... 

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). 

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. 

Information on the SMSI State Estimated from the Consumption Measurements of Pt/Al203 and Pt/Ti02 Catalysts4... 

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).

In spite of numerous investigations of metals in the SMSI state (cf. bibliography of Ref. 153), the exact nature of the phenomenon is still controversial (131), although there exists a fairly compelling relation to support reducibility (14, 71, 128, 153, 288,290). Changes in chemisorptive, catalytic, and structural properties in the SMSI state strongly suggest an electronic interaction at the metal-oxide interface with (whole or partial) electron transfer between a subjacent cation and a supported metal. Since the SMSI state apparently encompasses both a structural and an electronic... 

Studies by Short et al. (260) of the intensity of the white line of Pt on Ti02 indicate that the charge transfer from metal to support is essentially quite small, in accordance with XPS data. In fact, no Siegbahn chemical shift of the metal in the SMSI state has been observed (128a), nor are there XPS indications for electron transfer from titania to the metal (55, 257,290). 

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. 

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