The Strong Metal-Support Interaction

Not surprisingly, it was soon found that metals in this state had also lost much if not all their activity for reactions performed under strictly reduc- 

This brief survey must suffice to introduce a pervasive phenomenon which must often affect the structure and composition of small metal particles, and hence their catalytic behaviour this will be a recurring item in later chapters. Nunc est bibendum. 

The concern of this section is with incestuous reactions of hydrogen, that is to say, with reactions between the various distinguishable forms of the molecule rather than of the molecule with the catalyst or anything else. They will be treated only briefly (see Further Reading section), because they do not bear importantly on reactions of hydrocarbons although they may sometimes occur in parallel with them. 

Hydrogen Effects in Catalysis, (Z. Paal and P.G. Menon, eds.), Marcel Dekker New York (1988). 

---

Salama, T. M., Hattori,H., Kita,H., Ebitani, K., and Tanaka, T., X-ray adsorption spectroscopic and electron paramagnetic resonance studies on the strong metal-support interaction of platinum supported on titania dispersed on silica, J. Chem. Soc. Faraday Trans. 89(12), 2067 (1993). 

Dulub O, Hebenstreit W, Diebold U. Imaging cluster surfaces with atomic resolution the strong metal-support interaction state of Pt supported on TiO2(110). Phys Rev Lett. 2000 84 3646. 

Bowker M, Stone P, Morrall P, et al. Model catalyst studies of the strong metal-support interaction surface structure identified by STM on Pd nanoparticles on TiO2(110). J Catal. 2005 234 172-81. 

It is also possible to simulate supported metal catalysts by the vapor deposition of metal on a flat surface of silica, alumina, etc. The particle size distribution can be closely controlled and the results verified by various electron spectroscopies, for example (SI). For the reverse situation of a flat metal surface decorated by oxide particles, one can simulate catalysts in the strong metal-support interaction state (32). 

Finally, some concluding remarks summarising the progress made in the understanding of the strong metal/support interaction phenomena exhibited by the NM/Ce(M)02.x catalysts will be presented in section 4.3.4. 

The Nature of the Strong Metal/Support Interaction Effects in Nh4/Ce(h3)02.x catalysts. 

These results are a small fraction of the published data concerning metal-support interactions. While, as discussed later, there are some logical conclusions that can be drawn concerning the nature of the strong metal-support interactions there are too many factors involved to be able to sort through the mass of data concerned with the interaction of non-reducible or non-reduced supports and the metal particles on them. 

We have presented evidence that the strong metal-support interactions observed with titania and niobia are due to an oxide layer over the metal catalyst. This layer interacts chemically with the metal, as evidenced by the fact that the titania layers on Pt, Rh, and Pd do have slightly different properties. The fact that the methanation rates for a titania-covered and a niobia-covered Pt foil are identical indicates that the reason for enhanced methanation activity on the oxide-covered surface is likely due to geometric and not electronic cons id er at ions. 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

In this section we will focus on the description of several aspects relevant to the preparation of both catalytically active metal particles and metal-supported catalysts via the microemulsion technique. Regarding the metal supported catalysts, in some cases both the metallic particles and the support were synthesized by microemulsions. However, in general metallic particles prepared from microemulsions were deposited on commercial supports. The catalytic behaviour of these microemulsion-derived materials will be commented and, when possible, compared to catalysts obtained from traditional techniques under similar reaction conditions. Selected results concerning the study of the strong metal-support interaction effect (SMSI) obtained with catalysts prepared by microemulsion will be detailed . Several papers dealing with the preparation of immobilized metal particles on supports have been described although the catalytic behaviour of the solids was not studied. However, their potential catalytic ability led us to include those papers within this chapter. 

A study of the metal-support interaction effect has been carried out for a Pt-Ti02 catalyst prepared by microemulsion. Isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane were chosen as model reactions for studying the influence of the catalyst nature on the strong metal-support interaction (SMSI) effect. A comparison between the cata-lyticbehaviour of similar catalysts prepared by microemulsion and incipient wetness method,was reported. The catalysts were reduced at 200 and 390 °C since it is well known that the reduction temperature plays a predominant role in the SMSI effect l The behaviour of thePt-TiO microemulsion-prepared catalyst was found to be similar to that of a Pt/AbOs catalyst. Also, it was found that the microemulsion-prepared catalysts require a higher temperature of reduction to induce metal-support interactions. 

C. Ocal and S. Ferrer. The Strong Metal-Support Interaction (SMSI) in Pt-Ti02 Model Catalysts. A New CO Adsorption State on Pt-Ti Atoms. J. Chem. Phys. 89 4974 (1985). 

H.R. Sadeghi and V.E. Henrich. Rh on Ti02. Model Catalyst Studies of the Strong Metal-Support Interaction. Appl. Surf. Sd. 19 330 (1984). 

The good stability of Pt phase, and the absence of appreciable coking allows to perform regeneration by a simple oxidation-reduction treatments of the used catalysts without altering the catalytic properties, as it is shown in table 5. The strong metal/support interactions prevent any leaching of volatile Pt species, as it was instead observed for Pd. 

Catalyst deactivation and resistance to coking are two important issues of the methane reforming reaction with CO2 over Ni based catalysts because of their potential industrial application. Chen and Wren (9) and Bhattacharya and Chang (10) have recently proposed that the nickel aluminate spinel produced by interaction between nickel and alumina has a positive effect on the suppression of carbon deposition in CO2 reforming of methane. On the other hand, the formation of various types of nickel silicate species between the nickel and the support, attributed to the strong metal-support interaction, has been reported in Ni-silica catalysts (11,12). From these conclusions, it seems interesting to study the influence of Ni-silica interaction on carbon deposition. 

D) Spillover to a support that contains cations that are partially reducible, the former leading very often to the Strong Metal-Support Interaction to be discussed in the next section. 

---