Oxide support effect reactivity

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

Essentially the same methanol oxidation TOFs were obtained on the different oxide supports. The Degussa P-25 titania support (90% anatase 10% rutile) was also examined, as shown in Figure 6, because it possesses very low levels of surface impurities and represents a good reference sample. The invariance of the methanol oxidation TOF with the specific phase of the titania support reveals that the oxidation reaction is controlled by a local phenomenon, the bridging V-O-Support bond, rather than long range effects, the structure of the 2 support. Thus, the phase of the oxide support does not appear to influence the molecular structure or reactivity of the surface vanadia species. 

Lewandowski M, Sun YN, Qin ZH, Shaikhutdinov S, Freund HJ. Promotional effect of metal encapsulation on reactivity of iron oxide supported Pt catalysts. Appl Catal A. 2011 391 407-10. 

We investigated two methods of preparation observing that the wet impregnation produces a more stable catalyst with an increased dispersion of the metal due to the Ni(OH)2 precursor. About the support, La203 used as pure support or as a promoter on Si02 puts in evidence the support effect as it modifies the conversions and the carbon deposition. Indeed investigating more deeply about the differences of CO2 reactivity between silica and lanthana supported Ni catalysts, particularly from the tests on reduced and unreduced catalyst, it appears that the oxidation degree of the catalytic surface plays a relevant role on the reactivity. 

One of the most problematic questions in heterogeneous catalysis is the cooperative effect of different phases present in a given catalytic system and, in particular, the so-called metal-support interaction [15]. In the case of gold catalysis, interaction of the metal with an oxidic support seems to be of fundamental importance in determining the extraordinary reactivity observed during the low temperature oxidation of CO [14]. 

AW, Au ) at the metal-support interface or on the oxide support as isolated entities, and to calculate the effect on the reactivity in CO oxidation, and work of this type is commencing. One can cite DFT calculations showing the oxidation of Au to Au after adsorption on Ce02(lll) and its reactivity in the water gas shift reaction [190]. Another DFT study compares the stabUily of gold species in a variety of oxidation states (Au ° " and Au(OH)i,o3) on ZnO(OOOl)... 

Mavrikakis et al.I have nicely shown that the strain induced on the metal-metal bonds by the misalignment of the metal lattice to the registry of the oxide support leads to a shift in the center of the d-band. This change in the electronic structure alters the adsorbate bond strength at these sites, which ultimately dictates the reactivity of the metal. While these effects may die off for large particles on the support, they can clearly play a role for smaller nanoparticles that are in direct contact with the support. 

Lewandowska, A., Calatayud, M., Lozano Diz, E., et al. (2008). Comhining Theoretical Description with Experimental In Situ Studies on the Effect of Alkali Additives on the Structure and Reactivity of Vanadium Oxide Supported Catalysts, Catal. Today, 139, pp. 209-213. 

In the future, experiments at higher temperatures, as well as under reactive gas atmosphere (H2 or CO) and/or oxidative (O2) are planned to further understand the stability of selected clusters. Also, support effects will be addressed as samples on 5/3A4 and Si02 were prepared and are scheduled for measurements. 

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