Supported bimetallic clusters, effect

The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

A brief overview of bimetallic catalysts is presented. Electronic vs. ensemble effects are discussed, and literature is reviewed on single crystal bimetallics, and supported bimetallic clusters. Bimetallic cluster compounds are considered as models. Structural considerations, effects of potential poisons, particles from bimetallic cluster compounds, and catalytic activity/selectivity studies are briefly reviewed and discussed. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

Figure 4.24 X-ray diffraction pattern showing the effect of calcining (heating) a silica-supported platinum-iridium sample in air at 500°C prior to reduction in hydrogen at 500°C, illustrating the importance of the preparative conditions in the formation of highly dispersed platinum-iridium bimetallic clusters (4). (Reprinted with permission from Academic Press, Inc.)...

This effect has some industrial relevance. Thus the hydrogenolysis activity of supported Ru/Os reforming catalysts can be reduced by adding small amounts of copper, so that more alkenes are formed. These high surface area catalysts (ca. 300 m /g) contain the metal in the form of mixed crystals, often less than 5 nm in diameter ( bimetallic clusters ). Here, too, the Cu is found exclusively on the surface of the noble metal Ru. 

When a second metallic element is added to the common single metal-supported catalyst, bimetallic clusters of particles in the size range 1-50 nm are formed. About 50-95% of the metal atoms in the range of 1-3 nm are exposed to the surface. A particularly important effect of such microparticle clusters supported on silica or alumina is on selectivity, which can often be enhanced to significantly higher levels. 

Also, the bimetallic cluster HRuCo3(CO)22 supported on amorphous carbon showed high chemoselectivity for the formation of alcohols in the hydroformylation- hydrogenation of ethylene (172 °C, C2H4/CO/H2 = 20 20 20 mlmin" latm) and propylene (203 C, C3Hg/CO/H2 = 20 20 20mlmin , 1 atm), respectively [99]. A cooperative effect of both metals was assumed, since Co4(CO) 2 alone proved to be inactive. 

TiFe alloy turned out to be an active ammonia catalyst which is composed of the mixture of Fe, TiN and TiOx surface phases mounted on a TiFe bulk phase [30]. However, TiRu has no activity because TiN and Ti02 cover the surface of TiRu and Ru bulk phase [90]. FegiZj-g alloy is found to be an active catalyst, Fe-ZrOx, under the reaction conditions [91]. A Re-Pt bimetallic cluster is thought to be formed on an AI2O3 support [92]. It does not seem that alloying induces drastic effects. 

The cluster compounds (RCN)2M2Ru6C(CO)16 where R=C1-C6 alkyl, Ph, or 6 C 0 aralkyl, and M = Cu, Ag, or Au, are useful bimetallic systems for converting CO + H2 to CH3OH.(74) No catalyst support was employed, and a static reactor operating at 275 °C at moderate pressures was effective (no hydrocarbons were produced). 

Properties of supported catalysts by bimetallic substrates depend on the changes in geometry of the catalyst material by the strain of the substrate. Using a bimetallic substrate multiphes the possibilities to tune the catalyst to specific requirements. The chemistry of the nanosized overlayer is affected by the different orbital overlaps of atoms from the catalyst cluster and those from the substrate. Additionally, small supported metallic islands show low coordination and reduced near-neighbor distances thus their chemical properties are different with respect to those of flat surfaces. " Reactivity of several bimetallics were also studied by Balbuena et al., including bimetallics systems . Norskov et al. found several relations for the bimetallic systems considering local and nonlocal effects have also been reported. ... 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

---