Catalysts, bimetallic

During the 1950s and 1960s the performance of platinum reforming catalysts was considerably improved as operating procedures were optimized. More economic catalysts became available as the platinum content was reduced from more than 0.6 wt% to about 0.35 wt% and the sulfur content of feed naphtha was considerably reduced. Higher purity alumina supports, with better activity and lower levels of sodium and iron could be synthesized reproducibly. An optimum amount of chloride added to the support limited coke formation. 

The success of the platinum/rhenium catalyst was evident, as more than 80% of all catalyst replacement charges were bimetallic by 1972. By then 30% of all reformers were using the rhenium catalyst. Acceptance continued in all reforming processes so that more than two-thirds of installed catalyst capacity was bimetallic by 1979. Catalyst improvements also continued during this period, and the original Chevron catalyst. Grade A, was replaced several times up to Grade F.  

A disadvantage of platinum/rhenium catalysts is their sensitivity to sulfur poisoning. This requires that naphtha feed must be carefully hydrotreated before use. 

The very few plants still using platinum catalysts (with no rhenium) are those without the necessary hydrotreating facilities. 

It is necessary to pre-sulfide new or regenerated catalysts with a suitable sulfur compound prior to use when working with a low sulfur-content feed, to reduce excessive hydrogenolysis activity of the catalyst. Presulfiding causes deactivation of the super-active edges on the metal sites that are responsible for 

There are many examples for bimetallic catalysts which are applied in industrial processes (Table 5-16). 

Ni/Cu-Si02 hydrogenation of aromatics and long-chain olefins in the solvent industry 

Pd/Fe-SiOj hydrogenation of 2.4-dinitrotoluene to 2.4-diaminotoluene (through 2-nitro-4-aminotoluene and 2-amino-4-nitrotoluene) 

We will discuss some special reactions with bimetalhc catalysts in more detail. For example, the hydrogenation of ethyl acetate to ethanol has been studied with Rh/Sn-Si02 catalysts (Table 5-17). Widi increasing Sn content the following results were described [40]. 

The experimental results have been explained by following assumptions  

This effect is apparently normal for bimetallic catalysts. For example, Rodriguez (1996), analyzing the interaction of Co with Pd-based bimetallic catalysts, estabhshed that bimetallic surfaces have lower CO desorption temperature than Pd (see Table 10.4). 

It is important to emphasize that the use of alloys of palladium for surface modification is the most promising direction of surface modification of sensitive materials. For example, Chaudhary et al. (1998b) have shown that alloys of Ru/Pd really are much more effective surface catalysts than the same metals alone for attaining the required selectivity of gas sensors. It was established that, by using alloys of Ru/Pd, it is possible to achieve a considerably greater increase of sensitivity and selectivity for hydrogen detection with sensors based on tin oxide than with sensors modified using Ru or Pd metal (1-2 wt%) separately (Chaudhary et al. 1999). The optimum seems to be at a ratio Ru/Pd 1.28. 

A considerable increase of the promotion effect was also observed using a Co/Au alloy. Studying the influence of additives on gas-sensing characteristics of In Oj-based sensors, Yamaura et al. (2000) established that a binary catalyst consisting of 0.5 wt% Co and 0.04 wt% Au gave dramatically improved sensitivity and selectivity to CO detection (see Table 10.6). 

10 Surface Modifiers for Metal Oxides in Conductometric Gas Sensors 

The previous examples are all supported gold catalysts. Supported bimetallic catalysts have been explored as well. Scurrell and coworkers developed a series of Au-M/ Fe Oj (M=Ag, Bi, Co, Cu, Mn, Ni, Pb, Ru, Sn, Tl) catalysts for WGS [70,71], These catalysts were prepared by deposition-coprecipitation using HAuCl, FeCNOjjj, and metal salts as precursors, followed by calcination in air at 400°C. Au-Ru/Fe Oj showed the highest activity. However, there is no evidence showing that the so-called bimetallic catalysts are bimetallic. The thermal decomposition of metal salts in air usually leads to the formation of metal oxides instead [61]. 

Many hydrolytic enzymes possess two or three metal ions in their active site. Common strategies in the mimicry of such enzymes are based on synthetic catalysts consisting of several ligated metal ions connected by suitable spacers [26]. 

AcO ion favorably competes with EtO ion for the crown-complexed Ba + [Eq. (8)], with an equilibrium constant J =69 [27]. The corresponding figure for the Sr complex is 35 [28]. 

Since the equilibrium constant for binding of EtO to the crown complexed Ba and Sr + ions is 10 m in both cases [9,28], the affinity of carboxylate anion for the ligated metal ions turns out to be large enough (K3 10 m ) to ensure a virtually complete anchorage to the metal in the dilute solutions of the catalytic experiments. 

As before, much benefit is derived from studying the uptake of Pd over C03O4 and carbon independently. The steps to apply the SEA method for the synthesis of bimetallic materials are as follows  

determination of support and second metal oxide PZC (with bulk powders of each) 

metal uptake-pH surveys over each pure component 

an adsorption experiment at the optimal pH over a physical mixture of the two oxides, which can be identified separately in electron microscopy, to confirm the partitioning of the metal onto the desired oxide 

synthesis over the supported oxide, reduction, and characterization via STEM, TPR, EXAFS, and ultimately activity and selectivity testing to 

This led in turn to the idea to also use combinations of different metals (e.g., Co/Rh, Co/Pt, Co/Fe, Co/Mo, Rh/Fe, Rh/Mn, Rh/Re, Rh/W, Rh/Mo) with the aim of creating synergy effects [2]. In the last decade, especially G arland and coworkers accumulated much evidence through spectroscopic measurements and density functional theory (DFT) calculations that in rhodium-catalyzed hydroformylation of non-isomerizable olefins (cyclopentene or 3,3-dimethylbut-l-ene), carbonyl complexes, which are less active in hydroformylation, such as HMn(CO)5 or FIRe(CO)5 [3], can support the reductive elimination of the aldehyde from 

During desorption experiments with PtieAuie (Fig. 7.7), the Au-CO band disappears quickly, and coincides with an increase in the Pt-CO band intensity. Closer inspection of the PtigAuig desorption experiment suggests that the broad room temperature Pt-CO band may be better described as two bands at 2065 and 2050 cm.  

Changes in the Pt-CO band are due to a substantial increase in the intensity of the 2050 band. Once CO completely desorbs from Au, the Pt-CO band becomes substantially more symmetric and the 2065 cm band is no longer distinguishable. Complete desorption of the 2050 cm band also occurs at a higher temperature than 

FIGURE 7.7. Infrared Spectroscopy during CO desorption from Ri6Aui6/Si02 (a) 30,70,90, 120°C and (b) 120,150, 170, 180 190°C. As the band at 2113 cm decreases, the band at 2055 increases. Once the 2113 band disappears, the 2055 band decreases in intensity, but does not shift. Reprinted with permission from J. Am. Chem. Soc. 2004, 126, 12949-12956. Copyright 2004 American Chemical Society. 

FIGURE 7.8. TEM particle size distributions for Pt32/Si02 and Pti6Aui6/Si02. Reprinted widi permission from 7. Am. Chem.Soc. 2004,126,12949-12956. Copyright 2004 American Chemical Society. 

Catalysis is a potentially sensitive probe for nanoparticle properties and surface chemistry, since catalytic reactions are ultimately carried out on the particle surface. In the case of bimetaUic DENs, catalytic test reactions have provided clear evidence for the modification of one metal by another. DENs also provide the opportunity to undertake rational control experiments, not previously possible, to evaluate changes in catalytic activity as a function of particle composition. 

As Table 4.1 shows, several bimetalhc DENs have been employed as homogeneous catalysts, predominately for hydrogenations. The most detailed studies have been with aUyl alcohol hydrogenation and the partial hydrogenation of 1,3-cyclooctadiene (1,3-COD) test reactions. In these studies, turnover frequencies (TOEs), which can be normalized per mole of nanoparticles, can be compared as a function of the metaUic atomic ratio in the DENs. Comparison with physical mixtures of monometalhc DENs with the same net atomic ratio allows investigators to directly compare both the magnitude and direction of changes to rationally prepared control materials. 

The represent TOP data for Pd-only DENs, while the o represent data for the bimetallic DENs. Reprinted with permission from J. Am. Chem. Soc., 2004, 126, 15583-15591. Copyright 2004 American Chemical Society. 

Efficient chemoselective hydroformylation of monosubstituted alkenes was observed at room temperature under atmospheric pressure of CO H2 = 1 1, without affecting functional groups such as disubstituted alkene moieties, aryl and alkenyl iodide moieties, and hydroxy and carboxy groups (Equations 7.9 and 7.10) [78[. 

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To proceed with the topic of this section. Refs. 250 and 251 provide oversights of the application of contemporary surface science and bonding theory to catalytic situations. The development of bimetallic catalysts is discussed in Ref. 252. Finally, Weisz [253] discusses windows on reality the acceptable range of rates for a given type of catalyzed reaction is relatively narrow. The reaction becomes impractical if it is too slow, and if it is too fast, mass and heat transport problems become limiting. 

J. H. Sinfelt, Bimetallic Catalysts Discoveries, Concepts, and Applications, Wiley, New York, 1983. 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

Bill oflading Bilopaque Biltricide solid Bimetallic catalysts... 

Z. Knor, and J. Sotola, Supported Bimetallic Catalysts, Coll. Czech. Chemical Comm. 

Heterogeneous catalysis by metals has been of long-standing interest, with bimetallic catalysts a particular focus.Transition metal carbonyls have... 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Physicochemical attributes of catalysts were mostly controlled by nitridation temperature although there was a little influence on catalyst reducibility and acidity, better nitride species were formed at 973 K and TGA results revealed that complete nitridation occurs between 750-973 K and the feed gas stream containing H2 NH3=1 1 is preferably better mixture for the nitridation of Co-Mo bimetallic catalysts. 

Valence State of Rhenium in Reduced Bimetallic Catalysts With and Widiout Alkali Metals... 

Sodium or potassium severely poisons Pt-Re catalysts but the manner in Which the alhali metal operates is not apparent. The present study was designed to use ESCA to determine the valence state of Re in Pt-Re bimetallic catalysts. The valence state would be determined in san les that had been reduced and transferred to the instrument without exposure to an oxidizing atmosphere. Catalysts with and without potassium would be examined. 

We have found EXAFS to be a very effective method for obtaining structural information on bimetallic cluster catalysts (8,12-15,17) These types of catalysts, and bimetallic catalysts in general, have been the subject of extensive research in the EXXON laboratories since the 1960 s (18-25). In this paper we present a brief review of the results of some ofour EXAFS investigations on bimetallic cluster catalysts. 

When the ruthenium EXAFS for the ruthenium-copper catalyst is compared with the EXAFS for a ruthenium reference catalyst containing no copper, it is found that they are not very different. This indicates that the environment about a ruthenium atom in the bimetallic catalyst is on the average not very different from that in the reference catalyst. This result is consistent with the view that a ruthenium-copper cluster consists of a central core of ruthenium atoms with the copper atoms present at the surface. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

Surface Chemistry and Catalysis on Some Platinum-Bimetallic Catalysts... 

As was the case for the silica-supported Ru-Rh bimetallic catalysts, there was no significant surface enrichment in either metal over the entire range of bimetallic catalyst compositions. 

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