Bimetallic catalysts determination

Table 6.3 includes particle sizes for bimetallic catalysts determined by TEM. Figure 6.4 shows particle size distribution histograms of PtSn-BM and NiSn-BM bimetallic catalysts and the corresponding Pt and Ni monometallic ones. In all the cases, the sohds were subjected to a hydrogen reduction pretreatment. These... 

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. 

Waszczuk et al., 2001b Tong et al., 2002]. Because Ru is deposited as nanosized Ru islands of monoatomic height, the Ru coverage of Pt could be determined accurately. In that case, the best activity with regard to methanol oxidation was found for a Ru coverage close to 40-50% at 0.3 and 0.5 V vs. RHE. However, the structure of such catalysts and the conditions of smdy are far from those used in DMFCs. Moreover, the surface composition of a bimetallic catalyst likely depends on the method of preparation of the catalyst [Caillard et al., 2006] and on the potential [Blasini et al., 2006]. 

The nanostructured bimetallic catalysts were characterized using several techniques, and some of the main results are summarized in this section. We first describe the size and composition of the AuPt nanoparticles determined from TEM and DCP-AES analysis. This description is followed by discussion of the phase properties based on XRD results. We further discuss the FTIR provbing of CO adsorption on the bimetallic nanoparticle catalysts. 

Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10 - lO " M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. 

Ge. Coke formation was significantly reduced on bimetallic catalysts compared to Pt-alumina the results show that it is not only the amount but the location of coke that is important in determining catalyst performance. The greater efficiency of the bimetallics was due to both the amount of coke formed and its location. 

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

The essential application of LEISS analysis concerns supported bimetallic catalysts. For these solids, XPS analysis can no longer be considered as a surface analysis. The sizes of the particles present on the surface of these catalysts are around a nanometre, similar to the mean free path of the photoelectrons that determines the thickness probed by XPS. Certain pairs of elements do not lend themselves to LEISS analysis. This is the case, for example, with Pt-Rc for which the mass difference of the two metals is small compared to the resolution. One example of a pair that lends itself to LEISS analysis is provided by a study of Pt-Sn reforming catalysts. LEISS is ten times more sensitive for platinum than for tin. Despite this, an intensity of the tin peak at least comparable to that of platinum can be observed on a catalyst with a Pl/Sn ratio of three (Fig. 6.5), This study can be used to show a surface segregation of tin that is dilTicult to detect by XPS because of the small size of the platinum particles (2 nm). 

An interesting application of adsorption microcalorimetry was used by these researchers to examine changes in adsorption behavior of graphite-supported iron/rhodium bimetallic catalysts as a function of oxidation and reduction treatments. The differential heat of oxygen adsorption on the bimetallic catalysts after various treatments was compared to the values obtained for the monometallic materials to determine the relative contributions to the total adsorption. Reduction at 673 K produced an alloy whose... 

At the initial reaction stage, the activity of oxide Mo-Ni catalysts is higher than that of Mo catalysts by a factor of 5. In three hours, this value decreases to 4 (Fig. la). For the sulfide samples, this effect is less pronounced (Fig. lb). In the next run, the catalyst activity was determined at 400 C, then the temperature was decreased to 300 C. This procedure was repeated for several times. For both oxide and sulfide molybdenum catalysts, activity and the product composition are reproduced with high accuracy in a number of experiments (Fig. 2). At high temperature, the bimetallic catalysts are quickly deactivated during the reaction (and the experimental cycles are not reproducible). 

The question naturally arising is how reliable are the physicochemical quantities determined by means of RF-GC. Eor this reason, the adsorption of CO, O2, and CO2, as well as the oxidation of CO, has been studied over well-studied, silica-supported, Pt-Rh bimetallic catalysts. The following are indicative conclusions extracted by using RF-GC, which are in agreement with the observations of other techniques ... 

The bimetallic catalysts exhibit higher catalytic activity at lower temperatures in comparison with pure Pt and Rh ones, as shown in Fig. 2. Otherf workers have also observed this synergism for Pt-Rl bimetallic catalysts.s The rate constants found by the RF-GC technique such as those in Table 1, are very close to thosm determined experimentally by the frequency response method " for the adsorption of CO on Pt/Si02. The values of the estimated activation energies for CC dissociative adsorption, given in Table 2, are low. ... 

Iron-Platinum or Palladium. - The nature of supported bimetallic catalysts can be probed by determining the chemical state of one metallic component,... 

The characterization of supported metal catalysts is a matter of some complexity and supported bimetallic catalysts even more so. Nevertheless the development and application of methods for determining catalyst structure is essential for an understanding of why the performance of a selected combination of metal(s) and support varies as a function of preparative variable, activation procedure, reaction conditions, or time. Although some aspects of catalyst structure can be routinely determined, the basic measurement of absolute metal dispersion by selective chemisorption/gas titration is still the subject of many publications and the necessity of cross-checking by instrumental methods is generally appreciated. The characterization of supported metal catalysts also involves some less accessible properties, e.g., the sites available on crystallites as a function of size, high-temperature... 

For bimetallic catalysts, the sulfur coverage (9s) is lower on catalysts exhibiting the highest Pt-Re interaction determined by cyclopentane hydrogenolysis (Table 2). 

In preliminary assays to establish the optimal reaction conditions, it was determined that ruthenium, at 573 K, is capable of cracking n-H, more stable than 2-MP and 3-MP isoparaffins. However, the ruthenium behaviour in bimetallic catalysts is different in the reaction temperature range chosen, 476-533 K, as the fraction of n-H in the distribution of products does not changed appreciably with the preparation procedure (see Fig. 3). So, in these conditions, the cracking products are considered to arise from Isoparaffinic products through consecutive reactions [12]. 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

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