PtSn catalysts

Vigier F, Coutanceau C, Hahn F, Belgsir EM, Lamy C. 2004a. On the mechanism of ethanol electro-oxidation on Pt and PtSn catalysts Electrochemical and in situ IR reflectance spectroscopy studies. J Electroanal Chem 563 81-89. 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

AU chemicals were analytically pure and used as received. H2PtCl6-6H20 and SnCl2-2H20 were used as precursors of PtSn catalysts. The solid electrolyte was... 

TABLE 15.2. Microanalysis of PtSn catalysts by EDX (atomic ratio). 

To clarify the valence state of metals in both of the PtSn catalysts, XPS analyses were carried out. Sn 3d and Pt 4f spectra for PtSn catalysts are shown in Fig. 15.5. All spectra have been deconvoluted into different components. The binding energies of Pt and Sn in various catalysts as obtained from their XPS spectra are given in Table 15.3... 

From the above experimental results, it can be seen that the both PtSn catalysts have a similar particle size leading to the same physical surface area. However, the ESAs of these catalysts are significantly different, as indicated by the CV curves. The large difference between ESA values for the two catalysts could only be explained by differences in detailed nanostructure as a consequence of differences in the preparation of the respective catalyst. On the basis of the preparation process and the CV measurement results, a model has been developed for the structures of these PtSn catalysts as shown in Fig. 15.10. The PtSn-1 catalyst is believed to have a Sn core/Pt shell nanostructure while PtSn-2 is believed to have a Pt core/Sn shell structure. Both electrochemical results and fuel cell performance indicate that PtSn-1 catalyst significantly enhances ethanol electrooxidation. Our previous research found that an important difference between PtRu and PtSn catalysts is that the addition of Ru reduces the lattice parameter of Pt, while Sn dilates the lattice parameter. The reduced Pt lattice parameter resulting from Ru addition seems to be unfavorable for ethanol adsorption and degrades the DEFC performance. In this new work on PtSn catalysts with more... 

Table 6.4 shows the composition corresponding to different PtSn catalysts submitted to XPS and EXAFS/XANES analyses, while Table 6.5 gives XPS results giving the position of all of the main photoelectron peaks after referencing them to the C Is BE of 284.6eV [30, 62]. Figure 6.8 depicts XPS spectra of the Sn3d5/2 level for the tin-modified catalysts. 

On Pt-Sn, assuming that ethanol adsorbs only on platinum sites, the first step can be the same as for platinum alone. However, as was shown by SNIFTIRS experiments [37], the dissociative adsorption of ethanol on a PtSn catalyst to form adsorbed CO species takes place at lower potentials than on a Pt catalyst, between 0.1 and 0.3 V vs RHE, whereas on a Pt catalyst the dissociative adsorption of ethanol takes place at potentials between 0.3 and 0.4 V vs RHE. Hence it can be stated that the same reactions occur at lower potentials and with relatively rapid kinetics. Once intermediate species such as Pt-(COCH3)adsand Pt-(CO)ads are formed, they can be oxidized at potentials close to 0.3 V vs RHE, as confirmed by CO stripping experiments, because OH species are formed on tin at lower potentials [39, 40] ... 

Dautzenberg et al. (65) tested a number of unsupported PtSn alloys as well as a number of alumina supported PtSn catalysts. n-Hexane conversion was effected at atmospheric pressure for the unsupported alloy catalysts and for some supported catalysts other supported catalyst studies were at 3 bar. These authors reported that the addition of tin decreased the amount of methylcyclopentane and that coke was dramatically reduced during the conversion of n-hexane. 

Catalysts. Some commercial PtRe catalysts listed in Table I were used in this study as well as some PtSn catalysts on which were reported elsewhere (7). The fresh catalysts B-6, B-7 and B-8 made in China are white because they are unreduced. Catalyst 803 made by Engelhard Corp. was grey because it had been reduced by the manufacturer. 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

The paradoxical electrochemical behavior of PtSn based catalysts makes its stndy more controversial. Nevertheless, the catalytic enhancement of the electro-oxidation of adsorbed CO on PtSn catalysts is generally accepted. To explain the difference in activi-... 

Figure 33 shows the SNIFTIR (Fig. 33a) and the SPAIR (Fig. 33b) spectra of the species obtained on a PtSn catalyst under the same experimental conditions as in Fig. 32 for a Pt catalyst. 

SPAIR spectra obtained on Pt and PtSn catalysts during ethanol electro-oxidation (Figs. 32 and 33) show that CO2 (wave-number of the absorption band located at 2345 cm ) is only detected at 0.6 V vs. RHE on both Pt and on PtSn catalysts. But, as shown in Fig. 34a, the intensity of the CO2 IR band is higher for PtSn than for Pt at potentials lower than 0.65 V vs. RHE. It is likely that in the detection limit of the IR set up it is difficult to detect CO2 at lower potentials on a PtSn electrode. 

On the other hand, the absorption band due to a carbonyl group (at about 1725 cm ) displays, at high potentials, a lower intensity on PtSn (the absorption band located close to 1725 cm remains as a shoulder even at high potentials) than on Pt (Fig. 34b), which indicates that the formation of C2 species (presumably acetaldehyde) resulting from a non-dissociative adsorption of ethanol is lower on a PtSn catalyst. 

The ratio of carbon deposits on metal surfaces to the deposits on Ihe support can be reduced by the addition of tin. The adsorption sites of PtSn catalysts can be divided into two types, namely, low temperature adsorption sites and high temperature sites. The low temperature adsorption sites, similar to the mono-metallic Pt sites, arc mostly covered by carbon under severe reaction conditions, whereas 50 % of the high temperature adsorption sites of PtSn remain uncovered at the same reaction conditions. 

Dautzenberg FM, HeUe JN, Biloen P, Sachtler WMH (1980) Conversion of n-hexane over monofunctional supported and unsupported PtSn catalysts. J Catal 63 119... 

Stability and Regeneration of Supported PtSn Catalysts for Propane Dehydrogenation... 

The catalytic performance in the propane dehydrogenation reaction of Pt and PtSn catalysts on inert supports (like K-doped alumina and ZnAlj04, and on an acidic AljOj,) under conditions of high severity was studied by means of pulse and flow reaction techniques to determine the effect of the nature of the support on the catalytic behavior. The... 

The geometric and promoter effects on the selectivity to furfuryl alcohol were studied in the hydrogenation of furfural over various series of platinum catalysts. The results obtained with the PtCu catalysts indicate that the size of the active site does not affect the selectivity. Promotion of the platinum catalyst can lead to a considerable rise in the selectivity to furfuryl alcohol. The hydrogenation over the series of PtSn catalysts showed the influence of the reaction conditions. The experiments under isothermic conditions resulted in up to 80% selectivity for furfuryl alcohol, while the selectivity dropped to approximately 40% if non-isothermic conditions were applied. This change in selectivity is attributed to the self-poisoning of the catalyst at high temperatures. 

Figure 3 shows the selectivities to furfuryl alcohol obtained in the hydrogenation of furfural over the series of PtSn catalysts. The main products formed from furfural were furan/2-methyl-furan (in approximately 1 4 ratio) and furfiiryl alcohol. An interesting shift in the selectivities was observed when the temperature of the catalyst was changed. In both types of experiments the shown selectivities were measured at 170°C after 16 hours on stream. The conditions of the reaction, however, were either isothermic or non-isothermic before the selectivity was determined (Figure 3). It can be seen in Figure 3 that the selectivities for furfuryl alcohol in the hydrogenation reaction under isothermic conditions are much higher than under non-isothermic conditions.

We have to admit that it is still an open question in which form e.g. Sn exerts its promotion effect. A recent discussion [8,9] revealed that both oxidic tin species as well as metallic tin alloyed with Pt can coexist in PtSn catalysts. We suggest that an oxidic form of tin is the promoter, acting in a similar way as the promoting oxides do in syngas reactions [12]. As already mentioned above the allo3dng of Pt with a much less active metal like Cu does not increase the selectivity to furfuryl alcohol. We expect that the inactive Sn atoms in Pt do not act differently. 

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