Adsorbed tin on platinum

Fig. 4.1. Current and mass signal during an on-line mass spectroscopic experiment showing the effect of adsorbed tin on platinum upon methanol electrooxidation. 1 M CH3OH/0.5 M H2S04 sweep rate 10 mV/s, 24 °C.

Fig. 4.3. Cyclic voltammogram for adsorbed tin on platinum, v = 10 mV/s. Experimental procedure 11 min. adsorption from a 4 x 10-4 M Sn(S04)2 solution in 0.5 M H2S04 at 0.5 V followed by electrolyte replacement with pure supporting electrolyte.

Bittins-Cattaneo B, Iwasita T (1987) Electrocatalysis of methanol oxidation by adsorbed tin on platinum. J Electroanal Chem Interfacial Electrochem 238 151-161... 

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] ... 

Adatoms produce a strong change in catalytic properties of the metal on which they are adsorbed. These catalytic effects are highly specific. They depend both on the nature of the metal and on the nature of the adatoms they also depend on the nature of the electrochemical reaction. For instance, tin adatoms on platinum strongly (by more than two orders of magnitude) enhance the rate of anodic methanol oxidation. 

Fig. 4.4. Potential dependence of (a) the coverage degree of platinum by tin and (b) charge transferred during tin adsorption on platinum (according to Eq. 4.1). Tin was adsorbed from Sn(S04)2 ( ) and from SnS04 (O).

From these results it appears that the addition of tin to platinum greatly favors the formation of AA eomparatively to AAL. This can be explained by the bifunctionnal mechanism where ethanol is adsorbed dissociatively at platinnm sites, either via an 0-adsorption or a C-adsorption process followed by the oxidation of these adsorbed residues by oxygenated species formed on Sn at lower potentials giving AA. 

Preparation of Alloy Type Sn-Pt/Si02 Catalysts. Supported bimetallic Sn-Pt catalysts can be prepared using different methods and approaches. However, exclusive formation of alloy type nanoclusters can be achieved by using methods of surface organometallic chemistry, namely by applying Controlled Surface Reactions (CSRs) between hydrogen adsorbed on platinum and tin tetraalkyls. 

On the other hand, the yield of CO2 with a Pt/C catalyst is double that with a Pt-Sn/C catalyst (see Table 1.2). This can be explained by the need to have several adjacent platinum sites to adsorb dissociatively the ethanol molecule and to break the C-C bond. As soon as some tin atoms are introduced between platinum atoms, this latter reaction is disadvantaged. 

There still stands valid for platinum alloys the two metals that are able to promote methanol oxidation are ruthenium and tin. The case of ruthenium is interesting since it was also studied under UHV conditions [43,44]. The reaction of methanol on Pt/Ru alloys results in the production of carbon dioxide at lower potentials than on pure platinum. However, the presence of tin in Pt3Sn alloys only enhances methanol oxidation at low potentials, increasing carbon dioxide production (and diminishing carbon monoxide production) [45]. The addition of tin (II) ions to previously adsorbed methanol produces a fast oxidation process, demonstrated by DEMS experiments [46]. 

In the presence of a metal adspecies on the surface, it was found that CO also adsorbs on top of any platinum atom. However, in the case of tin metal species, the CO coverage decreases as the tin coverage increases. Although only the on-top adsorption geometry is observed, several platinum atoms show very similar characteristics with respect to CO adsorption. 

CSRs between Sn(C2Hs)4 and hydrogen adsorbed on supported platinum (see reaction (1) below) has been first described in 1984. Under properly chosen experimental condition the reaction between Sn(C2Hs)4 and surface OH groups of the support has been completely suppressed. Consequently, reaction (1) provided direct tin-platinum interaction that was maintained upon decomposition of the primary surface complex (PSC) in a hydrogen atmosphere (see reaction (2) below). The net result is the formation of alloy type bimetallic surface entities... 

Table 7.6 lists the potentials for the catalytic oxidation of nitrites on electrodes modified with MP or MPc complexes. Chen and Chen reported on the use of FeTMPyP film adsorbed on DNA modified carbon, gold, platinum, or transparent semiconductor tin oxide electrodes for the catalytic reduction of nitrite, through the Fe(I) species. Oxidation (at 0.81 V, Table 7.6) of nitrite occurred through the Fe(IV) species as follows (Equation 7.15 and 7.16) ... 

As stated above, the dependence of the reaxrtion rate on the carbon monoxide content shows the two kinetic regimes well known in the case of platinum catalysts. The synergism between platinum and tin(IV)oxide is operative in the domain where the rate is zero order with respect to carbon monoxide. It is often attributed to the spillover of reactive species, either carbon monoxide [14,15] or oxygen (16). We have discussed elsewhere [8,9], why we consider that oxygen is moving from the oxide to the platinum surface or to the three phase boundary, where it immediately reacts with adsorbed carbon monoxide as it is known from pure platinum. 

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