6.2.1 Distribution of Sn and Its Effects on CO, Formaldehyde and Methanol Electrooxidation
122 Simplified illustrations of Pt-Sn surfaces in the abovementioned three categories of Pt-Sn catalysts based on reference [113] are shown in Table 6.1. Sn atoms or SnOx clusters are painted in pink. The Pt atoms next to them with weaker Pt&α-C interaction and stronger Pt&O interaction are painted in blue. The Pt atoms further away which behave like bulk Pt are painted in black.
Table 6.1. Distribution of Sn/SnOx among the Pt atoms on the catalyst surface and their effects on the electrooxidation of CO, aldehydes, and alcohols*
Pt-SnOx Sn ad-atoms /
partially alloyed
Pt-Sn alloys e.g.
Pt3Sn(111) Sn/SnOx clusters
(ready *OH sources)
Pt (weaker Pt&C but stronger Pt&O)
Normal Pt
CO Good Good Excellent
H2CO / H2C(OH)2 Good Good Not Studied
CH3OH Poor to moderate
(rls: adsorption at blue Pt & oxidation at isolated black Pt)
Moderate to good (optimized for adsorption &
oxidation)
Poor to moderate (rls: adsorption)
* The visualization is based on the core concepts and the ranking of the effects (Excellent, Good, Moderate or Poor) taken from [113]. rls: rate limiting step.
For fuel molecules with a strong Pt&α-C interaction such as CO and aldehydes, the *OH from Sn and a weaker Pt&α-C interaction at adjacent sites can improve the catalyst performance relative to monometallic Pt which is easily deactivated by *CO and *CRO.
Pt-Sn alloys which increases the Pt-Sn interfaces therefore show excellent activities.
However, for fuel molecules with a relatively weak Pt&α-C interaction, e.g. CH3OH, the
123 weaker Pt&α-C interaction (at the blue Pt atoms) may result in adsorption as the rate limiting step while the (black) Pt sites remote from Sn or SnOx are still susceptible to
*CO poisoning. A partially alloyed Pt-Sn or a Pt surface with Sn adatoms will be more preferable since it is neither like Pt-SnOx where the (black) Pt sites are easily poisoned by
*CO, nor like Pt-Sn alloys where the (blue) Pt sites are subjected to strong repulsive interaction from *OH and O*. This particular surface configuration therefore provides a better balance between the needs for CH3OH adsorption on Pt and *OH formation on Sn;
and is therefore more effective for CH3OH oxidation. Although Pt-Sn is recognized as the most active bimetallic catalyst for current generation in ethanol electrooxidation [64, 114], it is not as good as PtRu for CH3OH electrooxidation [113]. This is because of the distinct difference between methanol and ethanol electrooxidations as will be shown later.
6.2.2 Similarity between Methanol and Ethanol
6.2.2.1 Enhancement of the O-Addition Pathway for Alcohols by *OH on Sn/SnOx
and Weaker Pt&α-C Interaction
Only strongly bound *CO was found on alloyed Pt-Sn in methanol electrooxidation while two types of *CO, namely strongly and weakly bound *CO, were formed in the direct adsorption of CO on alloyed Pt-Sn at high *CO surface coverages [115]. This observation can be rationalized by our proposed direct O-addition pathways for alcohol electrooxidation. The absence of weakly bound *CO suggests the predominance of the non-CO direct O-addition pathway (reactions 1-2) on (blue) Pt sites promoted by an
124 abundance of *OH and weaker Pt&α-C interaction4. HCOOH and H2C(OH)2 are formed and desorbed after the weak Pt-C bond is broken.
→
→
→ (1)
→
→ (2)
Pt-Sn is more effective for C2H5OH electrooxidation than CH3OH electrooxidation, especially the 4e- pathway leading to CH3COOH formation. The incorporation of Sn however does not improve the selectivity for the complete electrooxidation of C2H5OH to CO2 [55, 56, 116]. This can be easily explained by the weaker Pt&α-C interaction and the abundance of *OH which collectively increase the contribution of the direct O-addition pathway (without C-C bond rupture) similar to the effects of Ru addition to Pt discussed in Chapter 5.
6.2.2.2 Adsorption is Rate-limiting on Pt-Sn Alloys
Similar to methanol electrooxidation, non-alloyed Pt-SnO2 is more active than alloyed Pt- Sn (e.g. Pt:Sn ~ 5:1) for ethanol electrooxidation at room temperature [117]. In reference [13], SnO2 was postulated as the main *OH provider to electrooxidize *C(CH3)O as opposed to the metallic Sn in alloy with Pt. The argument is incongruent with the trend in
4 The formation of strongly bound *CO in methanol electrooxidation on Pt-Sn catalysts is likely due to the presence of strong Pt&C site, forming *CO via CH3OH :CHOH *CHO *CO as discussed in chapter 2.
125 CO electrooxidation where surface reaction is rate-limiting and yet is more facile on the Pt-Sn alloys. We therefore counter propose that ethanol adsorption on Pt-Sn alloys may be rate-limiting whereas the electrooxidation of intermediate adsorbed species is relatively facile (see Supporting Information 6S for the different effects of alloying on ethanol electrooxidation at elevated temperatures).
6.2.3 Difference Between Ethanol and Methanol 6.2.3.1 Easier Adsorption of Ethanol
Although ethanol electrooxidation on Pt-Sn at room temperature may still be constrained by ethanol adsorption, it is nevertheless an easier process than methanol adsorption. This can be the main reason why Pt-Sn catalyzes C2H5OH electrooxidation more effectively than CH3OH electrooxidation. At the molecular level, the presence of a stronger electron donating group in C2H5OH (i.e –CH3 as compared with –H in CH3OH) increases the electron density at the α-C of C2H5OH; thereby facilitating the transfer of electrons from α-C to the Pt site during adsorption. Furthermore, based on the proposed direct O- addition pathway forming RCOOH from RCH2OH (reaction 1), the potentials of CH3OH and C2H5OH electrooxidations calculated from the Gibbs free energy changes (ΔG0, as discussed in Chapter 1) are around 0.110V and 0.057V respectively. The higher potential for CH3OH electrooxidation could also be an indication of the slowness in the first step of dehydrogenative adsorption.
6.2.3.2 Inhibition by *OC(CH3)O* during Ethanol Electrooxidation
126 The other difference between ethanol and methanol electrooxidations is the mechanism of intermediate product inhibition. *CO and *CRO are common inhibitors of the Pt surface during both methanol and ethanol electrooxidations. They can however be effectively oxidized after the addition of Sn to Pt. However, for ethanol electrooxidation where CH3COOH is one of the oxidized products, adsorbed acetate (*OC(CH3)O*) is another inhibiting species. The inhibition effect of CH3COOH on Pt/C catalysts in ethanol electrooxidation has already been shown in Chapter 4. In short, adsorbed acetate
*OC(CH3)O* from CH3COOH competes with *OH formation over a wide range of potentials, and is almost electrochemically non-oxidizable. On the other hand, HCOOH from methanol electrooxidation is not a strong catalyst inhibitor as it can be directly oxidized via the *COOH pathway (Chapter 2) and even *OCHO* electrooxidation is significantly easier than *OC(CH3)O* (Chapter 3).
Fig. 6.1 shows the effects of extraneously introduced CH3CHO and CH3COOH on an E- TEK Pt3Sn/C catalyst in 1M C2H5OH. At low potentials below 0.3V, the added CH3CHO was found to even increase the oxidation current. Parallel to this formaldehyde has also been shown as the most active fuel molecule with an onset potential close to 0V on Pt- SnO2 [118] or Sn ad-atom modified Pt [119, 120]. These observations are consistent with our hypothesis that *CO, *CHO and *C(CH3)O are active species on Pt3Sn but are surface inhibitors on Pt. On the other hand, Fig. 6.1B shows the inhibiting effect of CH3COOH on Pt3Sn in decreasing the I-V slope. The inhibition on Pt3Sn is however less severe than that on Pt/C. The greater tolerance of Pt3Sn for CH3COOH inhibition is likely
127 due to the more facile formation of Sn-OH and SnOx-OH which compete well with
*OC(CH3)O* adsorption.
Fig. 6.1. The 20th scan cyclic voltammograms of the electrooxidation of 1M C2H5OH in 0.1M HClO4 on an E-TEK Pt3Sn/C catalyst at 10mV/s in the presence of different concentrations of extraneously introduced CH3CHO (A) or CH3COOH (B). The catalyst loading was 5àg Pt3Sn/cm2. Argon was continuously purged to eliminate dissolved O2 and to generate turbulence to improve external diffusion of products.
6.2.4 Comparison between Pt-Sn and Pt-Ru
As mentioned in §6.1, Pt-Ru is generally a better selection for methanol electrooxidation while Pt-Sn generates higher current density for ethanol electrooxidation. This could be due to the difference between methanol and ethanol electrooxidations. Since ethanol adsorption is easier and is less affected by *OH and O* compared with methanol adsorption; and the electrooxidation of ethanol requires more facile *OH formation to compete with *OC(CH3)O* deactivation, the catalyst for ethanol electrooxidation needs
128 to supply *OH at lower potentials. Since Sn and SnOx are more effective in providing accessible *OH compared with Ru and RuOx, this makes Pt-Sn a more active catalyst than Pt-Ru for ethanol electrooxidation. On the other hand, since excessive *OH at low potentials easily affects the methanol adsorption, Pt-Ru offers a better balance between the weaker adsorption of methanol and the supply of *OH for methanol electrooxidation.