Monometallic Pt-catalyzed electrooxidation of different small fuel molecules in acidic solutions at room temperature was analyzed first based on the above fundamental considerations. Table 9.1 is the summary of the analysis. Catalytic activity (current density) can be improved by promoting the direct pathways selectively and suppressing the indirect reaction pathways. This is because the latter forms strongly adsorbed *CO or
*CRO species which inhibit the dehydrogenative adsorption of the fuel molecules and are only removable by oxidation at high potentials. The oxidation of HCOOH and H2C(OH)2 by the direct dehydrogenation pathways (via *COOH, *CH(OH)2, or :C(OH)2) can be enhanced by suppressing the C-OH bond cleavage on α-C. For RCH2OH, oxidation by the direct O-addition pathways (via *CHROH or :CROH) can be enhanced by promoting the addition of C-OH bond to α-C.
The key to promoting C-OH bond addition and suppressing *CO and *CRO formation is a weaker Pt&α-C interaction and facile *OH formation at low potentials. Table 9.2 shows examples of how these can be achieved with different Pt surface structures and the addition of adjuvant components such as Ru/RuOx and Sn/SnOx to Pt. Sn/SnOx could even help to oxidize *CO and *CRO but the Pt&α-C interaction may be over-weakened to affect alcohol adsorption negatively, especially methanol. For C2H5OH
179 electrooxidation, the Pt activity is not only affected by *CO and *CRO, but also by the re-adsorption of CH3COOH. Some examples given in Table 9.2 include i) the limited catalytic activity of thick catalyst layers due to slow product diffusion, and ii) the improved activity in strongly alkaline solutions where the inhibition effects of *C(CH3)O and CH3COOH are weaker. The selectivity for the complete oxidation of C2H5OH is an additional concern besides current density. These limitations are best overcome by elevated temperature operations to accelerate both *CO electrooxidation and C-C cleavage. C-C bond cleavage is more effective at elevated temperatures because of a higher [CH3CHO]/[CH3CH(OH)2] ratio which promotes more *C(CH3)O formation.
Table 9.1 Summary of Pt-catalyzed electrooxidation of different oxygenates in acidic solutions at room temperature
Oxygenate Direct Pathways Major Limitations Causes for Limitations HCOOH *COOH CO2 *CO(*OH)*CO Strong Pt&C and Pt&O
interaction for C-OH cleavage H2C(OH)2 /
H2CO
*CH(OH)2 HCOOH :C(OH)2 CO2
*CH(OH)(*OH) :CHOH
*CHO *CO
H2CO *CHO *CO Presence of H2CO CH3CH(OH)2
/ CH3CHO
Insignificant due to fast
*C(CH3)O inhibition
CH3CHO *C(CH3)O
*CO + *CHx
High [CH3CHO] / [CH3CH(OH)2]
CH3OH
*CH2(OH) + *OH H2C(OH)2 H2CO
:CH(OH) + *OH
HCOOH
:CH(OH) *CHO *CO
Strong Pt&C interaction :CH(OH) COH *CO
CH3CH2OH
*CH(CH3)(OH) + *OH
CH3CH(OH)2
CH3CHO :C(CH3)(OH) + *OH
CH3COOH
:C(CH3)(OH)*C(CH3)O
*CO + *CHx
CH3CHO *C(CH3)O
*CO + *CHx
Adsorption
inhibition Dissolved product inhibition CH3COOH *OC(CH3)O* Competition
with *OH
Notes: the reactions above only show the principal reactants, products and intermediates.
For simplicity balance of H+, e-, *, H2O, and *OH (for C-OH cleavage) has been omitted.
180 Table 9.2 Summary of the effects of different Pt-based catalysts and operating conditions
Effect 1 Effect 2 Results
Pt(110) Strong Pt&C at *T Strong Pt&O at *S Easy formation of surface- blocking *CRO, *CO
Pt(111) Moderate Pt&C
Dilution of the sites for
*CO(*OH) formation by
*OCRO*
Most selective for the direct dehydrogenation pathways for HCOOH &
H2C(OH)2
Pt(553) or Pt(554)
Provide (111) terrace width where Pt&C is
not as strong
Provide S*OH for the O- addition step
Optimized the direct O- addition pathways for
RCH2OH
Addition of Ru/RuOx to
Pt
Provides *OH at lower potentials
Weakens Pt&C interaction (ligand effect + repulsion
by *OH)
Enhanced direct O-addition pathway
Slightly improved *CO /
*CRO electrooxidation
Addition of Sn/SnOx to
Pt
Provides *OH at very low potentials
Weakens Pt&C interaction more so than Ru/RuOx (ligand effect +
repulsion by *OH)
Higher selectivity for the direct O-addition pathway Easier oxidation of *CRO
& *CO Excess *OH and over- weakened Pt&C could limit alcohol adsorption Increased
Catalyst Loading
Increased [CH3COOH]
in the catalyst layer (for ethanol) suppresses
*OH formation
Increased [CH3CHO] in the catalyst layer (for ethanol) inhibits ethanol
adsorption.
Current density per Pt mass basis is reduced.
Strongly alkaline solutions
*CRO :CRO- CH3COOH CH3COO- Improved activity at the practical anode potentials
(<0.7V) unless pH is significantly lowered by accumulated anions, e.g.
CH3COO-.
*CRO inhibition is weakened since :CRO- can be oxidized directly
similar to :CR(OH) in acidic solutions
*OCRO* inhibition is weakened since CH3COO-
prefers to be in solution and only adsorbs at high
potentials (on the RHE scale)
Elevated Temperature
Enhanced *CO electrooxidation
Shift equilibrium from RCH(OH)2 towards
RCHO
Improved overall activity and CO2 selectivity (from
ethanol)
Notes: the reactions above only show the principal reactants, products and intermediates.
For simplicity balance of H+, e-, *, H2O, and *OH (for C-OH cleavage) has been omitted.
181 9.1.2 Pd-Based Catalysis
The two fundamental considerations also apply to Pd-catalyzed electrooxidation as well.
The main difference from Pt is the strong Pd&H interaction which weakens the Pd&C and Pd&O interactions relatively. Consequently *CO formation via *CO(*OH) is made more difficult during Pd-catalyzed HCOOH electrooxidation resulting in higher selectivity for the direct dehydrogenation pathway and good catalyst activity. However, the weak Pd&O interaction also postpones *OH formation to high potentials (onset ~ 0.7V-0.8V). Pd is therefore inactive for the electrooxidation of other fuel molecules which require facile *OH under acidic conditions.
In strongly alkaline solutions, *CRO could also be ionized to :CRO- as in the case of Pt catalysts. With a weaker Pd&C interaction than Pt&C interaction, the negatively charged
*OH- on Pd is also reactive for electrooxidizing :CRO- via the direct O-addition pathway.
Pd is therefore active towards ethanol electrooxidation unless pH is not high enough to form :C(CH3)O- from *C(CH3)O. The latter is an inhibitor which suppresses the adsorption of both ethanol and OH-.