2.2 The Proposed Unifying Mechanistic Framework
2.2.3 HCOOH Adsorption and Electrooxidation
HCOOH electrooxidation at low potentials via the direct dehydrogenation pathway (§
2.2.3.1.) is the most desirable. However it only occurs when the catalyst surface is not
*CO inhibited. The goal in HCOOH oxidation is therefore to promote the direct dehydrogenation pathway by suppressing *CO formation. This requires a good understanding of how the various pathways are related to each other and influenced by the Pt&α-C, Pt&O, and Pt&H interactions on different surfaces (Scheme 2.1).
30
HCOOH
C
O HO
C O
H
O
C
O O
H C
O O
H
C O
O
C O
O H
O H
H
solution catalyzed
strong Pt&-C, next to a sufficiently
strong Pt&O site weaker Pt&-C
than Pt&O
sufficiently strong Pt&-C No site blocking by *CO optimal @ ~ pztc
For T*OH stronger than T*CO direct pathway
Scheme 2.1.The proposed general reaction scheme for HCOOH electrooxidation. The direct dehydrogenation pathway (CO2 formation via *COOH) is the most desirable for current generation. It occurs when the surface is not blocked by *CO and is most favorable when adsorption as *COOH is least interfered by H* and *O-species (i.e. at around ptzc). T*CO formation can be minimized by a weaker Pt&α-C interaction; and by the competing adsorption of species in the blue boxes. Once T*CO is formed, it can only be removed effectively by oxidation when T*OH becomes abundant (i.e. at high V, via the pathway in red).
31 2.2.3.1 Dependence of Reaction Pathways on Pt&α-C, Pt&O, and Pt&H Interactions
In the absence of *CO inhibition, the adsorption of HCOOH as T*COOH requires a sufficiently strong T*Pt&α-C interaction. Subsequent cleavage of the O-H bond in
T*COOH releases the hydrogen atom as H+ to water, with e- passed to the electrode and CO2 desorbed from the catalyst surface. This is known as the “direct dehydrogenation pathway” of HCOOH electrooxidation [11].
The rate limiting step in the direct dehydrogenation pathway is the dehydrogenative adsorption of HCOOH where the C-H bond in HCOOH is cleaved [21] to form *COOH.
This is more viable around the pztc when adsorption via the carbon atom is least affected by H* and various *O-species (Supporting Information 2S3).
→
→ (2)
If the Pt&α-C interaction is strong and there is an adjacent site with a sufficiently strong Pt&O interaction (e.g. at moderate potentials), a bidentate transition state T*CO(*OH) may be formed after dehydrogenative adsorption. A subsequent cleavage of the C-OH bond leaves the surface with *OH and site-blocking T*CO. The *OH could also be reduced and desorb as H2O if the potential is not sufficiently high to stabilize the *OH.
32
→
→ (3)
Although T*CO(*OH) forms when a strong T*Pt&α-C site is adjacent to sites with sufficiently strong Pt&O interactions, it does not form when the adjacent sites are with too strong a Pt&O interaction that stabilizes the *OH from water dissociation. Increasing in potential increases the Pt&O interaction (Table 2.2). At potentials higher than the onset of *OH formation from water dissociation, *OH becomes increasingly abundant and reacts with T*CO to form T*COOH and CO2 in sequence (the reverse of the *CO formation process). The greater presence of *OH also decreases the available *T sites and adsorption as T*CO(*OH) (the precursor to T*CO formation) which requires two contiguous sites. The increase in Pt&O interaction also increases the likelihood of reversible adsorption as *OCHO*, which further diminishes the prospect of adsorption as
T*CO(*OH). By comparison the direct dehydrogenation pathway is not as adversely affected by the competing adsorption from the *O-species since *COOH adsorption requires only single site as opposed to T*CO(*OH) adsorption which requires dual sites.
As a consequence, the selectivity to the direct dehydrogenation pathway is enhanced relatively in the presence of *O-species, thus preventing *CO inhibition and resulting in an increase in current (Supporting Information 2S3).
On the other hand, *OCHO* is adsorbed with its C-H bond furthest away from the Pt surface and is therefore more difficult to oxidize than *CO. Hence if T*CO is already formed before Pt&O interaction is increased by raising the applied potential, the increase
33 in Pt&O interaction allows HCOOH to adsorb as *OCHO* and competes with *OH formation which is required for *CO removal. Hence T*CO electrooxidation is more difficult in the presence of HCOOH than in an electrolyte without it (Supporting Information 2S4).
- (4)
2.2.3.2 Observations of Surface Geometry Dependency
Tables 2.1 and 2.2 above and the discussion in the preceding section may be used to rationalize the following experimental observations:
1) Pt(110) has strong T*Pt&α-C and S*Pt&O sites next to each other and consequently the
T*CO(S*OH) adsorbed species can be established easily at low potentials. Recall that
T*CO(*OH) does not form when the Pt&O interaction is too strong that *OH from water dissociation is stable, As a result, with increase in potentials that stabilizes S*OH, T*CO can be formed via T*CO(T*OH) instead of T*CO(S*OH). Since the formation (stabilization) of T*OH from H2O dissociation requires a high potentials (greater than pme of ~0.65V in Table 2.2), T*CO formation via T*CO(T*OH) still proceeds at moderately high potentials. Hence Pt(110) is the most active surface for T*CO formation, as has been found in [22].
34 2) Pt(100), with only moderate T*Pt&O interaction, does not favor the formation of
T*CO(T*OH) at low potentials. With a T*Pt&C interaction which is weaker than that on Pt(110), the formation of T*CO(T*OH) is competed strongly by *O-species at high potentials. As a result, Pt(100) is only active for T*CO formation around its pztc between 0.2V to 0.5V [22, 23].
3) Pt(111) with relatively moderate Pt&α-C and Pt&O interactions favors the adsorption of HCOOH as *OCHO*, which is supported by the calculations in [24]. The adsorption as T*CO(T*OH) is therefore inhibited. Furthermore, with a moderate Pt&α-C interaction, desorption of *COOH as CO2 should be easier than on other planes. Pt(111) therefore has the highest selectivity for the direct dehydrogenation pathway, as observed in [22].
2.2.4 Aldehyde Adsorption and Electrooxidation
A significant fraction of formaldehyde and acetaldehyde is hydrated in water and exists in the diol forms of H2C(OH)2 and CH3CH(OH)2 [25]. The hydration and dehydration reactions are reversible and immediately reach the equilibrium in water, especially water with dissolved acidic and basic species [26]. This implies a fast hydration-dehydration reactions between RCHO and RCH(OH)2 and low activation barrier for C-OH and O-H bond cleavages from the >C(OH)2 structure in water. With this understanding and by analogy with HCOOH electrooxidation, the mechanism for H2C(OH)2 electrooxidation can be understood by means of Scheme 2.2.
35
(5)
[H2C(OH)2] / [H2CO] = 2.28 x 103, [CH3CH(OH)2] / [CH3CHO] = 1.1
at room temperature and pressure (r.t.p) [25, 27].
2.2.4.1 Major Difference between H2C(OH)2/ H2CO and HCOOH Electrooxidations
The adsorption and electrooxidation of H2C(OH)2/H2CO in Scheme 2.2 shares many common features with HCOOH electrooxidation. There are, however, additional pathways contributed by the dehydrogenative adsorption of H2CO (the dehydrated form) to *CHO (and then *CO). Even without decomposing to *CO, *CHO has adsorption strength comparable to that of *CO and is a site blocker except at high potentials where
*OH can add to it to form HCOOH (Supporting Information 2S5). As a result, catalyst deactivation is more pervasive in H2C(OH)2/H2CO electrooxidation than in HCOOH electrooxidation (as shown in [28]).
→ (6)
→ (7)
↔ (8)
36
H2C(OH)2
C
OH C HO
O H
O H
H2CO
H
C
OH O
H H
HCOOH C
OH HO
C
O
HO H
C O O
C
O H
C O
O
H H
H
C O
C
O HO
C O
O
H H
C O O
H
weak Pt&C
no surface blocking by *CO and *CHO.
strong Pt&C adjacent site has suf f iciently strong Pt&O direct pathway
high V
high V
Scheme 2.2. A proposed general reaction scheme for H2C(OH)2 electrooxidation. It is analogous to HCOOH oxidation in the following aspects: direct dehydrogenation pathways via O-H cleavage(s) in solution to form HCOOH and CO2, indirect pathways via surface catalyzed C-OH cleavage forming inhibiting *CHO and subsequently *CO.
The main difference is the added possibility of *CHO formation from H2CO, which makes surface inhibition an easier process.
37 2.2.4.2 Similarities between H2C(OH)2 and HCOOH Electrooxidations
Similar to HCOOH, H2C(OH)2 electrooxidation also has direct dehydrogenation pathways for current generation at low potentials, which occur through the dehydrogenative adsorption of H2C(OH)2 as *CH(OH)2 when the surface is not extensively blocked by *CO and *CHO. With subsequent O-H cleavage in solution and surface-catalyzed C-H cleavage, HCOOH (reaction 9) and CO2 (reaction 10) are eventually formed. Due to the ease of O-H cleavage in >C(OH)2 in water, the desorption of *CH(OH)2 and :C(OH)2 to HCOOH and CO2 should be as viable as the desorption of
*COOH to CO2.
→
→ (9)
- →
- - -
→ (10)
Similar to *CO formation via the *CO(*OH) intermediate in HCOOH electrooxidation, a strong Pt&α-C site next to a sufficiently strong Pt&O site can readily transform
*CH(OH)2 into *CH(OH)(*OH), followed by surface-catalyzed C-OH cleavage to :CH(OH) and then O-H cleavage to *CHO in solution. In analogy to *CO(*OH)
*CO with C=O converting to C≡O, *CH(OH)(*OH) *CHO with C-OH converting to C=O should also be feasible (See Supporting Information 2S6).
- →
- - - -
→ (11)
38 The similarities between H2C(OH)2 and HCOOH electrooxidations are reflected by their similar surface geometry dependence: Pt(110) is the easiest to be deactivated by *CO, followed by Pt(100) and Pt(111) in that order. The ranking is opposite to the viability of the direct pathway on these surfaces [29, 30]. Furthermore, *OCH2O* has been detected on Pt(111) at both low and high potentials (0.1V and 1.0V) [30]. The persistence of
*OCH2O* even at 1.0V indicates that it is as tenacious to oxidize as *OCHO*. Its function on Pt(111) could be similar to that of *OCHO* - competing against adsorption as *CH(OH)(*OH) and preventing the surface-catalyzed formation of *CHO and *CO at low potentials.
2.2.4.3 Comparison between CH3CHO and H2CO
For acetaldehyde, the much lower aqueous phase equilibrium constant of (~1.1) compared with (~2280) [25] suggests that there are more pristine CH3CHO to dehydrogenate to *C(CH3)O than for H2CO to dehydrogenate to *CHO. Due to the difficulty in *CRO oxidation, the surface is more readily deactivated in CH3CHO than in H2CO (Supporting Information 2S7). Comparing the *CO formation from *CHO and *C(CH3)O, the latter is more difficult because of the need to cleave the C-C bond of
*C(CH3)O to *CO and *CHx (reaction 12, most likely x=1 [31]). The cleavage of the C-C bond is an arduous undertaking and requires sufficient free sites to bind to β-C at
39 potentials below *OH formation [31-36]. The oxidation of *CO to CO2 and *CRO to RCOOH (e.g. reaction 8) are categorized as “indirect pathways”.
→
→ (12)