This thesis is set out in 9 Chapters. In the following Chapters, Chapter 2 is focused on development of a unifying framework for understanding the electrooxidation of formic acid, C1-C2 aldehydes and alcohols on Pt in acidic condition (only pathways not more
16 difficult than *CO oxidation are covered); and Chapter 3 investigates reaction pathways for the complete oxidation of ethanol and acetaldehyde to CO2 at high potentials, and proposes concern for catalyst design for ethanol electrooxidation. In Chapter 4, inhibition effect of acetaldehyde and acetic acid during ethanol electrooxidation on Pt is examined.
In Chapter 5 and 6, the effect of added Ru/RuOx and Sn/SnOx to Pt and their distribution are investigated, and the various effects of *OH in alcohol electrooxidation is thoroughly discussed. Chapter 7 is focused on HCOOH electrooxidation on Palladium (Pd) in acidic condition, with a same unifying understanding as in Chapter 2 is applied. Chapter 8 is devoted to an examination of effects of ionization on ethanol electrooxidation on Pt and Pd in strong alkaline solutions, as an extension from the unifying mechanism from acidic condition. Finally, Chapter 9 concludes this research work with summary table highlighting the major findings of this thesis work.
Due to the many conflicting mechanisms in the literature, this thesis will not have a specific chapter on literature review. Instead our proposed unifying mechanisms will be introduced first, followed by the presentation of experimental evidence (drawing from the literature and some of our own) supporting the various ramifications of the unifying mechanisms in different chapters. Often the most important observations and major arguments are given in the chapter main pages, with secondary observations and additional arguments in the Supporting Information of each Chapter. This thesis structure aims to facilitate the readers’ appreciation of how the unifying mechanism framework may be applied to different conditions and situations, without the burden of information overload. This particular way of organizing the information may however make it less
17 easy to identify the difference between current hypotheses in the research area and the mechanisms proposed in this thesis. In order to reduce such potential compromise and to more clearly differentiate the contributions of this thesis from previous research, Table 1.3 is provided as a checklist and roadmap for comparing between current and proposed mechanisms. It is recommended that the readers refer to this Table after completing each chapter as a summary of the major findings therein.
Among the findings in this checklist, readers may like to pay special attention to our proposed direct O-addition pathways for alcohol electrooxidation (in red fonts in Table 1.3) since they are the major pathways for current generation in the unifying mechanism for alcohols. Besides, their repeated occurrence in a wide range of conditions (including Pt with or without Ru or Sn addition in acidic conditions; and Pt and Pd in strongly alkaline conditions) also adds credence to the acceptance of these proposed pathways.
In Table 1.3, “*S”, “*T” are two specific types of adsorption sites, namely the step hollow site and the terrace top site respectively. The geometry of these sites is illustrated in Fig.2.1 (Chapter 2.) For simplicity the balance of * is omitted in some equations. “–H*”
represents surface catalyzed dehydrogenation, “–H+–e-” represents a proton release via interactions with surrounding H2O or OH- with the simultaneous transfer of an electron to the electrode.
18 Table 1.3. Comparison between Current and Proposed Mechanisms
Chapter 2 (Reaction Pathways on Pt at Practical Anode Potentials) Fuel
Molecule
Current Mechanisms Proposed Mechanisms Desorbed Product
Reactive Intermediate
Blocking Intermediate
Reactive Intermediate
Blocking Intermediate
HCOOH
*COOH *CO T*COOH
(*OCHO* is not reactive but suppresses *CO
formation)
T*CO (*OCHO*
only weakly suppresses the
*COOH pathway)
CO2
*OCHO* *CO
*COOH, *CO, *OCHO*
H2CO / H2C(OH)2
*OCH2O* *CO T*CH(OH)2
:C(OH)2
T*CO,
T*CHO
HCOOH CO2 CH3CHO /
CH3CH(OH)2
Non *CO,
*C(CH3)O
Non T*CO,
T*C(CH3)O
Non
CH3OH
:CHOH + *OH
*CH(OH)2
*CO
:CHOH + S*OH
T*CH(OH)2
T*CO,
T*CHO
HCOOH
*CH2OH (unknown
pathway) T*CH2OH +
S*OH H2C(OH)2
H2C(OH)2 CH3O* –
H*H2CO
CH3CH2OH
*C(CH3)O +
*OH CH3COOH
*CO
:C(CH3)OH +
S*OH
T*C(CH3)(OH)2
T*CO,
T*C(CH3)O
CH3COOH :C(CH3)OH +
*OH
*C(CH3)(OH)2 CH3CH2O* – H* CH3CHO
T*CH(CH3)OH + S*OH CH3CH(OH)2
CH3CH(OH)2
*CH(CH3)OH – H* CH3CHO
Chapter 3 (Reaction Pathways at Potentials Higher than *CO Electrooxidation on Pt) Fuel
Molecule
Current Mechanisms Proposed Mechanisms Desorbed Product
Intermediate at High Potentials
Conflicting Findings
Intermediate at High Potentials
Remark
CH3CH2OH,
CH3CHO *CHx
*CHx was found to convert to
*CO at low potentials
*O-species, e.g.
*OCHO*,
*O*OCCO*O*,
*C(CH3)O*
They suppress
*OH formation, but their oxidation require *OH
CO2
19 Chapter 4 (The Effects of *OC(CH3)O* and *C(CH3)O Blocking and Catalyst loading)
Current Proposed
No detailed study on the catalyst poisoning effect of CH3COOH and CH3CHO during CH3CH2OH
electrooxidation
*OC(CH3)O* competes with *OH formation;
T*C(CH3)O blocks reaction sites and suppresses CH3CH2OH adsorption.
No detailed study on the effect of catalyst loading per unit electrode surface area
Catalyst loading affects CH3COOH and CH3CHO diffusion, and therefore the Pt activity If / Ib ratio was thought to indicate the catalyst
tolerance to poisoning
If / Ib ratio is not a proper indicator to measure the catalyst tolerance to poisoning.
Chapter 5 - 6 (Effects of Ru & Sn Addition to Pt)
Current Proposed
Enhanced *CO oxidation by
i) (common reason): bi-functional effect with Ru or Sn sites providing *OH
ii) (common reason): (Surface) electronic ligand effect by weakening Pt-CO but strengthening Pt-OH iii) (unpopular reason): Sn-OH weakens adjacent Pt-
CO by intermolecular interaction
All three factors are valid, but their most important effect is not enhancing *CO oxidation, but suppressing *CO formation and facilitating alcohol
electrooxidation via
:CROH + *OH T*CR(OH)2 RCOOH + H+ + e-
T*CHROH + *OH RCH(OH)2 The intermolecular interaction with *OH is very
important and worthy of more attention.
Chapter 7 (HCOOH Electrooxidation)
Current Proposed
*COOH CO2, but lacks detailed explanation on good selectivity over *CO formation
With the proposed *CO formation mechanism in Chapter 2, and the well-known strong Pd&H interaction, the good selectivity is explained.
Chapter 8 (Electrooxidation in Strongly Alkaline Solutions) Fuel
Molecule
Current Mechanisms Proposed Mechanisms (for Pt)
Reactive Pathway Reactive Pathway Desorbed
Product
CH3CH2OH / CH3CH2O-
*C(CH3)O + *OH CH3COOH
:C(CH3)O- + S*OH
T*C(CH3)(OH)O- (negligible surface inhibition by
*C(CH3)O)
CH3COO-
For Pd, with to stronger Pd&H but relatively weaker Pd&C:
:C(CH3)O- + *OH-
T*C(CH3)(OH)O- + e-
CH3COO-
20
CH3CH2O* – H* CH3CHO T*CH(CH3)O- + S*OH CH3CH(OH)O-
CH3CH(OH)O-
*CH(CH3)OH– H* CH3CHO T*CH(CH3)O- CH3CHO + e- CH3CHO
CH3CH(OH)O- / CH3CH(OH)2 / CH3CHO /
CH2CHO-
*C(CH3)O + *OH CH3COOH CH3CH(OH)O- – H*
T*C(CH3)(OH)O- (very active pathway on Pd)
CH3COO-
CH2CHO- H2C**CHO- ? CH2CHO- H2C**CHO-? CO3-
It is reasonable that the reader may feel unconvinced by this unifying mechanism, due to the unavoidable conflicts to other proposed mechanisms in literature. However, readers are strongly encouraged to investigate and analyze the reported experimental observations (in literature and in this thesis work) and propose your own unifying mechanism to cover the same range of various reaction systems as in this thesis. A better buy-in of the unifying mechanistic framework in this thesis may only come after the reader experiences through a similar thinking process. Nevertheless, we are open to the possibility of a different unifying understanding.
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