The Capability of Proposed Unifying Mechanism and its Core Principles

Một phần của tài liệu An unifying framework for understanding the electrooxidation of small organic molecules for fuel cell applications (Trang 38 - 41)

1.4.1 Different Systems Examined in this Thesis

The variables in fuel cell reactions can first be organized into different categories by the type of fuel molecules used, the catalyst(s) involved and the operating conditions. Each category is then expanded into subcategories for different specific situations. There is therefore an almost infinite number of possible combinations that can be examined. This thesis will only look at the most representative systems over a sufficient variety of fuel- catalyst-operating condition combinations, as shown below.

Fuel molecule: CO, HCOOH, H2CO and its hydrate H2C(OH)2, CH3CHO and its hydrate CH3CH(OH)2, CH3COOH, HOOCCOOH, CH3OH and CH3CH2OH

Catalyst: monometallic Pt with different surface geometries, catalyst loading per unit electrode surface area (Chapters 2, 3, 4, 8), bimetallic Pt (Pt-Ru in Chapter 5 and Pt-Sn in Chapter 6) and monometallic Pd (Chapter 7, 8).

Operating condition: potential, pH (acidic in Chapters 2-7, alkaline in Chapter 8), temperature

13 Currently, there is no general mechanistic framework that can rationalize or reconcile the multitude of observations under such a wide variety of reaction systems. This is the unique contribution of this thesis project.

1.4.2 Core Principles for Deducing Unifying Mechanism Framework

To construct a building, beams and pillars are needed to strengthen the structure.

Similarly, to construct a unifying mechanism framework, core principles are needed to link up observations over various fuel-catalyst-operating condition combinations. There are two core principles being applied over this thesis:

I. interactions between the catalytic site and adsorbed *H, *C-species and *O-species II. interactions between and among adsorbed species and dissolved species

(In this thesis, * is used to represent a general adsorption site when there is no need to be specific about the site geometry. The adsorbate atom which is bound to surface site is identified next to the * symbol.)

The first core principle can be used to explain the effect of various catalyst geometries and the distribution of second metal (oxide) to the adsorption rate and selectivity among various adsorbed species, which will subsequently affect the overall reaction rate and reaction selectivity. The interactions between the catalytic site and adsorbed *H, *C- species and *O-species are also influenced by electrode potential. For example, a higher potential facilitates interaction to *O-species, e.g. the formation of *OH and *O.

14 The second core principle can be further categorized into i) interactions between adsorbed species, e.g. oxidation of adsorbed intermediate by *OH; ii) interactions between dissolved species, e.g. the equilibrium concentration ratio between hydrated and unhydrated aldehyde (RCH(OH)2  RCHO, R could be a H or an alkyl group); iii) interaction between adsorbed and dissolved species, e.g. a strongly adsorbed *CO and

*CRO will block the adsorption of other species from the solution.

To further zoom into these three categories, we would like to highlight some simple but important concepts which are first time suggested (or at least uncommon in literature):

1) Between strongly adsorbed and weakly (or unstably) adsorbed intermediates requiring oxidation by *OH, the weakly (or unstably) adsorbed intermediate is easier to be oxidized. This leads to impact to reaction selectivity in alcohol electrooxidation.

2) Comparing CH3CH(OH)2  CH3CHO to H2C(OH)2  H2CO, the acetaldehyde has a much higher equilibrium [RCHO] / [RCH(OH)2] concentration ratio, thus acetaldehyde is much easier to be adsorbed into *CRO as compared to formaldehyde. Similarly, a higher temperature enhance the dehydration (e.g.

RCH(OH)2 to RCHO), and hence the *CRO formation is also facilitated.

3) The role of *OH is not only in oxidation of other reaction intermediate, it also affects the reactant adsorption and the formation of certain critical transition intermediate. This is the major cause of hysteresis between forward and backward

15 scan during cyclic voltammetry (as well as other tests with step-up or step-down potential change). For example for HCOOH electrooxidation on Pt(100), the surface is inactivated by blocking *CO during forward scan, while with remained

*OH during the backward scan, the reaction favors direct pathway. Furthermore, the addition of Ruthenium (Ru), tin (Sn), and their oxides provides *OH for alcohol oxidation at lower potential during forward scan but retain the *OH on adjacent Pt affecting alcohol adsorption during backward scan.

4) With increase pH, the dissolved and adsorbed species will shift their equilibrium towards anion forms, e.g. RCOOH  RCOO-. This will shift the adsorption into

*OCRO* to a higher R.H.E. potential. A very high pH could even turn surface blocking *CRO into :CRO-, which we believe to be an reactive intermediate (:

represents bridge binding to two atoms). These are another examples how a dissolved species (e.g. increase pH by higher cation concentration) influences other dissolved and adsorbed species, and hence the reactions being affected.

From the above brief discussion, various fuel-catalyst-operating condition combinations are well linked by the two core principles.

Một phần của tài liệu An unifying framework for understanding the electrooxidation of small organic molecules for fuel cell applications (Trang 38 - 41)

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