Electrochemical Oxidation Of Small Organic Molecules Biology Essay

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Today's general concern for the problems inflicted upon the global climate through CO2 emissions by the current extensive use of fossil fuels as well as fossil resource issues has led to an intensive search for alternative energy systems, primarily based on nuclear or renewable energy sources. Energy storage remains a major challenge associated with large-scale deployment of renewable energy. It is therefore anticipated that electrochemical energy storage, such as represented by batteries or energy systems based on hydrogen as a storage medium, will play a central role in peak-levelling renewable energy production.

Hydrogen is the ideal energy carrier from an environmental viewpoint as it can be transformed into useful energy with pure water as the only product. The CO2 balance for the entire energy process depends on the fuel or primary energy source used to produce hydrogen. The main reason of the increased interest in the hydrogen economy over the last few decades is primarily due to the environmental benefit of introducing hydrogen as the fuel source for stationary and mobile applications. Hydrogen can be made from a variety of renewable energy sources, including water electrolysis driven by for example wind or photovoltaic sources, and biomass-derived hydrogen by gasification.

In a hydrogen energy-system water electrolysers and fuel cells are devices of critical importance. In both of these, the non-trivial part of the design is related to the electron transfer at interfaces between electronic and ionic conductors. If the rates of the electron transfer is dependent on the chemical composition of the electronic conductor or on its surface microstructure the process is said to be electrocatalytic, and the electrode is referred to as an electrocatalyst.

The low volumetric energy density of hydrogen and lack of a gaseous fuel infrastructure, however, complicates the process of introducing hydrogen as a fuel in the short run. Small organic molecules may serve as alternative fuels to hydrogen. These are are liquid at room temperature and can be introduced and stored more efficiently than hydrogen, and even to some degree use the existing infrastructure. Small organic molecules derived from biomass or other renewable energy sources are CO2 neutral and often termed biofuels. Methanol, formic acid, ethanol and dimethylether are all examples of small organic molecules that can be produced without generating more CO2 and could be fed directly into the fuel cell anode. In the short term methanol is emerging as the fuel of choice for fuel cell application first of all since it is a simple molecule, relatively easy to store with a high energy density, and is already a commodity product from the world's petrochemical industry.

Direct oxidation of small organic molecules in a fuel cell will in most cases involve many partial oxidized by-products or intermediates that can for example have a strong poisoning effect on the electrocatalyst, like CO(ads), thus reducing the energy efficiency severely. Mechanistic and kinetic considerations are in this context important in order to be able to determine what aspects of a catalyst that promote a negative or positive effect on the performance and efficiency. Since the electrocatalyst used are expensive platinum group metals, it is very important to optimize the kinetics in favour of reducing the amount of electrocatalyst needed. Although much progress has been made in understanding the nature of the surface-bound intermediates and the interaction with the surface, there are still important kinetic issues that needs to be dealt with.

Methanol as a fuel. Methanol oxidation

The most promising hydrogen carrier is methanol, which can be produced without net production of CO2 through gasification of biomass [FS1], thus making these fuel cells sustainable. Direct oxidation of methanol on the fuel cell anode is ideally a 6 electron transfer resulting in CO2 and water. However, as is the case of both alcohols and ethers, addition of extra oxygen atoms are needed to the carbon molecule in order to obtain the final oxidation product CO2. It is experimentally evident that oxidation of methanol may happen through two different pathways, one through short lived intermediates to stable compounds, and the other through a strongly adsorbed intermediate, believed to be CO(ads), to a stable oxidation product. The existence, and conditions, of a distinct pathway not involving a poisoning intermediate has been a hot topic in the fundamental research on methanol oxidation over the past 2 decades. Experimental evidence has been shown by for example differential elecectrochemical mass spectrometry (DEMS) [FS2] and multipulse potentiodynamic measurements [FS3]. Ongoing work focuses on mapping the conditions that enhances the parallel route by electrochemical and surface sensitive methods. The electrocatalyst and potential dependent occurrence of formaldehyde and formic acid, as stable oxidation products from the methanol oxidation process, represent a safety risk when operating direct methanol fuel cells.

The presence of a second metal on platinum surfaces affect the performance of methanol oxidation. Promotional behaviour of ruthenium to platinum has been thoroughly investigated by many research institutions using both well characterized PtRu alloys and carbon supported electrocatalysts. A bifunctional mechanism has generally been postulated whereby water is activated by ruthenium at lower overpotentials, with subsequent electrooxidation of CO to CO2 on a neighbouring platinum atom. The bifunctional mechanism thus promotes the pathway through a strongly adsorbed intermediate, increasing the performance by reducing the stability of the poison.

Formic acid as a fuel. Oxidation of formic acid

Formic acid is also a commodity product and can be produced through a light oil liquid-phase oxidation process or by heating CO and NaOH followed by careful treatment with sulphuric acid. Formic acid exists in the bodies of red ants and in the stingers of bees and is very easily oxidized. It is a simple molecule with only two electrons in its oxidation to carbon dioxide, and previously was considered as a model reaction in electrocatalysis. It differs significantly from methanol as it does not require addition of any oxygen donating species in order to obtain the complete oxidation product, CO2. Additionally, the amount and range of stable oxidation by-products are very small, thus reducing the safety risk of using formic acid as fuel in fuel cells. Even though formic acid is readily oxidized to CO2, it is complicated by the occurrence of a chemical step to form adsorbed CO(ads), which again blocks the active surface.

Palladium is considered to be the most efficient monometallic catalyst for formic acid oxidation, and does not suffer drastically from self-poisoning. Only sparse literature exists on the effect of adding a second metal to the palladium catalyst, although some improvement was obtained by using Pd-Au materials [FS4].

Ethanol as a fuel. Oxidation of ethanol

Ethanol, with its high energy density, low toxicity, small crossover in Nafion, facile production from renewable sources, and simplicity of storage and transportation, is almost the ideal fuel for fuel cells for direct conversion to electric energy (DEFC). However, commercialization of DEFC has been impeded by the inefficiency of oxidation of ethanol at even the best available electrocatalyst.

Many possible pathways for ethanol oxidation exits, but these can be classifies, as for other organic fuels, in a serial pathway involving a strongly bound poisonous intermediate and a parallel pathway with the final oxidation product as the only stable species. Again, the pathways depends on operational parameters (temperature, pressure, chemical composition etc.), and catalyst structure and composition.

The existence of a C-C bond in the ethanol molecule enhances the parallel path, and leads to soluble oxidation products such as acetaldehyde and acetic acid. While this prevents poisoning of the catalyst, the partial oxidation of ethanol is also less efficient than those for which the final oxidation product is CO2. A catalyst that effectively splits the C-C bond in ethanol at room temperature in acid solutions yet is not poisoned by stable intermediates such as linearly adsorbed CO would thus facilitate the complete oxidation of ethanol at low potential to CO2. Promising bimetallic catalysts in this respect are PtRu and PtSn.

Related electrochemical processes

If reformate gas is used in fuel ell anode, CO levels as low as 30 ppm result in degradation of cell performance. Carbon monoxide is present in both hydrogen obtained by reforming and CO-like intermediates formed in electrooxidation of hydrocarbons on anode catalysts. Oxidation of adsorbed CO is known as the limiting step in direct electrooxidation of small organic molecules. Strongly adsorbed CO-like species are the reason for decrease of fuel cells efficiency, remaining one of the first obstacles for the commercialization of these portable devices. There is considerable interest for new electrocatalysts tolerant to the presence of at least 100 ppm of CO, and the most promising in this aspect is a PtRu catalyst.

CO-adsorption, dissolved CO, other fuels, ORR, …

CO as a characterisation technique (ptzc etc.)

Synthesis of nano-structured noble-metal electrocatalysts. Supports

Synthesis of electrocatalyst should be a well-controlled process and result in high catalyst dispersion. This is usually achieved by tuning the synthesis conditions, in particular with respect to the catalyst precursors (usually a transition metal salt such as chlorides, nitrates etc) and the reducing agent [1] . The solvent will also play a central role, e.g. through its viscosity. The use of surfactants or polymer capping agents will also crucially influence the structure of the as-produced catalyst. Finally, the nature of catalyst support (carbon nanofibers, oriented graphite, carbon black black) and it's pre-treatment through oxidative and other procedures play an important role.

At Department of Materials Science and Engineering, NTNU, we are working actively in this area, collaborating closely with the Department of Chemical Engineering, NTNU, with an emphasis on carbon nanofiber supports. An example of supported catalysts produced at NTNU is given in the figure.

Stability of supported noble-metal electrocatalysts

Catalyst stability is an important issue for a large-scale diffusion of PEM fuel cells into consumer applications. Noble metals loss and aggregation of catalyst particles are highly dependent on operating conditions and protocols. A combination of electrochemical measurements, in-situ X-ray diffraction (XRD), EXAFS, electrochemical quartz crystal microbalance (EQCM), scanning-tunneling microscopy, and other techniques have disclosed two potential regimes for Pt. In the double-layer region (approximately 0.4 to 0.7 V vs the Normal Hydrogen Electrode (NHE)) loss of active area may be expected primarily due to particle growth. [2] Cycling the electrode in the region 0.7 through 0.9 V leads to loss of active area due to loss of catalyst by dissolution. Cycling may actually lead to dissolution rates several orders of magnitude larger than those observed under steady-state conditions. Hydrogen penetration from the anode may also have a decisive influence on re-precipitated catalyst particles under real-cell operation.

Additional problems may be associated with carbon corrosion, among other things due to spurious voltage excursions at the cathode during fuel-cell start-up or local fuel starvation. All of these issues are also being studied at the NTNU group.

Particle size, electronic structure, and electrocatalysis

An important research task is to establish theoretical guidelines and models that would allow for catalyst design on a predicitive basis. Such a framework for rational heterogeneous catalyst selection has been under development for a number of years, and also more recently for electrocatalysts. In general these efforts revolve around the Sabatier principle (1913), stating that a catalyst should adsorb a key intermediate neither to strongly nor too weakly. A too weakly adsorbed intermediate has a low reactivity because of its low concentration, and a too strongly adsorbed intermediate leads to a too low reactivity due to the difficulties with which it desorbs. One thus expects a volcano-shaped catalyst activity as a function of the heat of adsorption or any parameter that determines that heat of adsorption or is related to it.

At the nanoscale level the electronic structure is demonstrated to be crucially different from that of the corresponding bulk material for a number of materials. For example, the band gap of a number of semiconductor crystals increases with diminishing particle size as this reaches the nm-range. Similar phenomena will occur in metal catalyst particles. For example will the work function of iron clusters vary several eV's with cluster size below 20 atoms.

These changes are therefore expected to manifest themselves as changes in electrocatalytic and catalytic activity. [3] A well-known example is gold, which in bulk form is catalytically inactive (primarily owing to its filled d-band leading to a very weak interaction with adsorbates). Nano-sized gold, on the other hand, is catalytically active for the water-gas shift reaction. 1 Usually, although less dramatic, a similar effect is operative for fuel-cell supported Pt cathodes. A frequent finding is that electrocatalytic activity for oxygen reduction increases with increasing particle size, whereas that for methanol oxidation decreases with increaseing particle size. Both effects may be related to the adsorption of hydroxyl on Pt as a function of particle size. [4] 

In our group we have observed an opposite tendency for the electrocatalytic activity for oxygen reduction at CNF-supported Pt catalyst particles, supporting the findings of another recent report. The difference of these studies and those referred to above lies in the mode of particle growth. In the latter cases the particle size is induced by potential cycling, whereas the work referred to above employs pristine electrocatalysts with differences in average particle size. Thus, it appears that not only size but also particle history, presumably through differences in morphology, are decisive for catalyst activity.

Core-shell catalysts

Ad-atom, overlayer and alloy systems have also been studied in electrochemical systems. Kinetically controlled current densities for oxygen reduction show volcano behaviour on Pt alloys as a function of the content of the alloying metal (Ni, Fe and Co). [5] Analysis of reaction and bond-breaking mechanisms shows that the electrochemical behaviour may be rationalised with reference to the d-structure of the catalysts, both for oxygen reduction [6] and more directly for oxidation of formic acid. [7] 

From the above, it appears that significant degrees of freedom in designing catalysts and electrocatalyst may be achieved by combining the effects on catalytic action of size and composition. In principle one may therefore seek designs based on nano-sized alloys as well as overlayer structures, the latter usually being referred to as core-shell structures.1 Catalytic effects of the core in such structues have been demonstrated in electrochemical systems [8] as well as in heterogeneous catalysis. [9] 

At NTNU a project on core-shell structures was recently (2008) started in collaboration with the University of Maryland, aiming at producing and characterising core-shell electrocatalysts.

Research challenges and opportunities for oxidation of small organic molecules: Supported nanostructured bimetallic catalysts

From the above it is quite clear that the kinetics for the oxidation reactions are highly sensitive to the chemical composition, size of the catalyst particles, their architecture (alloys or core shell, facets etc.), and possibly catalyst-support interaction. A rational appplication of the principles and facts gained from the studies referred to above combined with colloidal synthesis techniques should result in improved catalysts for the oxidation of fuels such as methanol and formic acid.

A number of challenges remain for the implementation of such a programme. First of all, some of the results are not unambigous results, such as the effect of particle size, and conflicting reports have been published. Also, attempts at applying principles established from studies of smooth electrodes to nanostructured catalysts are not always straightforward. An example is given in the figure below, which displays chronoamperometric curves for oxidation of methanol for several core-shell catalysts synthesised at NTNU (FIGURE FROM José). As apparent from the figure, the catalyst activity depends on the thickness of the shell, which is not in line with recent theoretical estimates (REFERANSE FRA DE CHEN). Also, since these are synthesised according to protocols verified to give core-shell catalysts (REFERANSE UMD), any bi-functionaluty is absent and only the ligand effect should be operative. Yet, the perofrmance of the best catalyst formulation to a significant extent out-performs the commercial catalyst included in the curve.

These results allude to the fact that possible synergistic effects are difficult to account for, and this remains a central challenge in studies of these systems. As a consequence, reaction mechanisms and modes of catalyst enhancement may differ from catalyst to catalyst even if these are of a similar makeup, solely due to catalyst structure. It therefore becomes important to study the catalytic properties of the individual catalysts.

Also, in spite of the fact that quite much of the available literature appears to indicate the desirable aspects of increasing temperature beyond those usually employed (principally out of limitations associated with the Nafion electrolyte). Studies in a temperature range wider than this is therefore of immediate relevance, both fundamentally and in terms of applications. Due to the scarcity of such data even for smooth electrodes in the literature, such high-temperature catalyst studies should be supplemented by electrochemical measurements of model electrodes such as polycrystalline platinum.

As always with these types of structures, their verification remains a challenge in its own right.

Finally, verification of the performance in actual operation of the catalysts in a fuel cell is essential. In some cases catalysts of similar catalytic properties in aqeuous electrochemical cells have been shown to behave quite differently when used in catalytic layers in actual fuel cell operation (2 x Zawodzinski 2007).

Objectives

The main goal of the present project is to study mechanisms and rates of electrooxidation of small organic molecules on supported nano-structured electrocatalysts in the temperature range from room temperature through approximately 150 oC. Catalyst performance will be established with relation to electrocatalyst composition and structure.

Hypotheses and Methodology

With reference to the research challenges listed above, the most important subgoals of the project are to synthesise well-dispersed bimetallic catalysts of compositions based on Pt, Ru, Mo, Co, and Sn. Results from the literature on corresponding bulk electrodes as well as results generated internally will be used as a basis for comparison. In particular will the project seek to identify catalytic effects beyond those that would be expected on the basis of such results.

Electrochemical characterisation will include measurements in aqueous electrochemical cells (voltammetry and chronoamperometry, ring-disc measurements etc.), also at high temperature in an autoclave. A newly built test station will be employed for testing of the catalysts under fuel cell conditions at high temperature. The project will also establish and use differential electrochemical mass spectroscopy (DEMS) at the department (funding of equipment obtained elsewhere, purchase in progress). Use of flow cells is established in an interacting project on the oxidation of small organic molecules at high temperature (vide infra), and will also be used in the current project.

Synthesis (colloidal techniques) and characterisation (TEM, XRD, in-situ spectrocsopy, etc.) of catalysts will include techniques already established at the department or in collaboration with partners in Europe and the US..

As sample reactions we will employ the electropchemical oxidation of methanol, ethanol, and formic acid, supported with oxidation of adsorbed and dissolved carbon monoxide.

[FS1] J. Twidell, T. Weir, Renewable Energy Resources, E & FN SPON, London, 1998

[FS2] H. Wang, T. Löffler, H. Baltruschat, "Formation of intermediates during methanol oxidation: A quantitative DEMS study", J. Appl. Electrochem., 31 (2001) 759-765

[FS3] F. Seland, D.A. Harrington, R. Tunold, "Fast methanol oxidation on polycrystalline Pt", Electrochim. Acta, 52 (2006) 773-779

[FS4] J.K. Lee, J. Lee, J. Han, T.-H. Lim, Y.-E. Sung, Y. Tak, Electrochim. Acta, 53 (2008) 3474-3478

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