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"A catalyst accelerates a chemical reaction without affecting the position of the equilibrium", a definition of catalyst by Ostwald in 1895 which still rings true today. Many catalytic reactions were already in used in olden times such as the production of wine and beer, although the underlying principles of catalysis were not understood at the time.
It was not till the 1970s that catalysis has made tremendous progress, especially in the development in environmental protection, such as the catalytic converter for automobiles (Hagen, 2006) and more recently the PEM fuel cell applications. Besides accelerating reactions, catalysts also influence the selectivity of chemical reactions, which is often the more important property in certain industries.
Tungsten Carbide Catalyst
Tungsten carbide, a transition metal carbide, has been reported to possess specific activities and selectivity for hydrocarbon conversion reactions that are similar to those that are currently available commercial catalysts. Replacing Platinum and other precious metals in the electrocatalyst formulations would represent a major advancement towards commercialisation of fuel cells and could assure the sustainability of industrial-scale production for a variety of applications (Zhang, 2008).
While many studies have focused on tungsten carbide as a supplementary catalyst to enhance the performance of platinum catalyst, this report is focused on tungsten carbide as the key catalyst in any chemical reactions. The preparation of tungsten carbide in its various phases, its support, and its characterisation will be discussed. The application of tungsten carbide catalyst and its deactivation will also be covered within the scope of this report.
Preparation of Tungsten Carbide
Carbides of Group IV-VI transition metals have been studied extensively as they show catalytic behaviour similar to those of platinum group metals. Amongst those, tungsten carbide has been the promising catalyst due to its stability in acid solutions at anodic potentials, as well as its resistance to CO poisoning. There are three known crystalline phase of tungsten carbide, namely Î²-W2C, Î±-WC and Î²-WC1-x (Ganesan & Lee, 2005).
Direct Carburisation Method
Typically, WC were synthesised by reacting WO3 particles with flowing NH3, or H2S, or C2H5/H2 mixtures. To produce Î±-WC, quartz synthesis tube reactor is used for carburisation at a 20:80 wt% mixture of CH4 and H2. As for preparation of Î²-W2C, 10% of C2H5 in H2 is used. Carburisation occurs at temperature between 900 to 1173 K and holding its temperature for an hour (Hara, 2007).
Incipient Wetness Method
W2C can be synthesised by heating a liquid mixture of resorcinol-formaldehyde polymer (a carbon precursor) and ammonium metatungstate salt, (NH4)6H2W12O40âˆ™xH2O (a tungsten precursor) under reflux at 367 K for 24 hours. The resulting sol-gel was dried at room temperature and reheated to 1173K with argon flow followed by hydrogen flow to remove all free carbon deposited on the W2C catalyst (Ganesan & Lee, 2005).
If WO3 is used as a starting material, it has been found that the WC obtained would have a different crystalline phase which is dependent on the reaction conditions and the synthetic pathways (Hara, 2007):
WO3 + C2H6/H2 â†’ Î²-W2C (hexagonal close-packed) (Eq. 1)
WO3 + CH4/H2 â†’ Î±-WC (hexagonal) (Eq. 2)
WO3 + NH3 â†’ W2N (Eq. 3)
W2N + CH4/H2 â†’ Î²-WC1-x (face-centered cubic) (Eq. 4)
Î²-WC1-x + CH4/H2 â†’ Î±-WC (hexagonal) (Eq. 5)
Figure : Reaction Pathway of Î±-WC synthesis
As shown in , other than using WO3, Î±-WC can also be synthesised through a novel pathway from tungsten phosphide (WP) precursor through the following reaction (Hara, 2007):
WP + CH4/H2 â†’ Î±-WC (Eq. 6)
Preparation of Catalyst Support
Support for catalyst can be simile as the vehicle for the active catalyst. The main function of support is to maximise surface area of the active phase (catalyst) by providing a large area over which catalyst may be distributed throughout the support. In general the support should be inert to the reaction; however it can also participate in the total reaction when combined with the active phase (Moulijn, 1993). The most common supports for tungsten carbide are activated carbon (in fuel cell application) and alumina (in high end applications such as hydrazine decomposition in micro-thrusters).
Carbon support is the most suitable support for tungsten carbide as PEM fuel cell catalyst as it has high surface area and good stability in liquid media. Catalyst can also be reclaimed easily by combusting the carbon (Gaigneaux, 2010).
The starting material for support preparation is generally from carbon black. Carbon black would be thermally treated at 1273 K for an hour under helium flow. The support would then be subjected to an oxidation treatment with hydrogen peroxide to create surface oxygen group by immersing the support in an aqueous solution of H2O2. The oxidised support would be washed with deionised water to eliminate excess H2O2, and dried under air at 383 K (Rodriguez-Reinoso, 1998). The end product of carbon support give rise to a total surface area of approximately 635 m2 g-1.
Ï’-Al2O3 and È - Al2O3 are the important compounds as catalyst support as they exhibit high thermal stability and high surface area (15-300 m2/g). They can be prepared starting from an aqueous acidic Al+3 solutions by basic titration. At pH above 3, gel-like substance with minute crystals of boehmite (AlO(OH)) begins to form. This microcrystalline boehmite gel slurry is aged at 40oC to form bayerite (crystalline form of Al(OH)3). The final product È - Al2O3 is formed after bayerite is being filtered, dried and calcined (Moulijn, 1993).
Ï’-Al2O3 can be formed by further aging of bayerite at 80oC in pH 8. The intermediate product of this aging process is crystalline boehmite, which can be converted to Ï’-Al2O3 and Î´-Al2O3 by filtering and calcinating crystalline boehmite. The precipitation process can also be done starting from a basic AlO45- solution by the addition of acid. Gel-like bayerite occurs at pH below 11 (Moulijn, 1993).
It is important to characterise the catalyst being produced as better understanding of the catalyst fundamental properties would allows the improvements of future catalyst productions. There are five main objectives of catalyst characterisation (Fishwick, 2010):
Activity structure relationship: to understand and better design of catalyst with optimum performance based on its activity, selectivity and stability in any given application.
Understanding catalyst deactivation: to understand the cause of deactivation, thus identifying catalyst regeneration process and/or selection of catalyst.
Data for reactor and process design: Provides physical properties of catalyst to allow process optimisation and reactor design.
Quality control: Monitor changes in the physical and chemical properties of catalyst during its life-cycle to ensure quality of reaction products.
Fundamental mechanistic understanding: Information on catalyst would allow the development of more robust and predictable catalyst.
X-ray Diffraction (XRD)
X-ray Diffraction (XRD) can be used to determine the structure of carbides with different crystal structure. As shown in below, the peaks of the microsphere correspond to W2C as the major phase and to WC and WC1-x as the minor phase. The formation of the crystal structure is very sensitive to the heating rate, temperature, as well as amount of carbon present. There were no XRD peaks corresponding to tungsten trioxide, carbon and metallic tungsten as they are present as an amorphous phase (Ganesan & Lee, 2005).
Figure : XRD pattern of tungsten carbide in different phase
Transmission Electron Microscopy (TEM)
Transmission Electron Microscopy (TEM) can also be used to further examine the crystal size of WCx. In unison with the XRD results, the dispersion of the tungsten carbide follows the order of: WCx/Al2O3-CH > WCx/AC-CH > WCx/AC-H (Sun, et al., 2008).
BET theory can be used to determine the surface area of W2C, which is approximately 176 m2 g-1 (compared to 635 m2 g-1 for carbon microspheres). The CO uptake value, which indicates the number of CO molecules taken up by the tungsten atom at the active site of the catalyst, can be experimentally measured and calculated using temperature-programmed-desorption (TPD) which is approximately (956 mmolg-1) (Ganesan & Lee, 2005).
High Resolution Transmission Electron Microscope (HRTEM)
shows the microscopic images of carbon and W2C, which shows uniform morphology with diameters 2-4 Î¼m. The carbon microspheres appear to act as a template for the formation of W2C microspheres. The W2C microspheres were also observed under a high-resolution transmission electron microscope (HRTEM).
Figure : a) SEM image of carbon microsphere b) SEM image of W2C microsphere c) HRTEM image of a W2C microsphere d) HRTEM image of the surface of the W2C microsphere
As shown in d, the surface roughness gives rise to the high surface area of 176 m2g-1 found using BET in Section . HRTEM imaging also confirms the crystal size of W2C to be approximately 12 nm by applying the Debye-Scherrer equation to the XRD peaks as described in Section (Ganesan & Lee, 2005).
X-Ray Photoelectron Spectroscopy (XPS)
It is understood that the amount of free carbon deposited on the catalyst would greatly affect the electroactivity of the catalyst due to the blockage of active sites by free carbon. On the other hand, tungsten carbides obtained from different precursors (as described in Section ) would greatly affect the amount of free carbon. XPS can be used to determine these differences by examining the C 1s spectra in Î±-WC obtained from WO3, W2N and WS2 precursors (Hara, 2007).
As described by Hara (2007), the binding energy for the C 1s spectra for metal carbide and graphite-like carbide varies between 283.3 eV and 284.9 eV. The binding energy for other carbon species, such as the amorphous carbon is around 286 - 295 eV. These chemical shifts in binding energy differentiate carbide carbon from other free carbons.
Figure : XPS Spectra for C 1s peak in Î±-WC prepared from a) WO3 b) W2N c) WS2 precursors
shows that Î±-WC prepared by direct carburisation (peak a) is covered by up to 80% free carbon, thus explained the inactivity of tungsten carbide in DMFC conditions. In contrast, Î±-WC prepared from W2N and WS2 (peak b and c) have about 36 - 49% of carbide carbon exposed on the surface. The free carbon coverage is greatly influenced by the carburisation conditions (Hara, 2007).
Butane dehydrogenation activity measurements can be carried out in U-tube reactor. The catalyst were first pre-treated with H2 flow at 753 K for 3 hours, activities measurements were carried out using 4% C4H10 with He balance, or 4% C4H10, 36% He with H2 balance. Activity measurements can then be made after 4 to 6 hours on steam at temperature between 623 and 723 K. The products can then be separated and analysed using gas-chromatography (GC) (Thompson & Curry, 1994).
In general, the activity of the catalyst increases with increasing surface area. In the presence of H2, the activity of tungsten carbide is much higher than that of without H2. This phenomenon is again being attributed to the deposition of coke (free carbon) is prohibited in the presence of H2, thus leading to a much higher active surface area. The average apparent activation energy of tungsten carbide is found to be 23Â±5 kcal/mol, comparable to that of the Pt-Sn catalyst (28Â±2 kcal/mol) (Thompson & Curry, 1994).
Industrial Examples of Catalyst Use
Tungsten carbide is currently used in combustion power plant as a replacement for TiO2WO3V2O5 ceramics for the removal of nitrogen oxides (NOx) by selective catalytic reduction with ammonia or urea, producing harmless nitrogen and water vapour (ITIA, 2005).
Tungsten is also a catalyst for hydro-cracking, hydro-desulphurisation and hydro-denitrification of mineral oil product to increase the yield of gasoline and other light hydrocarbon in crude oil processing. The catalyst also reduces environmental risks by reducing the contents of aromatic hydrocarbons, sulphur and nitrogen compounds (ITIA, 2005).
Currently, extensive research have been attempted to identify cheaper substitutes for Platinum-based catalyst in PEM fuel cell applications. Amongst all the transition metals, tungsten carbides have been the most extensively studied as an electrocatalyst for the Hydrogen Oxidation Reduction (HOR) process in PEM fuel cell applications because of its Pt-like catalytic behaviour, its stability in acid solutions at anodic potentials as well as its resistance to CO poisoning (Zhang, 2008). Although tungsten carbide has been reported as the only carbide that shows low electrocatalytic activity under DMFC condition, it is explained that it may be caused by low specific surface area as tungsten carbide is being prepared using traditional methods. New preparations of tungsten carbide have shown great improvements on its catalytic activities (Hara, 2007).
Tungsten carbide with activated carbon support has also been reported as potential substitute for the Iridium-based catalyst for hydrazine decomposition in space technology (such as the micro-thrusters). The formation of well crystallised W2C phase, the restraining of the carbon deposition and the prohibition of methanation are attributed to the high activity and stability of the catalyst (Sun, et al., 2008).
There has been a data inconsistency between Regalbuto (2007) and Kolasa et. al. (2006) about the catalytic activity of metal carbides in hydrogenation processes. In comparing tungsten carbide and molybdenum carbide, Kolasa and colleagues claim that tungsten carbide possesses the strongest hydrogenation and isomerisation properties. In contrast, Regalbuto stated that tungsten carbide showed little catalytic activity and molybdenum carbide has the most promising catalytic properties. This difference is probably due to the different preparation methods of carbides used in their studies respectively. The inactivity of tungsten carbide as expressed by Regalbuto (2007) may be explained by the free carbon coverage phenomena as illustrated in Section .
In application of tungsten catalyst as shown in Section such as those used in gas fired plants, catalyst may go through deactivation mainly caused by the loss of surface area for sintering at constant high temperature conditions. Other source of deactivation may also include poisoning of catalyst by heavy metals or plugging of catalyst pores. However, a study by Nova et. al. (2001) shows that thermal sintering of NOx removal catalysts has a negligible effect on its efficiency. The overall decrease of the catalyst surface activity is counterbalanced by an increase of the rate of intra-porous diffusion due to the enlargement of pores (Nova, Lietti, Beretta, & Forzatti, 2001).
Currently there has not been any published study on the deactivation of tungsten carbide as catalyst in PEM fuel cell application.
It is evident that the activity of tungsten carbide as catalyst is highly dependent on its surface area, which in turn is highly dependent on its method of preparation.