Activity sintering, effect

Phenomena that arise in these materials include conduction processes, mass transport by convection, potential field effects, electron or ion disorder, ion exchange, adsorption, interfacial and colloidal activity, sintering, dendrite growth, wetting, membrane transport, passivity, electrocatalysis, electrokinetic forces, bubble evolution, gaseous discharge (plasma) effects, and many others. 

Wang X, Casolco SR, Xu G, Garay JE (2007) Finite element modeling of electric current-activated sintering the effect of coupled electrical potential, temperature and stress. Acta Mater 55 3611-3622... 

Precious metals such as platinum, palladium, rhodium and ruthenium on ceria supports are reported in the literature as formulations for a water-gas shift [164]. Certainly the most prominent formulation is platinum/ceria, which has the drawback of a particular activity towards methane formation [164] at temperatures exceeding 375-425 °C, depending on the catalyst formulation. Its long-term stability is usually limited to a maximum temperature of between 425 and 450 °C due to the sintering effects of platinum. 

Gene, A., Coskun, S., Ovecoglu, M. L. (2010). Decarburization ofTiC in Ni activated sintered W-jcTiC (jc = 0,5,10,15 wt%) composites and the effects of heat treatment on the microstructural and physical properties. International Journal of Refractory Metals and Hard Materials, 28(3), 451 58. doi 10.1016/j.ijrmhm.2010.02.004. 

M., Hirai, T. (2000). Effect of plasma activated sintering (FAS) parameters on densification of copper powder. Materials Research Bulletin, 35(4), 619-628. doi 10.1016/ S0025-5408(00)00246-4. 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

A TWC catalyst must be able to partition enough CO present in the exhaust for each of these reactions and provide a surface that has preference for NO adsorption. Rhodium has a slight preference for NO adsorption rather than O2 adsorption Pt prefers O2. Rh also does not cataly2e the unwanted NH reaction as does Pt, and Rh is more sinter-resistant than Pt (6). However, the concentrations of O2 and NO have to be balanced for the preferred maximum reduction of NO and oxidation of CO. This occurs at approximately the stoichiometric point with just enough oxidants (O2 and NO ) and reductants (CO, HC, and H2). If the mixture is too rich there is not enough O2 and no matter how active the catalyst, some CO and HC is not converted. If the mixture is too lean, there is too much O2 and the NO caimot effectively compete for the catalyst sites (53—58). 

Ashby pointed out diat die sintering studies of copper particles of radius 3-15 microns showed clearly the effects of surface diffusion, and die activation energy for surface diffusion is close to the activation energy for volume diffusion, and hence it is not necessarily the volume diffusion process which predominates as a sintering mechanism at temperatures less than 800°C. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

On the other hand, when the reaction temperature was increased fijrther to 400°C, the reactivity of the absorbent significantly dropped. It was previously reported that for absorbent prepared from coal fly ash, when the absorbent was dried at temperature above 400°C, the reactivity of the absorbent dropped due to the decomposition of the active materials in the absorbent [8]. Since the effect of drying the absorbent above 400°C is similar to exposing the absorbent to reaction temperature above 400°C, therefore it can be concluded that the active materials in absorbent prepared from oil palm ash also decompose at reaction temperature above 400°C resulting in lower reactivity. Apart from that, another possible explanation for the drop in the reactivity of the absorbent at 400°C could be due to the sintering of the absorbent that decreases the surface area of the absorbent. 

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