Catalyst degradation sintering

What are the principal fundamental mechanisms by which thermal degradation, sintering and redispersion of supported metals occur What factors determine which of these mechanisms predominates or controls the sintering process What do we leam from model catalyst studies regarding these mechanisms ... 

There are however concerns with durability or lifetime of the catalyst. There can be contaminants within the reformate, in particular sulfur compounds, that can render the catalyst completely inactive. In addition, if the reactor temperature operates too high or an unexpected over-temperature event occurs, the performance or durability of the catalyst can be seriously compromised. Additionally, the support of the catalyst can sinter over time on stream due to the presence of significant amounts of water. Fouling can be another issue, which impacts durability if very pure water or clean air is not used. While all these factors are present, catalyst formulations have been shown to operate for hundreds of hours with minimal to no degradation. 

Considerable effort has been put into the detailed examination of Pt catalyst degradation mechanisms during long-term operation. First, a pure Pt catalyst may be contaminated by impurities that originate from supply reactants or the fuel cell system. Second, the catalyst may lose its activity due to sintering or migration of Pt particles on the carbon support, detachment and dissolution of Pt into the electrolyte, and corrosion of the carbon support. Several mechanisms... 

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

The low melting point and high surface mobility of NiS also accelerate the sintering process of Ni crystallites. Since the formation of NiS is exothermic, activity loss can be partially recovered by raising the reaction temperature, which, however, also accelerates thermal degradation of the catalyst and increases carbon formation through cracking reactions. 

Thermal Degradation Catalyst sintering can occur at flue gas temperatures > 800°F. This will result in the pore distribution shifting to larger pores. The loss of small pores will generally not have a large effect on activity since diffusion is not a critical parameter. The majority of conversion occurs on the exterior surface of the catalyst. 

Continuous exposure of catalysts to high temperatures may cause an alteration in its components and gradually lead to its deactivation. Thermal degradation may have an undesirable impact on both the catalyst substrate and noble metal load in various ways. Thermal degradation covers two phenomena sintering and solid-state transformation. 

Sintering is an important mode of deactivation in supported metals. The high surface area support (carrier or substrate) in these catalysts serves several functions (l) to increase the dispersion and utilization of the catalytic metal phase, (2) to physically separate metal crystallites and to bind them to its surface, thereby enhancing their thermal stability towards agglomeration, and (3) in some cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

Second, during regeneration these metals oxidize and act as oxidation catalysts, leading to excessive combustion rates and sintering. Especially bad is V,Oi, not only because it is a strong oxidation agent but also because it melts and forms a flux to accelerate particle degradation. 

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