High-temperature WGSR

TABLE 9.1 Proposed Reaction Mechanisms for High-Temperature WGSR... 

Mechanism of the High-Temperature WGSR Over Iron Oxide Catalysts Using Characterization Techniques... 

One of the first expressions for high-temperature WGSR was proposed by Kulkova and Temkin [2a] and it was based on the regenerative mechanism... 

Most of the studies show that some of the kinetic expressions valid for high-temperature WGSR are also valid for low-temperature WGSR. Shchibiya et al. found a kinetic expression similar to high-temperature kinetic equation based on Cu0/Zn0/Cr203 [2b]. Later Uchida et al. [49] confirmed that the kinetic expression derived from redox mechanism accurately correlated the kinetic data obtained with the Cu-Zn catalysts. Cherednik et al. [50] proposed other equation for a commercial catalyst (2u0/Zn0/Cr203... 

The aim of the next topacs is to develop this set of equations to model an industrial high temperature WGSR. 

The thermodynamic analysis of loannides [193] on SRE in a solid polymer fuel cell indicated that the ethanol steam reforming reaction needs to be carried out in two steps a high-temperature endothermic step (steam reforming), in which ethanol is converted to a gaseous mixtures of H2, CO, CO2, CH4 and unreacted H2O, and a subsequent, low-temperature step (WGSR) in which CO reacts with water to form H2 and CO2. 

Notably, since high-temperature steam reforming enhances the r-WGSR, which would produce CO, the undesirable poison of fuel cells, low-temperature reforming is preferable. Low temperatures can be achieved over strong acid catalysts, although the strong acid at the same time tends to cause deactivation by coke formation. 

The thermodynamics of the WGSR are well known that at high temperatures the CO conversion is equilibrium limited and at low temperatures the reaction is kinetically limited. The Cu-based catalysts are not suitable active catalysts for single-stage WGS applications. For this reason industries first operate at higher temperatures to have 95-97% CO conversion, and then the remaining... 

In recent studies about new formulations of the high-temperature catalysts, diffuse reflectance infrared Fourier transform spectroscopy was used to elucidate the reaction mechanism. The results showed that the catalyst surface can provide oxygen to CO even in the absence of water. When the surface comes in contact with water, the surface of the catalyst recovers oxygen. The presence of intermediate species on the catalyst surface was not detected. This suggested that the WGSR over Fe based catalysts proceeds by redox mechanism. 

According to the reaction temperature (Chen et al, 2008), the WGSR falls into two categories high-temperature shift catalyst (HTC) and low-temperature shift catalyst (LTC). The catalyst commonly used in the former is an iron-chromium-based catalyst, whereas a copper-zinc-based catalyst is frequently adopted in the latter. 

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