421
CeO 2 -Al 2 O 3 , MgO-Al 2 O 3, etc. Here, the oxide catalysts mentioned are majorly used as
supports. Supports are thermally more stable and provide high surface area. The metal
particles are doped over the support to achieve high mechanical and thermal strength;
however, it comes at the cost of reduced metal dispersion and stability. Therefore, the
functionality of the catalyst also depends on the interaction mechanism between the
metal and the support [21]. Various supports are proposed by researchers depending
on their respective properties. For example, Frusteri et al. [22] postulated that CeO 2 or
MgO supports should not help the coke formation by ethylene production as dehydrogenation to acetaldehyde or dehydration to ethylene occurs depending on the nature
of support. It was also reported that if support has a high basic sites, and high electronic polarizability, it favors dehydrogenation over dehydration.
A few significant features of these supports can be mentioned as the following:
(a) ϒ-Al 2 O 3 has a high surface area and is widely used due to its thermal and mechanical stability. However, the acidic property of ϒ-Al 2 O 3 favors dehydration of alcohol to alkene which acts as a precursor to form coke on the catalyst surface. This
is completely undesirable and is more harmful to membrane reformer as coke can
be deposited on the membranes. This can affect the performance and life of the
membrane and hence the overall efficiency of the membrane reformer. To attenuate coking on the catalyst surface incorporation of various alkali metals such as K,
Li Ca, and Na is reported [23]. These basic sites not only act to eliminate acidity
but also favors the carbon oxidation and reduction in coke deposition by enhancing water adsorption and –OH mobility on the surface [24].
(b) Elias et al. [25] reported an increase in H 2 selectivity for ethanol steam reforming with increasing Ca content which leads to decreasing alkenes (like C 2 H 4 )
selectivity and thereby coking. Ca also enhances the stability of the catalysts
against the oxidation process. However, the decrease in acidity was more prominent for impregnation-based catalysts.
(c) MgO, ZnO, and CeO 2 too have basic properties that minimize the coke formation
on the surface of the catalyst and hence preferred for membrane reformer applications. Llorca et al. [26] proposed that increasing the basicity of the system might
also increase the selectivity of acetaldehyde for ethanol steam reforming. Amongst
the supports mentioned above, ZnO was reported to be one of the best oxide supports for the Co-based catalyst used for ethanol steam reforming.
(d) The size of cerium oxide crystallites plays a determinant role in increasing the
interaction with the noble metals. It promotes CO oxidation and water gas shift
(WGS) reaction and promotes the stability of the catalyst due to its higher
reducibility and oxygen storage-release capacity [17]. Addition of zirconium
enhances the redox property and oxygen adsorption capacity of the CeO 2 [27].
Srinivas et al. [28] have reported that even after several successive cycles of
reduction and re-oxidation, the mixed oxides of ceria-zirconia was thermally
stable. Such catalysts are preferred; however, the re-oxidation step in the case
of membrane reformer is quite challenging.
(e) Y 2 O 3 and La 2 O 3 have weak acidic and basic properties and exhibit long-term
stability to the process.
Steam Reforming Catalysts for Membrane Reformer
CeO 2 -Al 2 O 3 , MgO-Al 2 O 3, etc. Here, the oxide catalysts mentioned are majorly used as
supports. Supports are thermally more stable and provide high surface area. The metal
particles are doped over the support to achieve high mechanical and thermal strength;
however, it comes at the cost of reduced metal dispersion and stability. Therefore, the
functionality of the catalyst also depends on the interaction mechanism between the
metal and the support [21]. Various supports are proposed by researchers depending
on their respective properties. For example, Frusteri et al. [22] postulated that CeO 2 or
MgO supports should not help the coke formation by ethylene production as dehydrogenation to acetaldehyde or dehydration to ethylene occurs depending on the nature
of support. It was also reported that if support has a high basic sites, and high electronic polarizability, it favors dehydrogenation over dehydration.
A few significant features of these supports can be mentioned as the following:
(a) ϒ-Al 2 O 3 has a high surface area and is widely used due to its thermal and mechanical stability. However, the acidic property of ϒ-Al 2 O 3 favors dehydration of alcohol to alkene which acts as a precursor to form coke on the catalyst surface. This
is completely undesirable and is more harmful to membrane reformer as coke can
be deposited on the membranes. This can affect the performance and life of the
membrane and hence the overall efficiency of the membrane reformer. To attenuate coking on the catalyst surface incorporation of various alkali metals such as K,
Li Ca, and Na is reported [23]. These basic sites not only act to eliminate acidity
but also favors the carbon oxidation and reduction in coke deposition by enhancing water adsorption and –OH mobility on the surface [24].
(b) Elias et al. [25] reported an increase in H 2 selectivity for ethanol steam reforming with increasing Ca content which leads to decreasing alkenes (like C 2 H 4 )
selectivity and thereby coking. Ca also enhances the stability of the catalysts
against the oxidation process. However, the decrease in acidity was more prominent for impregnation-based catalysts.
(c) MgO, ZnO, and CeO 2 too have basic properties that minimize the coke formation
on the surface of the catalyst and hence preferred for membrane reformer applications. Llorca et al. [26] proposed that increasing the basicity of the system might
also increase the selectivity of acetaldehyde for ethanol steam reforming. Amongst
the supports mentioned above, ZnO was reported to be one of the best oxide supports for the Co-based catalyst used for ethanol steam reforming.
(d) The size of cerium oxide crystallites plays a determinant role in increasing the
interaction with the noble metals. It promotes CO oxidation and water gas shift
(WGS) reaction and promotes the stability of the catalyst due to its higher
reducibility and oxygen storage-release capacity [17]. Addition of zirconium
enhances the redox property and oxygen adsorption capacity of the CeO 2 [27].
Srinivas et al. [28] have reported that even after several successive cycles of
reduction and re-oxidation, the mixed oxides of ceria-zirconia was thermally
stable. Such catalysts are preferred; however, the re-oxidation step in the case
of membrane reformer is quite challenging.
(e) Y 2 O 3 and La 2 O 3 have weak acidic and basic properties and exhibit long-term
stability to the process.
Steam Reforming Catalysts for Membrane Reformer