337
4.3 Ceria, Zirconia and Their Combinations Are
Useful Supports
Several recent studies have focussed on using ceria, zirconia or their combinations
for the dry reforming of methane. In this context, ceria has been shown to improve
coking resistance in comparison to Ni/Al 2 O 3 system [24, 25]. Ceria provides the
lattice oxygen that, through redox reactions, prevents the formation of carbon
species from the methane decomposition and Boudard reactions [24]. In the Ni/
CeO 2 system, the addition of zirconia improved the activity of the catalyst, which
was attributed to the higher surface density of active sites [26]. Zirconia also
improved the resistance to coking [26]. Interestingly, coking resistance was
attributed to the formation of filamentous carbon, which was not seen in pure CeO 2 -
supported catalyst [26]. On the other hand, addition of CeO 2 in Ni/ZrO 2 catalyst
reduced the formation of filamentous carbon, improving coking resistance without
affecting the activity [27]. Thus, it appears that zirconia improves both coking
resistance and activity, and ceria improves the resistance to coking. In contrast,
addition of CeO 2 in Ni/SiO 2 system improved both the activity and coking resistance
[28]. Further, the effect of ZrO 2 has been studied for two different catalysts (Ni/
Al 2 O 3 and Pt/Al 2 O 3 ) [29]. In both systems, adding zirconia reduced carbon
deposition, suggesting that zirconia induced gasification of carbon-containing
intermediates adsorbed in the catalyst [29].
4.4 Different Supports and Promoters Are Also Used
Different supports and trace additives may also affect carbon deposition on catalyst.
Nagaoka et al. studied ruthenium catalyst with different supports (SiO 2 , Al 2 O 3 ,
MgO, TiO 2 ) and found that the order of coke deposition is Al 2 O 3 > SiO 2 > MgO > TiO 2
with TiO 2 having non-detectable amounts of coke deposited [30]. Similarly, Alipour
et al. studied the effect of alkaline promoters (MgO, CaO and BaO) on coking in Ni/
Al 2 O 3 system. The authors found that the highest amount of carbon deposited was
that of filamentous carbon, and concluded that the amount of filamentous carbon
decreased with addition of alkaline promoters and was in the order
MgO > CaO > BaO, which was the same order as their decreasing basicity [31].
However, no reduction was observed in the case of amorphous carbon, except for
CaO, which gave the same amount of amorphous carbon as unpromoted Ni/Al 2 O 3
system [31]. Similarly, no reduction was observed in the case of active carbon
species, except in the case of MgO [31]. Thus, while filamentous carbon reduces in
the order of basicity of alkaline promoters, no clear pattern could be observed in
case of amorphous carbon and active carbon species and more studies are needed to
classify the effect of alkaline promoters on gasification of these carbonaceous
species.
Flue Gas Treatment via Dry Reforming of Methane
4.3 Ceria, Zirconia and Their Combinations Are
Useful Supports
Several recent studies have focussed on using ceria, zirconia or their combinations
for the dry reforming of methane. In this context, ceria has been shown to improve
coking resistance in comparison to Ni/Al 2 O 3 system [24, 25]. Ceria provides the
lattice oxygen that, through redox reactions, prevents the formation of carbon
species from the methane decomposition and Boudard reactions [24]. In the Ni/
CeO 2 system, the addition of zirconia improved the activity of the catalyst, which
was attributed to the higher surface density of active sites [26]. Zirconia also
improved the resistance to coking [26]. Interestingly, coking resistance was
attributed to the formation of filamentous carbon, which was not seen in pure CeO 2 -
supported catalyst [26]. On the other hand, addition of CeO 2 in Ni/ZrO 2 catalyst
reduced the formation of filamentous carbon, improving coking resistance without
affecting the activity [27]. Thus, it appears that zirconia improves both coking
resistance and activity, and ceria improves the resistance to coking. In contrast,
addition of CeO 2 in Ni/SiO 2 system improved both the activity and coking resistance
[28]. Further, the effect of ZrO 2 has been studied for two different catalysts (Ni/
Al 2 O 3 and Pt/Al 2 O 3 ) [29]. In both systems, adding zirconia reduced carbon
deposition, suggesting that zirconia induced gasification of carbon-containing
intermediates adsorbed in the catalyst [29].
4.4 Different Supports and Promoters Are Also Used
Different supports and trace additives may also affect carbon deposition on catalyst.
Nagaoka et al. studied ruthenium catalyst with different supports (SiO 2 , Al 2 O 3 ,
MgO, TiO 2 ) and found that the order of coke deposition is Al 2 O 3 > SiO 2 > MgO > TiO 2
with TiO 2 having non-detectable amounts of coke deposited [30]. Similarly, Alipour
et al. studied the effect of alkaline promoters (MgO, CaO and BaO) on coking in Ni/
Al 2 O 3 system. The authors found that the highest amount of carbon deposited was
that of filamentous carbon, and concluded that the amount of filamentous carbon
decreased with addition of alkaline promoters and was in the order
MgO > CaO > BaO, which was the same order as their decreasing basicity [31].
However, no reduction was observed in the case of amorphous carbon, except for
CaO, which gave the same amount of amorphous carbon as unpromoted Ni/Al 2 O 3
system [31]. Similarly, no reduction was observed in the case of active carbon
species, except in the case of MgO [31]. Thus, while filamentous carbon reduces in
the order of basicity of alkaline promoters, no clear pattern could be observed in
case of amorphous carbon and active carbon species and more studies are needed to
classify the effect of alkaline promoters on gasification of these carbonaceous
species.
Flue Gas Treatment via Dry Reforming of Methane