422
Along with supports, the preparation method also affects the catalyst performance based on the difference in surface area. The two widely applied techniques
for catalyst preparation include co-precipitation and impregnation [29].
Co-precipitation provides high metal dispersion with high attainable metal loading
up to 60–80%, because of which catalysts prepared by this method are also termed
as bulk catalysts. However, this process creates a lot of problems with its supersaturation conditions above which the product is to be separated from the large
quantities of waste salt generation. Impregnation, on the other hand, is a widely
applied process not only on a lab scale but also industrially. In impregnation, liquid
phase gets adsorbed on the solid phase. However, the contacting of liquid and solid
phase is critical to achieve uniform metal dispersion. Dry incipient wetness refers to
the impregnation of the previously dried support with a solution containing a precursor of the active phase, i.e., V=V pore . The solution gets sucked in the pores by
capillary action and no excess solution is left in case of the proper wetness, henceforth called dry or incipient wetness [30].
Either prepared by impregnation or co-precipitation, the specific surface area
values for co-precipitation are proposed to be higher than impregnation due to thermal treatment which releases the gases because of the decomposition of the precursor and provides high catalyst porosity [25]. Besides, co-precipitation is a fast
precipitation process occurring in super-saturation conditions. Faster nucleation
than crystal growth results in an amorphous precipitate with a large disorder prompting a higher surface area. However, reduced acidity of the system was reported for
the catalysts synthesized through impregnation method. Active metals, be it noble
or non-noble, are normally chosen based on the reaction as well as the basic inherent property it must possess for H 2 generation. These properties comprise C–C bond
cleavage, its stability, and activity in the desired reaction conditions, and its resistance to coke, generated during the reaction [21]. Ni shows high selectivity to
hydrogen generation. The low cost of Ni and has high C–C bond cleavage activity
makes it more attractive. Haryanto et al. [23] compared Ni/ϒ-Al 2 O 3 , Ni/La 2 O 3 , Ni/
La 2 O 3 -Al 2 O 3 , Ni/MgO, and found La 2 O 3 -Al 2 O 3 as the best support. Sun et al. [31]
proposed that “La” promotes dehydrogenation of alcohols, thereby reducing the
formation of alkenes. The order of H 2 selectivity and activity of Ni by Sun et al. [31]
was reported as Ni/La 2 O 3 > Ni/Y 2 O 3 > Ni/Al 2 O 3 for ethanol steam reforming. Rh
and Co-based catalysts are also proposed to be promising in terms of activity and
selectivity. Likewise, Pt, Ni, NiPt/Al 2 O 3 were examined by Sanchez et al. [24] and
suggested the reaction pathway over these catalysts by ethanol dehydrogenation and
subsequent acetaldehyde decomposition. A complete conversion during 45 h on
stream was also reported by Cai et al. [32] using Ir/Ce700 with 59% H 2 , 19% CO 2 ,
17%CO, and 5%CH 4 selectivity. Cavallaro and Freni [33] illustrated the occurrence
of intermediates such as acetic acid, ethyl acetate, and acetaldehyde below 325 °C
when selectivity of H 2 and CO 2 is very low at 1 atm pressure; water: ethanol 6:1 &
10:1 using catalysts CuO/ZnO/Al 2 O 3 , NiO/CuO/SiO 2 , Pt/Al 2 O 3 , etc. It was also
identified that selectivity toward H 2 , CO, and CO 2 increases with an increase in
temperature and that all catalysts shift the system toward equilibrium above 360 °C
[34]. Aupretre et al. [35] tested a wide range of active metals (Rh, Pt, Pd, Ru, Ni, Cu,
R. K. Upadhyay
Along with supports, the preparation method also affects the catalyst performance based on the difference in surface area. The two widely applied techniques
for catalyst preparation include co-precipitation and impregnation [29].
Co-precipitation provides high metal dispersion with high attainable metal loading
up to 60–80%, because of which catalysts prepared by this method are also termed
as bulk catalysts. However, this process creates a lot of problems with its supersaturation conditions above which the product is to be separated from the large
quantities of waste salt generation. Impregnation, on the other hand, is a widely
applied process not only on a lab scale but also industrially. In impregnation, liquid
phase gets adsorbed on the solid phase. However, the contacting of liquid and solid
phase is critical to achieve uniform metal dispersion. Dry incipient wetness refers to
the impregnation of the previously dried support with a solution containing a precursor of the active phase, i.e., V=V pore . The solution gets sucked in the pores by
capillary action and no excess solution is left in case of the proper wetness, henceforth called dry or incipient wetness [30].
Either prepared by impregnation or co-precipitation, the specific surface area
values for co-precipitation are proposed to be higher than impregnation due to thermal treatment which releases the gases because of the decomposition of the precursor and provides high catalyst porosity [25]. Besides, co-precipitation is a fast
precipitation process occurring in super-saturation conditions. Faster nucleation
than crystal growth results in an amorphous precipitate with a large disorder prompting a higher surface area. However, reduced acidity of the system was reported for
the catalysts synthesized through impregnation method. Active metals, be it noble
or non-noble, are normally chosen based on the reaction as well as the basic inherent property it must possess for H 2 generation. These properties comprise C–C bond
cleavage, its stability, and activity in the desired reaction conditions, and its resistance to coke, generated during the reaction [21]. Ni shows high selectivity to
hydrogen generation. The low cost of Ni and has high C–C bond cleavage activity
makes it more attractive. Haryanto et al. [23] compared Ni/ϒ-Al 2 O 3 , Ni/La 2 O 3 , Ni/
La 2 O 3 -Al 2 O 3 , Ni/MgO, and found La 2 O 3 -Al 2 O 3 as the best support. Sun et al. [31]
proposed that “La” promotes dehydrogenation of alcohols, thereby reducing the
formation of alkenes. The order of H 2 selectivity and activity of Ni by Sun et al. [31]
was reported as Ni/La 2 O 3 > Ni/Y 2 O 3 > Ni/Al 2 O 3 for ethanol steam reforming. Rh
and Co-based catalysts are also proposed to be promising in terms of activity and
selectivity. Likewise, Pt, Ni, NiPt/Al 2 O 3 were examined by Sanchez et al. [24] and
suggested the reaction pathway over these catalysts by ethanol dehydrogenation and
subsequent acetaldehyde decomposition. A complete conversion during 45 h on
stream was also reported by Cai et al. [32] using Ir/Ce700 with 59% H 2 , 19% CO 2 ,
17%CO, and 5%CH 4 selectivity. Cavallaro and Freni [33] illustrated the occurrence
of intermediates such as acetic acid, ethyl acetate, and acetaldehyde below 325 °C
when selectivity of H 2 and CO 2 is very low at 1 atm pressure; water: ethanol 6:1 &
10:1 using catalysts CuO/ZnO/Al 2 O 3 , NiO/CuO/SiO 2 , Pt/Al 2 O 3 , etc. It was also
identified that selectivity toward H 2 , CO, and CO 2 increases with an increase in
temperature and that all catalysts shift the system toward equilibrium above 360 °C
[34]. Aupretre et al. [35] tested a wide range of active metals (Rh, Pt, Pd, Ru, Ni, Cu,
R. K. Upadhyay