190
ported on cement clinker (Ni-CC). Cement clinker composed of a mixture of CaO
and SiO has been reported to release CO 2 . Thus, the main reason of utilizing the
particular support is to reduce the effect of catalyst deactivation due to carbon deposition. Whisker carbon was detected on the spent Ni-CC catalyst after the dry
reforming of glycerol, yet the carbon is easily gasified by O 2 oxidation (Lee et al.
2014a, b). Later from the same group, Siew et al. (2014, 2015a, b) employed
La-promoted Ni−Al 2 O 3 catalyst for glycerol dry reforming at 650–850 °C. It is suggested that La promotion has successfully improved carbon resistance in Ni − Al 2 O 3
catalyst as La offers redox properties which provides additional route for carbon
removal, provides better Ni metal dispersion, and reduces the acidity of the Ni catalyst. In contrary to the earlier thermodynamic analysis, the group proposed that the
high CO 2 -to-glycerol ratio is essential for carbon mitigation as CO 2 effectively acts
as a gasification agent in carbon gasification pathway.
Similarly, in another study using Ni catalyst supported on Al 2 O 3 of a nanocrystalline structure by Tavanard et al. (2018), the increase of CO 2 content has improved
the occurrence of reverse Boudouard reaction that allows more for gasification of
carbonaceous species on the catalyst surface. Additionally, in a recent work using
Ni − Al 2 O 3 catalyst, Ag has been employed as a promoter in glycerol dry reforming.
The promoted catalyst has given better glycerol conversion due to the ability of Ag
to enhance the dispersion on Ni particles on the Al 2 O 3 support surface. Two types of
carbon were observed after 72 h of longevity study, encapsulated and whisker-type
carbons, yet they were easily removed through oxidation during TPO analysis at
temperatures below 680 °C (Harun et al. 2019).
Dry reforming of glycerol also has been conducted on Ni catalyst supported on
ZrO 2 , CaO, and La 2 O 3 supports (Mohd Arif et al. 2017). Ni−CaO with 15% Ni provides the highest hydrogen yield and glycerol conversion as the catalyst shows a
higher metal dispersion and smaller crystallite size of NiO species compared to that
with other catalysts (Ni−ZrO 2 and Ni−La 2 O 3 ). CaO support has been suggested to
reduce the accumulation of carbon on the catalyst surface due to its basic site properties. Basic functional group on support is beneficial for CO 2 adsorption and thus
assists in gasification of carbon on the catalyst surface. Further, the group has incorporated Re as a promoter on the same catalyst (Ni-CaO), and the addition of Re has
been anticipated to enhance the surface adsorption of OH group of the glycerol
(Mohd Arif et al. 2018). The addition of Re also has contributed to the lesser amount
of carbon on the Ni-CaO catalyst during glycerol dry reforming.
Besides the Ni-based catalysts, noble catalysts such as Rh, Ru, Ir, Pd, and Pt supported on alumina-stabilized alumina (MgAl 2 O 4 ) also have been investigated for
glycerol dry reforming (Tavanarad et al. 2017). Among the catalysts, Rh − MgAl 2 O 4
has displayed the highest glycerol conversion and stability for 20 h of operation.
The authors attributed the decent activity of Rh catalyst to its high BET surface area
and high active metal area. Similar to previous work using Ni − Al 2 O 3 , the presence
of whisker carbon was also observed on the spent Rh − MgAl 2 O 4 . Additionally, the
excess of CO 2 also has suppressed the carbon formation on the noble catalysts.
Rh catalyst has been further employed for dry reforming of glycerol (Bulutoglu
et al. 2018). Two catalysts, Rh − ZrO 2 and Rh − CeO 2 , have been investigated for
S. Z. Abidin et al.
ported on cement clinker (Ni-CC). Cement clinker composed of a mixture of CaO
and SiO has been reported to release CO 2 . Thus, the main reason of utilizing the
particular support is to reduce the effect of catalyst deactivation due to carbon deposition. Whisker carbon was detected on the spent Ni-CC catalyst after the dry
reforming of glycerol, yet the carbon is easily gasified by O 2 oxidation (Lee et al.
2014a, b). Later from the same group, Siew et al. (2014, 2015a, b) employed
La-promoted Ni−Al 2 O 3 catalyst for glycerol dry reforming at 650–850 °C. It is suggested that La promotion has successfully improved carbon resistance in Ni − Al 2 O 3
catalyst as La offers redox properties which provides additional route for carbon
removal, provides better Ni metal dispersion, and reduces the acidity of the Ni catalyst. In contrary to the earlier thermodynamic analysis, the group proposed that the
high CO 2 -to-glycerol ratio is essential for carbon mitigation as CO 2 effectively acts
as a gasification agent in carbon gasification pathway.
Similarly, in another study using Ni catalyst supported on Al 2 O 3 of a nanocrystalline structure by Tavanard et al. (2018), the increase of CO 2 content has improved
the occurrence of reverse Boudouard reaction that allows more for gasification of
carbonaceous species on the catalyst surface. Additionally, in a recent work using
Ni − Al 2 O 3 catalyst, Ag has been employed as a promoter in glycerol dry reforming.
The promoted catalyst has given better glycerol conversion due to the ability of Ag
to enhance the dispersion on Ni particles on the Al 2 O 3 support surface. Two types of
carbon were observed after 72 h of longevity study, encapsulated and whisker-type
carbons, yet they were easily removed through oxidation during TPO analysis at
temperatures below 680 °C (Harun et al. 2019).
Dry reforming of glycerol also has been conducted on Ni catalyst supported on
ZrO 2 , CaO, and La 2 O 3 supports (Mohd Arif et al. 2017). Ni−CaO with 15% Ni provides the highest hydrogen yield and glycerol conversion as the catalyst shows a
higher metal dispersion and smaller crystallite size of NiO species compared to that
with other catalysts (Ni−ZrO 2 and Ni−La 2 O 3 ). CaO support has been suggested to
reduce the accumulation of carbon on the catalyst surface due to its basic site properties. Basic functional group on support is beneficial for CO 2 adsorption and thus
assists in gasification of carbon on the catalyst surface. Further, the group has incorporated Re as a promoter on the same catalyst (Ni-CaO), and the addition of Re has
been anticipated to enhance the surface adsorption of OH group of the glycerol
(Mohd Arif et al. 2018). The addition of Re also has contributed to the lesser amount
of carbon on the Ni-CaO catalyst during glycerol dry reforming.
Besides the Ni-based catalysts, noble catalysts such as Rh, Ru, Ir, Pd, and Pt supported on alumina-stabilized alumina (MgAl 2 O 4 ) also have been investigated for
glycerol dry reforming (Tavanarad et al. 2017). Among the catalysts, Rh − MgAl 2 O 4
has displayed the highest glycerol conversion and stability for 20 h of operation.
The authors attributed the decent activity of Rh catalyst to its high BET surface area
and high active metal area. Similar to previous work using Ni − Al 2 O 3 , the presence
of whisker carbon was also observed on the spent Rh − MgAl 2 O 4 . Additionally, the
excess of CO 2 also has suppressed the carbon formation on the noble catalysts.
Rh catalyst has been further employed for dry reforming of glycerol (Bulutoglu
et al. 2018). Two catalysts, Rh − ZrO 2 and Rh − CeO 2 , have been investigated for
S. Z. Abidin et al.