574
in comparison with bare In 2 O 3 due to the strong basic sites present on the catalyst
surface. Moreover, hexagonal-phase In 2 O 3 (H-In 2 O 3 ) structured onto thin-felt Al 2 O 3 /
Al-fiber matrixes via the mix-solvothermal preparation method has been used for
CO 2 hydrogenation to methanol. The H-In 2 O 3 /Al 2 O 3 /Al-fiber catalyst showed positive results in terms of CO 2 conversion and methanol selectivity of 4.4% and 67.6%,
respectively, at 325 °C and 40 bars. The In 2 O 3 /Al 2 O 3 /Al-fiber catalyst was stable for
200 h of run time without deactivation. Men [91] et al. synthesized Pt/In 2 O 3 hybrid
films by cold-plasma/peptide-assembly (CPPA) method and showed appreciable
results for CO 2 conversion.
DFT study revealed that formation of Pd-In bimetallic site improved (compared
to In 2 O 3 ) the catalytic performance by providing metal (Pd) sites generation of
atomic H 2 and interfacial sites for CO 2 adsorption and hydrogenation. However,
poor methanol yield was observed in experimental studies. The H 2 chemisorption
results revealed that the Pd-In bimetallic interface has fewer active sites for adsorption of atomic hydrogen compared to Pd; therefore, the rate of hydrogenation
decreased in the experiments [92]. To increase the methanol yield over the Pd/In 2 O 3
catalyst, one must minimize formation of Pd-In bimetallic during the catalyst synthesis. Thus Qingfeng et al. [93] research groups synthesized highly dispersed Pd
NPs (3.6 nm) on In 2 O 3 using peptide template. The Pd-In bimetallic sites formed
appreciably enhanced dissociative adsorption of hydrogen, with preserving the density of the oxygen vacancies. The CO 2 conversion >20% with methanol selectivity
>70% was observed for the catalyst (STY 0.89 g MeOH h
−1
gcat
−1
at 300 °C
and 5 MPa).
Metal organic frameworks (MOFs) also termed as “porous coordination polymers” (PCP) are three-dimensional porous crystalline materials. MOF consists of
regular arrangement of a metal cation or cluster (secondary building units (SBUs))
and a polydentate organic ligand with coordination-type connections. MOFs have
attracted attention for their application in catalysis due to their metal center, highly
ordered porous structure, uniform pore size, and functional organic linkers.
Therefore, Rungtaweevoranit et al. [94] synthesized Cu nanoparticles encapsulated
within framework of UiO-66 and reported 100% methanol selectivity (175 °C and
10 bar). The variable oxidation state of Cu and increased interfacial contact between
Cu NCs and Zr oxide SBUs enhanced TOF for methanol formation (>20 × 10
3
s
−1
at 200 °C). The degree of charge transfer from Cu nanoparticle to the Zr
4+
at the
interface of Cu-UiO66 is linearly related to the activity of MOF-based catalyst for
methanol synthesis reaction [95]. An et al. [96] reported Cu/ZnO x supported on
UiO-bpy (synthesized by solvothermal process 2,2′-bipyridine-5,5′-dicarboxylate
(bpydc) bridging ligand and Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 SBUs) MOF, owing to the high
thermal stability (>250 °C). The well-mixed Cu, ZnO x , and Zr SBUs surface/interfacial sites contributed to the adsorption and activation of H 2 and CO 2 in the CuZn@
UiO-bpy catalyst. The Cu/ZnOx@MOF catalysts show very high activity with STY
of up to 2.59 g MeOH kg Cu
−1
h
−1
, 100% methanol selectivity at 180 °C, and high
stability for about 100 h.
The surface chemistry of transition metal carbides (TMCs) is alike to that of Pt
group metals like Pd, Rh, or Ru during the transformation of hydrocarbons. Thus,
K. Tripathi et al.
in comparison with bare In 2 O 3 due to the strong basic sites present on the catalyst
surface. Moreover, hexagonal-phase In 2 O 3 (H-In 2 O 3 ) structured onto thin-felt Al 2 O 3 /
Al-fiber matrixes via the mix-solvothermal preparation method has been used for
CO 2 hydrogenation to methanol. The H-In 2 O 3 /Al 2 O 3 /Al-fiber catalyst showed positive results in terms of CO 2 conversion and methanol selectivity of 4.4% and 67.6%,
respectively, at 325 °C and 40 bars. The In 2 O 3 /Al 2 O 3 /Al-fiber catalyst was stable for
200 h of run time without deactivation. Men [91] et al. synthesized Pt/In 2 O 3 hybrid
films by cold-plasma/peptide-assembly (CPPA) method and showed appreciable
results for CO 2 conversion.
DFT study revealed that formation of Pd-In bimetallic site improved (compared
to In 2 O 3 ) the catalytic performance by providing metal (Pd) sites generation of
atomic H 2 and interfacial sites for CO 2 adsorption and hydrogenation. However,
poor methanol yield was observed in experimental studies. The H 2 chemisorption
results revealed that the Pd-In bimetallic interface has fewer active sites for adsorption of atomic hydrogen compared to Pd; therefore, the rate of hydrogenation
decreased in the experiments [92]. To increase the methanol yield over the Pd/In 2 O 3
catalyst, one must minimize formation of Pd-In bimetallic during the catalyst synthesis. Thus Qingfeng et al. [93] research groups synthesized highly dispersed Pd
NPs (3.6 nm) on In 2 O 3 using peptide template. The Pd-In bimetallic sites formed
appreciably enhanced dissociative adsorption of hydrogen, with preserving the density of the oxygen vacancies. The CO 2 conversion >20% with methanol selectivity
>70% was observed for the catalyst (STY 0.89 g MeOH h
−1
gcat
−1
at 300 °C
and 5 MPa).
Metal organic frameworks (MOFs) also termed as “porous coordination polymers” (PCP) are three-dimensional porous crystalline materials. MOF consists of
regular arrangement of a metal cation or cluster (secondary building units (SBUs))
and a polydentate organic ligand with coordination-type connections. MOFs have
attracted attention for their application in catalysis due to their metal center, highly
ordered porous structure, uniform pore size, and functional organic linkers.
Therefore, Rungtaweevoranit et al. [94] synthesized Cu nanoparticles encapsulated
within framework of UiO-66 and reported 100% methanol selectivity (175 °C and
10 bar). The variable oxidation state of Cu and increased interfacial contact between
Cu NCs and Zr oxide SBUs enhanced TOF for methanol formation (>20 × 10
3
s
−1
at 200 °C). The degree of charge transfer from Cu nanoparticle to the Zr
4+
at the
interface of Cu-UiO66 is linearly related to the activity of MOF-based catalyst for
methanol synthesis reaction [95]. An et al. [96] reported Cu/ZnO x supported on
UiO-bpy (synthesized by solvothermal process 2,2′-bipyridine-5,5′-dicarboxylate
(bpydc) bridging ligand and Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 SBUs) MOF, owing to the high
thermal stability (>250 °C). The well-mixed Cu, ZnO x , and Zr SBUs surface/interfacial sites contributed to the adsorption and activation of H 2 and CO 2 in the CuZn@
UiO-bpy catalyst. The Cu/ZnOx@MOF catalysts show very high activity with STY
of up to 2.59 g MeOH kg Cu
−1
h
−1
, 100% methanol selectivity at 180 °C, and high
stability for about 100 h.
The surface chemistry of transition metal carbides (TMCs) is alike to that of Pt
group metals like Pd, Rh, or Ru during the transformation of hydrocarbons. Thus,
K. Tripathi et al.