2
Wang et al. Sci China Chem
RhFeLi/TiO2 nanorods can stabilize formate and protonate
methanol, which is easily dissociated to CHx then is inserted
to CH3CO and hydrogenated to ethanol [19].
Germany), Fe(NO3)3•9H2O (98%, Alfa Aesar Chemical Co.
Ltd., China), 50% Manganous nitrate water solution (99.0%,
Tianjin kemi’ou Technology Development Co. Ltd., China),
cerium nitrate hexahydrate (98%, Aladdin Chemical Co.
Ltd., China) and ammonium niobate(V) oxalate (98%, Alfa
Aesar Chemical Co. Ltd., China) as precursors. Then MCM-
41 supports (1 g) were impregnated with 2 mL solution of
the precursor by using ultrasonication for 1 h. Subsequently,
the sample is dried at 80 °C for 10 h, calcined at 500 °C for
4 h and reduced in pure H2 at 400 °C for 1 h.
Generally, the CO2 hydrogenation to ethanol reaction
pathway contains reverse water gas shift (RWGS), CO dis-
sociation, CO insertion and hydrogenation [15,16,19]. The
promoting effect of metal oxide over Rh-based catalysts for
CO2/CO hydrogenation to ethanol was extensively in-
vestigated. Several studies have suggested that FeOx can
promote RWGS to activate CO2 [19], Fe0 promote CO dis-
sociation and hydrogenation [15] and RhFe alloy was cor-
related with ethanol selectivity [20,21] in CO2/CO
hydrogenation. MnOx can also promote CO dissociation by
formation of tilted CO species at Rh-MnO interface [22,23].
Generally, the effect of promoter can be ascribed to the
interfacially active sites between Rh and metal oxide
Since the size effect of Rh nanoparticle can often influence
the CO2 conversion and product selectivity [28,29], MCM-
41 was used as support to control the Rh nanoparticle size.
Firstly, the influence of various promoters was studied. A
significant improvement of CO2 conversion and ethanol se-
lectivity was observed when vanadium oxide (VOx) was
added to Rh/MCM-41. Furthermore, the VOx promoting
effect on the electronic state of Rh and the reaction me-
chanism of ethanol formation was investigated.
2.2 Characterization
X-ray photoelectron spectroscopy (XPS) measurements
were taken on a PHI 1600 ESCA instrument (PE Company)
which is equipped with an Al Kα X-ray radiation source (hν=
1,486.6 eV). Before measurements, all the samples were
dried at 80 °C for 12 h. The binding energies were calibrated
using the C 1s peak at 284.6 eV as a reference. Raman
spectra were recorded under ambient conditions using a
Renishaw inVia reflex Raman spectrometer with a 325 nm
Ar ion laser beam and a 633 nm He-Ne ion laser beam.
Before each measurement, the samples were dried at 80 °C
for 12 h. The morphology of catalysts was characterized by
transmission electron microscopy (TEM; FEI Tecnai G
2 F20, 200 kV, USA) and field emission scanning electron
microscopy (FE-SEM; Hitachi S-4800, 5 kV, Japan). The
average particle diameter was obtained, after counted over
300 particles. Before each measurement, the samples were
dissolved in ethanol and dispersed by using ultrasonication.
Powder X-ray diffraction (XRD) patterns were performed
with 2θ values between 1° and 60° using a Bruker-D8 dif-
fractormeter (λ=1.54056 Å, Germany). In order to measure
the weight content of element Rh and V, 5 mg of each cat-
alyst dissolved in a perchloric/nitric acid mixture, was
measured by the inductively coupled plasma optical emis-
sion spectroscopy (ICP-OES; Varian 720-ES, USA).
H2 temperature-programmed reduction (H2-TPR) and CO2
temperature-programmed desorption (CO2-TPD) were per-
formed on a Micromeritics AutoChem II 2920 apparatus
equipped with HIDEN QIC-20 mass spectrometer (MS). For
H2-TPR experiment, 100 mg sample was pretreated at
300 °C for 1 h under flowing Ar to remove water and other
contamination. After cooling to 50 °C, 10 vol% H2/Ar was
introduced and the temperature was increased from 50 to
800 °C with the ramp rate of 10 °C/min. The signal was
recorded online by thermal conductivity detector (TCD). For
CO2-TPD experiment, 100 mg sample was pre-reduced at
400 °C for 1 h under 10 vol% H2/Ar. When the temperature
was cooled and kept stable at 50 °C, pure CO2 was in-
troduced for 0.5 h. Subsequently, the purging was carried out
by Ar for 1 h. Then the temperature was increased from 50 to
600 °C with the ramp rate of 10 °C/min. The gas component
2 Experimental
2.1 Preparation of catalysts
The catalysts were prepared by the incipient wetness im-
pregnation method. RhCl3•nH2O (99.9%, Huaweiruike
Chemical Co., China), NH4VO3 (99.0%, Tianjin Guangfu
Technology Development Co. Ltd., China), oxalic acid
(99.0%, Aladdin Industrial Corp., China) were used as pre-
cursors. MCM-41 (99.9%, NanJingJiCang Nanotechnology
Corp., China) with channels diameter between 1.7 to 3.8 nm
were used as the support. MCM-41 (1 g) were impregnated
with an aqueous solution of oxalic acid (2 mL) containing
the precursor [NH4VO3/oxalic acid=0.5 (mole ratio)] by
using ultrasonication for 1 h. Subsequently, the sample is
dried at room temperature overnight and then at 80 °C for
10 h. Finally, the sample is calcined in air at 500 °C for 4 h
and reduced in pure H2 at 400 °C for 1 h. The loading of Rh
was controlled at 2 wt%. The loading of VOx was based on
the molar ratio between V and Rh. We named the catalyst as
Rh-(y)VOx/MCM-41, where the y in the bracket repesents
the weight percent of V. We prepared Rh-MOx/MCM-41
(MOx=WOx, FeOx, MnOx, CeOx and NbOx) catalysts by ap-
plying the co-impregnation method. We used ammonium
metatungstate hydrate (98%, Adamas Reagent Co. Ltd.,