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RSC Advances
selecting a suitable promoter for catalyst. Alkaline-earth oxide, a
well-known solid base, has been employed in a variety of
organic reactions as the main component or promoter of cata-
lyst.21 Moreover, alkaline-earth oxide exhibits a high thermal
stability, which can prevent the agglomeration of catalyst and
increase the surface area of catalyst.22,23 Thus, the introduction
of alkaline-earth oxide into CuO–ZrO2 may regulate the surface
basicity and improve the Cu dispersion of catalysts. However, to
date, a systematic examination of CuO–ZrO2 catalyst doped with
alkaline-earth oxides for CO2 hydrogenation is absent.
The primary purpose of the present work is to explore the
inuence of alkaline-earth oxides (MgO, CaO, SrO and BaO)
doping on the properties of CuO–ZrO2 catalysts. The physico-
chemical properties of alkaline-earth oxides doped CuO–ZrO2
catalysts were characterized by XRD, BET, N2O titration, TPR,
XPS and CO2-TPD techniques, and the catalytic activity for
methanol synthesis from CO2 hydrogenation was evaluated.
Based on the catalytic mechanism of methanol synthesis, the
catalytic activity and selectivity of the doped catalysts were
discussed in relation to the physicochemical properties
including the copper surface area, the reducibility of CuO and
the surface basicity. In addition, the effects of doping amount
was emphasized for MgO-doped catalyst.
2.2 Catalyst characterization
The X-ray diffraction (XRD) analysis of the sample was carried
out on a PANalytical X'Pert diffractometer using nickel-ltered
Cu Ka radiation at 40 kV and 40 mA. Two theta angles ranged
from 10 to 70ꢀ with a speed of 6ꢀ per minute.
The BET surface area (SBET) of sample was determined by a
Micromeritics ASAP2020M + C adsorption apparatus with
nitrogen adsorption/desorption isotherms. Before each anal-
ꢀ
ysis, samples were dried at 200 C under vacuum for 3 h.
Copper surface area (SCu) in the reduced catalyst was deter-
mined using the N2O titration method similar to that described
by Chinchen et al.25 The catalyst (0.2 g) was reduced in an H2/He
mixture at 300 ꢀC for 1 h. Then, it was purged with He and
ꢀ
cooled to 60 C. A ow of 1 vol% N2O/He gas mixture was fed
into the reactor. The N2 produced by the decomposition of N2O
on the exposed Cu atoms was detected using a mass spec-
trometer (Pfeiffer Vacuum Quadstar, 32-bit). The copper surface
area was calculated assuming an atomic copper surface density
of 1.46 ꢁ 1019 Cu atoms per m2 and a molar stoichiometry of
N2O/Cu ¼ 0.5.7
Temperature-programmed reduction (TPR) measurements
were performed in a linear quartz microreactor fed with a
10 vol% H2/N2 mixture owing at 50 ml minꢂ1 and heated at a
rate of 5 ꢀC minꢂ1. A ca. 30 mg of a freshly calcined catalyst was
placed on top of glass wool in the reactor. The outlet of the
reactor was connected to a glass column packed with molecular
2. Experimental
2.1 Catalyst preparation
˚
sieve 5 A in order to remove the moisture produced from
CuO–ZrO2 (CZ) and alkaline-earth oxides doped CuO–ZrO2
(MCZ, M ¼ Mg, Ca, Sr, Ba) catalysts were prepared by the urea-
nitrate combustion method. All chemicals used were of
analytical reagent grade (Shanghai Chemical Reagent Corpora-
tion, Shanghai, China). Firstly, the required amounts of metal
nitrates were dissolved in deionized water in a basin to form a
mixed solution with a total cation concentration of 3.0 M. Then,
the solution of urea (3.5 M) was slowly added to the metal
nitrate solution under constant stirring. The resulting mixture
was kept in an ultrasound bath operating at 47 kHz with a power
of 30 W for 0.5 h until a pale-blue slurry was obtained. Aer-
ward, the slurry was transferred to an open muffle furnace
preheated at 300 ꢀC. The slurry started boiling with frothing and
foaming, and ignition took place. Along with rapid evolution of
a large quantity of gases, a foamy, voluminous powder was
produced. Because the time for the autoignition was rather
short, to remove traces of undecomposed urea, nitrates and
their decomposition products, the powder was further calcined
in air at 500 ꢀC for 4 h. To obtain an optimum catalytic
performance, the amount of urea used in the combustion
process was 50% of the stoichiometric amount, which can be
calculated according to propellant chemistry.24 A more detailed
reduction. The amount of consumed H2 was measured by a
thermal conductivity detector (TCD).
The surface electronic states were investigated by X-ray
photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo
Scientic Escalab) with Al Ka (1486.6 eV) radiation as the X-ray
excitation source. All the binding energy values were calibrated
by using C 1s ¼ 284.8 eV as a reference.
The basicity of the catalysts was measured by CO2
temperature-programmed desorption (CO2-TPD). Prior to the
adsorption of CO2, the catalysts were reduced at 300 ꢀC for
60 min in a ow of 10% H2/N2 mixture. Aer cooling toꢀroom
temperature, the catalyst was saturated with CO2 at 50 C for
60 min, and then ushed with He ow to remove any phys-
isorbed molecules. Aerward, the TPD experiment was started
ꢂ1
ꢀ
with a heating rate of 5 C min under He ow, and the des-
orbed CO2 was detected by a mass spectrometer. The amount of
the desorbed CO2 was quantied by comparing the integrated
area of the TPD curves to the peak area of the injected CO2
calibration pulse.
2.3 Catalytic testing
description of the preparation process was given in ref. 8. Catalytic activity and selectivity tests for methanol synthesis
Unsupported CuO powder in this study was prepared via the from CO2 hydrogenation were carried out in a continuous-ow,
thermal decomposition of Cu(NO3)2 at 500 ꢀC. The synthesized xed-bed reactor. Prior to the catalytic measurements, the fresh
catalysts are denoted as MxCyZz, where x, y and z represent the catalyst was reduced in a stream of 10 vol% H2/N2 at 300 ꢀC for
atomic concentration of alkaline-earth metal, Cu and Zr, 3 h under atmospheric pressure. Then the reactor was cooled to
ꢀ
respectively. The sum of atomic concentration of metal was 180 C and the reactant gas (CO2 : H2 ¼ 1 : 3, molar) ow was
taken as 1.0, and a constant atomic concentration of Cu (0.5) introduced, raising the pressure to 3.0 MPa and the tempera-
was employed in this study.
ture to a given temperature. The transfer line from the reactor to
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 52958–52965 | 52959