G Model
APCATA-15702; No. of Pages13
ARTICLE IN PRESS
K. Kousi et al. / Applied Catalysis A: General xxx (2016) xxx–xxx
3
Table 1
Materials: their notation and textural properties.
Catalysts
Notation
B2O3 or La2O3 wt% content
SBET of supporta (m2/g)
SBET,(m2/g)
Calc.b
Average pore
diameter (nm)
Pore volume
(cm3/g)
Red.c
Calc.b
Red.c
Calc.b
Red.c
Ni/␥–Al2O3
Ni/B2O3–Al2O3
Ni/La2O3–Al2O3
NiAl
NiBAl
NiLaAl
–
5.6
16.8
194
205
83
160
192
83
152
185
75
12.4
9.2
16.1
12.8
12.2
16.0
0.60
0.53
0.37
0.60
0.68
0.35
a
Specific surface area of the corresponding support.
Calcined catalyst.
Reduced catalyst.
b
c
2.2. Catalyst characterization techniques
(GC 8610 Chemito). A cold trap (liquid nitrogen/ethanol bath) was
used to remove water from the reactor effluent before it reached
the TCD detector. Various amounts of CuO were reduced under the
same experimental conditions in order to calibrate the method and
quantify hydrogen consumption.
High Resolution Transmission Electron Microscopy (HR-TEM)
micrographs were obtained on a JEOL JEM-2100 system. For nickel
particle size distributions more than 240 individual particles per
specimen were measured.
Textural properties were measured based on the nitrogen
adsorption–desorption isotherms at 77 K using a Micromeritics
TriStar 3000 apparatus. For calculation of the specific surface areas
the standard B.E.T. equation was applied within the nitrogen rela-
tive pressure range of 0.06 < P/P0< 0.20. Pore size distribution was
estimated based on the Barret–Joyner–Halenda (BJH) method and
the adsorption branch of the isotherms.
X-ray diffractograms (XRD patterns) of the fresh and reduced
catalysts were recorded on a Bruker D8 Advance diffractome-
ter equipped with a Ni-filtered Cu K␣ radiation (ꢁ = 0.15418 nm).
The following operating parameters were selected: 40 kV, 40 mA,
anglerange 2◦ < 2ꢂ < 85◦, step size 0.015◦ and scan speed 0.3 s/step.
JCPDF (Joint Committee on Powder Diffraction Standards) data files
were used for the phase identification. The average size of metal
crystallites was calculated following the Scherrer equation.
Diffuse reflectance spectra (DRS) of the calcined materials
were recorded using a UV–vis spectrophotometer (Varian Cary
3) equipped with an integration sphere coated with BaSO4. The
powder materials were mounted in an appropriate quartz cell
which provides an “infinite” sample thickness and the spectra were
recorded at room temperature and in the range of 200–800 nm.
These are presented as the Schuster–Kubelka–Munk (SKM) func-
2.3. Catalytic performance evaluation
The catalytic performance for the steam reforming of glyc-
erol was evaluated following two different experimental protocols,
using a fixed bed plug-flow reactor, operating at atmospheric
pressure. The catalysts were submitted to ex-situ reduction. Tem-
perature was increased gradually (10 ◦C/min) from RT to 800 ◦C
under N2 flow (50 mL/min) and then the catalyst was reduced under
a stream of 100% H2 (100 mL/min) for 1 h at this temperature, fol-
lowed by cooling to RT under N2 flow. The sample was then stored.
200 mg of the pre-reduced catalyst (particle size 180–250 m)
were reduced in situ under a stream of H2 (50 mL/min), while tem-
perature was increasing following a ramp of 10 ◦C/min, from RT
to 750 ◦C, where it was kept for 2 h. The catalyst was then purged
with helium for 15 min and temperature was increased to 800 ◦C
and the reaction feed was introduced into the catalyst bed. The lat-
ter is prepared as follows: a mixture of 20:80 wt% glycerol/water is
kept in a tank under continuous stirring at room temperature. The
mixture is introduced into the evaporator using an HPLC pump.
At the same time a Ar/He mixture (1:40), controlled by two sepa-
rate mass flow controllers, is introduced into the evaporator from
another inlet. The evaporator outlet, consisting of 20% He, 1% Ar,
75% H2O and 4% glycerol, at W/F = 1.05 mg min mL−1, is fed into
the reactor. Evaporator and tubing are held at 350 ◦C in order to
make sure that glycerol is completely in the gas phase. Catalytic
activity and selectivity were assessed in the temperature range of
400–800 ◦C, at steady state conditions for each temperature.
Catalysts were also evaluated following a different reaction pro-
tocol, according to which catalytic tests were performed for exactly
3 h, at the temperatures of 400, 500 and 600 ◦C, using a fresh catalyst
sample at each temperature. Activation was performed in exactly
the same way as described above. Temperature was then lowered
to reaction temperature and the reaction feed was introduced into
the reactor (feed composition and W/F are the same as previously
stated).
tion F (R ) = (1 − R
)
2/2R = K/S, where R is the reflectance of
∞
∞ ∞ ∞
a thick solid layer and K and S are the absorption and diffusion
coefficients, respectively.
Surface acidity was quantified by ammonia temperature pro-
grammed desorption (NH3-TPD). All samples were pre-reduced in
order to evaluate their acidity at a similar state as they meet the
reactant stream. The pre-reduced catalyst was dried overnight at
120 ◦C and then cooled slowly to room temperature (RT). 100 mg
of the catalyst (particle size 180–250 m) were introduced in
the reactor and then reduced in situ under a stream of pure H2
(30 mL/min) at 400 ◦C for 30 min and then cooled to RT under a flow
of He (30 mL/min) to remove adsorbed H2. Next, a stream of NH3/He
mixture was introduced in the reactor for 1 h, and then switched
to He (30 mL/min) to remove ammonia physically adsorbed on the
catalyst surface. When the signals, monitored by an MS detector
(FL-9496 Balzers), were stabilized, temperature was increased from
RT to 750 ◦C at a rate of 10 ◦C/min. Signal calibration was performed
using known quantities of ammonia to the mass spectrometer.
Integration of the TPD curves was used to quantify the amount of
desorbed ammonia. With this method, the exact nature of adsorp-
tion sites (Brønsted or Lewis) can not be defined, but only the total
acidity of the samples.
Feed and gas-phase reaction products were analyzed by 3 gas
chromatographs: (a) a Shimadzu GC-8A, equipped with a packed
Carbosieve SII column (spherical carbon molecular sieve), a TCD
detector, N2 as carrier gas and a data processor (HP 3395), (b) a Shi-
madzu GC-9A equipped with a packed Porapak Q column (spherical
carbon molecular sieve), an FID detector, He as carrier gas and a data
processor (Shimadzu C-R6A) and (c) a Shimadzu GC-14A with a
packed Carboxen 1000 column (spherical carbon molecular sieve),
Reducibility of supports and catalysts was studied by temper-
ature programmed reduction (TPR) with hydrogen. 300 mg of the
material, previously dried at 393 K, was heated under a stream of
10% H2/Ar (40 mL/min) from RT to 800 ◦C, with a temperature ramp
of 10 ◦C/min. The sample remained at 800 ◦C under the same gas
stream until the signal reached baseline (at least for 1 h). The con-
sumption of H2 was measured by a thermal conductivity detector