Y. Kanda et al. / Applied Catalysis A: General 475 (2014) 410–419
411
and Bussell et al. [3,39] have reported that rhodium phosphide
(Rh2P) supported on SiO2 exhibits high and stable catalytic activ-
ity toward the HDS reaction. As with the Ni2P catalysts, the P/Rh
activity of Rh catalysts. Previously, we reported that the support
strongly affects the formation of Rh2P and its HDS activity and that
SiO2, TiO2, and Al2O3 supports showed superior HDS activity com-
pared to MgO and ZrO2 supports [37]. However, the formation of
Rh2P on Al2O3 is difficult because of the formation of AlPO4 [3], and
the turnover frequency (TOF) of Rh2P is remarkably enhanced by
the interaction between the active phase and TiO2 [37]. Thus, SiO2,
which does not exhibit a strong interaction with Rh2P or P when
used as a support, is a superior support for clarifying the effects of
the P loading on Rh2P formation and on its catalytic HDS activity.
As previously noted, numerous reports have described the effects
of P loadings on the formed phases and on their HDS activities.
However, the effect of P loading on the reducibility of phosphates
and on the formation temperature of phosphides, especially noble
metal phosphides, has scarcely been reported. In addition, reduc-
tion temperature is one of the most important factors in evaluating
the reducibility of phosphates and the formation of phosphides.
Herein, the effects of reduction temperature and P loading on the
formation of rhodium phosphides (RhXPY) and on the HDS activity
of Rh2P/SiO2 catalysts were examined to enable the preparation of
highly active phosphided HDS catalysts.
The rate constant was calculated from the following equation
under the assumption of a pseudo-first-order reaction:
−ln(1 − x/100)
=
(1)
W/F
where kHDS is the reaction rate of thiophene HDS (mol h−1 g−1) and
x is the conversion rate at 3 h (%).
2.3. Catalyst characterization
The Rh and Rh-xP catalysts were characterized using N2 adsorp-
tion, temperature-programmed reduction (TPR), X-ray diffraction
(XRD), transmission electron microscopy (TEM), and carbon
monoxide (CO) adsorption analyses. Measurements of N2 adsorp-
tion at −196 ◦C were performed using a Micromeritics ASAP 2010.
The catalysts were evacuated at 300 ◦C for 10 h prior to the N2
adsorption measurements. The surface area of the catalysts was
calculated by the Brunauer–Emmett–Teller (BET) method. TPR
measurements were performed using a Shimadzu GC-8A gas chro-
matograph. The supported Rh or Rh-xP catalysts (0.1 g) were heated
in a He stream (30 ml min−1) from room temperature to 500 ◦C at
10 ◦C min−1, followed by He treatment at 500 ◦C for 1 h. After this He
treatment, the catalysts were cooled to 30 ◦C in a He stream, and the
He was switched to a hydrogen–nitrogen (5 vol%H2–N2) gas mix-
ture at 30 ◦C for 0.5 h before the measurement was performed. The
TPR spectrum was recorded over the temperature range of 30 to
800 ◦C at 10 ◦C min−1, using a thermal conductivity detector (TCD)
to monitor H2 consumption. Water was removed using a molecular
sieve trap. The XRD patterns of the calcined and reduced catalysts
in air were measured using a Rigaku MiniFlex equipped with a Cu
K␣ radiation source operated at 30 kV and 15 mA. The crystallite
size of the metallic Rh and RhXPY were calculated using Scherrer’s
equation:
2. Experimental
2.1. Catalyst preparation
Silica (SiO2, BET surface area 295 m2 g−1) was supplied by
Nippon Aerosil Co. The Rh/SiO2 catalyst was prepared by an impreg-
nation method described previously [36–38]. Rhodium(III) chloride
trihydrate (RhCl3·3H2O, Kanto Chemical Co.) was used as a precur-
sor for the catalysts and was dissolved in water. The Rh loading
amount was 5 wt%. After impregnation, the catalyst was dried at
110 ◦C for 24 h, followed by heat treatment under nitrogen (N2)
stream at 450 ◦C for 1 h in order to decompose the Rh salts. The
sieved catalysts (30- to 42-mesh-size granules) were calcined in
air at 500 ◦C for 4 h. The ramp rate for the heat treatment and cal-
cination was 10 ◦C min−1. P-added 5 wt% Rh (Rh-P)/SiO2 catalysts
were prepared using the same procedure, except with aqueous
solutions of RhCl3·3H2O and ammonium dihydrogen phosphate
(NH4H2PO4, Kanto Chemical Co.). The P concentration was varied
from 0.8 to 3.0 wt%. These catalysts were labeled as Rh-xP, where
“x” denotes the P loading (wt%). The P/Rh molar ratio in the cat-
alysts with 0.8, 1.5, 2.2, and 3.0 wt% P was 0.5, 1.0, 1.5, and 2.0,
respectively.
Kꢀ
d =
(2)
B
cos ꢁ
where d is the crystallite size (nm), B is the full-width at half maxi-
mum of the selected peak (FWHM, radians), K is shape factor (0.9),
and ꢀ is the wavelength of the X-ray radiation (0.154184 nm). The
XRD peaks at 34.2◦ (Rh2O3, (1 1 4) plane), 40.9◦ (Rh, (1 1 1) plane),
46.7◦ (Rh2P, (2 2 0) plane), and 23.9◦ (RhP2, (1 1 1) plane) were used
to calculate the B parameter.
TEM observations were performed using a JEOL JEM-2000FX.
The conditions of TEM operation were an acceleration voltage of
200 kV and a magnification of 120,000×. The particle size dis-
tribution and average particle size were determined from the
measurements of 1000 particles in the TEM micrographs. The CO
uptake of the supported Rh and Rh-P catalysts was measured using
the pulse method. The supported Rh or Rh-xP catalysts (0.1 g) were
treated in He at 500 ◦C (10 ◦C min−1) for 1 h, followed by reduc-
tion in H2 at 350–700 ◦C for 1 h. CO was injected onto the catalyst
layer at 25 ◦C using a sampling loop (1.0 ml). The amount of CO
adsorbed was measured with a Shimadzu GC-8A gas chromato-
graph equipped with a TCD.
2.2. Hydrodesulfurization of thiophene
The HDS of thiophene was performed at 350 ◦C under 0.1 MPa
using a conventional fixed-bed flow reactor. The calcined catalyst
(0.1 g) was charged into the quartz reactor and heated (10 ◦C min−1
)
3. Results and discussion
in a helium (He) stream (30 ml min−1) at 500 ◦C for 1 h, fol-
lowed by reduction in H2 (30 ml min−1) at 350–700 ◦C for 1 h.
A hydrogen–thiophene gas mixture (H2/C4H4S = 30), obtained by
passing a H2 stream through a thiophene trap cooled at 0 ◦C, was
then introduced into the reactor (W/F = 37.9 g h mol−1). The reac-
tion products were analyzed using a gas chromatograph equipped
with a flame ionization detector (FID) and a silicone DC-550
(length: 2 m, temperature: 110 ◦C) and Al2O3/KCl plot (ID: 0.53 mm,
length: 25 m, film thickness: 10 m, temperature: 60–190 ◦C, rate:
7.5 ◦C min−1) columns.
3.1. HDS of thiophene over Rh-xP catalysts
Previously, we reported that reduction temperature strongly
affects the HDS activity of Rh-1.5P catalysts [36–38]. Thus, the HDS
activity of Rh-xP catalysts reduced at various temperatures was
examined in this work. Fig. 1 shows the effect of reduction tem-
perature on the HDS activity (rate constant) of the Rh-xP catalysts
after reaction for 3 h. The HDS activity of the Rh catalyst barely