H. Song et al. / Catalysis Communications 63 (2015) 52–55
53
of 40 eV. The typical physico-chemical properties of catalysts were ana-
lyzed by BET method using micromeritics adsorption equipment of
NOVA2000e. All the samples were outgassed at 200 °C until the vacuum
pressure was 6 mm Hg. The adsorption isotherms for nitrogen were mea-
sured at −196 °C. CO chemisorption uptake measurements were per-
formed in a Micromeritics ASAP 2010 apparatus under static volumetric
conditions. After pretreatment in a continuous N2 flow, CO pulses were
injected into N2, and the CO uptake was measured using a TCD until
there was no further CO uptake after consecutive injections.
increasing Y content, indicating that the incorporation of Y could pro-
mote better dispersion of Ni2P, which may also influence the DBT HDS
conversion and product distribution. Wang et al. [14,15] have studied
the effect of rare earth metal Ce on the particle size of the Ni2P, they
found that the presence of Ce have a positive effect on the dispersion
of the Ni2P particles. The strong interaction between nickel metals and
certain reducible oxides (such as CeO2) may constrain the growth of
the particle size of nickel and thus the final Ni2P particles. [15]
3.2. BET
2.3. Catalytic activities
Table 1 summarizes the textural properties of YxNi2P catalysts as
well as bulk Ni2P. The surface area and pore volume of the Ni2P sample
were determined as 6.1 m2 · g−1 and 0.025 cm3 · g−1, respectively. It is
worth noting that all of the YxNi2P catalysts showed a gradual increase
in specific surface area and pore volume with increasing Y content.
Compared with the bulk Ni2P catalyst, the surface area and pore volume
of Y0.10Ni2P (16.0 m2 · g−1 and 0.086 cm3 · g−1, respectively) were
almost twofold higher. This indicates that Y could promote greater dis-
persion of the Ni2P phase, which is consistent with the results obtained
by XRD.
The HDS of DBT was carried out in a flowing high-pressure fixed-bed
reactor using a feed consisting of a decalin solution of DBT (1 wt.%).
Catalytic activities were measured at 340 °C, 3.0 MPa, hydrogen/oil
ratio of 500 (V/V) and weight hourly space velocities (WHSV) of
1.5 h−1. Prior to reaction, 0.8 g of the catalysts were pretreated in situ
with flowing H2 (40 mL/min) at 500 °C for 2 h. Sampling of liquid prod-
ucts was started 2 h after the steady reaction conditions had been
achieved. The liquid samples which mainly contain the formed biphenyl
(BP) and cyclohexylbenzene (CHB) were collected every hour and ana-
lyzed by FID gas chromatography with a GC-14C-60 column.
3.3. CO uptake
3. Results and discussion
CO uptake measurements were used to effectively “titrate” the sur-
face metal atoms and to provide an estimate of the number of active
sites and their dispersion on the catalysts, ignoring the very low degree
of adsorption at P sites [18,20]. The measured CO adsorption capacities
of Ni2P and YxNi2P catalysts are presented in column 7 of Table 1. The
3.1. XRD
As can be seen from Fig. 1, XRD diffractograms of Ni2P and YxNi2P
precursor present broad feature peak centered at 2θ = 32°, which can
be assigned to the amorphous NixPyOz phases [19]. For all of the
catalysts, the peaks at 2θ = 40.6°, 44.5°, 47.1°, and 54.1° (PDF no.
03–0953) can be ascribed to Ni2P, indicating this to be the principal
active phase for all samples. Bulk Ni2P shows weak diffraction peaks at
2θ = 28.8°, 30.2°, 31.6°, 36.1°, 43.9°, 45.1°, 47.8°, and 53.0° (PDF no.
18–0883), which can be assigned to a small amount of Ni5P4 phase.
However, YxNi2P exhibits a broad peak due to an amorphous phase at
the corresponding position. This suggests that Y suppresses formation
of the Ni5P4 phase. The diffraction peaks of the Ni2P phase in YxNi2P be-
come more broadened with increasing Y content. In general, an increase
in peak width can be attributed to a decrease in the crystallite size of the
Ni2P. The calculated mean crystallite sizes (Dc) of Ni2P in the catalysts
based on Scherrer's equation are listed in column 2 of Table 1. The
Ni2P crystallite size in the YxNi2P catalysts is seen to decrease with
CO uptake of the bulk Ni2P sample was determined as 171 μmol · g−1
.
The CO uptake of the YxNi2P samples was clearly considerably higher
than that of bulk Ni2P, and increased with increasing Y content, indicat-
ing that the addition of Y can improve the dispersion of active Ni2P. This
may be because for bulk Ni2P the lower surface area leads to a poor dis-
persion of the particles and the accumulation of phosphate leads to less
nickel sites being exposed on the surface (see discussion of the XPS
results). For YxNi2P, the higher CO uptake can be attributed to the smaller
Ni2P particles, as observed in the XRD analysis. It should also be noted
that the CO uptakes are below the theoretical surface metal site density
calculated from the average particle size determined by XRD analysis.
This indicates that some nickel sites were covered by excess phosphorus,
which was enriched on the surfaces of all of the catalysts. This will be
discussed further in relation to the XPS results.
3.4. XPS
The XPS spectra for Ni2P and YxNi2P catalysts in the Ni(2p) and P(2p)
regions are shown in Fig. 2, and the binding energies are presented in
Table 2. As shown in Fig. 2(a), all spectra were decomposed taking
into account the spin-orbital splitting of Ni 2p3/2 and Ni 2p1/2 lines
(17.5 eV) and the presence of satellite peaks at about 5 eV higher than
the binding energy of the parent signal [21,22]. For all catalysts, the Ni
2p3/2 spectrum consists of three contributions. The bands centered at
852.5–852.7 eV and 856.1 ~ 856.7 eV can be attributed to the Niδ+ spe-
cies in Ni2P phase and Ni2+ species interacting with phosphate as a con-
sequence of a superficial passivation, respectively [22–24]. The bands
centered at 860.8–861.2 eV can be assigned to the shake-up satellite
[4,21]. As shown in Fig. 2(b), the peaks centered at 128.8–130.3 eV
can be assigned to Pδ+ species in the Ni2P phase [22,25], and the peak
at 134.5–134.8 eV can be attributed to phosphate (P5+) due to superfi-
cial oxidation of the Ni2P particles [24]. It is worth noting that the P 2p3/2
binding energy corresponding to phosphide phase increases with in-
creasing Y content in the catalyst due to the electron transfer from phos-
phide species to yttrium.
The ratios of the Niδ+ species to total Ni of Ni2P phase (R Niδ+) in Ni
2p3/2 are also illustrated in column 7 Table 2. It is worth noting that the
Fig. 1. XRD patterns of the Ni2P and YxNi2P catalysts and precursors.