E.V. Golubina et al. / Journal of Catalysis 344 (2016) 90–99
91
mechanism of precursor reduction and formation of the catalyti-
cally active sites on the surface of NDs is still missing in the
literature.
was treated under Ar flow (12 ml/min) at 900 °C for 3 h (desig-
nated as ND_Ar).
Catalysts with 5 wt.% Ni loading were prepared by the wet
impregnation of ND with nickel (II) nitrate solution. A suspension
of raw or Ar-treated ND in distilled water was stirred for 10 min
using a magnetic stirrer. A water solution of a required amount
of Ni(NO3)2 (Reachem, Russia) was slowly added to the ND suspen-
sion under continuous stirring. The solvent was slowly evaporated
at 80 °C. The impregnated catalyst was then dried in air at room
temperature for 12 h, heated to 150 °C, calcined at 150 °C for 2 h
to decompose nitrates, and finally cooled to room temperature.
The prepared samples were denoted as NiO/ND and NiO/ND_Ar.
Catalysts were reduced at 280 °C for 2 h by hydrogen (12 ml/min
flow rate). Reduced catalysts were named Ni/ND and Ni/ND_Ar,
respectively.
Metal content in catalysts was measured by atomic absorption
spectrometry (AAS) on a Thermo iCE 3000 spectrometer (Thermo
Fisher Scientific Inc., USA). Metal was dissolved at 90 °C in a mix-
ture of HNO3/H2SO4 (1:1 weight ratio). The relative error in the
AAS measurements of metal concentration in solution was 1%.
The BET surface area (SBET), pore diameter, and volume
(Vpore) were determined by low-temperature N2 adsorption using
an Autosorb–1 physisorption analyzer (Quantachrome, USA).
High-resolution transmission electron microscopy (HRTEM)
investigation was carried out with a JEOL JEM 2100F (Jeol, Japan)
microscope operated at 200 kV. Energy-dispersive X-ray spec-
troscopy (EDX) was applied to assess the catalyst composition.
The diffuse reflectance IR spectra were measured on a Bruker
Equinox 55/S Fourier transform IR spectrometer (Bruker, USA).
Quantitative X-ray fluorescence analysis was carried out on a ‘‘Re
spect’’ X-ray spectrometer (LDC Tolokonnikov, Russia) equipped
with two X-ray sources with Ag, Ti, Cu, Mo, and Re anodes. Zeta
potential in a deionized water suspension was measured by a
Malvern ZETASIZER nano-ZS instrument (Malvern, UK).
Temperature-programmed reduction (TPR) was performed in a
fixed-bed flow apparatus. The outlet of the reactor was connected
directly to a thermal conductivity detector. A sample charge of
25 mg loaded into the reactor was heated from room temperature
to 900 °C at a heating rate of 12 °C/min in a flow of 5%H2–95%Ar
mixture.
Semihydrogenation of phenylacetylene to styrene is a process of
great industrial importance, because phenylacetylene is a poison-
ing impurity in styrene feedstocks that causes deactivation of the
styrene polymerization catalyst [7,8]. Styrene selectivity in this
reaction is sensitive to catalysts’ active site composition in
bimetallic catalytic systems [9] and metal–support interaction
[10]. Semihydrogenation of phenylacetylene is also a good model
reaction for the evaluation of selective hydrogenation catalysts
under very mild conditions.
Many technologically important solid materials are amorphous
or poorly crystalline, and therefore their structure cannot be char-
acterized by diffraction techniques. For example, heterogeneous
catalysts comprising metal or metal oxide nanoparticles dispersed
on a high-surface-area support commonly fall into this category.
Even when a catalyst support is crystalline, active sites at the sur-
face represent a very small atomic fraction of the material, and
therefore have to be probed by a spectroscopic technique that
offers selectivity to these particular sites. Variability in the active
sites often increases with metal loading, while dilute systems pose
challenges for spectroscopic sensitivity even when they are more
uniform structurally.
Extended X-ray absorption fine structure (EXAFS) spectroscopy
has proven to be a powerful technique for probing local structure
in noncrystalline materials. EXAFS spectroscopy yields structural
information within a sphere with a radius of about 5–7 Å around
the central atom selected by the energy of the specific X-ray
absorption edge. On the basis of EXASF data, adequate and reliable
information about the environment of the atoms forming the
active sites of heterogeneous catalysts can be obtained. Usually
EXAFS measurements of the absorption spectrum
imated by the function
v(k) are approx-
n
X
2R
k
eꢀ eꢀ2 2 sinð2kRi þ
WÞ;
ð1Þ
Ni FiðkÞ
r2i k
i
v
ðkÞ ¼ S20
Ri2
k
i¼1
where Ni is the number of atoms in the ith coordination sphere, Ri is
the distance from the central atom to the ith coordination sphere,
r2 is the Debye–Waller factor, Fi(k) is the photoelectron backscat-
i
The Ni K-edge EXAFS spectra were acquired at the ‘‘Structural
Materials Science” beamline of the Kurchatov Synchrotron Radia-
tion Source (NRC ‘‘Kurchatov Institute,” Moscow, Russia) [15].
The electron storage ring operated at energy 2.5 GeV and current
120–150 mA was used as a source of radiation. The incident
X-ray beam was monochromatized with a Si(111) channel-cut
monochromator slightly detuned to suppress higher harmonics.
All spectra were recorded in the transmission mode using a pair
of ion chambers filled with the appropriate N2/Ar mixtures to pro-
vide 20% and 80% absorption. The radial pair distribution functions
around Ni atoms were obtained by the Fourier transformation of
k2-weighted EXAFS functions over the range of photoelectron wave
numbers 1.0–11.0 Åꢀ1. The structural parameters were found by
the nonlinear fit of theoretical spectra to experimental ones.
Wavelet transform of EXAFS spectra was performed using an ad
hoc routine programmed in the Math Lab software. In situ reduc-
tion of NiO/ND with H2 (5% H2 in Ar) was performed in the EXAFS
spectrometer cell at 150, 300, and 900 °C.
Catalytic tests were performed in the range 50–300 °C in a
packed-bed reactor equipped with a hydrogen supply line at con-
stant atmospheric pressure and a downstream trap for products
accumulation cooled with ice water. H2 was fed through the top
of the reactor. At each reaction temperature, phenylacetylene
was injected by a syringe into a flow of hydrogen by pulses
(0.23 mmol, 3 pulses at a given temperature at 10 min intervals).
After three phenylacetylene pulses, the reactor was kept at a given
tering amplitude, and k is the photoelectron wavenumber.
A traditional way to determine the interatomic distance R from
EXAFS data utilizes the Fourier transform [11]. However, if two dif-
ferent atoms are located at similar distances from the absorber
atom, their contributions in the R-space overlap and thus become
indistinguishable. The wavelet transform (WT) is known to be use-
ful for EXAFS signal extraction and for the discrimination of atoms
by their atomic numbers in cases of heavily overlapping contribu-
tions. The WT has proven a valuable tool for EXAFS data analysis
for structures, where two types of backscattering atoms, e.g., a
heavy and a light one, are located at the same distance from the
central atom [12–14].
This work focuses on the metal anchored to the ND surface. The
effect of the nature of the ND surface on active site formation in
supported Ni-containing catalysts, the mechanism of metal precur-
sor coordination, and active sites evolution were investigated by
in situ EXAFS combined with TPR. The influence of the surface
structure characteristics of various types of NDs on the catalytic
activity and stability in the selective phenylacetylene (PhA) hydro-
genation was studied as well.
2. Experimental
ND was obtained from ‘‘Sinta” JS Company (Belarus; sample
was kindly donated by Dr. A. Korzhenevsky). Part of the ND sample