44
DEDOV et al.
in these temperatures in the case of replacement of a compounds with a K2NiF4 structure (space group
portion or the entire amount of cobalt in NdCaCoO4
with nickel.
I4/mmm) is observed.
For the sample containing no cobalt, the formation
of rhombically distorted K2NiF4 structure is observed.
All the nickel-containing samples comprise a small
amount of NiO. In addition, the cationic composition
of the resulting compounds with a K2NiF4 structure
changes with an increase in the nickel content in the
reaction mixture, as evidenced by a shift of reflections
(101) and (103) with an increase in the Ni/Co ratio.
The aim of this study is to explore the possibility of
synthesizing POM catalysts by the complete or partial
replacement of cobalt in the NdCaCoO4 matrix with
nickel and analyze the conversion and catalytic prop-
erties of the resulting materials in a methane–oxygen
mixture that is not diluted with an inert gas.
Comparison of the phase composition of the cata-
lysts before and after the catalytic conversions of the
methane–oxygen mixture (Fig. 1b) shows that the
POM reaction leads to the formation of Nd2O3, CaO,
and Co3O4 in the case of NdCaCoO4 and also NiO in
all other cases. At x ≥ 0.6, the presence of a metallic
phase is also observed; owing to the structural similar-
ity of the respective modifications, the reflections of
this phase can be attributed to nickel, cobalt, their
mixture, or solid solution. It should be noted that,
according to most of the researchers, it is the presence
of metal particles on the catalyst surface that is respon-
sible for the activity of nickel- and cobalt-containing
POM catalysts [1–8].
Results of the POM tests (see Table 1) show that all
the studied materials catalyzed the formation of syn-
thesis gas. For all the samples, an increase in the
methane conversion and the synthesis gas selectivity
with increasing temperature is observed; however, for
samples with different Ni/Co ratios, these parameters
increase in somewhat different ways. At 829–860°C,
the cobalt-containing samples provide an almost
complete oxidation of methane to CO2, as evidenced
by the minimum CO yield at a significant degree of
conversion of methane (23–28%) and a CO2 yield of
18–21%. At the same time, an increase in temperature
to 895–900°C leads to a change in the behavior of the
processes occurring on the catalyst surface and
thereby to a significant increase in the CO and H2
yields with a respective decrease in the CO2 yield. This
finding suggests that the complete catalytic oxidation
of methane is replaced by partial oxidation (POM).
EXPERIMENTAL
NdCaCo1–xNixOn (x = 0, 0.2, 0.4, 0.6, 0.8, 1)
materials were prepared by solid state synthesis. Mix-
tures of Nd2O3, CaCO3, Co3O4, and NiO in ratios cor-
responding to the cationic composition of the final
products were wetted with acetone and ground in a
Fritsch Pulverisette 5 planetary mill for 1 h. The
resulting powders were heat-treated at 1100°C for 24 h,
pelletized, annealed at 1200°C for 10 h, ground, repel-
letized, and annealed at 1200°C for 20 h. The long-
term synthesis procedure was used to achieve the max-
imum homogeneity of the chemical composition of
the precursor.
The phase composition of the resulting materials was
studied on a Rigaku MiniFlex 600 diffractometer; the
diffraction data were processed using the WinXPOW
software package.
The morphology of the catalysts before and after
use in POM was studied using a Carl Zeiss NVision 40
scanning electron microscope (SEM) equipped with
reflected (InLens) and backscattered electron detec-
tors (ESB) at an accelerating voltage of 7 and 1 kV,
respectively, and a magnification of up to 200000×.
The catalytic properties of the samples in the POM
reaction were studied in a heated flow-type quartz
reactor made in the form of a U-shaped tube with an
internal diameter of 5 mm and a thermocouple pocket
located between the inlet and outlet tubes of the reac-
tor. A catalyst was placed in the lower part of the reac-
tor; the free volume of the reactor below and above the
catalyst was filled with quartz chips to limit the contri-
bution of noncatalytic gas-phase methane conversion
processes and more correctly characterize the catalytic
properties of the samples. The catalyst loaded into the
reactor had a particle size of 0.5–1.0 mm and a weight
of 0.2 g. The reactor was fed with a mixture of methane
(99.99% purity) and oxygen (99.999% purity), which
was not diluted with an inert gas. The CH4/O2 ratio
The authors of [9] showed that the NdCaCoO4
compound with a K2NiF4 structure is capable of cata-
lyzing the complete oxidation of methane, whereas the
products of the reductive decomposition of
NdCaCoO4 catalyze the POM process [7, 8]. Taking
into account these features, a change in the behavior
of the methane oxidation processes with increasing
temperature is apparently attributed to a vigorous
reductive decomposition of the studied complex
oxides.
was 2; the gas mixture space velocity was 9 L (g h)–1.
The POM product composition was analyzed by gas
chromatography as described in [7, 8].
The relatively high CO yield observed at T < 900°C
in the presence of catalysts with a high nickel content
(x ≥ 0.4) shows that the reductive decomposition of the
oxide precursors of this compound occurs at signifi-
cantly lower temperatures than for NdCaCoO4 and
RESULTS AND DISCUSSION
Analysis of the phase composition of the products
of the solid state synthesis of the catalyst precursors
(Fig. 1a) showed that, at x = 0–0.8, the formation of
PETROLEUM CHEMISTRY
Vol. 58
No. 1
2018