52
SHESHKO, SEROV
nitrogen by BET measured ~30 m2/g, while the mean
size of the particles was 20 nm.
n × )
106, mol/(h gact.ph.
250
200
150
100
50
All of the investigated monometallic catalysts were
5% Fe or Ni, while bimetallic catalysts were 10%
active phase.
1
2
3
Before catalytic experiments, the catalyst samples
were subjected to reductive processing in a hydrogen
flow at 573 K and a bulk velocity of hydrogen of 1.5–
2.0 l/h. In the gas phase, we registered methane, CO,
and СО2, due to the presence of carbon and oxygenꢀ
containing particles adsorbed during the production
and storage of the samples. Attempts to completely
remove the abovementioned particles from the sample
surface were futile, although the yield of methane
upon reduction decreased.
RESULTS AND DISCUSSION
Performing the reaction of hydrogenation by mixꢀ
ing carbon oxides at the ratio of (СО : СО2) : Н2 = 1 : 2
and 1 : 4 on Ni UDP, Fe UDP, and their equivalent
mixtures showed that the main product was methane
(Fig. 1), the formation of which began at 403 K and
increased as the temperature grew. In performing the
reaction on ironꢀcontaining samples, were we detected
ethylene and ethane among the reaction products along
with methane (Fig. 1).
6
7
4
5
The qualitative and quantitative compositions of
the products, along with the values of the calculated
activation energies of methane formation and the preꢀ
exponential factor logarithm, are listed in the table.
From the table it follows that the addition of an equivꢀ
alent amount of nickel to iron nanopowders substanꢀ
tially enhanced the specific catalytic activity (SCA)
relative to the conversion of carbon oxide to methane
and ethane: on the bimetallic catalyst, the maximum
yield of hydrocarbons was several times higher than
the amount of СН4 obtained on samples containing
only iron nanoparticles. Enhancement of the logaꢀ
rithm of the preexponential factor in the Arrhenius
equation when performing the reaction in a dearth of
hydrogen indicates a rise in the number of catalytically
active centers of the surface, i.e., a synergetic effect
was observed. The 28% increase in olefin selectivity
(Fig. 2) in the range of low temperatures was also worꢀ
thy of note.
0
293
393
493
593
693
, K
T
Fig. 1. Temperature dependence of the yield of hydrocarꢀ
bons upon performing the reaction of hydrogenation in the
ratio of (СО + СО ) : Н = 1 : 4. (
2) Fe/Al O ; (3) Fe–Ni/Al O ; (4) С Н on Fe/Al O
1)
СН on Ni/Al O
;
;
2
2
4
2
3
3
(
2 3 2 3 2 6 2
(5
) Fe–Ni/Al O ; (6) С Н on Fe/Al O ; (7) Fe–
2 3 2 4 2 3
Ni/Al O
.
3
2
of Fe pentacarbonyl, the required amount of alumiꢀ
num oxide was placed into the plasma chemical reacꢀ
tor. The immediate matrixing of the metallic particles
in the carrier during their formation led to the iron
particles being fixed on the carrier.
Xꢀray studies revealed the presence of
α
and g
phases of iron; FeO, Fe3O4, and Fe2O3 oxides in powꢀ
ders; and trace amounts of Fe3C and free carbon.
Chemical analysis of the iron powders showed that
they contained up to 3% of free carbon and up to 2%
of bound carbon, and were ~80% metal. According to
the transmission electron microscopy data, the powꢀ
ders contained both separate particles and their
agglomerates. The smallest particles (2–5 nm) were
not faceted and formed large agglomerates. Particles
with the sizes of 5–15 nm had fair faceting, while the
larger particles with sizes of 15–25 nm were clearly
faceted. The specific surface of ironꢀcontaining cataꢀ
The ratio of saturated and unsaturated hydrocarꢀ
bons in the hydrogenation products was determined
mainly by the amount of atomic hydrogen, which was
able to migrate to the active centers of the surface, and
by the structure of these centers [4, 8]. Upon the
adsorption of hydrogen on metals able to dissolve it,
there is a possibility that two forms will emerge on the
surface [8, 9], one of which is associated only with one
metal atom (НI) while the other (НII), being strongly
adsorbed, is associated with several atoms. The formaꢀ
tion of hydrocarbons occurs through the stage of the
lysts defined by the lowꢀtemperature adsorption of formation of active carbon; however, the olefin selecꢀ
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 1
2011