9
30
SHESHKO et al.
4
+
was introduced into GdFeO . Selectivity toward ole- atomic hydrogen goes for the partial reduction of Fe
3
3+
fins on perovskite-like ferrites reached 40% and to Fe ; i.e., the heterovalent state of iron atoms favors
increased in the order SrFeO
(n = ∞) < Fe O < the formation of an unsaturated compound. At the
3
– x
2
3
4+
GdFeO (n = ∞) < GdSrFeO (n = 1) < Gd SrFe O
7
same time, a sample’s Fe content is greatest in
SrFeO , and the selectivity toward olefins is mini-
3
4
2
2
(
n = 2).
3
– x
mal. This can be explained by the lack of free atomic
In contrast to iron oxide, two linear regions were
3+
observed on the Arrhenius dependences of the rates of hydrogen and a reduction in the number of Fe sites
product formation when using gadolinium ferrites as active toward olefins.
catalysts. The presence of the two linear regions with
Perovskite-like gadolinium ferrite prepared using
different activation energies Е can be explained by the
а
ceramic technology was tested to determine the effect
n+
change in active centers Мe as a result of the altered
temperature of the state or by the change of the reac-
tion mechanism. Temperature T* at the bend was cal-
the method of a catalyst’s preparation has on its activ-
ity. The composition of the reaction products was the
same (С1–С5 hydrocarbons), but the component
ratio was different. A substantial drop in the rate of
ethylene formation was observed, the rates of methane
a1 − a2
culated using the formula T* =
as the solution
b − b
2
1
formation being close at all СО and Н ratios in the
to a system of two equations: y = a + bx, where a is the
2
logarithm of the pre-exponential factor, b = E /R. The reaction mixture (Table 1, Fig. 3).
a
Е values for methane and ethylene formation changed
а
The differences between the catalytic properties of
upon the transition at Т* = 523–548 K (Table 1). This
could have been due to a change in the mode of carbon
monoxide adsorption. At temperatures lower than Т*,
the mode of CO adsorption on the active catalytic cen-
ters is linear [5]; when Т > Т*, it is a bridge (probably
the samples obtained by various methods could be due
to several reasons. First, the samples obtained using
ceramic technology were sub-microcrystalline, while
ferrites synthesized with the sol–gel process were
nanocrystalline with a porous structure that facilitated
n+
0
carboxylate) mode on Ме –Ме centers with stron-
the transfer of atomic hydrogen and the CH -particles
x
ger bonds (heat of adsorption, Q ). The experimental
ads
formed in the reaction from one site to another. Sec-
ond, as was noted above, the heterovalent state of the
iron atoms suitable for the formation of olefins was
observed only for sol–gel products.
value of the reaction activation energy thus fell upon
the transition at Т* according to the well-known
Polani–Semenov correlation E = Е – αQ , where
а
0а
А
Q is the heat of chemical transformation, Е is the
А
0а
The content of reactant in the gas phase near the
catalyst surfaces was analyzed. It was shown that there
was intense adsorption of CO at room temperature,
true activation energy of the reaction, and αQ is the
А
contribution from the heat effect.
In addition, oxides SrFeO , GdFeO , GdSrFeO , and the shape of the curve was identical for all of the
3
– x
3
4
and Gd SrFe O are supposed to have several types of studied samples. After adsorption equilibrium was
2
2
7
active sites, some of which work at temperatures lower achieved, the composition of the gas phase (CO + H )
2
than 548 K while others work at higher temperatures. was stable up to 573 K, and the subsequent rise in tem-
As was mentioned above, the iron atoms in complex perature and transition to the catalytic range was
3
+
oxide GdFeO are in the Fe state in two fields of dif- accompanied by the formation of CO (Table 2). CO
2
3
4
+
conversion on all gadolinium-containing ferrites was
thus 55–75% and varied only slightly with tempera-
ture. It was higher on Fe O when the reaction was
ferent symmetry, while Fe with oxygen vacancies is
found in layered oxides SrFeO , GdSrFeO , and
3
+
– x
4
3
2
3
Gd SrFe O , in addition to Fe in fields of different
2
2
7
conducted in both a deficit and a surfeit of hydrogen.
symmetries. The heterovalent coordinationally unsat-
urated state of iron could be another reason for the
emergence of different active catalytic sites.
The temperature dependences of the carbon oxide
content in the reaction mixture when conducting the
process in stoichiometric amounts and a deficit of
hydrogen on the catalysts were found to be similar,
with CO conversion being nearly the same. We may
therefore assume that only carbon particles resulting
from the dissociative adsorption of carbon monoxide
at noncatalytic temperatures were involved in the for-
mation of reaction products.
The ratio of paraffins and olefins in the reaction
products was mainly determined by the amount of
atomic hydrogen able to migrate from one active sur-
face site to another [6, 7]. As was shown in [6, 8, 9],
two forms of chemisorbed hydrogen can exist on a
metal surface: Н (weakly bound) and Н (strongly
I
II
adsorbed). Hydrocarbons form via a stage of active
carbon formation, but the selectivity of the process
toward olefins is determined by the Н : Н ratio on
CO can form as a result of the interaction between
2
I
II
adsorbed molecules of CO (ads.) and either perovskite
surface oxygen (O (S)) or oxygen released upon the
dissociative adsorption of CO:
the catalyst’s surface. The non-isovalent substitution
3+
2+
of Gd for Sr is likely to distort the perovskite struc-
ture, reduce the symmetry of iron atoms, and lead to
the emergence of oxygen vacancies. Most of the labile
СО (ads.) + О(S) → СО .
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 90
No. 5
2016