New prospective Ni-catalytic materials

Article abstract of DOI:10.1023/A:1010140803562

IR spectroscopy was applied to investigate the non-isothermal redox processes in Ni/NiO-gas phase system involving the elimination of CO₂. At the Curie temperature (T_c=633 K for Ni), a change in the mechanism of CO catalytic oxidation on NiO was observed. It was established by means of temperature-programmed desorption with mass-spectroscopic registration that fluctuations in the oxygen density on the NiO surface are generated at this temperature. Under the given chemical reaction conditions, these fluctuations bring about the formation of Ni microheterogeneity on the NiO surface. X-ray diffraction, electronic and magnetometric measurements were employed to determine the dimensions of the Ni particles. It was shown that an increase in the Ni dispersity in Ni/NiO may be responsible for an increased number of active surface sites. This was verified for the NiO/thermo-exfoliated graphite system.

Full text of DOI:10.1023/A:1010140803562

Journal of Thermal Analysis and Calorimetry, Vol. 62 (2000) 345–348
NEW PROSPECTIVE Ni-CATALYTIC MATERIALS
V. L. Budarin, V. Diyuk, L. Matzui, L. Vovchenko, T. Tsvetkova and
M. Zakharenko
Kyiv Shevchenko University*, 64 Vladimirskaya str., Kyiv 252033, Ukraine
Abstract
IR spectroscopy was applied to investigate the non-isothermal redox processes in Ni/NiO–gas phase
system involving the elimination of CO2. At the Curie temperature (Tc=633 K for Ni), a change in
the mechanism of CO catalytic oxidation on NiO was observed. It was established by means of tem-
perature-programmed desorption with mass-spectroscopic registration that fluctuations in the oxy-
gen density on the NiO surface are generated at this temperature. Under the given chemical reaction
conditions, these fluctuations bring about the formation of Ni microheterogeneity on the NiO sur-
face. X-ray diffraction, electronic and magnetometric measurements were employed to determine
the dimensions of the Ni particles. It was shown that an increase in the Ni dispersity in Ni/NiO may
be responsible for an increased number of active surface sites. This was verified for the
NiO/thermo-exfoliated graphite system.
Keywords: catalysis, Curie temperature, nickel, temperature-programmed desorption,
thermo-exfoliated graphite
Introduction
The structure of the active sites present in heterogeneous catalysts is of importance in
catalysis. A change in the active site structure brings about change in the reaction
mechanism. Ni and Ni compounds supported on carriers are used both for technology
and for theoretical research. In the theory of catalysis, study of the influence of the
carrier on the disperse and surface energy of a supported catalyst, and thereby on the
active site structure and catalytic activity, is important [1, 2].
It is well known that two mechanisms of CO catalytic oxidation on NiO exist. These
mechanisms are manifested at different temperatures and have substantially different ac-
tivation energies. At high temperature, the mechanism of oxidation-reduction of the cata-
lyst surface is realized; at low temperature, the associative mechanism is observed. The
nature of this mechanism change has not yet been clarified. The change in mechanism
occurs at 623–673 K. For the solution of this problem, it is necessary to use tempera-
ture-programmed desorption (TPD) with mass-spectroscopic registration, ESCA, X-ray
diffraction, electronic and magnetometric methods.
*
E-mail: yats@chem.univ.kiev.ua
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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BUDARIN et al.: Ni-CATALYTIC MATERIALS
Experimental
Two kinds of systems containing nanoscale Ni particles were prepared and studied: Ni
particles grown on NiO surfaces by the reduction of NiO with CO at 623–703 K, and Ni
particles chemically deposited on the surface of thermo-exfoliated graphite (TEG).
The reaction of catalytic CO oxidation on NiO and the reaction of NiO reduction
with CO were studied in the closed set-up with a circulating argon flow. Pulse injec-
tion of the reactants was performed. The product concentration (CO2) in the IR cell
(cell length 10 cm) was determined with an IR spectrometer (Specord 71). A sensitiv-
ity in excess of 10–7 mol l–1 was obtained. TPD with mass-spectroscopic (MI-1201)
registration was used to study the removal of O and O2 from NiO. The surfaces of
NiO and NiO/TEG catalysts were analyzed by ESCA (Kratos 800XPS). By means of
X-ray diffraction, electronic and magnetometric measurements, the average size of
the Ni particles in both systems was determined.
Results and discussion
A change in the mechanism of catalytic CO oxidation on NiO was found to occur at
633 K (Fig. 1, curve 2). At high temperature, the mechanism involves the alternate
oxidation–reduction of the catalyst surface; at low temperature, the associative mech-
anism is observed. The associative mechanism includes the adsorption of compo-
nents on the catalyst surface, the following reaction and product desorption.
Fig. 1 Change in Arrhenius activation energy at the Curie temperature for 1 – NiO re-
duction with CO; 2 – catalytic CO oxidation on NiO and 3 – catalytic CO oxida-
tion on NiO/TEG
To explain the change in mechanism of the catalytic action of Ni and NiO at a
temperature near 633 K, the kinetics of the oxidation of Ni with molecular oxygen at
473–973 K and the reduction of NiO with CO at 423–703 K were studied in detail.
There are no critical phenomena at 633 K for Ni oxidation with O2. Under
non-isothermal conditions and without metallic Ni, NiO reduction with CO occurs
only at >633 K. At 633±4 K, a horizontal part is observed in the lnr=f(1/T) curve
(Fig. 1, curve 1). At 633 K, the mechanism of NiO reduction changes. This shows that
J. Therm. Anal. Cal., 62, 2000

BUDARIN et al.: Ni-CATALYTIC MATERIALS
347
at <633 K nuclear growth is possible. The nucleation of Ni is possible only when the
temperature passes the Curie temperature of Ni. This phenomenon is connected with
different states of the Ni particles [3].
On the whole, the kinetic regularities of NiO reduction with CO completely cor-
responded with those for catalytic CO oxidation on NiO: activation energy Ea at
>633 K is more than at <633 K (Table 1).
Table 1 Activation energies (Ea) of catalytic CO oxidation and NiO reduction with CO
System
Ea (T<633 K)/kJ mol–1
Ea (T>633 K)/kJ mol–1
200±8
NiO+CO (reduction)
NiO+CO+O2 (catalysis)
NiO/TEG+CO+O2 (catalysis)
120±5
50±5
58±4
125±7
178±14
For an explanation of the mechanism of Ni-phase particle formation, the kinetic
data must be compared with the TPD data [4]. The TPD data and Ea obtained from ki-
netic studies are shown in Fig. 2.
Fig. 2 Comparison of the data on oxygen desorption with Ea for catalytic CO
oxidation: a – Ea vs. temperature; b – TPD of O from NiO surface and c – TPD
of O2 from NiO surface
The sharp decrease in O and O2 emission from the NiO surface (Fig. 2b, c) oc-
curs at the same temperature as the abrupt change in Ea for catalytic CO oxidation
(Fig. 2a). The increase in Ea at >633 K is connected with the formation of Ni nuclei.
The TPD data indicate that Ni nucleation on NiO is connected with fluctuations in ox-
ygen density at 633 K. The mechanism of Ni fluctuation (Ni* formation) may be rep-
resented as follows:
NiO ↔ Ni*+NiO1+x x>0
The existence of NiO1+x on the NiO surface has been established by ESCA [5].
Thus, the mechanism of oxidation–reduction is possible only at >633 K. Under these
conditions, the stabilization of Ni fluctuation occurs in the initial stage of the process,
due to CO adsorption. The sizes of these clusters should be between 1 and 10 nm [6].
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X-ray diffraction, electronic and magnetometric measurements were employed to de-
termine the average size of the Ni particles in Ni/NiO. It is 5–12 nm when the Ni con-
centration is less than 5%. The sharp increase in the activation energy of catalytic CO
oxidation at a temperature above 633 K is connected with the formation of these clus-
ters. On the other hand, the number of small active surface sites increases with the
catalyst dispersity [7]. It can be assumed that the number of active sites and conse-
quently the activation energy for the NiO chemically deposited on the surface of TEG
will be increased. This was observed in our experiment (Fig. 1, curve 3, and Table 1).
The rate of catalytic CO oxidation on NiO/TEG and Ea are higher than those for NiO.
Conclusions
TEG is concluded to be a perspective carrier for the preparation of metal and metal
oxide catalysts. The chemical deposition of NiO on TEG offers increases in the num-
ber of active surface sites and in the catalytic activity.
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