(141d) Role of CeO2 In the Total Oxidation of Toluene Over CuO-CeO2/Al2O3

Introduction

Benzene, toluene and xylene
(BTX), are considered to be three major carcinogens and their emission in the
environment has to be controlled. Among the destructive technologies, catalytic
oxidation is the most promising approach for the abatement of Volatile Organic
Compounds (VOCs) owing to its advantages including high efficiency, low
operating temperature and no secondary pollution[1].
The performance of the catalyst determines the effectiveness of this technique.

Supported or unsupported noble
metal catalysts are used for complete oxidation of VOCs, but high costs limit
their wide application, thus giving way to transition metal oxides such as
copper oxide. These are less active at lower temperatures but present comparable
activity at higher temperatures and have high catalyst loading capabilities[2-6].

However, a pure copper-based
catalyst is less active and stable in the presence of water vapor and/or CO₂
in the gas mixture. At the same time, copper catalysts promoted by ceria show
better catalytic performance for the complete oxidation of toluene, propane, benzene
and p-xylene than unpromoted copper catalysts[7,8]
as well as improved activity in the presence of oxidation products CO₂
and water[9]. Ceria as a promoter for
supported CuO has shown several advantages: (a) ceria stabilizes the dispersion
of the active component[10,
11]; (b) metal/ceria
interactions strongly affect their redox and catalytic properties; (c) ceria
also acts as an oxygen storing component due to the presence of mixed oxidation
states (3+/4+) of cerium; (d) copper-doped CeO₂ can improve the
oxygen storage capacity, diffusivity and redox properties, which are attributed
to the formation of structural defects.

However, it is still unclear how
copper and ceria interact with each other and promote catalytic activity. The
questions addressed in this study are: (1) Assistance of CeO₂ in the
oxidation of reduced copper species, (2) the crucial copper species inducing
the copper?ceria interaction, (3) the active sites of copper?ceria system for
toluene oxidation, (4) the role of ceria in decreasing the negative effect of the
presence of water and CO₂, (5) the enhancement of catalytic activity
due to copper?ceria interaction

In this study the binary mixed
oxide, CuO-CeO₂/g-Al₂O₃, has
been investigated in detail in comparison to its corresponding single-oxide
components CuO/g-Al₂O₃, CeO₂/g-Al₂O₃
in order to correlate catalytic activity towards total oxidation of
toluene with their physicochemical characterization. A transient response
technique with millisecond time scale[12] was used to investigate the
catalytic activity and influence of H₂O and CO₂ on the
catalytic behavior. The structure of the catalysts was investigated by the use
of high resolution electron microscopy (HRTEM), selected area electron
diffraction (SAED), X-ray diffraction (XRD) and X-ray absorption Spectroscopy
(XAS).

Experimental procedure

The CuO-CeO₂/γ-Al₂O₃,
CuO/γ-Al₂O₃ and CeO₂/γ-Al₂O₃
catalysts were synthesized via impregnation of γ-Al₂O₃
with Cu(NO₃)₂×2.5H₂O  and/or Ce(NO₃)₄×9H₂O precursors, followed by drying at 357K for 8h and
calcination above 973 K for 8h in air. 10mg of 250-500mm particles were used for experiments to determine the catalytic
activity.

The selected area
electron diffraction (SAED) and high resolution electron microscopy (HREM) of
the samples were carried out in a transmission electron microscope (TEM) operating at 300 kV (Jeol, JEM-2200FS).

The temporal analysis of products
(TAP) experiments are performed in a quartz micro-reactor, with 33mm bed-length
and 4.75mm inner diameter, evacuated to 10^-4 ? 10^-5 Pa. A
very small amount of reactant molecules (~10^-9 mol), which is ~5
orders less than the active sites in the catalyst, is pulsed into the reactor by
means of two high-speed pulse valves. The products and reactants at the outlet
of the reactor are monitored by a quadrupole mass spectrometer. A temperature
range of 723K -873K is covered. To study the activity of the CuO-CeO₂/γ-Al₂O₃,
CuO/γ-Al₂O₃ and CeO₂/γ-Al₂O₃ catalysts towards toluene total
oxidation, experiments were performed over O₂ pre-treated catalysts
by pulsing C₇H₈ with and without di-oxygen in the feed. Typically, a stoichiometric ratio
of di-oxygen to toluene, 9:1, was used in the mixture C₇H₈/O₂/Ar,
when experiments were conducted in the presence of di-oxygen.

In situ XAS experiments were
performed at the SAMBA beam line of the SOLEIL synchrotron. X-ray absorption
near edge (XANES) spectra were recorded with 1s time resolution during H₂
reduction and reoxidation with CO₂ and water at both the Cu K (8979 eV)
and Ce L_III edge (5723 eV).

Results

XRD analysis of the
CuO-CeO₂/γ-Al₂O₃ catalyst shows that
alumina and ceria have a crystallite diameter of about 5 nm. At the same time CuO presents single
crystals of about 100nm. From the STEM and SAD images the CuO single crystal
units of ~100nm were confirmed, while CeO₂
appeared as 100nm clusters of  nanocrystallites. EDX line scans were undertaken
through specific regions of a STEM frame, displaying the elements present along
the line (see Fig.1). The line scan proves that in addition to the large CuO crystallites,
 CuO is also present inside the CeO₂ clusters as small sized
species. In situ XANES measurements at the CeL_III and Cu K edge
yield direct evidence that CeO₂ in the CuO-CeO₂/g-Al₂O₃ catalyst
is reduced by H₂ at similar temperature as the CuO phase. Fig. 2
shows XANES results which confirms partial reoxidation of ceria by water. Whereas,
CuO is not reoxidised by water.

The toluene oxidation experimental
data indicate that the reaction is carried out according to a redox Mars-van
Krevelen mechanism. Lattice oxygen atoms from the surface of the catalyst are
consumed by toluene and oxygen vacancies are created. These vacancies are
filled by the oxygen atoms that diffuse from the bulk to the surface of the
catalyst or by di-oxygen. It is shown that the degree of reduction of the metal
oxide determines to a large extent its  performance for VOCs oxidation. The
presence of water or CO₂  on the catalyst surface decreases the rate
of regeneration of the reduced catalyst by di-oxygen. Furthermore, if water
and/or CO₂ are present in the gas phase, the rate of desorption from
the surface becomes slower due to the high partial pressure of water in the gas
phase.

The activity of the
CuO-CeO₂/Al₂O₃ catalyst increases in the presence
of H₂O or CO₂. On the other hand, CuO/Al₂O₃
showed loss of activity in the presence of H₂O. There was no
significant changes in activity on CuO/Al₂O₃ and CeO₂/Al₂O₃
when co-feeding toluene with CO₂.

Conclusions

Two CuO phases exist
in the CuO-CeO₂/Al₂O₃ catalyst: large ~100nm crystals, not contacting CeO₂ and small CuO species
dispersed upon and incorporated into clusters of CeO₂
nano-crystallites with size ~5nm.

The
effect of CeO₂ on the catalytic activity can be explained by two
factors. First, ceria stabilizes the dispersion of the active component, i.e. a
smaller crystallite size of the CuO phase on the support becomes one of the key
factors in determining the chemical reactivity. Second, incorporation of copper
into the CeO₂ lattice can form Ce_1-xCu_xO_2-d
solid solution which also has high activity and reducibility at lower
temperatures compared to CuO or CeO₂ alone. The promoting effect of
ceria is found to be more pronounced at low temperatures.

Adding
H₂O or CO₂ into the C₇H₈/O₂
mixture increases the catalytic activity of CuO-CeO₂/Al₂O₃
catalyst by producing lattice oxygen species  from  reoxidation of ceria by CO₂
or H₂O. Over the CuO/Al₂O₃
catalyst the re-oxidizing role of CO₂ or H₂O
co-fed with toluene is insignificant.

Acknowledgement

This work was supported
by a Concerted Research Action (GOA) of Ghent University and the ?Long Term
Structural Methusalem Funding by the Flemish Government'.

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