FTIR studies on the selective oxidation and combustion of light hydrocarbons at metal oxide surfaces: Part 3. - Comparison of the oxidation of C3 organic compounds over Co3O4, MgCr2O4 and CuO

Article abstract of DOI:10.1039/a605341f

Oxidation of the C₃ organic compounds propane, propene, acrolein, propan-2-ol and acetone has been investigated over three transition-metal oxide catalysts, Co₃O₄, MgCr₂O₄ and CuO, in a flow reactor and using FTIR spectroscopy to study the adsorbed species. Co₃O₄ and MgCr₂O₄ are very active in propane and propene catalytic combustion. FTIR studies suggest that adsorbed isopropoxide species and adsorbed acetone and acetates are intermediates in propane oxidation while adsorbed acrolein and acrylates are intermediates in propene oxidation. Flow reactor studies support these hypotheses. It is suggested that the reaction rates in propane and propene total oxidation can be influenced, at low temperature, by the rate of oxidation of adsorbed acetate and acrylate intermediates, respectively. Co₃O₄ and MgCr₂O₄ are also active and quite selective catalysts for the oxydehydrogenation of propan-2-ol to acetone at low conversion, suggesting that the same oxygen species are involved in total and partial oxidation of organic compounds. CuO, as such, is not active in the adsorption and oxidation of C₃ hydrocarbons and oxygenates, at low temperature. At higher temperatures the reactants reduce the catalyst and catalytic activity starts. The oxidation state of the CuO_x catalyst can be evaluated by IR studying the transmittance of the radiation upon different treatments.

Full text of DOI:10.1039/a605341f

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FTIR studies on the selective oxidation and combustion of light
hydrocarbons at metal oxide surfaces
Part 3.¤ÈComparison of the oxidation of C organic compounds over Co O ,
3
3 4
MgCr O and CuO
2
4
Elisabetta Finocchio,a,b Ronald J. Willey,a Guido Buscab and Vincenzo Lorenzellib
a Department of Chemical Engineering, Northeastern University, Boston, Ma 02115, USA
b Istituto de Chimica, Universita, P. le J. F. Kennedy, I-16129 Genova, Italy
`
Oxidation of the C organic compounds propane, propene, acrolein, propan-2-ol and acetone has been investigated over three
3
transition-metal oxide catalysts, Co O , MgCr O and CuO, in a Ñow reactor and using FTIR spectroscopy to study the
3
4
2 4
adsorbed species. Co O and MgCr O are very active in propane and propene catalytic combustion. FTIR studies suggest that
3
4
2 4
adsorbed isopropoxide species and adsorbed acetone and acetates are intermediates in propane oxidation while adsorbed acrol-
ein and acrylates are intermediates in propene oxidation. Flow reactor studies support these hypotheses. It is suggested that the
reaction rates in propane and propene total oxidation can be inÑuenced, at low temperature, by the rate of oxidation of adsorbed
acetate and acrylate intermediates, respectively. Co O and MgCr O are also active and quite selective catalysts for the oxy-
3
4
2 4
dehydrogenation of propan-2-ol to acetone at low conversion, suggesting that the same oxygen species are involved in total and
partial oxidation of organic compounds. CuO, as such, is not active in the adsorption and oxidation of C hydrocarbons and
3
oxygenates, at low temperature. At higher temperatures the reactants reduce the catalyst and catalytic activity starts. The oxida-
tion state of the CuO catalyst can be evaluated by IR studying the transmittance of the radiation upon di†erent treatments.
x
Total oxidation processes of organic compounds for environ-
mental protection are mostly carried out industrially on the
quite unstable and expensive noble-metal-based catalysts,
such as those used in the catalytic converters of cars.1
However, the abatement of organic compounds from indus-
trial gaseous effluents can also be achieved with far less expen-
sive transition-metal-based catalysts.2,3 Some transition-metal
oxide systems can also be successfully applied to selective oxi-
dation processes where hydrocarbons or oxygenated com-
pounds are converted selectively into other organic
compounds with higher degrees of oxidation. In e†ect, total
oxidation is directly related to partial oxidation. While total
oxidation is the main factor limiting selectivity in processes
aimed at the production of partially oxidized compounds,
partial oxidation (even in traces) must be avoided in the cata-
lytic abatement of organic compounds, because the partially
oxidized compounds are frequently very toxic (e.g. aldehydes).
Many aspects of the catalytic selective oxidation mechan-
isms appear to be rather well established in the scientiÐc liter-
ature; among them, the involvement of lattice catalyst oxygen,
acting as nucleophilic oxygen4 in MarsÈvan Krevelen type
redox cycles.5 However, other aspects are still incompletely
explained, e.g. the nature of the active sites for and the mecha-
nism of CwH hydrocarbon activation. Conversely, the
mechanism of total oxidation over transition-metal oxides has
been poorly studied.2 In particular, the respective roles of
adsorbed oxygen species acting as electrophilic oxygen,4 and
of lattice “nucleophilcÏ oxygen, in catalytic combustion are not
fully clariÐed.6
Cobalt oxide Co O , chromium oxide Cr O and copper
oxide CuO are reported to be among the most active binary
3
4
2 3
oxides in catalytic combustion.8h11 MgCr O has been
2
4
reported to be more active and more stable than Cr O .12,13
2
3
In the present paper, some results of an IRÈÑow reactor
study of the oxidation of C organic compounds over two
3
typical p-type semiconductors (the spinels MgCr O and
2
4
Co O ) and an n-type semiconductor, CuO7 are reported.
The oxidation of the C hydrocarbons, propane and propene,
and of the oxygenates propan-2-ol, acetone and acrolein have
3
4
3
been chosen as test reactions to cover a wide range of VOC to
be oxidized. The mechanism of the oxidation of these com-
pounds has been studied using FTIR spectroscopy. The aim
was to establish better selective and total oxidation at the
transition-metal oxide catalyst surfaces.
Experimental
MgCr O (53 m2 g~1) was produced as an aerogel as reported
2
4
previously.14,15 Co O (15 m2 g~1) was a commercial com-
3
4
pound from Carlo Erba. Both were characterized as crystallo-
graphically pure spinel-type phases. CuO (18 m2 g~1) was
produced by thermal decomposition of commercial malachite
CuCO É Cu(OH) from Merck (Darmstadt, Germany), at 773
3
2
K. XRD analysis shows it to consist of pure CuO-tenorite.
Catalytic combustion experiments have been performed in a
Ñow reactor with 2 : 10 : 88 vol. % HCÈO ÈHe mixtures (HC
2
hydrocarbon or organic compound). The total Ñow was 500
Transition-metal oxides can be broadly divided into n-type
and p-type semiconductors. According to Bond,7 p-type semi-
conductors should be more active in catalytic combustion
than n-type semiconductors. On the other hand, most selec-
tive oxidation catalysts, e.g. those based on molybdates and
vanadates, should be classiÐed as n-type semiconductors.
ml min~1, with catalyst weight varying from 0.1 g for
MgCr O to 0.25 g for Co O and CuO, according to the
2
4
3 4
di†erent surface areas. The product analysis was carried out
using gas chromatography (HP 5890). FTIR spectra were
recorded with a Nicolet Magna 750 instrument, using conven-
tional IR cells connected to evacuation-gas manipulation
apparatuses. For IR analyses the powders were pressed into
self-supporting disks, calcined in air at 773 K and outgassed
at 773 K for 20 min.
¤
Part 2: Ref. 16
J. Chem. Soc., Faraday T rans., 1997, 93(1), 175È180
175

View Article Online
Results and Discussion
Conversion of propane and propene over MgCr O and Co O
2
4
3 4
catalysts
The light-o† curves relative to the oxidation of propane over
Co O , MgCr O and CuO catalysts are shown in Fig. 1.
3
4
2 4
Cobalt oxide is the most active, giving rise to substantial con-
version of propane from ca. 523 K. This agrees with literature
data that reported Co O to be, among binary oxides, the
3
4
most active in the oxidation of several organic com-
pounds.8h11 Over Mg chromite, propane conversion becomes
signiÐcant above 600 K.
The “light-o†Ï curves relative to the oxidation of propene
over the same catalysts are reported in Fig. 2. Over
MgCr O , propene conversion is deÐnitely higher than
Scheme 1
2
4
propane conversion under the same conditions. This is not
true for Co O , where the conversion of propane and of
However, we cannot exclude, at the moment, that temperature
gradients in the catalyst bed also have a role under these con-
ditions.
According to our previous IR studies, Mg chromite is less
active in CwH bond activation than Co O ; we detect
adsorbed species with a lower oxidation degree and at higher
3
4
propene are similar or that of propane is apparently even
higher at low temperature. On the other hand, while Co O is
3
4
far more active than MgCr O in propane conversion, the
2
4
behaviour of these catalysts is similar in propene oxidation,
with MgCr O being even more active than Co O .
3 4
2
4
3 4
temperatures. On MgCrO , propene activation is detected
In previous studies, we investigated the interaction of
propane and propene, and of possible intermediates in their
oxidation, over preoxidized MgCr O 14,15 and on Co O 16
4
from 373 K while propane activation occurs from 423 K.15 In
the case of propane oxidation, acetone is detected, later giving
rise to acetate and formate species. From propene, traces of
allyloxy species and acrolein were found, later giving rise to
acrylate species.
2
4
3 4
by IR spectroscopy. On Co O 16 we found that propene is
3
4
oxidized at the allylic position giving rise to acrylate species
already at room temperature, likely following the reaction
Scheme 1. On this catalyst, propane is also activated at a very
low temperature, at C(1) and at C(2). Activation at C(2) gives
rise to acetates (Scheme 2). This route may be the fastest reac-
tion route from propane at low temperature and could explain
the slightly higher rate of propane vs. propene oxidation.
Comparison of the Ñow reactor and IR data for the low-
temperature oxidation of propane and propene over Co O
3 4
and MgCr O suggests that, under these conditions, the oxi-
2 4
dation rate can be inÑuenced not only by the Ðrst CwH acti-
vation step (as it likely occurs for high-temperature
combustion) but also by other steps including, most probably,
the burning of carboxylate species, recognized as interme-
diates in catalytic combustion long ago17,18 and detected by
us to be quite stable intermediates, but disappearing under
these reaction conditions.
Conversion of propan-2-ol and acetone on MgCr O and
2
4
Co O catalysts
3
4
The oxidation path shown in Scheme 2 has been proposed by
us14,16 to represent the predominant surface mechanisms for
propane combustion on transition-metal oxides. To check this
idea, the behaviour of Co O and MgCr O in the oxidation
3
4
2 4
of propan-2-ol and acetone is compared in Fig. 3. Cobalt
oxide is more active than Mg chromite in propan-2-ol oxida-
tion, as the conversion curve for Co O [Fig. 3, solid curve
3 4
a)] is shifted down by nearly 60 K compared to the Mg chro-
(
mite conversion curve [Fig. 3, solid curve (b)]. In both cases,
high selectivities to acetone are found at low propan-2-ol con-
versions. The curve of acetone selectivity for propan-2-ol oxi-
dation decreases at the same temperature at which the
conversion of acetone in acetone oxidation increases. This is
Fig. 1 “Light-o† Ï curves for oxidation of propane over (a) Co O
4 (b)
3
MgCr O and (c) CuO
2
4
x
Fig. 2 “Light-o†Ï curves for oxidation of propene over (a) Co O ,
3
4
(
b) MgCr O and (c) CuO
Scheme 2
2
4
x
176
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Fig. 3 Conversion of propan-2-ol (ÈÈ), selectivities to acetone
upon propan-2-ol oxidation (É É É É É) and conversion of acetone (È È È)
over (a) Co O and (b) MgCr O
3
4
2 4
not unexpected, and suggests that acetone is overoxidized to
CO at high temperature in the propan-2-ol oxidation experi-
x
ments.
These data show that cobalt oxide and magnesium chro-
mite can act as selective oxidation catalysts towards propan-2-
ol, under very similar conditions where they are unselective
oxidation catalysts for hydrocarbons and acetone. This
strongly suggests that the active oxygen species involved in
selective and non-selective oxidation of organic compounds
are the same. According to our previous studies, and to the
established mechanisms of partial oxidation, these species
should be “nucleophilicÏ lattice oxygen.
It can be remarked that the curve of acetone oxidation (Fig.
3
) is, for the Co O catalyst, almost coincident with the curve
3
4
of propane oxidation (Fig. 1), at least in the low-conversion,
low-temperature region. IR studies16 have shown that, over
the same temperature range, acetates (that are formed from
both propane and acetone adsorption) are, on Co O , decom-
posed and burn. This supports the idea that adsorbed acetone
and acetates are intermediates in the propane combustion
mechanism (Scheme 2) and that the rate-determining step of
both reactions can be, at low temperature, a late step, very
likely the burning of the last detectable intermediates, the
acetate ions. This does not seem to be the case for MgCr O
3
4
2
4
where acetone combustion is faster than propane combustion
at any temperature. To gain more information, we investi-
gated, by FTIR spectroscopy, the stability of acetates over Mg
chromite (Fig. 4). These species, characterized by bands at
1
575, 1430 and 1350 cm~1 [l and l , (COO) and d ], are
Fig. 4 FTIR spectra of the adsorbed species arising from adsorption
as
sym
CH3
and oxidation of (a) acetic acid and (b) acrylic acid over MgCr O
partly decomposed in the temperature range 573È673 K. In
this temperature range acetone “lights o† Ï while conversion of
propane is still very low. This suggests that, in the case of Mg
chromite catalysts owing to the lower activity of the surface
sites in CwH bond activation, the propane oxidation rate is
determined by the Ðrst CwH activation step also at low tem-
perature.
2
4
upon outgassing at the given temperatures
just starting, in the Ñow reactor. On Mg chromite (Fig. 6B)
acrolein is adsorbed as such at room temperature but is con-
verted to acrylates near 423 K. Acrylates are oxidized in the
range 523È623 K on MgCr O (Fig. 4), which is the range
2
4
where catalytic conversion of acrolein starts and increases to
70%. The comparison of these data with those for propene
oxidation indicate that adsorbed acrolein and acrylates can be
intermediates in propene oxidation. Moreover, owing to the
near coincidence of the curves relative to propene and acrolein
oxidation on both catalysts, it is likely that the rate determin-
ing step is the same for both acrolein and propene oxidation
on both catalysts, under these conditions. IR data strongly
suggest that this step is associated with acrylate ion com-
bustion.
Conversion of acrolein on MgCr O and Co O catalysts
2
4
3 4
The oxidation path shown in Scheme 1 has been proposed by
us14,16 to represent the predominant surface mechanism for
propene combustion on transition-metal oxides. To check
this, the oxidation of acrolein has been investigated. The con-
version curves measured on MgCr O and Co O catalysts
2
4
3 4
are reported in Fig. 5. As in the case of propene conversion,
and unlike the propane, propan-2-ol and acetone conversions,
cobalt oxide is not more active than Mg chromite in acrolein
conversion. The IR spectra relative to acrolein adsorption and
oxidation on the two catalysts are compared in Fig. 6. On
cobalt oxide (Fig. 6A) acrolein is already transformed to acryl-
ate species at room temperature. These species burn below
Conversion of propane, propene and isopropyl alcohol on CuO
According to Germain and Perez,8 the most active binary
oxides in propene combustion are Co O and CuO. Since
3
4
5
23 K, i.e. at a temperature where conversion of acrolein is
CuO is formally an n-type semiconductor,7 unlike Co O and
3
4
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
177

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chromites that are p-type semiconductors, it seemed of interest
to study also the oxidation of C compounds on CuO. The
3
curves relative to propane and propene conversion over CuO
are shown in Fig. 1(c) and 2(c). The catalyst is far less active
than cobalt oxide and Mg chromite. The conversion of
propene starts near 573 K and is always greater than that of
propane, that starts near 623 K. IR spectroscopy conÐrms the
weak reactivity of the CuO surface with propane and propene.
We did not detect any reactive adsorption from either C3
hydrocarbon up to ca. 623 K. At this temperature both mol-
ecules react with the catalyst surface which becomes opaque.
This is interpreted as being due to the partial reduction of
CuO to copper metal. Accordingly, it is well known that
copper oxide based catalysts are good catalysts for propene
allylic oxidation to acrolein, but that the real catalyst is
reduced “in situÏ, at least in part, to copper metal.19
Fig. 5 “Light-o† Ï curves for the oxidation of acrolein over (a) Co O
3
4
and (b) MgCr O
2
4
In contrast, copper oxide appears to be quite a good cata-
lyst for propan-2-ol oxidation (Fig. 7), similar to cobalt oxide
and better than Mg chromite. Acetone is formed with quite
high selectivity at low conversion, as in the previous cases. At
higher temperatures, total oxidation predominates.
The spectra of the adsorbed species arising from contact of
propan-2-ol with CuO are reported in Fig. 8. The spectra
show bands at 1460 cm~1 (asymmetric CH bending), a
3
doublet at 1380 and 1370 cm~1 (symmetric CH bending),
3
together with a strong complex band with maxima at 1158,
1
145 and 1135 cm~1, typically due to coupled CwC and
CwO stretchings of alkoxide species (prop-2-oxides). These
species persist up to temperatures [380 K, while they almost
completely disappear by outgassing at 423 K. After this treat-
Fig. 7 Conversion of propan-2-ol (È È È) and selectivity to acetone
(
ÈÈÈ) over CuO
x
Fig. 6 FTIR spectra of the adsorbed species arising from acrolein
Fig. 8 FTIR spectra of the adsorbed species arising from propan-2-
adsorption and oxidation on (a) Co O and (b) MgCr O . Bands due
ol and acetone adsorption on CuO , after outgassing at the given
3
4
2
4
x
to adsorbed acrolein (…) and acrylates (X).
temperatures
178 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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ment, no traces of adsorbed species can be found. In particu-
lar, the IR spectra do not show any evidence of propan-2-ol
oxidation at the surface under these conditions. This behav-
iour is markedly distinct from that observed on Mg
chromite14 and on Co oxide,16 where the isopropoxy group
gives rise to strongly adsorbed acetone that can later desorb
or be further oxidized. To obtain more information, we also
studied acetone adsorption on CuO (Fig. 8, upper spectra).
Molecular species can be found at room temperature. These
species are mainly characterized by the CxO stretching band
at 1713 cm~1, i.e. only shifted down slightly with respect to
the value for gas-phase acetone (1731 cm~1), and easily desorb
on outgassing. The value of l(CxO) of acetone adsorbed on
Co O and MgCr O is 1700 cm~1 and 1710, 1695 cm~1
3
4
2 4
(
split), respectively. This shows that acetone is only slightly
more strongly adsorbed on cobalt oxide and Mg chromite
than on CuO. However, even on heating CuO in the presence
of propan-2-ol, we cannot Ðnd any adsorbed acetone up to
400 K. On further heating CuO in contact with propan-2-ol
above 450 K the sample becomes opaque to IR radiation.
The adsorption and the complete desorption without oxida-
tion of propan-2-ol and acetone on CuO are observed in a
temperature range where catalytic activity still does not occur.
These processes are likely not related to catalysis. On the con-
trary, at a temperature higher than 450 K propan-2-ol reduces
the catalyst. This is likely related to the catalytic reaction and
would support a MarsÈvan Krevelen-type mechanism where
the reactant reduces the oxide catalyst. The conclusion of our
study, in agreement with previous studies concerning propene
oxidation19 and propan-2-ol (oxy-)dehydrogenation20,21 on
copper oxide, is that CuO is almost inactive in oxidation, but
when reduced, at least in part, to copper metal, the activity
rises signiÐcantly. Unfortunately, because of the opacity of the
sample in these conditions, we cannot follow the oxidation
mechanism by IR spectroscopy.
Characterization of semiconductor/conductor properties of
CuO (0 Æ x91) by IR spectroscopy
x
In previous papers we applied IR spectroscopy to give infor-
mation on the p-type semiconductor properties of Co O
3 4`x
and of MgCr O
catalysts, in relation to their catalytic
2
4`x
activity. The IR data gave some evidence on the
semiconductor/conductor transition of our CuO disks on
reduction by organic compounds. As discussed previously22
CuO is quite a basic surface and is reduced partly by simple
outgassing, as deduced by the IR spectra of adsorbed CO that
provide evidence of Cu(I)-carbonyls. To get a clearer picture,
we investigated the adsorption of a more acidic and oxidizable
molecule, i.e. acetic acid (Fig. 9). Acetic acid is adsorbed quite
x
Fig. 9 FTIR spectra of a CuO pressed disk (a) after activation, (b)
x
after adsorption of acetic acid and after outgassing at (c) 393, (d) 423,
(
e) 473, (f) 533 and (g) 573 K
According to the literature data, CuO is a semiconductor,
with the absorption edge centred near 850 nm,24 i.e. in the
near-IR region. In fact, it is black, owing to the almost total
strongly on outgassed CuO (x B 1) at room temperature
x
giving mainly acetate ions, characterized by bands at 2988,
visible light absorption. On the contrary, Cu O is red and
2
broad, and 2935 cm~1 (CH stretching), 1570 cm~1
accordingly, its absorption edge falls in the visible region (610
3
(
asymmetric COO~ stretching), 1433 cm~1 (symmetric COO~
nm).25 The increase in transmission of our disks in the IR
region upon adsorption/outgassing of acetic acid can be
stretching), 1340 cm~1 (CH deformation), 1052 and 1025
3
cm~1 (CH rocking) and 924 cm~1 (CwC stretching).23
associated with a CuO ] Cu O progressive transformation.
3
2
Bands at 1720, 1626 and 1290 cm~1, present before outgas-
In contrast, the opacity of the samples when they are involved
sing, are related to non-dissociative adsorption of acetic acid
in monomeric and dimeric forms. Outgassing at temperatures
in the range 300È473 K leads to disappearance of the bands
due to the undissociated acid forms and to progressive forma-
tion of new bands in the region 1800È1600 cm~1, likely
associated with carbonate species. Upon this treatment, the
transmission of the IR light by the sample increases strongly.
The maximum transmittance near 1100 cm~1 passes from
near 12% for the activated sample to near 45% after outgass-
ing at 473 K. At even higher temperatures, the bands due to
carboxylates progressively disappear and bands due to car-
bonates grow. In these conditions the transmission of the IR
light by the sample weakens strongly.
in deeper oxidations of organic compounds is indicative of
partial transformation to Cu metal.
These data allow us to conclude that upon interaction with
propane, propene and propan-2-ol, our CuO sample is only
x
partly reduced, with 0.5 \ x \ 1. When catalytic reaction
occurs, however, the sample is deeply reduced, with
0 \ x \ 0.5.
The much higher catalytic activity of CuO for propan-2-ol
x
oxidation than for propane and propene oxidation, that dis-
tinguishes this material from cobalt oxide and Mg chromite,
can be assigned to the poor activity of Cu` sites (likely pre-
dominant in CuO with 0 \ x \ 0.5) for activation of the
x
CwH hydrocarbon bond. Previously, we proposed that such
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
179

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reduced and deeply reduced pressed disks. Slightly reduced
copper oxide disks transmit IR radiation better than oxidized
CuO. Deeply reduced samples, in part containing Cu metal,
are opaque. Nearly stoichiometric CuO adsorbs the organics
investigated here very weakly and is essentially inactive in
catalytic oxidation. However, it apparently becomes active in
oxidation when it is reduced, at least in part, to Cu metal.
activation can be associated with a “polarizationÏ of such
CwH bonds over Lewis acid cations, followed by a direct
electron exchange with the passage of two electrons from the
CwH p bond to the reducible cation.15,26 It is possible that,
owing to their very poor Lewis acidity, oxidized Cu centres
are poorly e†ective in polarizing CwH bonds of hydrocar-
bons. The higher activity in propan-2-ol oxidation can be
associated with the weakly BrÔnsted acidic character of such a
molecule in relation to the relevant basicity of Cu oxides,
allowing a stronger interaction and no necessary CwH bond
activation.
The authors acknowledge MURST (Rome, Italy) for Ðnancial
support.
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10 J. E. Germain and L. Laugier, Bull. Soc. Chim. Fr., 1972, 541 and
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T rans., 1996, 92, 1587.
1
The data presented here give three di†erent scales for the
catalytic activity in the oxidation of the three C organic com-
7
V. A. Kutznetsov, S. V. Gerei and Ya. B. Gorokhovatskii, Kinet.
Katal., 1977, 18, 710.
3
pounds: for propane oxidation the scale is Co O [
3
4
18 A. A. Davydov, Infrared Spectroscopy of Adsorbed Species on the
Surface of T ransition Metal Oxides, Wiley, New York, 1990.
19 H. H. Voge and C. R. Adams, Adv. Catal. Relat. Subj., 1967, 17,
151.
MgCr O [ CuO ; for propene oxidation the scale is
2
4
4
x
Co O B MgCr O [ CuO ; for propan-2-ol oxidation the
2 4
conversion scale is Co O P CuO [ MgCr O .
3
x
3
4
x
2 4
20
21
22
23
A. Gil, P. Ruiz and B. Delmon, J. Catal., 1996, 159, 496.
J. C. Volta, P. Turlier and Y. Trambouze, J. Catal., 1974, 34, 329.
G. Busca, J. Mol. Catal., 1987, 43, 225.
K. Nakamoto Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, Wiley, New York, 4th edn., 1986, p. 232.
Comparison of the conversion of propane and acetone, and
of propene and acrolein supports our previous proposal,
based on IR experiments, that adsorbed acetone and acrolein
are intermediates in the total oxidation of propane and
propene, respectively. The data also suggest that, at low tem-
perature, the rate of propane and propene oxidation can be
inÑuenced by late steps, such as the burning of adsorbed
acetate and acrylate species, respectively.
24 F. S. Stone, Bull. Soc. Chim. Fr., 1966, 819.
25 J. Bloem, Phillips Rev. Rept., 1958, 13, 167.
2
6
G. Busca, E. Finocchio, G. Ramis and G. Ricchiardi, Catal.
T oday, in the press.
2
7
A. Terlecki-Baricevic, B. Grbic, D. Jovanovic, S. Angelov, D.
Mehandziev, C. Morinova, and P. Kirilov-Stefanov, Appl. Catal.,
The relative activity of Co O and MgCr O seems to
3
4
2 4
parallel the similar stabilities of acrylate species on the two
surfaces, in contrast to the very di†erent stabilities of acetate
ions.
1
989, 47, 145.
28 K. R. Barnard, K. Foger, T. W. Tunney and R. D. Williams, J.
Catal., 1990, 125, 265.
2
9
H. G. Lintz and K. Wittstock, Catal. T oday, 1996, 29, 457.
Paper 6/05341F; Received 30th July, 1996
The n-type semiconducting oxide CuO behaves di†erently.
IR spectroscopy allow us to evaluate the oxidation state of
CuO , owing the di†erent transparencies of oxidized, slightly
x
180
J. Chem. Soc., Faraday T rans., 1997, V ol. 93