CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels

Article abstract of DOI:10.1016/j.catcom.2016.09.016

Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent activity for CO-O₂ and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is having an enhancing effect on the catalytic conversion. Best a

Full text of DOI:10.1016/j.catcom.2016.09.016

CO-NO/O2 redox reactions over Cu substituted cobalt oxide spinels
A.V. Salker, M.S. Fal Desai
PII:
DOI:
Reference:
S1566-7367(16)30333-8
doi: 10.1016/j.catcom.2016.09.016
CATCOM 4785
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
31 May 2016
5 August 2016
12 September 2016
Please cite this article as: A.V. Salker, M.S. Fal Desai, CO-NO/O2 redox reac-
tions over Cu substituted cobalt oxide spinels, Catalysis Communications (2016), doi:
1
0.1016/j.catcom.2016.09.016
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CO-NO/O redox reactions over Cu substituted cobalt oxide spinels
2
A. V. Salker* and M. S. Fal Desai
Department of Chemistry, Goa University, Goa 403206, India
Abstract
Catalytic conversion of NO and CO over Cu substituted cobalt oxide spinels show excellent
activity for CO-O and NO-CO reactions. Lower concentration of Cu in cobalt oxide spinel is
2
having an enhancing effect on the catalytic conversion. Best activity among the tested
◦
catalyst was found for Co2.9 Cu O and complete conversion (100%) is observed at 93 C for
0.1
4
◦
CO oxidation by O and 209 C for NO reduction by CO. Prepared catalysts show promising
2
activity compared to few of the precious metal based catalysts reported in the literature. The
influence of moisture and oxygen on catalytic conversion has been studied.
Keywords
Cobalt oxide spinels, Copper substitutions, Inverse spinels, Sol-gel, CO-NO reaction.
------------------------------------------------------------------------------------------------------------
*
Corresponding author: Tel: +91-832-6519315; Mobile: +91-7083740482
E-mail address: sav@unigoa.ac.in; avsalker@gmail.com
1

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1
Introduction
CO oxidation over Cu substituted cobalt oxide spinels has been studied extensively by
various research groups [1-4]. Cu is an electron rich transition metal which is very closely
related to the precious group metals. It is well known for good CO chemisorption property [5,
6
]. Majority of the work published show that there is increase in the catalytic activity with
increase in Cu substitution for various catalytic reactions. Recent experiment and DFT
calculation for CO oxidation by O done by M. Zhou et al. showed an increase in activity
2
only up 15% of Cu in Co O nano-rods, as higher concentrations have resulted in biphasic
3
4
materials which were found to be less active [7]. The reason stated for lower activity by the
respected group, is the formation of biphasic materials of CuO and Co O above 15% of Cu.
3
4
Catalytic activity is not only enhanced by nano size and morphology of the catalyst but also
on the availability of surface active sites for the redox reaction. Increase in the concentration
3
+
of Cu tends to change its normal cobalt spinel to inverse spinel structure with Co at the
tetrahedral sites [8-10]. Thus, increase in Cu concentration in cobalt oxide spinel has a lot of
scope for further investigations.
3
+
The literature insights, Co is an active site for CO conversion to CO and subsequently
2
3
+
2+
Co gets reduced to Co [11, 12]. Thus, change in active site should result in change in the
catalytic activity. Our investigation over two catalytic reactions showed that the decrease in
activity with the increase in concentration of Cu, supports that there is a decrease in
availability of active sites. Change in spinel structure from normal to inverse has a significant
effect on the catalytic activity. Catalyst activity in presence of moisture and oxygen is also
investigated which showed their temporary effect on the activity.
2
Experimental
2
.1 Preparation of Co3-XCuxO4
2

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Co Cu O (where, X = 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared using
3
-x
x
4
Co(NO ) .6H O (Sigma Aldrich) and Cu(NO ).5H O (Sigma Aldrich). Cobalt and copper
3
2
2
3
2
salts in calculated amounts were dissolved in distilled water and citric acid monohydrate
Thomas Baker, AR) was added in the molar ratio 1: 1 (Co/Citric acid). Solution was stirred
(
for 1 h and then evaporated to get a viscous gel. This gel was dried in the oven at 150 ˚C for 3
h which resulted in a foamy metal citrate precursor. This foamy complex was ground and
then sintered at 600 ˚C for 7 h in air to get final copper substituted cobalt spinel oxides.
2
.2 Characterization
The spinel phase of Cu Co O4 was analyzed using X-ray powder diffractometer
x
3-x
(Regaku Ultima IV) with Cu-Kα source, filtered through Ni absorber, at a scanning rate
o
1
/min. BET surface areas were measured with the help of QUANTACHROME
AUTOSORB IQ-MP-C surface area analyzer at liquid nitrogen temperature. XPS studies of
the catalysts were carried out using PHOIBOS HSA3500 DLSEGD 150 R7 with Al Kα
-11
excitation source (E=1486.74eV). The chamber vacuum was 6x10
mbar. Magnetic
susceptibility (χ ) of the spinels in ambient air was determined by Guoy method at room
g
temperature with the field of 0.75 T (7500 G), using Hg[Co(SCN) ] as a standard material.
4
2
.2 Catalytic tests
The reactions of CO - O and CO - NO were carried out in a continuous flow, fixed bed
2
glass reactor. The CO oxidation by O was analyzed with the feed gas composition of 5% CO
2
-1
and 5% O in N at the rate of 5000 ml h . CO oxidation by NO was studied with feed gas
2
2
5
% NO and 5% CO in Ar at the same flow rate. 1 g of the catalyst samples was supported in
between the glass wool plugs in a glass reactor. Reactor was placed in a temperature
programed electrical furnace and activities of the catalysts were measured with respect to
increasing temperature. The feed gases and products were analyzed using online Gas
3

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Chromatograph employing Molecular sieve 13x and Porapak Q columns with TCD detector,
H was used as a carrier gas. Details about the analysis are given in the supplementary
2
information. Stability of the catalyst was also analyzed in the presence of 2.5% moisture and
2
-
.5% O with the same feed gas composition. Simultaneous effect of moisture and O for NO
2 2
CO redox reaction was also investigated using 2.5% O with 2.5% of moisture with the
2
same feed gas composition.
3
Result and discussion
XRD pattern of Co O in Figure 1 reveals monophasic nature of all polycrystalline
3
-x
4
catalyst samples except for the value of X = 1 (JCPDS card no. 42-1467). A tiny impurity
peak could be noticed in the pattern of X = 1, which is attributed to CuO (JCPDS card no.48-
1
548). No extra reflections of CuO in the XRD patterns of other composition were observed
which suggests incorporation of Cu in the spinel lattice. The pattern substantiates the
formation of cubic spinel phase of cobalt oxide. Lattice parameter is given in Table 1 for all
the compounds. Formation of spinel phase was also evident from the IR studies qualitatively
(Supplementary Figure S1).
BET surface area showed that the catalysts have fairly good and uniform surface areas,
represented in Table 1. X-ray photoelectron spectra showed prepared catalyst were pure in
composition. Only Co, Cu and oxygen reflections are seen and no other elemental peaks were
noticed in the full scan (Supplementary Figure S ). De-convoluted cobalt 2p shows the
3
3/2
3
+
2+
presence of Co at 781.5 eV and Co at 779.7 eV which is attributed to the tetrahedral and
3
+
2+
octahedral Co, respectively (Figure 2a). Co and Co reflections are also seen in cobalt
2
p1/2 peak. De-convoluted XPS peaks of copper 2p3/2 at 932.9 eV and 2p1/2 953.0 eV showed
the presence copper in 2+ oxidation state (Figure 2b) [13].
4

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CO oxidation by O and NO reduction by CO were studied with respect to increase in
2
temperature, depicted in Figure 3 and Figure 4, respectively. There was a gradual decrease
in CO conversion observed with the increase in Cu substitution. The prepared catalyst
showed better activity with 1: 1, CO to O ratio. M. Zhou et al. reported CO oxidation over
2
Cu substituted Co O at 120 ˚C for complete conversion of CO in excess of O [8]. In the
3
4
2
present study high activity was found for Co Cu O and complete conversion (100%) is
2
.9
0.1
4
◦
◦
observed at 93 C for CO oxidation by O and 209 C for NO reduction by CO. NO
2
conversion was observed in-line with CO conversion till 350 ˚C (Supplementary Figure S5).
Marginal fall in NO conversion was observed above 350 ˚C due to increase in the stability of
NO at higher temperature. Selectivity for N O is represented in Supplementary Figure S .
2
6
Lower concentration of Cu (Co2.95Cu0.05O ) also showed improvement in the activity
4
compared to the pristine oxide, however it was observed lower compared to Co Cu O
4
2
.9
0.1
(
Supplementary Figure S ). Good selectivity for N and very low selectivity (< 15%) for
13 2
N O is observed for the catalyst. Prepared catalysts have shown promising activity compared
2
to the some precious metal based catalysts [14-16]. Catalyst activities for both the redox
reaction over pristine CuO is given in supplementary information. Probably, the
3
+
chemisorption of CO enhanced by the Cu substitution but decrease in availability of Co has
lowered the conversion. Franken and R. Palkovits studied the N O catalytic decomposition
2
and found there was decrease in activity with the increase in Cu in cobalt spinel oxide and
resultant trend in activity was attributed to change in cobalt spinel from normal to inverse
type [15]. 100% CO conversion by O was observed at lower temperature compared to CO
2
oxidation by NO. NO has the bond order 2.5 while O have 2, possibly higher energy is
2
required to activate dissociation of N-O than O-O bond.
3
+
3+
Co at octahedral site is an active site for CO conversion to CO as Co gets reduced to
2
2
+
Co with loss of lattice oxygen [11, 12]. This is based on Mar’s Van Krevelen mechanism
5

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3+
which is also valid for NO reduction by CO. Decrease in availability of Co should have a
same effect on the CO oxidation by NO as that of CO oxidation by O , as it follows the same
2
mechanism as that of CO oxidation over metal oxide. Reduction in number of available
active sites for CO oxidation has same effect on the catalytic conversion of NO which is
reflected in both the catalytic reactions.
The prepared cobalt oxide spinel compounds are found to be paramagnetic at room
3
+
temperature. Higher concentration of Cu in cobalt oxide spinel forces Co to the tetrahedral
site. This change is reflected in effective magnetic moment in magnetic susceptibility (χg)
represented in Table 1 [18, 19]. Increase in magnetic moment suggests a gradual increase in
3
+
Co in tetrahedral site thus changing normal spinel structure of cobalt oxide to inverse
3+
spinel. Theoretically in cobalt oxide spinel, Co has zero magnetic moment at octahedral site
as all the electrons are paired and observed magnetic moment is mainly contributed by to
2
+
2+
4
3
Co in tetrahedral site in the cubic spinel lattice. Co in tetrahedral has e and t2 where
3
+
there are three unpaired electrons which will result in magnetic moment of 3.87 BM. Co at
the tetrahedral site results in increase in magnetic moment with four unpaired electron due to
3
3
2
e t electronic configuration.
Stability of the catalyst in various reaction conditions are represented in Figure 4.
Highly active Co Cu O was found stable in the reaction condition for 12 h for NO
2
.9
0.1
4
reduction by CO and CO oxidation by O . Moisture reduced the conversion of NO by
2
marginal value due to condensation of H O on the surface of catalyst. O has decreased the
2
2
activity to about 75% as O in feed gas oxidizes NO to NO and compete with NO in
2
2
oxidation of catalyst surface. Simultaneous effect of O and moisture shows further decline in
2
the activity to about 70%. However it was observed that the catalyst regained its activity to
1
00% when flow of O and moisture were stopped from the feed gas studied as seen in
2
6

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Figure 5. These results indicate that the effect of O and moisture is temporary, as on
2
removal of these, catalyst regains the original activity.
4
Conclusions
Monophasic Co Cu O catalysts prepared by citric acid assisted sol-gel method have
3
-x
x
4
2
-1
fairly good surface area in the range 65 - 85 m g . Increase in the amount of Cu in cobalt
3
+
spinel has increased the magnetic moment by one electron suggesting Co at the tetrahedral
site in cobalt spinel oxide. Inverse spinels have shown low catalytic activity for CO
conversion for both the reactions studied. Catalytic activity is not only attributed to the high
surface area and presence of Cu which is an active metal that facilitates CO adsorption but
3
+
also to the availability of Co which has a significant role on the CO conversion. Moisture
and O have temporary effect on the catalyst activity.
2
Acknowledgment
Authors would like to thank UGC - New Delhi, for the financial assistance under SAP DSA-I
program (No. F.540/14/DSA-I/2015).
7

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[
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Figure Caption
Figure 1. X-Ray diffraction pattern of Co3-XCu O .
X
4
Figure 2. XPS spectra of Co2.9 Cu O a) Co 2p and b) Cu 2p.
0.1
4
-1
Figure 3. CO oxidation by O (5% CO and 5% O in N at the rate of 5000 ml h ).
2
2
2
-1
Figure 4. CO oxidation by NO (5% CO and 5% NO in Ar at the rate of 5000 ml h ).
Figure 5. Catalyst stability in various reaction conditions Co Cu O (5% CO and 5% O in
2.9
0.1
4
2
-1
N and 5% CO and 5% NO in Ar at the rate of 5000 ml h ).
2
Table 1. Latice parameter, BET surface area and magnetic moment.
1
0

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1
1

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2

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3

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4

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5

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Table 1. Latice parameter, BET surface area and magnetic moment.
Lattice
parameter
BET surface
area
Magnetic moment
Catalyst sample
Co2.9 Cu O
µB
(B. M.)
2
-1
(Å)
(m g )
0.1
4
8
8
8
8
8
8
.09
.08
.09
.09
.07
.07
74.3
78.2
72.1
80.6
74.3
83.4
3.73
3.99
4.51
4.62
4.70
4.57
Co2.8 Cu O
0.2
4
Co Cu O
4
2
.6
0.4
Co Cu O
4
2
.4
0.6
Co2.2 Cu O
0.8
4
Co Cu O
4
2
1
1
6

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Graphical abstract
1
7

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Highlights
◦





Complete CO oxidation by O was observed below 100 C.
2
◦
100% N selectivity observed below 275 C for NO – CO redox reaction.
2
Greater than x = 0.2 of Cu in Co O decreased the catalytic activity.
3
4
Catalysts are highly stable in the reaction conditions.
Presence of moisture has a temporary effect on the catalyst activity.
1
8