Copper oxide on Cu/Al2O3-TiO2 catalysts TG, FTIR-CO absorption and catalytic activity in the NO reduction by CO

Article abstract of DOI:10.1023/A:1025174617195

TG, FTIR-(CO absorption), and catalytic activity in the NO reduction by CO were used to characterize Cu/Al₂O₃-TiO₂ catalysts prepared by co-gelling aluminum tri-sec-butoxide and titanium iso-propoxide at pH 9 and at pH 3 gelling conditions. Under nitrogen flow, copper oxide decomposition, oxygen storage capacity (OSC) and sample dehydroxylation (total mass loss) was followed by TG. The CuO decomposition forming Cu⁰, Cu⁺¹ was observed by means of FTIR (CO absorption) spectra. In pH 9 sample the large amount of Cu⁰ was observed. At low total mass loss and high Cu⁰/Cu⁺¹+Cu ⁺² ratio (pH 9 sample) a lowest light-off in the NO reduction by CO was observed.

Full text of DOI:10.1023/A:1025174617195

Journal of Thermal Analysis and Calorimetry, Vol. 73 (2003) 341–346
COPPER OXIDE ON Cu/Al2O3–TiO2 CATALYSTS
TG, FTIR-CO absorption and catalytic activity in the NO
reduction by CO
M. Morán-Pineda 1*, S. Castillo 1,2, M. Asomoza 1,3 and R. Gómez 1,3
1Instituto Mexicano del Petróleo, Simulación Molecular, Eje Central Lázaro Cárdenas 152,
07730 México D. F., México
2Instituto Politécnico Nacional, Edif. 7, Unidad Profesional Adolfo López Mateos,
07738 México D. F., México
3Universidad Autónoma Metropolitana-I, Chem. Dept.. AP 55-534, 09340 México D. F., México
(Received October 19, 2002; in revised form March 18, 2003)
Abstract
TG, FTIR-(CO absorption), and catalytic activity in the NO reduction by CO were used to character-
ize Cu/Al2O3–TiO2 catalysts prepared by co-gelling aluminum tri-sec-butoxide and titanium
iso-propoxide at pH 9 and at pH 3 gelling conditions. Under nitrogen flow, copper oxide decomposi-
tion, oxygen storage capacity (OSC) and sample dehydroxylation (total mass loss) was followed by
TG. The CuO decomposition forming Cu0, Cu+1 was observed by means of FTIR (CO absorption)
spectra. In pH 9 sample the large amount of Cu0 was observed. At low total mass loss and high
Cu0/Cu+1+Cu+2 ratio (pH 9 sample) a lowest light-off in the NO reduction by CO was observed.
Keywords: alumina–titania sol–gel catalysts, CO adsorption on alumina–titania catalysts, copper
alumina–titania oxidation state, NO reduction by CO, TG sol–gel alumina–titania
Introduction
Supported noble metals like platinum, palladium and rhodium or combinations be-
tween them were reported as the most effective catalysts for the NO reduction [1–2].
The essential support is alumina or ceria and lanthanum doped alumina. The main
role of rare earth oxides has been related to improving the textural properties of alu-
mina, however oxygen storage capacity (OSC) on doped alumina was observed [3].
The OSC effect increases CO oxidation rate, while the NO reduction by CO is inhib-
ited [4]. Then a compromise between the improvement of the textural properties of
supports and the importance of the OSC effect in the NO reduction must be reached.
Less expensive catalysts like supported copper have been reported as alternative
ones to substituted noble metal supported catalysts [5]. Copper–lanthanum–manganite
has been reported as a good catalyst the CO oxidation and NO reduction by CO. The
composite shows oxygen storage capacity [6].
*
Author for correspondence: E-mail: fmarina@imp.mx
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

342
MORÁN-PINEDA et al.: Cu/Al2O3–TiO2 CATALYSTS
Titania is known as a reducible oxide [7]. When metals are deposited on its surface
metal oxidation and titania reduction processes occurs. The oxidizing reducing cycle
Ti4+ to Ti3+ and Cu0 to Cu+1 have been reported on reduced Cu/TiO2 catalysts [8, 9].
In the present work we study Cu/Al2O3–TiO2 catalysts, with the aim to stabilize
titania on the alumina matrix and to study the oxidation reducing cycles of copper on
the support. The mixed oxides were prepared by the sol–gel method using titanium
and aluminum alkoxides as starting materials and CuCl2 2H2O as copper precursor.
The catalysts characterization was made by means of TG-FTIR, FTIR (CO absorp-
tion) and by the NO reduction by CO as catalytic test.
Experimental
The Al2O3–TiO2 supports were synthesized as following: 0.64 mol of aluminum
tri-sec-butoxide (Aldrich, 99.9% purity) was dissolved in 100 mL of ethanol (Baker
99.9%) and refluxed under constant stirring. For basic medium preparation, the solu-
tion was adjusted to pH 9 with NH4OH (Baker 35% NH3 in water), while for acidic
gelation the solution was adjusted to pH 3 with HCl (Baker, 36.5% in water). Under
continuous stirring 0.07 mol of titanium isopropoxide (Aldrich, 99.9%) was added
drop by drop to the ethanolic solution (10 mass% TiO2). Hydrolysis was accom-
plished by adding 0.83 mol of bidistilled dropwise water. The solution was main-
tained under reflux until the gel was formed. Samples were dried at 70°C for 12 h and
calcined at 550°C for 4 h in a static oven. Metal supported catalysts were prepared by
impregnation of the calcined Al2O3–TiO2 supports with a CuCl2 2H2O aqueous solu-
tion, the concentration was in order to obtain 1.0 mass% Cu on the catalysts. Impreg-
nated catalysts were dried at 70°C overnight and calcined at 550°C in air for 4 h.
Thermogravimetric analyzer, TG 2950 TA Instruments, coupled to a FTIR
Nicolet Protege 460 spectrophotometer was used. The system allows the analysis by
FTIR of the evolved gases from room temperature to 550°C. In previously calcined
samples placed on the TG pan, a heating rate of 10°C min–1 from 25 to 150°C in N2
carrier flow was applied. Then the temperature was hold for 60 min at 150°C and af-
ter the sample was heating to 800°C. Afterwards, instead of N2 flow, air flowing was
admitted in the system and the sample cooled from 800 to 50°C by shooting off the
heating program [10].
The catalytic activity was measured under atmospheric pressure in the non-selective
reduction of NO by CO using a fixed bed quartz reactor applying a heating program rate
of 5°C min–1. The reactant mixture containing 1.5 vol% CO, 0.5 vol% NO, balance N2,
flow LHSV=3600 h–1 (liquid hourly space velocity), catalyst mass 50 mg. Temperature
range 100–400°C. The catalytic system was coupled to a FTIR spectrometer
Nicolet-8220 gas analyzer permitting the identification of desorbed species. The activity
was followed as NO total conversion.
The CO adsorption on the metallic catalysts was study by means of FTIR-CO
absorption spectra using a Nicolet model 710 apparatus. Samples pressed as thin self-
supported wafers were placed in a Pyrex glass cell equipped with CaF2 windows. The
calcined samples were reactivated ‘in-situ’ at 400°C for 30 min under vacuum
J. Therm. Anal. Cal., 73, 2003

MORÁN-PINEDA et al.: Cu/Al2O3–TiO2 CATALYSTS
343
(1 10–5 torr); afterwards, the cell was cooled to –85°C with liquid nitrogen. CO
(20 torr) was then admitted into the cell. The excess of CO was evacuated during fif-
teen minutes and then the FTIR spectra were recorder.
Results and discussion
Before thermogravimetric analysis the samples were previously calcined at 800°C and
re-exposed to air after the thermal treatment. In Fig. 1, two mass loss steps can be ob-
served. The first mass loss comprised from room temperature to 150°C correspond to
H2O desorption as it was identified by the FTIR corresponding bands. The second mass
loss comprised between 150 and 800°C was assigned to copper oxide decomposition, ox-
ygen storage capacity (OSC) and dehydroxylation of the mixed oxide. The mass loss ob-
served after water release, (6.7 mass%) for pH 9 sample and 8.7 mass% for pH 3 sample
is due to the big capacity of sol–gel materials to be re-hydroxylated. Alumina or titania
sol–gel are oxides who show high hydroxylation after air exposure. The evolution of the
copper oxidation states from initial Cu2+ to Cu1+ and Cu0 as the thermal treatment in-
creases, could be supported by the coloration of the samples observed when they were
annealed. Initially (150°C) the samples show green color which turns to dark green color
and finally around 800°C the samples became bluish gray color. Similar effect of color
evolution has been reported by Del Arco et al. [9], on Cu/TiO2 catalysts thermally
treated. The evolution of the coloration of the samples was attributed to Cu2+ to Cu1+ and
Cu0 oxidation states and is a reversible process, since when the samples were cooling in
air flow from 800 to 50°C the samples acquires the initial green color. This behavior was
repetitive for each heating–cooling cycle given to the sample. The mass variation (W) di-
vided by the initial mass of the sample (W0) in function of the temperature is shown in
Fig. 2. We can see that the pH 3 sample loss is 20% higher than that observed for the
pH 9 sample. The total mass loss involves copper oxide reduction, support oxygen re-
lease (OSC), and dehydroxylation, then, we are not in the possibility to quantify the
amount corresponding to the oxygen storage capacity. But this phenomenon cannot be
Fig. 1 TG on Cu/Al2O3–TiO2 catalysts showing the decomposition and reoxidation cy-
cle: a – pH 9; b – pH 3. The mass loss from 150 to 800°C will correspond to
copper oxide decomposition, OSC phenomenon, and dehydroxylation (total
mass loss) of the samples after exposure to air
J. Therm. Anal. Cal., 73, 2003

344
MORÁN-PINEDA et al.: Cu/Al2O3–TiO2 CATALYSTS
Fig. 2 TG mass variation on function of temperature for the Cu/Al2O3–TiO2 pH 9 and
Cu/Al2O3–TiO2 pH 3. The Fig. shows a higher total mass loss for the sample
prepared at pH 3
discarded and it would be higher in the pH 3 sample. The OSC phenomenon has been ob-
served on TiO2 [9].
The CO absorption spectra of Cu/Al2O3–TiO2 pH 9 and Cu/Al2O3–TiO2 pH 3 at
–85°C are shown in Fig. 3. Three bands around 2104, 2117 and 2150 cm–1 can be ob-
served and they were assigned to CO absorption on copper Cu0, Cu1+and Cu2+respec-
tively [10]. In the sample prepared at pH 9 it can be observed greater density of Cu0
and Cu1+ if compared with the pH 3 sample. Thus, in the sample prepared at pH 9 the
amount of copper on low oxidation state is indicative that in such sample, the decom-
position of copper oxide is faster achieved.
Fig. 3 FTIR (CO adsorption) on Cu/Al2O3–TiO2 catalysts: a – prepared at pH 9;
b – prepared at pH 3; Cu0 (2104–2110 cm–1), Cu1+ (2117–2122 cm–1) and Cu2+
(2150–2154 cm–1)
The catalytic activity of the samples evaluated on the NO reduction by CO with-
out oxygen is shown in Fig. 4. The activity on function of the temperature shows that
the catalyst prepared at pH 9 reach 50% light-off conversion at 280°C, while for the
samples prepared at pH 3 50% is reached at 370°C. We can assume then that the low
light-off showed by the pH 9 catalyst could be related to a low OSC effect. The reac-
tion of NO with CO in absence of oxygen can be described as follows:
2NO+CO→N2O+CO2
N2O+CO→N2+CO2
J. Therm. Anal. Cal., 73, 2003

MORÁN-PINEDA et al.: Cu/Al2O3–TiO2 CATALYSTS
345
Fig. 4 NO conversion on Cu/Al2O3–TiO2 catalysts as a function of the temperature dur-
ing the NO reduction by CO. Light-off for NO conversion (50%) at 260 and at
370°C for pH 9 and pH 3 samples respectively can be observed
When the reaction occurs in presence of oxygen as reactant or with oxygen re-
leased from the support, the large affinity of CO for oxygen inhibit the reduction of
NO due to stoichiometric conditions [11].
Conclusions
Even when a quantitative evaluation of the OSC phenomenon cannot be obtained, we can
assume that it is present in both samples and it can be detected by the catalytic activity be-
havior of the samples and by the Cu0/Cu1+ and Cu2+ ratio. In the first case a low OSC phe-
nomenon could present in pH 9 catalyst (low light-off). In the second case, the
Cu0/Cu1++Cu2+ ratio of pH 9 sample is higher than that obtained for the pH 3. At least
OSC will correspond a highest Cu0 oxidation concentration. Such results let us to propose
that the TG thermogravimetric analysis as a helpful technique for the interpretation of the
Cu/TiO2–Al2O3 sol–gel gas exhaust catalytic converter reactions.
* * *
We are indebted to IMP-Molecular Engineering Program for financial support. The authors thank
Tec. Rufino Vazquez for his assistance and technical support in this work.
References
1 G. P. Ansell, A. F. Diwell, S. E. Golunski, J. W. Hayes, R. R. Rajaram, T. J. Treux and
A. P. Walker, Appl. Catal. B: Environ., 2 (1993) 81.
2 M. Sirilumpen, R. T. Yang and N. Tharapiwattananon, J. Mol. Catal. A: Chemical,
137 (1999) 273.
3 V. I. Parvulescu. P. Grange and B. Delmon, Catal. Today, 46 (1998) 223.
J. Therm. Anal. Cal., 73, 2003

346
MORÁN-PINEDA et al.: Cu/Al2O3–TiO2 CATALYSTS
4 M. Iwamoto, T. Zengyo, A. Hernández and H. Araki, Appl. Catal B: Environmental,
17 (1998) 254.
5 M. Haneda, Y. Kintaichi, T. Mizushima, N. Kakuta and H. Hamada, Appl. Catal. B:
Environmental, 31 (2000) 81.
6 F. Patcas, F. C. Buciuman and J. Zsaco, Thermochim. Acta, 360 (2000) 71.
7 H. Bosch and F. Janssen, Catal. Today, 2 (1988) 364.
8 P. A. Sermon, K. Rollings, P. N. Reyes and M. J. Davies, J. Chem. Soc. Faraday Trans., I,
83 (1987) 134.
9 M. del Arco, A. Caballero, P. Malet and V. Rives, J. Catal., 113 (1988) 120.
10 O. Scarlatioana and M. Zaharuscu, J. Therm. Anal. Cal. 68 (2002) 861.
11 A. Dandekar and M. A. Vannice., Appl. Catal., B: Environmental, 22 (1999) 179.
12 M. Shelef and G. W. Graham, Catal. Rev. Sci. Eng., 36 (1994) 431.
J. Therm. Anal. Cal., 73, 2003