Ali H. Gemeay a,,, Mohamed A. Salem b, Ibrahim A. Salem a. " Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt b Department of ...
COLLOIDS AND Colloids and Surfaces A: Physicochemicaland Engineering Aspects 117 (1996) 245-252
ELSEVIER
A
SURFACES
Activity of silica-alumina surface modified with some transition metal ions Ali H. Gemeay a,,, Mohamed A. Salem
b,
Ibrahim A. Salem
a
" Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt b Department of Chemistry, Faculty of Science, University of Qatar, P.O. Box 2713, Doha, Qatar Received 20 October 1995; accepted 21 May 1996
Abstract
Kinetics of H202 decomposition have been investigated in the presence of a silica-alumina surface (25% A1203) modified with transition metal ions including Cu(II), Co(II), and Fe(III). The decomposition reaction proceeded with first-order kinetics with respect to the peroxide concentration. The rate of reaction depends upon the type of ions. The reaction rate and the energy of activation decrease in the order Cu(II) > Co(II) > Fe(III). The initial concentration of H202 has a great influence on the reaction rate and the coloration of the surface. The decomposition reaction involved the generation of free radical species as confirmed by using the reference chromogen 2,2'-azinobis(3-ethylbenzthiazoline)-6-sulphonate diammonium salt as a probe. An isokinetic relationship indicates that the reaction is entropy-controlled. A probable mechanism has been suggested for the decomposition process which agrees with the results obtained.
Keywords: Hydrogen peroxide; Isokinetic relationship; Kinetics; Silica-alumina; Transition metal ions
1. Introduction
The rapid decomposition of H 2 0 z has practical applications in fuel cells, rocket propellants and bleaching, as it is a source of oxygen. It has been reported that the decomposition of H2O 2 in the presence of metal ions as a homogeneous catalyst is slow, but that it is relatively fast when these ions are used along with a small amount of alumina or beryllium oxide [ 1 - 5 ] . For systems involving metal ions in solution, kinetic phenomena are more complex and are dependent on the transition metal ion present, and the reaction conditions. Studies using an alumina surface modified by ferric ions showed that the reaction * Corresponding author. s0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0927-7757 (96)03711-0
was first order with respect to [ H 2 0 2 ] , while alkaline Fe(III) solutions produced more complex behavior [6]. The catalytic activity of an alumina surface modified with some metal oxide was investigated [7]. The kinetics of heterogeneous decomposition of H202 by some metal complexes immobilized on alumina [4,5], silica-alumina [8,9] and cation-exchange resin [ 1 0 - 1 4 ] have also been studied in an aqueous medium. Shirasaki et al. [15] pointed out that Bronsted acid sites on the surface of silic~alumina can undergo cation exchange in aqueous solution without affecting the surface hydrogen bond. As a continuation of our previous studies on the heterogeneous decomposition of HzO 2 [8,9], we report results obtained from the study of H202 decomposition by the silica-alumina surface modified with Cu(II), Co(II), and Fe(III).
246
A.H. Gemeay et al./Colloids Surfaces A. Physicochem. Eng. Aspects 117 (1996) 245-252
2. Experimental 2.1. Materials
Silica-alumina (25% AlzO3) was supplied by Joseph Crossfield & Son Ltd. It has a pore volume of 0 . 8 0 m l g - 1 and an average particle size of 63 ~tm. The surface area was 250 m 2 g 1 as determined by a high speed surface area analyzer (Shimadzu model 2205). It was washed thoroughly in hot bidistilled water and dried at 120°C. To immobilize the metal ions on the surface, the dried silica-alumina was treated with an aqueous solution of metal chloride (0.05 mol dm -3) at ambient temperature for 2 h. The surfaces modified by Cu(II), Co(II) and Fe(III) were filtered, washed with bidistilled water to remove the excess metal chloride, and then dried at 120°C. The concentration of metal ions per gram of air-dried silica alumina was determined via the liberation of these metal ions. An excess volume of HC1 (0.5 mol dm -3) was added to a definite amount of catalyst and the concentration of liberated metal ion was measured by an atomic absorption spectrophotometer (Shimadzu A-670). The concentrations of metal ions were: 0.015, 0.011 and 0.0041 mol g 1 for Cu(II), Co(II) and Fe(III) respectively. H 2 0 2 (30% volume) was A.R. grade from Merck. Initial solutions (0.1-0.4 mol d m - 3) were prepared by dilution. The exact concentration was estimated iodometrically using standard sodium thiosulphate and starch as an indicator. Reference chromogen, 2,2'-azino-bis(3-ethylbenz-thiazoline)-6-sulphonate diammonium salt (ABTS), was purchased from Aldrich. It was dissolved in hot freshly distilled water to give the desired concentration. Other chemicals were of analytical grade quality and used as received. The spectrophotometric measurements were performed with a Shimadzu UV-2100S spectrophotometer. For pH measurements, a Crison pH meter digit-501 that had been calibrated prior to the measurements was used. 2.2. Kinetic measurements
A number of flasks (volume 100 cm 3) containing a definite quantity of silica-alumina/M "+ catalyst
together with doubly-distilled water (17 cm 3) were placed in a water thermostat and shaken for 30 min. To each flask H202 (3 cm 3) was added within about 3 s by a micropipette and zero time was noted at half the addition point. At known intervals of time, the reaction was quenched by rapid filtration (the time of filtration was about 30 s) through G 4 sintered glass. An aliquot volume of the filtrate (5 cm 3) was taken and the concentration of the unreacted H202 was determined. The measurements were carried out at temperatures below 40°C in order to avoid the possible self-decomposition of H 2 0 2. In the absence of a silica alumina-metal ion catalyst, a mixture of ABTS and H202 was stable for several hours without the appearance of the green color characteristic of radical cations (ABTS+'). The same has been observed for a mixture of catalyst and ABTS solution. The concentration of H202 was kept equal to or below the concentration of ABTS in order to produce the radical cation. If the concentration of H 2 0 2 exceeded the concentration of ABTS, no radical cation formed [16 18].
3. Results and discussion There was no difference in the determination of metal ion concentration per gram of dry silicaalumina catalyst before and after the completion of reaction. This means that the immobilized metal ions were very stable on the surface, without being released during the reaction. Prior to the addition of H202 to the catalyst, the pH of the medium (silica-alumina metal ion + water) showed noticeable changes depending upon the transition metal ion present on the surface. The measured values of pH were: 6.5, 6.2 and 5.8 for Cu(II), Co(II) and Fe(III) modified surfaces respectively. These little changes in pH values can be attributed to the ability of transition metal ions to form aqua complexes. The higher the positive charge of the metal ion, the easier is the dissociation of coordinated water molecules to liberate protons into the medium [19]. On addition of H202, a further decrease of pH was observed particularly at the beginning of a reaction. This is due to the
247
A.H. Gemeay et al./Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 245 252
deprotonation of HzO2 (see Eq. (2)). Such a decrease of p H did not release the metal ions from the surface of silica-alumina and was in good agreement with results found in our previous work I-8,9]. With all modified silica-alumina surfaces, the reaction followed first-order kinetics, as shown in Fig. 1. The rate constant per gram of dry catalyst, k, was obtained from Eq. (1):
ItJg ianlounl ol catalystJ
-1.2
(1)
log(a/a - x) = k w t / 2 . 3 0 3
I
-1.0 [
I
-0.8 i
-0-6 I
12
-3.8
8
-/*.0
J
where a is the initial concentration of H202, x is the amount of hydrogen peroxide decomposed at time t, k is the observed rate constant, and w is the amount of dry catalyst in grams. While the concentration of H202 was held constant at 0.2 tool dm -3, the reaction was studied with variable amounts (0.1-0.3 g) of catalyst in the form of Cu(II), Co(II), and Fe(III). It has been found that the quantity of H202 decomposed increases with an increasing amount of catalyst. The plot of kobs against the amount of catalyst is linear, as shown in Fig. 2(a). This linear proportionality indicates that the present reaction is a true heterogeneous catalytic one. Extrapolating through the origin shows that the uncatalyzed reaction is negligible. The order of reaction with respect to the amount of catalyst was calculated from the slope of the
-¢.2
I
! 0'1
~
I 0-2
I
I 0 '3
J
.td,
Amount of catalyst (g)
Fig. 2. (a) Plot of ]Cobs against amount of catalyst, silicaalumina-Cu(II), at constant H 2 O 2 concentration (0.2tool dm -3) at 35°C. (b) Plot of log kobs against log [amount of catalyst] under the same conditions mentioned in (a). logarithmic plot of kobs and the amount of catalyst (see Fig. 2(b)). It is equal to 0.96 and indicates first-order kinetics with regard to the amount of catalyst.
03
0.2
0-1
~
l
I
60
120
,
Iime (min)
I
180
I
~
240
I
/ .....
1
300
Fig. 1. Illustration of the first-order rate equation for H20z (0.2 mol dm -3) decomposition in the presence of silica-alumina-Co(II) catalyst (0.2 g) at various temperatures: ([]) 25; (O) 30; (~) 35; and (O) 40°C.
A.H. Gemeay et al./Colloids Surfaces A: Physicochem. Eng. Aspects 117 (1996) 245-252
248
The dependence of the reaction rate on the initial concentration of hydrogen peroxide, [H202]o, with the three supported metal ions has been investigated. In all cases the observed rate constant was measured at a fixed amount of catalyst (0.2 g) at 35°C and calculated per dry gram of support. With silica-alumina-Cu(II) k increases linearly with [H202]o, attaining a maximum, and then decreases thereafter (see Fig. 3). With silica alumina-Fe(III) and Co(II), only a decrease in the rate constant was obtained (data not shown), It is worthwhile mentioning that some color changes are noticed during the experiment. A light brown color on the silica-alumina-Cu(II) surface was invariably seen in the reaction vessel when [ H 2 O z ] 0 > 0 . 2 m o l dm -3. The color persisted as long as any residual H202 remained undecomposed. However, no other color was observed except the light blue color initially observed on the surface when [H202]o
