The influence of La2O3-doping on structural, surface and catalytic properties of nano-sized cobalt–manganese mixed oxides

Structural, textural and catalytic activity of CoO–Mn2O3 system as being influenced by La2O3-doping (0.75–3 mol%) and calcination temperatures (300–500 °C) were investigated. The techniques employed were XRD, N2-adsorption–desoprtion at −196 °C, EDX and catalysis of H2O2-decomposition in aqueous solution at 30–50 °C. The results revealed that the investigated system consisted of nano-sized Co2MnO4 as a major phase together with un-reacted portion of Co3O4 and γ-Mn3O4. Doping with the smallest amount of La2O3 greatly increased the surface molar ratio of Mn/Co (88 %) for the solid calcined at 500 °C with subsequent increase of the catalytic activity more than 12-fold for solids calcined at 500 °C. The increase of La2O3-dopant above 0.75 mol% decreased progressively the surface molar ratio of Mn/Co with subsequent decrease in the catalytic activity which still measured higher values than that measured for the un-doped catalyst.


Introduction
Manganese oxides are reported to be considered as environment-friendly materials. MnO 2 and Mn 3 O 4 were found to be active and stable catalysts for the combustion of organic compounds [1,2].
Nanocrystalline manganese oxide powders were synthesized in previous study by an inert gas condensation technique [3]. The manganese oxide, which is prepared, is a mixture of MnO and Mn 3 O 4 . The particle size of manganese oxides is greatly dependent on their preparation conditions. Dimesso et al. [4] claimed that manganese oxides can be prepared by an inert gas condensation technique followed by annealing in air and oxygen at various temperatures. The predominant phase of MnO and Mn 3 O 4 are obtained after annealing in air at 400°C.
Mixed oxides containing transition metal oxides are used to design the catalytic materials to replace noble metal catalysts. Lahousse et al. [1] have found that c-MnO 2 and Pt/TiO 2 catalysts measured high catalytic activities as compared to noble metals catalysts.
Mixing manganese with transition metal oxides in many catalytic systems modify the catalytic activity of individual components [5,6]. Mixed oxide materials are active for oxidation-reduction reactions and combustion processes. For example, cobalt-zinc manganites, manganese-CeO 2 mixed oxides and Co-containing mixed oxides prepared from hydrotalcite-like precursors were active catalysts in the reduction of nitrous oxide [7][8][9]. Also, Co-Mn mixed oxides were found to be active catalysts for oxidation of ethanol [10] and conversion of synthesis gas to light olefins [11]. However, Ag-Mn, Ag-Co and Ag-Ce composite oxides supported on Al 2 O 3 have been reported as catalysts for oxidation of volatile organic compounds [12]. In a previous study, Mn-Cu mixed oxides have been reported to & Abdelrahman A. Badawy aabadawy107@yahoo.com be catalytically more active towards ethanol oxidation as compared to individual Mn 2 O 3 and CuO [6]. Both copper and manganese mixed oxides catalysts were found to be more active catalysts in many industrial oxidation processes, such as CO oxidation by O 2 , combustion of toluene, methanol, ethylene, ammonia, NO 2 and other combustion reactions [13][14][15][16]. However, Mn-Zr mixed oxide samples are active towards dehydrogenation of isopropanol, giving rise to acetone with high selectivity at partial conversion [17]. Hydrogen peroxide and its solutions find use as antiseptic in medicine [18,19], other applications such as bleach in the textile and paper/pulp industry, in treatment of waste water [20]. However, the literature survey reveals that mixed oxide catalyst is more active in H 2 O 2 decomposition. These catalysts have attracted much attention of chemists due to their application as low-cost fuel cells, their stability and high activity [21,22]. The decomposition of hydrogen peroxide in presence of some metal oxides as LaMnO 3 at room temperature and nanocrystalline LaCrO 3 was investigated by Khetre et al. [23]. They found that the catalytic activity was increased by increasing both the amount of the catalyst and pH. The probable reaction mechanism has been suggested in which an intermediate surface complex is thought to be responsible for the enhancement of the decomposition of hydrogen peroxide.
The present work aimed at studying the effect of La 2 O 3doping of CoO/Mn 2 O 3 system prepared by coprecipitation method on its structural, surface and catalytic properties. The techniques employed were XRD, EDX, N 2 -adsorption isotherms carried out at -196°C and catalytic decomposition of H 2 O 2 in aqueous solution at 30-50°C.

Experimental
Materials Equimolar proportions of CoO/Mn 2 O 3 were prepared by coprecipitation method of their mixed hydroxides from their nitrates solution using 1 M NaOH solution at pH 8 and a temperature of 70°C. The carefully washed precipitate was dried at 110°C till constant weight, and then subjected to heating at 300, 400, and 500°C for 4 h. Three La 2 O 3 -doped samples were prepared by impregnating a given dry weight of the mixed hydroxides with calculated amount of lanthanum nitrate dissolved in the least amount of distilled water sufficient to make pastes. The pastes were dried at 110°C and then calcined at 300, 400 and 500°C for 4 h. The dopant concentrations in the calcined solids were 0.75, 1.5, and 3 mol% La 2 O 3 .
Techniques X-ray powder diffractograms of various investigated samples calcined at 300, 400 and 500°C were determined using a Bruker diffractometer (Bruker D 8 advance target). The patterns were run with copper K a with secondly monochromator (k = 1.5405 Å ) at 40 kV and 40 mA. The scanning rate was 0.8°in 2h min -1 for phase identification and line broadening profile analysis, respectively. The crystallite size of the phases present in pure and variously La 2 O 3 -doped solids was determined using the Scherrer equation [24]: where d is the mean crystallite diameter, k is the X-ray wave length of the X-ray beam, K is the Scherrer constant (0.89), b 1/2 is the full width at half maximum (FWHM) of the main diffraction peaks of the investigated phases, in radian and h is the diffraction angle.
Energy dispersive X-ray analysis (EDX) was carried out on a Hitachi S-800 electron microscope with a Kevex Delta system attached. The parameters were as follows: -15 kV accelerating voltage, 100 s accumulation time, 8 lm window width. The surface molar composition was determined by the Asa method (Zaf-correction, Gaussian approximation).
Different surface characteristics, namely specific surface area (S BET ), total pore volume (V p ), mean pore radius (r -) and pore volume distribution curves (Dv/Dr) of various solids were determined from nitrogen adsorption-desorption isotherms measured at -196°C using NOVA Automated Gas sorbometer. Before undertaking such measurements, each sample was degassed under a reduced pressure of 10 -5 Torr for 3 h at 200°C. The values of V p were computed from the relation: where V st is the volume of nitrogen adsorbed at P/P 0 tends to unity. The values of rwere determined from the equation: The catalytic activities of pure and variously La 2 O 3doped solids were determined by studying the decomposition of H 2 O 2 in their presence at temperatures within 30-50°C using 25, 50 and 100 mg of a given catalyst sample with 0.5 ml volume of H 2 O 2 of known concentration diluted to 20 ml with distilled water (initial concentration of H 2-O 2 = 0.01 mol/L). The reaction kinetics was monitored by measuring the volume of oxygen liberated at different time intervals until no further O 2 was liberated. The volume of the liberated oxygen was recalculated under STP.
Results and discussion X-ray investigation of various solids X-ray diffractograms of un-doped and variously La 2 O 3doped solids calcined at 300-500°C were determined from the recorded diffractograms of these solids are illustrated in Figs. 1, 2, 3 for the solids calcined at 300, 400 and 500°C, respectively. The different structural characteristics of the solids investigated are given in Table 1. Table 1 includes the peak area of the main diffraction lines of different phases present and the crystallite size of CoMn 2 O 4 phase formed calculated from the Scherrer equation.
Examination of Figs. 1, 2, 3 and Table 1  These results show clearly the role of La 2 O 3 in hindering the solid-solid interaction between cobalt and manganese oxide to yielding cobalt manganite. The increase in calcination temperature within 300-500°C stimulated the formation of Co 2 MnO 4 . The formation of Co 2 MnO 4 took place according to the reaction: The addition of smallest amounts of La 2 O 3 (0.75-3 mol%) followed by heating at 300-500°C hindered Co 2 MnO 4 formation to an extent proportional to its amount added. 20 [25] according to:  [26] having hysteresis loops of small areas closing at P/P 0 at about 0.5. Figure 4 depicts representative N 2adsorption-desorption isotherms measured over pure and variously doped solids calcined at 500°C. Figure 5 depicts Dv/Dr curves of various solids calcined at 300 and 500°C. Analysis of the recorded adsorption-desorption isotherms permitted us to calculate different surface characteristics, namely specific surface area (S BET ), total pore volume (V p ) and mean pore radius (r -). The computed values of S BET , V p and rare given in Table 2. Examination of Table 2 and Fig. 5 shows the following: (1) the S BET and V p values of pure and variously doped solids increased progressively by increasing the calcination temperature within 300-500°C. The increase was, however, more pronounced for pure mixed solids which attained 113 and 295 % for S BET and V p , respectively [27]. (2) The rvalues cited in the last column of Table 2 show that the investigated solids are mesoporous adsorbents measuring (r -) values varying between 30 and 88 Å . (3) La 2 O 3 -doping decreased effectively the S BET to an extent directly proportional to its amount present. The doping process did not affect the rvalues which remained almost constant for heavily doped sample (3 mol% La 2 O 3 ). (4) Most of the investigated solids exhibit bimodal pore volume distribution curves except the heavily doped sample (3 mol% La 2 O 3 ) exhibited tri-modal distribution curves (c.f. Figure 5). The maximum hydraulic pore radius was located at 11.5 and 27.5 Å for pure solids and found at 10 and 23 Å for solids doped by 0.75 mol% La 2 O 3 and 9.5 and 30 Å for solids doped by 1.5 mol% La 2 O 3 and 10, 16 and 26 Å for solids doped by 3 mol% La 2 O 3 being calcined at 500°C. These results show clearly the role of La 2 O 3 -doping in modifying the various surface characteristics of the system investigated. Comparison of (r -) values of pure and variously doped solids calcined at 300-500°C decreased effectively the calculated (r -) values. This decrease might be followed by an increase in the S BET values opposite to what was found. So, one might expect that the dopant process decreased the concentration of the narrowest pore located at 11.5 Å .
Energy dispersive X-ray analysis of various solids EDX investigation of pure and doped solids calcined at 300-500°C was determined. The relative atomic abundance of manganese, cobalt, oxygen and lanthanum species present in the uppermost surface layers of the calcined solids is given in Table 3. It is well known that EDX technique supplies an accurate determination of relative atomic concentration of different elements present on their outermost surface layers [28][29][30][31][32][33][34]. In fact, this technique (EDX) has been successfully employed in determining the surface composition of a big variety of catalytic systems such as CuO/Mn 2 O 3 [28], CuO/ZnO [29,31,33], TiO 2 / Al 2 O 3 [30], CuO/NiO [32] and Co 3 O 4 /Fe 2 O 3 [34]. The thickness of these layers is bigger than those measured by using XPS technique. XPS is a well-known surface-sensitive technique that supplies very accurate relative atomic abundance of cationic and anionic species on the surfaces of investigated solids. Surface and bulk compositions of various solids are given in Table 3. Inspection of the results given in Table 3    bulk of pure and variously doped solids. (3) The concentration of surface manganese was several folds that of cobalt. This finding might suggest that cobalt hydroxide was precipitated much earlier than manganese hydroxides. This conclusion seems logical because of the significant difference between the values of the solubility products of Co(OH) 3 and Mn(OH) 2 which measured 5.9 9 10 -15 and 1.6 9 10 -44 , respectively. (4) Lanthanum species present in the uppermost surface layers of all doped solids calcined at temperature within 300-500°C is bigger than the amount present in the bulk of solids. This finding is expected because all doped solids were prepared by wet impregnation method [35,36]. Furthermore, the surface concentration of lanthanum increases by increasing the calcination temperature of the doped solids. (5) The addition of the smallest amount of La 2 O 3 (0.75 mol%) much increased the surface concentration of manganese present in all solids calcined at 300-500°C. The increase in surface concentration of manganese species attained 4, 25 and 13 % for the solids calcined at 300, 400 and 500°C, respectively. The increase of the dopant concentration above 0.75 mol% La 2 O 3 decreased the surface concentration of manganese which still remained bigger than that measured for the un-doped samples calcined at the same temperatures.
It is well known that manganese species present in the uppermost surface layers in pure and doped solids are considered as the most catalytically active constituent involved in H 2 O 2 -decomposition. This assumption is based on the possible presence of manganese cations in different oxidation states varying between di-and hepta-valence states leading to a significant increase in the concentration of manganese ion pairs participating in the catalytic decomposition process. So, the bigger the surface concentration of manganese the bigger will be the concentration of the most catalytically active constituent and the bigger the catalytic activity. This speculation will be confirmed in the next section of the present work dealing with the catalytic decomposition of H 2 O 2 on pure and variously La 2 O 3 -doped solids calcined at 300-500°C.

Catalytic activity of pure and variously doped solids
The catalytic decomposition of H 2 O 2 in aqueous solution was studied at 30, 40 and 50°C over pure and variously doped solids precalcined at 300, 400 and 500°C. Firstorder kinetics was observed in all cases. In fact, straight line was found upon plotting ln a/ax against the time intervals t, where a is the initial concentration of H 2 O 2 and ax is its concentration at time t. The slopes (d ln a/ax/ dt) of plots determine the reaction rate constant (k) for the reaction conducted at a given temperature over a given catalyst sample. The reaction kinetics were monitored using 25, 50 and 100 mg of catalyst sample. The results, not given, showed that k value increases linearly by increasing the catalyst mass indicating the absence of any possible solid gas diffusion of liberated oxygen. Figures 6 and 7 depict representative first-order plots of the catalyzed reaction conducted at 30 and 50°C over pure and variously La 2 O 3 -doped solids pre-calcined at 300 and 500°C, respectively. The values of the reaction rate constant per unit mass for the reaction carried out at 30, 40 and 50°C were calculated from the slope of the first-order plots.  (4) The observed increase in the catalytic activity of pure and variously doped solids by increasing their calcination temperature within 300-500°C might be attributed to the observed increase in surface concentration of manganese species (considered as the most catalytically active constituent) and the observed increase in the specific surface areas (c.f. Tables 2, 3). (5) The observed significant increase in the catalytic activity due to doping with 0.75 mol% La 2 O 3 could be also attributed to the observed decrease in the crystallite size of Co 2 MnO 4 phase (c.f. Table 1). (6) The decrease in the catalytic activity of the heavily doped solids might result from the observed increase in the crystallite size of Co 2 MnO 4 (the major phase present in pure and doped solids) and also due to the observed decrease in surface concentration of manganese species besides the significant decrease in the S BET (c.f. Tables 1, 2, 3).
In order to throw more light about the role of both calcination temperature and dopant concentration (La 2 O 3 ) in the mechanism of the catalyzed reaction the activation energy of which (DE) was determined for pure and doped solids calcined at 300-500°C. DE values were calculated from the values of k measured for the reaction carried out at 30, 40 and 50°C by direct application of the Arrhenius equation. The computed DE values are given in Table 4. Examination of Table 4 shows that DE values for pure and variously doped solids decreased progressively as a function of dopant concentration. This trend ran parallel to observed increase in the catalytic activity, expressed as k 50°C values (c.f. Table 4). This finding expresses the observed increase in the catalytic activity of sample of 0.75 mol% La 2 O3-doping. On the other hand, increasing the dopant concentration above this limit increased DE values which remained almost smaller than DE values measured for the pure catalyst samples.