Effects of La2O3-doping on physicochemical surface and catalytic properties of nickel and manganese oxides supported on alumina

Applied Catalysis A: General 257 (2004) 177–191

Effects of La2 O3-doping on physicochemical surface and catalytic properties of nickel and manganese oxides supported on alumina Nagi R.E. Radwan Department of Chemistry, Faculty of Education, Suez Canal University, Suez, Egypt Received 14 January 2003; received in revised form 28 July 2003; accepted 28 July 2003

Abstract The effects of doping of Al2 O3 -supported nickel and manganese oxides with La2 O3 on their surface and catalytic properties were investigated using nitrogen adsorption at −196 ◦ C, the oxidation of CO by O2 at 150–200 ◦ C, and the decomposition of H2 O2 at 30–50 ◦ C. The dopant concentration was varied between 1.0 and 6.0 mol%. Pure and variously doped solids were prepared by wet impregnation method using finely powdered Al(OH)3 solid followed by treating with different amounts of lanthanum nitrate. The lanthanum nitrate-treated were divided into two portions. The first portion was treated with a fixed amount of nickel nitrate dissolved in the least amount of distilled water. The second portion was treated with a known amount of manganese nitrate. Pure and doped solids were calcined at 400–700 ◦ C. The results obtained revealed that the doping process of the investigated solids decreased the degree of crystallinity and particle size of both NiO and Mn2 O3 phases. La2 O3 -doping of investigated systems increased their catalytic activities towards CO-oxidation by O2 and H2 O2 decomposition to an extent proportional to the amount of dopant added. The doping process did not modify the activation energy of the catalyzed reaction but increased the concentration of catalytically active constituents taking part in the catalytic processes without changing their energetic nature. © 2003 Elsevier B.V. All rights reserved. Keywords: Nickel oxide; Manganese oxide; La2 O3 -doping; Alumina-support; CO-oxidation; H2 O2 -decomposition

1. Introduction Catalysis plays a major role in minimizing pollution. Catalytic oxidation is an effective way to control air pollution. The different applications use oxidation catalysts to control or remove CO, NO, volatile organic compounds (VOCS), automobile exhaust emission, byproducts from chemicals production, odour and toxic organics in waste water [1–6]. The complete oxidation of carbon monoxide is very important in automotive emission control. Precious metals have long been used as most efficient three-way catalysts to control automotive exhaust. Considerable attention has been paid to use metal oxides [7–16] or supported metal oxides as oxidation catalysts [17–21]. The high cost of precious metals, their limited availability and their sensitivity to high temperatures have stimulated the search to substitute catalysts. Metal oxides are alternative to noble metals as oxidation catalysts. They have sufficient

E-mail address: nagi r [email protected] (N.R.E. Radwan). 0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.07.006

activity, although they are less active than noble metals at low temperatures. However, at high temperatures the activities are similar [22–25]. They may also react with Al2 O3 to form metal aluminates, MAl2 O4 , of low catalytic activity [25–27]. However, some combinations of oxides may have high catalytic performance and high thermal stability as compared to single components [7,8,28–31]. The oxidation of CH4 and CO over CuO, CuCr2 O4 , Co3 O4 , Fe2 O3 , MnO2 , SnO2 and ZrO2 had been studied and it was reported that CuO and CuCr2 O4 exhibited the highest activity [32]. The most active single metal oxide catalysts for complete oxidation for a variety of oxidation reactions are oxides of Ag, V, Cr, Mn, Fe, Co, Ni and Cu [33,34]. Among the transition metal oxides, Mn oxides are recognised as being very active for total oxidation of CO and hydrocarbons [35,36]. Manganese oxides react to a lower extent with Al2 O3 to form spinel aluminate, MnAl2 O4 , of low activity [37]. For the oxidation of CO and CH4 , CuO exhibited a better activity compared to MnOx [38]. Similar results have been reported for unsupported metal oxides [32]. The activities of cobalt and nickel oxides

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supported on Al2 O3 suffered a significant decrease towards CO-oxidation due to the formation of their aluminates at 500 ◦ C [26]. In fact, it has been reported that cobalt and nickel aluminates exhibit low activities for CO-oxidation by O2 [39,40] as compared to the other oxides. MnOx is known to have several oxidation states, redox reactions are thus of great importance. When heated in air, MnOx undergoes phase transitions, at temperatures between 500 and 600 ◦ C. MnO2 is converted into Mn2 O3 and above 890 ◦ C into Mn3 O4 [41]. Lanthanum appeared to be one of the best additives for inhibiting the sintering of high surface area Al2 O3 [42]. The main role of La is to stabilize Al2 O3 , i.e. is to decrease the rate of surface diffusion that hinders sintering and loss of surface area [43]. At low La loadings and low temperatures, La3+ ions dissolve in the Al2 O3 lattice and hinders both bulk and surface diffusion [44]. There is a concentration limit, termed saturation value, to which the Al2 O3 support can accommodate dispersed La in the form of a two-dimensional over layer undetectable by XRD. Above the critical concentration La would only form crystalline oxides which in general at calcination temperatures below 800 ◦ C, exist as La2 O3 , whereas at higher temperatures LaAlO3 could be observed. Indeed, at higher temperatures or higher La concentration the formation of LaAlO3 phase has been reported [45,46]. Oudet et al. claimed that the presence of La strongly modifies the morphological features of the Al2 O3 support, which appears poorly crystallized and composed of particles of undefined shape [47]. This provides an increased number of nucleation sites for metals during the first steps of deposition, leading to an enhancement of the initial repartition and dispersion of the metallic phase on the doped samples. Xie et al. suggested that the promotion by La changes the surface property of the support and the interface energy between Ni and the support effected a significant decrease in the particle size of nickel crystallites [48]. The present work aimed to investigate the role of La2 O3 -doping in modifying the surface and catalytic properties of NiO/Al2 O3 and Mn2 O3 /Al2 O3 systems. The techniques employed were XRD, IR, nitrogen adsorption at −196 ◦ C, oxidation of CO by O2 at 150–200 ◦ C and H2 O2 decomposition at 30–50 ◦ C.

2. Experimental 2.1. Materials Nickel and manganese oxides supported alumina samples having the molar composition 0.2 NiO/Al2 O3 and 0.2 Mn2 O3 /Al2 O3 , respectively was prepared by wet impregnation method using finely powdered Al(OH)3 with solutions containing known amounts of Ni(NO3 )2 and Mn(NO3 )2 dissolved in the least amounts of bidistilled water making a paste to avoid any possible sedimentation and to ensure the homogeneity of the samples. The paste was dried

at 120o C, then calcined at 400, 500, and 700 ◦ C for 5 h. The doped solid samples were prepared by treating known amounts of Al(OH)3 with solutions containing different amounts of La(NO3 )3 , dried at 120 ◦ C. The obtained doped alumina samples were treated with solutions containing fixed amounts of Ni(NO3 )2 and Mn(NO3 )2 . The obtained solids were dried at 120 ◦ C, then calcined at 400, 500 and 700 ◦ C for 5 h. The dopant concentrations were 1.0, 2.0, 4.0 and 6.0 mol% La2 O3 . The chemicals employed were of analytical grade and supplied by BDH Company. 2.2. Techniques An X-ray powder diffraction patterns of pure and variously doped solid samples calcined in air at 400– 700 ◦ C was carried out using a Philips diffractometer (type PW 1390). The patterns were run with Ni-filtered copper radiation (λ = 1.5405 Å) for pure and treated NiO/Al2 O3 solids at 30 kV and 10 mA with a scanning speed of 2◦ in 2θ min−1 , while with unfiltered Fe radiation (λ = 1.9373 Å) for pure and treated Mn2 O3 /Al2 O3 solids. The particle size of the investigated solids was calculated from the line broadening of some diffraction lines of NiO and Mn2 O3 phases using Scherer equation (Cullity, 1967) [49]; d=

Kλ B1/2 cos θ

where d is the mean crystallite diameter, λ the X-ray wavelength, k the Scherer constant (0.89), B1/2 the full width half maximum (FWHM) of the NiO and Mn2 O3 diffraction peaks and θ the diffraction angle. In line broadening profile analysis the scanning rate was fixed at 0.2◦ in 2θ min−1 . Infrared transmission spectra of various solids were determined using Perkin-Elmer spectrophotometer (type 1430). The IR spectra were measured from 400 to 4000 cm−1 . Two milligrams of each solid sample were mixed with 200 mg of vacuum-dried IR-grade KBr. The mixture was dispersed by grinding for 3 min in a vibratory ball mill and placed in a steel die 13 mm in diameter and subjected to a pressure of 12 ton cm−2 . The sample disks were placed in the holder of a double-grating IR spectrometer. The specific surface areas (SBET ), total pore volume (Vp ) and mean pore radius (r) of the various catalysts were determined form nitrogen adsorption isotherms measured at −196 ◦ C using a conventional volumetric apparatus. Before under taking such measurements, each sample was degassed under pressure of 10−5 Torr for 3 h at 200 ◦ C. The catalytic oxidation of CO by O2 was carried out on various catalysts at 150–200 ◦ C using a static method. A fresh 200 mg catalyst sample was employed in each kinetic experiment and was activated by heating at 300 ◦ C for 2 h under a reduced pressure of 10−6 Torr. The kinetics of the catalytic reaction were monitored by measuring the pressure of the reaction mixture (2 Torr of CO + 1/2O2 ) at different time intervals until equilibrium was attained. The reaction product (CO2 ) was removed from the reaction atmosphere

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by freezing in a liquid nitrogen trap. So the percentage decrease in the pressure of the reaction gases at a given time interval determines the percentage conversion of the catalytic reaction at that time. The catalytic activities of different catalyst samples were also determined by studying the decomposition of H2 O2 in their presence at 30–50 ◦ C using 100 mg of a given catalyst sample with 0.5 ml volume of H2 O2 of known concentration diluted to 20 ml with distilled water. The reaction kinetic was monitored by measuring the volume of oxygen gas liberated at different time intervals until equilibrium was attained. 3. Results and discussion 3.1. X-ray investigation of pure and doped solids precalcined at different temperatures X-ray diffractograms of pure and doped solids precalcined at 400–700 ◦ C were determined. Fig. 1 shows X-ray diffrac-

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togram of pure NiO/Al2 O3 and those doped with 2.0 and 6.0 mol% La2 O3 precalcined at 400 and 700 ◦ C. Inspection of this figure revealed that the solids investigated preheated at 400 ◦ C consisted of NiO and AlO(OH) phases while the solids preheated at 700 ◦ C consisted of NiO and poorly crystalline ␥-Al2 O3 phases. The absence of any diffraction line of La2 O3 phase in the heavily doped solid samples (6.0 mol% La2 O3 ) might suggest its complete dissolution in the lattices of NiO and Al2 O3 and/or its presence in an amorphous state. The addition of small amounts of La2 O3 (1.0– 6.0 mol%) as dopant resulted in a progressive decrease in the relative intensity of the diffraction lines corresponding to NiO and AlO(OH) or ␥-Al2 O3 phases. In other words, the doping process resulted in an increase in the degree of dispersion of NiO and ␥-Al2 O3 crystallites. The observed increase in the dispersion of these phases as a result of La2 O3 treatment of the NiO/Al2 O3 system precalcined at 400 and 700 ◦ C has been tentatively attributed to a possible

Fig. 1. X-ray diffractograms of pure and doped NiO/Al2 O3 solid samples precalcined at 400 and 700 ◦ C.

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Fig. 2. X-ray diffractograms of pure and doped Mn2 O3 /Al2 O3 solid samples precalcined at 400 and 700 ◦ C.

coating of NiO and Al2 O3 crystallites with a La2 O3 film which hinders the particle adhesion process of the treated oxides, thus limiting their grain growth during the course of heat treatment. The peak height of the main diffraction line corresponding to NiO phase (d = 2.09 Å ) was 125, 43 and 37 a.u. for the pure sample and those treated with 2.0 and 6.0 mol% La2 O3 , respectively, thereby suggesting that La2 O3 treatment led to a decrease in the degree of crystallinity of the NiO phase to an extent proportional to the amount of dopant present. The heat treatment of pure and variously doped solids at 700 ◦ C did not affect the interaction between NiO and Al2 O3 yielding to the formation of crystalline NiAl2 O4 compound. The effect of this treatment on the particle size of NiO phase has been studied for the investigated system precalcined at 400 and 700 ◦ C. The results obtained revealed that the particle size of NiO phase was 98, 73 and 63 Å for pure sample and those treated with 2.0 and 6.0 mol% La2 O3 , respectively for the solids calcined at 400 ◦ C. While for the solids calcined at 700 ◦ C the particle size measured 172, 98 and 57 Å for pure specimen and those doped with 2.0 and 6.0 mol% La2 O3 , respectively. Fig. 2 depicts X-ray diffractograms of pure and doped Mn2 O3 /Al2 O3 solids precalcined at 400 and 700 ◦ C. It is seen from this figure that pure solid sample and those doped with 2.0 mol% La2 O3 precalcined

at 400 ◦ C consisted of Mn2 O3 and AlO(OH), while the solids doped with 6.0 mol% La2 O3 consisted only of AlO(OH). The addition of 2.0 mol% La2 O3 to Mn2 O3 /Al2 O3 led to decrease the degree of crystallinity of Mn2 O3 and AlO(OH) phases. The increase in the amount of La2 O3 added led to disappearance of all diffraction lines of Mn2 O3 phase besides an effective decrease in the degree of crystallinity of AlO(OH) phase. The disappearance of all diffraction peaks characteristics for manganese oxide phase in the solids treated with La2 O3 might suggest the existence of very fine particles of Mn2 O3 whose crystallite size became very small below the detection limit of X-ray diffractometer. These results clearly indicate the role of La2 O3 in increasing the degree of dispersion of manganese oxide crystallites leading to the formation of small sized crystallites. It can also seen from Fig. 2 that pure and 2.0 mol% La2 O3 doped Mn2 O3 /Al2 O3 solids calcined at 700 ◦ C composed of Mn2 O3 and ␥-Al2 O3 phases, while solids treated with 4.0 and 6.0 mol% La2 O3 calcined at the same temperature were composed only of Mn2 O3 phase. The increase in the amount of La2 O3 added from 4.0 to 6.0 mol% led to an effective decrease in the degree of crystallinity of Mn2 O3 phase. In fact, the addition of 2.0 mol% La2 O3 to the investigated system followed by calcination at 700 ◦ C resulted in a

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significant decrease in the degree of crystallinity of Mn2 O3 and ␥-Al2 O3 . While the addition of 4.0 or 6.0 mol% La2 O3 led to the disappearance of all diffraction lines of ␥-Al2 O3 phase and an effective weakening in the peak height of the diffraction lines of Mn2 O3 phase. The peak height of the main diffraction line of Mn2 O3 phase (d = 2.72 Å) was 79, 63, 36 and 10 a.u. for the pure sample and those treated with 2.0, 4.0 and 6.0 mol% La2 O3 , respectively. The increase in the degree of dispersion of all phases due to treatment with La2 O3 might be expected to be accompanied by an increase in the specific surface area of the treated solids. This expectation is additionally supported by the absence of any appreciable solid–solid interaction between manganese oxide and the alumina support material to produce manganese aluminate. The effect of this treatment on the particle size of Mn2 O3 phase has been investigated in the case of pure and doped solids precalcined at 400 and 700 ◦ C. The results obtained revealed that the particle size of Mn2 O3 phase in the investigated solids calcined at 400 ◦ C measured 183 and 114 Å for pure sample and that doped with 2.0 mol% La2 O3 . The computed values of particle size of Mn2 O3 phase in different solids calcined at 700 ◦ C were 270, 260, 240 and 117 Å for the pure sample and those treated with 2.0, 4.0 and 6.0 mol% La2 O3 . These results showed the role of La2 O3 -doping in much increasing the degree of dispersion of different catalyst constituents and decreasing their degree of crystallinity. The observed changes in the degree of crystallinity and particle size of NiO and Mn2 O3 phases due to the treatment of solid samples with La2 O3 precalcined at 400–700 ◦ C are expected to modify the surface and catalytic properties of the treated solids. 3.2. IR spectrophotometric investigation of various solids The IR transmission spectra were measured for Al2 O3 precalcined at 700 ◦ C and NiO, Mn2 O3 , pure and heavily doped NiO/Al2 O3 solids calcined at 500 and 700 ◦ C. The spectra obtained are depicted in Figs. 3 and 4. Inspection of Fig. 3 shows that: (1) all the spectra of solid samples exhibit bands characteristic for the individual oxides, viz. NiO, Al2 O3 . (2) Treatment of NiO/Al2 O3 solids with 6.0 mol% La2 O3 followed by calcination at 500 ◦ C resulted in an increase in the intensity of the transmission bands located at 1380 and 1080 cm−1 . (3) The very strong bands at 1183, 1125 and 1080 cm−1 characteristic for NiAl2 O4 structure of pure NiO/Al2 O3 solid calcined at 700 ◦ C disappeared due to the treatment of solid sample with 6.0 mol% La2 O3 calcined at the same temperature. These bands should characterize the nickel aluminate structure which disappeared upon doping NiO/Al2 O3 with 6.0 mol% La2 O3 . These results show clearly the role of La2 O3 in increasing the dispersion of NiO and Al2 O3 and hindering their solid–solid interactions. (3) The transmission bands located at 3470–3425 cm−1 and 1650–1635 cm−1 corresponding to

Fig. 3. Infrared transmission spectra for Al2 O3 calcined at 700 ◦ C and NiO, pure and heavily doped NiO/Al2 O3 with La2 O3 precalcined at 500–700 ◦ C.

the OH groups in the sample. Fig. 4 shows the transmission spectra of Mn2 O3 , pure and heavily doped Mn2 O3 /Al2 O3 calcined at 500 and 700 ◦ C. It is observed from this figure that all the spectra exhibit bands characteristic for the individual oxides (Mn2 O3 and Al2 O3 ). The transmission bands located at 3450–3400 cm−1 and 1643–1635 cm−1 corresponding to the OH groups in samples. The observed hindrance of nickel aluminate formation due to La2 O3 -doping of NiO/Al2 O3 and modification of the intensity of bands characteristic for surface OH groups as a result of doping process are expected to induce significant changes in their catalytic activities towards CO-oxidation by O2 and H2 O2 decomposition, respectively.

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Fig. 4. Infrared transmission spectra for Mn2 O3 , pure and heavily doped Mn2 O3 /Al2 O3 with La2 O3 precalcined at 500 and 700 ◦ C.

3.3. Surface properties of pure and doped solid catalysts preheated at 400–700 ◦ C The different surface characteristics of pure and doped solids with La2 O3 precalcined at 400, 500 and 700 ◦ C were determined from nitrogen adsorption conducted at −196 ◦ C. These characteristics include the specific surface areas (SBET and St ), total pore volume (Vp ) and mean pore radius (r). The adsorption isotherms, not given here, are of type II of Brunauer classification [50]. The BET-surface areas of the investigated adsorbents were calculated from the linear BET plots and the data obtained is given in Table 1, which includes also the values of Vp and r. Another series of specific surface areas St were computed from volume-thickness

curves (Vl−t plots) of the various investigated adsorbents. These plots were constructed using the de Boer-t plot [51]. The Vl−t plots of pure and doped solids (NiO/Al2 O3 and Mn2 O3 /Al2 O3 ) treated with La2 O3 precalcined at 400, 500 and 700 ◦ C are similar for each other. Fig. 5 shows a representative Vl−t plot of pure and variously doped solids precalcined at 500 ◦ C. The Vl−t plots of pure and doped solids precalcined at 400 ◦ C (not given) showed downward deviation while the Vl−t plots of pure and doped solids precalcined at 500 ◦ C (c.f. Fig. 5) and 700 ◦ C (not given) exhibited an upward deviation. So, the investigated solids precalcined at 400 ◦ C contained narrow pores as dominant porosity, while the solids calcined at 500 and 700 ◦ C contained mainly wide pores. However, these solids are considered as mesoporous adsorbents [52]. In fact, the r values of all the investigated solids vary between 21 and 74 Å. It is seen from Table 1 that the values of SBET and St are close to each other which justifies the correct choice of standard t-curves for pore analysis and indicates the absence of ultramicropores. The doping of NiO/Al2 O3 and Mn2 O3 /Al2 O3 solids with La2 O3 followed by calcination at 400–700 ◦ C resulted in a progressive increase in their specific surface areas. This increase depends upon the dopant concentration and calcination temperature. The maximum increase in the SBET of NiO/Al2 O3 solids due to doping with 6.0 mol% La2 O3 attained 20, 18.8 and 28.3% for the adsorbents calcined at 400, 500 and 700 ◦ C, respectively. While for Mn2 O3 /Al2 O3 solids followed by calcination at 400, 500 and 700 ◦ C the maximum increase in their SBET due to doping with 6.0 mol% La2 O3 attained 25.8, 25.7 and 37%, respectively. The observed significant increase in the BET-surface areas of the investigated solids due to doping with La2 O3 could be attributed to—(1) creation of new pores resulting from liberation of nitrogen oxides during the thermal treatment of added lanthanum nitrate. (2) The observed significant decrease in the pore size (Table 1) of the investigated solids (pore narrowing process) due to this treatment should be accompanied by a significant increase in the specific surface areas of the treated solids. (3) The decrease in the degree of crystallinity of both phases (NiO and Mn2 O3 ) and the effective decrease in the particle size of these phases. This reflects the role of La2 O3 dopant in increasing the degree of dispersion of NiO and Mn2 O3 over the alumina support. In fact, X-ray line broadening analysis of variously doped solids calcined at different temperatures indicated a significant decrease in the particle size of NiO and Mn2 O3 phases. The observed decrease in the particle size of these two phases and the significant increase in the specific surface areas due to doping with La2 O3 are expected to be accompanied by measurable changes in the catalytic activity of the treated solids. 3.4. Catalytic activity of pure and treated solid catalysts The catalytic oxidation of CO by O2 and H2 O2 decomposition were carried out over pure and vari-

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Table 1 Surface characteristics for pure and variously doped solid samples precalcined at different temperatures Adsorbents

Calcination temperature (◦ C)

SBET (m2 g−1 )

St (m2 g−1 )

Vp (cm3 g−1 )

r (Å)

CBET

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

400 400 400 400 400

216 226 235 248 260

214 225 232 245 258

0.285 0.273 0.265 0.247 0.231

33 30 28 25 22

67 75 92 105 122

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

500 500 500 500 500

202 211 219 231 240

198 209 217 228 237

0.315 0.298 0.281 0.260 0.251

39 35 32 28 26

58 79 95 109 135

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

700 700 700 700 700

127 134 146 155 163

125 133 144 152 135

0.375 0.352 0.329 0.312 0.295

74 66 56 50 45

81 102 123 136 148

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

400 400 400 400 400

229 241 256 270 288

227 239 254 269 286

0.295 0.276 0.261 0.245 0.238

32 29 26 23 21

73 91 106 122 145

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

500 500 500 500 500

206 219 233 246 259

204 218 231 245 257

0.306 0.292 0.284 0.270 0.256

37 33 31 27 25

85 98 112 128 152

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 + 2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

700 700 700 700 700

135 148 162 179 185

134 147 160 176 184

0.364 0.348 0.329 0.317 0.304

67 59 51 44 41

98 117 138 169 185

ously doped solids precalcined at different temperatures. 3.4.1. Catalytic oxidation of CO by O2 over pure and treated solids The oxidation of CO by O2 was conducted over NiO/Al2 O3 solids treated with different amounts of La2 O3 (1.0–6.0 mol%) and subjected to heat treatment at 400, 500 and 700 ◦ C. First-order kinetics were observed in all cases, the slope of the first-order plot determines the values of reaction rate constant (k). Fig. 6 shows representative first-order plots of CO-oxidation by O2 conducted at 175 ◦ C over pure and doped NiO/Al2 O3 solids precalcined at 500 and 700 ◦ C and over pure and doped Mn2 O3 /Al2 O3 solids precalcined at 700 ◦ C. The value of k of the catalyzed reaction conducted over pure and doped solid catalysts calcined at 400, 500 and 700 ◦ C were determined. The role of La2 O3 treatment in the catalytic activity of NiO–La2 O3 /Al2 O3 and Mn2 O3 –La2 O3 /Al2 O3 systems is better investigated by comparing the value of k as a function of doped concentration in catalyst samples precalcined at 400, 500 and 700 ◦ C.

The variation of k for the reaction conducted at 150–200 ◦ C as a function of dopant is graphically illustrated in Fig. 7 for the different pure and doped solids precalcined at different temperatures. It is seen from Fig. 7 that La2 O3 -doping of NiO/Al2 O3 and Mn2 O3 /Al2 O3 followed by calcination at 400, 500 and 700 ◦ C resulted in a progressive increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activities, expressed as k150◦ C , k175◦ C and k200◦ C of the NiO/Al2 O3 solids investigated due to doping with 6.0 mol% La2 O3 followed by calcination at 500 ◦ C attained 533, 234 and 153.6%, respectively. For the Mn2 O3 /Al2 O3 treated with 6.0 mol% La2 O3 followed by calcination at 500 ◦ C the maximum increase in the value of k150◦ C , k175◦ C and k200◦ C attained 230, 163 and 105%, respectively. The significant effective increase in the catalytic activity of the investigated catalyst samples due to doping with La2 O3 can be attributed to: (1) the presence of most active sites (NiO or Mn2 O3 ) on the top surface layers of the treated solids which enhanced the chemisorption and catalysis of CO-oxidation and facilitated desorption of the produced CO2 . (2) The presence of lanthanum in the alu-

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Fig. 5. Plots of volume vs. thickness for pure and various La2 O3 -doped solids calcined at 500 ◦ C.

mina serves to stabilize it, thus allowing alumina to remain in amorphous structure under high temperature conditions [16]. At the same time lanthanum stabilizes the oxidation activity of these catalysts by hindering the interaction of metal oxide with alumina to form less active bulk spinel compounds. (3) The lanthanum affects the increase of dispersion of metal oxides on the alumina support [22], which acting as the active phases for the oxidation of CO. (4) The role of La2 O3 dopant in decreasing the particle size and the degree of crystallinity of metal oxide phase (NiO or Mn2 O3 ). (5) The addition of La2 O3 stabilizes catalyst surface metal oxides in which metal ions-oxygen bond energy is lowered (Mn+1 –O), and meanwhile Mn+1 ions also quickly reduce into active Mn ions, therefore easily forming a redox cycle of metal ions (Mn to Mn+1 ), there is a close relation between catalytic activity of metal oxides and their reduction ability. So, the addition of La2 O3 increases the reduction speed of the catalysts and therefore increases their catalytic activity [53]. On the other hand, the catalytic activity of variously

doped solids suffered small decrease upon increasing their calcination temperature from 400 to 700 ◦ C. A decrease of about 22, 26 and 28.6% in the catalytic activity measured at 150, 175 and 200 ◦ C, respectively were observed for NiO/Al2 O3 samples doped with 6.0 mol% La2 O3 due to increasing their calcination temperature from 400 to 700 ◦ C. While the observed decrease in the catalytic activity of Mn2 O3 /Al2 O3 solids treated with 6.0 mol% La2 O3 measured at 150, 175 and 200 ◦ C were 12.5, 9.2 and 12.7%, respectively. The observed changes in the catalytic activity of pure and doped solids due to calcination at temperatures ranged from 400 to 700 ◦ C could result from the induced changes in their specific surface areas. The comparison between the effects of La2 O3 -doping of the investigated systems showed clearly that NiO/Al2 O3 is more susceptible for increasing its catalytic activity in CO-oxidation by O2 than Mn2 O3 /Al2 O3 system. The determination of activation energy (E) of CO-oxidation by O2 over pure and treated solids with La2 O3 can

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Fig. 6. First-order plots of CO-oxidation by O2 conducted at 175 ◦ C over pure and variously doped NiO/Al2 O3 and Mn2 O3 /Al2 O3 solid catalysts.

throw more light on the role of lanthanum oxide in changing the mechanism of the catalyzed reaction. This has been done by determining the catalytic reaction rate constant (k) at different temperature on various catalyst samples via applying the Arrhenius equation. The computed values of E are listed in Table 2, which includes the values of the preexponential factor (ln A) for the Arrhenius equation. Inspection of the data listed in Table 3 shows that La2 O3 -doping (1.0–6.0 mol%) of NiO/Al2 O3 and Mn2 O3 /Al2 O3 systems calcined at 400, 500 and 700 ◦ C brought about a progressive decreased in the E values. These results express the observed progressive increase in the catalytic activity of the investigated solids. This apparent discrepancy is resolved when the value of the logarithm of pre-exponential constant (A) in the Arrhenius equation is taken into account. Such

data which is listed in Table 2 shows that, for the two systems investigated, the values of ln A varied considerably for each system calcined at a given temperature but with different concentrations of the catalytically active constituent, thereby demonstrating the heterogeneity of the catalyst surface. To account for this heterogeneity, the activation energies (E) for the catalytic reaction were recalculated (E∗ ) for the doped solids adopting ln A values of the undoped solids calcined at 400, 500 and 700 ◦ C to the variously doped solids calcined at the same temperatures. The computed E∗ values are given in the last column of Table 2. The resulting E∗ values obtained were virtually the same for the pure and variously doped solids precalcined at the same temperatures. This indicates that La2 O3 -doping of NiO/Al2 O3 and Mn2 O3 /Al2 O3 solids did not change the mechanism of the

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Fig. 7. Variation of reaction rate constant (k) as a function of dopant concentration for CO-oxidation by O2 conducted at 150, 175 and 200 ◦ C over pure and variously doped NiO/Al2 O3 and Mn2 O3 /Al2 O3 solid catalysts precalcined at 400, 500 and 700 ◦ C.

catalytic reaction but rather increased the concentration of catalytically active sites on the catalyst surface. 3.4.2. Catalytic decomposition of H2 O2 over the investigated solid catalysts The kinetics of H2 O2 decomposition in the presence of the various solid catalysts calcined at 400, 500 and 700 ◦ C were monitored by measuring the volume of oxygen liberated at different time intervals at 30–50 ◦ C until equilibrium was attained at each temperature. The catalytic decomposition was found to follow first-order kinetics in all cases. In fact, the plots of ln (a/a−x) versus time give straight lines, where “a” is the initial concentration of H2 O2 solution (mmol l−1 ) and x is the fraction of H2 O2 (mmol l–1 )

at a given time. The slopes of such plots allow the ready determination of the reaction rate constant (k) measured at a given temperature over a given catalyst sample. Fig. 8 depicts representative first-order plots of the catalytic reaction carried out at 40 ◦ C over pure and variously doped NiO/Al2 O3 with La2 O3 precalcined at 400, 500 and 700 ◦ C. The different value of k for the reaction carried out at 30, 40 and 50 ◦ C over the investigated solid catalysts were calculated. The variation of reaction rate constant k as a function of La2 O3 concentration for H2 O2 decomposition carried out at 30, 40 and 50 ◦ C over pure and variously doped NiO/Al2 O3 and Mn2 O3 /Al2 O3 solid catalysts precalcined at 400-700 ◦ C is graphically illustrated in Fig. 9. The treatment of the investigated solid catalysts with La2 O3 followed by

N.R.E. Radwan / Applied Catalysis A: General 257 (2004) 177–191 Table 2 Activation energies (E and E∗ ) and frequency factor (ln A) for the catalytic oxidation of CO by O2 over pure and doped solid catalysts calcined at different temperatures

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Table 3 Activation energies (E and E∗ ) and frequency factor (ln A) for the catalytic decomposition of H2 O2 over pure and doped solid catalysts calcined at different temperatures

Catalyst

Calcination temperature ( ◦ C)

E (kJ mol−1 )

ln A

E∗ (kJ mol−1 )

Catalyst

Calcination temperature ( ◦ C)

E (kJmol –1 )

ln A

E∗ (kJ mol−1 )

NiO/Al2 O3 +1.0 mol% La2 O3 + 2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

400 400 400 400 400

37 26 19 20 14

37 24 16 18 12

37 37 37 37 36

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

400 400 400 400 400

72 72 71 71 70

113 111 113 111 115

72 72 72 72 72

NiO/Al2 O3 +1.0 mol% La2 O3 + 2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

500 500 500 500 500

43 31 20 17 14

43 30 18 14 12

43 43 43 43 42

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

500 500 500 500 500

72 71 71 71 70

112 111 112 118 120

72 71 71 71 71

NiO/Al2 O3 +1.0 mol% La2 O3 + 2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

700 700 700 700 700

38 38 24 18 12

36 37 32 16 9

38 37 37 36 36

NiO/Al2 O3 +1.0 mol% +2.0 mol% +4.0 mol% +6.0 mol%

La2 O3 La2 O3 La2 O3 La2 O3

700 700 700 700 700

78 73 72 72 71

123 115 113 113 114

78 78 78 78 78

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 + 2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

400 400 400 400 400

24 18 16 13 11

22 16 14 10 9

24 23 23 23 23

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

400 400 400 400 400

43 33 32 33 33

68 53 51 53 54

43 42 42 42 42

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

500 500 500 500 500

25 17 16 13 11

23 15 13 10 8

25 25 24 24 24

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

500 500 500 500 500

43 35 33 37 31

70 56 54 60 51

43 43 43 43 43

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

700 700 700 700 700

28 17 17 14 11

26 14 15 11 9

28 28 27 27 27

Mn2 O3 /Al2 O3 +1.0 mol% La2 O3 +2.0 mol% La2 O3 +4.0 mol% La2 O3 +6.0 mol% La2 O3

700 700 700 700 700

21 20 22 21 19

35 33 36 35 32

21 21 21 21 21

calcination at 400–700 ◦ C resulted in a progressive increase in the catalytic activity of the treated solids. The maximum increase in the value of k30◦ C attained 40, 44 and 67% for NiO/Al2 O3 solids doped with 6.0 mol% La2 O3 calcined at 400, 500 and 700 ◦ C, respectively, while the maximum increase in the value of k30◦ C attained 170, 167 and 101 % for Mn2 O3 /Al2 O3 solids doped with 6.0 mol% La2 O3 calcined at 400, 500 and 700 ◦ C, respectively. On the other hand, the catalytic activity of the investigated solids increased by increasing their calcination temperature from 400 to 700 ◦ C. An increase of about 49, 22 and 45% in the catalytic activity measured at 30, 40 and 50 ◦ C, respectively were observed for NiO/Al2 O3 solids doped with 6.0 mol% La2 O3 due to increasing their calcination temperature from 400 to 700 ◦ C. The observed increase in the catalytic activity of Mn2 O3 /Al2 O3 solids treated with 6.0 mol% La2 O3 for the reaction conducted at 30, 40 and 50 ◦ C were 70, 38 and 22%, respectively. This significant increase in the catalytic activity of solid catalysts reflects a significant increase in the concen-

tration of catalytically active constituents in the uppermost surface layers which take part in catalyzing the decomposition of H2 O2 . The results of many studies [54–57] indicate that one of the most factor influencing the activity of oxide catalysts is the oxidation state of their surface atoms. The oxidation state of nickel (Ni2+ –Ni3+ ) in nickel oxide and oxidation state of manganese (Mn2+ –Mn3+ ), (Mn3+ –Mn4+ ) and (Mn2+ –Mn4+ ) in manganese oxide determine the catalytic activity of the oxide, where the ions in different oxidation state play an important role in the catalytic activity of oxide catalysts. There is a relation between the oxidation state and the catalytic activity of oxide catalysts and the ratio of two different oxidation states of the ions, one of which is able to oxidize the hydrogen peroxide and the other is able to reduce it determines the catalytic activity of the oxide catalysts [54–57]. From the electronic theory of catalysis and the principle of bivalent catalytic sites [58] there are two kinds of catalytic sites in equilibrium on the catalyst surface, i.e. donor and acceptor sites which may be

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Fig. 8. First-order plots of H2 O2 decomposition at 40 ◦ C over pure and variously doped NiO/Al2 O3 catalysts precalcined at 400–700 ◦ C.

formed by metal catalyst ions in various valence states or by charge defects stabilized on the catalyst surface. Therefore, the catalytic reaction proceeds on the catalytic sites constituted from the ion pairs which one of the ion occurs in a lower and the second ion in a higher oxidation state [54–58]. So, the observed significant increase in the catalytic activity of the treated solids due to doping with La2 O3 conducted at 400–700 ◦ C in H2 O2 decomposition could be attributed to creation of ion pairs in the catalytic reaction. The ion pairs in the NiO/Al2 O3 solids are Ni2+ –Ni3+ while in Mn2 O3 /Al2 O3 solids are Mn2+ –Mn3+ , Mn3+ –Mn4+ , and

Mn2+ –Mn4+ which controlling the catalytic activity of these solid catalysts. On comparing the catalytic activity of these two solid catalysts (NiO/Al2 O3 and Mn2 O3 /Al2 O3 ), it was found that pure and variously doped Mn2 O3 /Al2 O3 solids have higher catalytic activity than pure and variously doped NiO/Al2 O3 towards H2 O2 decomposition. This may be attributed to—(1) Alumina is incapable of dispersing NiO on its surface with the same degree of dispersion as Mn2 O3 and that is clear from XRD measurements of investigated solids. (2) The reaction proceeds on acceptor and donor sites and that depend on the standard oxidation or reduction potentials of the systems (NiO and Mn2 O3 ) relative to H2 O2 .

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Fig. 9. Variation of reaction rate constant (k) as a function of dopant concentrations for H2 O2 decomposition at 30, 40 and 50 ◦ C over pure and variously doped NiO/Al2 O3 and Mn2 O3 /Al2 O3 catalysts precalcined at 400–700 ◦ C.

The observed changes in the catalytic activity of pure and doped solids due to calcination temperatures ranged from 400–700 ◦ C could result from the induced changes in their specific surface areas. Determination of the apparent activation energy (E) for the catalysis of H2 O2 decomposition over different investigated solids shed some light on the possible change in the mechanism of the catalyzed reaction and hence gives useful information about possible changes in the concentration and nature of the catalytically active constituents. The value of k measured at 30, 40 and 50 ◦ C over pure and variously doped solids enable E to be calculated by direct application of the Arrhenius equation. The calculated values of E are listed in Table 3, which also includes the pre-exponential

factor (A) for the Arrhenius equation. Inspection of the data listed in Table 3 revealed that the doping of NiO/Al2 O3 solids with La2 O3 led to the observed E values for the solids calcined at 400 and 500 ◦ C (71 ± 1 kJ mol−1 ) remain virtually unchanged, while for the solids calcined at 700 ◦ C led to a progressive decrease in the E values ( from 78 to 71 kJ mol−1 ). These findings indicate that the effect of doping in the catalytic activity of NiO/Al2 O3 solids was limited. The data listed in Table 3 showed also that the doping of Mn2 O3 /Al2 O3 solids with La2 O3 brought about a progressive decrease in the E values for the solids calcined at 400 ◦ C (from 43 to 32 kJ mol−1 ) and at 500 ◦ C (from 43 to 31 kJ mol−1 ), while at 700 ◦ C (21 ± 1 kJ mol−1 ) remained virtually unchanged. These results express the

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observed progressive increase in the catalytic activity of Mn2 O3 /Al2 O3 solids calcined at 400 and 500 ◦ C due to doping with La2 O3 , while the solids calcined at 700 ◦ C the effect of doping was limited in their catalytic activity. This apparent discrepancy is resolved when the value of the logarithm of pre-exponential constant (A) in the Arrhenius equation is taken into account. Such data which is listed in Table 3 shows that, for the two systems investigated, the values of ln A varied considerably for each system calcined at a given temperature but with different concentrations of the catalytically active constituent, thereby demonstrating the heterogeneity of the catalyst surface. To account for this heterogeneity, the activation energies (E) for the catalytic reaction were recalculated (E∗ ) for the doped solids adopting ln A values of the undoped solids calcined at 400, 500 and 700 ◦ C to the variously doped solids calcined at the same temperatures. The computed E∗ values are given in the last column of Table 3. The recalculated activation energy (E∗ ) were virtually the same for pure and variously doped solids precalcined at the same temperatures. This indicates that the doping of the investigated solids with La2 O3 did not change the mechanism of the catalytic reaction but rather increased the concentration of catalytically active sites on the catalyst surface. The comparison between the effects of doping of NiO/Al2 O3 and Mn2 O3 /Al2 O3 solids with La2 O3 on their catalytic activities towards CO-oxidation by O2 and H2 O2 decomposition suggested that the catalytically active sites involved in both catalytic reactions are different from each others. This conclusion is based on the fact that the doping process effected different changes in their catalytic activities in catalyzing both reactions.

4. Conclusions The following are the main conclusions that can be drawn from the results obtained. 1. The doping of the investigated solids NiO/Al2 O3 and Mn2 O3 /Al2 O3 with La2 O3 (1.0–6.0 mol%) resulted in an effective decrease in the degree of crystallinity of NiO and Mn2 O3 phases to an extent proportional to the amount of dopant present. The results clearly indicate the role of La2 O3 -doping in increasing the degree of dispersion of NiO and Mn2 O3 over the alumina support, decreasing their degree of crystallinity and decreasing their particle size. 2. The doping process hindered the nickel and manganese aluminates formation and effected a modification in the intensity of IR bands characteristic for OH groups. 3. The doping of both NiO/Al2 O3 and Mn2 O3 /Al2 O3 solids with La2 O3 followed by calcination at 400–700 ◦ C resulted in a significant increase in their specific surface areas.

4. Doping of NiO/Al2 O3 and Mn2 O3 /Al2 O3 with La2 O3 followed by calcination at 400–700 ◦ C brought about a significant increase in their catalytic activities towards CO-oxidation by O2 and H2 O2 decomposition. The increase was, however, more pronounced for NiO/Al2 O3 in case of CO-oxidation while in H2 O2 decomposition reaction the increase was limited. 5. The doping process did not modify the mechanism of the two catalyzed reactions but rather increased the concentration of the catalytic active sites without changing their energetic nature.

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