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Al hydrotalcite-like compounds
Applied Catalysis B: Environmental 60 (2005) 289–297 www.elsevier.com/locate/apcatb
Catalytic decomposition of nitrous oxide over catalysts prepared from Co/Mg-Mn/Al hydrotalcite-like compounds L. Obalova´ a,*, K. Jira´tova´ b, F. Kovanda c, K. Pacultova´ a, Z. Lacny´ a, Z. Mikulova´ b b
a Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojova´ 135, 165 02 Prague 6, Czech Republic c Department of Solid State Chemistry, Institute of Chemical Technology, Technicka´ 5, 160 00 Prague, Czech Republic
Received 2 December 2004; received in revised form 7 April 2005; accepted 9 April 2005 Available online 13 May 2005
Abstract Co/Mg-Mn/Al hydrotalcite-like compounds with Co:Mg:Mn:Al molar ratios of 4:0:2:0, 2:2:2:0, 2:2:1:1, 4:0:1:1, 4:0:0:2 and 2:2:0:2 were prepared by coprecipitation method. The mixed oxides obtained by calcination at 500 8C were characterized using various techniques (XRD, IR, BET surface area measurements, TPD and TPR). The activities of calcined hydrotalcites were tested for N2O decomposition in an inert gas, and in the presence of oxygen and water vapor as well. Among the catalysts screened, the catalyst prepared from Co hydrotalcite containing Al and Mn as the trivalent cation showed the highest activity, which was retained in the presence of oxygen but was inhibited by water vapor. Redox properties of the catalysts play an important role in the reaction and optimum extent of reduction is necessary for achievement of the highest catalytic activity. # 2005 Elsevier B.V. All rights reserved. Keywords: Nitrous oxide; Catalytic decomposition; Layered double hydroxides; Mixed oxide catalysts
1. Introduction Nitrous oxide is known as a strong greenhouse gas that contributes to catalytic destruction of stratospheric ozone. Therefore, it is of interest to reduce effectively the N2O emissions. Catalytic decomposition offers a simple solution of the N2O emission control in waste gases arising from combustion processes (fluidized bed combustion) and chemical industry (e.g. production of nitric or adipic acids). The N2O decomposition was studied over many different catalysts [1,2] in the last four decades. N2O decomposition is most efficiently catalyzed by transition metals of the Group VIII (Co, Ni, and Fe) and by Rh, Ru, Pd, Ir, Cu, La and Mn as implies from the published results. The studies from recent years deal especially with Fe-zeolites [3–13], precious metals Rh, Ru on various supports [14–21] and supported Cu [22–27]. The key parameters determining the rate of N2O decomposition are the type of support or matrix and the * Corresponding author. Tel.: +420 59 6991532; fax: +420 59 7323396. E-mail address: [email protected] (L. Obalova´). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.04.002
oxidation state and coordination of the transition metal cations. In this contribution, we focused on catalytic properties of cobalt and manganese cations in the mixed oxide based catalysts, with the aim to study the synergic effect of these two transition metals in N2O decomposition. In the 1970s, Cimino [28] reported the N2O catalytic decomposition over Mn containing catalysts. The studies of Mn ions in very dilute concentration dispersed in MgO matrix have revealed that the Mn3+ ions are more active than Mn4+ which is in good agreement with observation of Yamashita and Vannice [29]. Raj et al. [30] concluded that the Mn3+/Mn4+ couple could be the active cluster site for the N2O decomposition at high concentrations of Mn. Manganese-based mixed oxide systems serve as efficient catalysts in many oxidation processes [31–34] and its reduction–oxidation chemistry is especially important in electron transfer reactions. Decomposition of N2O belongs to such type of reactions [35]. Cimino and Pepe [36] reported the kinetic studies of N2O decomposition over CoO-MgO solid solution (cobalt content from 0.05 to 50 atoms/100 Mg atoms). Reaction was studied
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under conditions giving 1% N2O conversion and the activity per cobalt was reported to decrease with increasing cobalt concentration. Distribution of cobalt ions between octahedral and tetrahedral position in CoxMg1xAl2O4 spinel changes with temperature and catalytic activity of spinel increases with increasing portion of Co ions in octahedral positions [37]. Recently, different Co-containing spinels prepared by co-precipitation method were tested in N2O decomposition [38–42]. Drago et al. [43] studied catalytic decomposition of N2O over various supported oxides. They concluded that CoO is much more active than CuO and Fe2O3 and when supporting on MgO, very active catalyst was attained (95% N2O conversion was achieved at 40,000 h1 and 500 8C). Mixed oxide based catalysts, especially the multicomponent ones, can be obtained by a controlled thermal decomposition of hydrotalcite-like precursors. Hydrotalcitelike compounds, a class of layered double hydroxides, consist of positively charged metal hydroxide layers separated each other by anions and water molecules. Their chemical composition can be represented by the general formula [MII1xMIIIx(OH)2]x+[Anx/nyH2O]x, where MII and MIII are divalent and trivalent metal cations, An is an n-valent anion (often carbonate), and x has usually values approximately between 0.25 and 0.33. By thermal treatment of hydrotalcitelike precursors, the MII and MIII mixed oxides with a high surface area and good thermal stability can be prepared. Both cobalt and manganese cations may be incorporated into hydroxide layers of hydrotalcite-like compounds. Co-containing mixed oxides prepared from hydrotalcite-like precursors were tested in the catalytic decomposition of nitrous oxide [44–51] or in the selective catalytic reduction of NO with ammonia [52]. The substitution of cobalt by magnesium in CoAl catalyst considerably influenced its catalytic behavior [53]. Kustrowski et al. [54] found the descending order of the transition metal oxides dispersed in the MgO-Al2O3 matrix (Cu > Co > Fe > Ni) in N2O decomposition. Only a few papers have reported the hydrotalcite-like compounds containing manganese. Preparation and thermal decomposition of Mg-Mn and Mg-Mn-Al layered double hydroxides are described in [55] and [56]. Synthesis and thermal behaviour of Ni-Mn hydrotalcite-like compounds are reported by Barriga et al. [57] and Kovanda et al. [58]. Formation of mixed oxides during heating of layered double hydroxides containing simultaneously copper and manganese cations in hydroxide layers is described in recent papers [59,60]. However, until now, no report is available on the synthesis, physicochemical and catalytic properties in N2O decomposition of catalysts prepared by thermal treatment of hydrotalcite-like compounds containing cobalt and manganese simultaneously. In the present study, we have prepared the series of catalysts from Co-(Mg)-Mn-(Al) hydrotalcite-like precursors. The catalysts were characterized using various techniques (AAS, XRD, IR, BET surface area measurement, TPR, TPD) and tested in N2O decomposition under conditions approaching those in industrial units. For that
reason, the catalytic performance in the presence of oxygen and water vapor was also studied in order to find out the extent of inhibition of reaction rate by the gases.
2. Experimental methods 2.1. Preparation and characterization of catalysts Samples of Co/Mg-Mn/Al layered double hydroxides with Co:Mg:Mn:Al molar ratios of 4:0:2:0, 2:2:2:0, 2:2:1:1, 4:0:1:1, 4:0:0:2 and 2:2:0:2 were prepared by coprecipitation of nitrate solutions and Na2CO3/NaOH solution at 25 8C and pH 10. The washed and dried products were formed into extrudates and calcined at 500 8C for 4 h in air. The prepared catalysts were crushed and sieved to obtain the fraction of particle size 0.160– 0.315 mm, which was used in catalytic measurements. The samples were denoted as Co4Mn2, Co2Mg2Mn2, Co2Mg2MnAl, Co4MnAl, Co4Al2 and Co2Mg2Al2. The powder XRD patterns of the prepared precursors and calcined samples were recorded using a Seifert XRD 3000P instrument with Co Ka radiation (l = 0.179 nm, graphite monochromator, goniometer with Bragg-Brentano geometry) in 2u range 128–758, step size 0.058 2u. Fourier-transform infrared absorption spectra were recorded using the KBr pellet technique on the spectrometer FTIR 2000 Perkin-Elmer in the range 4000–400 cm1 and the resolution of 4 cm1. The surface area measurements were carried out by nitrogen adsorption at 77 K and evaluated by one point BET method. Prior to each experiment, the samples were activated by heating (0.5 h) at 350 8C. The detailed analysis of the porous structure was obtained from the measurement of adsorption–desorption isotherms of nitrogen at 77 K on an ASAP 2010 instrument (Micromeritics, USA). Prior to the measurement, the samples were activated at 130 8C. The TPR measurements were carried out in an apparatus described earlier [20]. Hydrogen/nitrogen mixture (10 M% H2) was used to reduce the catalysts at the flow rate of 15 cm3 min1. The temperature was linearly raised at the rate of 20 8C min1 up to 850 8C. The TPD measurements were carried out using 0.025 g of the catalyst and helium as carrier gas (flow rate 15 ml/h). Prior to the CO2 desorption all catalysts were calcined to 500 8C at a linear temperature rate increase 10 8C min1. Then, after cooling the sample to ambient temperature, ten doses of CO2 (840 ml) were injected into the helium stream at 30 8C. The excess of CO2 was removed by helium flowing through the catalyst for one hour. Then temperatureprogrammed desorption of CO2 was started and finished when temperature 450 8C was reached. 2.2. Catalytic measurements The N2O decomposition reaction was performed in fixedbed stainless steel reactor of 5 mm internal diameter in
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temperature range 330–450 8C. It was verified that stainless steel did not contribute to the catalyst’s performance at used conditions. Total flow rate was kept at 100 ml min1 NTP (273 K, 101,325 Pa). The catalyst bed contained 0.1–0.3 g of sample with particle size 0.160–0.315 mm. The space velocity (GHSV) of 20,000–60,000 cm3 g1 h1 was applied. The inlet concentration of N2O was 1000 ppm balanced by helium; oxygen (5 M%) and water vapor (2.5 M%) were added to some experimental run. The Gas Humidifier with Nafion1 membrane (Perma Pure Inc.) was used for saturation of reaction gas by water vapor. Before chromatographic analysis, the humidity was removed by Peltier cell and then the gas was final dried in a packed column with CaCl2 granule. A temperature-controlled furnace was used to heat the reactor. Before each run, the catalyst was pre-treated by heating in a He flow (50 cm3 min1) at 10 8C min1 up to 450 8C and maintaining the temperature for 1 h. Then the catalyst was cooled to the reaction temperature and the steady state of the N2O concentration level at the reactor output was measured. The gas chromatograph Hewlett Packard (model 5890, series II GC) equipped with a gas sample loop preceding the split/split less injector system and electron capture detector was used to analyze the nitrous oxide concentration in the reactor inlet and outlet. Data were acquired with HP Chemstation. The chromatographic column used was PoraPlot Q (30 m 0.53 mm 40 mm). An electron capture detector in overheating mode (330 8C) was applied for detection. The two-points calibration with calibration gases (70 ppm, 1000 ppm) was performed before each experimental run. The sample loop was filled with constant gas flow controlled by rotameter. The rate constants were determined according to the 1st order kinetic expression d pN2 O =dt ¼ k pN2 O by an integral method; for the evaluation of the kinetic data the conversions were restricted to less than 30%. The activation energies and preexponential factors were evaluated according to Arrhenius plot. Rate constants k at temperature of 420 8C were extrapolated using Arrhenius equation.
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Table 1 Molar ratios of metal cations determined by chemical analysis and lattice parameters of formed spinels evaluated from XRD patterns of catalysts Catalyst
Molar ratio Co:Mg:Mn:Al
Spinel lattice ˚) parameter a (A
Co4Mn2 Co4MnAl Co4Al2 Co2Mg2Mn2 Co2Mg2MnAl Co2Mg2Al2
2.16:0:1.00:0 4.12:0:1.00:1.07 1.92:0:0:1.00 1.03:0.77:1.00:0 2.07:1.96:1.00:0.97 1.00:1.00:0:1.00
8.184 8.141 8.083 8.248 8.160 Not determined
were identified as predominant phases in the Co4Mn2/nc precursor. Thermal decomposition of Co-based LDHs results in the formation of spinel-type oxides. The study of the thermal decomposition of Co-Al hydrotalcite published by Pe´rezRamı´rez et al. [61] documented that dehydroxylation accompanied by a collapse of hydrotalcite structure was complete at 250–300 8C in air and at 350–400 8C in the inert gas. Thermal treatment in air led to a solid solution of cobalt spinels [Co(Co, Al)2O4] whereas mixtures of CoO and CoAl2O4 were formed upon treatment in the inert gas. The lower thermal stability of the Co-Al hydrotalcite in air was explained by the presence of oxidizable Co2+ cations in the octahedral sheets and the diffusion of Co3+ and Al3+ to the interlayer space in the dehydrated layered structure. A highly stable and homogeneous solid solution of cobalt spinels started to form already at relatively low temperature of 150 8C. A formation of the spinel-like phase during
3. Results and discussion 3.1. Catalysts characterization The molar ratios of metal cations in prepared catalysts presented in Table 1 were calculated from results of chemical analysis and were close to those in the nitrate solutions used for coprecipitation of precursors. A trace amount of Na was also detected in the analyzed samples. Only a well-crystallized hydrotalcite-like phase was found in the powder XRD patterns of the dried precursors except Co4Mn2/nc and Co2Mg2Mn2/nc samples, nc denotes noncalcined sample (Fig. 1). In the Co2Mg2Mn2/nc precursor, MnOOH (feitknechtite) was detected together with hydrotalcite-like phase. Cobalt and manganese (oxo)hydroxides
Fig. 1. X-ray powder diffraction patterns of the non-calcined precursors: H, hydrotalcite-like phase; F, MnOOH (feitknechtite); C, CoOOH; M, (Co, Mn)(OH)2.
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Fig. 3. Infrared spectra of prepared catalysts.
Fig. 2. X-ray powder diffraction patterns of the catalysts prepared by calcination of precursors at 500 8C: S, spinel.
heating of cobalt-rich Co-Mg-Al layered double hydroxides was reported by Ribet et al. [62]. Calcination of Co-Fe and Co-Cr LDHs yielded also non-stoichiometric spinel-type phases [63,64]. The powder XRD patterns of the catalysts prepared by calcination of precursors at 500 8C are shown in Fig. 2. All calcined samples exhibited a relatively low crystallinity. The observed diffraction lines were ascribed to the spinel-like oxides. The lattice parameters of formed spinels presented in Table 1 were evaluated using DiffracPlus Topas 2000 software. The FTIR spectra of the prepared catalysts are shown in Fig. 3. The mixed oxides are formed during thermal decomposition of layered double hydroxides but a trace amount of remaining carbonate was found in the spectra of calcined samples (the bands at about 1500 and 1320 cm1). The absorption bands at 3420 and 1620–1640 cm1 indicated the presence of OH and/or water but the dehydroxylation of samples should be complete at applied calcination temperature 500 8C. An adsorption of air humidity during preparation and handling of KBr pellet can be expected. The two absorption bands in the area of wavenumbers below 900 cm1 are characteristic for n1, n2 vibration of lattice octahedra of spinel. Their values for Co4Al2 sample (n1 = 671 cm1, n2 = 570 cm1) resemble those of Co3O4 (the values from literature [65]: n1 = 672 cm1, n2 = 590 cm1). Dissolving of Al3+ and/or non-stoichiometry of spinel phase can cause the observed shift in n2 compared to the value given in literature (the band is ascribed to Co3+ in octahedral coordination) [47]. The values of absorption bands n1 = 649 cm1 and n2 = 557 cm1 were published [66,67] for spinel-type oxide
MnCo2O4, and these values are very close to n1 = 647 cm1, n2 = 555 cm1 found in the prepared Co4Mn2 catalyst. Substitution of manganese by aluminium led to a shift in the n1 band only, the n2 value being the same for Co4Mn2 and Co4MnAl. The substitution of cobalt by magnesium caused broadening of both, n1 and n2 in the Co2Mg2Al2, Co2Mg2Mn2 and Co2Mg2MnAl catalysts, which corresponds to their less ordered structure. Surface areas determined by means of physical sorption and desorption of nitrogen moved from the highest value (106.3 m2 g1) for Co2Mg2Al2 catalyst to the lowest value 43.6 m2 g1 for Co4Mn2 catalyst (Table 2). Introduction of both magnesium and aluminium into the hydroxide layers of hydrotalcite-like compounds leads to an increase of the catalyst surface area. Temperature-programmed reduction curves of all catalysts are shown in Fig. 4. The Co4Mn2 catalyst is reduced in two main temperature regions, the first one (200–400 8C) belonging to reduction of Co compounds, while the second one (400–700 8C) mainly representing reduction of Mn compounds. The low-temperature peak consists of two peaks representing the reduction of CoIII to CoII and the second part of this peak belongs to reduction of CoII–Co0. The findings are in accord with the data in the chemical literature. The reduction of Co3O4 normally occurs in two steps [68], but Arnoldy et al. [69] observed the reduction of Co3O4 around 330 8C and both corresponding reduction peaks were almost indistinguishable. Reduction of Mn oxides is complicated and can proceed via different reduction stages. In principle, the reduction of b-MnO2 manifests itself in two peaks; the first peak at lower temperature around 250 8C corresponds to reduction of MnO2 or Mn2O3–Mn3O4 and reduction of Mn3O4–MnO occurs at 360–440 8C [70]. Substitution of Co for Mg and/or Mn for Al in our Co4Mn2 catalyst did not change the position of the first reduction peak too much but the second peak was shifted to higher temperatures indicating worse reducibility of the
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Table 2 Rate constant k (420 8C) determined from the 1st order kinetic expression and the consumption of H2 and the amount of desorbed CO2 in the temperature range in which N2O decomposition reaction proceeds Catalyst
SBET (m2 g1)
k 109 ðmolN2 O s1 g1 Pa1 Þ
k 1011 ðmolN2 o s1 m2 Pa1 Þ
TPRa (mmol H2.g1)
TPRb (mmol H2.g1)
Co + Mn (mmol g1)
TPDa (mmol CO2.g1)
Co4Mn2 Co4MnAl Co4Al2 Co2Mg2Mn2 Co2Mg2MnAl Co2Mg2Al2
43.6 92.7 84.0 57.3 91.7 106.3
1.78 12.95 1.76 1.51 1.73 2.32
4.08 13.97 2.10 2.64 1.89 2.18
1.81 0.96 0.66 1.57 1.77 0.80
11.58 11.26 9.16 9.75 7.47 6.69
9.9153 7.5167 6.2108 7.2588 3.4821 3.4617
0.02 0.03 0.025 0.03 0.05 0.03
a b
In the region 350–450 8C. In the temperature region 20–800 8C.
arising components of the catalysts (CoMn; CoAl spinel). Temperature-programmed reduction of the calcined Co-Al and Co-Mg-Al hydrotalcite was previously published by Ribet et al. [62] and occurrence of two distinct reducible cobalt species in both samples was shown. The species below 350 8C were assigned to Co3O4, those reducible around 730 8C were ascribed to a spinel-like phase. The addition of magnesium to the Co3O4 did not change the course of reduction of CoIII to CoII (>300 8C), but it influenced the reduction of CoII to Co0. The reason is the occurrence of reaction between CoO and MgO resulting in new phases CoO–MgO solid solution. Cobalt in CoO–MgO solid solution is reduced completely at much higher temperatures (500–700 8C) in comparison with CoO (400 8C) [68]. Al3+ ions influenced the reducibility of Co ions strongly and caused an increase in reduction temperature. This is explained by polarization of Co–O bonds by Al3+ ions [69]. In Table 2 there are summarized the data on TPR of all catalysts in temperature range 20–800 8C. The total amounts of hydrogen consumed during reduction in 20–800 8C were correlated with the amounts of Co a Mn found in the catalysts and linear dependence was proved. Correlation equation was y = 0.8437x + 3.7607, R2 = 0.892. However, catalytic components reducible at the temperatures higher than reaction temperatures cannot likely contribute to the
Fig. 4. Temperature-programmed reduction of the prepared catalysts.
catalytic reaction and for that reason we determined the amounts of components reducible in the reaction temperature region as that of hydrogen consumed in the range 350– 450 8C. The data are shown in Table 2. Temperature-programmed desorption of the preadsorbed CO2 obtained in the experiments with linearly increased temperature (TPD) was used to characterize basicity of the catalysts. The Co4Mn2 catalyst exhibited basic sites of low strength (Tmax 140 8C) and introduction of Mg made basic sites of a catalyst stronger (Tmax is shifted to higher temperatures of about 150–350 8C). The lowest amount of desorbed CO2 was determined with the Co4Al2 catalyst. CO2 can be bonded to a catalyst surface as monodentate species, chelating bidentate, and bridging bidentate structures [71]. Monodentate species are formed on the strongest basic oxygen ions and chelating and bridging bidentate carbonates on the sites of decreasing basic strength with the participation of an adjacent cationic sites. Similarly as in the case of previously calculated number of reduction sites we determined number of basic sites (possibly active in the N2O conversion) as the amount of CO2 desorbed in the temperature range 350–450 8C. The data are shown in Table 2 and will be discussed together with catalytic activity. 3.2. Catalytic activity The temperature dependences of N2O conversion using 1000 ppm N2O are given for each catalyst in Fig. 5. The activity of Co4MnAl catalyst was much higher than that of the other catalysts. It began to show appreciable conversion at temperatures as low as 330 8C and reached 100% at 450 8C. Other samples showed lower and almost the same activity. Different kinetic expressions describing the kinetics of N2O decomposition can be found in the literature, depending on the fact whether the inhibition of reaction rate by oxygen occurred or not [1]. The kinetic data of the hydrotalcite based catalysts can be fitted with 1st rate law corresponding to no inhibition by oxygen [44]; therefore, the rate constants k (molN2 O s1 g1 Pa1) were determined according to the first order kinetic expression. Since the catalytic reaction proceeds on the catalyst surface area that differs for the samples, the rate constants were also expressed per m2 of
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Fig. 5. Temperature dependence of N2O conversion. Conditions: 1000 ppm N2O balanced He; total flow: 100 cm3/min; weight of catalyst: 0.1 g.
catalyst to get the right evaluation of the effect of the chemical composition to the reaction rate (Table 2). It was found that these reaction rate constants decreased in order Co4MnAl > Co4Mn2 > Co2Mg2Mn2 > Co2Mg2Al2 Co4Al2 > Co2Mg2MnAl. Although surface area is usually critical in determining catalytic activity, a variation in specific surface area has been shown to have a little effect. It was reported in literature that transition metals, Co and Mn in our case, are very active in decomposition of nitrous oxide, while Mg and Al are much less active as determined by the catalytic measurements over MgO and Al2O3 or calcined Mg-Al hydrotalcite-like compound [72]. Another parameter influencing catalytic activity is oxidation state of transition metal ion. It was established that the most active state of manganese is Mn3+. From XPS data, we determined Mn3+ on the surface of Co4Mn2 catalyst [73]. An increase in conversion of N2O over Co4Mn2 against Co4Al2 could be explained by substitution of Al3+ for active Mn3+. It is interesting that partial substitution of Mn for Al led to an increase in conversion of N2O despite the fact that the amount of Mn in the sample was decreased. The phenomenon could be connected with the changes of degree of substitution and the degree of spinel inversion. These two factors were related to affect essentially catalytic activity of the spinels [39]. The spinel matrix has the peculiarity of containing cation sites of both octahedral and tetrahedral symmetry. Octahedral coordinated cobalt is more active than the tetrahedral cobalt in the spinel matrix because of difference in the cobalt-oxygen bond strength, which is larger for tetrahedral cobalt [37]. The decisive parameter for decomposition rate of N2O resulting from the published mechanism of nitrous oxide decomposition [1] is the bond strength between oxygen and metal on the catalyst surface. The oxygen release is easier when octahedral cobalt is involved. Therefore, one can suppose that the presence of Al in the spinel structure caused a decrease in bond energy of surface oxygen.
Catalytic N2O decomposition over the calcined Co and Mn hydrotalcites has not been published in literature yet, but we can compare catalytic activities with those of other hydrotalcite-based catalysts. Kannan et al. tested calcined Co-Al hydrotalcite with various Co/Al molar ratios in N2O decomposition under static recirculation conditions [44,48], but also under identical flow conditions as those in our experiments [46,47]. The calcined Co-Al hydrotalcite tested by Kannan under flow conditions showed about two time’s higher N2O conversion in comparison with our Co-Al catalyst. It is not surprising when we take into account lower Co/Al molar ratio in our catalysts and findings of Kannan [44] confirming increasing activity with an increase in cobalt concentration in the sample. It is interesting that Chang et al. [49] published quite contrary conclusion regarding the influence of Co/Al ratio on catalytic activity in N2O decomposition: From their results implies that catalytic activity decreases with an increase in cobalt concentration. Co-Al hydrotalcite based catalyst (Co/Al = 3) prepared by Pe´rez-Ramı´rez et al. [72] showed better catalytic performance than both our catalyst with Co/Al = 2 and Kannan’s one with Co/Al = 3, despite Pe´rez-Ramı´rez et al. carried out their catalytic experiments with higher space velocity. Such differences can be caused by different conditions in the catalyst preparations (e.g. resulting in higher specific surface area). Since the Co-Al hydrotalcite precursor decomposes at much lower temperature (200–250 8C) than is the temperature used in catalyst calcination, the difference between our used calcination temperature (500 8C) and that of Pe´rezRamı´rez (450 8C) could be important because in this temperature region a sharp decrease in surface area occurs with increasing calcination temperature. The comparison of catalytic performances over various Co-Mg-Al calcined hydrotalcite is interesting because quite different results were obtained. Pe´rez-Ramı´rez [72] published 14% and 4% N2O conversion over the Co-Mg-Al calcined hydrotalcite with molar ratio Co/Mg/Al = 2/1/1 and 1/2/1, respectively (temperature 650 K, 0.05 g of catalyst and total flow rate 100 ml/min). Contrary to this, 83% N2O conversion over Co-Mg-Al calcined hydrotalcite with molar ratio Co/Mg/Al = 2/0.94/1 (650 K, 0.1 g of catalyst and 100 ml/min total flow rate) was estimated from the data of Kannan et al. [46]. Incorporation of Mg in hydrotalcite precursor caused significant increase in N2O conversion compared to Co-Al system in the Kannan experiments and the decrease in Pe´rez-Ramı´rez experiments. In our case, moderately higher N2O conversion was reached over Co2Mg2Al2 catalyst than over Co4Al2. Comparing N2O conversions over the hydrotalcite-based catalysts containing Co and Mn with those reported in literature for other catalysts, the catalysts examined in the paper is possible to consider as highly active when they are tested in an inert gas. For example, the N2O conversion over Co4MnAl at 450 8C is 97% (0.1 M% N2O balanced by He, F = 100 cm3/min, weight 0.1 g) while that for the state-of-
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the-art Fe-zeolite catalysts [29] is 75–88% (0.15 M% N2O balanced by He, F = 100 cm3/min, weight 0.1 g). The catalytic activity under conditions simulating real off-gases is, however, predominant for the practical use. For that reason, effect of O2 and H2O addition on N2O decomposition was also studied. The results obtained for N2O decomposition at 450 8C using 1000 ppm N2O and 2.5 M% H2O, 1000 ppm N2O and 5 M% O2 and also 1000 ppm N2O with both 2.5 M% H2O and 5 M% O2 are given in Fig. 6. The activities of all CoMn-containing catalysts, except the most active Co4MnAl catalyst, decreased with addition of O2; similarly, addition of water vapor caused a decrease in N2O conversion over all catalysts. After removal of H2O from the feed gas, the activity returned in all cases to the initial value indicating that water vapor has a reversible effect resulting from the competitive chemisorption with N2O. Similar behavior showed other oxide catalysts, like Cu/Al2O3 [23], Rh, Ag/ Al2O3 [20], Ru/Al2O3 [74], the oxides derived from hydrotalcite precursors containing Cu, Co, Ni, or Fe [54], Co-La-Al oxides [49] or Ni, Mg, Co spinels [38]. Inhibition by oxygen and water vapor is a weak point of our catalysts when compared them with the results obtained for the zeolite catalysts CuAPSO-34 [27] or ex-FeZSM-5 [75]. Some trends can be observed in the found inhibition of N2O conversion by water vapor and oxygen. The catalyst containing only transition metals (Co4Mn2) shows smaller decrease in N2O conversion in the presence of H2O, but higher decrease in activity in the presence of O2. Contrary to this fact, the remaining catalysts containing also other components, Mg and/or Al, show smaller decrease in N2O conversion in the presence of oxygen, but higher decrease in the presence of water. Differences in the sensitivity of catalysts to water vapor and oxygen could be explained by the difference in the affinity of the catalyst material to gaseous components O2 and H2O. The activity of the most active Co4MnAl catalyst was followed in the long-lasting experiment (120 h) with the presence of water and oxygen and no decrease in N2O conversion was detected.
Physical–chemical properties of the prepared catalysts were studied by TPR and TPD methods to find a relation between the amounts of reducible components, basicity of the catalyst surface and catalytic activity in N2O decomposition. Alini et al. [76] found that changes in the Mg/Al ratio (i.e. basicity of the oxide matrix) did not cause significant differences in the activity for N2O decomposition of the calcined hydrotalcite-based catalysts. On the contrary, some alkaline metals (Na, Li, K) can act as basic centers and influence catalytic activity [77]. Better catalytic performance in the presence of residual Na remaining in the trace amounts after washing step was already published. A small but a critical amount of activator metal is supposed to promote the decomposition of N2O [78]. Tabata et al. [79] found out that decomposition of N2O over the murdochite type catalysts (Mg6xLix)MnO8 increases with increasing amount of Li. Our observation is in agreement with the Alini’s findings on correlation of basicity and catalytic activity: Activity of the catalysts (determined in helium) is not in any correlation with the amount of CO2 desorbed in the temperature interval in which decomposition reaction proceeds (Table 2). From the dependence of N2O conversion on the amount of reducible components (Fig. 7) follows that the highest activity of the catalyst can be achieved when optimum amount of reducible components is present in the catalyst. It is not surprising as decomposition of N2O proceeds by oxidation–reduction mechanism. Based on the solid-state physics, the existence of the optimum amount of reducible components can be explained using the concept of polaron formation around the transition metal (polaron consists of delectron and its polarized environment) in an inert matrix [80]. The N2O chemisorption is supposed to be the first step in N2O decomposition, accompanied by transfer of an electron. The N2 molecule is released to the gas phase and oxygen remains adsorbed on the catalyst surface. The next step – oxygen desorption – is connected with an electron
Fig. 6. The N2O conversion dependence on the composition of input mixture. Conditions: 1000 ppm N2O; total flow: 100 cm3/min; weight of catalyst: 0.3 g, temperature: 450 8C.
Fig. 7. Dependence of the N2O conversion carried out in helium on the amount of reducible components in the interval 350–450 8C. Conditions: 1000 ppm N2O; total flow: 100 cm3/min; 450 8C.
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transfer from the adsorbed oxygen back to the solid and reoccupation of vacant site occurs. It was found that on doping with transition metal cations from zero to the certain concentration (dependent on the matrix), the catalytic activity for N2O decomposition, and some other simple reactions like oxidation of CO, isopropanol decomposition or oxidation of H2 increases until each polaron has at least one polaron nearest neighbor. The critical concentration at which the phenomenon proceeds is called as the localized– nonlocalized point (LNL point). After LNL point the movement of polarons is relatively easy, the bulk electron will tend to fill in the empty orbital, this restricts the desorption step and causes the decrease in catalytic activity.
4. Conclusions The series of Co/Mg-Mn/Al mixed oxide based catalysts prepared by calcination of layered double hydroxides with MII/MIII molar ratio of two were characterized by various methods and tested for their activity in the N2O decomposition. The spinel structures were identified in all samples by X-ray diffraction and infrared spectroscopy. Surface area of the studied catalysts varied between 44 and 106 m2g1. The catalytic activity of the prepared catalysts expressed as the rate constants (molN2 O s1 m2 Pa1) decreased in order Co4MnAl > Co4Mn2 > Co2Mg2Mn2 > Co2Mg2Al2 Co4Al2 > Co2Mg2MnAl. Combination of cobalt, manganese and aluminium in the catalyst structure increased considerably catalytic activity in N2O decomposition compared to the other tested catalysts. The activities of all samples decreased with addition of O2 with exception of the most active Co4MnAl catalyst; addition of water vapor caused decrease in N2O conversion over all catalysts. TPR measurements indicated that an optimum amount of components reducible in the temperature region 350–450 8C is necessary for achieving high catalytic activity in N2O decomposition. The existence of such optimum can be explained by using the concept of polaron hopping mechanism.
Acknowledgements This work was supported by the Grant Agency of Czech Republic (grants nos. 106/02/0523, 104/04/2116 and 106/ 05/0366) and by the Czech Ministry of Education, Youth and Sports (research project nos. MSM 6046137302 and 6198910016).
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Catalytic decomposition of nitrous oxide over catalysts prepared from Co/Mg-Mn/Al hydrotalcite-like compounds L. Obalova´ a,*, K. Jira´tova´ b, F. Kovanda c, K. Pacultova´ a, Z. Lacny´ a, Z. Mikulova´ b b
a Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Rozvojova´ 135, 165 02 Prague 6, Czech Republic c Department of Solid State Chemistry, Institute of Chemical Technology, Technicka´ 5, 160 00 Prague, Czech Republic
Received 2 December 2004; received in revised form 7 April 2005; accepted 9 April 2005 Available online 13 May 2005
Abstract Co/Mg-Mn/Al hydrotalcite-like compounds with Co:Mg:Mn:Al molar ratios of 4:0:2:0, 2:2:2:0, 2:2:1:1, 4:0:1:1, 4:0:0:2 and 2:2:0:2 were prepared by coprecipitation method. The mixed oxides obtained by calcination at 500 8C were characterized using various techniques (XRD, IR, BET surface area measurements, TPD and TPR). The activities of calcined hydrotalcites were tested for N2O decomposition in an inert gas, and in the presence of oxygen and water vapor as well. Among the catalysts screened, the catalyst prepared from Co hydrotalcite containing Al and Mn as the trivalent cation showed the highest activity, which was retained in the presence of oxygen but was inhibited by water vapor. Redox properties of the catalysts play an important role in the reaction and optimum extent of reduction is necessary for achievement of the highest catalytic activity. # 2005 Elsevier B.V. All rights reserved. Keywords: Nitrous oxide; Catalytic decomposition; Layered double hydroxides; Mixed oxide catalysts
1. Introduction Nitrous oxide is known as a strong greenhouse gas that contributes to catalytic destruction of stratospheric ozone. Therefore, it is of interest to reduce effectively the N2O emissions. Catalytic decomposition offers a simple solution of the N2O emission control in waste gases arising from combustion processes (fluidized bed combustion) and chemical industry (e.g. production of nitric or adipic acids). The N2O decomposition was studied over many different catalysts [1,2] in the last four decades. N2O decomposition is most efficiently catalyzed by transition metals of the Group VIII (Co, Ni, and Fe) and by Rh, Ru, Pd, Ir, Cu, La and Mn as implies from the published results. The studies from recent years deal especially with Fe-zeolites [3–13], precious metals Rh, Ru on various supports [14–21] and supported Cu [22–27]. The key parameters determining the rate of N2O decomposition are the type of support or matrix and the * Corresponding author. Tel.: +420 59 6991532; fax: +420 59 7323396. E-mail address: [email protected] (L. Obalova´). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.04.002
oxidation state and coordination of the transition metal cations. In this contribution, we focused on catalytic properties of cobalt and manganese cations in the mixed oxide based catalysts, with the aim to study the synergic effect of these two transition metals in N2O decomposition. In the 1970s, Cimino [28] reported the N2O catalytic decomposition over Mn containing catalysts. The studies of Mn ions in very dilute concentration dispersed in MgO matrix have revealed that the Mn3+ ions are more active than Mn4+ which is in good agreement with observation of Yamashita and Vannice [29]. Raj et al. [30] concluded that the Mn3+/Mn4+ couple could be the active cluster site for the N2O decomposition at high concentrations of Mn. Manganese-based mixed oxide systems serve as efficient catalysts in many oxidation processes [31–34] and its reduction–oxidation chemistry is especially important in electron transfer reactions. Decomposition of N2O belongs to such type of reactions [35]. Cimino and Pepe [36] reported the kinetic studies of N2O decomposition over CoO-MgO solid solution (cobalt content from 0.05 to 50 atoms/100 Mg atoms). Reaction was studied
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under conditions giving 1% N2O conversion and the activity per cobalt was reported to decrease with increasing cobalt concentration. Distribution of cobalt ions between octahedral and tetrahedral position in CoxMg1xAl2O4 spinel changes with temperature and catalytic activity of spinel increases with increasing portion of Co ions in octahedral positions [37]. Recently, different Co-containing spinels prepared by co-precipitation method were tested in N2O decomposition [38–42]. Drago et al. [43] studied catalytic decomposition of N2O over various supported oxides. They concluded that CoO is much more active than CuO and Fe2O3 and when supporting on MgO, very active catalyst was attained (95% N2O conversion was achieved at 40,000 h1 and 500 8C). Mixed oxide based catalysts, especially the multicomponent ones, can be obtained by a controlled thermal decomposition of hydrotalcite-like precursors. Hydrotalcitelike compounds, a class of layered double hydroxides, consist of positively charged metal hydroxide layers separated each other by anions and water molecules. Their chemical composition can be represented by the general formula [MII1xMIIIx(OH)2]x+[Anx/nyH2O]x, where MII and MIII are divalent and trivalent metal cations, An is an n-valent anion (often carbonate), and x has usually values approximately between 0.25 and 0.33. By thermal treatment of hydrotalcitelike precursors, the MII and MIII mixed oxides with a high surface area and good thermal stability can be prepared. Both cobalt and manganese cations may be incorporated into hydroxide layers of hydrotalcite-like compounds. Co-containing mixed oxides prepared from hydrotalcite-like precursors were tested in the catalytic decomposition of nitrous oxide [44–51] or in the selective catalytic reduction of NO with ammonia [52]. The substitution of cobalt by magnesium in CoAl catalyst considerably influenced its catalytic behavior [53]. Kustrowski et al. [54] found the descending order of the transition metal oxides dispersed in the MgO-Al2O3 matrix (Cu > Co > Fe > Ni) in N2O decomposition. Only a few papers have reported the hydrotalcite-like compounds containing manganese. Preparation and thermal decomposition of Mg-Mn and Mg-Mn-Al layered double hydroxides are described in [55] and [56]. Synthesis and thermal behaviour of Ni-Mn hydrotalcite-like compounds are reported by Barriga et al. [57] and Kovanda et al. [58]. Formation of mixed oxides during heating of layered double hydroxides containing simultaneously copper and manganese cations in hydroxide layers is described in recent papers [59,60]. However, until now, no report is available on the synthesis, physicochemical and catalytic properties in N2O decomposition of catalysts prepared by thermal treatment of hydrotalcite-like compounds containing cobalt and manganese simultaneously. In the present study, we have prepared the series of catalysts from Co-(Mg)-Mn-(Al) hydrotalcite-like precursors. The catalysts were characterized using various techniques (AAS, XRD, IR, BET surface area measurement, TPR, TPD) and tested in N2O decomposition under conditions approaching those in industrial units. For that
reason, the catalytic performance in the presence of oxygen and water vapor was also studied in order to find out the extent of inhibition of reaction rate by the gases.
2. Experimental methods 2.1. Preparation and characterization of catalysts Samples of Co/Mg-Mn/Al layered double hydroxides with Co:Mg:Mn:Al molar ratios of 4:0:2:0, 2:2:2:0, 2:2:1:1, 4:0:1:1, 4:0:0:2 and 2:2:0:2 were prepared by coprecipitation of nitrate solutions and Na2CO3/NaOH solution at 25 8C and pH 10. The washed and dried products were formed into extrudates and calcined at 500 8C for 4 h in air. The prepared catalysts were crushed and sieved to obtain the fraction of particle size 0.160– 0.315 mm, which was used in catalytic measurements. The samples were denoted as Co4Mn2, Co2Mg2Mn2, Co2Mg2MnAl, Co4MnAl, Co4Al2 and Co2Mg2Al2. The powder XRD patterns of the prepared precursors and calcined samples were recorded using a Seifert XRD 3000P instrument with Co Ka radiation (l = 0.179 nm, graphite monochromator, goniometer with Bragg-Brentano geometry) in 2u range 128–758, step size 0.058 2u. Fourier-transform infrared absorption spectra were recorded using the KBr pellet technique on the spectrometer FTIR 2000 Perkin-Elmer in the range 4000–400 cm1 and the resolution of 4 cm1. The surface area measurements were carried out by nitrogen adsorption at 77 K and evaluated by one point BET method. Prior to each experiment, the samples were activated by heating (0.5 h) at 350 8C. The detailed analysis of the porous structure was obtained from the measurement of adsorption–desorption isotherms of nitrogen at 77 K on an ASAP 2010 instrument (Micromeritics, USA). Prior to the measurement, the samples were activated at 130 8C. The TPR measurements were carried out in an apparatus described earlier [20]. Hydrogen/nitrogen mixture (10 M% H2) was used to reduce the catalysts at the flow rate of 15 cm3 min1. The temperature was linearly raised at the rate of 20 8C min1 up to 850 8C. The TPD measurements were carried out using 0.025 g of the catalyst and helium as carrier gas (flow rate 15 ml/h). Prior to the CO2 desorption all catalysts were calcined to 500 8C at a linear temperature rate increase 10 8C min1. Then, after cooling the sample to ambient temperature, ten doses of CO2 (840 ml) were injected into the helium stream at 30 8C. The excess of CO2 was removed by helium flowing through the catalyst for one hour. Then temperatureprogrammed desorption of CO2 was started and finished when temperature 450 8C was reached. 2.2. Catalytic measurements The N2O decomposition reaction was performed in fixedbed stainless steel reactor of 5 mm internal diameter in
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temperature range 330–450 8C. It was verified that stainless steel did not contribute to the catalyst’s performance at used conditions. Total flow rate was kept at 100 ml min1 NTP (273 K, 101,325 Pa). The catalyst bed contained 0.1–0.3 g of sample with particle size 0.160–0.315 mm. The space velocity (GHSV) of 20,000–60,000 cm3 g1 h1 was applied. The inlet concentration of N2O was 1000 ppm balanced by helium; oxygen (5 M%) and water vapor (2.5 M%) were added to some experimental run. The Gas Humidifier with Nafion1 membrane (Perma Pure Inc.) was used for saturation of reaction gas by water vapor. Before chromatographic analysis, the humidity was removed by Peltier cell and then the gas was final dried in a packed column with CaCl2 granule. A temperature-controlled furnace was used to heat the reactor. Before each run, the catalyst was pre-treated by heating in a He flow (50 cm3 min1) at 10 8C min1 up to 450 8C and maintaining the temperature for 1 h. Then the catalyst was cooled to the reaction temperature and the steady state of the N2O concentration level at the reactor output was measured. The gas chromatograph Hewlett Packard (model 5890, series II GC) equipped with a gas sample loop preceding the split/split less injector system and electron capture detector was used to analyze the nitrous oxide concentration in the reactor inlet and outlet. Data were acquired with HP Chemstation. The chromatographic column used was PoraPlot Q (30 m 0.53 mm 40 mm). An electron capture detector in overheating mode (330 8C) was applied for detection. The two-points calibration with calibration gases (70 ppm, 1000 ppm) was performed before each experimental run. The sample loop was filled with constant gas flow controlled by rotameter. The rate constants were determined according to the 1st order kinetic expression d pN2 O =dt ¼ k pN2 O by an integral method; for the evaluation of the kinetic data the conversions were restricted to less than 30%. The activation energies and preexponential factors were evaluated according to Arrhenius plot. Rate constants k at temperature of 420 8C were extrapolated using Arrhenius equation.
291
Table 1 Molar ratios of metal cations determined by chemical analysis and lattice parameters of formed spinels evaluated from XRD patterns of catalysts Catalyst
Molar ratio Co:Mg:Mn:Al
Spinel lattice ˚) parameter a (A
Co4Mn2 Co4MnAl Co4Al2 Co2Mg2Mn2 Co2Mg2MnAl Co2Mg2Al2
2.16:0:1.00:0 4.12:0:1.00:1.07 1.92:0:0:1.00 1.03:0.77:1.00:0 2.07:1.96:1.00:0.97 1.00:1.00:0:1.00
8.184 8.141 8.083 8.248 8.160 Not determined
were identified as predominant phases in the Co4Mn2/nc precursor. Thermal decomposition of Co-based LDHs results in the formation of spinel-type oxides. The study of the thermal decomposition of Co-Al hydrotalcite published by Pe´rezRamı´rez et al. [61] documented that dehydroxylation accompanied by a collapse of hydrotalcite structure was complete at 250–300 8C in air and at 350–400 8C in the inert gas. Thermal treatment in air led to a solid solution of cobalt spinels [Co(Co, Al)2O4] whereas mixtures of CoO and CoAl2O4 were formed upon treatment in the inert gas. The lower thermal stability of the Co-Al hydrotalcite in air was explained by the presence of oxidizable Co2+ cations in the octahedral sheets and the diffusion of Co3+ and Al3+ to the interlayer space in the dehydrated layered structure. A highly stable and homogeneous solid solution of cobalt spinels started to form already at relatively low temperature of 150 8C. A formation of the spinel-like phase during
3. Results and discussion 3.1. Catalysts characterization The molar ratios of metal cations in prepared catalysts presented in Table 1 were calculated from results of chemical analysis and were close to those in the nitrate solutions used for coprecipitation of precursors. A trace amount of Na was also detected in the analyzed samples. Only a well-crystallized hydrotalcite-like phase was found in the powder XRD patterns of the dried precursors except Co4Mn2/nc and Co2Mg2Mn2/nc samples, nc denotes noncalcined sample (Fig. 1). In the Co2Mg2Mn2/nc precursor, MnOOH (feitknechtite) was detected together with hydrotalcite-like phase. Cobalt and manganese (oxo)hydroxides
Fig. 1. X-ray powder diffraction patterns of the non-calcined precursors: H, hydrotalcite-like phase; F, MnOOH (feitknechtite); C, CoOOH; M, (Co, Mn)(OH)2.
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Fig. 3. Infrared spectra of prepared catalysts.
Fig. 2. X-ray powder diffraction patterns of the catalysts prepared by calcination of precursors at 500 8C: S, spinel.
heating of cobalt-rich Co-Mg-Al layered double hydroxides was reported by Ribet et al. [62]. Calcination of Co-Fe and Co-Cr LDHs yielded also non-stoichiometric spinel-type phases [63,64]. The powder XRD patterns of the catalysts prepared by calcination of precursors at 500 8C are shown in Fig. 2. All calcined samples exhibited a relatively low crystallinity. The observed diffraction lines were ascribed to the spinel-like oxides. The lattice parameters of formed spinels presented in Table 1 were evaluated using DiffracPlus Topas 2000 software. The FTIR spectra of the prepared catalysts are shown in Fig. 3. The mixed oxides are formed during thermal decomposition of layered double hydroxides but a trace amount of remaining carbonate was found in the spectra of calcined samples (the bands at about 1500 and 1320 cm1). The absorption bands at 3420 and 1620–1640 cm1 indicated the presence of OH and/or water but the dehydroxylation of samples should be complete at applied calcination temperature 500 8C. An adsorption of air humidity during preparation and handling of KBr pellet can be expected. The two absorption bands in the area of wavenumbers below 900 cm1 are characteristic for n1, n2 vibration of lattice octahedra of spinel. Their values for Co4Al2 sample (n1 = 671 cm1, n2 = 570 cm1) resemble those of Co3O4 (the values from literature [65]: n1 = 672 cm1, n2 = 590 cm1). Dissolving of Al3+ and/or non-stoichiometry of spinel phase can cause the observed shift in n2 compared to the value given in literature (the band is ascribed to Co3+ in octahedral coordination) [47]. The values of absorption bands n1 = 649 cm1 and n2 = 557 cm1 were published [66,67] for spinel-type oxide
MnCo2O4, and these values are very close to n1 = 647 cm1, n2 = 555 cm1 found in the prepared Co4Mn2 catalyst. Substitution of manganese by aluminium led to a shift in the n1 band only, the n2 value being the same for Co4Mn2 and Co4MnAl. The substitution of cobalt by magnesium caused broadening of both, n1 and n2 in the Co2Mg2Al2, Co2Mg2Mn2 and Co2Mg2MnAl catalysts, which corresponds to their less ordered structure. Surface areas determined by means of physical sorption and desorption of nitrogen moved from the highest value (106.3 m2 g1) for Co2Mg2Al2 catalyst to the lowest value 43.6 m2 g1 for Co4Mn2 catalyst (Table 2). Introduction of both magnesium and aluminium into the hydroxide layers of hydrotalcite-like compounds leads to an increase of the catalyst surface area. Temperature-programmed reduction curves of all catalysts are shown in Fig. 4. The Co4Mn2 catalyst is reduced in two main temperature regions, the first one (200–400 8C) belonging to reduction of Co compounds, while the second one (400–700 8C) mainly representing reduction of Mn compounds. The low-temperature peak consists of two peaks representing the reduction of CoIII to CoII and the second part of this peak belongs to reduction of CoII–Co0. The findings are in accord with the data in the chemical literature. The reduction of Co3O4 normally occurs in two steps [68], but Arnoldy et al. [69] observed the reduction of Co3O4 around 330 8C and both corresponding reduction peaks were almost indistinguishable. Reduction of Mn oxides is complicated and can proceed via different reduction stages. In principle, the reduction of b-MnO2 manifests itself in two peaks; the first peak at lower temperature around 250 8C corresponds to reduction of MnO2 or Mn2O3–Mn3O4 and reduction of Mn3O4–MnO occurs at 360–440 8C [70]. Substitution of Co for Mg and/or Mn for Al in our Co4Mn2 catalyst did not change the position of the first reduction peak too much but the second peak was shifted to higher temperatures indicating worse reducibility of the
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Table 2 Rate constant k (420 8C) determined from the 1st order kinetic expression and the consumption of H2 and the amount of desorbed CO2 in the temperature range in which N2O decomposition reaction proceeds Catalyst
SBET (m2 g1)
k 109 ðmolN2 O s1 g1 Pa1 Þ
k 1011 ðmolN2 o s1 m2 Pa1 Þ
TPRa (mmol H2.g1)
TPRb (mmol H2.g1)
Co + Mn (mmol g1)
TPDa (mmol CO2.g1)
Co4Mn2 Co4MnAl Co4Al2 Co2Mg2Mn2 Co2Mg2MnAl Co2Mg2Al2
43.6 92.7 84.0 57.3 91.7 106.3
1.78 12.95 1.76 1.51 1.73 2.32
4.08 13.97 2.10 2.64 1.89 2.18
1.81 0.96 0.66 1.57 1.77 0.80
11.58 11.26 9.16 9.75 7.47 6.69
9.9153 7.5167 6.2108 7.2588 3.4821 3.4617
0.02 0.03 0.025 0.03 0.05 0.03
a b
In the region 350–450 8C. In the temperature region 20–800 8C.
arising components of the catalysts (CoMn; CoAl spinel). Temperature-programmed reduction of the calcined Co-Al and Co-Mg-Al hydrotalcite was previously published by Ribet et al. [62] and occurrence of two distinct reducible cobalt species in both samples was shown. The species below 350 8C were assigned to Co3O4, those reducible around 730 8C were ascribed to a spinel-like phase. The addition of magnesium to the Co3O4 did not change the course of reduction of CoIII to CoII (>300 8C), but it influenced the reduction of CoII to Co0. The reason is the occurrence of reaction between CoO and MgO resulting in new phases CoO–MgO solid solution. Cobalt in CoO–MgO solid solution is reduced completely at much higher temperatures (500–700 8C) in comparison with CoO (400 8C) [68]. Al3+ ions influenced the reducibility of Co ions strongly and caused an increase in reduction temperature. This is explained by polarization of Co–O bonds by Al3+ ions [69]. In Table 2 there are summarized the data on TPR of all catalysts in temperature range 20–800 8C. The total amounts of hydrogen consumed during reduction in 20–800 8C were correlated with the amounts of Co a Mn found in the catalysts and linear dependence was proved. Correlation equation was y = 0.8437x + 3.7607, R2 = 0.892. However, catalytic components reducible at the temperatures higher than reaction temperatures cannot likely contribute to the
Fig. 4. Temperature-programmed reduction of the prepared catalysts.
catalytic reaction and for that reason we determined the amounts of components reducible in the reaction temperature region as that of hydrogen consumed in the range 350– 450 8C. The data are shown in Table 2. Temperature-programmed desorption of the preadsorbed CO2 obtained in the experiments with linearly increased temperature (TPD) was used to characterize basicity of the catalysts. The Co4Mn2 catalyst exhibited basic sites of low strength (Tmax 140 8C) and introduction of Mg made basic sites of a catalyst stronger (Tmax is shifted to higher temperatures of about 150–350 8C). The lowest amount of desorbed CO2 was determined with the Co4Al2 catalyst. CO2 can be bonded to a catalyst surface as monodentate species, chelating bidentate, and bridging bidentate structures [71]. Monodentate species are formed on the strongest basic oxygen ions and chelating and bridging bidentate carbonates on the sites of decreasing basic strength with the participation of an adjacent cationic sites. Similarly as in the case of previously calculated number of reduction sites we determined number of basic sites (possibly active in the N2O conversion) as the amount of CO2 desorbed in the temperature range 350–450 8C. The data are shown in Table 2 and will be discussed together with catalytic activity. 3.2. Catalytic activity The temperature dependences of N2O conversion using 1000 ppm N2O are given for each catalyst in Fig. 5. The activity of Co4MnAl catalyst was much higher than that of the other catalysts. It began to show appreciable conversion at temperatures as low as 330 8C and reached 100% at 450 8C. Other samples showed lower and almost the same activity. Different kinetic expressions describing the kinetics of N2O decomposition can be found in the literature, depending on the fact whether the inhibition of reaction rate by oxygen occurred or not [1]. The kinetic data of the hydrotalcite based catalysts can be fitted with 1st rate law corresponding to no inhibition by oxygen [44]; therefore, the rate constants k (molN2 O s1 g1 Pa1) were determined according to the first order kinetic expression. Since the catalytic reaction proceeds on the catalyst surface area that differs for the samples, the rate constants were also expressed per m2 of
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Fig. 5. Temperature dependence of N2O conversion. Conditions: 1000 ppm N2O balanced He; total flow: 100 cm3/min; weight of catalyst: 0.1 g.
catalyst to get the right evaluation of the effect of the chemical composition to the reaction rate (Table 2). It was found that these reaction rate constants decreased in order Co4MnAl > Co4Mn2 > Co2Mg2Mn2 > Co2Mg2Al2 Co4Al2 > Co2Mg2MnAl. Although surface area is usually critical in determining catalytic activity, a variation in specific surface area has been shown to have a little effect. It was reported in literature that transition metals, Co and Mn in our case, are very active in decomposition of nitrous oxide, while Mg and Al are much less active as determined by the catalytic measurements over MgO and Al2O3 or calcined Mg-Al hydrotalcite-like compound [72]. Another parameter influencing catalytic activity is oxidation state of transition metal ion. It was established that the most active state of manganese is Mn3+. From XPS data, we determined Mn3+ on the surface of Co4Mn2 catalyst [73]. An increase in conversion of N2O over Co4Mn2 against Co4Al2 could be explained by substitution of Al3+ for active Mn3+. It is interesting that partial substitution of Mn for Al led to an increase in conversion of N2O despite the fact that the amount of Mn in the sample was decreased. The phenomenon could be connected with the changes of degree of substitution and the degree of spinel inversion. These two factors were related to affect essentially catalytic activity of the spinels [39]. The spinel matrix has the peculiarity of containing cation sites of both octahedral and tetrahedral symmetry. Octahedral coordinated cobalt is more active than the tetrahedral cobalt in the spinel matrix because of difference in the cobalt-oxygen bond strength, which is larger for tetrahedral cobalt [37]. The decisive parameter for decomposition rate of N2O resulting from the published mechanism of nitrous oxide decomposition [1] is the bond strength between oxygen and metal on the catalyst surface. The oxygen release is easier when octahedral cobalt is involved. Therefore, one can suppose that the presence of Al in the spinel structure caused a decrease in bond energy of surface oxygen.
Catalytic N2O decomposition over the calcined Co and Mn hydrotalcites has not been published in literature yet, but we can compare catalytic activities with those of other hydrotalcite-based catalysts. Kannan et al. tested calcined Co-Al hydrotalcite with various Co/Al molar ratios in N2O decomposition under static recirculation conditions [44,48], but also under identical flow conditions as those in our experiments [46,47]. The calcined Co-Al hydrotalcite tested by Kannan under flow conditions showed about two time’s higher N2O conversion in comparison with our Co-Al catalyst. It is not surprising when we take into account lower Co/Al molar ratio in our catalysts and findings of Kannan [44] confirming increasing activity with an increase in cobalt concentration in the sample. It is interesting that Chang et al. [49] published quite contrary conclusion regarding the influence of Co/Al ratio on catalytic activity in N2O decomposition: From their results implies that catalytic activity decreases with an increase in cobalt concentration. Co-Al hydrotalcite based catalyst (Co/Al = 3) prepared by Pe´rez-Ramı´rez et al. [72] showed better catalytic performance than both our catalyst with Co/Al = 2 and Kannan’s one with Co/Al = 3, despite Pe´rez-Ramı´rez et al. carried out their catalytic experiments with higher space velocity. Such differences can be caused by different conditions in the catalyst preparations (e.g. resulting in higher specific surface area). Since the Co-Al hydrotalcite precursor decomposes at much lower temperature (200–250 8C) than is the temperature used in catalyst calcination, the difference between our used calcination temperature (500 8C) and that of Pe´rezRamı´rez (450 8C) could be important because in this temperature region a sharp decrease in surface area occurs with increasing calcination temperature. The comparison of catalytic performances over various Co-Mg-Al calcined hydrotalcite is interesting because quite different results were obtained. Pe´rez-Ramı´rez [72] published 14% and 4% N2O conversion over the Co-Mg-Al calcined hydrotalcite with molar ratio Co/Mg/Al = 2/1/1 and 1/2/1, respectively (temperature 650 K, 0.05 g of catalyst and total flow rate 100 ml/min). Contrary to this, 83% N2O conversion over Co-Mg-Al calcined hydrotalcite with molar ratio Co/Mg/Al = 2/0.94/1 (650 K, 0.1 g of catalyst and 100 ml/min total flow rate) was estimated from the data of Kannan et al. [46]. Incorporation of Mg in hydrotalcite precursor caused significant increase in N2O conversion compared to Co-Al system in the Kannan experiments and the decrease in Pe´rez-Ramı´rez experiments. In our case, moderately higher N2O conversion was reached over Co2Mg2Al2 catalyst than over Co4Al2. Comparing N2O conversions over the hydrotalcite-based catalysts containing Co and Mn with those reported in literature for other catalysts, the catalysts examined in the paper is possible to consider as highly active when they are tested in an inert gas. For example, the N2O conversion over Co4MnAl at 450 8C is 97% (0.1 M% N2O balanced by He, F = 100 cm3/min, weight 0.1 g) while that for the state-of-
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the-art Fe-zeolite catalysts [29] is 75–88% (0.15 M% N2O balanced by He, F = 100 cm3/min, weight 0.1 g). The catalytic activity under conditions simulating real off-gases is, however, predominant for the practical use. For that reason, effect of O2 and H2O addition on N2O decomposition was also studied. The results obtained for N2O decomposition at 450 8C using 1000 ppm N2O and 2.5 M% H2O, 1000 ppm N2O and 5 M% O2 and also 1000 ppm N2O with both 2.5 M% H2O and 5 M% O2 are given in Fig. 6. The activities of all CoMn-containing catalysts, except the most active Co4MnAl catalyst, decreased with addition of O2; similarly, addition of water vapor caused a decrease in N2O conversion over all catalysts. After removal of H2O from the feed gas, the activity returned in all cases to the initial value indicating that water vapor has a reversible effect resulting from the competitive chemisorption with N2O. Similar behavior showed other oxide catalysts, like Cu/Al2O3 [23], Rh, Ag/ Al2O3 [20], Ru/Al2O3 [74], the oxides derived from hydrotalcite precursors containing Cu, Co, Ni, or Fe [54], Co-La-Al oxides [49] or Ni, Mg, Co spinels [38]. Inhibition by oxygen and water vapor is a weak point of our catalysts when compared them with the results obtained for the zeolite catalysts CuAPSO-34 [27] or ex-FeZSM-5 [75]. Some trends can be observed in the found inhibition of N2O conversion by water vapor and oxygen. The catalyst containing only transition metals (Co4Mn2) shows smaller decrease in N2O conversion in the presence of H2O, but higher decrease in activity in the presence of O2. Contrary to this fact, the remaining catalysts containing also other components, Mg and/or Al, show smaller decrease in N2O conversion in the presence of oxygen, but higher decrease in the presence of water. Differences in the sensitivity of catalysts to water vapor and oxygen could be explained by the difference in the affinity of the catalyst material to gaseous components O2 and H2O. The activity of the most active Co4MnAl catalyst was followed in the long-lasting experiment (120 h) with the presence of water and oxygen and no decrease in N2O conversion was detected.
Physical–chemical properties of the prepared catalysts were studied by TPR and TPD methods to find a relation between the amounts of reducible components, basicity of the catalyst surface and catalytic activity in N2O decomposition. Alini et al. [76] found that changes in the Mg/Al ratio (i.e. basicity of the oxide matrix) did not cause significant differences in the activity for N2O decomposition of the calcined hydrotalcite-based catalysts. On the contrary, some alkaline metals (Na, Li, K) can act as basic centers and influence catalytic activity [77]. Better catalytic performance in the presence of residual Na remaining in the trace amounts after washing step was already published. A small but a critical amount of activator metal is supposed to promote the decomposition of N2O [78]. Tabata et al. [79] found out that decomposition of N2O over the murdochite type catalysts (Mg6xLix)MnO8 increases with increasing amount of Li. Our observation is in agreement with the Alini’s findings on correlation of basicity and catalytic activity: Activity of the catalysts (determined in helium) is not in any correlation with the amount of CO2 desorbed in the temperature interval in which decomposition reaction proceeds (Table 2). From the dependence of N2O conversion on the amount of reducible components (Fig. 7) follows that the highest activity of the catalyst can be achieved when optimum amount of reducible components is present in the catalyst. It is not surprising as decomposition of N2O proceeds by oxidation–reduction mechanism. Based on the solid-state physics, the existence of the optimum amount of reducible components can be explained using the concept of polaron formation around the transition metal (polaron consists of delectron and its polarized environment) in an inert matrix [80]. The N2O chemisorption is supposed to be the first step in N2O decomposition, accompanied by transfer of an electron. The N2 molecule is released to the gas phase and oxygen remains adsorbed on the catalyst surface. The next step – oxygen desorption – is connected with an electron
Fig. 6. The N2O conversion dependence on the composition of input mixture. Conditions: 1000 ppm N2O; total flow: 100 cm3/min; weight of catalyst: 0.3 g, temperature: 450 8C.
Fig. 7. Dependence of the N2O conversion carried out in helium on the amount of reducible components in the interval 350–450 8C. Conditions: 1000 ppm N2O; total flow: 100 cm3/min; 450 8C.
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transfer from the adsorbed oxygen back to the solid and reoccupation of vacant site occurs. It was found that on doping with transition metal cations from zero to the certain concentration (dependent on the matrix), the catalytic activity for N2O decomposition, and some other simple reactions like oxidation of CO, isopropanol decomposition or oxidation of H2 increases until each polaron has at least one polaron nearest neighbor. The critical concentration at which the phenomenon proceeds is called as the localized– nonlocalized point (LNL point). After LNL point the movement of polarons is relatively easy, the bulk electron will tend to fill in the empty orbital, this restricts the desorption step and causes the decrease in catalytic activity.
4. Conclusions The series of Co/Mg-Mn/Al mixed oxide based catalysts prepared by calcination of layered double hydroxides with MII/MIII molar ratio of two were characterized by various methods and tested for their activity in the N2O decomposition. The spinel structures were identified in all samples by X-ray diffraction and infrared spectroscopy. Surface area of the studied catalysts varied between 44 and 106 m2g1. The catalytic activity of the prepared catalysts expressed as the rate constants (molN2 O s1 m2 Pa1) decreased in order Co4MnAl > Co4Mn2 > Co2Mg2Mn2 > Co2Mg2Al2 Co4Al2 > Co2Mg2MnAl. Combination of cobalt, manganese and aluminium in the catalyst structure increased considerably catalytic activity in N2O decomposition compared to the other tested catalysts. The activities of all samples decreased with addition of O2 with exception of the most active Co4MnAl catalyst; addition of water vapor caused decrease in N2O conversion over all catalysts. TPR measurements indicated that an optimum amount of components reducible in the temperature region 350–450 8C is necessary for achieving high catalytic activity in N2O decomposition. The existence of such optimum can be explained by using the concept of polaron hopping mechanism.
Acknowledgements This work was supported by the Grant Agency of Czech Republic (grants nos. 106/02/0523, 104/04/2116 and 106/ 05/0366) and by the Czech Ministry of Education, Youth and Sports (research project nos. MSM 6046137302 and 6198910016).
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