Study on the Mechanism and Kinetics of the Thermal Decomposition

Aug 30, 2008 - The dehytroxylation follows a two-dimensional diffusion-controlled mechanism with instantaneous nucleation represented by the first-ord...
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Ind. Eng. Chem. Res. 2008, 47, 7211–7218

7211

Study on the Mechanism and Kinetics of the Thermal Decomposition of Ni/Al Layered Double Hydroxide Nitrate Liren Wang, Zhi Lu¨,* Feng Li,* and Xue Duan State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China

In the paper, the mechanism and kinetics of thermal decomposition of Ni/Al layered double hydroxide nitrate with a Ni/Al molar ratio of 3.0 (NiAl-LDH) were studied by thermogravimetry/differential thermal analysis coupled with mass spectrometry (TG/DTA-MS). The results indicated that the thermal decomposition of NiAl-LDH proceeds in three individual processes, i.e., removal of the physisorbed and interlayer water, dehydroxylation of the layers, and decomposition of the interlayer nitrate ions (denitration). The mechanism and kinetics of dehydroxylation and denitration, which take place simultaneously almost in the same region of temperatures, were separately achieved by only using MS data sets recorded at different heating rates. The dehytroxylation follows a two-dimensional diffusion-controlled mechanism with instantaneous nucleation represented by the first-order Avrami-Erofe’ev equation with an average activation energy of ca. 129.0 kJ · mol-1, while the denitration obeys a three-dimensional diffusion-controlled mechanism represented by the Zhuralev-Lesokin-Tempelman equation with an average activation energy of ca. 137.2 kJ · mol-1. 1. Introduction The kinetics of thermal decomposition of materials always attracts the interest of both chemical engineers and materials scientists because of the fact that kinetic data are essential to chemical engineering design and fundamental to the design and optimization of materials. In addition to obtaining a model to predict the behavior of a solid material under certain conditions, kinetic analysis is also the starting point to hypothesize the decomposition mechanism.1 Layered double hydroxides (LDHs, [MII1-xMIIIx(OH)2]x+(An-)x/ n · mH2O), also known as anionic clays or hydrotalcite-like compounds, consist of a positively charged brucite-like layer ([MII1-xMIIIx(OH)2]x+) and a charge-balancing anion (An-) in the hydrated interlayer galleries.2 The identities of the divalent and trivalent cations (MII and MIII, respectively) and the interlayer anion (An-) together with the value of the stoichiometric coefficient (x) may be varied over a wide range, giving rise to a large class of isostructural materials with different physicochemical properties,3 which have attracted increasing interest in recent years for their potential applications in catalysis, adsorption, photochemistry, electrochemistry, and other areas.2,4,5 One of the most interesting features of LDHs is their role as precursors of mixed-oxide catalysts. Upon calcination at intermediate temperatures (450-600 °C), LDHs undergo decomposition, thus giving poorly crystallized mixedmetal oxides with large specific surface areas and homogeneous dispersion of the metal components.5 Recently, considerable attention has been paid to Ni-containing LDHs because of the high catalytic activity of the deriving homogeneous Ni-Al oxides, with high thermal and chemical stability,6 for a variety of reactions such as steam reforming of methanol,7–9 oxydehydrogenation of ethylbenzene,10 hydrogenation of acetonitrile,6,11 and aldol condensation of acetone.12,13 The thermal decomposition of LDH materials is an essential step for the preparation of these mixed oxides. Therefore, investigation into the mechanism and kinetics of the thermal decomposition of LDH * To whom correspondence should be addressed. Tel.: 861064451226. Fax: 8610-64425385. E-mail: [email protected] (Z.L.), [email protected] (F.L.).

materials can provide valuable information about the behavior of their thermal decomposition, which is also one of the important aspects that must be considered in the study of mixed oxide preparation and their catalytic performances. In recent years, besides common differential scanning calorimetry (DSC) and thermogravimetry/differential thermal analysis (TG/DTA), some in situ techniques have also been used to characterize the thermal evolution of the LDH structure, such as high-temperature X-ray diffraction (HT-XRD),14–17 infrared emission spectroscopy,18 in situ Fourier transform infrared (FT-IR),15–17 in situ Raman,15 diffuse-reflectance infrared Fourier transform spectroscopy,14 and thermogravimetry coupling with mass spectrometry (TG-MS).14,15,19 In general, the fundamental processes involved in the thermal decomposition of LDHs with volatile inorganic interlayer anions (carbonate, nitrate, sulfate, chloride, etc.) are20,21 (i) the removal of adsorbed water happening below 100 °C, (ii) the removal of interlayer water occurring until ca. 250 °C, (iii) the dehydroxylation of the layers, and (iv) the decomposition of the interlayer anions taking place up to around 500 °C. However, because the layer dehydroxylation and interlayer anion decomposition always happen simultaneously in the same region of temperatures, TG and DSC data, which are related to mass loss and reaction heat, respectively, are unable to be employed to distinguish the two processes from each other. In this paper, we established a facile method for studying the mechanism and kinetics of the thermal decomposition of Ni/Al LDH nitrate (NiAl-LDH) by only using MS data recorded at different heating rates in TG/DTA-MS. In the system, H2O molecules are released from OH- during the dehydroxylation and NO or NO2 from NO3- during the denitration. Therefore, MS peaks of different released species during the dehydroxylation and denitration are recorded separately, and correspondingly the mechanism and kinetics are separately determined. To the best of our knowledge, no report has been published on the individual mechanism and kinetics of the layer dehydroxylation and the interlayer anion decomposition of LDH materials.

10.1021/ie800609c CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

7212 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 1. Mathematical Expressions of the Commonly Used Kinetic Models mechanism

symbol

one-dimensional diffusion two-dimensional diffusion (Valensi equation) three-dimensional diffusion (Jander equation) three-dimensional diffusion (G-B equationa) three-dimensional diffusion (Z-L-T equationb) two-dimensional phase-boundary reaction three-dimensional phase-boundary reaction one and a half order phase-boundary reaction second-order phase-boundary reaction nucleation and nuclei growth (A-E equationc, n ) 1) nucleation and nuclei growth (A-E equation, n ) 1.5) nucleation and nuclei growth (A-E equation, n ) 2) nucleation and nuclei growth (A-E equation, n ) 3) exponential nucleation (Mample equation, n ) 1) exponential nucleation (Mample equation, n ) 2) exponential nucleation (Mample equation, n ) 3) exponential nucleation (Mample equation, n ) 4) a

D1 D2 D3 D4 D5 R2 R3 C1.5 C2 A1 A1.5 A2 A3 P1 P2 P3 P4

f(R)

g(R)

0.5/R [-ln(1 - R)]-1 1.5(1 - R)2/3[1 - (1 - R)1/3]-1 1.5[(1 - R)1/3 - 1]-1 1.5(1 - R)4/3[(1 - R)-1/3 - 1]-1 2(1 - R)1/2 3(1 - R)2/3 (1 - R)3/2 (1 - R)2 1-R 1.5(1 - R)[-ln(1 - R)]1/3 2(1 - R)[-ln(1 - R)]1/2 3(1-R)[-ln(1-R)]2/3 1 2R1/2 3R2/3 4R3/4

R R + (1 - R) ln(1 - R) [1 - (1 - R)1/3]2 (1 - 2R/3) - (1 - R)2/3 [(1 - R)-1/3 - 1]2 1 - (1 - R)1/2 1 - (1 - R)1/3 (1 - R)-1/2 (1 - R)-1 -ln(1 - R) [-ln(1 - R)]2/3 [-ln(1 - R)]1/2 [-ln(1 - R)]1/3 R R1/2 R1/3 R1/4 2

Ginstling-Brounstein equation. b Zhuralev-Lesokin-Tempelman equation. c Avrami-Erofe’ev equation.

2. Thermal Decomposition Kinetics The rate-determining step in any decomposition reaction of solid-state materials can be either diffusion, i.e., the transportation of matter to or from the reaction zone, or a chemical reaction, i.e., the breaking and forming of bonds, generally occurring at a reaction interface. Accordingly, the known theoretical kinetic models are usually classified into three groups: the diffusion models, the chemical reaction models, and the nucleation models.22,23 In addition, some empirical and semiempirical models have also been used to describe solid-state reactions. The reaction rate of solid-state decomposition can be represented by the general equation (1) dR ⁄ dt ) k(T) f(R) where R, called the decomposition fraction or extent of reaction, is a function of time t, the kinetic model (also called the kinetic mechanism) f(R) is a function of R, whose analytical form depends on the reaction mechanism and rate-determining step, and k(T) is a rate constant that accounts for the temperature dependence of the reaction rate, usually assumed to be independent of the decomposition fraction. Rate constant k(T) is usually expressed by an Arrhenius equation and eq 1 can be rewritten as (2) dR ⁄ dt ) Ae-Ea⁄RTf(R) where A is the preexponential factor, Ea the activation energy for the reaction, and R the universal gas constant. Separation of variables in eq 2 gives g(R) )

∫ dR ⁄ f(R) ) A∫ e

-Ea⁄RT

dt

(3)

where g(R) denotes the integral form of the kinetic model f(R). The commonly used expressions for f(R) and g(R) are listed in Table 1.24–26 One of the main targets of kinetics is to determine the kinetic triplet factors, i.e., the analytical form of the kinetic model, and the values of the activation energy Ea and preexponential factor A. Two alternative methods have been used in kinetic analysis of thermal decomposition and, indeed, other reactions of solid state:27,28 the isothermal method, in which the reaction is carried out at a constant temperature, and the nonisothermal method, in which the reaction is subjected to a controlled rising temperature. At constant temperature, eq 3 can be simplified as g(R) ) k(T) t, which is the fundamental for isothermal kinetic analyses. Compared with isothermal experiments, however,

nonisothermal runs are more convenient to carry out because it is not necessary to perform a sudden temperature jump of the sample at the beginning. Thus, most experiments of solid-state decomposition are performed nonisothermally, and the heating rate is kept constant. Defining the heating rate as β ) dT/dt, eqs 2 and 3 can be rewritten as A dR ⁄ dT ) e-Ea⁄RTf(R) β

(4)

∫e

(5)

and g(R) )

A β

-Ea⁄RT

dT

where ∫e-Ea/RT dT is called the Arrhenius integral or exponential temperature integral. Equations 4 and 5 are the fundamentals for nonisothermal kinetic analyses. The nonisothermal methods are usually classified as differential (based on eq 4), integral (based on eq 5), or their combinations.29 It is considered that the integral methods are less subjected to experimental errors (noise of the device, etc.), giving more reliable results. The reported integral methods may be further classified as single-scan methods (by a single R-T curve obtained under one heating rate) and multiscan methods (by different R-T curves obtained under different heating rates). The kinetic compensation effect, i.e., the activation energy and preexponential factor are mutually correlated,30 makes it possible to obtain the kinetic triplet factors with one single-scan curve. However, a single R-T curve under one heating rate can be satisfactorily fitted using different sets of kinetic parameters, or by different kinetic models, unless one of f(R) and Ea is previously known. It seems like that the analytical form of the kinetic model depends on Arrhenius parameters.31 That is obviously not correct. The multiscan method is one way to get around this problem, giving more reliable results. It has been suggested to use at least three different heating rates.32 Some multiscan integral methods have been proposed for the determination of the kinetic triplet factors, in which the Ozawa-Flynn-Wall (OFW)33,34 and Kisinger-AkahiraSunose35 methods are independent of the Arrhenius parameters and are the most popular ones. Recently, Popescu36 has proposed a variant OFW method independent of the kinetic model. In the present work, the Popescu method was exploited for the determination of the kinetic model and the OFW method for the determination of Arrhenius parameters.

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7213

Figure 1. Powder XRD pattern of the NiAl-LDH sample.

3. Experimental Section Preparation of the Sample. The sample NiAl-LDH was prepared by coprecipitation from solutions of appropriate metal nitrates and NaOH, Ni(NO3)2 · 6H2O (13.954 g, 0.048 mol) and Al(NO3)3 · 9H2O (6.0013 g, 0.016 mol) in 80 mL of deionized water, and NaOH (4.0091 g, 0.1 mol) in 100 mL of deionized water. The solutions of nitrates and NaOH were simultaneously added to a vessel containing 150 mL of deionized water under a nitrogen atmosphere and vigorous stirring at room temperature. The rate of NaOH dosing was carefully controlled in order to keep the pH at a constant level of 5.5 ((0.5). The slurry obtained was stirred at 80 °C for a further 20 h, filtered, washed with deionized water, and dried in air at 60 °C overnight. The deionized water used above was decarbonated before use. Characterization. The powder XRD pattern of the sample was recorded on a Shimadzu XRD-6000 diffractometer using Cu KR radiation (λ ) 1.542 Å, 2θ ) 3-70°, 40 kV, 30 mA). Elemental analyses for metal ions in NiAl-LDH were performed using a model ICPS-7500 inductively coupled plasma emission spectrometer, with the sample being dissolved in dilute hydrochloric acid. The thermal behavior of the as-prepared NiAl-LDH was measured using a PerkinElmer Pyris Dismond TG/DTA under a flow of pure helium (200 cm3 · min-1) in the temperature range of 303-873 K at various heating rates of 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, and 20.0 K · min-1. The gases evolved during the thermal decomposition were continuously monitored with a ThermalStar mass spectrometer connected online to the microbalance. 4. Results and Discussion Crystalline Structure and Metallic Composition of the Sample. The powder XRD pattern of the NiAl-LDH sample was shown in Figure 1. The symmetric 00l reflection peaks and the broad and less symmetric 0kl reflection peaks revealed a typical LDH phase with highly turbostratic disorders of NO3anions and water molecules in the interlayer galleries,21,37,38 a profile similar to those presented by hydrotalcite-like compounds. Figure 1 shows that no excess phase was detected in the sample. The peaks were indexed in a hexagonal cell with rhombohedral symmetry, where c ) 2.491 nm (c ) 3d003) and a ) 0.301 nm (a ) 2d110). The d003 parameter, which corresponds to an interlayer distance of 0.830 nm, is typical for hydrotalcite-like materials with tilted nitrate anions.39,40 Elemental analyses for metallic ions by ICP showed that the Ni/Al molar ratio in the NiAl-LDH sample was about 2.81,

which was slightly lower than that in the starting mixed aqueous solution (3.00). This suggests that, under the preparation conditions, not all metal cations in the precursor solution can be completely coprecipitated. Thermal Behavior of the NiAl-LDH. Decomposition of the NiAl-LDH sample under an inert gas helium atmosphere was followed by TG from 303 to 873 K, and the released gaseous species were monitored by MS online following m/e ) 18 (H2O), 30 (NO), 44 (CO2), and 46 (NO2). Only negligible trace amounts of CO2 and NO2 were detected, indicating that the reaction system was well isolated from CO2 and the interlayer NO3- ions were converted mainly to NO as previously reported.11,41 The TG/DTA plots of the sample at different heating rates are given in Figure 2, as well as the corresponding MS curves of m/e ) 18 and 30. As a whole, there are two mass losses at whatever heating rate, as can be seen from Figure 2. Details of the mass losses are given in Table 1. The total mass losses fall in the range ∼37.24 to ∼ 37.73 wt %. The first mass loss, ∼7.3 wt % until the temperature of ∼463 to ∼513 K as the heating rate was increased, is accompanied by the only release of H2O (MS) and one heat flow (DTA). It should be due to the removal of the physisorbed water and interlayer water. The second distinct mass loss region of ∼30.2 wt % is in the temperature range until ∼900 K. As can be seen from the DTA and MS plots (Figure 2), it is accompanied by one peak of heat flow and two MS peaks of H2O (from HO-) and NO (from NO3-). It is clear that the second mass loss results from two processes, the dehydroxylation of the NiAl-LDH layers and the decomposition of the interlayer NO3- anions. The DTG peaks, as well as the peaks of heat flow and evolution of H2O and NO species, move toward high temperature with an increase in the heating rate, as expected. It is noticed that, at whatever heating rate, NO is released at a slightly higher temperature than H2O. This suggests that the NO3- species decompose only after the destruction of the lamella structure of LDH. Determination of the Kinetic Model. As discussed above, the kinetic model may be determined only after the values of the activation energy and preexponential factor are calculated, so that the analytical form of the kinetic model seems to depend on Ea and A. Popescu36 proposed an integral method for the determination of the kinetic model by using multiscan curves obtained at different heating rates without any preliminary computation or assumption of the values of Ea or A. Equation 5 can be rewritten as

∫

Rn

Rm

dR ⁄ f(R) )

A β

∫

Tn -E ⁄RT a

Tm

e

dT

(6)

where Rm and Rn are two different decomposition fractions and Tm and Tn are their corresponding reaction temperatures. By using the notations gmn )

∫

Rn

Rm

dR ⁄ f(R) and Imn ) A

∫

Tn -E ⁄RT a

Tm

e

dT

eq 6 can be rewritten in a short form as 1 gm ) Imn (7) β Because the Arrhenius integral only depends on the temperature and is independent of decomposition fraction R, for a certain couple of temperatures (Tm and Tn), Imn for each R-T curve obtained at different heating rates βi is a constant. Therefore, a plot of the values of gmn versus 1/β has to lead to a straight line with an intercept of zero if the analytical form of f(R) is properly chosen, which is the principle of the Popescu method.

7214 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 2. TG, DTG, DTA, and MS plots of the NiAl-LDH sample under a helium atmosphere at different heating rates: 2.5 K · min-1 (A), 5.0 K · min-1 (B), 7.5 K · min-1 (C), 10.0 K · min-1 (D), 12.5 K · min-1 (E), 15.0 K · min-1 (F), and 20.0 K · min-1 (G).

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7215

Figure 3. Plots of the dehydroxylation extent (A) and the denitration extent (B) versus temperature at different heating rates.

The present work is concerned with the simultaneous dehydroxylation and denitration of NiAl-LDH. As can be seen from Figure 2, these two processes cannot be distinguished from each other by means of TG or DTA measurements. However, the evolution MS peaks of H2O and NO, which are associated with these two processes, respectively, are separately recorded. Therefore, the MS data could be used for the kinetic analyses of these two processes in detail. The second MS peak of H2O and the evolution peak of NO are related to the dehydroxylation and denitration, respectively. For each process at different heating rates, the total integral area of the corresponding MS peak of H2O or NO can be regarded as the decomposition extent of 1, and thus the extent of decomposition after temperature T can be simply defined as the fraction of the integral area of the MS curve from initial temperature T0 to temperature T divided by the total integral area. The lowest point between the two MS peaks of H2O is regarded as the initial temperature T0 of the dehydroxylation. The R-T curves of the dehydroxylation and denitration at different heating rates are given in parts A and B of Figure 3, respectively. For either dehydroxylation or denitration, a given pair of temperatures (Tm, Tn) can determine pairs of decomposition fractions (Rm,βi, Rn,βi) on different curves obtained at different heating rates (βi) by using the R-T data (Figure 3A,B), and the corresponding gmn for every commonly used kinetic model (Table 1) at different heating rates can be calculated as gmn(βi) ) g(Rm,βi) - g(Rn,βi). The pair of temperatures (Tm, Tn) was given by considering that the corresponding extents of decomposition (Rm,βi, Rn,βi) on each curve fall in the range of

0.05-0.95. A pair of temperatures, Tm ) 553 K and Tn ) 603 K, were selected for the dehydroxylation, and Tm ) 603 K and Tn ) 673 K were selected for the denitration. The plots of gmn against 1/β for every commonly used kinetic model were shown in parts A and B of Figure 4 for the dehydroxylation and denitration, respectively. Using the gmn-1/β data (Figure 4A,B), the degree of linearity of eq 7 was examined by the least-squares method for every commonly used kinetic model (Table 1). The most suitable kinetic model for each process in question was determined by considering the following factors: (i) the correlation coefficient R should be greater than 0.99, and the closer the value of R to unity, the more suitable the kinetic model; (ii) the straight line should pass through the origin of the axes; (iii) the standard deviation should be less than 0.1; (iv) the degree of consistency with the state of the reaction system. The fitting results for every commonly used kinetic model were shown in Table 3. It can be seen from Figure 4A and Table 3 that the best-fitting kinetic model for the dehydroxylation was the A1 mechanism, given by the first-order Avrami-Erofe’ev equation as g(R) ) -ln(1 - R). The Avrami-Erofe’ev equation can be generally represented as g(R) ) [-ln(1 - R)]1/n appropriate to a random nucleation and nuclei growth process, where n is usually considered as the reaction order. The value of parameter n is in range of 0.5-4, depending on the nucleation rate, the geometry of the nuclei, and the growth mechanism (either diffusion-controlled or phaseboundary reaction controlled).23 The nuclei should be grown two-dimensionally according to the lamellar structure of LDH. The A1 mechanism can be derived by assuming that the nucleation rate is instantaneous and the nuclei growth is controlled by the diffusion of migrating species. Therefore, the dehydroxylation obeys a two-dimensional diffusion-controlled mechanism with instantaneous nucleation. For the denitration, it can be seen from Figure 4B and Table 3 that the best-fitting kinetic model was the Zhuralev-LesokinTempelman equation: g(R) ) [(1 - R)-1/3 - 1]2, which is appropriate for a three-dimensional diffusion-controlled process. Although it is also diffusion-controlled, the geometrical structure of the material is changed. As discussed above, the NO3- species are decomposed after the dehydroxylation of the LDH layers. The dehydroxylation can completely destruct the structure of the LDH layers, resulting in the formation the Ni-containing mixed oxides and free NO3- anions. Therefore, after dehydroxylation, random stacking of the deriving mixed oxides can give rise to a three-dimensional diffusion of the migrating species NO. Determination of the Arrhenius Parameters. As discussed above, multiscan integral methods are considered to be more reliable. Application of integral methods, however, requires evaluation the exponential temperature (Arrhenius) integral. Unfortunately, the Arrhenius integral has no exact solution; hence, numerical or approximate solutions are required. There have been many solutions proposed, which have been excellently reviewed by Flynn and Wall.34 Among them, the OFW method,33,34 based on the approximation of the Arrhenius integral proposed by Doyle,42 is the most popular one for determining the Arrhenius parameters. The OFW method is based on eq 8. Ea

( RgAER ) - 2.315 - 04567 RT

log(β) ) log

( )

(8)

Because g(R) is only a function of decomposition fraction R, for a certain R, although the corresponding temperatures

7216 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 4. Plots of gmm against 1/β with different kinetic models for the dehydroxylation (A) and denitration (B). Table 2. TG/DTA Analyses of the NiAl-LDH Sample under Different Heating Rates first mass loss β/K · min-1 2.5 5.0 7.5 10.0 12.5 15.0 20.0

second mass loss

peak loss peak loss total loss temperature/K mass/% temperature/K mass/% mass/% 366.0 388.1 392.8 397.1 399.2 402.2 412.3

6.99 7.09 7.03 7.38 7.31 7.52 7.52

589.4 603.9 611.7 616.0 620.0 625.8 632.9

30.55 30.31 30.21 30.06 30.30 30.22 30.01

37.54 37.40 37.24 37.44 37.61 37.73 37.53

under different heating rates are different, the value of log[AEa/ Rg(R)] is a constant for each kinetic model. Therefore, a plot of log(β) versus 1/T has to lead to a straight line, independent of the analytical form of g(R). For this reason, the OFW method is also called the isoconversion method. Practically, given R and according to eq 8, the value of activation energy Ea can by evaluated from the slope of the straight line and preexponential factor A can be calculated from the intercept with respect to the probable kinetic model g(R) determined above.

Table 3. Fitting Results of the Dehydroxylation and Denitration for Every Commonly Used Kinetic Model dehydroxylation

denitration

symbol

correlation coefficient

standard deviation

correlation coefficient

standard deviation

D1 D2 D3 D4 D5 R2 R3 C1.5 C2 A1 A1.5 A2 A3 P1 P2 P3 P4

0.9944 0.9948 0.9801 0.9913 0.9347 0.9844 0.9944 0.9830 0.9559 0.9995 0.9873 0.9587 0.8990 0.9177 0.7220 0.6062 0.5387

0.0273 0.0376 0.0292 0.0133 0.2199 0.0461 0.0222 0.1671 1.1500 0.0251 0.1098 0.1466 0.1529 0.1292 0.1783 0.1648 0.1455

0.3636 0.5942 0.8431 0.6934 0.9961 0.2547 0.3822 0.9104 0.9907 0.6445 0.3490 0.1718 -0.0015 -0.0419 -0.2497 -0.3086 -0.3349

0.4133 0.2916 0.0916 0.0732 0.0792 0.3458 0.2544 0.5750 1.0026 0.9239 0.7913 0.6915 0.5501 0.5182 0.4831 0.4116 0.3520

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7217 Table 4. Determination of the Arrhenius Parameters dehydroxylation

denitration

Ra

Ea/ kJ · mol-1

A/min-1

correlation coefficient

Ea/kJ · mol-1

A/min-1

correlation coefficient

0.3 0.4 0.5 0.6 0.7

121.5 129.8 132.6 131.8 129.3

7.01 × 108 4.86 × 109 1.06 × 1010 1.09 × 1010 7.84 × 109

0.99435 0.99695 0.99777 0.99789 0.99758

136.4 133.6 132.0 136.4 147.4

1.27 × 109 9.90 × 108 8.75 × 108 2.07 × 109 1.29 × 1010

0.99645 0.99635 0.99676 0.99704 0.99652

a

Extent of reaction.

5. Conclusions TG/DTA-MS study of NiAl-LDH indicated that the thermal decomposition of this compound proceeds in three individual stages including the removal of the physisorbed and interlayer water and almost simultaneous dehydroxylation of the LDH layers and decomposition of the interlayer NO3- ions and that the interlayer NO3- ions decompose only after the destruction of the lamellar LDH structure. The mechanism and kinetics of the dehydroxylation and denitration were analyzed based on MS data sets recorded at different heating rates. The dehytroxylation follows the nucleation and nuclei growth mechanism represented by the first-order Avrami-Erofe’ev equation, i.e., a twodimensional diffusion-controlled mechanism with instantaneous nucleation, and the average activation energy is about 129.0 kJ · mol-1. The denitration obeys a three-dimensional diffusioncontrolled mechanism represented by the Zhuralev-LesokinTempelman equation, and the average activation energy is about 137.2 kJ · mol-1. Acknowledgment The authors gratefully acknowledge financial support from the National Science Foundation of China, the 111 Project (Project B07004), the National Basic Research Program of China (Project 2009CB939802), and the Program for New Century Excellent Talents in University (Project NCET-04-0120). Literature Cited

Figure 5. Plots of ln(β) against 1/T at different extents of reaction for the dehydroxylation (A) and denitration (B).

The reaction temperatures at extents of decomposition (R ) 0.3, 0.4, 0.5, 0.6, and 0.7) under different heating rates are determined by using the R-T data of the dehydroxylation and denitration (parts A and B of Figure 3, respectively), and the plots of log(β) versus 1/T at different extents of decomposition are shown in Figure 5A,B. The log(β)-1/T data of these two processes at different R are linearly regressed according to eq8 by the least-squares method, the regressing parameters are shown in Table 4, and the activation energies at different R determined from the slopes and the preexponential factors calculated from the intercepts are also given in Table 4. It can be seen from Table 4 that, for both the dehydroxylation and denitration, the correlation coefficients for different extents of decomposition R from 0.3 to 0.7 are all greater than 0.99, indicating the high linearity of log(β)-1/T. The average value of the activation energies at R ) 0.3-0.7 is about 129.0 kJ · mol-1. Except for the one at R ) 0.3, the activation energies are approximately equal, and the average value is about 130.9 kJ · mol-1. The exception maybe resulted from measuring error. For the denitration, the average value of activation energies at R ) 0.3-0.7 is about 137.2 kJ · mol-1.

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ReceiVed for reView April 15, 2008 ReVised manuscript receiVed July 8, 2008 Accepted July 20, 2008 IE800609C