Thermoanalytic study of some metal propionates synthesised by sol–gel route: a kinetic and thermodynamic study

Journal of Analytical and Applied Pyrolysis 65 (2002) 253–267

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Thermoanalytic study of some metal propionates synthesised by sol–gel route: a kinetic and thermodynamic study A. Kaddouri *, C. Mazzocchia Chemical Engineering and Industrial Chemistry Department, Politecnico di Milano, P.za L. Da Vinci, 32, 20133 Mi, Italy Received 30 July 2001; accepted 20 December 2001

Abstract The thermal decomposition reactions of lanthanum (III) and nickel (II) propionates, under different atmospheres, were studied in the temperature range of 25 – 800 °C using a thermogravimetry and differential thermal analysis. Thermal decomposition runs were performed under air, nitrogen and hydrogen. Solids analysis resulting from the precursors decomposition were characterised by infrared spectroscopy and X-ray diffractometry while gases were analysed using gas chromatography combined with mass spectrometry. Non-isothermal kinetic (A and DE) and thermodynamic parameters (DH, DS and Cp) for each process were determined. Under N2, the final products NiO and La2O3 crystallised at temperatures of : 310 and 720 °C respectively. Mixed metal carbonate– oxide phases were observed as intermediate solid products. Gas phase products included mainly CO and CO2 with a ratio which vary with decomposition temperature and atmosphere. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ni(II)-propionates; La(III)-propionates; Pyrolysis; Kinetics; Thermodynamics; XRD; DSC; TG-DTA

* Corresponding author. Tel.: + 39-02-23993247; fax: +39-02-70638173. E-mail address: [email protected] (A. Kaddouri). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 2 ) 0 0 0 0 4 - 9

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1. Introduction Supported metal catalysts are used in different applications such as plants exhaust emission control, petroleum reforming among others are reasonably resistant to sintering at temperatures up to : 800 °C. However, higher temperatures lead to metal crystallite growth. Sintering does not only result in the loss of exposed metal surface area but it may also lead to changes in the catalytic properties of the supported metal. In the past few years research on sintering on supported metal catalyst stability has been the subject of intense study. In order to overcome this problem, efforts have been focussed on the preparation methods. Between the several methods analysed to prepare supported metal catalysts, the most important are: precipitation or co-precipitation [1], impregnation [2,3], Ion-exchange [4,5] and sol– gel processing [6– 10]. The last procedure was found to be the most promising route for obtaining a better dispersion of the active phase on the catalyst support. The molecular compounds formed as a result of the copolymerisation of the organic and inorganic precursors using the sol–gel synthesis approach, have unusual physical and chemical properties. They form stable solid structures at low reaction temperatures with unusually high surface areas. Particle sizes are in the nano size range and they have additional physical and chemical properties which make them useful as either catalysts or catalysts supports [11– 14]. Therefore once the metal precursors are formed it is of importance to know their thermal decomposition behaviour. To our knowledge data on thermal decomposition of propionates precursors synthesised by sol–gel route is lacking [15– 17]. The metallic nickel and lanthanum oxide supported together on alumina, silica or titania are often used as catalysts systems for methane reforming [18]. This paper deals with investigations of the thermal analysis decomposition of nickel and lanthanum propionates under oxidant, inert and reducing atmospheres using thermogravimetry-differential thermal analysis (TG-DTA), XRD, Mass spectrometry and IR techniques. Kinetic parameters of the observed thermal phenomena are calculated from the experimental data using the non-isothermal method [19] and thermodynamic parameters are calculated from DSC curves [20].

2. Experimental Materials used in the present study were prepared in laboratory using the sol–gel method.

2.1. Sol –gel preparation The catalyst was prepared (Fig. 1) by dissolving at 90 °C, pure lanthanum oxide La2O3, nickel acetate or basic nickel carbonate Ni(OH)2 –NiCO3 under constant stirring in liquid propionic acid. The dissolution of the nickel and lanthanum compounds was very slow. After some hours, the solutions become transparent. Particularly for the lanthanum solution the dissolution occurs after a long period

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under acidic conditions. The cited solutions are hydrolysed with a few drops of distilled water and evaporated; the obtained gel (a polymer composed of MOM or M(mOH)M bonds) is frozen with liquid nitrogen, thus powdering the gel (Ni and La propionate precursor; NiPP and LaPP) into solid state.

2.2. Characterisation The decomposition of the precursors was studied under air, nitrogen and hydrogen using a Seiko instruments thermogravimetric analyser (TG-DTA-DTG). The samples, weighing :20 mg, were placed in quartz crucible and heated up to 800 °C (with a heating rate of 2, 5, 10 and 20 °C min − 1). The total gas feed was 60 ml min − 1. DSC experiments were carried out using a DSC 6200 S II Seiko instruments EXSTAR 6000 apparatus. For all the tests carried out using DSC or TG-DTA, the experiments were repeated several times to assess the reproducibility of the data. X ray diffraction patterns of the samples were recorded using a Siemens D 5000 diffractometer with filtered CuKa radiation (count time of 1 s in the range 2U 10–70°). The samples have also been characterised by FT-IR (Perkin Elmer Mod. 1760)

Fig. 1. Summarised scheme for sol gel synthesis of nickel and lanthanum propionates.

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Gas phase products were analysed using a DANI 8520 Gas chromatograph with a TCD detector.

3. Data analysis The kinetic parameters for thermal events were calculated from TG, and/or DTA data. The temperatures (Tmax) at which weight-variant (DTA) thermal events are maximised, were determined as a function of the heating rate. The kinetic activation energy DE (kJ mol − 1) was then calculated for each event from a plot of log i against 1/Tmax, according to the following relationship [19]: DE = − R/l d logi/d(1/T),

(1)

where R is the gas constant (8.31 J mol K ), i is the heating rate (°C min ) and l is a constant (0.457). Calculation of the frequency factor A (S − 1) for the weight-variant events was carried out, assuming first-order kinetics, using the following equation [19]: −1

−1

log[−ln(1 −h)/T 2]= logAR/iE − DE/2.303RT,

−1

(2)

where h is the fraction decomposed at Tmax. Thermodynamic parameters (DH, DS and Cp) for each process were calculated from DSC curves using Cp=DH/DT and DS = 2.303·Cp·log(T2/T1) equations [20].

4. Results and discussion

4.1. Precursors decomposition 4.1.1. Under air 4.1.1.1. Ni propionate precursor. The decomposition of Ni propionate (Fig. 2a) shows two endothermic peaks between ambient temperature and 150 °C. These are attributed to a weight loss of : 16.3% (H2O) (theor. 16%). Next a decrease in mass occurs between 150 and 250 °C followed by a sharp decrease in mass in the temperature interval of 250– 325 °C. The first weight loss of 20.8% (theor. 23.2%) represents a transformation of C3H5O2 groups into C2H5O ones as determined by mass spectra analysis. The second mass loss of 29.6% (theor. 30.7%) is accompanied by an exothermic peak at 295 °C which is attributed to the decomposition of C2H5O groups and crystallisation of NiO. The heat produced during this exothermic phenomenon corresponds to : −3.98 kJ g − 1. 4.1.1.2. La propionate precursor. Under air the decomposition of LaPP (Fig. 2b) shows an endothermic peak accompanied by a weight loss of 5% (H2O), followed by a second weight loss of about 14.7% and a sharp exothermic peak at 343.4 °C

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Fig. 2. Thermal decomposition of (a) NiPP and (b) LaPP under air (heating rate 10 °C min − 1, F= 60 ml min − 1).

(weight loss of 28.2%) which is due to the simultaneous decomposition of propionate groups and crystallisation of lanthanum oxide-lanthanum oxycarbonate. This peak is preceded by a small exothermic one at : 275 °C which is due to the formation of CH3CH2O or CH3CH2CO2 radicals [17]. The heat of the precursor decomposition at 343.4 °C was estimated to be − 10 kJ g − 1. The overall decomposition process of the Ni propionate precursor (NiPP) and LaPP can be exemplified by the following equations: Ni(C3H5O2)2·2H2O +7O2 “NiO +6CO2 + 7H2O 2La(C3H5O2)3·H2O +21O2 “La2O3 +18CO2 + 17H2O under air the percentage of weight loss during the decomposition of NiPP and LaPP precursors are 66.9% (69% theor.) and 57.0% (56.7% theor.), respectively in agreement with the cited equations.

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4.1.2. Under nitrogen 4.1.2.1. Ni propionate precursor. From the thermoanalytical curve of NiPP (Fig. 3a), it follows that the complex starts to lose mass (: 14.8%) at low temperature up to :130 °C. Next a two decrease in mass occur in the temperature range 130–250 °C ( : 9.4% and 7.9%) followed by another rapid decrease in mass in the interval temperature of 250– 325 °C. TG curve proceeds then horizontally and the loss in mass in the latter range of temperature is : 35%. Such a decrease in mass is related to the decomposition of the organic ligand. A strong endothermic effect is in fact observed in DTA curve (Fig. 3b), and a sharp DTG peak is also detected. Above 400 °C, no change in the TG curve is observed and NiO, the final product of Ni propionate decomposition, is obtained. With respect to the calculated mass loss 69% the experimental value was 67%. 4.1.2.2. La propionate precursor. After loss of water (5% weight), the La propionate is stable up to 250 °C, and then decomposes to the oxide with the intermediate formation of lanthanum oxycarbonates. From the thermoanalytical curves (Fig. 4a and Fig. 4b), it follows that the complex starts to decompose at : 250 °C. Next, in the temperature range 250– 430 °C, a sudden decrease in mass, in two times (16.8 and 30%), occurs and the TG curve proceed horizontally up to 575 °C. The total mass loss for this range of temperature is about 46.8%. This decrease in mass is ascribed to the elimination of the organic ligand and the formation of La2O2CO3. The decomposition process is connected with the endothermic effect observed in the DTA curve (Fig. 4b), and a sharp DTG peak (not reported) corresponds to the rapid mass loss. In the temperature range 575–750 °C, the oxycarbonate of lanthanum decomposes (mass loss 6%) to La2O3. The calculated and experimental mass losses are 56.7 and 57.3% respectively. 4.1.3. Under hydrogen The decomposition of the LaPP and NiPP under hydrogen are shown in Fig. 3a and Fig. 4a, which compare results of six TG experiments performed at 5 °C min − 1 under different atmospheres: air, N2 and H2. While for LaPP (Fig. 4a) TG curve is broadly similar to that obtained in nitrogen, with NiPP (Fig. 3a), the mass loss step in H2 in the interval of temperature 250–350 °C is accompanied by a 42.1% mass loss compared with 35% mass loss in N2 in a similar temperature range. Therefore, the total mass loss in H2 was found to be 74.1% (theor. 75.6%) compared with 67% (theor. 69%) in N2 atmosphere (the difference between experimental mass loss obtained under the two atmospheres H2 and N2 (DH2 – N2 = 7.1 exp.) is fairly in good agreement with the theoretical value (6.65 theor.). Fig. 3b and Fig. 4b show the DTA curves of the decomposition of NiPP and LaPP in H2 and air at 5 °C min − 1 together with a curve obtained under N2 introduced for comparison. The DTA curve of NiPP under H2 (Fig. 3b) displayed six peaks at 60, 90, 184, 236, 300 and 325 °C while for LaPP (Fig. 4b) only five peaks are observed at 110.5, 263.2, 300, 389.5 and 642 °C. In air, the decomposition of NiPP (Fig. 2a) becomes very rapid at around 295 °C and the reaction was almost completed at

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Fig. 3. Comparison of thermal decomposition of NiPP under different atmospheres (air, N2 or H2) (a) TG, (b) DTA curves (heating rate 5 °C min − 1).

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Fig. 4. Comparison of thermal decomposition of LaPP under different atmospheres (air, N2 or H2) (a) TG, (b) DTA curves (heating rate 5 °C min − 1).

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310 °C (i.e. about 15 °C lower than the temperature observed when the reaction is carried out under N2). Only five peaks appeared in the DTA curve of the decomposition under air. These were located at 45.5, 77.3, 172.7, 211.4 and 295 °C. The total mass loss under air was measured as 67% vs. : 74.1 under hydrogen. When comparing DTA and TG curves (Fig. 3a, Fig. 3b) with those performed under N2 important differences are observed due to the use of air or H2, they consist essentially in: (a) under air at 295 °C, a greater NiPP decomposition rate relative to the decomposition rate under N2 and (b) an exothermic peak that appears under H2 at : 325 °C (Fig. 3b) which is attributed to the phenomena that take place during NiO reduction to metallic Ni: the process undertaken under hydrogen is considered to involve the elimination of oxygen and also carbon residues (due to organic precursors decomposition), leading to formation of metallic Ni. Based on literature reports, carburization of nickel supported on oxides in temperature-programmed carburization (TPC) experiments in flowing H2 and CO produced only apparent weight changes and no carbide Ni3C or coke was observed. Since nickel carbide is easily reduced to methane and elemental nickel in hydrogen [21], it seems clear that any carbon formed by NiPP decomposition rapidly reacts with hydrogen to form methane. Methanation, the hydrogenation of carbon oxides to methane, has been the subject of a large number of catalytic studies and nickel was found to be a very efficient catalyst. The most essential results with regard to the mechanism of methanation are: (a) hydrogenation of carbon atoms deposited on the surface in an easy reaction, faster than the methanation in the reaction mixture under the same temperature and hydrogen pressure (b) dissociation of CO (and at low hydrogen pressure also the production of CO2) can also proceed in the presence of hydrogen (c) isotopic labeling revealed that hydrogen from a mixture of CO and H2 prefers to react with carbon already present on the surface from preceding disproportionation (dissociation) rather than to react with CO from the gas phase and subsequently adsorbed on the surface [22]. These facts (a– c) indicate that, in one hand, at the reaction temperatures the primary processes are dissociative (H2 “2Hsurf and CO “ Csurf + Osurf) and, on the other hand, because the adsorption of CO is stronger than adsorption of hydrogen, we can combine these pieces of information and say that hydrogen reacts with Cs in an adsorbed state: Cs+ Hs “(CHsurf). We may conclude therefore that dissociative chemisorption of CO by contact with a nickel surface represents an energetically possible mechanism for the formation of the surface carbon intermediate. This carbon species are highly reactive towards hydrogenation and methane formation and should be distinguished from bulk nickel carbide, (Ni3C), whose activity in methanation catalysis has been examined and found to be relatively low [23,24]. The conversion of CO and H2 to methane can be described by the following reactions: 3H2 + CO “ CH4 +H2O

(3)

2H2 + 2CO “ CH4 +CO2

(4)

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4H2 + CO2 “CH4 +2H2O

(5)

However, hydrogenation of carbon dioxide (Eq. (5)), does not occur in the presence of CO. Also reaction (Eq. (4)) can be considered to be a combination of reaction (Eq. (3)) and the water gas shift reaction (Eq. (6)): CO +H2O “CO2 +H2

(6)

From the enthalpy values for reactions (3– 6), reported in [25], it is observed that all the reactions are exothermic and, indeed all except reaction (6) are highly exothermic. Generally this high heat release makes difficult to prevent over heating and inactivation of the catalyst. It is further noted that heat of reactions are not greatly influenced by temperature (between 300 and 600 °C, DHf (kCal) vary from−49.3 to −52.1 for reaction (3) from−59.1 to − 61.4 for reaction (4), from − 39.5 to −42.8 for reaction (5), and finally from−9.8 to − 9.30 for reaction (6). In contrast, changes in free energy and equilibrium constants for methanation reactions are quite dependent on temperature. Thus, during the NiPP decomposition, the exotherm observed in DTA of the sample treated under hydrogen could be due to the CO disproportionation and hydrogen dissociative adsorption on nickel together with surface carbon hydrogenation which occur simultaneously with the endothermic deoxygenation process of NiO. Similar phenomena, involving iron organic precursor decomposition under hydrogen has been observed [26]. These were accompanied by an exothermic peak at 490 °C which was attributed to Fe2O3 reduction to metallic Fe. Under hydrogen, the overall decomposition process of the NiPP can be exemplified by the following equation: Ni(C3H5O2)2·2H2O “Ni + 6CO +7H2 For LaPP precursor the decomposition under air (Fig. 4b) becomes rapid at :340 °C and the reaction was found to be faster relative to those observed under N2 (368 °C) or H2 (389.5 °C) respectively. Taking into account the initial decomposition temperature and XRD spectra, it may be assumed that La and Ni propionates are less stable under air, ( while they are a little more stable under nitrogen or hydrogen. The thermal stabilities of these two precursors increase in the following sequence: LaPPair B LaPPN2 B LaPPH2 and NiPPair B NiPPH2 BNiPPN2. In any case the stability of LaPPs was found to be greater than the stability of Ni propionate.

4.1.3.1. Kinetic and thermodynamic analysis of thermoanalytical data. Values of the kinetic parameters (i.e. activation energy, Ea and the frequency factor, ln A) together with thermodynamic parameters of the dehydration and the decomposition steps are listed in Tables 1 and 2 respectively. The data reported in Table 1 show that the activation energy of the LaPP dehydration step (the two H2O molecules) is : 133 kJ mol − 1 which is in the range characteristic of dehydration processes involving loosely bound water molecules (60 kJ mol − 1) [27]. On the contrary

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Table 1 Non-isothermal kinetic parameters of the thermal events occurring throughout the decomposition course of LaPP and NiPP under dry nitrogen or hydrogen atmospheres Event

I II III IV V

LaPP under N2

NiPP under N2

NiPP under H2

Log A

DE (kJ mol−1)

Log A

DE (kJ mol−1)

Log A

DE (kJ mol−1)

21.2 19.3 32.1 16.4 5.3

133.5 86.6 286.2 295.8 154.6

21.9 17.2 10.2 18.1 17.9

173.7 180.9 296.9 170.6 189.7

30.2 17.6 18.2 32.9 34.3

128.6 132.4 168.4 154.4 136.4

activation energy for NiPP dehydration step is much higher : 173.7 and 180 kJ mol − 1 under N2 and 128.6 and 132.4 kJ mol − 1 under H2. Under N2 the activation energy Ea of the decomposition process of LaPP (step IV) (295.8 kJ mol − 1) is relatively close to the value obtained for the decomposition of NiPP (step III) (296.9 kJ mol − 1). Under H2 atmosphere the activation energies of the thermal decomposition process of NiPP are sensibly lower than the corresponding values calculated under N2. This can be ascribed to the ability of H2 to react with the nickel leading to a readiness of the reduction processes. Comparison with other works [28] involving nickel salts decomposition under N2 atmosphere shows that the activation energy calculated for dehydration and decomposition processes of NiPPs is relatively higher than the corresponding reported values for nickel acetate tetrahydrate (93 kJ mol − 1 for dehydration and, 170 and 157 kJ mol − 1 for decomposition). Thermodynamic parameters of NiPP thermal decomposition calculated in N2 atmosphere (Table 2) are greater than those calculated for LaPP.

4.1.3.2. IR spectroscopy. The IR peaks observed for nickel and lanthanum precursors are reported in Fig. 5. The IR study of the crystals obtained starting from the lanthanum oxide and nickel acetate, dissolved in propionic acid solution, showed Table 2 Thermodynamic parameters of the thermal events occurring throughout the decomposition course of LaPP and NiPP Event

I II III IV V

LaPP

NiPP

DH (J g−1)

Cp (J deg−1 g) DS (J deg−1 g)

DH (J g−1)

Cp (J deg−1 g) DS (J deg−1 g)

128.9 36.2 113.0 114.5 97.3

1.62 0.90 1.44 0.81 0.84

192.4 193.7 149.4 145.1 387.7

6.10 4.20 3.42 2.93 6.0

1.30 0.42 0.35 0.22 0.15

4.20 2.32 0.82 0.61 1.43

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Fig. 5. FTIR spectra of (a) NiPP and (b) LaPP precursors.

the formation of a monodentate propionate precursor. However starting from the nitrate salts, the propionate to nitrate substitution is more difficult. This substitution depends on the nature of the element and on its hydration rate as shown in Table 3. In the case of lanthanum nitrate, the IR band corresponding to nitrate has been found to be intense (fig. not reported) indicating that the substitution of nitrate by propionate is more difficult than for Ni. Table 3 Nature of the precursors in propionic acid versus the starting materials Starting materials

Type of precursors in propionic acid

Lanthanum nitrate Lanthanum oxide Nickel nitrate Nickel acetate

Nitrates and few bidentate propionates Monodentate propionates Nitrates and few bidentate propionates Monodentate propionates

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Fig. 6. XRD spectra of (a) NiPP precursor, (b) heated at 150 °C, (c) at 200 °C, (d) at 300 °C.

4.1.3.3. Products analysis. A typical quartz micro-reactor flow system was used to identify the products generated during precursors decomposition. The flow rates of the gases (Air, H2, N2) were controlled by mass flow meters (Brooks 5850). Samples (1 ml each) of the effluent gas were collected automatically with a Valco 16 multiport heated valve then analysed on-line by GC. Different decomposition temperatures were used and gas chromatographic analisis showed that CO and CO2 were the two main gaseous products together with traces of CH3CH2CO2H and CH3CH2OH. Their relative ratio varied both with the prevailing atmosphere and the decomposition temperature. 4.1.3.4. XRD analysis. Structure analysis by XRD and IR techniques were carried out after the samples were heated to specific temperatures in the TG-DTA apparatus then cooled to ambient temperature. The XRD patterns, shown in Figs. 6 and 7 for the solid products at 200 and 300 °C, respectively for NiPP and LaPP, also indicate the disappearance of the characteristic peaks of the parent propionate. The XRD pattern of the LaPP (b) product (Fig. 7) is typical of an amorphous material. In contrast, the pattern of the

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NiPP (c) (Fig. 6) shows diffraction lines of a partially crystalline product different from those reported for NiPP (a) or NiO (d) (ASTM card no. 22– 1189). These suggest that the propionate intermediates, as identified by IR spectra (figures not reported), are amorphous in nature. This is in agreement with the values of activation energy determined for event IV involving LaPP (b) (295.8 kJ mol − 1) and for event III involving NiPP (c) (296.9 kJ mol − 1) that have been found to be similar. Sol – gel processing is a relevant method for the synthesis of Ni and LaPPs for the preparation of nickel metal (active phase) and lanthanum oxide (as support or promoter) both used successfully in methane dry reforming. Relative to the traditionally preparation from nitrates or chlorides salts several advantages are obtained with propionate precursors such as nano-particle size (5–8 nm for metallic nickel determined by TEM analysis), high dispersion and low sintering of the nickel metal and also a high surface area and porosity of the oxides (La2O3 and NiO) after thermal decomposition of their corresponding La and NiPPs.

Fig. 7. XRD spectra of (a) LaPP precursor, (b) heated at 300 °C, (c) at 500 °C, (d) at 750 °C.

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5. Conclusions Metal-propionates is a most promising route for the preparation of solid structures with better homogeneous particle size distribution. The final solids obtained after thermal treatment of LaPP and NiPP can posses additional physical and chemical properties which make them useful as either catalysts or catalysts supports. Thermal decomposition which involves several pathways depends on the atmosphere used. The conversion of CH3CH2CO2 groups first to CH3CH2O then into CO and CO2 shows that the gaseous decomposition products are not always formed directly from the initial products. Thermal stability of propionate precursors (NiPP and LaPP) is affected both by varying the heating rate and also by the prevailing atmosphere. However, LaPP was found to be more thermally stable than NiPP.

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