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Mechanism of low temperature decomposition of NiO single crystals
Solid State Sciences 11 (2009) 1686–1691
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Mechanism of low temperature decomposition of NiO single crystals Filippo Maglia*, Giorgio Spinolo, Umberto Anselmi-Tamburini Department of Physical Chemistry, University of Pavia and IENI–CNR, V.le Taramelli 16, I-27100 Pavia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2008 Received in revised form 15 April 2009 Accepted 27 May 2009 Available online 6 June 2009
The decomposition of NiO single crystal was investigated under dissociative conditions in the temperature interval between 330 and 850 C in the absence of reducing gas species. An unusually fast and constant decomposition rate was measured at the lowest temperatures, coupled with an unusual largely porous microstructure of the metallic product layer. This anomalous high reaction rate was interpreted on the basis of a decomposition mechanism implying the dissociative vaporization of the oxide followed by the condensation of the metal. The proposed mechanism is supported by the microstructure of the product and of the reacting interface. The complex dependence of reaction rate from temperature was shown to be related to a collapse of the porous product to form a compact metal layer at higher temperatures due to sintering. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Endothermal decomposition Interface microstructure NiO
1. Introduction Numerous studies have been devoted, since the beginning of the last century, to the heterogeneous reduction of NiO and transition metals oxides [1–5]. Most of the work was aimed to the interpretation of the complex kinetic behavior observed experimentally, but a clear understanding of the overall reaction mechanism involved in this process is still missing. This is particularly true in the case of the carbothermal reduction that represents the most known, investigated, and technologically relevant of all the reduction processes [6,7]. The carbothermal reduction of the transition metal oxides (i.e. their reduction by solid carbon) does not require physical contact between the reactants to occur. All the proposed reaction mechanisms, in fact, involve the presence of gaseous intermediate species. The most known of such mechanisms involves the presence of CO(g) [6]: MOðsÞ D CO / MðsÞ D CO2 CO2 D CðsÞ / 2CO
(1)
other authors have on the other hand proposed a mechanism based on the reducing action of H2 adsorbed on the carbon surface [8]: MOðsÞ D H2 / MðsÞ D H2 O CðsÞ D H2 O / H2 D CO
(2)
* Corresponding author. Tel.: þ39 0382 987208; fax: þ39 0382 987575. E-mail address: fi[email protected] (F. Maglia). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.029
However, as pointed out by L’vov [6], although being able to account for some experimental evidences, these reaction schemes cannot be applied for a full interpretation of the reaction mechanism. In particular the amount of H2 is probably too small in comparison to the observed reaction kinetics and the restoration of CO is too slow at low temperature. Other authors have interpreted the carbothermal reduction of NiO through a dissociative scheme [6,9,10]: MOðsÞ / MðsÞ D ½ O2
(3)
where no reducing gas is involved and carbon simply acts as an oxygen getter maintaining a low oxygen partial pressure in the reaction environment. Despite its simplicity the dissociative mechanism received very little or no attention at all, as most of the literature is focused on the processes involving the presence of a reducing gas, such as CO or H2. The dissociative evaporation scheme was denied by the majority of researchers for many years since the equilibrium oxygen partial pressures required to perform this reaction were considered too low to account for the high reduction rate experimentally measured [6]. Despite that, of all possible mechanisms involved in the reduction of the metal oxide the dissociative process appears to be particularly attractive because of its simplicity and of its similarity with other processes, such as the dissociative vaporization and the endothermic decomposition of carbonates and hydroxides, whose reaction mechanism is better understood from the thermodynamic and kinetic point of view [11–14]. Aim of the present work was to investigate the decomposition of NiO under dissociative conditions, in order to investigate the
F. Maglia et al. / Solid State Sciences 11 (2009) 1686–1691
fundamental steps involved in the reduction reaction. The main focus was on the investigation of the reaction kinetics and of the product microstructure. 2. Experimental methods The reduction kinetics was measured with a gravimetric method under isothermal conditions. Two different approaches have been used, one allowing continuous and one based on discontinuous measurements. In the first approach the reaction was performed in a homemade apparatus built around a Cahn RG automatic balance having a sensitivity of 107 g and a maximum load of 1 g (Fig. 1). The apparatus was designed to keep a strict symmetric arrangement. The quartz tubes enclosing sample and tare were symmetrically fastened to the stainless steel container of the balance (on top) and to a Pyrex connection with a common inlet (on bottom). Therefore the gas flux was symmetrically divided between sample and tare. Heating was provided by a twin furnace operated by two control/ power units in a master/slave configuration and driven by two thermocouples. An Analog Devices Input/Output (I/O) processor collected analog inputs from the balance and the thermocouples and sent the data to a personal computer through a serial interface. Since no gaseous reducing agent was employed in this study the reduction process could be obtained only when the P(O2) was below the Ni/NiO equilibrium value. Fig. 2 shows the dependence of the P(O2) on temperature for the Ni/NiO equilibrium in comparison with other metal/metal oxide equilibria. It can be noted that for temperatures below 1000 K the reduction of NiO requires extremely low partial pressures. In order to achieve such low oxygen contents a purification system based on metal getters was used. The metals were placed in two quartz tubes placed in the same furnace kept at 800 C under a flow of high purity argon (99.9998% nominal). A mixture of Fe/FeO was placed in the first tube and a mixture of Ti/TiO (or Zr/ZrO2) in the second one. This arrangement allowed to achieve oxygen partial pressures below the
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detection limit of our oxygen sensor (1025 atm) for a period of 10– 15 minutes. The experimental procedure was as follows. Each sample was heated to the desired temperature under a flux of argon coming directly from the cylinder. Under these conditions no reduction was observed at any temperature. As the sample reached the desired temperature, the gas flow was switched in order to let the purified gas to reach the sample. The reduction kinetics was also investigated with a discontinuous method. In this case the sample was placed at one end of a quartz tube sealed under vacuum. Few grams of Fe/FeO powders were placed at the other end of the tube. Quartz wool was placed in the middle in order to prevent any contact between the sample and the getter. The two ends of the tube (100 mm long and 5 mm in diameter) were inserted into two independent furnaces in order to control the sample and the getter temperature independently. The kinetics of the reduction was studied by rapidly cooling the sample, opening the vial, and weighing the sample at selected time intervals. After each measurement the sample was placed in a new vial. Microstructural characterization of the reacted samples was made by optical and scanning electron microscopy (SEM). A Zeiss Axioplan optical microscope and a Cambridge SEM Stereoscan 200 operated at 30 kV and equipped with a backscattered electron detector and a Link microprobe were used. Prior to SEM analysis the sample surface was coated with 20 nm of sputtered gold. 3. Results The experiments were conducted on monocrystalline samples (30–40 mg of weight) obtained by cutting a NiO single crystal prepared with a single ellipsoid image furnace. The weight loss vs. time measured by the Cahn electrobalance at different temperatures is reported in Fig. 3. The weight variations have been normalized to an initial external surface of NiO (in single crystal form) equal to 100 mm2 in order to have directly comparable results and have been shifted along the vertical axis for clarity.
Fig. 1. Schematic of the experimental set up: (1) microbalance, (2) sample, (3) tare, (4) furnaces, (5) temperature controllers, (6) oxygen sensor, (7) data acquisition system, (8) gas cylinder, (9) iron getter, (10) titanium getter, (11) thermocouples.
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The dependence of the reduction kinetics from temperature is peculiar. No significative weight loss was observed for temperatures below 330 C. Increasing the temperature the decomposition rate at first increased, but for temperatures above 540 C it started decreasing. A very slow decomposition rate, comparable with the one observed at 330 C, was observed at temperatures above 800 C. In all cases, however, the weight loss slowed down after 10–15 minutes because of the increase in the oxygen partial pressure produced by the reduced efficiency of the getters. This conclusion was confirmed by decomposition experiments performed with the discontinuous setup. The single crystal of NiO was in this case placed in a sealed quartz tube at a temperature of 540 C corresponding to a partial pressure for the equilibrium Ni/NiO equal to 8.5 1022 atm. At the other side of the quartz sealed tube were placed few grams of Fe/FeO powders. This end of the tube was kept at the temperature of 721 C corresponding to a partial pressure for the equilibrium Fe/FeO equal to 8.3 1022 atm. The weight loss vs. time at selected time intervals is shown in Fig. 4(a). It is evident as, differently from the reduction in the thermobalance, which slows down quickly, the sample reduced in the sealed tube keeps decomposing even after several hours. In Fig. 4(b) the data of Fig. 4(a) have been plotted taking into account the decrease of the NiO/Ni interface on the basis of a shrinking core model. The data, extending up to a 74% of reduction of the sample, show a marked linear trend. The superficial layer of metal produced by the reduction process shows a quite peculiar microstructure. In the samples reduced at the lowest temperatures (Figs. 5 and 6) the metallic layer shows an extremely high porosity characterized by a typical interconnected microstructure. The size of the pores is about 1 mm, but the thickness of the wall separating the pores is in the nanometric range (Fig. 7). When the reaction temperature is increased the pore size also increases and a collapse in the microstructure is observed (Fig. 8). In the samples reduced at the highest temperature (850 C) the porosity is almost completely absent and a fairly compact metal layer adherent to the NiO surface is observed (Fig. 9).
Fig. 2. Equilibrium oxygen partial pressure for the following equilibria: (1) Ni/NiO, (2) Fe/FeO, (3) Ti/TiO, (4) Zr/ZrO2.
Fig. 3. Weight loss for NiO single crystal as a function of time for different annealing temperatures. Data normalized to an initial external area for NiO equal to 100 mm2.
4. Discussion The experimental results reported in this work show how the reduction of single crystals of NiO can be performed to a significant extent in the absence of any reducing gas at temperatures as low as 330 C. This result allows to gain some significant information on the mechanism and the kinetics of the reduction of transition metal
Fig. 4. (a) Weight loss kinetics obtained by the discontinuous (sealed tube) at 540 C (data normalized to an initial external area for NiO equal to 100 mm2); (b) reduction kinetics (discontinuous method) taking into account the decrease of the NiO/Ni interface on the basis of a shrinking core model.
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Fig. 5. SEM image of the product (Ni) zone for the sample reduced at 330 C. The NiO– Ni interface is visible in the lower left corner.
Fig. 7. Higher magnification SEM image of the NiO–Ni interface for the sample reduced at 330 C.
oxides in its simplest form and in its earliest stages, showing characteristics that are generally not evident when higher temperatures or reducing gases are used. In our experimental approach, in fact, the reduction derives only from a simple dissociative mechanism producing only one gaseous species. Furthermore, because of the low temperatures, far away from the melting point of the reduced metal, the diffusivity in the product layer is very limited, allowing to observe the microstructure of the product layer in its early form, as it is first produced by the chemical process, with very limited subsequent modification. The microstructure of the produced metallic layer shows some very peculiar characteristics. The most striking morphological feature is represented by the continuous and interconnected isotropic nature of this microstructure. As shown in Figs. 5–7 the layer is made by a metallic sponge whose porosity appears to be completely open, as all the cavities seem to be connected to each other. When the reaction is performed at the lowest temperatures, the layer microstructure appears to be remarkably isotropic with no apparent correlation with the layer direction of growth. The Ni/NiO interface (Fig. 7) appears to be quite irregular. It is remarkable, however, as even in this region the porosity seems to maintain in the same scale. Observations performed at very high magnification failed to show any evidence of smaller porosity even at the very interface. This observation seems to exclude the possibility of a mechanism of nucleation and growth for these pores. The lack of evolution with time seems to confirm that the observed microstructure represents the primary product of the chemical decomposition step. Complex
microstructures are often the result of decomposition reactions when the process is performed in conditions very far from the equilibrium. A typical example is represented by the microstructures deriving from the endothermal decomposition of carbonates or hydroxides performed in high vacuum [12,13,15,16]. These complex microstructures originate from the requirement to accommodate the large decrease in molar volume without the possibility of a significant decrease in the overall volume due to the limited bulk diffusivity. In the case of the NiO decomposition, though, the microstructure does not seem to be compatible with a mechanism involving only local rearrangements. As noted in the case of carbonates and hydroxides’ decomposition [12,13,15,16] and, more recently, in the case of void organization in partial reduction of NiO [17], the microstructures deriving from these local rearrangements are always nanometric in nature, with a pore size that is in the range of few nanometers and show always some sort of correlation with lattice structure of the parent material. In some cases a topotactic growth is observed [18,19]. The microstructure observed in our study, though, is quite large. Although the thickness of the walls separating the pores is surely in the nanometric range, the pores themselves are well in the micrometric range. It is hard to believe that such a large microstructure could be the result of a near-diffusionless process. Note that as in the proximity of the interface, where the decomposition process is still uncompleted, the internal surface of the pores is coated by a thin, well adherent, and apparently compact layer of metal (Fig. 7), a feature that does not seem to be compatible with
Fig. 6. Higher magnification SEM image of the product (Ni) zone for the sample reduced at 330 C.
Fig. 8. SEM image of the product (Ni) zone and the NiO–Ni interface for the sample reduced at 830 C.
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Fig. 9. SEM image of the product (Ni) zone and the NiO–Ni interface for the sample reduced at 850 C.
a scenario that excludes the possibility of long range diffusion. Furthermore, as we noted before, the product microstructure seems to be isotropic and independent from the direction of growth and lacks any correlation with the NiO lattice. Although no clear interpretation for the actual morphology can be put forward at the moment, we will return on this point in the following. As the temperature is increased clear evidence of modification in this microstructure starts to appear due to the increased diffusivity in the metallic layer (Fig. 8). Eventually a complete collapse in the porous structure is observed at the highest temperatures, leaving an almost completely compact layer of metal (Fig. 9). This evolution in the microstructure explains some of the kinetic features shown by the reaction at the different temperatures. At the lowest temperatures a remarkably linear kinetics is observed (Fig. 4). This implies a kinetics totally controlled by an interfacial step. Such a process allows for a fast reaction rate even at temperatures as low as 330 C. This is a quite surprising result, considering that traditionally the reduction of transition metal oxides is a considered reaction characterized by a very high activation energy, requiring high temperatures to be performed with an appreciable kinetics. Our results, however, confirm that these characteristics are not related with the intrinsic properties of the chemical process, but are the result of a complex mechanism mostly controlled by the ability of the produced layer to allow the oxygen release. The most important parameter that controls the overall reaction kinetics is in fact represented by the porosity of the metal product layer. If a compact surface layer of metal is formed the reduction rate is completely determined by the transport of oxygen through the metallic layer and a parabolic growth of the metal layer can be forecast [11]. If the product layer is not compact, on the other hand, as long as the porous layer is sufficiently thin to oppose negligible resistance to the gas evolution, the kinetics will be controlled by the interface reaction [11]. As a result, temperature plays a complex role by controlling not only the intrinsic kinetics of the primary interfacial chemical process, but also the rate and degree of sintering of the product layer. The importance of the porosity of the product layer on the overall kinetics has been recognized before. In several systems, the reaction rate, after an initial increase with temperature due to the increased surface reaction rate, shows a marked reduction due to the increased product sintering [2]. Several kinetic models, based on the well-known shrinking core scheme [1–3,20,21], have been proposed for the description of the reduction of metal oxides. These models are characterized by different levels of complexity with the aim of taking into account the complexity of the process described above plus other features such as kinetics of nucleation of the metal phase, autocatalytic
effects, multiple product phase, etc. In all these cases, however, it has never been realized that the primary chemical step is characterized by a high intrinsic chemical kinetics that appears to be controlled only by an interfacial process. Another noteworthy characteristic of the kinetic data of Figs. 3 and 4 is represented by the complete absence of any induction or acceleratory period. This absence is particularly relevant as the presence of these two characteristics is considered typical for this type of reduction process. In some cases they have been used to support some specific reaction mechanisms. L’vov [9] suggested that an initial induction and a subsequent acceleratory (autocatalytic) period are related with the formation and growth of nuclei of the metallic product from the gas phase. These interpretations, however, are based on data on the kinetics of carbothermic reduction at high temperatures where, as we discussed earlier, the intrinsic kinetics is strongly influenced by the sintering of the metallic layer. Our data, on the opposite, show that despite the low temperatures the decomposition process proceeds with a very regular kinetics excluding the possibility that heterogeneous nucleation can be a process controlling the overall kinetics. More information on the nature of the mechanism responsible of the chemical decomposition can be obtained from the quantitative analysis of the kinetic data. When the mechanism of the reaction is controlled by a simple decomposition step like Equation (3) the maximum reaction rate can be calculated on the basis of a Hertz–Knudsen–Langmuir [22–24] model. In such approach the maximum reaction rate is controlled by the maximum flux of gas molecules, J, that can emerge from the solid phase:
Peq J ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRT
(4)
M is the molar mass of the gas, and Peq is its equilibrium pressure. This flux identifies an upper limit for the reaction rate in the approximation that no other mechanism, besides the pure gas effusion, is controlling the rate of oxygen release. Obviously in the presence of an impervious layer of product or of a kinetically limiting chemical step a slower kinetics is expected. In such cases a vaporization coefficient av is introduced that equals the ratio between the measured pressure and the equilibrium one:
av ¼
P : Peq
(5)
The maximum reaction will be given by:
Peq J ¼ av pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRT
(6)
In almost all cases av 1. This dissociative evaporation scheme has been proven to be able to explain the mechanism and kinetics of thermal decomposition of various oxides as well as nitrates, carbonates, etc. av values of 102–103 are found for many oxides while values as low as 1040 are typical of other substances such as oxalates and azides [9]. Table 1 shows for our experiments’ calculated and experimental values for J together with the relative values of av in the case of the decomposition of a single crystal of NiO at various temperatures. Surprisingly, the observed experimental fluxes appear to be anomalously high and the resulting av values are higher than 108. Values of av 1 are extremely rare, as they imply the presence of a partial pressure of the gaseous component higher than the equilibrium value; an eventuality quite unrealistic. A similar anomaly is mentioned by L’vov [9] in the case of the carbothermal reduction of NiO, CuO and FeO, although no direct experimental data are presented. No direct evidence of abnormally high values of av in the case of direct decomposition has ever been presented before.
F. Maglia et al. / Solid State Sciences 11 (2009) 1686–1691 Table 1 Calculated and experimental values for the oxygen flux, J, and the relative values of av for the decomposition of NiO single crystals at different temperatures. Sample
T ( C)
Equilibrium P(O2) (atm)
Calculated J ¼ dn/dt (mol cm2 s1)
Experimental J ¼ dn/dt (mol cm2 s1)
av
M1 M2 M3 M4 M5 M6 M7
830 680 600 540 450 380 330
9.76 1014 2.74 1017 1.10 1019 8.52 1022 1.27 1025 2.49 1029 1.64 1032
3.26 1014 9.83 1018 4.12 1020 3.31 1022 5.23 1026 1.08 1029 7.40 1033
1.04 108 6.25 108 1.30 107 3.38 107 1.72 107 3.64 108 2.60 108
3.19 105 6.40 109 3.16 1012 1.02 1015 3.29 1018 3.37 1021 3.51 1024
L’vov provides an interpretation of this anomalous behavior introducing a slight but important modification in the mechanism of the direct decomposition reaction as presented in Equation (3): MOðsÞ / MY ðgÞ D ½ O2 ðgÞ
(7)
here the symbol MY(g) refers to the metal formed in gas form followed immediately by its condensation. In this approach the controlling step would be a dissociative vaporization of the oxide, followed by the condensation of the metal. In L’vov model half of the energy released by the condensation of the metal would be returned to the reactants, increasing the kinetics of the decomposition process itself. The author demonstrates that av > 1 can be derived from this model when the following thermodynamic requirements are met:
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in condition far from the equilibrium, as it would be ejected from the solid with limited chances of equilibration, making the application of the Hertz–Knudsen–Langmuir Equation (4) questionable and, suggesting a possible interpretation for the abnormally high value of av. 5. Conclusions The presented experimental results confirm that the decomposition of single crystals of NiO can be achieved through a dissociative mechanism at temperatures as low as 330 C. At temperatures below 500 C the decomposition proceeds with a surprisingly fast kinetics, showing decomposition rates that are orders of magnitude higher than the values calculated on the basis the Hertz–Knudsen– Langmuir theory. When not limited by the efficiency of the oxygen getter the reaction shows a constant decomposition rate, suggesting an interface-controlled mechanism. In these conditions the Ni metal is produced with a remarkable spongy microstructure. At higher temperatures this porous microstructure collapses, due to sintering, reducing the rate of further decomposition. As a result, the slow kinetics and the high activation energy generally observed in the high temperature reduction of NiO appear to be determined by the transport of the gaseous products through the metal layer and not by the intrinsic characteristics of the chemical process. Since at the lowest temperatures no significant bulk diffusivity is expected in the solid state, it is suggested that in these conditions the reaction proceeds through a mechanism involving a dissociative vaporization followed by a condensation of the metallic product. Acknowledgments
DH298 ðMOÞ DH298 ðMÞ 4DH298 ðMOÞ where the above terms represent the enthalpies of formation of gaseous M and solid MO at 298 K. Besides his thermodynamic soundness this model has the advantage to allow a possible interpretation for the puzzling microstructure of the product layer shown in Figs. 5 and 6. One of the remarkable features of this microstructure is represented by the complete lack of correlation with the direction of growth, together with the absence of any correlation between the crystal lattice of the reactants and of the products. As we noted earlier, the microstructure of Figs. 5 and 6 seems to imply a large scale reconstruction of the Ni lattice, a process deriving from a massive bulk diffusion. The temperature, however, is too low to justify massive solid-state diffusion, but it could be acceptable if a gas-phase transfer process is involved. In other words, it can be suggested that the observed microstructure derives from the sudden vaporization of fairly large regions of NiO close to the interface, followed by immediate condensation of the metal producing a uniform coating of the internal surface of the produced pore. The layer of metal prevents any further decomposition and could explain the lack of evolution in the pore morphology. The possibility of a decomposition process occurring in a sequence of bursts has been proposed in a recent review on the decomposition mechanism in calcium carbonate by Beruto et al. [25]. These authors suggested that release of CO2 from a fairly large region close to the interface can produce a layer on nonporous CO2 deficient calcite that can then undergo a structural collapse releasing the stress and ejecting any remaining CO2. In the case of the decomposition of NiO it could probably be coupled with the process represented by the reaction (7) producing bursts of vaporization that involve fairly large interfacial regions. It must also be noted that in this view the gas phase deriving from this process would probably be
We thank Prof. Klaus Becker for kindly supplying the NiO single crystals. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14]
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J. Szekely, J.W. Evans, Metall. Trans. 2 (1971) 1691. J. Szekely, J.W. Evans, Metall. Trans. 2 (1971) 1699. J.J. Heizmann, J. Bessieres, A. Bessieres, J. Chem. Phys. 83 (1986) 725. J.J. Scholz, M.A. Langell, Surf. Sci. 164 (1985) 543. J.T. Richardson, R. Scates, M.V. Twigg, Appl. Catal., A 246 (2003) 137. B.V. L’vov, Thermochim. Acta 360 (2000) 109. S.K. Sharma, F.J. Vastola Jr., P.L. Walker, Carbon 34 (1996) 1407. S.V. Digonskii, New Methods of Metal Production from Their Oxidised Compounds, Nauka, St. Petersburg, 1998, (in Russian). B.V. L’vov, Thermochim. Acta 373 (2001) 97. M.M. Kurchatov, Kinetic and mechanism of the reduction of metal oxides by solid reducing agents, Communications of the Department of Chemistry, Bulgarian Academy of Sciences, 11 (1969) 401. H. Schmalzried, Solid State Reactions, Verlag Chemie, Basel, 1981. A.W. Searcy, D. Beruto, J. Phys. Chem. 80 (1976) 425. A.W. Searcy, D. Beruto, J. Phys. Chem. 82 (1976) 163. G. Watelle, G. Bertrand, M. Lallemant, J.C. Mutin, J.C. Niepce, New data and concepts on the mechanism of endothermic decomposition of ionocovalent solids eliminating a gas, Mater. Sci. Monogr. 10 (1982) 1011. J.C. Niepce, G. Wattelle, N.H. Brett, J. Chem. Soc., Faraday Trans. 1 74 (1978) 1530. G. Spinolo, U. Anselmi Tamburini, Solid State Ionics 32–33 (1988) 413. M.A. Monge, A.I. Popov, C. Ballestreros, C. Gonzalez, Y. Chen, E.A. Kotomin, Phys. Rev. B: Condens. Matter 62 (2000) 9299. M. Figlarz, J. Guenot, F. Fievet-Vincent, J. Mater. Sci. 11 (1976) 2267. H. Naono, K. Nakai, J. Colloid Interface Sci. 128 (1989) 146. K.H. Spitzer, P.S. Manning, W.O. Philbrook, Trans. Met. Soc. AIME 236 (1966) 726. D. Beruto, L. Barco, G. Belleri, L. Fortina, J. Mater. Chem. 2 (1977) 203. H. Hertz, Ann. Phys. Chem. 17 (1882) 177. I. Langmuir, Phys. Rev. 2 (1913) 329. I. Langmuir, Phys. Rev. 8 (1916) 149. D. Beruto, A.W. Searcy, M.G. Kim, Thermochim. Acta 424 (2004) 99.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Mechanism of low temperature decomposition of NiO single crystals Filippo Maglia*, Giorgio Spinolo, Umberto Anselmi-Tamburini Department of Physical Chemistry, University of Pavia and IENI–CNR, V.le Taramelli 16, I-27100 Pavia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2008 Received in revised form 15 April 2009 Accepted 27 May 2009 Available online 6 June 2009
The decomposition of NiO single crystal was investigated under dissociative conditions in the temperature interval between 330 and 850 C in the absence of reducing gas species. An unusually fast and constant decomposition rate was measured at the lowest temperatures, coupled with an unusual largely porous microstructure of the metallic product layer. This anomalous high reaction rate was interpreted on the basis of a decomposition mechanism implying the dissociative vaporization of the oxide followed by the condensation of the metal. The proposed mechanism is supported by the microstructure of the product and of the reacting interface. The complex dependence of reaction rate from temperature was shown to be related to a collapse of the porous product to form a compact metal layer at higher temperatures due to sintering. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Endothermal decomposition Interface microstructure NiO
1. Introduction Numerous studies have been devoted, since the beginning of the last century, to the heterogeneous reduction of NiO and transition metals oxides [1–5]. Most of the work was aimed to the interpretation of the complex kinetic behavior observed experimentally, but a clear understanding of the overall reaction mechanism involved in this process is still missing. This is particularly true in the case of the carbothermal reduction that represents the most known, investigated, and technologically relevant of all the reduction processes [6,7]. The carbothermal reduction of the transition metal oxides (i.e. their reduction by solid carbon) does not require physical contact between the reactants to occur. All the proposed reaction mechanisms, in fact, involve the presence of gaseous intermediate species. The most known of such mechanisms involves the presence of CO(g) [6]: MOðsÞ D CO / MðsÞ D CO2 CO2 D CðsÞ / 2CO
(1)
other authors have on the other hand proposed a mechanism based on the reducing action of H2 adsorbed on the carbon surface [8]: MOðsÞ D H2 / MðsÞ D H2 O CðsÞ D H2 O / H2 D CO
(2)
* Corresponding author. Tel.: þ39 0382 987208; fax: þ39 0382 987575. E-mail address: fi[email protected] (F. Maglia). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.05.029
However, as pointed out by L’vov [6], although being able to account for some experimental evidences, these reaction schemes cannot be applied for a full interpretation of the reaction mechanism. In particular the amount of H2 is probably too small in comparison to the observed reaction kinetics and the restoration of CO is too slow at low temperature. Other authors have interpreted the carbothermal reduction of NiO through a dissociative scheme [6,9,10]: MOðsÞ / MðsÞ D ½ O2
(3)
where no reducing gas is involved and carbon simply acts as an oxygen getter maintaining a low oxygen partial pressure in the reaction environment. Despite its simplicity the dissociative mechanism received very little or no attention at all, as most of the literature is focused on the processes involving the presence of a reducing gas, such as CO or H2. The dissociative evaporation scheme was denied by the majority of researchers for many years since the equilibrium oxygen partial pressures required to perform this reaction were considered too low to account for the high reduction rate experimentally measured [6]. Despite that, of all possible mechanisms involved in the reduction of the metal oxide the dissociative process appears to be particularly attractive because of its simplicity and of its similarity with other processes, such as the dissociative vaporization and the endothermic decomposition of carbonates and hydroxides, whose reaction mechanism is better understood from the thermodynamic and kinetic point of view [11–14]. Aim of the present work was to investigate the decomposition of NiO under dissociative conditions, in order to investigate the
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fundamental steps involved in the reduction reaction. The main focus was on the investigation of the reaction kinetics and of the product microstructure. 2. Experimental methods The reduction kinetics was measured with a gravimetric method under isothermal conditions. Two different approaches have been used, one allowing continuous and one based on discontinuous measurements. In the first approach the reaction was performed in a homemade apparatus built around a Cahn RG automatic balance having a sensitivity of 107 g and a maximum load of 1 g (Fig. 1). The apparatus was designed to keep a strict symmetric arrangement. The quartz tubes enclosing sample and tare were symmetrically fastened to the stainless steel container of the balance (on top) and to a Pyrex connection with a common inlet (on bottom). Therefore the gas flux was symmetrically divided between sample and tare. Heating was provided by a twin furnace operated by two control/ power units in a master/slave configuration and driven by two thermocouples. An Analog Devices Input/Output (I/O) processor collected analog inputs from the balance and the thermocouples and sent the data to a personal computer through a serial interface. Since no gaseous reducing agent was employed in this study the reduction process could be obtained only when the P(O2) was below the Ni/NiO equilibrium value. Fig. 2 shows the dependence of the P(O2) on temperature for the Ni/NiO equilibrium in comparison with other metal/metal oxide equilibria. It can be noted that for temperatures below 1000 K the reduction of NiO requires extremely low partial pressures. In order to achieve such low oxygen contents a purification system based on metal getters was used. The metals were placed in two quartz tubes placed in the same furnace kept at 800 C under a flow of high purity argon (99.9998% nominal). A mixture of Fe/FeO was placed in the first tube and a mixture of Ti/TiO (or Zr/ZrO2) in the second one. This arrangement allowed to achieve oxygen partial pressures below the
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detection limit of our oxygen sensor (1025 atm) for a period of 10– 15 minutes. The experimental procedure was as follows. Each sample was heated to the desired temperature under a flux of argon coming directly from the cylinder. Under these conditions no reduction was observed at any temperature. As the sample reached the desired temperature, the gas flow was switched in order to let the purified gas to reach the sample. The reduction kinetics was also investigated with a discontinuous method. In this case the sample was placed at one end of a quartz tube sealed under vacuum. Few grams of Fe/FeO powders were placed at the other end of the tube. Quartz wool was placed in the middle in order to prevent any contact between the sample and the getter. The two ends of the tube (100 mm long and 5 mm in diameter) were inserted into two independent furnaces in order to control the sample and the getter temperature independently. The kinetics of the reduction was studied by rapidly cooling the sample, opening the vial, and weighing the sample at selected time intervals. After each measurement the sample was placed in a new vial. Microstructural characterization of the reacted samples was made by optical and scanning electron microscopy (SEM). A Zeiss Axioplan optical microscope and a Cambridge SEM Stereoscan 200 operated at 30 kV and equipped with a backscattered electron detector and a Link microprobe were used. Prior to SEM analysis the sample surface was coated with 20 nm of sputtered gold. 3. Results The experiments were conducted on monocrystalline samples (30–40 mg of weight) obtained by cutting a NiO single crystal prepared with a single ellipsoid image furnace. The weight loss vs. time measured by the Cahn electrobalance at different temperatures is reported in Fig. 3. The weight variations have been normalized to an initial external surface of NiO (in single crystal form) equal to 100 mm2 in order to have directly comparable results and have been shifted along the vertical axis for clarity.
Fig. 1. Schematic of the experimental set up: (1) microbalance, (2) sample, (3) tare, (4) furnaces, (5) temperature controllers, (6) oxygen sensor, (7) data acquisition system, (8) gas cylinder, (9) iron getter, (10) titanium getter, (11) thermocouples.
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The dependence of the reduction kinetics from temperature is peculiar. No significative weight loss was observed for temperatures below 330 C. Increasing the temperature the decomposition rate at first increased, but for temperatures above 540 C it started decreasing. A very slow decomposition rate, comparable with the one observed at 330 C, was observed at temperatures above 800 C. In all cases, however, the weight loss slowed down after 10–15 minutes because of the increase in the oxygen partial pressure produced by the reduced efficiency of the getters. This conclusion was confirmed by decomposition experiments performed with the discontinuous setup. The single crystal of NiO was in this case placed in a sealed quartz tube at a temperature of 540 C corresponding to a partial pressure for the equilibrium Ni/NiO equal to 8.5 1022 atm. At the other side of the quartz sealed tube were placed few grams of Fe/FeO powders. This end of the tube was kept at the temperature of 721 C corresponding to a partial pressure for the equilibrium Fe/FeO equal to 8.3 1022 atm. The weight loss vs. time at selected time intervals is shown in Fig. 4(a). It is evident as, differently from the reduction in the thermobalance, which slows down quickly, the sample reduced in the sealed tube keeps decomposing even after several hours. In Fig. 4(b) the data of Fig. 4(a) have been plotted taking into account the decrease of the NiO/Ni interface on the basis of a shrinking core model. The data, extending up to a 74% of reduction of the sample, show a marked linear trend. The superficial layer of metal produced by the reduction process shows a quite peculiar microstructure. In the samples reduced at the lowest temperatures (Figs. 5 and 6) the metallic layer shows an extremely high porosity characterized by a typical interconnected microstructure. The size of the pores is about 1 mm, but the thickness of the wall separating the pores is in the nanometric range (Fig. 7). When the reaction temperature is increased the pore size also increases and a collapse in the microstructure is observed (Fig. 8). In the samples reduced at the highest temperature (850 C) the porosity is almost completely absent and a fairly compact metal layer adherent to the NiO surface is observed (Fig. 9).
Fig. 2. Equilibrium oxygen partial pressure for the following equilibria: (1) Ni/NiO, (2) Fe/FeO, (3) Ti/TiO, (4) Zr/ZrO2.
Fig. 3. Weight loss for NiO single crystal as a function of time for different annealing temperatures. Data normalized to an initial external area for NiO equal to 100 mm2.
4. Discussion The experimental results reported in this work show how the reduction of single crystals of NiO can be performed to a significant extent in the absence of any reducing gas at temperatures as low as 330 C. This result allows to gain some significant information on the mechanism and the kinetics of the reduction of transition metal
Fig. 4. (a) Weight loss kinetics obtained by the discontinuous (sealed tube) at 540 C (data normalized to an initial external area for NiO equal to 100 mm2); (b) reduction kinetics (discontinuous method) taking into account the decrease of the NiO/Ni interface on the basis of a shrinking core model.
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Fig. 5. SEM image of the product (Ni) zone for the sample reduced at 330 C. The NiO– Ni interface is visible in the lower left corner.
Fig. 7. Higher magnification SEM image of the NiO–Ni interface for the sample reduced at 330 C.
oxides in its simplest form and in its earliest stages, showing characteristics that are generally not evident when higher temperatures or reducing gases are used. In our experimental approach, in fact, the reduction derives only from a simple dissociative mechanism producing only one gaseous species. Furthermore, because of the low temperatures, far away from the melting point of the reduced metal, the diffusivity in the product layer is very limited, allowing to observe the microstructure of the product layer in its early form, as it is first produced by the chemical process, with very limited subsequent modification. The microstructure of the produced metallic layer shows some very peculiar characteristics. The most striking morphological feature is represented by the continuous and interconnected isotropic nature of this microstructure. As shown in Figs. 5–7 the layer is made by a metallic sponge whose porosity appears to be completely open, as all the cavities seem to be connected to each other. When the reaction is performed at the lowest temperatures, the layer microstructure appears to be remarkably isotropic with no apparent correlation with the layer direction of growth. The Ni/NiO interface (Fig. 7) appears to be quite irregular. It is remarkable, however, as even in this region the porosity seems to maintain in the same scale. Observations performed at very high magnification failed to show any evidence of smaller porosity even at the very interface. This observation seems to exclude the possibility of a mechanism of nucleation and growth for these pores. The lack of evolution with time seems to confirm that the observed microstructure represents the primary product of the chemical decomposition step. Complex
microstructures are often the result of decomposition reactions when the process is performed in conditions very far from the equilibrium. A typical example is represented by the microstructures deriving from the endothermal decomposition of carbonates or hydroxides performed in high vacuum [12,13,15,16]. These complex microstructures originate from the requirement to accommodate the large decrease in molar volume without the possibility of a significant decrease in the overall volume due to the limited bulk diffusivity. In the case of the NiO decomposition, though, the microstructure does not seem to be compatible with a mechanism involving only local rearrangements. As noted in the case of carbonates and hydroxides’ decomposition [12,13,15,16] and, more recently, in the case of void organization in partial reduction of NiO [17], the microstructures deriving from these local rearrangements are always nanometric in nature, with a pore size that is in the range of few nanometers and show always some sort of correlation with lattice structure of the parent material. In some cases a topotactic growth is observed [18,19]. The microstructure observed in our study, though, is quite large. Although the thickness of the walls separating the pores is surely in the nanometric range, the pores themselves are well in the micrometric range. It is hard to believe that such a large microstructure could be the result of a near-diffusionless process. Note that as in the proximity of the interface, where the decomposition process is still uncompleted, the internal surface of the pores is coated by a thin, well adherent, and apparently compact layer of metal (Fig. 7), a feature that does not seem to be compatible with
Fig. 6. Higher magnification SEM image of the product (Ni) zone for the sample reduced at 330 C.
Fig. 8. SEM image of the product (Ni) zone and the NiO–Ni interface for the sample reduced at 830 C.
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Fig. 9. SEM image of the product (Ni) zone and the NiO–Ni interface for the sample reduced at 850 C.
a scenario that excludes the possibility of long range diffusion. Furthermore, as we noted before, the product microstructure seems to be isotropic and independent from the direction of growth and lacks any correlation with the NiO lattice. Although no clear interpretation for the actual morphology can be put forward at the moment, we will return on this point in the following. As the temperature is increased clear evidence of modification in this microstructure starts to appear due to the increased diffusivity in the metallic layer (Fig. 8). Eventually a complete collapse in the porous structure is observed at the highest temperatures, leaving an almost completely compact layer of metal (Fig. 9). This evolution in the microstructure explains some of the kinetic features shown by the reaction at the different temperatures. At the lowest temperatures a remarkably linear kinetics is observed (Fig. 4). This implies a kinetics totally controlled by an interfacial step. Such a process allows for a fast reaction rate even at temperatures as low as 330 C. This is a quite surprising result, considering that traditionally the reduction of transition metal oxides is a considered reaction characterized by a very high activation energy, requiring high temperatures to be performed with an appreciable kinetics. Our results, however, confirm that these characteristics are not related with the intrinsic properties of the chemical process, but are the result of a complex mechanism mostly controlled by the ability of the produced layer to allow the oxygen release. The most important parameter that controls the overall reaction kinetics is in fact represented by the porosity of the metal product layer. If a compact surface layer of metal is formed the reduction rate is completely determined by the transport of oxygen through the metallic layer and a parabolic growth of the metal layer can be forecast [11]. If the product layer is not compact, on the other hand, as long as the porous layer is sufficiently thin to oppose negligible resistance to the gas evolution, the kinetics will be controlled by the interface reaction [11]. As a result, temperature plays a complex role by controlling not only the intrinsic kinetics of the primary interfacial chemical process, but also the rate and degree of sintering of the product layer. The importance of the porosity of the product layer on the overall kinetics has been recognized before. In several systems, the reaction rate, after an initial increase with temperature due to the increased surface reaction rate, shows a marked reduction due to the increased product sintering [2]. Several kinetic models, based on the well-known shrinking core scheme [1–3,20,21], have been proposed for the description of the reduction of metal oxides. These models are characterized by different levels of complexity with the aim of taking into account the complexity of the process described above plus other features such as kinetics of nucleation of the metal phase, autocatalytic
effects, multiple product phase, etc. In all these cases, however, it has never been realized that the primary chemical step is characterized by a high intrinsic chemical kinetics that appears to be controlled only by an interfacial process. Another noteworthy characteristic of the kinetic data of Figs. 3 and 4 is represented by the complete absence of any induction or acceleratory period. This absence is particularly relevant as the presence of these two characteristics is considered typical for this type of reduction process. In some cases they have been used to support some specific reaction mechanisms. L’vov [9] suggested that an initial induction and a subsequent acceleratory (autocatalytic) period are related with the formation and growth of nuclei of the metallic product from the gas phase. These interpretations, however, are based on data on the kinetics of carbothermic reduction at high temperatures where, as we discussed earlier, the intrinsic kinetics is strongly influenced by the sintering of the metallic layer. Our data, on the opposite, show that despite the low temperatures the decomposition process proceeds with a very regular kinetics excluding the possibility that heterogeneous nucleation can be a process controlling the overall kinetics. More information on the nature of the mechanism responsible of the chemical decomposition can be obtained from the quantitative analysis of the kinetic data. When the mechanism of the reaction is controlled by a simple decomposition step like Equation (3) the maximum reaction rate can be calculated on the basis of a Hertz–Knudsen–Langmuir [22–24] model. In such approach the maximum reaction rate is controlled by the maximum flux of gas molecules, J, that can emerge from the solid phase:
Peq J ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRT
(4)
M is the molar mass of the gas, and Peq is its equilibrium pressure. This flux identifies an upper limit for the reaction rate in the approximation that no other mechanism, besides the pure gas effusion, is controlling the rate of oxygen release. Obviously in the presence of an impervious layer of product or of a kinetically limiting chemical step a slower kinetics is expected. In such cases a vaporization coefficient av is introduced that equals the ratio between the measured pressure and the equilibrium one:
av ¼
P : Peq
(5)
The maximum reaction will be given by:
Peq J ¼ av pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRT
(6)
In almost all cases av 1. This dissociative evaporation scheme has been proven to be able to explain the mechanism and kinetics of thermal decomposition of various oxides as well as nitrates, carbonates, etc. av values of 102–103 are found for many oxides while values as low as 1040 are typical of other substances such as oxalates and azides [9]. Table 1 shows for our experiments’ calculated and experimental values for J together with the relative values of av in the case of the decomposition of a single crystal of NiO at various temperatures. Surprisingly, the observed experimental fluxes appear to be anomalously high and the resulting av values are higher than 108. Values of av 1 are extremely rare, as they imply the presence of a partial pressure of the gaseous component higher than the equilibrium value; an eventuality quite unrealistic. A similar anomaly is mentioned by L’vov [9] in the case of the carbothermal reduction of NiO, CuO and FeO, although no direct experimental data are presented. No direct evidence of abnormally high values of av in the case of direct decomposition has ever been presented before.
F. Maglia et al. / Solid State Sciences 11 (2009) 1686–1691 Table 1 Calculated and experimental values for the oxygen flux, J, and the relative values of av for the decomposition of NiO single crystals at different temperatures. Sample
T ( C)
Equilibrium P(O2) (atm)
Calculated J ¼ dn/dt (mol cm2 s1)
Experimental J ¼ dn/dt (mol cm2 s1)
av
M1 M2 M3 M4 M5 M6 M7
830 680 600 540 450 380 330
9.76 1014 2.74 1017 1.10 1019 8.52 1022 1.27 1025 2.49 1029 1.64 1032
3.26 1014 9.83 1018 4.12 1020 3.31 1022 5.23 1026 1.08 1029 7.40 1033
1.04 108 6.25 108 1.30 107 3.38 107 1.72 107 3.64 108 2.60 108
3.19 105 6.40 109 3.16 1012 1.02 1015 3.29 1018 3.37 1021 3.51 1024
L’vov provides an interpretation of this anomalous behavior introducing a slight but important modification in the mechanism of the direct decomposition reaction as presented in Equation (3): MOðsÞ / MY ðgÞ D ½ O2 ðgÞ
(7)
here the symbol MY(g) refers to the metal formed in gas form followed immediately by its condensation. In this approach the controlling step would be a dissociative vaporization of the oxide, followed by the condensation of the metal. In L’vov model half of the energy released by the condensation of the metal would be returned to the reactants, increasing the kinetics of the decomposition process itself. The author demonstrates that av > 1 can be derived from this model when the following thermodynamic requirements are met:
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in condition far from the equilibrium, as it would be ejected from the solid with limited chances of equilibration, making the application of the Hertz–Knudsen–Langmuir Equation (4) questionable and, suggesting a possible interpretation for the abnormally high value of av. 5. Conclusions The presented experimental results confirm that the decomposition of single crystals of NiO can be achieved through a dissociative mechanism at temperatures as low as 330 C. At temperatures below 500 C the decomposition proceeds with a surprisingly fast kinetics, showing decomposition rates that are orders of magnitude higher than the values calculated on the basis the Hertz–Knudsen– Langmuir theory. When not limited by the efficiency of the oxygen getter the reaction shows a constant decomposition rate, suggesting an interface-controlled mechanism. In these conditions the Ni metal is produced with a remarkable spongy microstructure. At higher temperatures this porous microstructure collapses, due to sintering, reducing the rate of further decomposition. As a result, the slow kinetics and the high activation energy generally observed in the high temperature reduction of NiO appear to be determined by the transport of the gaseous products through the metal layer and not by the intrinsic characteristics of the chemical process. Since at the lowest temperatures no significant bulk diffusivity is expected in the solid state, it is suggested that in these conditions the reaction proceeds through a mechanism involving a dissociative vaporization followed by a condensation of the metallic product. Acknowledgments
DH298 ðMOÞ DH298 ðMÞ 4DH298 ðMOÞ where the above terms represent the enthalpies of formation of gaseous M and solid MO at 298 K. Besides his thermodynamic soundness this model has the advantage to allow a possible interpretation for the puzzling microstructure of the product layer shown in Figs. 5 and 6. One of the remarkable features of this microstructure is represented by the complete lack of correlation with the direction of growth, together with the absence of any correlation between the crystal lattice of the reactants and of the products. As we noted earlier, the microstructure of Figs. 5 and 6 seems to imply a large scale reconstruction of the Ni lattice, a process deriving from a massive bulk diffusion. The temperature, however, is too low to justify massive solid-state diffusion, but it could be acceptable if a gas-phase transfer process is involved. In other words, it can be suggested that the observed microstructure derives from the sudden vaporization of fairly large regions of NiO close to the interface, followed by immediate condensation of the metal producing a uniform coating of the internal surface of the produced pore. The layer of metal prevents any further decomposition and could explain the lack of evolution in the pore morphology. The possibility of a decomposition process occurring in a sequence of bursts has been proposed in a recent review on the decomposition mechanism in calcium carbonate by Beruto et al. [25]. These authors suggested that release of CO2 from a fairly large region close to the interface can produce a layer on nonporous CO2 deficient calcite that can then undergo a structural collapse releasing the stress and ejecting any remaining CO2. In the case of the decomposition of NiO it could probably be coupled with the process represented by the reaction (7) producing bursts of vaporization that involve fairly large interfacial regions. It must also be noted that in this view the gas phase deriving from this process would probably be
We thank Prof. Klaus Becker for kindly supplying the NiO single crystals. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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