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Sintering of nickel particles supported on γ-alumina in ammonia
Applied Catalysis A: General 228 (2002) 145–154
Sintering of nickel particles supported on ␥-alumina in ammonia Johan Lif a,b,∗ , Magnus Skoglundh a , Lars Löwendahl a a
Department of Applied Surface Chemistry, Chalmers University of Technology, S-41296 Göteborg, Sweden b Akzo Nobel Functional Chemicals AB, S-44485 Stenungsund, Sweden Received 6 September 2001; received in revised form 8 November 2001; accepted 9 November 2001
Abstract Sintering of nickel on ␥-alumina in hydrogen, ammonia, ammonia + hydrogen and ammonia + nitrogen was investigated isocoric in the temperature range 483–523 K. The sintering process was followed by hydrogen chemisorption, specific surface area measurements, X-ray diffraction and transmission electron microscopy. After exposure to ammonia + hydrogen, hydrogen chemisorption revealed a fast decline of the nickel surface area. The results from X-ray diffraction and transmission electron microscopy are consistent with the result from the chemisorption experiments. The BET analysis showed that there are no losses of the total surface area. The sintering rate was significantly slower if the heat treatment was performed in the presence of only one of the gases, ammonia or hydrogen, or in ammonia + nitrogen. This indicates that the sintering in the temperature range 483–523 K only occurs with reduced nickel particles in ammonia + hydrogen. The prevailing mechanism for the sintering seems to be sintering by particle migration. The kinetics of the sintering process indicates that the rate constant is correlated to more factors than the temperature. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sintering; Nickel; Alumina; Catalyst deactivation; Ammonia; Hydrogen; Chemisorption
1. Introduction Supported nickel catalysts are widely used in various industrial processes like hydrogenation, dehydrogenation, methanation, amination and petroleum reforming [1,2]. Exposure of supported nickel catalysts to high temperatures in different chemical atmospheres leads to a decrease of the number of active sites and, hence, loss of catalytic activity [3–6]. One of the most serious aspects of thermal degradation manifests itself as growth of the metal particles with concomitant loss of active metal surface area. This phenomenon is normally referred to as sintering ∗ Corresponding author. Tel.: +46-303-853-29; fax: +46-303-887-86. E-mail address: [email protected] (J. Lif).
of small metal particles into larger ones with lower surface-to-volume ratio. Two different mechanisms have been proposed for sintering of supported metal catalysts, particle migration and atomic migration. Particle migration includes migration of particles on the support, collisions between the particles and their coalescence [6–11]. Atomic migration, often called Ostwald ripening, includes escape of atoms or molecules from the particle, transport of these clusters across the support or in the gas phase and capture of these clusters by collision with a metal particle [6,12–16]. The driving force for sintering is to minimise the surface energy by increasing the co-ordination number of the surface atoms. Larger particles are more stable than smaller ones due to the fact that the metal–metal bond energies often are considerable
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higher than the metal–support interactions. Typical values for detachment of a nickel atom from a nickel cluster is 431 kJ/mol [17] whereas the binding energy between metal particles and supports are in the range 5–15 kJ/mol [13,18]. Sintering is generally observed at temperatures above 700 K and the process shows strong temperature dependence [4,19]. The activation energy for atomic migration is normally much higher than for particle migration [4], sintering will consequently proceed via particle migration at low temperatures. Factors affecting the stability of the metal particle towards sintering include surrounding atmosphere (gas composition), metal–support interaction, particle shape, particle size, support roughness, pore size and impurities of the support or in the metal. The literature gives contradictory information of the importance of the atmosphere, several authors report that oxidising atmospheres give more severe sintering compared to inert or reducing atmospheres [15]. Bartholomew, however, observed that this is not a general case, he also found that adsorbed species like H, H2 and H2 O change the surface structure and, therefore, the rate of sintering [18]. For example, water vapour is proposed to increase the mobility of metal atoms or metal particles on the support due to adsorbed water or hydroxyl groups and, hence, accelerate the sintering [18]. This points out the importance of the surface energy, which is dependent on the gas composition. For instance, nickel oxide interacts strongly with SiO2 and is more stable in air than in H2 [18]. The presence of metal–support interactions affects the spreading, wetting and re-dispersion of the metal on the support. The pore structure of the support can determine the final particle size of the nickel particles [10], and reactions between the support and the active nickel particles or phase transitions of the support also affect the sintering process [20]. Reactions between the nickel particles and the feed, for instance carbon monoxide, can also accelerate the sintering [20,21]. The kinetics of catalyst deactivation is a function of time, temperature, pressure, concentration of different substances, etc. This change of activity can be an effect of one or several of the earlier mentioned processes. For sintering, the kinetics can easily be derived from time versus metal surface area measurements at constant temperature if all other earlier mentioned
processes are controlled. The activity plot usually shows two different regimes, one fast exponential loss during the initial stage and, thereafter, a slower linear loss [5,6,19] or an asymptotic approach of the dispersion towards infinite time [17,22–24]. Fuentes describes the kinetics of the sintering process by the generalised power law equation (GPLE) [4,22–24]. This equation fits sintering rate data closely and gives first- or second-order reactions for most published sintering data [4], which enables a direct quantitative comparison between different studies of sintering [18]. The number of published investigations of sintering is large and there are several good reviews in this area [4,6,15–17,24–29]. The number of papers that concerns sintering of nickel particles on ␥-alumina is, however, few and all of them concern atmospheres like He, H2 , N2 , H2 /H2 O and H2 /CO [4,5,16,20,21,25]. Presently, we have not found any papers dealing with the sintering of nickel particles on ␥-alumina in the presence of ammonia or ammonia + hydrogen. The present study aims to study the sintering behaviour of nickel particles on ␥-alumina in ammonia and/or hydrogen atmospheres, in view of the industrial application as an amination catalyst. We recognise that our work only concerns hydrogen and ammonia atmospheres and not any species that could react during the sintering. Hydrogen chemisorption, specific surface area measurements, X-ray diffraction and transmission electron microscopy were used to follow the sintering process.
2. Experimental 2.1. Catalyst preparation A 10 wt.% nickel on alumina catalyst was prepared by the incipient wetness method. The support material, ␥-alumina tablets Al-4504T 1/8 in. with 90 m2 /g received from Engelhard, used as received, was impregnated with an aqueous solution of nickel nitrate (Ni(NO3 )2 ·6H2 O) from Merck. The impregnation was done with a Ni(NO3 )2 concentration of 85% at a pH of 2.3. The impregnated sample was then dried at 393 K for 2 h and finally calcined in air at 673 K for 4 h. The preparation procedure yielded a homogenous nickel distribution in the alumina tablets. A large amount of
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catalyst was prepared and samples for all the experiments were taken from the same batch. To establish homogeneous sampling and to avoid mass and heat transfer limitations in the experiments, the catalyst was crushed and sieved and a particle size fraction between 150 and 300 m was used. The nickel dispersion and the specific surface area of the fresh catalyst was 7.2% and 87.5 m2 /g, respectively.
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and the tube was evacuated. Ammonia, 10.5 g, was introduced and the tube was pressurised with hydrogen or nitrogen to 4.2 MPa at room temperature. The tube was placed in a hot furnace at 483, 498 or 523 K, and kept there for the desired time. The pressure at the highest temperature, 523 K, in the ammonia + hydrogen atmosphere was 22.0 MPa. After the heat treatment, the tube was cooled to room temperature. The pressure was registered and the tube was evacuated. The catalyst was then taken out into air.
2.2. Heat treatment 2.3. Sample characterisation The catalyst was subjected to heat treatment in ammonia, ammonia +hydrogen, ammonia +nitrogen and hydrogen atmospheres for up to 160 h at the temperatures 483, 498 and 523 K (see Table 1). The heat treatment conditions, i.e. atmosphere, temperature and pressure, were chosen to simulate sintering of supported nickel under reaction conditions typically for amination. The general applied reaction conditions for amination over supported metal catalyst are, thus, in the temperature range 453–523 K under a total pressure between 5 and 30 MPa [30]. The heat treatments were performed in a stainless steel tube with a volume of 95 ml. The catalyst, a fresh sample for each heat treatment, was placed in the tube
After the heat treatment, the samples were divided into several portions where different portions were used for different analysis. This was done to ensure no correlation between the treatments in the different analyse methods. 2.3.1. Chemisorption measurements An ASAP 2010C instrument from Micromeritics was used to determine the nickel surface area of the catalysts by hydrogen chemisorption. The measurements were carried out volumetrically using a constant volume chemisorption system capable of a dynamic vacuum of 0.05 Pa. The catalyst was reduced at 723 K
Table 1 Specific uptake of chemisorbed hydrogen and dispersion of nickel for fresh and sintered samples after thermal treatments in different atmospheres Sample
Atmospherea
Temperaturea (K)
Timea (h)
1 (Reference)d 2 3 4d 5 6 7 8d 9d 10 11 12 13
NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 NH3 /N2 H2
483 483 483 498 498 498 523 523 523 523 523 523
23 73 124 23 66 122 23 90 162 144 112 92
a
H2 uptakeb (mol H2 /g sample)
Dispersionc (%)
61.6 48.2 37.9 37.5 44.2 28.6 29.0 34.4 21.4 18.7 46.0 50.9 51.3
7.22 5.67 4.45 4.40 4.96 3.38 3.38 4.04 2.54 2.18 5.40 5.97 6.01
Heat treatment conditions. Volume of hydrogen uptake at 308 K extrapolated to zero. Mean value of four repeated measurements. c Metal dispersion is calculated with the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms, and assuming a nickel cross-section area of 0.0649 nm2 [31]. d Mean values of two to four different analyses. b
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Table 2 Experimental procedure during the chemisorption measurements 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Flow of He at 373 K for 10 min Flow of He at 623 K for 10 min Evacuation at 673 K for 5 min Reduction in flowing H2 at 673 K for 120 min Evacuation at 673 K for 20 min Evacuation at 308 K for 10 min Leak test at 308 K First analysis with H2 at 308 K Evacuation at 308 K for 30 min Second analysis with H2 at 308 K
with hydrogen at atmospheric pressure for 2 h, thereafter, evacuated at 723 K for 20 min. The chemisorption measurements were performed at 308 K. A detailed description of the experimental procedures during the chemisorption is given in Table 2. As seen in the following sections, this high temperature (723 K) is needed to obtain reproducible analysis, it may, however, also affect the dispersion of the supported nickel particles. The decrease of metal surface area during the reduction was controlled by a heat treatment experiment with hydrogen atmosphere (sample 13, Table 1). The reduction of metal surface area was 17% after heat treatment for 92 h in hydrogen at 523 K with a pressure of 22.0 MPa. Bartholomew reported a reduction of metal surface area by approximately 5% after heat treatment for nickel on alumina in hydrogen at 923 K for 25 h [25]. To elucidate the needed reduction temperature, chemisorption measurements were also performed at a lower reduction temperature, 623 respective 673 K. However, the results were unreliable as the deviation between different measurements was too large, which probably is due to partial reduction of nickel. The chemisorption isotherm are obtained as the difference between the first adsorption measurement, the sum of both chemisorption and physisorption, and of the second adsorption measurement, which consist of only physisorption. After the first adsorption measurement, the sample is evacuated at 623 K to release all physisorbed hydrogen followed by the second adsorption measurement. The chemisorption isotherm is obtained by plotting the adsorbed amount of hydrogen versus the hydrogen pressure and then extrapolated to zero pressure to obtain the hydrogen uptake due to chemisorption on nickel. The nickel dispersion and
nickel particle size are calculated assuming the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms [31]. Each sample was analysed in a sequence of four individual measurements, where each measurement was pre-treated identically (see earlier sections). The hydrogen uptake by chemisorption was calculated as a mean value of the three last measurements for each sample. The deviation between those measurements was approximately 3%. 2.3.2. Specific surface area measurements The specific surface areas of the fresh and the most sintered sample, treated at 523 K for 162 h in ammonia + hydrogen, were determined by nitrogen adsorption according to the BET method using an ASAP 2010C instrument from Micromeritics. The BET surface area determinations were based on five experiments at relative pressures of nitrogen in the range of 0.05–0.21. 2.3.3. X-ray diffraction (XRD) The crystalline phases present in the fresh sample and the samples treated at 498 and 523 K in ammonia+ hydrogen were identified by XRD. The analysis was performed in air at room temperature with a Siemens D5000 powder diffractometer, with Cu K␣ radiation and a grazing incidence beam equipped with a Göbel mirror. The obtained diffraction patterns were matched to literature data [32]. The nickel particle size is calculated using line broadening with the Debye–Scherrer equation for the main peak indicating metallic nickel, θ = 44.5◦ . 2.3.4. Transmission electron microscopy (TEM) The fresh catalyst and the sample treated at 573 K for 162 h in ammonia + hydrogen, were studied by TEM. The catalyst was ground to an alumina particle size of a few micrometers, suspended in methanol and placed on a copper grid. Several different areas in each sample were studied with different magnifications. The TEM technique was not used to statistically determine the size distribution of nickel particles, but only for observing the size and shape of individual particles. The instrument used was a Philips CM200 FEGTEM furnished with a Link Isis EDX system. The digital micrographs were taken by a slow scan CCD camera from Gatan.
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3. Results The hydrogen uptake due to chemisorption for the fresh and for samples exposed to different atmospheres at different temperatures is listed in Table 1. The normalised metal surface area, i.e. metal surface area after heat treatment divided by initial metal surface area (sample 1), is plotted as a function of time in Figs. 1 and 2. The hydrogen uptake for the fresh catalyst and for samples 4, 8 and 9 was analysed several times. Those analyses were used to establish the deviation between the different chemisorption analyses. The derivation of the hydrogen uptake in the different measurements was ±5%. Samples 2–10, which are heat treated at 483, 498 or 523 K in the presence of both ammonia and hydrogen, show a rapid decline of the normalised metal surface area during the first 25 h followed by a more gradual decline (Fig. 1). The loss of metal surface area increases with increasing temperature during the heat treatment. After 150 h at 483, 498 and 523 K with both ammonia and hydrogen present the metal surface area has declined with 39, 53 and 70%, respectively. Sample 11 with only ammonia present during the heat treatment shows a relatively small reduction (25%) of the metal surface area after 144 h at 523 K
Fig. 1. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at various temperatures in ammonia+hydrogen. Symbols correspond to experimental data points, the curves to calculated data based on first-order GPLE kinetics and kd is 0.0391, 0.0451 and 0.0388 for 483, 498 and 523 K, respectively.
Fig. 2. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K in different atmospheres. Symbols correspond to experimental data points, the curve to the calculated data based on first-order GPLE kinetics for sintering at 523 K in ammonia + hydrogen.
compared to the heat treatment in the presence of both ammonia and hydrogen at 523 K (70% for sample 10, see Fig. 2). The same behaviour can be seen for heat treatments in a mixture of ammonia + nitrogen, which gives a decrease of 17% after 112 h at 523 K (sample 12), or in only hydrogen, which gives a decrease of 17% after 92 h at 523 K (sample 13, see Fig. 2). The measured total specific surface area of the fresh catalyst (sample 1) and the sample heat treated at 523 K for 162 h in ammonia + hydrogen (sample 10) is almost equal, 87.5 and 86.2 m2 /g, respectively. X-ray diffraction was performed of the fresh catalyst (sample 1) and for the heat treated samples at 498 and 523 K in ammonia + hydrogen (samples 5–10). Fig. 3 shows X-ray diffractograms for the fresh catalyst (sample 1) and for the three samples (8–10) that have been heat treated at 523 K. For all of the samples the diffractogram shows a typical pattern for ␥-alumina. For the fresh catalyst, no other peaks could be revealed. However, for the heat treated samples, the diffractogram shows peaks at 44.5 and 51.8 which are attributed to metallic nickel. Those nickel peaks become more intense with the heat treatment time, which indicates that the amount of detectable crystalline nickel is enhanced. The nickel particle size is calculated from line broadening of the main X-ray diffraction peak for metallic nickel and compared
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4. Discussion
Fig. 3. Powder X-ray pattern during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K during different times. The different profiles has an offset of 15 in-between and the literature data is indicated for (䉱) metallic nickel; (䉲) ␥-alumina; (䊉) nickelaluminate; and (䊏) nickel oxide.
with the particle size calculated from the hydrogen chemisorption [31], the result is listed in Table 3. Two representative micrographs were chosen from the TEM measurements (Figs. 4 and 5). The TEM micrographs of the fresh catalyst (sample 1) show nickel particles with a diameter of approximately 10 nm (Fig. 4). For the sample heat treated at 523 K for 162 h in ammonia + hydrogen (sample 10), only large, faceted, well-dispersed particles are found. The diameter of those large nickel particles was 50–100 nm (Fig. 5).
The results of this investigation describes the sintering behaviour of a supported nickel on alumina catalyst in an ammonia + hydrogen. The different analytical methods used here have provided a picture of a rapid growth of the metal particles during the first 25 h. At longer heat treatment times will the growth of the metal particle decline. The hydrogen chemisorption provides data for the different heat treatment times, which have been fitted with good correlation to the first-order GPLE: −d(D/D0 ) D D∞ m − (1) = kd dt D0 D0 where D is the dispersion, D0 the initial dispersion, D∞ the account for the observed asymptotic approach of the dispersion at infinite time, kd the rate constant, and m is the sintering exponent. The correlations have been done with a D∞ set to the highest heat treatment time for each temperature, respectively [4,22–24]. The assumption that D∞ is at 120–160 h gives a good correlation to experimental data. Correlation to the second-order GPLE does not give as good fit as the first-order GPLE correlation (Fig. 6). The measured decrease in hydrogen uptake with increasing temperature and exposure time in ammonia + hydrogen using hydrogen chemisorption by itself is not an evidence for the sintering. The reduction of metal surface area could also be due to other mechanisms like coking, volatilisation, reaction between nickel and alumina or collapse of the
Table 3 Nickel particle size calculated from hydrogen chemisorption and X-ray diffraction Sample
Atmospherea
Temperaturea (K)
Timea (h)
Particle size calculate from H2 chemisorptionb (nm)
Particle size calculate from XRDc (nm)
5 6 7 8 9 10
NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2
498 498 498 523 523 523
23 66 122 23 90 162
38.3 56.2 56.2 47.0 74.8 87.2
21.4 44.0 – 38.4 56.3 66.7
a
Heat treatment conditions. The mean value of the equivalent spherical particle diameter is calculated with the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms, a nickel cross-section area of 0.0649 nm2 and a nickel density of 8.9 g/cm3 [31]. c Particle size the Debye–Scherrer equation [31]. b
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Fig. 4. Transmission electron microscopy micrograph of nickel particles supported on ␥-alumina in the fresh sample (sample 1).
support with entrapment of nickel in the pores. To verify that the reduction of metal surface area essentially is caused by particle growth, some of the samples were analysed using TEM, nitrogen adsorption and XRD. The TEM measurements show the same trend as the hydrogen chemisorption with larger metal particles in the sample heat treated in ammonia + hydrogen than in the fresh catalyst. There are no small particles to be found in the sintered sample, only large crystallites of single crystals. The lack of small particles indicates that the sintering is caused by particle migration [3,28]. The specific surface area measurements showed that the total surface area in the catalyst is almost constant during the heat treatment in ammonia + hydrogen, indicating that there is no support collapse of the
␥-alumina. For the nickel on ␥-Al2 O3 systems, support collapse has been reported to occur above 800 K in the presence of water and at even higher temperatures in absence of water [25]. The XRD results support the results obtained by the chemisorption study (Fig. 3). The fresh sample does not provide any peak for nickel oxide, which is due to the fact that nickel oxide diffraction pattern is covered by the ␥-alumina diffraction pattern [32]. The ␥-alumina support is to a large extent amorphous and will, therefore, give broad peaks. The heat treated samples have two peaks indicating metallic nickel. Those nickel peaks has a correlation between the magnitude of the peaks and the time of the heat treatment, which verifies the result from the chemisorption measurements, i.e. growth of the nickel particles.
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Fig. 5. Transmission electron microscopy micrograph of a sintered nickel particle on ␥-alumina, after heat treatment in ammonia + hydrogen at 523 K for 162 h (sample 10).
When the sample is pre-treated in ammonia + hydrogen at 523 K, the nickel particle size calculated from hydrogen chemisorption is 47, 75 and 87 nm after 23, 90 and 162 h, respectively. The corresponding values from the XRD measurements are 38, 56 and 67 nm. Although the absolute values of the calculated particle size differ, the both methods show the same trend. The particle size increases with increasing exposure time in ammonia + hydrogen. The reaction between metallic nickel and alumina resulting in formation of nickel aluminate is a problem at high temperatures, above 800 K [6,24]. The absence of new diffraction peaks, except for metallic nickel, in the X-ray diffractogram indicates that no new crystalline phases have been formed in the catalyst during
the heat treatments. The XRD has also a limitation of the detectable particle size of 2–4 nm [16]. 4.1. The effect of temperature Although previous studies of nickel on ␥-Al2 O3 have shown that sintering generally becomes important at temperatures above 700 K [4,5,16,20,21,25]. The present work has shown that sintering could be a severe problem already at 500 K in ammonia + hydrogen atmosphere. When the different heat treatments are correlated to the first-order GPLE, the different temperatures gives a sintering rate constant, kd , of 0.0391, 0.0451 and 0.0388 h−1 for 483, 498 and 523 K, respectively. If kd
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Fig. 6. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K in ammonia + hydrogen atmosphere. Symbols correspond to experimental data points, the curves to the calculated data based on first- and second-order GPLE kinetics.
is assumed to be the k in the Arrhenius equation, calculation gives an apparent activation energy close to zero (−1.5 kJ/mol), which not is likely. This surprisingly low Ea is not likely when there is a clear correlation between the temperature and the final particle size at infinity time and it is not consistent with previous studies [14]. One explanation for the low activation energy could possibly be found if particle migration is the prevailing sintering mechanism. The particle migration process consists of transportation of nickel particles on the surface and coalescence of those nickel particles. The movement of the nickel particles is probably a slightly endothermic process due to that bonds between the nickel particle and the support are broken and reformed during the movement. The coalescence of the nickel particle has a low activation energy and is an exothermic reaction, which gives a large energy contribution to the sintering process and, therefore, could the apparent activation energy be close to zero. The Ea could also be affected by the deviation in measured hydrogen uptake. The deviation was determined to be ±5%, which would give as most an activation energy of 4 kJ/mol. This activation energy is quite low for a process like this, other investigations of nickel sintering have achieved activation energies between 25 and 160 kJ/mol depending on the experimental parameters [4]. Considering the low temperature and that no small particles were found in the TEM study indicate that
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the sintering proceeds via particle migration rather than atomic migration. This is not fully verified in this investigation and will be clarified in a coming investigation by direct examination of metal crystallite location and size as a function of time, temperature and atmosphere. The decrease of normalised metal surface area, i.e. the final particle size at infinity time, seems to be dependent on the temperature, which is consistent with previous investigations [18,22–24,33]. This could be explained by the fact that particles of a certain size need a certain temperature to move [33]. When the heat treatment starts, there is a particle size distribution of small particles. During the initial stage of the heat treatment, those small particles migrate, collide and coalescence with other particles. Particles can move if they have a size smaller than the maximum mobile size [33]. Pore structure, particle size, particle shape and bonds between the particle and the support affects the mobility of the particles. The particle migrating process continues as long as there exist moving particles. 4.2. The effect of atmosphere The sintering is less sever for heat treatments with only ammonia, only hydrogen or ammonia + nitrogen at 523 K than for the heat treatment with ammonia + hydrogen present at 523 K, i.e. the pressure of both ammonia and hydrogen is crucial for the sintering behaviour. Since almost no sintering takes place during heat treatment in ammonia+nitrogen, at high pressure, sintering in ammonia is not induced by high pressure without hydrogen. With the reducing atmosphere of hydrogen, the nickel particles will be reduced to metallic nickel (Ni0 ), ammonia by itself will not be able to reduce nickel oxide particles under the reaction conditions used in this study. Ammonia will adsorb to the reduced metallic nickel and the binding energy will be in the range of 60–90 kJ/mol [34]. Metallic nickel can chemisorb and dehydrogenate–re-hydrogenate ammonia [30,35]. This reaction probably changes the wetting and spreading of the metal crystallites to the alumina support and, therefore, weakens the interacting bonds between the particle and the surface. The wetting has previously been shown to play an important role for the sintering [6,11]. The ammonia could also adsorb to the acid sites on the support and, therefore, change the surface properties of the support.
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With weakened bonds between the particle and the support the particle could start to migrate on the surface already at low temperature. The factors that limit the growth could then be the size and shape of particles, pits and pores at the support.
5. Conclusions Nickel supported on ␥-alumina is sensitive to sintering in the presence of ammonia + hydrogen under mild temperature conditions (483–523 K). About 70% of the initial nickel, surface area is lost after exposure to ammonia + hydrogen at 523 K for 100 h. With only one of the two gases, ammonia or hydrogen, present during the heat treatment the loss in metal surface area is small. The sintering rate is significantly higher for reduced nickel particles compared to oxidised ones. The presence of ammonia might increase the sintering rate of reduced nickel particles by chemisorption of ammonia on nickel, which weakens the interactions between the particles and the support.
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Sintering of nickel particles supported on ␥-alumina in ammonia Johan Lif a,b,∗ , Magnus Skoglundh a , Lars Löwendahl a a
Department of Applied Surface Chemistry, Chalmers University of Technology, S-41296 Göteborg, Sweden b Akzo Nobel Functional Chemicals AB, S-44485 Stenungsund, Sweden Received 6 September 2001; received in revised form 8 November 2001; accepted 9 November 2001
Abstract Sintering of nickel on ␥-alumina in hydrogen, ammonia, ammonia + hydrogen and ammonia + nitrogen was investigated isocoric in the temperature range 483–523 K. The sintering process was followed by hydrogen chemisorption, specific surface area measurements, X-ray diffraction and transmission electron microscopy. After exposure to ammonia + hydrogen, hydrogen chemisorption revealed a fast decline of the nickel surface area. The results from X-ray diffraction and transmission electron microscopy are consistent with the result from the chemisorption experiments. The BET analysis showed that there are no losses of the total surface area. The sintering rate was significantly slower if the heat treatment was performed in the presence of only one of the gases, ammonia or hydrogen, or in ammonia + nitrogen. This indicates that the sintering in the temperature range 483–523 K only occurs with reduced nickel particles in ammonia + hydrogen. The prevailing mechanism for the sintering seems to be sintering by particle migration. The kinetics of the sintering process indicates that the rate constant is correlated to more factors than the temperature. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sintering; Nickel; Alumina; Catalyst deactivation; Ammonia; Hydrogen; Chemisorption
1. Introduction Supported nickel catalysts are widely used in various industrial processes like hydrogenation, dehydrogenation, methanation, amination and petroleum reforming [1,2]. Exposure of supported nickel catalysts to high temperatures in different chemical atmospheres leads to a decrease of the number of active sites and, hence, loss of catalytic activity [3–6]. One of the most serious aspects of thermal degradation manifests itself as growth of the metal particles with concomitant loss of active metal surface area. This phenomenon is normally referred to as sintering ∗ Corresponding author. Tel.: +46-303-853-29; fax: +46-303-887-86. E-mail address: [email protected] (J. Lif).
of small metal particles into larger ones with lower surface-to-volume ratio. Two different mechanisms have been proposed for sintering of supported metal catalysts, particle migration and atomic migration. Particle migration includes migration of particles on the support, collisions between the particles and their coalescence [6–11]. Atomic migration, often called Ostwald ripening, includes escape of atoms or molecules from the particle, transport of these clusters across the support or in the gas phase and capture of these clusters by collision with a metal particle [6,12–16]. The driving force for sintering is to minimise the surface energy by increasing the co-ordination number of the surface atoms. Larger particles are more stable than smaller ones due to the fact that the metal–metal bond energies often are considerable
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 9 5 7 - 7
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higher than the metal–support interactions. Typical values for detachment of a nickel atom from a nickel cluster is 431 kJ/mol [17] whereas the binding energy between metal particles and supports are in the range 5–15 kJ/mol [13,18]. Sintering is generally observed at temperatures above 700 K and the process shows strong temperature dependence [4,19]. The activation energy for atomic migration is normally much higher than for particle migration [4], sintering will consequently proceed via particle migration at low temperatures. Factors affecting the stability of the metal particle towards sintering include surrounding atmosphere (gas composition), metal–support interaction, particle shape, particle size, support roughness, pore size and impurities of the support or in the metal. The literature gives contradictory information of the importance of the atmosphere, several authors report that oxidising atmospheres give more severe sintering compared to inert or reducing atmospheres [15]. Bartholomew, however, observed that this is not a general case, he also found that adsorbed species like H, H2 and H2 O change the surface structure and, therefore, the rate of sintering [18]. For example, water vapour is proposed to increase the mobility of metal atoms or metal particles on the support due to adsorbed water or hydroxyl groups and, hence, accelerate the sintering [18]. This points out the importance of the surface energy, which is dependent on the gas composition. For instance, nickel oxide interacts strongly with SiO2 and is more stable in air than in H2 [18]. The presence of metal–support interactions affects the spreading, wetting and re-dispersion of the metal on the support. The pore structure of the support can determine the final particle size of the nickel particles [10], and reactions between the support and the active nickel particles or phase transitions of the support also affect the sintering process [20]. Reactions between the nickel particles and the feed, for instance carbon monoxide, can also accelerate the sintering [20,21]. The kinetics of catalyst deactivation is a function of time, temperature, pressure, concentration of different substances, etc. This change of activity can be an effect of one or several of the earlier mentioned processes. For sintering, the kinetics can easily be derived from time versus metal surface area measurements at constant temperature if all other earlier mentioned
processes are controlled. The activity plot usually shows two different regimes, one fast exponential loss during the initial stage and, thereafter, a slower linear loss [5,6,19] or an asymptotic approach of the dispersion towards infinite time [17,22–24]. Fuentes describes the kinetics of the sintering process by the generalised power law equation (GPLE) [4,22–24]. This equation fits sintering rate data closely and gives first- or second-order reactions for most published sintering data [4], which enables a direct quantitative comparison between different studies of sintering [18]. The number of published investigations of sintering is large and there are several good reviews in this area [4,6,15–17,24–29]. The number of papers that concerns sintering of nickel particles on ␥-alumina is, however, few and all of them concern atmospheres like He, H2 , N2 , H2 /H2 O and H2 /CO [4,5,16,20,21,25]. Presently, we have not found any papers dealing with the sintering of nickel particles on ␥-alumina in the presence of ammonia or ammonia + hydrogen. The present study aims to study the sintering behaviour of nickel particles on ␥-alumina in ammonia and/or hydrogen atmospheres, in view of the industrial application as an amination catalyst. We recognise that our work only concerns hydrogen and ammonia atmospheres and not any species that could react during the sintering. Hydrogen chemisorption, specific surface area measurements, X-ray diffraction and transmission electron microscopy were used to follow the sintering process.
2. Experimental 2.1. Catalyst preparation A 10 wt.% nickel on alumina catalyst was prepared by the incipient wetness method. The support material, ␥-alumina tablets Al-4504T 1/8 in. with 90 m2 /g received from Engelhard, used as received, was impregnated with an aqueous solution of nickel nitrate (Ni(NO3 )2 ·6H2 O) from Merck. The impregnation was done with a Ni(NO3 )2 concentration of 85% at a pH of 2.3. The impregnated sample was then dried at 393 K for 2 h and finally calcined in air at 673 K for 4 h. The preparation procedure yielded a homogenous nickel distribution in the alumina tablets. A large amount of
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catalyst was prepared and samples for all the experiments were taken from the same batch. To establish homogeneous sampling and to avoid mass and heat transfer limitations in the experiments, the catalyst was crushed and sieved and a particle size fraction between 150 and 300 m was used. The nickel dispersion and the specific surface area of the fresh catalyst was 7.2% and 87.5 m2 /g, respectively.
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and the tube was evacuated. Ammonia, 10.5 g, was introduced and the tube was pressurised with hydrogen or nitrogen to 4.2 MPa at room temperature. The tube was placed in a hot furnace at 483, 498 or 523 K, and kept there for the desired time. The pressure at the highest temperature, 523 K, in the ammonia + hydrogen atmosphere was 22.0 MPa. After the heat treatment, the tube was cooled to room temperature. The pressure was registered and the tube was evacuated. The catalyst was then taken out into air.
2.2. Heat treatment 2.3. Sample characterisation The catalyst was subjected to heat treatment in ammonia, ammonia +hydrogen, ammonia +nitrogen and hydrogen atmospheres for up to 160 h at the temperatures 483, 498 and 523 K (see Table 1). The heat treatment conditions, i.e. atmosphere, temperature and pressure, were chosen to simulate sintering of supported nickel under reaction conditions typically for amination. The general applied reaction conditions for amination over supported metal catalyst are, thus, in the temperature range 453–523 K under a total pressure between 5 and 30 MPa [30]. The heat treatments were performed in a stainless steel tube with a volume of 95 ml. The catalyst, a fresh sample for each heat treatment, was placed in the tube
After the heat treatment, the samples were divided into several portions where different portions were used for different analysis. This was done to ensure no correlation between the treatments in the different analyse methods. 2.3.1. Chemisorption measurements An ASAP 2010C instrument from Micromeritics was used to determine the nickel surface area of the catalysts by hydrogen chemisorption. The measurements were carried out volumetrically using a constant volume chemisorption system capable of a dynamic vacuum of 0.05 Pa. The catalyst was reduced at 723 K
Table 1 Specific uptake of chemisorbed hydrogen and dispersion of nickel for fresh and sintered samples after thermal treatments in different atmospheres Sample
Atmospherea
Temperaturea (K)
Timea (h)
1 (Reference)d 2 3 4d 5 6 7 8d 9d 10 11 12 13
NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 NH3 /N2 H2
483 483 483 498 498 498 523 523 523 523 523 523
23 73 124 23 66 122 23 90 162 144 112 92
a
H2 uptakeb (mol H2 /g sample)
Dispersionc (%)
61.6 48.2 37.9 37.5 44.2 28.6 29.0 34.4 21.4 18.7 46.0 50.9 51.3
7.22 5.67 4.45 4.40 4.96 3.38 3.38 4.04 2.54 2.18 5.40 5.97 6.01
Heat treatment conditions. Volume of hydrogen uptake at 308 K extrapolated to zero. Mean value of four repeated measurements. c Metal dispersion is calculated with the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms, and assuming a nickel cross-section area of 0.0649 nm2 [31]. d Mean values of two to four different analyses. b
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Table 2 Experimental procedure during the chemisorption measurements 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Flow of He at 373 K for 10 min Flow of He at 623 K for 10 min Evacuation at 673 K for 5 min Reduction in flowing H2 at 673 K for 120 min Evacuation at 673 K for 20 min Evacuation at 308 K for 10 min Leak test at 308 K First analysis with H2 at 308 K Evacuation at 308 K for 30 min Second analysis with H2 at 308 K
with hydrogen at atmospheric pressure for 2 h, thereafter, evacuated at 723 K for 20 min. The chemisorption measurements were performed at 308 K. A detailed description of the experimental procedures during the chemisorption is given in Table 2. As seen in the following sections, this high temperature (723 K) is needed to obtain reproducible analysis, it may, however, also affect the dispersion of the supported nickel particles. The decrease of metal surface area during the reduction was controlled by a heat treatment experiment with hydrogen atmosphere (sample 13, Table 1). The reduction of metal surface area was 17% after heat treatment for 92 h in hydrogen at 523 K with a pressure of 22.0 MPa. Bartholomew reported a reduction of metal surface area by approximately 5% after heat treatment for nickel on alumina in hydrogen at 923 K for 25 h [25]. To elucidate the needed reduction temperature, chemisorption measurements were also performed at a lower reduction temperature, 623 respective 673 K. However, the results were unreliable as the deviation between different measurements was too large, which probably is due to partial reduction of nickel. The chemisorption isotherm are obtained as the difference between the first adsorption measurement, the sum of both chemisorption and physisorption, and of the second adsorption measurement, which consist of only physisorption. After the first adsorption measurement, the sample is evacuated at 623 K to release all physisorbed hydrogen followed by the second adsorption measurement. The chemisorption isotherm is obtained by plotting the adsorbed amount of hydrogen versus the hydrogen pressure and then extrapolated to zero pressure to obtain the hydrogen uptake due to chemisorption on nickel. The nickel dispersion and
nickel particle size are calculated assuming the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms [31]. Each sample was analysed in a sequence of four individual measurements, where each measurement was pre-treated identically (see earlier sections). The hydrogen uptake by chemisorption was calculated as a mean value of the three last measurements for each sample. The deviation between those measurements was approximately 3%. 2.3.2. Specific surface area measurements The specific surface areas of the fresh and the most sintered sample, treated at 523 K for 162 h in ammonia + hydrogen, were determined by nitrogen adsorption according to the BET method using an ASAP 2010C instrument from Micromeritics. The BET surface area determinations were based on five experiments at relative pressures of nitrogen in the range of 0.05–0.21. 2.3.3. X-ray diffraction (XRD) The crystalline phases present in the fresh sample and the samples treated at 498 and 523 K in ammonia+ hydrogen were identified by XRD. The analysis was performed in air at room temperature with a Siemens D5000 powder diffractometer, with Cu K␣ radiation and a grazing incidence beam equipped with a Göbel mirror. The obtained diffraction patterns were matched to literature data [32]. The nickel particle size is calculated using line broadening with the Debye–Scherrer equation for the main peak indicating metallic nickel, θ = 44.5◦ . 2.3.4. Transmission electron microscopy (TEM) The fresh catalyst and the sample treated at 573 K for 162 h in ammonia + hydrogen, were studied by TEM. The catalyst was ground to an alumina particle size of a few micrometers, suspended in methanol and placed on a copper grid. Several different areas in each sample were studied with different magnifications. The TEM technique was not used to statistically determine the size distribution of nickel particles, but only for observing the size and shape of individual particles. The instrument used was a Philips CM200 FEGTEM furnished with a Link Isis EDX system. The digital micrographs were taken by a slow scan CCD camera from Gatan.
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3. Results The hydrogen uptake due to chemisorption for the fresh and for samples exposed to different atmospheres at different temperatures is listed in Table 1. The normalised metal surface area, i.e. metal surface area after heat treatment divided by initial metal surface area (sample 1), is plotted as a function of time in Figs. 1 and 2. The hydrogen uptake for the fresh catalyst and for samples 4, 8 and 9 was analysed several times. Those analyses were used to establish the deviation between the different chemisorption analyses. The derivation of the hydrogen uptake in the different measurements was ±5%. Samples 2–10, which are heat treated at 483, 498 or 523 K in the presence of both ammonia and hydrogen, show a rapid decline of the normalised metal surface area during the first 25 h followed by a more gradual decline (Fig. 1). The loss of metal surface area increases with increasing temperature during the heat treatment. After 150 h at 483, 498 and 523 K with both ammonia and hydrogen present the metal surface area has declined with 39, 53 and 70%, respectively. Sample 11 with only ammonia present during the heat treatment shows a relatively small reduction (25%) of the metal surface area after 144 h at 523 K
Fig. 1. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at various temperatures in ammonia+hydrogen. Symbols correspond to experimental data points, the curves to calculated data based on first-order GPLE kinetics and kd is 0.0391, 0.0451 and 0.0388 for 483, 498 and 523 K, respectively.
Fig. 2. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K in different atmospheres. Symbols correspond to experimental data points, the curve to the calculated data based on first-order GPLE kinetics for sintering at 523 K in ammonia + hydrogen.
compared to the heat treatment in the presence of both ammonia and hydrogen at 523 K (70% for sample 10, see Fig. 2). The same behaviour can be seen for heat treatments in a mixture of ammonia + nitrogen, which gives a decrease of 17% after 112 h at 523 K (sample 12), or in only hydrogen, which gives a decrease of 17% after 92 h at 523 K (sample 13, see Fig. 2). The measured total specific surface area of the fresh catalyst (sample 1) and the sample heat treated at 523 K for 162 h in ammonia + hydrogen (sample 10) is almost equal, 87.5 and 86.2 m2 /g, respectively. X-ray diffraction was performed of the fresh catalyst (sample 1) and for the heat treated samples at 498 and 523 K in ammonia + hydrogen (samples 5–10). Fig. 3 shows X-ray diffractograms for the fresh catalyst (sample 1) and for the three samples (8–10) that have been heat treated at 523 K. For all of the samples the diffractogram shows a typical pattern for ␥-alumina. For the fresh catalyst, no other peaks could be revealed. However, for the heat treated samples, the diffractogram shows peaks at 44.5 and 51.8 which are attributed to metallic nickel. Those nickel peaks become more intense with the heat treatment time, which indicates that the amount of detectable crystalline nickel is enhanced. The nickel particle size is calculated from line broadening of the main X-ray diffraction peak for metallic nickel and compared
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4. Discussion
Fig. 3. Powder X-ray pattern during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K during different times. The different profiles has an offset of 15 in-between and the literature data is indicated for (䉱) metallic nickel; (䉲) ␥-alumina; (䊉) nickelaluminate; and (䊏) nickel oxide.
with the particle size calculated from the hydrogen chemisorption [31], the result is listed in Table 3. Two representative micrographs were chosen from the TEM measurements (Figs. 4 and 5). The TEM micrographs of the fresh catalyst (sample 1) show nickel particles with a diameter of approximately 10 nm (Fig. 4). For the sample heat treated at 523 K for 162 h in ammonia + hydrogen (sample 10), only large, faceted, well-dispersed particles are found. The diameter of those large nickel particles was 50–100 nm (Fig. 5).
The results of this investigation describes the sintering behaviour of a supported nickel on alumina catalyst in an ammonia + hydrogen. The different analytical methods used here have provided a picture of a rapid growth of the metal particles during the first 25 h. At longer heat treatment times will the growth of the metal particle decline. The hydrogen chemisorption provides data for the different heat treatment times, which have been fitted with good correlation to the first-order GPLE: −d(D/D0 ) D D∞ m − (1) = kd dt D0 D0 where D is the dispersion, D0 the initial dispersion, D∞ the account for the observed asymptotic approach of the dispersion at infinite time, kd the rate constant, and m is the sintering exponent. The correlations have been done with a D∞ set to the highest heat treatment time for each temperature, respectively [4,22–24]. The assumption that D∞ is at 120–160 h gives a good correlation to experimental data. Correlation to the second-order GPLE does not give as good fit as the first-order GPLE correlation (Fig. 6). The measured decrease in hydrogen uptake with increasing temperature and exposure time in ammonia + hydrogen using hydrogen chemisorption by itself is not an evidence for the sintering. The reduction of metal surface area could also be due to other mechanisms like coking, volatilisation, reaction between nickel and alumina or collapse of the
Table 3 Nickel particle size calculated from hydrogen chemisorption and X-ray diffraction Sample
Atmospherea
Temperaturea (K)
Timea (h)
Particle size calculate from H2 chemisorptionb (nm)
Particle size calculate from XRDc (nm)
5 6 7 8 9 10
NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2 NH3 /H2
498 498 498 523 523 523
23 66 122 23 90 162
38.3 56.2 56.2 47.0 74.8 87.2
21.4 44.0 – 38.4 56.3 66.7
a
Heat treatment conditions. The mean value of the equivalent spherical particle diameter is calculated with the stoichiometry factor of 2 between chemisorbed hydrogen molecules and surface nickel atoms, a nickel cross-section area of 0.0649 nm2 and a nickel density of 8.9 g/cm3 [31]. c Particle size the Debye–Scherrer equation [31]. b
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Fig. 4. Transmission electron microscopy micrograph of nickel particles supported on ␥-alumina in the fresh sample (sample 1).
support with entrapment of nickel in the pores. To verify that the reduction of metal surface area essentially is caused by particle growth, some of the samples were analysed using TEM, nitrogen adsorption and XRD. The TEM measurements show the same trend as the hydrogen chemisorption with larger metal particles in the sample heat treated in ammonia + hydrogen than in the fresh catalyst. There are no small particles to be found in the sintered sample, only large crystallites of single crystals. The lack of small particles indicates that the sintering is caused by particle migration [3,28]. The specific surface area measurements showed that the total surface area in the catalyst is almost constant during the heat treatment in ammonia + hydrogen, indicating that there is no support collapse of the
␥-alumina. For the nickel on ␥-Al2 O3 systems, support collapse has been reported to occur above 800 K in the presence of water and at even higher temperatures in absence of water [25]. The XRD results support the results obtained by the chemisorption study (Fig. 3). The fresh sample does not provide any peak for nickel oxide, which is due to the fact that nickel oxide diffraction pattern is covered by the ␥-alumina diffraction pattern [32]. The ␥-alumina support is to a large extent amorphous and will, therefore, give broad peaks. The heat treated samples have two peaks indicating metallic nickel. Those nickel peaks has a correlation between the magnitude of the peaks and the time of the heat treatment, which verifies the result from the chemisorption measurements, i.e. growth of the nickel particles.
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Fig. 5. Transmission electron microscopy micrograph of a sintered nickel particle on ␥-alumina, after heat treatment in ammonia + hydrogen at 523 K for 162 h (sample 10).
When the sample is pre-treated in ammonia + hydrogen at 523 K, the nickel particle size calculated from hydrogen chemisorption is 47, 75 and 87 nm after 23, 90 and 162 h, respectively. The corresponding values from the XRD measurements are 38, 56 and 67 nm. Although the absolute values of the calculated particle size differ, the both methods show the same trend. The particle size increases with increasing exposure time in ammonia + hydrogen. The reaction between metallic nickel and alumina resulting in formation of nickel aluminate is a problem at high temperatures, above 800 K [6,24]. The absence of new diffraction peaks, except for metallic nickel, in the X-ray diffractogram indicates that no new crystalline phases have been formed in the catalyst during
the heat treatments. The XRD has also a limitation of the detectable particle size of 2–4 nm [16]. 4.1. The effect of temperature Although previous studies of nickel on ␥-Al2 O3 have shown that sintering generally becomes important at temperatures above 700 K [4,5,16,20,21,25]. The present work has shown that sintering could be a severe problem already at 500 K in ammonia + hydrogen atmosphere. When the different heat treatments are correlated to the first-order GPLE, the different temperatures gives a sintering rate constant, kd , of 0.0391, 0.0451 and 0.0388 h−1 for 483, 498 and 523 K, respectively. If kd
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Fig. 6. Hydrogen chemisorption data during sintering of 10 wt.% nickel on ␥-Al2 O3 at 523 K in ammonia + hydrogen atmosphere. Symbols correspond to experimental data points, the curves to the calculated data based on first- and second-order GPLE kinetics.
is assumed to be the k in the Arrhenius equation, calculation gives an apparent activation energy close to zero (−1.5 kJ/mol), which not is likely. This surprisingly low Ea is not likely when there is a clear correlation between the temperature and the final particle size at infinity time and it is not consistent with previous studies [14]. One explanation for the low activation energy could possibly be found if particle migration is the prevailing sintering mechanism. The particle migration process consists of transportation of nickel particles on the surface and coalescence of those nickel particles. The movement of the nickel particles is probably a slightly endothermic process due to that bonds between the nickel particle and the support are broken and reformed during the movement. The coalescence of the nickel particle has a low activation energy and is an exothermic reaction, which gives a large energy contribution to the sintering process and, therefore, could the apparent activation energy be close to zero. The Ea could also be affected by the deviation in measured hydrogen uptake. The deviation was determined to be ±5%, which would give as most an activation energy of 4 kJ/mol. This activation energy is quite low for a process like this, other investigations of nickel sintering have achieved activation energies between 25 and 160 kJ/mol depending on the experimental parameters [4]. Considering the low temperature and that no small particles were found in the TEM study indicate that
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the sintering proceeds via particle migration rather than atomic migration. This is not fully verified in this investigation and will be clarified in a coming investigation by direct examination of metal crystallite location and size as a function of time, temperature and atmosphere. The decrease of normalised metal surface area, i.e. the final particle size at infinity time, seems to be dependent on the temperature, which is consistent with previous investigations [18,22–24,33]. This could be explained by the fact that particles of a certain size need a certain temperature to move [33]. When the heat treatment starts, there is a particle size distribution of small particles. During the initial stage of the heat treatment, those small particles migrate, collide and coalescence with other particles. Particles can move if they have a size smaller than the maximum mobile size [33]. Pore structure, particle size, particle shape and bonds between the particle and the support affects the mobility of the particles. The particle migrating process continues as long as there exist moving particles. 4.2. The effect of atmosphere The sintering is less sever for heat treatments with only ammonia, only hydrogen or ammonia + nitrogen at 523 K than for the heat treatment with ammonia + hydrogen present at 523 K, i.e. the pressure of both ammonia and hydrogen is crucial for the sintering behaviour. Since almost no sintering takes place during heat treatment in ammonia+nitrogen, at high pressure, sintering in ammonia is not induced by high pressure without hydrogen. With the reducing atmosphere of hydrogen, the nickel particles will be reduced to metallic nickel (Ni0 ), ammonia by itself will not be able to reduce nickel oxide particles under the reaction conditions used in this study. Ammonia will adsorb to the reduced metallic nickel and the binding energy will be in the range of 60–90 kJ/mol [34]. Metallic nickel can chemisorb and dehydrogenate–re-hydrogenate ammonia [30,35]. This reaction probably changes the wetting and spreading of the metal crystallites to the alumina support and, therefore, weakens the interacting bonds between the particle and the surface. The wetting has previously been shown to play an important role for the sintering [6,11]. The ammonia could also adsorb to the acid sites on the support and, therefore, change the surface properties of the support.
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With weakened bonds between the particle and the support the particle could start to migrate on the surface already at low temperature. The factors that limit the growth could then be the size and shape of particles, pits and pores at the support.
5. Conclusions Nickel supported on ␥-alumina is sensitive to sintering in the presence of ammonia + hydrogen under mild temperature conditions (483–523 K). About 70% of the initial nickel, surface area is lost after exposure to ammonia + hydrogen at 523 K for 100 h. With only one of the two gases, ammonia or hydrogen, present during the heat treatment the loss in metal surface area is small. The sintering rate is significantly higher for reduced nickel particles compared to oxidised ones. The presence of ammonia might increase the sintering rate of reduced nickel particles by chemisorption of ammonia on nickel, which weakens the interactions between the particles and the support.
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