Rheological characteristics of nickel–alumina sol–gel catalyst

Fuel Processing Technology 102 (2012) 85–89

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Rheological characteristics of nickel–alumina sol–gel catalyst Basim Abu-Jdayil a,⁎, Mohamed A. Al-Nakoua a, Muftah H. El-Naas a, Abbas Khaleel b a b

Chemical and Petroleum Engineering Department, United Arab Emirates University, P.O. Box: 17555, Al Ain, United Arab Emirates Chemistry Department, United Arab Emirates University, P.O. Box: 17555, Al Ain, United Arab Emirates

a r t i c l e

i n f o

Article history: Received 18 October 2011 Received in revised form 12 March 2012 Accepted 18 April 2012 Available online 16 May 2012 Keywords: Thin layer catalysts Catalytic coating Nickel–alumina sol–gel Sol–gel rheology Non-Newtonian fluid

a b s t r a c t A sol–gel of nickel–alumina catalysts can be employed as a thin layer (b 100 μm) on the surface of plate reactors to reduce mass and heat transfer limitations compared with pellet catalysts. The knowledge of flow behavior of sol–gel systems is an important tool to characterize the sol–gel properties, stability, and applications. In this study, the steady rheological properties of the nickel-alumina sol–gel have been investigated as a function of nickel content (0–75 wt.%). Increasing the Ni content led to an increase in the apparent viscosity of sol–gel samples to reach the maximum at 30 wt.% Ni content, which in turn decreased upon further addition of Ni. The Herschel–Bulkley model fitted well the flow curves of sol–gel samples. Increasing the Ni content in the mixture led to behavior transition from Newtonian to shear thinning behavior. High content of Ni particles contributed to significant yield stress. Samples with nickel-free and low Ni content (5 wt.%) showed Newtonian behavior that passed into Bingham plastic at 10 wt.% and 15 wt.% Ni content. However, this in turn ran into shear thinning behavior with a yield stress at Ni content of ≥ 20 wt.%. On the other hand, adding Ni to the sample beyond 60 wt.% led to forming a weak gel. Shear thinning behavior accompanied with time independent behavior observed for nickel–alumina sol–gel are important properties for coating process. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nickel oxide has been used extensively as a catalyst in a wide variety of reactions [1,2]. Nickel supported alumina catalyst has been applied in the selective hydrogenation of aromatics and steam reformation of methane in natural gas for use in steel and petrochemical industries [3]. Recently, nickel–alumina catalysts have been used in hydrogen production by steam reforming of liquefied natural gas [4,5]. The catalytic plate reactor offers several advantages over conventional packed bed, slurry, or tubular reactors. These include major reduction in reactor size and much better control of the catalyst temperature, offering improved selectivity. Employing the catalyst as a thin layer (b100 μm) coated on the plate reactor surface reduces mass and heat transfer limitations compared with pellet catalysts and can improve the effectiveness factor, leading to further intensification [6]. The sol–gel method had been applied for the preparation of a wide variety of catalysts [4,7–9]. It has numerous advantages such as well-defined pore size distribution, high purity control of reactants, homogeneity, very large and controlled porosity combined with the ability to form large surface area. High metal dispersions and thermal resistant catalysts can be accomplished by the sol gel technique in the synthesis of supported metal catalyst; these ⁎ Corresponding author. Tel.: + 971 3 7133552; fax: + 971 3 7624262. E-mail address: [email protected] (B. Abu-Jdayil). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.04.025

properties lead to maximizing support metal precursor interactions, BET surface areas and support pore structure. Gels of the nickel oxide are attractive practical materials for catalysis due their beneficial properties like remarkably high surface area and open porous structure [9]. On the other hand, alumina prepared by the sol–gel process has a wide range of engineering applications in different areas such as catalysis, membrane separation processes, catalytic membrane reactors, adsorbents, composites, and coating, fiber, electronic and optic fields [10]. The sol gel method can be employed to prepare a variety of mixed oxide supports that are based on different oxides other than alumina and silica [11]. Sol–gel technique has been adopted for stainless steel substrate and micro-channel reactor coating [12–15]. In another application, Luo et al. [16] have investigated the preparation and characterization of sol–gel alumina/nickel composite coatings on carbon steel. In a recent work, nickel–alumina thin catalyst coats were prepared by the sol–gel method and calcined at different conditions to use in a plate reactor [17]. Flow and rheological characterization is an effective tool in evaluating the properties and compositions of catalyst coatings. As complex fluids, coatings need to be characterized by their structure and flow properties in order to be applied successfully. Flow properties of coatings are important in determination of the stability and thickness of the coating layer. Previous studies have evaluated the flow behavior of sol–gel systems to characterize sol–gel properties, transition and applications [8,18,19].

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Many rheological studies are devoted to sol–gel materials [10,18–23], but the sol–gel of nickel–alumina catalysts has not received much attention. The present study, therefore, focuses on examining the flow behavior of the nickel–alumina sol–gel catalyst. The effect of nickel content on the steady shear properties of the sol–gel catalyst was investigated. 2. Experimental 2.1. Synthesis of sol–gel Nickel–alumina suspension was prepared by dispersing alumina supplied by Sasol, Germany in a solution of dilute nitric acid (1 wt.%, 0.11 M) to give 5 wt.% alumina suspension. Alumina powder, with 78 wt.% alumina, was poured into a 100 ml measuring beaker containing the acid aqueous solution, then the suspension was mixed for 10 to 15 min. A glass propeller, with a spinning speed of approximately 200 rpm provided by rotating motor, was used for mixing. Nickel precursor, Ni(NO3)2.6H2O, was added to the alumina suspension to give the desired nickel/alumina ratio. After vigorous stirring for 2 min, the sample was subjected to rheological tests or stored for days to be tested later. The nickel fraction in the nickel/alumina mixture was varied from 0 to 75 wt.%. 2.2. Rheological measurements The rheological measurements, which were performed to determine sol–gel flow behavior, were carried out using a Rheolab QC viscometer from Anton Paar, Germany. Coaxial cylinder measuring system used is according to ISO 3219 and DIN 53019. The dimensions corresponding to the geometry were 14.460 mm for the radius of the measuring cup, 13.329 mm for the radius measuring bob and 120° cone angle. This geometry allows a gap width of 1.132 mm. All rheological tests were performed at 25 °C. The temperature control was achieved with a thermostatedcirculating bath. In the flow curve measurements, the shear stress _ (τ) of the samples was measured as a function of shear rate ( γ) at a constant temperature. The measurements were carried out with increasing (forward measurements) and decreasing (backward measurements) shear rates. In time dependent measurements, the apparent viscosity of the samples was measured as a function of time at constant shear rate.

3. Results and discussion 3.1. Viscosity versus shearing time This study aimed to investigate the steady rheological properties of stable nickel–alumina sol–gel catalyst. In order to avoid the measurement in the transition period, the prepared samples were subjected firstly to the rheological tests immediately after mixing, where the apparent viscosity of samples with different Ni content was measured versus the shearing time at constant shear rate. Fig. 1 shows the dependence of the apparent viscosity of sol–gel samples with different Ni content on shearing time measured at constant shear rate of 10 s − 1. In general, the gelation point is defined as the time when the viscosity of sol has a sharp increase [19]. However, this behavior could not be observed in our samples. The apparent viscosity of 10 wt.% Ni content sample increased gradually without sharp points to reach some kind of constant viscosity within 5000 s. On the other hand, the viscosity of samples with higher Ni content (50 wt.% and 70 wt.%) decreased first with shearing time to reach a minimum before increasing gradually without reaching a constant value. This increase in viscosity was a sign for the starting of gelation process. The slow and continuous increase in viscosity could be due to the fact that the gelation in this case resulted from the linking of charged colloidal particles to form a continuous network. The colloidal particles are charged as a result of reaction with the acid which eventually leads to protonated alumina surface. The observed slow and continuous increase in the viscosity indicates that the gelation takes place very slowly. This behavior is different from the gelation that is usually accompanied by a sharp increase in viscosity in typical sol–gel processes which can be referred to starting with a solution of molecular species, instead of colloidal suspension in the current study. The starting time of gelation increased with Ni content. The 50 wt.% Ni content samples needed 300 s to start gelation while the 70 wt.% Ni content samples needed 510 s. This behavior could be referred to a delay in the condensation reactions that eventually lead to gelation. It seems that the presence of large amounts of nickel ions initially hinders the contact between the binding sites, and only after some time such reactions start. The gradual increase of viscosity with time without reaching a steady state condition within 2 h suggests that the gelation process of the nickel-alumina catalyst is a slow process and may take hours or days to complete the reaction [9].

600 Measured immediately after mixing Shear rate = 10 s-1

50 wt% Ni

500

η(mPa s)

400 70 wt% Ni 300

200 10 wt% Ni

100

0

1000

2000

3000

4000

5000

6000

7000

Time (s) Fig. 1. Apparent viscosity as a function of time for Ni–Al2O3 samples aged for 0 h.

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87

1000

50 wt% Ni 800 Measured 2 days after mixing

η(mPa s)

Shear rate = 10 s-1 600

70 wt% Ni 400

10 wt% Ni

200

0

1000

3000

2000

4000

5000

7000

6000

8000

Time (s) Fig. 2. Apparent viscosity as a function of time for Ni–Al2O3 samples aged for 48 h.

Following mixing, the suspensions were aged for 48 h under ambient conditions in closed vials. Then the apparent viscosity of the samples as a function of time was measured again at constant shear rate. As can be seen in Fig. 2, the viscosity of the sol–gels with different Ni content was nearly constant and did not change with time. It has been observed, that the viscosity of gels increased slightly at the beginning of the shearing process to reach quickly the steady state condition. This increase seems to be due to some kind of particle arrangement as a result of shearing to reach more stable conditions. Based on the results illustrated in Figs. 1 and 2, the flow curve measurements were conducted on samples aged for 48 h to ensure complete gelation. Fig. 3 shows the dependence of the apparent viscosity of sol–gel samples aged for 48 h on the shear rate and Ni content. The apparent viscosity increased with Ni content up to 30 wt.% and then it decreased. The apparent viscosity of 75 wt.% Ni content samples lies between the viscosities of 15 wt.% and 20 wt.% Ni content samples. For boehmite systems, it has been reported [24] that the viscosity of the dispersion is affected not only by the amount

10000

but also by the nature of the gel phase present in the suspension. Specifically, the apparent viscosity was reported to increase on increasing the gel strength, i.e., the degree of interaction in the gel network. It is expected that the viscosity increases up to the maximum level was due to the increase of the gel strength and to the formation of the flocks. On the other hand, the decrease of viscosity was due both to the decrease of gel strength and to the re-dispersion of the flocks [24]. The observed effect of nickel ions in the current study indicates that the presence of small amounts of Ni 2 + ions enhances the linking of the alumina colloidal particles resulting in an increase in viscosity and in the formation of a colloidal gel. This can be explained by the possible formation of bonds between nickel ions and oxygen sites of alumina species in the suspension. On the other hand, high concentration of nickel precursor results in considerable adsorption of nickel ions on the surface of the colloidal particles preventing binding and network formation. In dip coating applications, increasing the viscosity increases the coating load, i.e. increasing the layer thickness.

8 6

Nickel content

4

0 wt%

40 wt%

5.0 wt%

50 wt%

8 6

10 wt%

75 wt%

4

15 wt%

2

20 wt%

2

η (mPa s)

1000

100

30 wt%

25 wt% 8 6 4 2

10

8 6 4 2

1 0

100

200

300

400

500

.

600

700

800

900

γ (s-1) Fig. 3. Dependence of the apparent viscosity of Ni–Al2O3 samples on shear rate, effect of Ni content.

1000

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30

Nickel content 0.0 wt% 5.0 wt%

25

10.0 wt% 15.0 wt%

τ (Pa)

20

15

10

20.0 wt% 25.0 wt% 30.0 wt%

5

H-B model

0

100

200

300

500

400

600

.

700

800

900

1000

γ (s-1) Fig. 4. Flow curves of Ni–Al2O3 samples, effect of Ni content: 0–30 wt.%.

The flow curves of sol–gel samples with different nickel contents were measured in the shear rate range of 10–1000 s − 1. Fig. 4 shows the shear stress–shear rate relationship for samples with Ni content of 0 wt.%–30 wt.%, while Fig. 5 shows the flow curves for samples with Ni content of 40 wt.%–75 wt.%. The Herschel–Bulkley (H–B) model was used to describe the flow curves of sol–gel samples: n τ ¼ τ0 þ mγ_

5 wt.% Ni content samples is linear, indicating that both samples behaved like a Newtonian fluid. However, the presence of Ni in the sample led to an increase in the Newtonian viscosity. On the other hand, the samples with Ni content of 10 wt.% and 15 wt.% exhibited a Bingham behavior, i.e. linear shear stress–shear rate relationship with a yield stress, which increased with Ni content. It is clear that these two samples represent a transition state between the Newtonian behavior exhibited by the low Ni content samples, and the shear thinning behavior observed in the high Ni content samples (discussed below). The samples of Ni content varying between 20 wt.% and 75 wt.% showed shear thinning behavior with a yield stress. In shear thinning behavior, the viscosity decreases with shear rate. The viscosity at low shear rates was very high, which was evidence of a strong, cohesive structure. Any breaking up of network structure by shearing was more than balanced by the formation of new structures. The low viscosity at high shear rate can be explained by a breaking up of the nickel–alumina gel network [18]. As shown in Table 1, the flow behavior index for all samples in this range of Ni content was less than one. In addition, a significant yield stress has been observed for

ð1Þ

where τ is the shear stress in Pa, τ0 is the yield stress in Pa, γ_ is the shear rate in s − 1, m is the consistency coefficient and n is the flow behavior index. Typically the H–B model is used for many materials as the Newtonian, shear thinning, shear-thickening and Bingham plastic may be considered as special case. The rheological parameters obtained for all samples are listed in Table 1. Figs. 4 and 5 show that the H–B model fitted well the flow curves of nickel–alumina sol–gel with R 2 values (reported in Table 1) varied between 0.981 and 0.999. The results depicted in Figs. 4 and 5 and reported in Table 1 show that the shear stress–shear rate relationship for pure alumina and 30 Nickel content

75.0 wt%

0.0 wt% 25

H-B model

40.0 wt% 50.0 wt%

τ (Pa)

20

65.0 wt%

15

10

5

0

100

200

300

400

500

. γ (s-1)

600

700

800

900

Fig. 5. Flow curves of Ni–Al2O3 samples, effect of Ni content: 0 wt.%, 40 wt.%–75 wt.%.

1000

B. Abu-Jdayil et al. / Fuel Processing Technology 102 (2012) 85–89 Table 1 Herschel–Bulkley parameters for sol–gel Ni–Al2O3 samples with different Ni content. Sample

τ0 (Pa)

m (Pa sn)

n

R2

0 wt.% Ni: 100 wt.% Al2O3 5 wt.% Ni: 95 w% Al2O3 10 wt.% Ni: 90 wt.% Al2O3 15 wt.% Ni: 85 wt.% Al2O3 20 wt.% Ni: 80 wt.% Al2O3 25 wt.% Ni: 75 wt.% Al2O3 30 wt.% Ni: 70 wt.% Al2O3 35 wt.% Ni: 65 wt.% Al2O3 40 wt.% Ni: 60 wt.% Al2O3 45 wt.% Ni: 55 wt.% Al2O3 50 wt.% Ni: 50 wt.% Al2O3 55 wt.% Ni: 45 wt.% Al2O3 60 wt.% Ni: 40 wt.% Al2O3 65 wt.% Ni: 35 wt.% Al2O3 70 wt.% Ni: 30 wt.% Al2O3 75 wt.% Ni: 25 wt.% Al2O3

0.0 0.0 3.78 8.98 9.20 12.40 13.00 10.10 9.60 9.60 9.20 9.45 9.25 6.80 4.50 3.45

0.0067 0.0079 0.0062 0.0046 0.500 0.882 0.914 1.189 0.927 0.822 0.321 0.827 0.832 0.439 0.374 0.355

1.0 1.0 1.0 1.0 0.40 0.37 0.38 0.36 0.40 0.41 0.51 0.41 0.41 0.48 0.51 0.51

0.998 0.999 0.981 0.991 0.987 0.986 0.995 0.981 0.989 0.991 0.994 0.989 0.980 0.983 0.983 0.980

all samples. It is known that for easy application, coatings should be shear thinning. The yield stress provides further insight into the interactive forces present in the coating materials as well as the force required to initiate flow during application. As can be seen in Table 1, below Ni content of 10 wt.%, there is no detectable yield stress. For Ni content greater than 10 wt.% a certain shear stress has to be overcome before any flow is induced. The yield stress increased from 3.78 Pa at 10 wt.% Ni to 13.0 Pa at 30 wt.% Ni content. Then, the yield stress decreased at 35 wt.% Ni and stayed nearly constant till 60 wt.% Ni sample. After that, the yield stress decreased again with the Ni content. Moreover, the gel samples exhibited low yield stress and low m values at high Ni content (60 wt.%–75 wt.%), which indicate weak gel structure. Adding more Ni to the sample disturbed the network structure of the gel leading to reduction in the rheological parameters as well as in the apparent viscosity, see Fig. 3. The dependence of the apparent viscosity of nickel–alumina sol– gel on shearing time was investigated by different ways. The results presented in Fig. 2 show that the apparent viscosity of stable gel is independent of shearing time. In addition, the hysteresis tests of flow curves and step test with three intervals were conducted to investigate the thixotropic behavior of sol–gel samples, but all samples did not show noteworthy time-dependent behavior. Thus, it seems that the prepared nickel–alumina sol–gel samples were stable homogeneously dispersed samples, since the thixotropic behavior has not been observed. Paint and coating stability is important, since unstable dispersions lead to large shrinkage and surface cracking. 4. Conclusions The nickel–alumina sol–gel dispersions with Ni content of 0– 75 wt.% have been prepared. The flow properties measurements were conducted on sol–gel samples aged for 48 h, which were enough to complete gelation process and get stable gel. The sol–gel dispersion exhibited Newtonian behavior for Ni content of 0 and 5 wt.% and non-Newtonian with noteworthy time-dependent behav-

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ior for Ni content range of 10 wt.%–75 wt.%. Increasing the Ni content led to an increase in the apparent viscosity of sol–gel samples to reach the maximum at 30 wt.% Ni content. Further increase in Ni content resulted in slight decrease in the apparent viscosity. This decrease was more pronounced for samples with Ni content of ≥65 wt.%. The Herschel–Bulkley model fitted well the flow curves of sol–gel samples. Increasing the Ni content in the mixture led to behavior transition from Newtonian to shear thinning behavior. High content of Ni contributed to significant yield stress. Samples with nickel-free and low Ni content (5 wt.%) showed Newtonian behavior that passed into Bingham plastic at 10 wt.% and 15 wt.% Ni content. However, this in turn ran into shear thinning behavior with a yield stress at Ni content of ≥20 wt.%. On the other hand, adding Ni to the sample beyond 60% led to forming a weak gel. Acknowledgments The authors would like to acknowledge the financial support provided by the Research Affairs at the UAE University. Special thanks go to Hassan Kamal for his help with the experimental work. References [1] P. Gelin, M. Primet, Applied Catalysis B: Environmental 39 (2002) 1. [2] Y.Q. Song, H.M. Liu, S.Q. Liu, D.H. He, Energy & Fuels 23 (2009) 1925. [3] A. Aminzadeh, H. Sarikhani-fard, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 55 (1999) 1421. [4] J.G. Seo, M.H. Youn, J.C. Jung, I.K. Song, International Journal of Hydrogen Energy 35 (2010) 6738. [5] J.G. Seo, M.H. Youn, I.K. Song, Catalysis Surveys from Asia 14 (2010) 1. [6] M.V. Twigg, Catalyst Handbook, Wolfe Publishing, London, UK, 1989. [7] G.M. Pajonk, Heterogeneous Chemistry Reviews 2 (1995) 129. [8] D.J. Suh, T.J. Park, J.-H. Kim, K.-L. Kim, Journal of Non-Crystalline Solids 225 (1998) 168. [9] S.K. Gill, P. Brown, M.T. Ogundiya, L.J. Hope-Weeks, Journal of Sol-Gel Science and Technology 53 (2010) 635. [10] B. Ksapabutr, E. Gulari, S. Wongkasemjit, Colloids and Surfaces A: Physicochemical and Engineering Aspects 233 (2004) 145. [11] R.D. Gonzalez, T. Lopez, R. Gomez, Sol–gel preparation of supported metal catalysts, Catalysis Today 35 (1997) 293. [12] R. J. Charlesworth, Ph D thesis, Department of Chemical and Process Engineering, University of Newcastle, Newcastle upon Tyne, UK, 1996. [13] M. Babovic, Ph D thesis, School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne, UK, 2003. [14] M. Nakoua, Ph D thesis, School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne, UK, 2004. [15] D. Truyen, M. Courty, P. Alphonse, F. Ansart, Thin Solid Films 495 (2006) 257. [16] L. Luo, J. Yao, J. Li, J. Yu, Ceramics International 35 (2009) 2741. [17] M.A. Al-Nakoua, M.H. El-Naas, B. Abu-Jdayil, Fuel Processing Technology 92 (2011) 1836. [18] I. Meleshevych, S. Pakhovchyshyn, V. Kanibolotsky, V. Strelko, Colloids and Surfaces A: Physicochemical and Engineering Aspects 298 (2007) 274. [19] Y. Xu, D. Wu, Y. Sun, W. Chen, H. Yuan, F. Deng, Z. Wu, Colloids and Surfaces A: Physicochemical and Engineering Aspects 305 (2007) 97. [20] W. Vogelsberger, A. Seidel, R. Fuchs, Journal of Colloid and Interface Science 230 (2000) 268. [21] A. Ponton, S. Warlus, P. Griesmar, Journal of Colloid and Interface Science 249 (2002) 209. [22] N. Phonthammachai, M. Rumruangwong, E. Gulari, A.M. Jamieson, S. Jitkarnka, S. Wongkasemjit, Colloids and Surfaces A: Physicochemical and Engineering Aspects 247 (2004) 61. [23] L. Günther, W. Peukert, G. Goerigk, N. Dingenouts, Journal of Colloid and Interface Science 294 (2006) 309. [24] C. Cristiani, M. Valentini, M. Merazzi, S. Neglia, P. Forzatti, Catalysis Today 105 (2005) 492.