Multivariate Analysis of the Role of Preparation Conditions on the

Aug 18, 2004 - University of New South Wales, Sydney, New South Wales 2052, ... site strength and concentration were unaffected by variations in the t...
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Ind. Eng. Chem. Res. 2004, 43, 6006-6013

Multivariate Analysis of the Role of Preparation Conditions on the Intrinsic Properties of a Co-Ni/Al2O3 Steam-Reforming Catalyst Kelfin M. Hardiman, Cyrus G. Cooper, and Adesoji A. Adesina* Reactor Engineering and Technology Group, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia

The effect of major preparation variables on the innate characteristics of an alumina-supported Co-Ni catalyst system has been examined via a factorial design. Both calcination temperature and time have statistically significant (negative) effects on the Brunauer-Emmett-Teller surface area, with variation in the heating rate registering little or no influence. Similarly, the acidsite strength and concentration were unaffected by variations in the three preparation variables. However, the heating rate was a strong determinant of the metal particle size, dispersion, and metal surface area. Even so, analysis of the calcination kinetics revealed that a high rate of metal aluminate formation was associated with poor dispersion, low metal surface area, and large crystallite size especially for low-temperature calcination. The solid-state metal nitrate decomposition implicated an Avrami-Erofeev kinetics with an activation energy of 82.6 kJ mol-1. H2 temperature-programmed reduction and X-ray diffraction revealed the existence of multiple oxide phases, namely, Co3O4, NiO, NiCo2O4, Ni2O3, and Ni(Co)Al2O4, whose magnitude and stability depended on the calcination heating rate. Phase changes during reduction were supported by scanning electron microscopy and transmission electron microscopy images as well as surface elemental profiles. 1. Introduction The application of bi(multi)metallic systems to petrochemical catalysis as originally suggested by Sinfelt1-3 has gained considerable support because of the demonstrated improvements in activity and stability.4,5 Hydrocarbon reforming reactions are traditionally carried out over transition monometallic catalysts. Although single-metal steam-reforming catalysts often suffer from severe deactivation due to carbon deposition, the dilution of the active metal surface by another metal may yield sites inadequate for coke formation (arising from dehydropolymerization of surface carbon species) and, hence, bimetallic catalysts offer greater resistance to coke-induced deactivation than monometallic systems. The physicochemical features of the bimetallic catalyst are, however, strongly related to the preparation factors. For example, Mile et al.6 observed that the alloying and formation of new metallic phases, which may be more catalytically active than either of the individual metals, are influenced by the thermal treatment conditions. This catalyst synergism is controlled by the calcination temperature, time, and heating rate, which have therefore been selected as the principal preparation variables in this investigation. Prior studies in our laboratory by Opoku-Gyamfi et al.7 have shown that an alumina-supported Co-Ni catalyst exhibited synergistic effects during methane steam reforming. The catalyst also has superior carbon deposition resistance compared to monometallic Ni/ Al2O3. To evaluate its suitability for industrial applications, propane was used as the hydrocarbon substrate to increase the carbon deposition tendency without the complication of a higher reaction temperature that may * To whom correspondence should be addressed. Tel.: +612-9385-5268. Fax: +61-2-9385-5966. E-mail: a.adesina@ unsw.edu.au.

induce sintering (because propane steam reforming proceeds at a lower temperature than methane reforming).8 Choudhary and co-workers9-11 have also employed the Co-Ni system for methane partial oxidation and CO2 reforming and observed a substantial reduction in carbon deposition. These previous investigations did not, however, provide quantitative information on the relation between catalyst characteristics and synthesis conditions. Consequently, a two-level factorial experimental design strategy was employed to explore the influence of the calcination temperature, time, and heating rate. A statistical approach is useful in the analysis of complex multivariable systems especially when the possibility of significant interaction effects on the chosen response variable (physicochemical attributes) is foreshadowed by earlier studies.12 The advantage of a two-level factorial design to multivariable process analysis includes the potential to reveal major trends with promising directions with a minimum number of runs in factor space.13 Moreover, by means of an orthogonal main effect plan, experimental data from factorial design runs may be adequately represented by polynomial regression models for descriptive, predictive, and optimization purposes even without the need for detailed understanding of the mechanism connecting the input to the response variables. For that reason, multivariate analysis is a useful tool for combinatorial catalyst investigation in order to develop correlations between preparation variables and the intrinsic properties of the catalyst for tailored applications. As demonstrated in this work, this quantitative strategy complements information from qualitative catalyst characterization methods, viz., X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with an energy-dispersive X-ray (EDX) analysis unit, transmission electron microscopy (TEM), and temperature-programmed oxidation (TPO) and

10.1021/ie049760z CCC: $27.50 © 2004 American Chemical Society Published on Web 08/18/2004

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6007 Table 1. 23-Factorial Design Plan for Catalyst Calcination sample

holding time (h)

heating rate (K min-1)

temperature (K)

1 2 3 4 5 6 7 8

1 5 1 5 1 5 1 5

5 5 20 20 5 5 20 20

873 873 873 873 973 973 973 973

reduction (TPR) spectra. Moreover, transient thermogravimetric analysis (TGA) obtained during calcination was used to estimate the kinetics of the solid-state reaction for the catalyst production because this impacts our understanding of the solid-phase characteristics and subsequent catalytic performance. Optimal preparation conditions were then suggested based on statistically derived polynomial regression models correlating intrinsic catalytic properties to the preparation variables. 2. Experimental Details 2.1. Catalyst Preparation. The bimetallic 5:15:80 Co-Ni/Al2O3 was prepared via double impregnation. Commercial γ-Al2O3 (Norton, Worcester, MA) was pretreated at 1073 K for 6 h to ensure thermal stability of the alumina (δ phase) under calcination and steamreforming conditions. Impregnation was initiated by mixing the calculated amount of an aqueous Co(NO3)2 solution with the thermally conditioned Al2O3 support, followed by 3 h of stirring at 303 K at a pH of 2. The resulting solution was dried overnight in an oven at 393 K. The dried solid was subsequently impregnated with the required quantity of an aqueous Ni(NO3)2 solution under conditions identical with those of the first impregnation. The final product was calcined in accordance with the 23-factorial design plan outlined in Table 1. The calcined solid was subsequently crushed and sieved to 212-250 µm for further use. A lower limit of 873 K was chosen for the calcination temperature to ensure that there will be no structural changes in the catalyst within the temperature range (773-873 K) used for subsequent propane reforming. The upper limit at 973 K was selected to explore the effect of reaction phase changes on the catalyst attributes without excessive Brunauer-Emmett-Teller (BET) area loss, which would accompany the transition from the δ to R phase in the alumina support at about 1100 K. Heating rates of 5 and 20 K min-1 were implemented to examine the solid-phase transformation reaction while permitting possible internal restructuring (at the slower heating rate) and high formation rate of metal aluminate phase (at the higher heating rate). The calcination period was set at a lower limit of 1 h because preliminary TGA runs showed that the weight drop was constant only after about 50 min. The upper limit (5 h) was used to prevent excessive thermal conditioning. 2.2. Characterization of Bimetallic Catalysts. The BET surface area was measured using a Phlosorb apparatus for nitrogen adsorption at 77 K. An Autochem 2910 (Micromeritics, Norcross, GA) unit was used to conduct H2 chemisorption and NH3 temperature-programmed desorption (TPD) experiments. The chemisorption runs were carried out with temperature ramping at 2 K min-1 to 873 K, holding constant for 2 h followed by 20 doses of hydrogen at 373 K via a 1-mL

Table 2. Measured (Response) Variables Associated with the Physicochemical Properties and Solid-State Calcination Kinetic Constant of the Catalyst System ks SBET D Sm d Ed A/C sample (m2 g-1) (%) (m2 g-1) (nm) (kJ mol-1) × 1012 (h-1) 1 2 3 4 5 6 7 8

120.40 118.07 119.82 117.70 109.47 108.62 108.79 106.56

6.19 6.06 3.95 3.75 8.08 5.62 5.55 4.96

8.28 8.11 5.28 5.02 10.81 7.52 7.41 6.63

16.29 16.64 25.54 26.88 12.48 17.94 18.19 20.32

-16.29 -16.25 -15.83 -16.27 -15.67 -17.38 -15.22 -11.94

91.76 92.84 99.29 92.27 108.38 68.91 122.34 304.92

4.04 3.81 8.24 8.69 3.99 4.45 8.71 8.40

injection loop. A metal/H atom ratio of 1 was used in the treatment of the chemisorption data. The TPD analysis included pretreatment with pure H2 at 873 K for 2 h, and subsequent NH3-desorption measurements were performed using a 0.4% NH3/Ar mixture at different heating rates. TGA was performed using a ThermoCahn 2121 system. High-purity air at 55 mL min-1 was used for calcination and TPO, while TPR was conducted with 50% H2/N2 at 55 mL min-1. The TPRTPO procedures involved an initial moisture removal step using inert nitrogen flow at 423 K for 1 h. Subsequently, for TPR and TPO runs, the temperature was ramped at 5 K min-1 to 973 K and held at this level for 1 h. XRD measurements of the freshly calcined and freshly reduced catalysts were recorded on a Philips X’pert system using Ni-filtered Cu KR (λ ) 1.542 Å) at 40 kV and 30 mA. Catalyst reduction was carried out in a fluidized-bed rig operating under a well-mixed and smooth fluidization regime with a flow of pure hydrogen at 873 K for 2 h (5 K min-1 heating rate). The microscopic images were taken using a scanning electron microscope (Cambridge S360) equipped with EDX microanalysis. TEM micrographs were obtained from a Hitachi H-7000 microscope. Each characterization technique required typically around 0.15 g of the catalyst sample. 3. Results and Discussion 3.1. Physicochemical Properties. Table 2 displays the intrinsic characteristics of all eight catalysts evaluated in this study. It is evident from column 2 that variation in the preparation conditions registered only moderate changes in the total BET surface area, SBET. Even so, it would seem that specimens calcined at the lower temperature limit (873 K) have higher surface area than those obtained at the upper level (973 K) regardless of the heating rate and calcination period employed. Because all samples were prepared from alumina initially pretreated at 1073 K, this difference probably arose from crystallite size variations due to disparity in metal oxide particle distribution, dispersion, and reagglomeration rates at different temperatures rather than structural changes in the support. The companion F values computed from Yates’ analysis of these data (provided in Table 3, column 3) reveal that both calcination temperature, T, and time, t, exhibited statistically significant effects at the 95% confidence level while the heating rate, r, and the factor interactions played negligible roles because calculated F values for these effect identifications were all lower than F4,4 ) 6.39 (obtained from standard tables13 at a 95% confidence level). In fact, the effects of the calcination temperature and time are negatively signed (although not shown), suggesting that a decrease in each of these variables would yield a higher surface area.

6008 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 Table 3. Calculated F Values for the Response Variables (Physicochemical Properties)a sample effect ID 1 2 3 4 5 6 7 8

average t r tr T tT rT trT

SBET

D

6.44 3.15 1 36.37 1.17 1.53 1.36

2.48 5.71 1.33 3.13 2 1 1.43

Sm

d

Ed

A/C

ks

2.48 3.98 1 1 4.94 5.71 11.81 5.42 1.87 245.23 1.33 1 3.85 1.56 1.12 3.14 7.03 3.80 1.66 10.42 2 2.53 1.69 1.09 1 1 4.88 4.66 1.77 5.76 1.43 1.85 4.69 1.68 20.11

a Statistically significant values (at a 95% confidence level) are shown in bold font.

H2 chemisorptive parameters, namely, metal dispersion (i.e., proportion of active metal atoms on the catalyst surface), D, metal surface area, Sm, and metal particle diameter, d, seen in Table 2 display a striking consistency. At each temperature level, the dispersion and metal surface area decreased with the heating rate, while there is a corresponding increase in the particle diameter of the metal with the heating rate. Closer inspection further shows that the change in each of these response variables is relatively small with respect to the calcination time at each temperature level. It is therefore conceivable that the heating rate and calcination temperature would be the principal determinants of the H2 chemisorptive parameters. In consonance with this intuition, the associated Yates’ analysis in Table 3 confirms that, among all seven effect identifications, only r and T have statistically relevant roles on d at a 95% confidence level (shown in bold font). The same may be said of D and Sm, albeit the estimated F values are lower than those in standard tables. This is simply an artifact of the denominator variance: the trend is manifestly in agreement. Because the interface between the two oxides is usually acidic, it is expected that changes in the CoNi oxide (alloying) reaction or metal support interaction resulting from preparation conditions will be reflected in the acidic site strength (heat of desorption for NH3, -Ed) and concentration, A/C, of the supported bimetallic catalyst. Thus, data from NH3-TPD runs would offer useful insights into these solid-state attributes. As may be seen from Table 2, column 6, the NH3 heat of desorption is almost invariant with the preparation conditions. It would seem that solid solution species were formed in significant amounts under calcination conditions employed. Similarly, there is no discernible trend in the acid-site concentration (A/C) with changing preparation conditions. Not surprisingly, computed F values for both response variables confirmed that none of the seven effect identifications were statistically significant in the determination of both the heat of desorption, -Ed, and acid-site population, A/C. However, Figure 1 shows that the heat of desorption is well correlated with the active-site concentration, although both are response variables. This indicates that the relationship is most likely associative rather than causal and may be symptomatic of a compensation-effect phenomenon, which ensures that the preparation condition favoring a strong acid-site strength, high -Ed values, is counterbalanced by a low site density and vice versa. The TPD profile is characterized by two peaks at 500 and 750 K as shown in Figure 2. The specific chemical signature of these two types of acid sites is yet to be confirmed. 3.2. Solid-State Calcination Kinetic Analysis. The TGA profile in Figure 3 reveals a 26% weight drop

Figure 1. Associative correlation between Ed and A/C.

Figure 2. Representative NH3-TPD profile for the Co-Ni/alumina catalyst.

Figure 3. Typical transient weight profile during calcination.

during analysis. Theoretically, at least a 25.5% weight drop was required for the complete calcination of the catalysts with the specified metal loading prepared from nitrate precursors. Indeed, weight drops between 25.5 and 26.5% were observed in all eight catalysts, indicating complete calcination. Furthermore, transient weight measurements obtained during isothermal calcination of the catalyst specimens in air were used to estimate solid conversion, R, from the profile shown in Figure 3; hence

R)

wi - w wi - wf

(1)

The thermal spectra in Figure 4 show that metal nitrate decomposition rates are strongly dependent on the calcination temperature and heating rate. This is consistent with the significant effects of r and T on d arising from H2 chemisorption analysis. Although the mechanism of the solid-state reaction is substantially

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6009

Figure 6. Solid-state reaction dynamics for coupled TPR-TPO thermal analysis.

Figure 4. TGA profiles of calcination runs.

Figure 5. Weight-based conversion profile during the calcination run.

more complicated than gas- and/or liquid-state reactions because of nucleation, crystallization, and other peculiarities, kinetic data may be analyzed in terms of one or more of the various models classified as AvramiErofeev (A), Prout-Tompkins (B), geometrical (R), diffusion (D), and order of reaction (F) models, as proposed by Brown.14 The experimental conversion-time data (over 100 points because data acquisition was carried out at 12 min-1) plotted in Figure 5 reveal a sigmoidal shape, which is characteristic of an Avrami-Erofeev model. Thus, the applicable kinetic model for the solidstate reaction is

-ln[(1 - R)]1/3 ) kst

(2)

where ks is the solid-state kinetic constant for the

calcination reaction. This sigmoidal-shape conversiontime profile suggests that the metal nitrate decomposition to the oxidic phase proceeds via an initial induction period followed by reaction-controlled oxide evolution before a final relaxation (intraphase restructuring). Table 2, column 8, hosts the ks values for all eight specimens. It is apparent from this table that the heating rate and temperature have a strong influence on ks. Although fluid-solid transport resistances were negligible under the conditions used in the ThermoCahn TGA (55 mL min-1 with 230 µm average particle size), the associated calcination activation energy of 82.6 kJ mol-1 is somewhat lower than typical values for noncatalytic gas-solid reactions and is an indication of the combination of chemical and physical (internal restructuring) steps involved in the nitrate decomposition to its oxides.14 The effect of the calcination time is, however, somewhat less clear. Accordingly, the statistical treatment in Table 3 attests to the significant role played by r, T, and the three-factor interaction trT at a 95% confidence level. 3.3. Temperature-Programmed Studies. In another set of runs, H2 TPR followed by air TPO of each of sample was performed. The TPR scheme involved heating of the sample to 423 K and holding for 1 h in inert flowing N2 until physisorbed and interstitial water molecules have been removed. For both TPO and TPR, the temperature was ramped from 423 to 973 K at 5 K min-1 and then held constant for 1 h at this temperature as illustrated in Figure 6. During TPR, all catalysts exhibited a 6-6.5% weight drop as a result of oxide removal by hydrogen reduction. This is consistent with a 5.6% weight drop for complete reduction, signaling that the catalysts were fully reduced under the conditions used. From Figure 7, it is evident that the TPR spectrum is characterized by five peaks, although the size and temperature at which each peak appeared varied with the calcination conditions. The lowest temperature peak (440-490 K) may be due to the reduction of Ni3+ species (Ni2O3) to Ni2+ or Ni species.6 The next three peaks correspond to the reduction of NiCo2O4, Co3O4, and NiO in order of increasing temperature, which are in agreement with the findings of Haenen et al.,15 Jacobs et al.,16 and Li and Chen.17 NiCo2O4 is a “metastable” spinel structure, which breaks down at a lower temperature than Co3O4 and NiO.15 However, the disappearance of the NiO peak (810-850 K) in curves for higher calcination temperature and the increased intensity of the highest temperature peak in these profiles suggest the interaction between Ni and the alumina support, yielding a NiAl2O4

6010 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004

Figure 8. TPO spectra of various catalysts.

Figure 7. TPR spectra of various catalysts. Table 4. Positions of the TPR and TPO Peaks as a Function of the Temperature TPR (K)

TPO (K)

sample peak I peak II peak III peak IV peak V 1 2 3 4 5 6 7 8

449.4 451.2 493.7 479.6 483.4 483.9 480.6 482.0

561.2 569.9 554.4 557.8 564.3 582.2 571.7 570.8

689.2 698.0 699.5 711.8 614.5 641.3 642.1 641.3

809.7 851.7 777.6 854.2 699.9 730.6 723.5 716.0

942.6 946.0 935.0 954.5 963.3 965.4 960.3 965.8

peak I 470.5 470.9 454.0 462.1 475.0 495.9 470.9 469.9

peak (last peak) at around 973 K, which was also observed by Roh et al.18 Similarly, a higher calcination temperature resulted in the reduction of the NiCo2O4 peak intensity, which suggests that this less stable species has probably been incorporated into Co3O4, NiO, or NiAl2O4 phases. Table 4 displays the position of the TPR and TPO peaks as a function of the temperature. The samples calcined at lower temperature evidenced widely distinct peaks II-IV. However, in the samples with a higher calcination temperature, these peaks were located in much more similar temperature ranges and hence seem to resemble a single broad peak between 560 and 730 K. This shows that the metal oxide structure becomes more complex with increased temperature. This is in agreement with the report of Arnoldy and Moulijn,19 who observed similar TPR patterns for CoO/Al2O3 catalysts calcined between 380 and 1290 K. They found that five broad peaks corresponding to various CoO’s were obtained for specimens

Figure 9. X-ray diffractograms of (a) a freshly calcined catalyst at 973 K and (b) a freshly reduced catalyst at 873 K.

calcined at lower temperatures. A higher calcination temperature gave a well-integrated and sharper peak. Nevertheless, at calcination temperatures above 1025 K, a single sharp peak attributed to metal-support interaction was obtained at around 1200 K. TPO analysis, on the other hand, gave similar profiles for all eight catalysts, as shown by a broad asymmetrical peak at around 470 K (cf. Figure 8). This may contain two or more peaks from reoxidation of previously reduced phases. The TPO peak position in Table 4 also indicates that oxidation occurred at lower temperature for catalysts calcined using a higher heating rate. Oxide phases were reduced during TPR and were converted back to a single, thermally stable oxide phase during TPO.

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6011

Figure 10. SEM images of (a) a freshly calcined catalyst at 973 K and (b) a freshly reduced catalyst at 873 K.

3.4. Catalyst Morphology. X-ray diffractograms of an unreduced calcined catalyst, presented in Figure 9, show a strong signal for CoAl2O4 (2θ ) 36.7°) and NiAl2O4 (37° and 44.8°). Smaller Co3O4 (31.2°) and NiO (43.2°) peaks indicate the strong tendency of these oxides to react with the alumina support to form

aluminates (CoAl2O4 and NiAl2O4). Spinel-type NiCo2O4 (31°) was also observed; however, upon reduction this species along with Co3O4 and NiO was converted to metallic phases of Co (44.2° and 51.5°) and Ni (44.5° and 51.8°). Reduction at 873 K, however, has less influence on the more resistant metal aluminate species. In the reduced catalyst, the peaks of CoAl2O4 and NiAl2O4 were still evident, although with significantly reduced intensities. The SEM images in Figure 10 evince uniform distributions of alumina particles and metal crystallites, suggesting high-level dispersion at both micro- and nanolevels. The surface of the freshly reduced particles appeared to be “rougher”, probably because of the conversion of the more structured and ordered aluminates to Co and Ni atoms. The surface elemental profiles of the freshly calcined and freshly reduced catalysts recorded using EDX analysis are presented in Figure 11. The strong signal for Al in the freshly calcined catalyst is consistent with a high aluminate composition. However, in the reduced catalyst, there is practically no O on the surface, which contains high proportions of Co and Ni. Modest quantities of Al were present as a result of unreduced cobalt and nickel aluminates. These findings are in agreement with independent XRD data. The TEM images in Figure 12 featured a wide distribution of metals on the catalyst surface. A lowcontrast shell surrounding a core of metal particles in the freshly calcined catalyst corresponds to a layer of oxidized metals formed during calcination.20 Although the XRD data confirmed the presence of cobalt and nickel aluminates, it was more difficult to identify these species from TEM images because they would normally emerge as low-contrast phases. 3.5. Model Building. In addition to qualitative insights into the phenomenon or process in question, data derived from a full factorial experimental design may also be used for descriptive and deductive model building using orthogonal polynomials. For a 23-factorial design, the response variable, yi, may be written in terms of the three main factors and four factor-inter-

Figure 11. Surface elemental profiles of (a) a freshly calcined catalyst at 973 K and (b) a freshly reduced catalyst at 873 K.

6012 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 Table 5. Coefficients of the Regression Polynomials for Each Response Variable polynomial coefficient

SBET (m2 g-1)

a0 a1 a2 a3 a4 a5 a6 a7

119.94 -1.88

5.96

-10.64

-1.94 1.06

D (%)

d (nm)

Ed (kJ mol-1)

A/C × 1012 (arbitrary unit)

5.96

17.89

-15.61

96.54

-1.94 1.06

6.90 -4.10

Sm (m2 g-1)

ks (h-1) 3.94 4.52 0.27

-0.33

where subscripts U and L correspond to the upper and lower values of each variable, respectively. Thus, at Xt ) Xr ) XT ) 0, yi is equal to a0 (the value of the response variable for a sample conditioned at the lower limits of all preparation variables). In particular, eq 3 may be further simplified by deleting terms (factors and interactions) deemed statistically insignificant. In view of these considerations, data belonging to each measure of the physicochemical properties given in Table 2 were used to estimate the coefficients a0-a7 in eq 3, as summarized in Table 5. The relevant regression model was then used for constrained optimization to obtain the “best” preparation conditions for the particular catalyst property. For example, the optimum conditions for the BET surface area were obtained as Xt ) 0 (t ) 1 h), XT ) 0 (T ) 873 K), and a surface area, SBET, of 119.94 m2 g-1, while for optimal ks, the required preparation conditions are Xr ) 1 (r ) 20 K min-1), XT ) 1 (T ) 973 K), and Xt ) 0 (t ) 1 h), yielding an optimal kinetic coefficient, ks ) 8.73 h-1. This analysis shows that a quantitative catalyst design can be used for tailored catalyst synthesis. 4. Conclusions

Figure 12. TEM images of (a) a freshly calcined catalyst at 973 K and (b) a freshly reduced catalyst at 873 K.

action effects as

yi ) a0 + a1Xt + a2Xr + a3XT + a4XtXr + a5XtXT + a6XrXT + a7XtXrXT (3) where Xt, Xr, and XT are appropriately scaled independent variables (to minimize numerical errors) for the calcination time, heating rate, and calcination temperature, respectively. Additionally, XtXT represents the factor interaction between the calcination time, t, and calcination temperature, T.

t - tL r - rL T - TL ; Xr ) ; XT ) Xt ) tU - tL rU - rL TU - T L

This investigation has dealt with the systematic evaluation of the role of preparation (calcination) variables on the physicochemical properties of a Co-Ni/ Al2O3 catalyst. Statistical analysis showed that the calcination temperature and time played important roles in the total surface area creation while the heating rate and calcination temperature are the main determinants of the metal particle size, dispersion, and metal surface area. The acid-site strength and concentration are, however, unaffected by all of the factors studied. These results are in agreement with temperatureprogrammed data, which implicated the existence of five metal oxide phases including a metal aluminate phase. These oxide phases were irreversibly reduced by H2 and could not be regenerated in a subsequent TPO. SEM and TEM micrographs revealed the highly dispersed phase of metal deposition on the surface. Calcination kinetics also showed that metal nitrate decomposition involved a complex combination of chemical and physical (probably internal restructuring) steps because it was characterized by an Avrami-Erofeev model and an activation energy of about 82.6 kJ mol-1. The “best” preparation conditions for a specific catalyst property were obtained from a constrained optimization of the associated polynomial model. Acknowledgment

(4)

The authors are grateful to the Australian Research Council for financial support. K.M.H. also appreciates

Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6013

provision of a University Postgraduate Award from the University of New South Wales. Nomenclature A/C ) relative acid-site concentration a0-a7 ) coefficients of the polynomial regression model (cf. eq 3) D ) metal dispersion (%) d ) metal particle diameter (nm) -Ed ) heat of NH3 desorption (kJ mol-1) ks ) solid-state kinetic constant (h-1) r ) calcination heating rate (K min-1) SBET ) BET surface area (m2 g-1) Sm ) metal surface area (m2 g-1) T ) calcination temperature (K) t ) calcination time (h) Xi ) dimensionless independent variable, where subscript i ) t, r, or T w ) instantaneous sample weight (mg) wf ) final (end of peak) sample weight (mg) wi ) initial (start of peak) sample weight (mg) yi ) response variable of property i in the polynomial regression model (cf. eq 3) Greek Letter R ) solid-state conversion

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Received for review March 25, 2004 Revised manuscript received May 21, 2004 Accepted July 1, 2004 IE049760Z