Physico–chemical properties of CdO–Al₂O₃ catalysts. I – Structural characteristics

2.1 Materials

Pure aluminum hydroxide was precipitated from 1 M ${\text{Al}}_2{({\text{SO}}_4)}_3·16{\text{H}}_2\text{O}$ (BDH) using 3 M NaOH (BDH) at room temperature and pH 6 with continuous mechanical stirring for 3 h. The gel was left to settle down for 24 h and then filtered using a Buchner funnel. The precipitate was washed several times with distilled water until it was free from ${\text{SO}}_4^{-2}$ ions and then dried at 125 °C to a constant weight. The dried precipitate was ground and reserved in stoppered glass bottle and designated as AN6. A second sample (AN7) was prepared following the same procedure but at pH 7. A third sample (AA7) was prepared by precipitation from 1 M ${\text{Al}}_2{({\text{SO}}_4)}_3·16{\text{H}}_2\text{O}$ at pH 7 using 30% ammonium hydroxide.

Cadmium hydroxide was precipitated from 1 M ${\text{CdSO}}_4·8/3{\text{H}}_2\text{O}$ (Merck) using 2 M NaOH at pH 8 during continuous stirring for 3 h. The gel was allowed to settle down over night, washed, dried to constant weight at 125 °C and then reserved and designated as C8. A second Cd(OH)₂ sample (C9) was similarly prepared but precipitation was made at constant pH 9.

Mechanically mixed gels were prepared using aluminum hydroxide gel attained by the addition of 3 M NaOH solution to 1 M aluminum sulfate at constant pH 7 and Cd(OH)₂ by the addition of 2 M NaOH to 1 M cadmium sulfate at constant pH 8 with continuous stirring. Five CdO/Al₂O₃ samples with Al₂O₃/CdO molar ratio = 1:0.25, 1:0.5, 1:1, 0.5:1 and 0.25:1, which contain weight% of alumina = 80, 66.7,50, 33.3 and 20, respectively, were prepared by mixing the appropriate amounts of alumina and Cd(OH)₂ for 3 h using a mechanical mixed gel followed by settling over night, washing with distilled water, drying at 125 °C until constant weight was obtained. ACM is the general designation of these mechanically mixed samples. The molar ratio following this designation determines the individual designation. Thus sample ACM (1:0.5) stands for a mechanically mixed oxide prepared by mixing 1.0 M of alumina gel with 0.5 M of CdO gel.

Five coprecipitated samples of varying composition were prepared by dropping simultaneously the appropriate amounts of 1 M aluminum sulfate, 1 M cadmium sulfate and 3 M NaOH from three separate funnels into a beaker containing 100 ml of distilled water with continuous stirring for 3 h at pH 8. The coprecipitates were allowed to settle down overnight and then washed till free from ${\text{SO}}_4^{2-}$ ions and dried at 125 °C until constant weight was obtained and then reserved in stoppered bottles until used. The molar ratios were Al₂O₃/CdO = 1:0.25, 1:0.5, 1:1, 0.5:1 and 0.25:1. The general designation is ACC, which when followed by the molar ratio will give the respective sample. The sample ACC (0.5:1) stands for coprecipitated oxide containing 0.5 M Al₂O₃ and 1 M CdO.

Calcination was made at 400, 500, 600, 800 and 1000 °C with the rate of heating at 10°/min and soaking for 4 h at the maximum temperature. To refer to the thermal treatment temperature, the calcination temperature is added to the designation of the sample. Thus for example AN7-400 refers to alumina precipitated with NaOH at pH 7 and then calcined at 400 °C for 4 h.

2.2 Techniques

Thermogravimetric analysis TG and differential thermal analysis DTA of uncalcined samples was carried in argon atmosphere using a thermal analyzer model LABSYS^TM (TG–DTA) produced by SETARAM (France). A weight of 50–70 mg was placed in a crucible of 100 μL capacity. The run was followed between 25 and 1000 °C at a rate of heating 10°/min.

X-ray diffraction XRD was carried out for all the 1000 °C-calcination products and for ACM (1:0.25)-400, ACM (1:0.25)-600, ACM (1:1)-800, ACC (1:0.25)-400, ACC (1:0.25)-600 and ACC (1:1)-800. The apparatus was Philips Diffractometer Type PW (1830). The pattern was obtained with nickel filtered copper radiation (λ = 15405 Ǻ) at 40 kV and 30 mA with scanning speed of 1° in 2θ min⁻¹. The spacing d corresponding to 2θ of the peaks were calculated and correlated with those of ASTM to determine the phases existing.

3.1 Structural properties

TG–DTA was carried out for pure and mixed gels in an argon atmosphere in the temperature range of 20–1000 °C at a heating rate of 10°/min. The % of ignition loss (I.L. wt%) of all the samples was also determined via calcination in the temperature range of 400–1000 °C for 4 h. Representative TG–DTA curves are illustrated, but the analysis of all the TG–DTA patterns is considered.

TG–DTA curves of pure alumina gels AN6 and AN7 (Fig. 1) depict four endothermic effects with their location depending on the pH of the precipitation. The first endotherm covers the range 60–110 °C with the maximum located at 88 °C, this endotherm is associated with ∼5 wt% loss ascribed to the loss of physisorbed water. The first endotherm is followed by a broad endotherm covering the range 130–300 °C with its maximum at 182 °C, the second endotherm is associated with 22 wt% loss attributed to the removal of chemically combined water and to the formation of γ-AlOOH. The 310–500 °C endotherm is broad with its maximum at 400 °C and is associated with ∼9 wt% loss attributed to the loss of chemisorbed water and hydroxyl groups and to the complete transformation to γ-Al₂O₃. The TG curve of these three endotherms is smooth indicating the overlap of these endothermic effects. The fourth endotherm 920–970 °C is sharp and well developed with its maximum centered at 940 °C and is associated with ∼6 wt% loss. This endotherm corresponds to the loss of the combined OH groups on alumina surface and the transformation of active alumina to θ-Al₂O₃ (Mackenzie and Berggren, 1970). The DTA curve of AN7 shows a shift of the first three endotherms to higher temperatures compared with their corresponding endotherms exhibited by AN6, the maxima of these effects are located at 103, 206 and 410 °C. Also, the fourth endotherm exhibited by AN7 is broad covering the temperature range 700–900 °C and its minimum is located at 800 °C. The TG curve of AN7 is also smooth and shows weight losses of 7, 10, 11 and 12 wt% for the first-four endotherms, respectively.

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Figure 1

TG–DTA curves of AN6 and AN7 samples.

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The DTA of AA7 precipitated with ammonia is different compared with that of AN7 precipitated at the same pH with NaOH. Thus, the DTA of AA7 (not illustrated) shows the first endotherm at 90 °C associated with ∼5 wt% loss. This endotherm was followed by a broad endotherm covering the range 153–400 °C with its maximum at 237 °C and is associated with 21 wt% loss. The DTA of AA7 does not show any other endothermic effects up to 900 °C where two interfering endotherms are shown at 905–980 °C, these two endotherms are associated with a sharp weight loss of 9% as depicted by the TG curve. Evidently, the DTA curve of alumina gels precipitated at pH = 8 following the same procedure are different compared with those precipitated at pH = 7. This refers to the importance of pH of precipitation of aluminas (El-Nabarawy et al, 1995; Alaya and Soubahy, 1993).

The TG–DTA curves of the two Cd(OH)₂ samples C8 and C9 (Fig. 2) are different. C8 exhibited five endothermic effects. The first covers the range 50–120 °C with the minimum centered at 96 °C. The second endotherm covers the range 139–255 °C with its minimum located at 235 °C. The second endotherm is directly followed by a sharp endotherm covering the range 255–330 °C with the minimum centered at 283 °C. The fourth endotherm is relatively broad covering the range 340–470 °C with its minimum at 456 °C. The fifth endotherm extends from 870 to 960 °C. These endotherms are associated with a gradual weight loss of 25.9 wt%. The TG curves indicate that a small weight loss of 1% is observed in the temperature range 450–850 °C. The weight loss is probably due to the removal of physisorbed water than chemically combined water followed by dehydroxylation and hydroxide–oxide transformation and finally to the removal of the OH groups on the oxide surface. The poorly developed exotherm at 400–850 °C may be ascribed to the transformation of the shapeless CdO to the cubic phase (Grge, 2009).

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Figure 2

TG–DTA curves of C8 and C9 samples.

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Sample C9 exhibits three endothermic effects. The first covers the range 40–100 °C and the second endothermic effect covers the range 255–340 °C as it appears in the DTA curve (Fig. 2) as a well developed sharp endotherm with a minimum at 294 °C and is associated with 9 wt% loss. The third endotherm covers the range 350–460 °C with a minimum centered at 394 °C. A fourth endotherm associated with 8 wt% loss starts to appear at 920 °C. The TG curve of the sample C9 shows a total gradual weight loss of 21.8%. The above-mentioned results indicate that the thermal behavior of pure gels depends on the pH of precipitation, the reagent of precipitation and the original salt used. This is agreeable with a previously reported suggestion (Mackenzie and Berggren, 1970).

Figs. 3 and 4 depict the TG–DTA curves of mechanically mixed and coprecipitated gels precipitated at pH 8 and mixed with a different percentage to contain between 20 and 80 mol% Al₂O₃. For samples containing 80 mol% Al₂O₃, i.e. samples 1:0.25 M and 1:0.25 C, endotherms with minima centered at 100 °C are observed and are associated with 4 wt% loss attributed to the removal of humidity and physisorbed water. These low temperature endotherms are followed by relatively sharp endotherms with minima located at 293 °C and associated with ∼28 wt% loss as shown by their corresponding TG curves. The third endotherm at 370–430 °C is sharp with a minimum centered at 400 °C. These two samples also exhibited a weak endothermic effect in the range 860–960 °C with minima located at 895 °C for the sample 1:0.25 M and at 920 °C for the sample 1:0.25 C. No exothermic effects have been shown by DTA curves of 1:0.25 M and 1:0.25 C.

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Figure 3

TG–DTA curves of mechanically CdO–Al₂O₃ gels.

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Figure 4

TG–DTA curves of coprecipitated CdO–Al₂O₃ gels.

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For the samples 1:0.5 M and 1:0.5 C, the endotherms with minima at 100, 296 and 900 °C, which are attributed to the loss of humidity and physisorbed water, decomposition of hydroxides to oxides and the removal of the OH groups on the oxide surface respectively are shown in the DTA curves of these samples. Evidently, for these two samples an exotherm covering the range 610–650 °C and centered at 627 °C is shown for each sample and is attributed to cadmium aluminate. The DTA curve of the sample with composition 1:0.5 C shows an endothermic effect with its minimum located at 130 °C. Figs. 3 and 4 also show that the endotherms appearing at 150 and 900 °C become more pronounced with the increase of cadmium oxide content and the minima of the peaks also shift to higher temperature with the increase of CdO content. The exothermic effect centered at 627 °C increases also with the increase of CdO content.

The DTA curve of sample 0.5:1 M shows two interfering endothermic effects with minima at 245 and 276 °C, whereas the endothermic effect of the samples containing 20 mol% Al₂O₃ covers the range 230–310 °C with minimum at 245 °C for the sample 0.25:1 M and at 276 °C for 0.25:1 C. These endotherms are associated with stepwise TG curves.

Table 1 lists the weight loss due to ignition (I.L. wt%) and those determined from TG (TG%) of the prepared samples, at different temperatures together with theoretically calculated values. Table 1 reveals that (i) the values of weight loss determined from TG at a certain temperature are always higher than those determined on ignition at the same temperature, which may be attributed to the sorption of humidity. (ii) TG loss and ignition loss are both higher than theoretically calculated values as a result of the decomposition of corresponding hydroxides. This indicates that the water contents of the gels are higher than those of the hydroxides. The pores in the gels retain some water specifically held on their walls. (iii) The rise of the precipitation pH decreased the water content and consequently the water loss upon thermal treatment. (iv) The water content of ammonia-precipitated samples is higher than the water content of NaOH- precipitated equivalents. (v) Coprecipitated oxides exhibited higher weight loss compared to mechanically mixed oxides of the same chemical composition. (vi) The weight loss at the temperature range 400–800 °C is small in case of mixed oxides whereas the weight loss is more or less gradual in the case of alumina gel. (vii) The precipitation pH affects the course of water loss.

3.2 X-ray diffraction (XRD)

The XRD of 1000 °C calcination products of all the samples and of 400, 600 and 800-calcination products was carried out. Fig. 5 shows the XRD pattern of 400, 600 and 800-calcination products. It is shown that the samples calcined at 400 and 600 °C are poorly crystalline whereas those calcined at 800 °C are well crystalline. Thus, for example the 800-calcination products of samples 1:1 C and 1:1 M are well crystalline showing the spectral lines and d-spacing characteristic of cadmium aluminate, CdAl₂O₄, (card no. 12252-16-3). This agrees with the DTA tracings of the samples, which showed exothermic effects at temperatures higher than 600 °C. The XRD studies indicated that heating of alumina gel at 400 °C led to the formation of the active γ-Al₂O₃ phase whereas the calcinations at about 900 °C gave the δ phase. Calcination at 1000 °C was found to be associated with the formation of θ-alumina as a major phase together with α-Al₂O₃ as a minor phase. Above 1000 °C, α-Al₂O₃ as the most stable phase predominates (Lippens and Steggerda, 1970).

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Figure 5

XRD patterns of some calcined products at 400,600 and 800 °C.

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Thermal treatment of cadmia gel at 500 °C results in the transformation to CdO. Further rise of the calcination temperature at ⩾850 °C resulted in the dehydroxylation of the oxide and to the gradual change of the oxide color from pale brown to black at 1000 °C. These changes are in good agreement with the XRD of 800 and 1000 °C products, Fig. 6, which depict the crystalline characteristics of Monteponite (card no. 05-0640).

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Figure 6

XRD patterns of the 1000 °C-products of ACM samples.

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Figs. 6 and 7 show the XRD patterns of 1000 °C- calcination products of mechanically mixed and co-precipitated CdO/Al₂O₃ systems. It is shown that for the sample 1:0.25 M, the principal spectral lines of θ-Al₂O₃ predominate whereas those characteristic of CdAl₂O₄ are less predominating. On the other hand, the principal lines characteristic of corundum (card no. 42-1468) predominate in the XRD pattern of the sample 1:0.25 C. This refers to the role of the method of preparation of mixed oxide in determining its structural characteristics. The XRD of 1000 °C-samples containing 66.7 mol% Al₂O₃ whether they are mechanically mixed or coprecipitated was the same. Thus the patterns showed θ-Al₂O₃ and CdAl₂O₄ phases. For 1000 °C-samples containing 50 mol% Al₂O₃, only CdAl₂O₄ showed its existence. The decrease of Al₂O₃ content to 33.3 mol%, the XRD patterns of both CdAl₂O₄ θ-Al₂O₃ and montiponite were shown. Further decrease of Al₂O₃ content to 20 mol% in samples thermally treated at 1000 °C gave XRD patterns showing CdAl₂O₄ as a minor phase and monteponite as a major one.

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Figure 7

XRD patterns of the 1000 °C-products of ACC samples.

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