ORIGINAL PAPER Pigmentary properties of rutile ...

Chemical Papers 67 (11) 1386–1395 (2013) DOI: 10.2478/s11696-013-0396-7

ORIGINAL PAPER

Pigmentary properties of rutile TiO2 modified with cerium, phosphorus, potassium, and aluminium Marta Gle´ n*, Barbara Grzmil Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland Received 9 November 2012; Revised 28 January 2013; Accepted 13 February 2013

The influence of different modifiers, phosphorus, potassium, aluminium, and cerium on the pigmentary properties of TiO2 was studied. The phase composition and distribution of modifiers in prepared TiO2 products was investigated using XRD analysis, the selective leaching method, and ICP–AES technique. The optical properties, photoactivity, morphology, and surface area of modified TiO2 were determined by spectrophotometric, fluorescent, SEM, and BET measurements. The research was directed towards obtaining a pigmentary TiO2 with the highest possible photostability. It was found that the final calcination temperature, at which the anatase–rutile transformation rate was > 97 %, depended on the kind and amount of the modifiers introduced into hydrated titanium dioxide. In comparing the colour of TiO2 products modified with Ce, it was found that the addition of K to the TiO2 series caused an increase in all the optical properties examined. The presence of K and Al in TiO2 modified with Ce resulted in decreased photocatalytic activity. The photostability of TiO2 modified with Ce and K improved with an increase in P2 O5 content. The highest photostability was measured for the TiO2 –CePKAl series. It was concluded that the differences in both the optical properties and photoactivity of TiO2 depended on its phase composition and the distribution of modifiers in the products obtained. c 2013 Institute of Chemistry, Slovak Academy of Sciences Keywords: titanium dioxide, modification, photostability, optical properties, pigmentary properties

Introduction In nature, titanium dioxide crystallises in three forms of brookite, anatase, and rutile (Diebold, 2003). Brookite is difficult to obtain, hence has no value in the TiO2 industry (Bellussi et al., 2002). Anatase is a superior photocatalytic material for air and water purification, water disinfection, and hazardous waste remediation; it is applied to thin films and batteries (Fu et al., 2006; Li et al., 2011). Rutile is the most widely used white pigment nowadays (Woditsch & Westerhaus, 1993; Tayade et al., 2007). Inorganic TiO2 pigments have applications in a variety of products including paints, inks, plastics, paper, rubber, ceramics, enamels, textiles, food, glasses, and pharmaceuticals (Lewis, 1988; Rao & Reddy, 2007). It is

worth noting that the increase in demand for TiO2 pigments in Europe is estimated at 3 % per year, while in Asia double-digit rates are projected. Global TiO2 pigment consumption increased by 9 % in 2010 (Elsevier, 2011). The more compact structure of rutile in comparison with anatase affords the differences between these two forms. Rutile is the most thermodynamically stable form of the TiO2 polymorphs. In addition, rutile possesses greater brightness, hiding power, tinting strength, whitening ability, and opacity (D˛abrowski et al., 2006; Reidy et al., 2006). As a consequence, this form of titanium dioxide is preferred in the pigment industry for its outstanding optical properties. The colour of TiO2 is decisive for its optical performance since the titanium dioxide pigment is used in a wide

*Corresponding author, e-mail: [email protected]

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M. Gle´ n, B. Grzmil/Chemical Papers 67 (11) 1386–1395 (2013)

variety of applications and in many binder and plastic environments. In commercial practice, alumina, silica, zirconia, tin oxides, and phosphates are introduced to improve the optical characteristics of the applied pigment (Braun et al., 1992). Apart from its optical properties, the durability of TiO2 under ultraviolet radiation in the presence of the organic matrix to which the pigment is applied is a most important issue. Titanium dioxide acts either as a strong UV absorber or as a UV-activated oxidation photocatalyst (Braun et al., 1992). In the first case, TiO2 protects the binder and in the second it degrades the organic matter. The photocatalytic degradation causes deterioration in mechanical strength, changes in colour, chalking, impairment, and embrittles organic binders, so represents a major problem to the pigment industry (Diebold, 1995). The rutile phase is characterised by the lowest photoactivity of titanium dioxide polymorphs (Jung & Park, 2004). As a consequence, rutile TiO2 is used when highly photostable pigments are required. Moreover, the titanium dioxide pigment requires a special treatment which affects the durability of organic coatings (Jesionowski et al., 2007). This involves the modification of TiO2 with colourless inorganic compounds (Bellussi et al., 2002). Certain substances can be introduced into TiO2 during the manufacture of white pigments and thus can increase the stability of the materials produced. The stabilised pigment particles absorb UV light and provide UV protection for the products pigmented with them. Allen et al. (2004) claimed that the destructive oxidation of the binder could be inhibited when alumina, silica, zirconia, and phosphates were used. The activity of the modified TiO2 has been the subject of a number of studies and has attracted attention in many applications (Wold, 1993; Choi et al., 1994; Karvinen, 2003). The photocatalytic performance of titanium dioxide appears to be a complex function of the modifiers’ concentration, their energy level within the TiO2 lattice, electronic configurations, and distribution (Yu et al., 1998). The optical and photocatalytic properties of TiO2 attract a great deal of attention (Colón et al., 2006; K¨ or¨ osi & Dékány, 2006; Du et al., 2008; Gle´ n et al., 2011; Grzmil et al., 2011). The present study focuses on the modification of titanium dioxide in order to obtain the pigmentary rutile with high photostability and improved optical properties. TiO2 modified with different compositions of phosphorus, potassium, aluminium, and cerium was determined and compared with the properties of unmodified TiO2 . The starting material was technical-grade hydrated titanium dioxide (HTD).

Experimental The starting material was the concentrated suspension of technical-grade hydrated titanium diox-

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ide (Chemical Plant “POLICE” S.A., Police, Poland). The HTD, a semi–product from the industrial sulphate process, contained 40 mass % of TiO2 which consisted of 3.0 mass % of rutile nuclei, 78 % of amorphous anatase, and 19 % of crystalline anatase. Aqueous solutions of appropriate modifiers (calculated to CeO2 , P2 O5 , K2 O, and Al2 O3 ) were prepared and introduced into the pulp of hydrated titanium dioxide. The molar fractions of CeO2 , P2 O5 , K2 O, and Al2 O3 in relation to TiO2 were 0.07 mole %, 0.03– 0.52 mole %, 0.18 mole %, and 0.24 mole %, respectively. The pulp obtained after thorough mixing was inserted into a laboratory muffle furnace (LM 312.13) and heated to the required temperature. The products thus prepared were calcined for 3 hours at 900–1120 ◦C with the process temperature gradually increasing corresponding to the conditions of commercial calcination. The final calcination temperature, at which the anatase-rutile transformation rate was > 97 %, depended on the type and amount of modifiers introduced into the hydrated titanium dioxide. The modified titanium dioxide products, with different compositions of 0.7–1.39 mole % of CeO2 , 0.68– 5.63 mole % of P2 O5 , 1.78–3.56 mole % of K2 O, and 2.35–4.70 mole % of Al2 O3 , were calcined at 1000 ◦C for 2 h for the purpose of X-ray diffraction analysis. The products were investigated using XRD analysis, ICP-AES technique, selective leaching method, spectrophotometric, fluorescent, SEM, and BET measurements. The contents of the modifiers were verified by ICPAES analysis (Optima 5300 DV, Perkin–Elmer, USA) and their distribution in TiO2 was determined using a selective leaching method described elsewhere (Gle´ n et al., 2011). The phase composition of the rutile TiO2 products was examined by powder X-ray diffraction (X’Pert PRO Philips diffractometer, Netherlands, CuKα radiation). The relative amounts of the anatase and rutile phases were calculated from the diffraction intensities corresponding to (101) reflection of anatase and (110) reflection of rutile. The mass fraction of rutile in titanium dioxide powders, WR , was determined from the following equation: WR = 100/(1 + IA /kI R )

(1)

where: IA and IR are the peak intensities of anatase (101) and rutile (110) and k is the coefficient (the ratio of peak intensity (101) 100 % of anatase to the peak intensity (110) 100 % of rutile). The optical properties of rutile TiO2 powders were characterised by measuring the colour in the white and grey systems. In the white system, the brightness and white tone and, in the grey system, the relative lightening power (tinctorial strength – TcS) and grey tone (spectral characteristics – SCx) of TiO2 products were determined. The test procedure using a Konica Mi-


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Table 1. Influence of modifiers, Ce, P, K, and Al, on final calcination temperature (TF ) of TiO2 x/mole %

TF / ◦C

No. P2 O5 1 2 3 4 5 6 7 8

– 0.03 0.10 0.16 0.03 0.10 0.16

K2 O

Al2 O3

unmodified – – – – 0.18 0.18 0.18

TiO2 – – – – – – –

x/mole % WR /%

No.

CeO2

0.07 0.07 0.07 0.07 0.07 0.07 0.07

900 930 1010 1020 1050 1020 1040 1090

98.6 98.5 99.9 98.7 98.6 99.4 98.1 98.8

nolta CM-600d spectrophotometer, Japan (Standard Illuminant C, 2◦ Standard Observer) was described previously (Gle´ n et al., 2011). The relative lightening power (TcS) and grey tone (SCx) were calculated using the following equations: TcSs = TcSr + 100(Ys − Yr )

(2)

SCxs = SCxr + (Zs − Xs ) − (Zr − Xr )

(3)

where TcSs , SCxs are the relative lightening power and grey tone of the examined TiO2 , TcSr , SCxr are the known values of the relative lightening power and grey tone of the reference TiO2 , Xs , Ys , Zs are the average values of trichromatic components X, Y, Z of the examined TiO2 and Xr , Yr , Zr are the known average values of trichromatic components X, Y, Z of the reference TiO2 . The brightness and white tone were calculated using the equations: Bs = Br + (Ys − Yr )

(4)

WTs = WTr + (Zs − Xs ) − (Zr − Xr )

(5)

where Bs , WTs are the brightness and white tone of the examined TiO2 , Br , WTr are the known values of the brightness and white tone of the reference TiO2 . The photocatalytic activity of the TiO2 products was assessed by a white lead-glycerine test (Gle´ n et al., 2011) using CIE L∗ a∗ b∗ system (Konica Minolta CM600d spectrophotometer, Japan, Standard Illuminant C, 2◦ Standard Observer). The photoactivity ∆E∗ was calculated from the measurements of L∗ , a∗ , b∗ values of TiO2 products prior to and after UV-VIS irradiation using the equation: ∆E ∗ = ∆L∗2 + ∆a∗2 + ∆b∗2 (6) where ∆L∗ is the lightness change of the TiO2 product after irradiation with X dose, ∆a∗ , ∆b∗ are the colour changes of in the TiO2 product after irradiation with X dose. The formation of OH radicals on the TiO2 surface under UV irradiation was analysed by the fluores-

9 10 11 12 13 14 15 16

P2 O5

K2 O

Al2 O3

CeO2

0.21 0.28 0.03 0.10 0.16 0.21 0.45 0.52

0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18

– – 0.24 0.24 0.24 0.24 0.24 0.24

0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07

TF / ◦C

WR /%

1110 1120 1010 1050 1070 1090 1110 1110

97.1 99.1 99.5 99.1 98.5 98.7 99.5 99.8

cence technique using terephthalic acid. Terephthalic acid reacts readily with OH radicals and produces a highly fluorescent product, 2-hydroxyterephthalic acid (Tryba et al., 2007). The intensity of the peak attributed to 2-hydroxyterephtalic acid is known to be proportional to the amount of OH radicals formed. In this study, the selected concentration of terephthalic acid solution of 5 × 10−4 mol L−1 was prepared in a diluted aqueous NaOH solution with a concentration of 2 × 10−3 mol L−1 . It has been shown that, under these experimental conditions, the hydroxylation reaction of terephthalic acid proceeds mainly with OH radicals (Ishibashi et al., 2000). TiO2 products of 0.2 g were stirred magnetically in 100 mL of the prepared terephthalic acid solution under UV-VIS irradiation (200–800 nm, climatic chamber with 6 Philips lamps of 20 W) with an intensity of 166 W m−2 for 90 min. Samples were taken every 10 min. After filtration through a 0.2 mm membrane filter, the solution was investigated using a fluorescence spectrophotometer (F-2500 Hitachi, Japan). The product of terephthalic acid hydroxylation, 2-hydroxyterephthalic acid, exhibits a peak at a wavelength of approximately 425 nm upon excitation with the wavelength of 315 nm. The peak areas were calculated in order to determine the amount of OH radicals. Brunauer–Emmett–Teller surface area (SBET ) measurements of the TiO2 samples selected were conducted using a Micrometrics Quadrasorb SI Quantachrome Instrument (USA). Nitrogen was used as the gas in the analysis and N2 adsorption–desorption measurements were performed at liquid N2 temperature. The surface morphologies of the TiO2 products were observed and assessed by scanning electron microscopy operating at 20 kV (SEM, JEOL JSM-6100, Japan).

Results and discussion In the following investigations, a pigmentary rutile TiO2 modified with cerium and different compositions of phosphorus, potassium, and aluminium was obtained. The contents of modifiers in relation to TiO2 (Table 1) were determined using the ICP–AES



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Fig. 1. Influence of different modifiers’ compositions on colour in the white system (brightness (a) and white tone (b)) of TiO2 .

method. The results of the analysis were consistent with the theoretical assumptions. X-ray diffraction analysis was used in order to determine the influence of modifiers on the anatase– rutile phase transformation. The results are shown in Table 1. The final calcination temperature, at which the anatase–rutile transformation rate was > 97 %, was found to depend on the kind and amount of modifiers introduced into the hydrated titanium dioxide. Unmodified titanium dioxide containing 98.6 mass % of rutile was obtained at 900 ◦C. The addition of 0.07 mole % CeO2 to TiO2 led to an increase in the final calcination temperature, so cerium acted as an inhibitor of the anatase–rutile phase transformation. The introduction of phosphorus to TiO2 modified with cerium (TiO2 –Ce product) caused a decrease in the degree of rutilisation, hence the final calcination temperature of TiO2 increased in comparison with unmodified TiO2 and TiO2 modified with Ce. Moreover, the final calcination temperature of modified TiO2 products with a degree of rutilisation > 97 % increased when a higher amount of P was introduced. However, it was observed that the addition of 0.24 mole % of Al2 O3 to TiO2 modified with Ce, P, and K (TiO2 – CePKAl series) slightly accelerated the anatase–rutile phase transformation in comparison with TiO2 –CePK products without aluminium. The optical properties and photocatalytic activity of the modified TiO2 products were determined. The objective was to observe the influence of modifiers on the colour and photostability of the TiO2 products assigned for pigment applications. The optical properties of unmodified and modified TiO2 products were characterised by measuring the colour in the white and the grey systems. The optical

properties of titanium dioxide products determined in the white system were brightness and white tone (Fig. 1), whereas the relative lightening power and grey tone were measured in the grey system (Fig. 2). The brightness (B) and white tone (WT) of unmodified TiO2 were 93.20 and –8.33, respectively. The brightness values for the modified TiO2 products depended on the type and amount of modifiers in the titanium dioxide. It was observed that the addition of K to the TiO2 –CeP series caused an increase in brightness but the addition of P to TiO2 modified with Ce and Al to the TiO2 –CePK series resulted in a decrease in B values. However, the increasing amount of phosphorus in the modified TiO2 products produced different effects. The increasing P molar fraction caused a decrease in brightness for the TiO2 –CePK series, an increase in B for the TiO2 –CeKAl series and an insignificant change in B values for the TiO2 –CeP series. The best brightness value of B = 93.85 had TiO2 modified with 0.07 mole % of CeO2 , 0.18 mole % of K2 O, 0.24 mole % of Al2 O3 , and 0.45 mole of % P2 O5 . Fig. 1 shows that the TiO2 –CePK series with molar fraction of P2 O5 in the range of 0.16–0.28 mole % had the highest white tone values. The lowest white tone value was recorded for the TiO2 –Ce product, due to the formation of a yellow cerium dioxide. The increasing amount of P resulted in an increase in WT values for all TiO2 series. The addition of K caused an increase in white tone but Al introduced into the TiO2 –CePK series negatively affected the WT values. The better white tone values for the TiO2 products with the increasing P molar fraction are presumably associated with the bleaching effect caused by formation of the white cerium compound CePO4 . From the TiO2 colour analysis in the grey system (Fig. 2), it was found that K caused a great in-


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Fig. 2. Influence of different modifiers’ compositions on colour in the grey system (relative lightening power (a) and grey tone (b)) of TiO2 .

Fig. 3. Influence of different modifiers’ compositions on photoactivity (a) and specific surface area (b) of TiO2 .

crease in the relative lightening power (TcS) in comparison with unmodified TiO2 , the TiO2 –Ce product and the TiO2 –CeP series. The addition of Al to the TiO2 –CePK series resulted in slightly lower TcS values. The introduction of phosphorus to TiO2 –Ce negatively affected the TcS values, which were even lower than the TcS values for unmodified titanium dioxide. With the increasing P content, the relative lightening power for the TiO2 –CePK series did not change significantly, whereas it decreased for the TiO2 –CeP series and the TiO2 –CePKAl series with P2 O5 molar fractions of 0.45 mole % and 0.52 mole %. It was observed that the grey tone values were the

lowest when the TiO2 –CePK series was further modified with Al. The addition of K to the TiO2 –CeP series led to an increase in SCx. The grey tone of the TiO2 products increased with an increase in phosphorus in the TiO2 –CeP series, in the TiO2 –CePK series up to 0.16 mole % of P2 O5 and in the TiO2 –CePKAl series up to 0.21 mole % of P2 O5 . The highest value of SCx = 1.41 was measured for TiO2 modified with 0.07 mole % of CeO2 , 0.18 mole % of K2 O, and 0.16 mole % of P2 O5 . The photoactivity experiments were performed using the white lead–glycerin test with the results given in Fig. 3. The photocatalytic activity values, ∆E∗ ,



Fig. 4. Influence of different modifiers’ compositions on formation of OH radicals on surface of TiO2 under UV-VIS irradiation (IF = fluorescence intensity).

measured in the presence of UV-VIS light, for the TiO2 –Ce and TiO2 –CeP series were similar to the value of ∆E∗ = 19.72 obtained for unmodified titanium dioxide. The presence of potassium in the TiO2 – CePK series and aluminium in the TiO2 –CePKAl series resulted in decreased photocatalytic activity in comparison with the ∆E∗ values received for the corresponding TiO2 series without these modifiers. The photostability improved with the increasing P content in the TiO2 –CePK series,. A similar relationship was observed for the TiO2 –CePKAl series, although above 0.45 mole % of P2 O5 , the ∆E∗ values began to increase. The highest photostability, which varied from 2.49 to 5.00, was measured for the TiO2 –CePKAl products with a P2 O5 molar fraction ranging from 0.10 mole % to 0.52 mole %. Although there is a relation between the photocatalytic activity and the modifiers introduced, there is no significant dependence between the ∆E∗ and SBET values obtained. The specific surface area of the samples investigated varied within the narrow range of ca 5–10 m2 g−1 (Fig. 3). In addition, the highly reactive OH radicals formed on the TiO2 surface under UV-VIS light may be responsible for the different photocatalytic activities of the unmodified and modified TiO2 in the study. Fig. 4 shows the formation of hydroxyl radicals for the different TiO2 products obtained. The highest amount of OH radicals with time of UV-VIS light was calculated for the unmodified TiO2 . The remaining materials in the study were characterised by lower values close to each other. Hence, in order to facilitate a comparison of the formation of OH radicals on the surface of the investigated products, an area under each curve was calculated. The area for unmodified TiO2 was set as 100 %.

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Fig. 4. shows that the formation of OH radicals is very similar for the modified titanium dioxide. Therefore, the relation between the photocatalytic activity of the modified TiO2 products and the OH formation on their surface is insignificant. However, it may be concluded from these results that the formation of reactive hydroxyl groups on the surface of the TiO2 materials in the study modified with different phase compositions of P, K, and Al is blocked by the formation of a protective layer which precludes the production of OH radicals. The pigment properties are highly dependent on the morphology of the TiO2 particles. The studies revealed that the surface morphology of all the TiO2 products examined was similar and the shape of the grains was irregular. Examples of SEM images of unmodified and modified TiO2 are shown in Fig. 5. It can be assumed from the SBET measurements, fluorescence results, and SEM images that the differences in the photocatalytic activity and optical properties of titanium dioxide depended on the type and contents of the introduced additives (in the range examined) but not on the TiO2 surface, OH formation, or morphology of titanium dioxide. Differences in the effect of modifiers on the optical properties and photoactivity of TiO2 result from the different phase compositions. A selective leaching method was used in order to determine the phase composition of the TiO2 products. The characteristics of unmodified and modified rutile TiO2 and the results from leaching these materials with various solutions are shown in Table 2. XRD analysis was also conducted to verify phase formation. To perform this analysis, the contents of the modifiers in the titanium dioxide products selected for leaching was increased ten-fold in relation to their initial values. The objective of the XRD studies was to observe the formation of different phases in a modified titanium dioxide. Thus, the contents of P2 O5 , K2 O, Al2 O3 , and CeO2 in TiO2 –CeP, TiO2 –CePK, and TiO2 –CePKAl series were 1.0–5.2 mole %, 1.8 mole %, 2.4 mole %, and 0.7 mole %, respectively. All the products were calcined at 1000 ◦C for 2 h. The XRD patterns for unmodified TiO2 and TiO2 –Ce are shown elsewhere (Gle´ n et al., 2011). Fig. 6 shows the XRD patterns for TiO2 –CeP1.0, TiO2 –CeKP1.0, and TiO2 – CeKP2.8. Fig. 7 shows the XRD patterns for TiO2 – CeKAlP0.3, TiO2 –CeKAlP2.1 and TiO2 –CeKAlP5.2. In our previous work (Gle´ n et al., 2011), the influence of Ce on the properties of titanium dioxide was determined. It was concluded that cerium formed a separate phase, CeO2 , and reacted partly with titanium, probably creating a co–phase, Ce0.8 Ti0.2 O2 . These phases were proved to be soluble in an acidic solution. However, Ce0.8 Ti0.2 O2 was shown to be insoluble in EDTA due to the lower content of titanium in this solution. The introduction of P into the TiO2 – CeP series led to the formation of cerium phosphate,


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Fig. 5. SEM images of unmodified TiO2 (a), TiO2 –Ce (b), TiO2 –CeP0.16 (c), TiO2 –CeKP0.16 (d), and TiO2 –CeKP0.03 (e), calcined at 900 ◦C, 930 ◦C, 1050 ◦C, 1090 ◦C, and 1010 ◦C, respectively.

which was identified by XRD analysis (Fig. 6A). The formation of the white cerium compound CePO4 is

also evident from the results of WT values (Fig. 1) where the white tone improved after the addition


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Table 2. Contents of elements in different solutions after leaching of TiO2 products Contents of modifiers

Contents of element in solution

Leaching degree of element

mmol L−1

%

Solvent mole % P

K

Al

Ce

Ti

P

K

Al

Ce

Ti

Unmodified TiO2

HCl EDTA NH4 OH

– – –

– – –

– – –

– – –

0.15 0.00 0.10

– – –

– – –

– – –

– – –

0.02 0.00 0.02

HCl EDTA NH4 OH

– – –

– – –

– – –

0.19 0.12 0

0.35 0 0.08

– – –

– – –

– – –

22 14 0

0.06

0.14 CeO2

0.10 P2 O5 ; 0.07 CeO2

HCl EDTA NH4 OH

0.42 0.11 0.70

– – –

– – –

0.19 0.17 0

0.53 0.07 0.02

33 8 55

– – –

– – –

43 39 0

0.09 0.01 0

0.10 P2 O5 ; 0.18 K2 O; 0.07CeO2

HCl EDTA NH4 OH

0.59 0.15 0.27

2.10 2.10 1.61

– – –

0.20 0.16 0

0.44 0.09 0

46 12 35

94 94 72

– – –

46 38 0

0.07 0.01 0

0.28 P2 O5 ; 0.18 K2 O; 0.07 CeO2

HCl EDTA NH4 OH

0.24 0.20 0.49

1.29 1.20 0.83

– – –

0.14 0.19 0

0.48 0.17 0.05

7 6 14

57 54 37

– – –

31 43 0

0.08 0.03 0.01

0.03 P2 O5 ; 0.18 K2 O; 0.24 Al2 O3 ; 0.07 CeO2

HCl EDTA NH4 OH

0.42 0.11 0.32

2.07 2.05 1.83

0.80 0.45 0.33

0.08 0.06 0

0.31 0.03 0

100 27 76

93 92 82

27 15 11

18 15 0

0.05 0.01 0

0.10 P2 O5 ; 0.18 K2 O; 0.24 Al2 O3 ; 0.07 CeO2

HCl EDTA NH4 OH

0.26 0.29 0.21

2.12 2.10 1.79

1.15 0.61 0.60

0.11 0.09 0

0.21 0.03 0

21 22 16

95 93 80

39 21 20

26 20 0

0.03 0 0

0.21 P2 O5 ; 0.18 K2 O; 0.24 Al2 O3 ; 0.07 CeO2

HCl EDTA NH4 OH

0.56 0.32 0.56

1.90 1.76 1.66

1.89 0.54 1.50

0.14 0.09 0

0.22 0.05 0

21 12 21

85 79 74

64 18 51

31 20 0

0.04 0.01 0

0.52 P2 O5 ; 0.18 K2 O; 0.24 Al2 O3 ; 0.07 CeO2

HCl EDTA NH4 OH

0.20 0.20 0.56

1.69 1.61 1.54

1.05 0.67 0.47

0.144 0.10 0

0.27 0.03 0

3 3 9

76 72 69

36 23 16

31 23 0

0.04 0.01 0

Fig. 6. X-ray diffraction patterns of TiO2 –CeP1.0 (a), TiO2 – CeKP1.0 (b), and TiO2 –CeKP2.8 (c); rutile ( ), KCe2 (PO4 )3 ( ), KTi2 (PO4 )3 ( ), CePO4 ( ), K(TiO)PO4 ( ).

•

◦

of phosphorus into TiO2 modified with cerium. The phase of phosphorus identified was cerium phosphate,

0.01

Fig. 7. X-ray diffraction patterns of TiO2 –CeKAlP0.3 (a), TiO2 –CeKAlP2.1 (b), and TiO2 –CeKAlP5.2 (c); rutile ( ), CeO2 (), CePO4 ( ), K2 SO4 (), KTi2 (PO4 )3 ( ), Al2 O3 (), AlPO4 ().

•

which is soluble only in EDTA (Onoda & Sakumura, 2006). However, the phosphorus in the TiO2 –CeP se-


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ries was also leached into HCl and NH4 OH but no additional amounts of titanium were released into these solutions in comparison with the TiO2 –Ce series. This indicates that phosphorus occurred as P2 O5 . Moreover, an incomplete release of P into the leaching solutions is presumably related to the formation of the insoluble co-phase of phosphorus with titanium. On the basis of the XRD studies (Figs. 6B and 6C) and the degrees of leaching of particular elements (Table 2), it was concluded that in the TiO2 –CeKP series potassium occurs in the form of KCe2 (PO4 )3 , KTi2 (PO4 )3 , K(TiO)PO4 , K2 SO4 , and KPO3 . Compounds of KCe2 (PO4 )3 , KTi2 (PO4 )3 , and K(TiO)PO4 , unlike in basic solution used, were proved to be soluble in acidic and complexing solvents (Gle´ n & Grzmil, 2012). Thus, a decrease in potassium and no additional titanium or cerium were observed in aqua ammonia. Potassium in the TiO2 –CeKP0.1 series was leached in proportions of 94 %, 94 %, and 72 % when HCl, EDTA, and NH4 OH were used, respectively. Nevertheless, in the TiO2 –CeKP0.28 series, a significant decrease in K and P released in the same solutions was observed. This is associated with the assumed formation of insoluble KPO3. It can also be observed that, with the increasing P content in the TiO2 –CeKP series, cerium was leached in higher amounts into EDTA and in smaller quantities into HCl. The results of the leaching and higher WT values (Fig. 1) confirm that more cerium phosphate was formed in the TiO2 –CeKP0.28 series. In the TiO2 –CeKAlP series, potassium leaching is similar to that of the TiO2 –CeKP series (Table 2). However, the decrease in K with the increasing P content in this series is not so considerable. This is due to the additional modification of the TiO2 –CeKAlP series with aluminium. An introduction of Al into the modified titanium dioxide resulted in the formation of AlPO4 (soluble in HCl and NH4 OH (Kolb et al., 1981) and Al2 O3 (soluble in all the solvents used) (Fig. 7)). The presence of aluminium phosphate caused an increased leaching of P in all solvents in the TiO2 – CeKAlP series in comparison with the other TiO2 series with phosphorus. Therefore, smaller amounts of KPO3 were formed in the TiO2 –CeKAlP series. However, the co–phase of P with Ti and potassium metaphosphate were presumably responsible for the decreasing amounts of P and K in the leaching solutions when higher contents of phosphorus were introduced into the TiO2 –CeKAlP series. Moreover, increasing the molar fraction of P2 O5 in the series in the study from 0.03 mole % to 0.21 mole % resulted in an increased degree of leaching of aluminium and cerium. This indicates that AlPO4 and CePO4 were formed in greater amounts. A further increase in P in the TiO2 –CeKAlP series had no effect on the release of Ce. However, the decrease in leaching of Al into HCl and NH4 OH can probably be observed due to the changes in the distribution of AlPO4, which

can occur not only on the TiO2 surface but also in the particle bulk. On the basis of the leaching of elements from the titanium dioxide samples and XRD analysis, the distribution of modifiers in titanium dioxide was determined. It can be concluded that the differences in both the photoactivity and optical properties of the modified titanium dioxide depended on its phase composition.

Conclusions The present work focused on the influence of different modifiers, P, K, Al, and Ce, on the phase transformation, optical properties and photoactivity of cerium–modified TiO2 . The research was directed towards obtaining the pigmentary rutile TiO2 with the highest possible photostability. The starting material used in the investigations was technical–grade hydrated titanium dioxide, which was a semi–product from the industrial sulphate process. In comparing the colour of the TiO2 products modified with Ce, it was found that the addition of K to the TiO2 series caused an increase in all the optical properties examined. On the other hand, the presence of the Al modifier in the TiO2 series negatively affected all the optical properties which were measured. The presence of K and Al in the TiO2 modified with Ce resulted in a decreased photocatalytic activity in comparison with the ∆E∗ values received for the corresponding TiO2 series without these modifiers. The photostability of TiO2 modified with Ce and K improved with an increase in P2 O5 content. The highest photostability was recorded for TiO2 –CePKAl products. It was concluded that the differences in both optical properties and photoactivity of TiO2 depended on its phase composition and distribution of modifiers in the products obtained. Due to the high photostability and promising values of the optical properties obtained for the TiO2 modified with Ce, P, K, and Al composition, it is worth studying this group of modifiers more extensively in order to obtain an excellent TiO2 pigment. Selected pigments may be found to be potential alternatives for use in paints, coatings, plastics, and other products which employ photostable titanium dioxide. Acknowledgements. This work was funded from financial support for science for 2008–2011.

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