Photocatalytic Behavior of Ceramic Compounds with Similar Structures: Ba3Li2Ti8O20 and AmM2nO4n+1(A=Li, Na, K; m=2, n= 3)

Leticia M. Torres-Martínez, Aracely Hernandez and J. C. Luna-Urzua

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Posted: September 2005

Topics Covered

Abstract

Keywords

Introduction

Experimental

Synthesis

Characterization

Photocatalytic Activity

Dinitroaniline Decomposition

Chromium VI reduction

Results and Discussion

X Ray Diffraction

Photocatalytic activity

Dinitroaniline Decomposition

Chromium VI Reduction

Conclusions

Acknowledgements

References

Contact Details

Abstract

Photocatalytic processes involving aqueous suspensions of semiconductors with analogous structures such as Li₂Ti₆O₁₃, Na₂Ti₆O₁₃, K₂Ti₆O₁₃ and Ba₃Li₂Ti₈O₂₀ have been studied in our laboratories.  The crystal structure of Ba₃Li₂Ti₈O₂₀ is essentially analogous to that of Na₂Ti₆O₁₃ in which edge-sharing trimers formed by three kinds of octahedra: the Ti(1)O6, Ti/Li(2)O6 and Ti(3)O6, share corners with other trimers forming infinite ribbons with a zig-zag arrangement.  The Ba atoms are inside the tunnels, which were formed between the ribbons of edge-sharing octahedral.  The synthesis of these compounds was achieved by sol-gel technique and by means of solid state reaction.  The solid products were characterized by X-ray diffraction (XRD), Differential Thermal analysis (DTA), Thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR).  The band gap energy (Eg) of the samples prepared was calculated from the UV-Vis absorption spectra.  The values obtained ranging from 3.4 to 3.0 eV.  The specific surface area was calculated by the BET method from Nitrogen adsorption isotherms.  The catalytic activity of the samples was studied using as a test reaction, the decomposition of 2,4-dinitroaniline at room temperature.  The decomposition rate was calculated applying the Langmuir-Hinshelwood model.  We have found that photocatalytic activity is favored when the structure of each compound has been formed.  The best activity for the binary titanates was performed by Li₂Ti₆O₁₃ sol-gel treated al 800^oC and for Na₂Ti₆O₁₃ and K₂Ti₆O₁₃ by solid state reaction.  In the case of Ba₃Li₂Ti₈O₂₀ sol-gel, the photocatalytic behavior on 2,4-dinitroaniline oxidation, was better for the solid obtained at pH 9-600^oC.  The crystalline phase was already formed on these materials.  Other isostructural ceramic materials such as: Na₂ZrTi₅O₁₃ and Na_1.8Ba_0.3Ti_5.9O₁₃, were also synthesized by both methods, solid state reaction and sol-gel.  We also present preliminary results of photocatalytic reduction of Cr(VI) with some of these semiconductor catalysts.

Keywords

Photocatalytic oxidation, ceramic semiconductors, tunnel crystal structures

Introduction

In recent years, it has been widely demonstrated that the interaction between ultraviolet radiation and a semiconductor catalyst has a strong potential for the destruction of toxic organics in industrial wastewater [1, 2, 3]. Photocatalysis has many potential applications for treating water containing organics, for removing metals from water, and for splitting water.  All these processes are interrelated.  Band gap illumination of a semiconductor particle suspended in water causes electronic transitions from valence band to conduction band creating electron-hole pairs.  Simultaneously, in the presence of a fluid phase, adsorption occurs and an electron transfer proceeds towards acceptor molecules (oxidizing species), whereas positive holes are transferred to donor molecules [4].

A major center of attention of current catalysis research is the elucidation of critical structure-function relationships for existing solid catalysts that can be subsequently utilized in the design of materials. These studies necessarily require the combination of materials synthesis, characterization and catalytic testing.

Inoue et al., have shown that sodium hexatitanate Na₂Ti₆O₁₃, combined with an active phase such as Ru, has the capability of invoking a photocatalytic decomposition of water to produce H₂ and O₂ at approximately the stoichometric ratio [5].  This mixed oxide is one of the alkali-metal titanates belonging to a chemical formula of M₂Ti_nO_2n+1 (alkali-metal atom).  

On the other hand, previous studies about the ternary phases formation in the system BaO-Li₂O-TiO₂ for compositions containing > 50% of TiO₂, demonstrated that the phase A, centered in Ba₃Li₂Ti₈O₂₀, showed electronic conductivity [6, 7], which indicates  that this compound can be used as semiconductor. The crystal structure of  Ba₃Li₂Ti₈O₂₀ is analogous to that of Na₂Ti₆O₁₃ and  Ba₂Ti₆O₁₃ [monoclinic, space group C2/m (no.12)] in  which edge-sharing  trimers formed by three kinds of octahedra: the Ti(1)O₆, Ti/Li(2)O₆ and Ti(3)O₆, share corners with other trimers forming infinite ribbons with a zig zag arrangement.  The Ba atoms are inside the tunnels which were formed between the ribbons of edge-sharing octahedra [8].

We assume that if the structure of the catalyst is analogous, the ternary titanate Ba₃Li₂Ti₈O₂₀ or other titanates with the same structure, should exhibit also photocatalytic behavior.  To probe this assumption we carried out the decomposition reaction of 2,4 dinitroaniline over the solids obtained under the same degradation conditions. However, in photocatalysis the catalytic activity of a semiconductor is not only determined by electronic properties, but also by particle size, surface area and its crystallinity.

The chemical history and thermal treatment of the sample is very important in the activity of a semiconductor [9-14]; hence, in this work, we have prepared the ceramic compounds Li₂Ti₆O₁₃, Na₂Ti₆O₁₃ and K₂Ti₆O₁₃ by sol-gel method and by solid state reaction in order to study their photocatalytic properties and the influence of the preparation method.  The Ba₃Li₂Ti₈O₂₀ was only prepared by sol-gel technique in acid and alkaline conditions.

Experimental

Synthesis

For A_mM_2nO_4n+1 (A= Li, Na, K; m= 2, n= 3) sol-gel synthesis, we use as metallic precursors titanium n-butoxide (98+%, Strem Chem) and the corresponding alkali-metal acetate (99.5%, Aldrich), t-butanol (99.5%, J. T. Baker) was used as solvent and glacial acetic acid (Aldrich 99%) was added under continuous stirring to reach pH 3.

For Ba₃Li₂Ti₈O₂₀ sol-gel, barium isopropoxide (Aldrich Chemicals 99.9%), lithium acetate dihydrate (Sigma 63.4%) and titanium isopropoxide (Aldrich Chemicals 97%) were the metallic precursors.  Ethanol (99.9% PROQUIM) was used as solvent and glacial acetic acid (PROQUIM 99.8%) was added under continuous stirring to reach pH 3 [15].  The reaction mixture was heated by reflux at 70°C.  The gels obtained were dried in air at 80°C for 48 h (fresh sample) and different portions were heated at 200, 400, 600 and 800°C for 6 h.  These ternary compounds were also prepared at pH 9 by sol-gel and ammonia hydroxide was used instead of acetic acid [16].

Only the binary compounds were synthesized by solid state reaction.  An intimate mixture of TiO₂ and the corresponding carbonates (Aldrich, 99%) was heated at 1250ºC over a period of 48-72 h.

Characterization

Infrared absorption spectroscopy was done in a Perkin Elmer Fourier transform infrared spectroscopy (FT-IR) Spectrometer Paragon 1000 PC in order to analyze the hydroxyl groups on the catalysts.  Nitrogen adsorption isotherms of the samples were measured by a gas sorption instrument (Micromeritics ASAP 2000).  The specific surface area was calculated by the BET equation.  The samples were analyzed by X-Ray Diffraction (XRD) technique in a Siemens D-5000 diffractometer with Cu Kα radiation ( λ = 1.5418Å ). The diffuse reflectance spectra of the solids were obtained with a Perkin Elmer spectrophotometer Lambda 12 coupled to an integration Labsphere RSA-PE-20.  A Spectralon standard USRS-99010 with 100% reflectance was used as a reference.

Photocatalytic Activity

Dinitroaniline Decomposition

Photocatalytic experiments were carried out at room temperature, 200 mg of each catalyst was added to a flask containing 500 ml of an aqueous solution of 2,4-dinitroaniline at 30 ppm.  Under stirring, the solution was irradiated in a closed box with a UV lamp Black-Ray model XX-15, which emits radiation at λ=254 nm at an intensity of 1100 μW/cm².  The reaction rate was followed by taking aliquots every 10 min and then analyzed in an UV-Vis spectrophotometer Perkin-Elmer Lambda-12.  The concentration of 2,4-dinitroaniline was calculated from the absorption band at 346 nm present in the UV-Vis spectra of this substance.  For a data evaluation we applied the Langmuir-Hinshelwood model, using the equation for heterogeneous photocatalytic reactions in a batch reactor [17].

To estimate the time in which C = Co/2, we use:

Where V is the liquid volume, C the substrate concentration, t the time, m the mass of catalyst, A the adsorption sites by g/cat, k₁ the apparent rate constant and k₂ the apparent adsorption constant.

Chromium VI reduction

The crystalline forms of Na₂Ti₆O₁₃ and Ba₃Li₂Ti₈O₂₀ and the ceramic compounds Na₂ZrTi₅O₁₃ and Na_1.8Ba_0.3Ti_5.9O₁₃ prepared by sol-gel, were used as catalysts in the photacatalytic reduction of Cr(VI) to Cr(III).  In these experiments 100-150 mg of each catalyst were added to a flask containing 200 ml of an aqueous solution of K₂Cr₂O₇ (reagent grade) at 30 ppm.  Initial pH of the solution was 1-2.  The liquid was maintained under continuous stirring while it was irradiated in the closed box with the same UV lamp used in 2,4 dinitroaniline degradation.  Concentration of Cr(VI) was analyzed by UV spectroscopy using diphenylcarbazide [18].

For all experiments, the starting solution with the catalyst was allowed to equilibrate in the absence of UV illumination for 30 min.

Results and Discussion

X Ray Diffraction

Figure 1 shows XRD patterns of the xerogel evolution of lithium titanate with temperature.    When the powder is heated at 200 and 400°C a few peaks are observed, which is indicative of a structure almost amorphous.  However, a small peak is observed at 25° associated to the main peak of TiO₂ anatase phase.  When fresh sample is heated at 800°C, the xerogel is transformed to yield a crystalline material with a powder pattern similar to that reported for Li₂Ti₆O_13.  By solid state method, according to XRD studies, it was not possible to obtain this compound, which is due to some loss of lithium by volatilization at higher temperatures.

The formation of Na₂Ti₆O₁₃ compound was achieved when the sample obtained by sol-gel was heated also at 800°C.  At 600ºC the anatase form is present.  By means of ceramic method, this compound was formed at 1250°C for 48 h.

The XRD patterns of thermal evolution of xerogel potasium titanate are shown in Figure 2.  We can see that the most important reflections for the compound K₂Ti₆O₁₃ were fitted at 600°C.  The small reflections point out the presence of potassium hexatitanate formed with a particle size in the nanocrystal interval.  It is evident in the sample heated at 800°C that the reflections observed do not correspond to the K₂Ti₆O₁₃ structure.  This difractogram is similar to that reported for TiO₂ rutile form which is stable at higher temperatures, it is possible that this sol-gel phase is metastable and up to 600ºC it is becoming into other crystalline specie, then the majority phase in the powder at 800°C is rutile.  By solid state method the K₂Ti₆O₁₃ compound was obtained at 1250°C by 72 h.

The formation of Ba₃Li₂Ti₈O₂₀ sol-gel was achieved at pH 3 when a fresh sample was heated at 800°C.  The crystalline structure was confirmed by the XRD pattern similar to that reported by Dussarrat and et al. [8]_.

When a ternary compound was synthesized by sol gel at pH 9, Rietveld studies demonstrated that Ba₃Li₂Ti₈O₂₀ is formed with a particle size in the nanocrystal interval at 600°C [19].  Nanocrystallinity of the compound is initiated at lower temperature when the sol-gel method is performed at alkaline pH. The sample annealed at 800°C showed a XRD pattern which corresponds to that reported in the literature to Ba₃Li₂Ti₈O₂₀ structure, but it has higher crystallinity.

Photocatalytic activity

Dinitroaniline Decomposition

Figure 3 shows the reduction of 2,4 dinitroaniline concentration with reaction time on sodium titanate catalyst prepared by sol-gel.  Similar graphs were done with the other solids prepared.  We can see that the more active sample is the solid obtained at 600ºC when the Na₂Ti₆O₁₃ structure was formed.

The values of k₁, k₂, k₁k₂ (apparent reactivity) and t_1/2 (apparent half-life) for each catalyst, on the decomposition of the 2,4-dinitroaniline,  were calculated applying the Langmuir-Hinshelwood model.   These data were estimated from a graph of (Ln(Co/C))/(Co-C) against   t/(Co-C). 

The photocatalytic activity of each semiconductor catalyst was, therefore, compared by use of the half life time values shown in Table 1.   

Table 1. Half life time values of the decomposition of 2.4-dinitroaniline on A_mM_2nO_4n+1 Sol-gel catalysts.

* TiO₂ as rutile form is the majority crystalline phase present

With sol-gel catalysts, we can observe, for lithium samples, that the most active catalyst is the material heated at 800ºC when the structure of lithium hexatitanate is already formed.  In the case of sodium sol-gel samples, the best activity was exhibited by the solid heated at 600ºC when the TiO₂ anatase form is present.  Previous works in the TiO₂ sol-gel synthesis [20] showed that at pH3 conditions, anatase is present with small percentages of rutile when the samples are annealed at lower temperatures (200-600ºC).  Some investigations on photocatalytic degradation of different organic compounds in water, showed that the anatase form is more active than the rutile phase [21, 22].  In this case the structure is formed from anatase to Na₂Ti₆O₁₃ at 800ºC.

For potassium materials, the activity is improved with the catalyst heated at 600ºC.  We have seen that at this temperature the K₂Ti₆O₁₃ structure is achieved with a small particle size which favored the photocatalytic activity.  At 800ºC, the rutile form is present and several works have reported a lower activity for this catalyst. The rate of the hole and electron recombination is higher for rutile than for anatase due to the semiconducting properties of two TiO₂ phases that are slightly different [23].

Among all sol-gel A_mM_2nO_4n+1 catalysts probed we can see that the best activity was achieved with lithium hexatitanate obtained at 800ºC.  The TiO₆ octahedra distortion is caused by the alkaline atom size.  For these experiments the less distorted structure exhibit the major activity on the photocatalytic degradation of 2,4-dinitroaniline.

On the other hand, according with half life time calculated with the catalyst prepared by ceramic method (Table 2), we can appreciate a great catalytic activity similar to lithium hexatitanate sol-gel.  In these materials the crystallinity is higher and the band gap energy values calculated from its UV-vis spectra were 3.04 and 3.08 eV for Na₂Ti₆O₁₃ and K₂Ti₆O₁₃  respectively, whereas for the same compounds obtained by sol-gel this values were 3.4 eV for Li₂Ti₆O₁₃ and Na₂Ti₆O₁₃ ; nevertheless  for K₂Ti₆O₁₃  was 3.1eV.

Table 2. Half life time values of the decomposition of 2.4-dinitroaniline on A_mM_2nO_4n+1  catalysts prepared by solid state reaction.

Once Ba₃Li₂Ti₈O₂₀ was used as a photocatalyst, we found a higher photocatalytic activity expressed as t_1/2 (Table 3), for the solid obtained at pH 3 and annealed at 800°C (t_1/2= 26 min).  The adsorption capacity of this catalyst is favored due to its higher degree hydroxylation, hence the OH groups behave as adsorption centres.  At this temperature (800°C), the crystalline structure of Ba₃Li₂Ti₈O₂₀ is formed according to XRD studies.

Table 3. Half life time values of the decomposition of 2.4-dinitroaniline on Ba₃Li₂Ti₈O₂₀ sol-gel catalysts.

For the ternary catalyst synthesized at pH9 the photodegradation is improved with the samples fired at 600 and 800ºC.  At basic conditions the sample calcined at 800ºC has the same crystalline structure than under acidic conditions, however, it lost almost all hydroxy groups causing high defects degree yielding vacancies.  Defects in an oxide semiconductor are linked to electronic properties.  According to the k₁ value calculated_, in this sample, electronic transference is favoured and it shows a good activity on 2,4-dinitroaniline decomposition (t_1/2 = 28 min).  However, the adsorption capacity is low due to material sintering that takes place by thermal treatment.  With the sample heated at 600ºC obtained at alkaline pH, the activity expressed as t_1/2=24min, is closer to those calculated for the samples annealed at 800ºC.  This behaviour is due to major specific surface area and the nanocrynstallinity of the ternary compound. 

Chromium VI Reduction

Table 4 shows the results of the initial studies on photocatalytic activity of Na₂Ti₆O₁₃ and Ba₃Li₂Ti₈O₂₀ compared with others isostructural compounds (Na₂ZrTi₅O₁₃ and Na_1.8Ba_0.3Ti_5.9O₁₃) on the hexavalent chromium photoreduction.  The activity was evaluated calculating the fraction of Cr(VI) reduced to Cr(III) after 3.5 h of UV illumination.  Kinetics studies are being done in our research group in order to evaluate the reaction rates with these compounds.

Table 4. Cr (VI) photoreduction percent of isostructural materials with monoclinic structure with space group C2/m.

Conclusions

When the sol-gel method was used to prepare ceramic powders, the acid conditions caused the formation of the compound with less surface area and less porosity, giving as a result lower catalytic activity.  The Eg values depend on both synthesis parameters: pH and thermal treatment.  However, according to these results, the photocatalytic activity is favored when the structure of each compound has been formed.  The best activity in 2,4-dinitroaniline decomposition, was performed by lithium hexatitanate sol-gel when the crystal structure was formed and also, by Na₂Ti₆O₁₃ and K₂Ti₆O₁₃ prepared by solid state reaction, therefore, it seems to be that the crystal structure is an important factor that has to be considered in the photocatalytic process.  In these isostructural compounds the atoms arrangement is similar but TiO₆ octahedra distortion caused by the alkaline atom size is different.  For these experiments the less distorted structure exhibits the major activity on the photocatalytic process.  On preliminary studies of chromium reduction, Na₂Ti₆O₁₃ sol-gel was the best catalyst.

Additionally, the synthesis method plays an important role determining the surface characteristics of the photocatalytic material.  It was found that the synthesis of the solid obtained by sol-gel technique under basic conditions improved the catalytic behavior of the material.  The sol-gel technique cause the structure formation with different kind of defects formed due to dehydroxylation process during thermal treatment.  The method and synthesis conditions cause different number, type and strength of surface active sites which determine the catalytic properties of the catalysts.

Acknowledgements

We acknowledged the financial support provided by Conact through grant 35415-U.  We are grateful to Dr. Tessy Lopez for her valuable supervision in sol-gel technique.  We also thank Brenda Fernandez and Lucia Castillo for performing the preliminary Cr(VI) photoreduction experiments.  J.C. Luna-Urzua wishes to thank the CONACT for awarding of the scholarship.

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Contact Details

Leticia M. Torres-Martínez

acultad de Ciencias Químicas
División de Estudios Superiores
Universidad Autónoma de Nuevo León
Apartado Postal 1625, Monterrey
Nuevo León, México

E-mail : [email protected]

Aracely Hernandez

Facultad de Ciencias Químicas,
División de Estudios Superiores,
Universidad Autónoma de Nuevo León, Apartado Postal 1625, Monterrey,
Nuevo León, México

J. C. Luna-Urzua

Facultad de Ciencias Químicas
División de Estudios Superiores
Universidad Autónoma de Nuevo León Apartado Postal 1625, Monterrey
Nuevo León, México

This paper was also published in print form in “Advances in Technology of Materials and Materials Processing”, 6[2] (2004) 184-191.