The photocatalysed reduction of aqueous sodium carbonate on LaCoO₃ nano particles

Cui Xiaoliang, Yang Qiuhua, Fu Xixian*
(School of Science, Tianjin University, Tianjin, 300072, China)

Received Mar. 4, 2003; Supported by the National Natural Science Foundation of China (No.59772019)

1 INTRODUCTION
The increase in carbon dioxide has resulted in some serious environment hazards which have already alarm for future generations. Hence, attention has been raised to carry out an intense search of an alternate environmental friendly technology for energy production.
    The reduction of carbon dioxide using semiconducting powders as photocatalysts has been reported by Aurian-blajeni ^[1], TiO₂ and SrTiO₃ are known for their high stabilities towards photo corrosion and their favorite band energies^[2]. Possible ways to extend the semiconductor photo response to light absorption involve doping with suitable foreign cations^[3] and sensitizing by coating with photoactive dyes ^[4].
    A detailed survey of literature reveals that little attention has been paid to the use of nano complex oxides LaCoO₃ semiconductor in aqueous media for the photocatalytic reduction of carbon dioxide. The present work on this topic was therefore undertaken.

2 EXPERIMENTAL
All the reagents were Analar grade. The LaCoO₃ particles were prepared by a citrate method.
    50mL of 0.1mol·L^-1 lanthanum nitrate and 50mL of 0.1mol·L^-1cobalt nitrate were mixed in 60mL of 0.5mol·L^-1 citric acid (mole ratio of La: Co: citric acid =1:1:6) under magnetic stirrer. The mixture was evaporated under infrared lamp (250W) to become a viscous liquid. The next drying was carried out at 95ºC in a vacuum of 79-80 Pa for 6-8h to produce the required pink foamy precursors. The precursors were decomposed in air at 400ºC for 4h.The minimum calcination temperature for obtaining phase-pure crystalline sample of LaCoO₃ was 700ºC^[5]. The product was black powder.
    The crystal structure of catalyst was examined on an X-ray diffraction instrument (Rigaku Geigerflex-2038) using monochromatic Cu Ka radiation.
    The photoacoustic spectra of LaCoO₃ were recorded on an OAS-400 photoacoustimeter with a scanning range of 300nm-3.2m m.
    The photocatalysed reduction of aqueous Na₂CO₃ was carried out in a 250mL tapered bottle containing 100mg of LaCoO₃ and 50mL of 0.01 mol·L^-1 Na₂CO₃. The bottle was set on a vibrator operated at 120 times per minute. The whole reaction mixture was exposed to the light of different intensity. For this purpose a 450W fluorescent Hg lamp or sunlight at 9:00-10:00 a.m. in the mid- September of Tianjin were used as the irradiator. The distance between the bottle and lamp was 25cm. The temperature was attained by a bath cooler to prevent the photo reduced products from evaporation.
    The amount of formaldehyde and formic acid were determined spectrophotometrically using Nash reagent at l_max 412nm. Nash reagent was prepared by adding 150.0mg of ammonium acetate, 3.0mL of acetic acid and 2.0mL of redistilled acetyl acetone in 1L of water. The reagent was kept in the dark to avoid any photochemical reaction.
    For measurement of the concentration of formaldehyde, the test solution (0.5mL) was taken in a test tube and 2.0mL of Nash reagent was added. The mixture was heated for 5 min in a water bath at temperature of 50-60ºC. A bright yellow color was developed in the solution. This solution was then allowed to cool at room temperature and then the optical density was measured at l_max 412nm. The concentration of formaldehyde in the test solution was determined using a calibration curve. The concentration of formic acid was measured by an indirect manner using Nash reagent. First, the formic acid was converted into formaldehyde by treating it with magnesium and hydrochloric acid. Other product was not measured.

3 RESULTS AND DISCUSSION
According to the XRD spectrum of the prepared LaCoO₃ (Figure 1), the crystal lattice parameters of LaCoO₃ were calculated (Table 1). Compared to the theoretical values (No: 25-1060), we can find that the experimental value is well consistent with the theoretical one, which suggests that the prepared LaCoO₃ is a pure phase compound. By the TEM photography (Figure 2), the particle size of LaCoO₃ is 40-50nm.

Fig 1 XRD spectrum of LaCoO₃ (q/º)

Fig 2 The TEM photograph of catalysts (×10⁵)

Table 1 The theoretical and experimental values of crystal lattice parameters of LaCoO₃

3.1 Effect of photocatalyst concentration
The dependence of HCHO and HCOOH yields on the amount of LaCoO₃ was investigated and the results are summarized in Figure 3.
    It can be seen that an increase in the amount of photocatalyst results in the increase of the yields of both HCHO and HCOOH. This increase may be attributed to the exposed surface area of the reaction vessel, which is completely covered by a specific number of LaCoO₃ particles. However, after a certain limit (100mg) has been reached, there is no increase in the exposed surface area and hence also in the yield. There is a saturation point above which any increase in the amount of photocatalyst has negligible effect on the yields of HCHO and HCOOH.
    This may also be explained on the basis of geometry of the reaction vessel. This was confirmed by taking reaction vessels of different geometry and it was observed that the saturation point was shifted to a higher value when reaction vessels of larger capacity were used. On the other hand, the plateau was observed when small reaction vessels were used.

Fig 3 Effect of amount of catalyst on the yield of reduced products(C:HCOOH, B:HCHO)

Fig. 4 The photoacoustic spectra of LaCoO₃

3.2 Effect of different light sources
The effect of light sources on the formation of HCHO and HCOOH was also investigated using high pressure mercury lamp and solar irradiation. The results are reported in Table 2. It was also observed that higher yields of HCHO and HCOOH were obtained under solar irradiation than upon the mercury lamp. This may be explained on the basis that the range of wavelength of solar light is larger than mercury lamp (>410nm), while the absorption peaks of LaCoO₃ situated in ultraviolet region and visible light region (Figure 4). When the suspensions are irradiated with sunlight, the number of photons per unit area striking the catalyst increases and there is a corresponding increase in the photoreduction of carbonates to formaldehyde and formic acid.

Table 2 Effect of different irradiator on the yield of reduced products formaldehyde and formic acid

Note* Illuminating time 5h; amount of catalyst 100mg; 0.01mol·L^-1 Na₂CO₃

Na₂CO₃在纳米LaCoO₃体系中的光催化还原
崔晓亮 杨秋华 傅希贤
(天津大学理学院，天津300072)
摘要  以纳米半导体粉末LaCoO₃为催化剂进行Na₂CO₃水溶液的光催化还原实验。采用Nash试剂以分光光度法检测光还原产物甲酸和甲醛。研究Na₂CO₃浓度、催化剂用量和不同光源对光还原产物产率的影响。结果表明纳米LaCoO₃具有光催化还原活性。辐射导致LaCoO₃导带中产生光生电子。光生电子首先将CO₃^2-还原为HCOO^-，进而还原为HCHO和CH₃OH。
关键词  光催化还原；碳酸钠；纳米粒子；LaCoO₃

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