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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.51 n.4 Concepción dez. 2006

http://dx.doi.org/10.4067/S0717-97072006000400012 

 

J. Chil. Chi. Soc., 51, N°.4 (2006), p.1049-1051

 

CATALYTIC OZONATION OF OXALIC ACID WITH MnO2/TiO2 AND Rh/TiO2

 

CRISTINA QUISPE1, JORGE VILLASEÑOR1*, GINA PECCHI2 AND PATRICIO REYES2.

1Instituto de Química de Recursos Naturales, Universidad de Talca, Casilla 747 Talca, Chile.
2Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C Concepción, Chile.


ABSTRACT

The catalytic ozonation of oxalic acid in a batch reactor using TiO2, MnO2/TiO2 and Rh/TiO2 as catalysts was studied. The presence of the catalyst significantly improves the ozonation rate in comparison with the non-catalytic reaction. The adsorption and the extent of degradation of oxalic acid were found to be dependent on the solution's pH. Higher degradation rates were found for catalytic systems at pH 2.5, where the oxalic acid concentration in the water phase was almost completely removed. The contribution of the MnO2/TiO2 and Rh/TiO2 catalysts as well the TiO2 support is discussed. The highest mineralization, expressed as CO2 evolution, was reached with the MnO2/TiO2 catalyst at pH 2.5.

Keywords: Oxalic acid, catalytic ozonation, MnO2/TiO2, Rh/TiO2.


1. INTRODUCTION

Conventional water and wastewater treatment processes (i.e. physical and microbiologic treatments) have been extensively used in the last century [1 . Nevertheless, due to their limited effectiveness, new treatment alternatives have been studied, where the AOPs (advanced oxidation processes) appear to be a promising field of study [2-6 . The AOPs are based on the generation of free radicals, principally the hydroxyl radical, a powerful oxidant that attacks organic pollutants generating smaller molecules. In some cases, the complete mineralization of the pollutants into CO2 and H2O is not achieved. Thus, one limitation of these processes is the partial oxidation of organic matter resulting in poor total organic compound (TOC) removal and only a slight decrease in chemical oxygen demand (COD). Some simple compounds, such as formic, acetic or oxalic acid -known as recalcitrant compounds-, are very resistant to further degradation and remain in solution [7-9] .

Ozonation is an AOP widely used in many wastewater treatment plants for the oxidation of organic contaminants and for disinfection. Ozone, compared to other oxidizing reagents such as H2O2 or chlorine, is more efficient in pollutant degradation and less harmful to most living organisms [10] . In water, ozone can oxidize contaminants by means of direct selective reactions [11] , by for example the addition to double bonds or it can decompose through a chain reaction mechanism resulting in the production of free hydroxyl radicals, which are more oxidant than molecular ozone. The hydroxyl radicals are generated from ozone decomposition; this process is catalyzed by the presence of hydroxyl anions or by traces of other substances, such as transition metal cations.

O3 + H2O2 ® HO" + O2 0(1)

O3 + OH- ®O2"- + HO2" (2)

O3 + HO" ®O2 + HO2" 00(3)

O3 + HO2" ®2 O2 + HO" (4)

The decomposition of ozone in aqueous solution proceeds through the chain reaction shown above in equations 1-4 [12] , and leads to an indirect attack on organic compounds, which is faster than a direct attack by molecular ozone.

Many methods have been studied to increase the production of OH" [13,14] . One way to achieve this goal is to promote the catalytic decomposition of molecule ozone in water. Metal oxides (e.g. TiO2, MnO2, Al2O3 or Fe- or Cu-supported catalysts) activate molecular ozone and generate free hydroxyl radicals. This process, known as catalytic ozonation, has received particular attention during the last few years due to its lower costs and simpler operation [15-19] . The improvement in the pollutant degradation processes by using catalytic ozonation has been the driving force in the research of new catalysts capable to accelerate the ozone decomposition and to oxidize recalcitrant compounds achieving a complete mineralization.

The aim of this work is to study the catalytic ozonation of oxalic acid in aqueous solution over MnO2 and Rh supported on TiO2. For comparative purposes, non-catalytic ozonation was also studied. The influence of the solution's pH on the adsorption of oxalic acid onto the catalyst surface and on the degradation rates was also investigated.

2. EXPERIMENTAL

MnO2/TiO2 catalyst was prepared by the impregnation at room temperature of commercial TiO2 Degussa P25 with an aqueous Mn(NO3)2 solution in the appropriate amount to get 1wt% of MnO2. The solid was then dried at 393 K for 12 h and calcined in air at 773 K for 4 h [20] . A similar procedure was used to obtain the 1wt% Rh/TiO2 catalyst but using an aqueous solution of RhCl3 in the impregnation. After drying and calcination the solid was reduced in hydrogen flow at 773 K for 2 h [21] .

The characterization was carried out by evaluating the specific area and porosity in an automatic Micromeritics system Model Gemini 2370, of the N2 adsorption isotherm at -196 ºC in the relative pressure range 0.05 - 0.995. TPR experiments were carried out in a TPR/TPD 2900 Micromeritics system provided with a thermal conductivity detector. The reducing gas was a mixture of 5% H2/Ar (40 cm3 min-1), and a heating rate of 10 k min-1 was used. TPD of ammonia was carried out using an Ar flow of 50 cm3 min-1 as carrier gas. Ammonia pulses were dosed in order to saturate the catalyst surface at 100 ºC; then the sample was cooled to room temperature, and once the baseline was restored the temperature was linearly increased (10 K min-1) up to 500 ºC. The zero point charge (ZPC) was obtained by measuring the zeta potentials as function of pH suspensions. To measure, a Zeta-Meter Inc. (Model ZM-77) was used with 20 mg of 2 m catalyst particles ultrasonically suspended in 200 ml of 10-3 M KCl solution. H2 adsorption measurements were performed in a conventional volumetric system. Temperature programmed desorption studies were conducted in a TPR/TPD 2900 Micromeritics apparatus. The extent of adsorbed ammonia was evaluated from TPD experiments of NH3 previously adsorbed at 373 K. The oxalic acid adsorption was performed in conditions similar to those used in the catalytic measurements, described later on, but in absence of ozone.

The reactants used in this work were HPLC grade-Merck. The degradation processes were performed in a Pyrex batch reactor [22 charged with 90 ml of an aqueous solution of oxalic acid, having an initial concentration of 35 mg L-1. The catalyst (2.2 g L-1) was maintained in suspension under magnetic stirring. To generate ozone, an ozonizer OZOCAV was fed with an oxygen flow of 50 ml/min to reach an ozone concentration of 22.1 mg L-1. The ozone concentration was measured by absorbance at 254 nm, considering a molar extinction coefficient of 2900 M-1 cm-1. The catalytic activity was measured at atmospheric pressure, maintaining a constant temperature of 20ºC during a period of 60 min. Aliquots of the samples were extracted from the reactor, filtered using a 0.20 Millipore filter, and analysed with a HPLC system Perkin Elmer Series 200 chromatograph provided with an UV-VIS detector monitoring at 200 nm. A transgenomic ORH-801 column (6.5 mm×300 mm) was used with 0.01 N sulfuric acid as eluent at a flow of 0.8 ml min-1. CO2 evolution was monitored by gas chromatography using a Shimadzu GC-8A chromatograph with a Porapak Q column and a thermal conductivity detector.

The influence of pH on the adsorption processes was studied by placing 200 mg of catalysts in 90 ml of oxalic acid (35 ppm) and adjusting the pH at 7, 5 and 2.5 with small aliquots of aqueous NaOH. After 60 minutes, the catalysts were filtered off using a Millipore filter (20 µm) and the solution was analyzed by HPLC.

3. RESULTS AND DISCUSSION

Table 1 summarizes some characterization results of the studied catalysts. The specific surface area of the solids obtained from nitrogen adsorption isotherms at 77 K showed the expected trends. Thus, TiO2 support displayed a value of 49 m2g-1 equivalent to the Rh/TiO2. The addition of manganese oxide to the support produces a slight decrease in the surface due to surface coverage.


Temperature-programmed profiles of TiO2 did not show any reduction peak. Whereas Rh/TiO2 displays a single peak centered at 390K and MnO2/TiO2 shows a doublet with the first one centered at 615 K and the second one at 680 K, where the intensity of the first is approximately twice that of the later. In both studies, hydrogen consumptions is close to the amount required for the complete reduction of the impregnated phases. It should be mentioned that the TPR of bulk MnO2 presented similar trends with maximum values at 610 and 745 K. The observed shift in the second peak towards lower temperatures is indicative of the presence of TiO2 supports, which play an important role in favoring MnO2 reduction.

Surface acidity obtained from the TPD of ammonia showed a single, wide peak centered at 533 K for TiO2. Rh/TiO2 also displayed a single but better defined peak centered at 546 K, whereas MnO2/TiO2 catalyst showed two peaks, the first one was broad and centred at 533 K while the second presented a higher intensity centred at 790 K. Table 1 presents surface acidity estimated from TPD data. The MnO2/TiO2 catalyst displays the highest value. If it is considered that the bulk MnO2 posses a surface acidity of 0.095 meq g-1 and the MnO2 loading is only 1wt%, only a slight increase should be expected. However, the obtained surface acidity of MnO2/TiO2 is approximately 50% higher than the pure support, which is indicative of MnO2's high surface coverage.

The ZPC values are also displayed in Table 1. Taking into account that the bulk TiO2 displays a ZPC of 6.1, a decrease in 0.5 pH units after the deposition of MnO2 on TiO2 is observed. Since the ZPC of bulk MnO2 is 3.4, an estimate of surface coverage indicates that approximately 20% of the TiO2 surface is coverage with 1wt% of MnO2, indicating a high dispersion of the oxide phase. With regard to Rh/TiO2 catalysts, only a small change was observed as a consequence of metallic catalysts in a strong metal support interaction (SMSI).

Hydrogen chemisorption was measured only on Rh/TiO2. The obtained H/Rh ratio was 0.050. This value is not indicative of a poor metal dispersion, but implies that the rhodium is partially covered by TiO2 species generated by the SMSI effect since the catalyst was reduced at 773 K. This interpretation is confirmed by TEM studies that revealed an Rh particle size of approximately 3.3 nm.

An important step in catalytic ozonation is substrate adsorption on the surface. Table 2 presents the oxalic acid adsorption percentage at 293K on the studied solids. The values were obtained at different pH values (2.5, 5.0 and 7.0) and correspond to the equilibrium values. Considering the low concentration of oxalic acid (35 ppm equivalent to 3.9 x 10-4 mole L-1) and the relatively high acid strength (pKa1 = 1.19), it is possible to deduce that there is 95% ionization at pH 2.5 and 97% at pH 7, indicating that principally hydrogen oxalate anions species exist in solution. After one hour at pH 2.5, it was found that approximately 31 % of oxalic acid was adsorbed by TiO2 Degussa P25, while 27 % and 47 % were adsorbed while by the Rh/TiO2 and MnO2/TiO2 catalysts, respectively. Comparing the adsorption experiments performed at pH 7 on TiO2, it is evident that the adsorption of oxalic acid increases as the solution's pH turns acid. At pH 7, the percentage of oxalic acid adsorbed on the catalyst was 9.6 % and increases to 31 % at pH 2.5. Similar trends were observed for MnO2/TiO2 and Rh/TiO2 catalysts. This behavior can be explained by the ZPC of the catalysts since the ZPC of MnO2/TiO2 and TiO2 catalysts are 5.6 and 6.1 respectively. At pH 7, the solid's surface is negatively charged, and electrostatic repulsion effects prevent adsorption of the predominant hydrogen oxalate anions. At pH 5, the surface is positively charged (below ZPC), favoring adsorption of hydrogen oxalate anions. At pH 2.5, this effect is stronger due to the large extension of positives charges on the catalyst's surface allowing more oxalate species to be adsorbed. Since the adsorption processes are similar in the three catalysts, the improvement observed on Rh/TiO2 and MnO2/TiO2 cannot be explained just in terms of interactions with the support.


The catalytic ozonation of oxalic acid at 293 K and different pH values for the studied solids are displayed in Figure 1. It has been expressed as the extent of substrate disappearance versus time on stream. For all the studied catalysts, the trend is similar, disappearing fast during the first five minutes, and then it remains almost stable. Still, significant differences can be observed in the extent of disappearance. A decrease in the pH value can be related to an increase in the extent of disappearance. This behaviour is displayed by the three studies catalysts. In the case of the reaction in absence of catalyst, it occurs at a much lower extent. Even though the studied catalysts did not show significant differences in the catalytic activity, higher activity at pH 5 and 7 is displayed by MnO2/TiO2 catalysts. At pH 2.5, MnO2/TiO2 catalysts exhibit almost the same activity. The catalytic processes produce an important enhancement in the activity especially at lower pH. Thus, the extent of disappearance in the catalytic system at pH 2.5 is twice the observed on the pure ozone degradation. At pH 5, the catalytic processes are approximately 6 to 9 times more effective.


It is well known that the solution pH may significantly affect the decomposition of molecular ozone, and consequently the generation of hydroxyl radicals, which are essentially the oxidant agent. At basic pH, the reaction rate is enhanced; whereas in acid medium, pH < 3 hydroxyl radicals do not affect the decomposition [13] . It can be seen that in pure ozonation experiments carried out at pH 7, 5 and 2.5, the concentration of oxalic acid remaining in water was almost constant. This result can be explained that for this non-catalytic system, the previous effect has only a slight importance. Nevertheless, after the addition of a catalyst, an increase in oxalic acid disappearance was observed. This fact may be explained considering that the catalytic ozonation is a complex process, where several processes such as diffusion, adsorption, electron transfer, and desorption are simultaneously taking place.

Thus, apart from the disappearance due to the ozonation process, substrate adsorption and catalytic degradation are also occurring. From the results given in Figure 1 and Table 2, it is clear that the disappearance of oxalic acid at pH 7 and 5 mainly occurs by direct ozonation and adsorption on the catalyst surface with almost no contribution of the catalytic processes. On the other hand, at a solution pH of 2.5, the catalytic degradation plays a role even more important than substrate adsorption and non-catalytic oxidation.

The obtained results can be explained considering that molecular ozone could be adsorbed on surfaces through one of its terminal oxygen atoms [12] as shown in Eq 5.

O3 + S Û O=O-O-S 000000000(5)

where S represents an active site at the catalyst surface. Several groups have reported that TiO2 has no or little participation in the decomposition of aqueous ozone [23 , while manganese dioxide and rhodium have been shown to improve its decomposition rate [24,25] .

It is well known that ligand exchange reactions can take place on metal oxide surfaces [12,26] . Eq. 6-8 represents a scheme of these reactions:

M(OH)(H2O) + L « M(H2O)L + OH 00(6)

M(OH)(H2O) + L « M(OH)L + H2O 00(7)

M(OH)L1 + L2 « M(OH) L2 + L1 0000(8)

where M is a metal, L is a Lewis base (e.g. L1 ozone L2 anionic species in water as Cl-). These reactions are highly pH dependent. At basic pH, the exchange reactions given in equations 6 to 8 are negligible due to the high concentration of the hydroxyl anion, which is the strongest monovalent Lewis acid. In contrast, at acid pH all the exchange reactions can take place.

It has been reported [18 that MnO2 can also include a few Mn(III) centres. These ions are capable to form actives sites type >Mn-OH2+ and >Mn-O- depending on the solution pH and ZPC. The first species is present at pH below ZPC and can undergo nucleophilic attack, as follows:

> Mn-OH2+ + HC2O4- « > Mn-O-C2O3H + H2O00(9)

Considering that both reactions can occur at the same time, i.e. oxalic acid and ozone adsorbed on the catalyst surface, the reactions (Eq. 5 to 9) may occur leading to a complete mineralization as can be observed in Figure 2 where a high evolution of CO2 is reached by the MnO2/TiO2 catalyst.


In this Figure, it is evident that the maximum evolution of the final mineralization product is observed in the O3+MnO2/TiO2 system, and is also important for the O3+Rh/TiO2. The maximum in both catalysts are in the range 15 to 20 min. For these samples, almost no intermediate products were detected after 90 min on stream. Therefore, the observed enhancement in the oxalic acid degradation when the Rh/TiO2 and MnO2/TiO2 have been used as catalyst can be explained by the combination of adsorption processes on the TiO2 support and the effective decomposition of ozone on the MnO2 and Rh sites.

4. CONCLUSIONS

The refractory organic compound oxalic acid was degraded using catalytic and non-catalytic ozonation. It was found that the catalysts, MnO2/TiO2 y Rh/TiO2 improved the degradation rate of oxalic acid. A key factor in the degradation process is the adsorption of ozone/oxalic acid on the catalyst support. Adsorption and degradation were found to be pH dependent. The highest adsorption and degradation values were found at pH 2.5.

ACKNOWLEDGMENTS

The authors thank to Dr. Alvaro Delgadillo for his help and useful comments on this manuscript. C. Q. thanks the Universidad de Talca for a Doctoral Grant and Enlace-Fondecyt Grant VAC 600355.

 

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* Corresponding author. Tel.: 56-71-200272; Fax: 56-71-200448 E-mail address: jvillase@utalca.cl
 

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