Nitrogenous pollutants are one of the most harmful contaminants in natural water, and large quantities of these substances are discharged directly into the environment, leading to serious pollution [1, 2]. Aniline, a model nitrogen-containing compound originating from different sources (e.g., dyes, polymers, pesticides, fertilizers and paintings) [3], is easily adsorbed through the skin, which can cause headaches, nausea, vomiting and other symptoms. According to the sewage treatment standard of the US Environmental Protection Agency, the emission standards for aniline is limited to 262 μg/Lin water [4]. Therefore, an effective water treatment method is imperative to detoxify aniline.

At present, the conventional treatment methods for organic sewage include physical adsorption, biological degradationand advanced oxidation processes [5-7]. The physical adsorption method simply transfers the pollutants onto a porous solid surface. However, the organic compounds cannot be degraded into non-toxic substances and still pose a threat to the environment. Although bio-degradation is widely used in organic sewage treatment, the highly toxic matter (e.g., aniline) always inactivates biological bacteria. As an advanced oxidation technology, catalytic wet air oxidation (CWAO) is promising to convert toxic, non-biodegradable and highly concentrated organic compounds to CO₂, H₂O and other innocuous end-products using oxygen or air as the oxidizing agent [8].

In practical sewage treatment, an ideal catalyst becomes the key factor of the CWAO technology for industrial application. Therefore, great efforts have been made to research highly efficient catalysts. Various types of homogeneous and heterogeneous catalysts for the oxidation of model compounds (e.g., phenol, carboxylic acids, N-containing compounds) have been developed [9, 10].Homogeneous catalysts (e.g., Cu²⁺, Fe³⁺, Mn²⁺) have some merits such as better diffusion and a more active center in solution, which are favorable for the catalytic performance [11]. However, an additional separate step is essential to prevent secondary pollution. Hence, the heterogeneous catalysts have attracted more and more attention over the years owing to their high activity and easy separation [12, 13]. Noble metal catalysts (such as Pt, Ru, Pd, Ir) supported on carbon materials or oxides (such as CeO₂, ZrO₂, TiO₂) have been widely investigated in the CWAO of organic compounds [14, 15]. Sergio et al. [16] considered that the conversion of aniline was enhanced by increasing the catalyst porosity and Pt dispersion. Barbier et al. [17] revealed that the addition of Pd into the Ru/CeO₂ catalyst improved the aniline conversion and the nitrogen selectivity. Castillejos-López et al. [18] compared the support effects in Ru catalysts applied for CWAO of aromatic compounds, and concluded that the use of activated carbon as a support enhanced the conversion and mineralization.

Although Ru is the most effective metal for CWAO of nitrogenous organics, the phenomenon of catalytic deactivation has inhibited its wide application. For example, Ru leaching has been verified since a complexation reaction occurs between the nitrogen atom and noble metal [19]. In addition, the formation of carbonaceous deposits on the catalyst surface decreases the collision probability between the active components and the organic matter [20]. Therefore, it is of great interest to develop a catalyst with the properties of high catalytic activity and excellent stability.

In this article, TiO₂ is modified by doping zirconium or cerium into the crystal lattice. To determine the optimum reaction conditions for the degradation of aniline, the influence of the reaction temperature and reaction pressure are investigated. In addition, the concentration of the reaction by-products with time on-stream is evaluated over Ru/Ti_0.9Zr_0.1O₂ so as to establish the reaction path. To explore the relationship between the catalytic physico-chemical properties and the reaction activity, the corresponding characterization techniques, such as X-ray diffraction (XRD), N₂ adsorption, high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H₂-TPR), are conducted.

According to the previously reported procedure [21], all of the investigated supports were prepared by the sol-gel method. Typically, a tetrabutyl orthotitanate (Ti(OC₄H₉)₄) solution, as titanic precursor, was quickly added into ethanol (95%) under continuous agitation until a white colloidal substance was generated owing to the reactant mixture hydrolysis reaction. Then, the obtained pseudo-gels were dried in an oven at 120 ℃ for 12 h. Finally, all samples were calcined at 500 ℃ for 6 h at a heating rate of 2 ℃/min. The preparation of Ti_0.9Zr_0.1O₂ and Ti_0.9Ce_0.1O₂ supports followed the same operation steps, where the alcohol solution, containing ZrOCl₂·8H₂O and Ce (NO₃)₃·6H₂O, was gradually transferred to the Ti(OC₄H₉)₄ precursor solution. After a period of time for stirring, the yielded pseudo-gels were further treated as the TiO₂ processing steps.

Catalysts containing 2 wt% Ru were prepared by the excessive impregnation method using aqueous solutions of ruthenium chloride. First, the prepared mixed oxides were immersed in the precursor solution of Ru salts under stirring for 6 h, then the excess water was removed by rotary evaporation. The obtained powders were separately reduced at 300 ℃ for 3 h under flowing H₂ (60 L/h) to achieve the as-prepared catalysts.

The catalytic wet oxidation reactions were performed in a 250 mL batch reactor which was equipped with a magnetically driven stirrer and an electric heating jacket. In a typical run, 0.3 g of the catalyst and 75 mL of the aniline model compound, with an initial chemical oxygen demand (COD) concentration of 5300 mg/L, were added into the reaction kettle. To avoid diffusion limitations, a high stirring speed (900 r/min) was selected. Before heating, the reaction kettle was repeatedly purged with nitrogen to replace the air. When the temperature was increased to the desired point, oxygenwas introduced into the reactor. This was taken as the "zero" time for the reaction. Then, samples were periodically acquired from the reactor. The non-catalyst oxidation reaction was also carried out under identical operating conditions. After the reaction, the used catalysts were recovered from the solution for the purpose of the cyclic experiment.

The evolution of the concentration of aniline and organic intermediates was analyzed by high-performance liquid chromatography (Agilent 1200) equipped with an Agilent ODS-C18 column and a UV-detector set at 254 nm. The isocratic elution was a mixture of methanol:water = 60:40 (vol/vol) with a flow rate of 0.45 mL/min.

The concentrations of ammonia, nitrite, and nitrate ions in the liquid samples were monitored using an ion chromatograph (HPIC, Dionex) equipped with a cationic column (CS12A, 4 mm × 250 mm) and an anionic column (AS14A, 4 mm × 250 mm). The mobile phases for the cations and anions were HNO₃ (1.7 mmol/L)/dipicolinic acid (0.7 mmol/L) and Na₂CO₃ (3.2 mmol/L)/NaHCO₃ (1 mmol/L), respectively.

The COD and total nitrogen content (TN) were determined by a 6B-2000 multiple parameter water quality analyzer. Prior to measurement, the samples were digested using the special reagent at 150 and 120 ℃, respectively. The TN was determined using the alkaline K₂S₂O₈ ultraviolet method. In this article, the TN represented the total amount of ammonium ions, nitrate ions, nitrite ions and all the nitrogenous organic matter in solution. The nitrogen balance was established by comparing the TN values and the calculated chromatography (both HPLC and HPIC) results. The maximum deviation was less than ±5% in both cases. The total organic carbon (TOC) was measured with a liquiTOC analyzer.

Nitrogen adsorption-desorption was performed at -196 ℃ using a NOVA 4200e apparatus to determine the special surface area by the Brunauer-Emmett-Teller (BET) model. Before the adsorption, the samples were degassed overnight at 150 ℃.

The crystalline structure of the catalysts was explored on a Rigaku D/MAX 2550 VB/PC X-ray diffractometer with Cu K_α. The operating voltage and current were at 40 kV and 40 mA, respectively. XRD patterns were recorded in the 2θ = 10°-80° range with a scanning speed of 6°/min. The average crystal size of the catalysts was calculated according to the Debye-Scherrer equation:

where D is the crystal size (nm), λ is the wavelength (nm), β is the peak width at half maximum (rad) and θ is the Bragg angle (rad).

The solution after reaction was analyzed by ICP-OES to check the leaching of the constituents of the catalyst (i.e., Ru, Ti, Zr and Ce) upon the CWAO.

The catalyst morphology was determined by HRTEM on a JEM-2100 equipped with a LaB₆filament and a Gatan Ultrascan CCD camera with a resolution of 2k × 2k. The catalysts were dispersed in ethanol solution by ultrasonication, and a drop of the suspension was deposited on a copper screen coated with carbon film (mesh 300).

The XPS spectra were recorded on a Thermo ESCALAB 250 spectrometer with a monochromatized Al K_α X-ray source (1486.6 eV) and a pass energy of 25 eV. The C 1s (binding energy 284.6 eV) of adventitious carbon was used as the reference.

H₂-TPR experiments were performed on a Pengxiang PX200 adsorption instrument. About 80 mg of the sample was placed in a U-shaped quartz tube, and then reduced under 5% H₂ gas flow (50 mL/min) from 50 to 800 ℃ at 10 ℃/min. The consumption of hydrogen was monitored by a thermal conductivity detector (TCD).

The BET surface areas of the catalysts are shown in Table 1. The BET surface areas of modified TiO₂ by Ce and/or Zr were larger than that of the original TiO₂. These results demonstrated that the dopant in the TiO₂ network had a positive effect on the surface area, which might facilitate the metal dispersion and/or provide more space for Ru loading [22]. After impregnation of a precious metal, the specific surface area of the catalysts decreased, especially for Ru/TiO₂. The real Ru loading on three supports ranged from 1.5 to 1.8 wt%, which was close to the calculated loading of 2 wt%.

Fig. 1 shows the XRD diffraction patterns of the catalysts. In all cases, the intensive diffraction peaks appearing around 25.3°, 37.0°, 38.6°, 48.0°, 54.0°, 55.1°, 62.1° and 68.7° were assigned to anatase phases (PDF21-1272). For the Ru catalysts supported on TiO₂, small peaks at 27.5° and 36° were detected, owing to the large particle size of RuO₂, which could be ascribed to the (110) and (101) planes of tetragonal RuO₂ (JCPDS 21-1172) [23]. After the incorporation of zirconium and cerium ions into the cubic lattice of TiO₂, no additional peaks were observed, indicating the formation of a solid solution to maintain the exclusive anatase crystal texture. Additionally, no diffraction peaks attributed to Ru species were observed in the Ru/Ti_0.9Ce_0.1O₂ and Ru/Ti_0.9Zr_0.1O₂ samples. This might arise from the low Ru content or the high dispersion of the metal species on the support surface. The other reason could be the presence of Ru in the form of the amorphous phase. Compared with the carrier TiO₂, the intensity of the diffraction peak appeared to decline over the Ru catalyst. This observation indicated the strong interaction between the active sites and the support maintained the integrity of the crystal shape. Table 1 also presents the crystal size of different catalysts as calculated by Scherrer's equation. The catalysts containing Zr or Ce with 10 wt% possessed a smaller crystal particle size (approximately 13 nm) compared with Ru/TiO₂ (approximately 25 nm). The above results further confirmed that the modified TiO₂ had a positive effect on reducing the crystal size of Ru-supported catalysts.

Fig. 2 shows HRTEM images of the Ru-based catalysts. The surface micrographs showed that the approximate sizes of metal particles ranged from 1 to 10 nm. The Zr dopant could effectively reduce the sizes of noble metal particles and enable a high metal dispersion on the support surface. In the image of Ru/TiO₂, metal coalescence occurred with a large particle size of approximately 6.2 nm. In accordance with the above results, the Ti_0.9Zr_0.1O₂ support was ideal for maintaining better Ru dispersion and smaller particle size.

Fig. 3 shows the O 1s XPS spectra of the catalysts, which could be deconvoluted into three peaks. The significant spectral region, at the low binding energy of 530.1-530.3 eV, was assigned to Ti-O in TiO₂. The second oxygen species, at approximately 531.4-531.6 eV, was attributed to Ti-O in Ti₂O₃, which was a so-called surface active oxygen species. The above two types of oxygen species originated from the lattice oxygen, while the last signal of O 1s, with a binding energy of 532.3-532.8 eV, was attributed to the surface hydroxyl group [24, 25]. The relative abundance of the oxygen species was included in the analysis (Table 2). The Ru/Ti_0.9Zr_0.1O₂ catalyst possessed the highest relative abundance of surface active oxygen species (36.7%), which was the main reason for the best catalytic activity.

Fig. 3 shows the XPS spectra of Ru 3d_3/2 and Ru 3d_5/2 for the fresh and spent catalysts. The peak for C 1s was centered at 284.6 eV, which was attributed to the standard carbon species. The other peaks for C-OR were observed at 286 eV and 288.5 eV as a result of carbonaceous pollution in the exposure of the environment and the incomplete calcination of the supports [26]. Additionally, they exhibited distinct Ru species peaks. The low binding energy of approximately 280.0-281.5 eV was ascribed to 3d_5/2 with a respective 3d_3/2 core level binding energy of approximately 285.5-287.0 eV, which corresponded to the reduced Ru⁰. The other peak, at approximately 282.0-283.5 eV, was assigned to 3d_5/2 with a respective 3d_3/2 core level binding energy of approximately 285.5-287.4 eV, which corresponded to oxidized Ru⁴⁺ [27, 28]. The curve-fitting results for the XPS spectra in the Ru 3d region are summarized in Table 2. The Ru⁴⁺ relative ratio in Ru/Ti_0.9Zr_0.1O₂, was increased to 59.2%, which was larger than the Ru/TiO₂catalyst with 25.4%. This could be explained by the strong interaction between the doped zirconium and the TiO₂ lattice causing a larger amount of surface reactive oxygen species, which oxidized the reduced species of ruthenium Ru⁰ into Ru⁴⁺.

The redox properties of Ru/TiO₂, Ru/Ti_0.9Ce_0.1O₂and Ru/Ti_0.9Zr_0.1O₂ catalysts were investigated by TPR. As shown in Fig. 4, the low-temperature reduction peaks, in the range of 70-140 ℃, were associated to the reduction of RO₂ to metallic Ru [29, 30]. For the Ru/Ti_0.9Ce_0.1O₂ catalyst, a main reduction peak at 130 ℃ was attributed to the reduction of poorly crystal or so-called amorphous RuO_x. Additionally, the shoulder peak at 105 ℃ was assigned to the reduction of highly dispersed amorphous RuO_x [31]. In this case, the reducibility of the RuO₂ clusters was strongly affected by the particle size of the catalyst. According to the above explanation, we ascribed the reduction peaks at 100 and 130 ℃ to different types of Ru species. On Ru/Ti_0.9Zr_0.1O₂, the reduction peak exhibited a narrow structure and high intensity, which was in agreement with the high metal dispersion and uniform particle size distribution. The reduction peaks of the Ru supported on modified TiO₂ at 300-400 ℃ were stronger than those of the Ru/TiO₂ catalyst, which was ascribed to the presence of a strong interaction between RuO_x and the support at the interface that remarkably enhanced the catalytic activity [32].Notably, the presence of zirconium in TiO₂ has been shown to reduce the Ti-O bonding energy and accelerate the flow of lattice oxygen [33]. The H₂ uptake during the reduction of RuO₂ was calculated in Table 3, which was close to the theoretical values of H₂ consumption on the basis of the reduction of RuO₂ to Ru. Among the catalysts, the Ru/Ti_0.9Zr_0.1O₂ exhibited the highest H₂ consumption, which was consistent with the XPS and ICP results (higher proportion of Ru⁴⁺ species and more ruthenium species on the support).

First, the thermal stability of the aniline molecule was evaluated in the absence of catalyst under a nitrogen atmosphere instead of oxygen. The thermal degradation of aniline was nearly negligible after 5 h of reaction at 180 ℃. After cooling to room temperature, the concentration of aniline was kept constant. Therefore, most of the organic compounds were in solution rather than volatilizing into the gas phase.

Fig. 5 shows the evolution of the TOC and TN concentration upon WAO at 180 ℃ at a 1.5 MPa oxygen pressure. The TOC removal was approximately 12.1% and an analogous horizontal line was observed for TN. From the chromatographic (LPLC-MS) analysis, some reaction intermediates (such as phenol, benzoquinone, hydroquinone, and nitrobenzene) were identified. Actually, some organic compounds were more difficult to degrade than aniline under such reaction conditions. Hence, harsher operating conditions were required to achieve a satisfactory conversion, even in the presence of noble metal catalysts [34-36]. The values of TOC and TN were directly obtained by using a multiple parameter controller, and were in good agreement with the calculated results. Consequently, most of the by-products could be accurately identified and quantified.

The catalytic wet oxidation of aniline over the supports and the supported Ru catalysts was further performed at 180 ℃ and 1.5 MPa total oxygen pressure. Fig. 6 shows the concentration of aniline, COD, and TN as a function of time. In the presence of the supports, the conversion of aniline was higher than the WAO of aniline. The Ti_0.9Zr_0.1O₂ support achieved 67.7% aniline conversion after 5 h, which exhibited better catalytic performance compared with the Ti_0.9Ce_0.1O₂ and TiO₂supports. However, the COD and TN conversion was only 20% and 15% after 5 h of reaction, respectively. This could be explained by the formation of intermediate species in the CWAO of aniline. Hence, the supports were unable to convert the intermediate species into eco-friendly matter under the current reaction conditions.

As a general observation, the Ru-based catalysts exhibited higher activity than the supports. As shown in Table 4, the initial reaction rate increased from 1.56 mmol_ani/(h∙g_cat) for Ti_0.9Zr_0.1O₂to 3.78 mmol_ani/(h∙g_cat)for Ru/Ti_0.9Zr_0.1O₂. Over the Ru/Ti_0.9Zr_0.1O₂catalyst, the complete aniline conversion was achieved after a short time and the COD removal reached 88.3% after 5 h. As reported earlier, the Ru-based catalyst exhibited better catalytic behavior owing to its higher C-C bond cleavage capacity [37]. The other Ru-based catalysts exhibited the same tendency.

Interestingly, the activity of the reaction was affected by the supports. Over the Ru/Ti_0.9Zr_0.1O₂ catalyst, the catalyst exhibited higher activities compared with those of the Ru/TiO₂ and Ru/Ti_0.9Ce_0.1O₂. The bare supports as catalysts presented the same tendency. These results indicated that the larger specific surface area and pore volume for the Ti_0.9Zr_0.1O₂ support could provide the precious metal with more space. Meanwhile, the characteristics of the supports made the Ru metal form larger particles. Hence, the Ti_0.9Zr_0.1O₂catalyst had more active sites for the oxidation of organic matter. Since the doping of zirconium into TiO₂ resulted in an increase of the redox of the Ru/Ti_0.9Zr_0.1O₂, this could explain the cause of the high degradation activities.

The selectivity towards N₂ was also an important factor for the CWAO process of nitrogen-containing species. Unfortunately, the TN abatement was low, even in the presence of a precious metal. Fig. 6(c) illustrates the N₂ selectivity over Ru-based catalysts, which was consistent with the results reported by Oliviero et al. [38]. This showed that the inorganic nitrogen species, such as NH₄⁺, and NO₃^-, was detrimental towards oxidation.

Indeed, the modified TiO₂, by doping with Zr and Ce ions, led to the more ideal structural properties (larger special surface area, smaller metal particle size, better metal dispersity) and more reactive oxygen species. Under an oxygen environment, these characteristics may promote the metal-support interaction to form more surface active species such as peroxides (O₂^-) and/or superoxides (O₂^2-) [39]. These results indicated that the Ti_0.9Zr_0.1O₂ solid solution was a better alternative as a support than TiO₂ in the CWAO of aniline. The catalytic activity on aniline was in the following order: Ru/Ti_0.9Zr_0.1O₂ > Ru/Ti_0.9Ce_0.1O₂ > Ru/TiO_2.

The reaction temperature was an important operation parameter, which remarkably affected the catalyst oxidation of organic matter. Hence, a series of experiments were performed at approximately 160, 170, 180, 190 and 200 ℃ at 1.5 MPa oxygen pressure. The obtained results over 2 wt% Ru/Ti_0.9Zr_0.1O₂ are shown in Fig 7. In this case, the activation energy was deduced from the Arrhenius plot and the result was approximately 30.46 kJ/mol. This value was comparable with the case of 0.5 wt% Ru/Ce_0.9Zr_0.1O₂108 kJ/moland Pt-Ru/C 34 kJ/mol [40, 41].

Interestingly, the initial reaction rate improved from 2.69 to 4.81 mmol_ani/(h∙g_cat)as the reaction temperature was increased from 160 to 200 ℃. However, in the absence of any catalyst, the initial reaction rate for the WAO of aniline was just 1.03 mmol_ani/(h∙g_cat)at 200 ℃. Consequently, the 2 wt% Ru/Ti_0.9Zr_0.1O₂ catalyst could efficiently decrease the reaction activation energy, and accelerate the oxidation of aniline. Fig. 7(b) and (c) shows that the variation scopes of ΔCOD and ΔTN were 63%-96% and 38%-75%, respectively, from 160 to 200 ℃. These results reflected that the reaction temperature had more of an obvious influence on the COD and TN removal than on the aniline conversion because a higher temperature was necessary to decompose the small molecule organic acids and inorganic ammonia. Additionally, the carbonaceous matters were more likely to deposit on the catalyst surface at low temperature and automatically be oxidized at high temperature [42]. The deposited matter covered the active sites and inhibited the adsorption of organics and/or oxygen on the catalyst surface. Therefore, the proper temperature was essential to not only guarantee the necessary energy to degrade the organic matter, but also to enhance the resistance capacity to carbon deposition. However, a higher temperature increases the cost of equipment and energy. Therefore, the optimal operating temperature was set 180 ℃.

To evaluate the influence of the oxygen pressure on the catalytic oxidation of aniline, Ru/Ti_0.9Zr_0.1O₂ was tested at 180 ℃, under an oxygen pressure of 0.5, 1.0, 1.5 and 2.0 MPa. As shown in Fig. 8(a), the complete aniline conversion was achieved even at a low oxygen pressure. However, the increase of the oxygen pressure obviously improved the COD removal as well as TN conversion, because increasing the oxygen pressure provided more active oxygen species to refractory matter. Hence, increasing the oxygen pressure could accelerate the transmission and activation process (from the gas to the liquid phase, from the liquid phase up to the catalyst surface, and from adsorbed oxygen to the active substance) [43]. Based on the above discussed results, the oxygen pressure was set to 1.5 MPa.

The identification of the by-products and intermediates formed upon reaction was essential to identify by-products so as to evaluate their toxicity and design more suitable sewage treatment processes. The residual COD and TN value could not provide sufficient information to assess the degree of degradation. Normally, the oxidation of aniline is not a direct process to form environmentally-friendly substances (organic matter), as it involves a complex process. During the oxidation process, the collected samples underwent a gradual change from an intense reddish-brown color to colorless. This suggested that the CWAO of aniline consisted of two-step paths, where the first step was to form nitrogen-containing organic matter, then further oxidization of the organic matter to colorless organic species occurred. Fig. 9 showed the concentration profiles versus reaction time for aniline and the main intermediate compounds (produced in the oxidation reaction) at 180 ℃ and a 1.5 MPa of oxygen pressure over Ru/Ti_0.9Zr_0.1O₂ catalyst.

In the CWAO of phenolic compounds, a free radical mechanism has already been determined [44-46]. During the catalytic oxidation of aniline, the active species (OH·) quickly attacks -NH₂ groups of aniline to form phenylhydroxyl. This intermediate is not detected owing to its quick disappearance in an aerobic environment [47]. The temporal existence of phenylhydroxyl arises from its transformation into p-aminophenol by isomerization or unceasing attack by reactive radicals to produce nitrosobenzene and nitrobenzene. The formed p-aminophenol is further oxidized to p-nitrophenol by the reactive oxygen species because amino group, as an electron-donating agent, increases the possibility of electrophilic attack [48]. Subsequently, the N-C bond on p-nitrophenol is broken by a denitration reaction and results in the formation of phenol within 2 h of reaction. Where after, hydroquinone and benzoquinone is successively derived. Finally, an amount of organic acid (such as acetic acid, succinic acid) is formed as a result of the opening of the aromatic ring. At high temperature, the produced acids accelerated the corrosion of the equipment and led to the dissolution of the active component of the catalyst. Therefore, a rapid removal of the above organic acids is critical to reduce the equipment burden and maintain the catalyst stability. According to the above analysis results and the reported results in the literature, the reaction pathway for the CWAO of aniline is proposed in Fig. 10.

Treatment of nitrogen-containing compounds through CWAO is a complicated process, where some inorganic compounds (such as ammonium, nitrites and nitrates ions) are generated. Fig. 11 shows the concentrations of the TN, N-organic and inorganic nitrogenous compounds as a function of time over Ru/Ti_0.9Zr_0.1O₂. Experimental results showed that a large amount of ammonium ions was quickly produced at the beginning of the reaction and the nitrogen concentration of NH₄⁺ achieved a maximum value (104 mg/L) in the reaction after 2 h. Afterwards, the ammonium ions were transformed into nitrogen and nitrate. Conversely, a conversion of N-organic matter was found, the concentration of which was 23.2 mg/L after 5 h of reaction. Hence, the concentration of nitrate ions increased gradually under the experimental conditions. A similar catalytic behavior has been reported by Oliviero et al. [49] where the N-species were mainly transformed into ammonium and nitrate ions using Ru/CeO₂ catalyst under the same operating conditions. However, a small amount of nitrite ions was observed owing to them being unstable during the CWAO process.

In the above mentioned catalysts, the Ru/Ti_0.9Zr_0.1O₂ sample showed the best catalytic stability in the CWAO of aniline after three cyclic experiments. The reduction of COD and TN with the regenerated catalyst was slightly lower than that achieved with fresh catalyst (Fig. 12). A complete conversion of aniline was still achieved after the 3rd run.

After CWAO of aniline at 180 ℃ and 1.5 MPa oxygen pressure, the used Ru/Ti_0.9Zr_0.1O₂was recovered and oven-dried at 120 ℃ overnight for analysis. The BET surface area of the spent catalyst decreased slightly to 85.3 m²/g, which was comparable to that of the fresh catalyst (94.1 m²/g). The decrease of the specific surface area might be caused by mechanical attrition or the carbon deposition on the catalyst surface. In XRD characterization, the spent catalyst still maintained an anatase crystal structure. Meanwhile, no obvious metal leaching in the ultimate solution revealed a stable activity in CWAO. In the XPS analysis, the content of the surface oxygen species on the spent catalyst was maintained at a high value. Although the formation of some carbonaceous deposits on the surface of the catalyst were evidenced (Fig. 13), these carbonaceous compounds could be removed by calcination and hydrogen treatment. Therefore, the characteristics of high activity, stability and regeneration property makes the Ru/Ti_0.9Zr_0.1O₂ sample a promising candidate for industrial sewage treatment.

Ru catalysts supported on Ce-and/or Zr-modified TiO₂ were synthesized and their catalytic behavior for the oxidation of aniline was studied. Doping zirconium into titanium could enhance the specific surface area and better disperse the metal. In addition, more of the surface oxygen species and a strong interaction between Ru and the support were confirmed in the Ru/Ti_0.9Zr_0.1O₂catalyst.

The Ru/Ti_0.9Zr_0.1O₂ catalyst demonstrated better performance in the CWAO of aniline, where complete aniline conversion and 88.3% COD conversion were obtained at 180 ℃ and a 1.5 MPa oxygen pressure. Based on the observed intermediate products in solution and the previous reported results, the reaction path for the catalytic oxidation of aniline was established.

A decline of the catalytic activity in the reaction process resulted from the deposition of carbonates on the surface of the catalysts. However, the aged catalysts could recover their activities after calcination and hydrogen treatment and maintain a high oxidation activity even after three consecutive tests.