Role of ceria-supported noble metal catalysts (Ru, Pd, Pt) in wet air oxidation of nitrogen and oxygen containing compounds

Catalytic wet air oxidation (CWAO) of aniline, phenol, carboxylic acids and ammonia was carried out in a batch reactor over noble metals (Ru, Pd, Pt) supported on ceria. Ruthenium is very active for the conversion of a wide range of organic compounds and selective into carbon dioxide. The ability of ceria to transfer oxygen is essential for good performances in CWAO. However, Ru/CeO₂ is not selective for ammonia oxidation into N₂. Addition of small amount of Pd enhances both activity and selectivity of Ru in this reaction. Finally, oxidation of nitrogenous organic compounds requires moderate temperature and oxygen pressure and needs to adjust the oxidizing capacity of the catalyst.     

Catalytic Wet Air Oxidation of Aqueous Solutions of Substituted Phenols

Catalytic wet air oxidation (CWAO) of aqueous solutions of phenol, 2-chlorophenol and 4-nitrophenol were studied using Cu/CeO₂ with 4% of copper as catalytic material. The catalyst was prepared by an impregnation method and characterized by X-ray diffraction (XRD), BET surface area, oxygen storage capacity (OSC), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR) and XPS. The reaction was carried out in a batch reactor at T=160 °C and 1.0 MPa. Cu/CeO₂ catalyst was found effective in CWAO. On the basis of characterization data, it is suggested that the high activity of the copper–ceria catalyst is related to the modification of the structural and redox properties of the cerium oxide on copper addition. The ratio BOD₅/COD was measured to evaluate the biodegradability. Pretreatment by CWAO under operating conditions resulted in effluents whose biodegradation rates were significantly higher than those of the original.     

水热电催化氧化法处理高浓度苯酚模拟废水的研究

结合催化湿式氧化法和电催化氧化法为水热电催化氧化法处理高浓度苯酚模拟废水((ρCOD)为7500mg/L):以C/Ru作催化剂(w(Ru)为0.5%),自制DSA阳极(釜体为阴极),加入NaCl作支持电解质(w(NaCl)为1%),充入氧气使PO2为3.5MPa,升温至设定温度后:开初0.5h进行苯酚的催化湿式氧化,后0.5h进行电催化氧化,并改变条件进行苯酚的单一催化湿式氧化和电催化氧化。结果表明:85℃时水热电催化氧化条件下,苯酚去除率为100%,COD去除率达92.34%,而单一催化湿式氧化和电催化氧化的COD去除率却依次为68.46%、39.73%,可见水热电催化氧化法利用了催化湿式氧化法和电催化氧化法的协同作用,取得了更佳的效果。该方法是一种新型、低温、高效的废水处理技术,为处理苯酚废水提供了另一种可能途径。     

Catalysts Ru–CeO2/Sibunit for catalytic wet air oxidation of aniline and phenol

Catalytic properties of carbon materials Sibunit (commercial samples) and ceria-promoted precious metals (Ru, Pt, Pd) supported on carbon were studied in the processes of catalytic wet air oxidation (CWAO) of aniline and phenol at elevated pressures and temperatures (T =433–473 K, P_O2 = 0.3–1.0 MPa). It was found that the activity increases when the catalyst is pretreated with hydrogen peroxide. An efficiency of Ru–CeO₂/Sibunit catalyst with a low ruthenium content (~0.6 Ru) for deep cleaning of polluted waters is demonstrated.     

Catalytic wet air oxidation of phenol and acrylic acid over Ru/C and Ru–CeO2/C catalysts

Ru/C catalysts promoted, or not, by cerium were prepared by impregnation of an active carbon (961 m² g⁻¹) with chlorine-free precursors of Ru and Ce. They were characterized by chemisorption of H₂ and of CO and by electron microscopy. TEM and H₂ chemisorption gives coherent results while CO chemisorption overestimates Ru dispersion. In Ru–Ce/C, Ce is in close contact with Ru and decreases Ru accessibility.

Catalytic wet air oxidation (CWAO) of phenol and of acrylic acid (160°C and 20 bar of O₂) was investigated over these catalysts and their performance (activity, selectivity to intermediate compounds) compared with that of a reference Ru/CeO₂ catalyst. Carbon-supported catalysts were very active for the CWAO of phenol but not for acrylic acid. Although high conversions were obtained, phenol was not totally mineralized after 3 h. It was shown that acrylic acid was more strongly adsorbed than phenol. Moreover, the number of contact points between Ru particles and CeO₂ crystallites constitutes a key parameter in these reactions. A high surface area of ceria is required to insure O₂ activation when the organic molecule is strongly adsorbed.     

Multi-walled carbon nanotubes (MWNTs) as an efficient catalyst for catalytic wet air oxidation of phenol

Multi-walled carbon nanotubes (MWNTs) were used as a catalyst for catalytic wet air oxidation (CWAO) of phenol in a batch reactor. SEM, TEM and FT-IR technique were applied to investigate the microstructure and the surface functional group of the MWNTs. When the carboxylic groups (–COOH) are grafted onto the surface of the MWNTs, the functionalized MWNTs exhibit a good catalytic activity in CWAO of phenol. At a reaction temperature of 160 °C, oxygen pressure of 2.0 MPa and a phenol concentration of 1000 mg/L, 100% phenol and 76% TOC are removed after 120 min reaction.     

Comparative study of support effects in ruthenium catalysts applied for wet air oxidation of aromatic compounds

Catalytic wet air oxidation (CWAO) reactions of aniline and phenol were conducted over supported ruthenium catalysts. Three support materials were employed: ZrO₂ and graphite, which exhibit medium adsorption capacities for pollutants and present mesopores in their texture, and an activated carbon. This latter has higher adsorption capacity for pollutants because of the large capability of the micropores for contaminant retention from water. The Ru catalysts supported on the activated carbon material showed the higher values of conversion in the oxidation of aniline and of conversion and mineralization in the reaction of phenol. Under our experimental conditions the role of micropores present on the support material seems to be relevant for improving catalytic performances. The incorporation of Ru nanoparticles from different precursors has been also evaluated. Even if the final Ru particle size is a key parameter for the catalytic mineralization, a cooperative effect with the activated carbon support has been established.     

Ru/TiO_2催化剂制备及其在有机酸废水湿式氧化中的应用

以纳米TiO_2为载体,通过优化制备方法,合成了高活性的Ru/TiO_2贵金属催化剂,采用X射线衍射(XRD)、N_2物理吸附、程序升温还原(H_2-TPR)和透射电镜(TEM)等手段对催化剂进行表征,并将制备的催化剂应用到湿式氧化处理3种有机酸模拟废水中。研究表明,氢气还原有助于Ru在TiO_2表面的分散,由此制备的催化剂氧化活性较高。在200℃,初始氧压3 MPa的条件下反应2 h,Ru/TiO_2在湿式氧化处理丙烯酸、丁二酸和乙酸有机酸模拟废水时,化学需氧量(COD)去除率分别达到95.3%,91.6%和70.1%。     

Airborne toluene degradation by using manganese oxide supported on a modified natural diatomite

The catalytic combustion of toluene over manganese oxide supported on natural diatomite has been investigated in the paper. The catalyst was prepared by the wet impregnation method and characterized by using the Brunauer Emmett Teller, field emission scanning electron microscopy, X-ray diffraction, X-ray fluorescence and temperature-programmed reduction analysis. The higher activity of the catalyst supported on diatomite for toluene oxidation was obtained at 12 wt% Mn loading. It was able to convert 90 % of toluene (T ₉₀) at 380 °C. The results indicated that the selectivity towards CO₂ was almost 100 % and no intermediates, such as CO or other hydrocarbons, were detected.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Catalytic activity, stability and structure of multi-walled carbon nanotubes in the wet air oxidation of phenol

Multi-walled carbon nanotubes (MWCNTs), with no supported metal, were used as catalysts in the wet air oxidation of phenol. The MWCNTs were chemically modified using HCl or HNO₃-H₂SO₄. They were characterized by BET, SEM, TEM, FT-IR and Raman spectroscopy. The functionalized MWCNTs exhibited both high activity and good stability in the wet air oxidation of phenol. At 160 °C and 2.0 MPa with an initial phenol concentration of 1000 mg/L, 100% phenol conversion and 76% total organic carbon abatement could be achieved after 120 min reaction. Upon reaction, the short chain carboxylic acids mainly maleic/fumaric, malonic, oxalic, formic and acetic acid were produced. Surface functional groups (-COOH) were shown to play a key role in the high activity of the functionalized MWCNTs. A mechanism for the CWAO of phenol was proposed.     

Catalytic wet peroxide oxidation of phenol solution over Fe–Mn binary oxides diatomite composite

Mn–Fe binary oxides incorporated into diatomite (denoted as FM-diatomite) was prepared by the redox reaction of KMnO₄ and FeSO₄ with pH ranging from 3 to 9. The catalytic activities of FM-diatomite were studied for phenol oxidation and were compared with iron oxide modified diatomite (F-diatomite) and manganese oxide modified diatomite (M-diatomite). The obtained catalysts were characterized by scanning electron microscope, powder X-ray diffraction, energy dispersive spectroscopy, transmission electron microscope, X-ray photoelectron spectroscopy, and nitrogen adsorption/desorption isotherms. The results show that Fe–Mn binary oxides were highly dispersed on the diatomite surface in which manganese oxide and iron oxide displayed multiple oxidation states including Mn⁴⁺, Mn³⁺, Fe²⁺ and Fe³⁺. The phenol oxidation by H₂O₂ through the use of Mn–Fe-diatomite as a catalyst was conducted. FM-diatomite exhibited as an excellent catalyst for the total oxidation of phenol and main intermediates (catechol and hydroquinone). The conversion of phenol and main intermediates by means of FM-diatomite was 100 % under 50 min while that by F-diatomite also was 100 % after 110 min but other intermediates still remained. While phenol conversion by M-diatomite was close to zero due to speedy hydroperoxide decomposition over the manganese oxide catalyst. These results show that there was a synergized effect of iron and manganese oxide present in FM-diatomite.     

Catalytic wet air oxidation of phenol using active carbon: performance of discontinuous and continuous reactors

Catalytic wet air oxidation (CWAO) of an aqueous phenol solution using active carbon (AC) as catalytic material was compared for a slurry and trickle bed reactor. Semi‐batchwise experiments were carried out in a slurry reactor in the absence of external and internal mass transfer. Trickle‐bed runs were conducted under the same conditions of temperature and pressure. Experimental results from the slurry reactor study showed that the phenol removal rate significantly increased with temperature and phenol concentration, whereas partial oxygen pressure had little effect. Thus, at conditions of 160 °C and 0.71 MPa of oxygen partial pressure, almost complete phenol elimination was achieved within 2 h for an initial phenol concentration of 2.5 g dm^?3. Under the same conditions of temperature and pressure, the slurry reactor performed at much higher initial rates with respect to phenol removal than the trickle bed reactor, both for a fresh active carbon and an aged active carbon, previously used for 50 h in the trickle bed reactor, but mineralisation was found to be much lower in the slurry reactor. Mass transfer limitations, ineffective catalyst wetting or preferential flow in the trickle bed alone cannot explain the drastic difference in the phenol removal rate. It is likely that the slurry system also greatly favours the formation of condensation polymers followed by their irreversible adsorption onto the AC surface, thereby progressively preventing the phenol molecules to be oxidised. Thus, the application of this type of reactor in CWAO has to be seriously questioned when aiming at complete mineralisation of phenol. Furthermore, any kinetic study of phenol oxidation conducted in a batch slurry reactor may not be useful for the design and scale‐up of a continuous trickle bed reactor. © 2001 Society of Chemical Industry     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

ACCELERATION OF THE WET OXIDATION REACTION OF PIPERAZINE BY HETEROGENEOUS Ru/TiO2 CATALYST

Wet air oxidation is a candidate technique for the effective treatment of wastewater contaminated by nitrogenous organic pollutants. Piperazine (PZ) is a cyclic diamine representing this class of compounds. In the present work, the wet oxidation reaction of PZ was studied for the first time. It was found that, in the studied range of temperatures of 180°–230°C and O₂ partial pressures of 0.69–2.07 MPa, the oxidation process was slow. Total organic carbon (TOC) conversion at 230°C and 0.69 MPa O₂ partial pressure was just 52% after 2 h. The investigated reaction was accelerated by a heterogeneous Ru/TiO₂ catalyst. Maximum TOC conversion (91%) was achieved during catalytic wet oxidation at 210°C and 1.38 MPa O₂ pressure. Kinetic data were collected over the range of temperatures 180°–210°C, O₂ partial pressures 0.34–1.38 MPa, and catalyst loading 0.11–0.66 kg/m³. The lumped TOC concentration decay was a two-step first-order process.     

Optimal design and operation of an industrial three phase reactor for the oxidation of phenol

Among several treatment methods catalytic wet air oxidation (CWAO) treatment is considered as a useful and powerful method for removing phenol from waste waters. In this work, mathematical model of a trickle bed reactor (TBR) undergoing CWAO of phenol is developed and the best kinetic parameters of the relevant reaction are estimated based on experimental data (from the literature) using parameter estimation technique. The validated model is then utilized for further simulation and optimization of the process. Finally, the TBR is scaled up to predict the behavior of CWAO of phenol in industrial reactors. The optimal operating conditions based on maximum conversion and minimum cost in addition to the optimal distribution of the catalyst bed is considered in scaling up and the optimal ratio of the reactor length to reactor diameter is calculated with taking into account the hydrodynamic factors (radial and axial concentration and temperature distribution).     

Surface modification of carbon-supported iron catalyst during the wet air oxidation of phenol: Influence on activity, selectivity and stability

Catalytic wet air oxidation (CWAO) of phenol with iron/activated carbon catalysts (Fe/AC) at temperature of 400 K and 8 atm of total pressure is an efficient treatment to oxidize a resistant pollutant such as phenol into biodegradable species, mainly short chain acids. Extended studies employing activated carbon catalysts point out significant changes in the carbon as a consequence of the CWAO process. After the long-term experiments carried out in this work it was concluded that these modifications consist of loss of microporosity, temporary decrease of the mesoporosity, decrease of the carbon/oxygen ratio on the catalyst surface, more acidic pH_slurry values, and aggregation of the -Fe₂O₃ crystallites. The causes that provoke these changes and the reasons why they do not alter significantly the CWAO efficiency were analyzed. The way of exposition of Fe/AC catalyst to the reactants plays an important role in its activity and selectivity towards complete mineralization, namely oxidation to CO₂ and H₂O.     

Zr- and Li-Modified Ru/Sio2 Catalysts for Fischer–Tropsch Synthesis

We have found that Zr- and Li-modified Ru/SiO₂ catalysts (Q-15) are extremely stable and can be used in FT synthesis to maintain the conversion rate of CO constant even after 33 h. Modification of Ru/SiO₂ by Zr (5 wt%) and Li (0.1 wt%) resulted in a remarkable increase in the stability of the catalyst. Taking into account surface acidity and reducibility, we assumed that this remarkable stability is due to the cooperative effects of Ru, Zr, Li, and the SiO₂ support.     

Preparation, characterization and catalytic properties of carbon nanofiber-supported Pt, Pd, Ru monometallic particles in aqueous-phase reactions

Carbon nanofibers (CNF) synthesized by catalytic chemical vapor deposition (CVD) method were used to prepare supported platinum, palladium and ruthenium monometallic (2.0 wt.%) catalysts by means of incipient-wetness impregnation method. The CNF support and catalysts were characterized by X-ray powder diffraction (XRD), nitrogen adsorption/desorption isotherms, volumetric chemisorption of hydrogen, temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM). Solids were tested in catalytic wet-air oxidation (CWAO) of phenol aqueous solution (180–240 °C and 10.0 bar of oxygen partial pressure) carried out in a continuous-flow trickle-bed reactor. Trends of phenol and total organic carbon (TOC) conversion demonstrate that the CNF support and CNF-Pt catalyst did not exhibit constant activity for CWAO of phenol. A decrease of catalyst activity, detection of carbon dioxide in the off-gas stream while examining catalyst stability and significant textural changes observed, provide an evidence that under net oxidizing reaction conditions gasification of the CNF support occurs. The prepared catalysts were also tested in liquid-phase thermal decarboxylation of formic acid in inert atmosphere (60–220 °C). Among solids examined, the CNF-Pd exhibited the highest activity. At the employed conditions, no decomposition of the CNF support was observed during the thermal decarboxylation of formic acid.     

Steam reforming of CH4 over Ni-Ru catalysts supported on Mg-Al mixed oxide

Ni_0.5/Mg_2.5(Al)O catalyst prepared from hydrotalcite precursors showed high and stable activity in the CH₄ steam reforming, but was severely deactivated in the daily start-up and shut-down (DSS) operation under steam purging. The addition of Ru drastically improved the behavior of Ni_0.5/Mg_2.5(Al)O catalyst for the DSS operation. During the wet Ru loading on the Ni_0.5/Mg_2.5(Al)O catalyst, the reconstitution of hydrotalcite took place by “memory effect,” resulting in the formation of Ru-Ni alloy as well as the strong interaction between Ru and Ni after the calcination followed by reduction. This provided the catalyst with high sustainability probably by suppressing the oxidation of Ni metal by steam by hydrogen spillover from Ru. Only 0.05 wt% of Ru loading was enough to effectively suppress the deactivation.