SYNTHESIS, PROPERTIES AND APPLICATION OF CuO-(CeO 2, La 2 O 3 )/SUPPORT CATALYSTS IN DECONTAMINATION TECHNOLOGIES OF VOLATILE AROMATIC COMPOUNDS

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2 KAUNAS UNIVERSITY OF TECHNOLOGY AURIMAS URBUTIS SYNTHESIS, PROPERTIES AND APPLICATION OF CuO-(CeO 2, La 2 O 3 )/SUPPORT CATALYSTS IN DECONTAMINATION TECHNOLOGIES OF VOLATILE AROMATIC COMPOUNDS Summary of Doctoral Dissertation Technological Sciences, Chemical Engineering (05T) 2014, Kaunas

3 The research was carried out in at Kaunas University of Technology, Faculty of Chemical Technology, Department of Physical Chemistry (from 2014 Department of Physical and Inorganic Chemistry). The research was supported by the scholarship (2014) and by the grant (No. MIP-025/2014) from Research Council of Lithuania. Scientific supervisor: Prof. Dr. Saulius KITRYS (Kaunas University of Technology, Technological Sciences, Chemical Engineering 05T). Dissertation Defense Board of Chemical Engineering Science Field: Prof. Dr. Raimundas ŠIAUČIŪNAS (Kaunas University of Technology, Technological Sciences, Chemical Engineering 05T) chairman; Prof. Dr. Kęstutis BALTAKYS (Kaunas University of Technology, Technological Sciences, Chemical Engineering 05T); Prof. Dr. Habil. Aivaras KAREIVA (Vilnius University, Physical Sciences, Chemistry 03P); Dr. Rimantas LEVINSKAS (Lithuanian Energy Institute, Technological Sciences, Materials Engineering 08T); Prof. Dr. Virgilijus VALEIKA (Kaunas University of Technology, Technological Sciences, Chemical Engineering 05T). The official defense of the dissertation will be held at the public meeting of the Board of Chemical Engineering Science Field at 1 p.m. on the 19 th of December, 2014 at the Dissertation Defense Hall of the Central Building of Kaunas University of Technology (K. Donelaičio g. 73, Kaunas). Address: K. Donelaičio st , LT 44029, Kaunas, Lithuania. Phone: (370) , fax: (370) , doktorantura@ktu.lt The Doctoral Dissertation is available at and at the library of Kaunas University of Technology (K. Donelaičio st. 20, Kaunas)

4 KAUNO TECHNOLOGIJOS UNIVERSITETAS AURIMAS URBUTIS KATALIZATORIŲ CuO-(CeO 2, La 2 O 3 )/NEŠIKLIS SINTEZĖ, SAVYBĖS IR TAIKYMAS LAKIŲJŲ AROMATINIŲ JUNGINIŲ ŠALINIMO TECHNOLOGIJOSE Daktaro disertacijos santrauka Technologijos mokslai, chemijos inžinerija (05T) 2014, Kaunas

5 Disertacija rengta metais Kauno technologijos universitete, Cheminės technologijos fakulteto Fizikinės chemijos katedroje (nuo 2014 m. Fizikinės ir neorganinės chemijos katedra). Mokslinius tyrimus rėmė Lietuvos mokslo taryba: mokslininkų grupių projektas (Nr. MIP-025/2014) ir doktoranto stipendija už akademinius pasiekimus (2014 m.). Mokslinis vadovas: Prof. dr. Saulius KITRYS (Kauno technologijos universitetas, technologijos mokslai, chemijos inžinerija 05T). Chemijos inžinerijos mokslo krypties taryba: Prof. dr. Raimundas ŠIAUČIŪNAS (Kauno technologijos universitetas, technologijos mokslai, chemijos inžinerija 05T) pirmininkas; Prof. dr. Kęstutis BALTAKYS (Kauno technologijos universitetas, technologijos mokslai, chemijos inžinerija 05T); Prof. habil. dr. Aivaras KAREIVA (Vilniaus universitetas, fiziniai mokslai, chemija 03P); Dr. Rimantas LEVINSKAS (Lietuvos energetikos institutas, technologijos mokslai, medžiagų inžinerija 08T); Prof. dr. Virgilijus VALEIKA (Kauno technologijos universitetas, technologijos mokslai, chemijos inžinerija 05T). Disertacija bus ginama viešame chemijos inžinerijos mokslo krypties disertacijos gynimo tarybos posėdyje, kuris įvyks 2014 m. gruodžio 19 d. 13 val. Kauno technologijos universitete, centrinių rūmų disertacijų gynimo salėje. Adresas: K. Donelaičio g , LT 44029, Kaunas. Tel.: (370) , faksas: (370) , e-paštas: Su disertacija galima susipažinti: interneto svetainėje ir Kauno technologijos universiteto bibliotekoje (K. Donelaičio g. 20, Kaunas).

6 1. INTRODUCTION Relevance of the work. Anthropogenic volatile organic compounds (VOC) are harmful and cause negative impact on air quality, human health and the surrounding environment. Therefore, the concentrations of these compounds emitted from industry are strictly controlled, and VOC removal technologies are significant environmental and chemical engineering problem. Volatile aromatic compounds such as benzene, toluene and xylenes (BTX) are ones of the most difficultly decontaminable substances. Benzene, toluene and xylene molecules are very stable because of their common chemical property - aromaticity and the removal technologies are more complicated when the concentrations of these gaseous emissions are low. One of the best available and most widely studied decontamination techniques of VOC is the heterogeneous catalytic oxidation by air oxygen. Such catalysts are used in both stationary and mobile (cars) source emissions decontamination. The best known active components of the oxidation catalysts are precious metals (Pt, Pd) dispersed on different supports. However, these catalysts are expensive. Transition metal (Cu, Co, Mn, Cr, etc.) oxides and other compounds are the most widely studied as an alternative to precious metals. According to literature data and the previous works carried out in KUT Department of Physical Chemistry, CuO is one of the most active single metal oxides in VOC oxidation reactions. CuO catalysts perform at lower temperatures compared with other metal oxides. The catalytic oxidation activity of alcohols is comparable to platinum group metal catalysts. However, CuO catalytic activity is lower in the oxidation technologies of aromatic compounds. Based on the literature, lanthanides or their oxide additives may increase the activity of CuO catalysts. Therefore, La, especially Ce and other rare-earth metals and their oxides have gained interest in low-temperature heterogeneous catalytic processes. CeO 2 and La 2 O 3 have many positive features which can be rationally exploited in multicomponent systems: CeO 2 is a strong oxidizing agent, has a large oxygen storage capacity and higher crystal lattice oxygen mobility compared to other metal oxides. La 2 O 3 increases thermal stability of supports and improves dispersity of other metal oxides on the surface of catalyst. Cu-Ce and Cu-La mixed oxides dispersed on high specific surface area support may be used as a dual function material adsorbent-catalyst. Such system concentrates VOC from contaminated air passing through the adsorptive reactor. The adsorbed VOC are then oxidized into CO 2 and water vapor by raising the temperature of the adsorbent-catalyst bed. Concentrating VOC on the surface of the adsorbent-catalyst enables to increase the oxidation reaction rate, since it is known that the reaction rate depends on the initial concentration of substance. Also, the heat released during the oxidation reaction becomes the driving force for the ongoing process. At low concentrations of contaminants this alternative 5

7 technology becomes more economical compared to conventional VOC catalytic oxidation. Aim of the work is to develop supported CuO-CeO 2 and CuO-La 2 O 3 catalysts, determine their properties and their usage in complete oxidation technologies of benzene, toluene and o-xylene. Following tasks had to be accomplished in order to achieve the aim of this work: 1. Evaluate adsorption-desorption properties of various supports and distinguish which of these supports are suitable for development of conventional and dual function catalysts. 2. Investigate the influence of La 2 O 3 and CeO 2 additives on CuO catalyst specific surface properties and activity in VOC oxidation reactions. Determine the optimal ratio of the active components in the catalyst. 3. Evaluate the activity of CuO-CeO 2 /(NaX, NaA) catalysts in benzene, toluene and o-xylene complete oxidation technologies and determine the optimal operating conditions. 4. Determine the kinetic parameters and the mechanism in the oxidation reactions of benzene, toluene and o-xylene. 5. Determine the structure and specific surface properties of dual function CuO-CeO 2 /NaX catalyst. 6. Propose technological recommendations for benzene, toluene and o-xylene complete oxidation technologies using developed catalysts. Scientific novelty of the dissertation. Influence of La 2 O 3 and CeO 2 additives on the activity of CuO supported catalysts was investigated in lowtemperature ( C) oxidation reactions of benzene, toluene, o-xylene and other VOC. Process kinetic parameters and reaction mechanisms were determined. Practical significance of the dissertation. Influence of initial saturation and regenerative air flow rate on the overall efficiency of VOC oxidation was investigated. Technological parameters were determined for benzene, toluene and o-xylene vapor decontamination using CuO-CeO 2 /NaX dual function adsorbent-catalyst. Approval and publication of research results. The research results are published in four articles. Two of them are published in cited journals corresponding to the list of Thompson Reuters Web of Science database. The research results were presented in five scientific conferences. Structure and contents of the dissertation. Dissertation consists of introduction, literature survey, experimental part, results and discussion, conclusions, list of 140 references and 9 publications. The main material is represented in 102 pages, including 56 figures and 30 tables. 6

8 Statements presented for defense: 1. CuO-CeO 2 supported catalysts surpass the activity of CuO-La 2 O 3 or single active component CuO catalysts in VOC oxidation reactions. The optimal ratio of CeO 2 /CuO amounts in the catalyst is equal to at temperatures of C. 2. The process of toluene and o-xylene oxidation undergoes according to the inherent parallel-sequential mechanism and intermediate products, CO, CO 2 and H 2 O vapor form on the surface of CuO-CeO 2 /NaX adsorbent-catalyst. Benzene vapor is oxidized directly into CO, CO 2 and H 2 O vapor. 2. EXPERIMENTAL Materials used in this work were chemically or analytically pure reagents and substances: Al(NO 3 ) 3 9H 2 O, Al 2 (SO 4 ) 3 18H 2 O, NaOH, CO(NH 2 ) 2, Ce(NO 3 ) 3 6H 2 O, La(NO 3 ) 3 6H 2 O, Cu(NO 3 ) 2 3H 2 O, HNO 3, HCl, H 2 O 2, methanol, methyl acetate, benzene, toluene, o-xylene, pelleted synthetic zeolites NaX and NaA, pelleted synthetic γ-al 2 O 3. γ-al 2 O 3 support and catalyst synthesis. Four different samples of alumina were made by precipitation or sol-gel methods from precursor salts. All catalysts were prepared by impregnation technique. Initial supports were impregnated with the solutions containing active catalyst metal components (Cu, Ce and La), dried and calcined. Catalytic oxidation of VOC was performed in a fixed-bed reaction system, containing three main parts: an effluent gas simulation block, fixed-bed quartz reactor and air preheater built in a heating chamber as well as the inlet and outlet gas analyzing system. Adsorption capacity and desorption parameters of supports were obtained with equilibrium research equipment under static conditions. Thermal desorption process was carried out under inert nitrogen atmosphere to eliminate theoretical oxidation reaction start at desorption temperature. Benzene was used as a reference material. Gas analysis. Concentrations of VOC and intermediate products of catalytic oxidation were determined using gas chromatography and mass spectrometry (GC/MS, Perkin Elmer Clarus 500). Tedlar 1 dm 3 bags were used for gas sampling. CO and CO 2 concentrations were monitored with an online gas analyzer TESTO 445. X-ray diffraction analysis (XRD) was performed with X-ray diffractometer DRON-6 using Cu K α radiation (λ= nm). Detector position step was Atomic absorption (AAS) and atomic emission (AES) spectroscopy. Analysis of copper, cerium and lanthanum content in the catalysts was carried out with Perkin Elmer AAnalyst 400 spectrometer by using atomic absorption and flame atomic emission spectroscopy methods 7

9 Simultaneous thermal analysis (STA) was performed to identify Ce(NO 3 ) 3 and La(NO 3 ) 3 thermal decomposition temperatures and properly estimate calcination temperature. NETZSCH STA 409 PC Luxx analyzer was used. Differential scanning calorimetry (DSC) and thermogravimetry (TG) curves were obtained. IR spectroscopy analysis was performed using Perkin Elmer FT-IR System spectrometer. Optically pure KBr was used to prepare samples for analysis. Analysis was carried out in wavenumber range between 4000 and 400 cm -1. Specific surface area (S BET ) and other surface properties of catalyst samples were determined by N 2 adsorption at 77 K with Sorptometer Kelvin Catalyst samples were outgassed for 3 h at 300 C prior N 2 adsorption. Optical microscopy (OM) was performed to determine cross-sectional dispersion of active components in the CuO-CeO 2 /NaX adsorbent-catalyst. Zeiss Axio Imager.Z2m optical microscope was used with magnification range between 50x and 400x. 3. RESULTS AND DISCUSSION 3.1. Support selection for dual function catalyst synthesis Comparison of different synthesized and industrial supports was conducted by determining benzene adsorption capacity and desorption rate under static isothermal conditions. Tested supports with assigned sample numbers are represented in Table 3.1. Table 3.1. Tested supports Sample No. Support 1 γ-al 2 O 3 2 γ-al 2 O 3 3 γ-al 2 O 3 Precursors and preparation method Al(NO 3 ) 3 9H 2 O, NaOH, precipitation reaction, without mixing Al 2 (SO 4 ) 3 18H 2 O, NaOH, precipitation reaction, without mixing Al(NO 3 ) 3 9H 2 O, NaOH, precipitation reaction, rigorous mixing 4 γ-al 2 O 3 Al(NO 3 ) 3 9H 2 O, urea, sol-gel method 5 Zeolite NaX Synthetic, pelleted, ready for use 6 Zeolite NaA Synthetic, pelleted, ready for use 7 γ-al 2 O 3 Synthetic, pelleted, ready for use Adsorption process was carried out for three hours. As seen from adsorption kinetic curves of different support samples (Fig. 3.1), industrial zeolites and alumina sample No. 4 have reached adsorption equilibrium at 25 C during the process time. Other supports were still unsaturated. 8

10 Fig Adsorption kinetic curves of benzene on support samples. Experimental results were fitted with exponential decay curves to calculate complete saturation of support samples. Adsorption capacities of tested supports are shown in Table 1. Calculated values are presented in brackets. Table 3.2. Experimentally determined and calculated adsorption capacities of support samples for benzene vapor Sample No Adsorption mg/g (334) (90) (349) (275) capacity, X m mmol/g (4.28) (1.15) (4.47) (3.53) According to the obtained results, alumina sample Nr. 3 had the higher adsorption capacity for benzene vapor than sample Nr. 1 (prepared under same conditions, except stirring). Vigorous stirring prevented from formation of larger agglomerates during precipitation stage and probably led to the formation of the higher specific surface area formation. Alumina precipitated from aluminum sulfate (sample Nr. 2) had the lowest adsorption capacity. Thus, aluminum sulfate is worse precursor than aluminum nitrate under the same experimental conditions. For further investigation only samples Nr. 3-7 were chosen. Thermal desorption process was carried out after support samples were saturated with benzene vapor for three hours. As seen in Fig. 3.2, almost all the amount of adsorbed benzene was desorbed from tested alumina samples during first ten minutes at 150 C. Though, zeolites NaX and NaA (samples No. 5 and 6) desorbed only less than half of adsorbed benzene during first five minutes. Even after one hour of desorption process at 150 C, the amount of adsorbed benzene for NaX and NaA remained 33 % and 38 % of complete saturation, respectively. 9

11 Fig Desorption kinetic curves of benzene on different support samples Ratio of adsorption capacity X m and desorption rate coefficient k t (Table 3.3) was used as a criteria determining the selection of support for adsorbent-catalyst synthesis. Table 3.3. Ratio of benzene adsorption capacity X m and desorption rate coefficient k t for tested supports Sample No. Support X m /k t 3 γ-al 2 O γ-al 2 O NaX NaA γ-al 2 O According to the obtained results, tested supports may be divided into two groups. γ-al 2 O 3 supports are more suitable for synthesis of conventional VOC oxidation catalysts. The highest values of X m /k t were obtained for zeolites. Zeolite supports are more suitable for dual function VOC oxidation catalyst synthesis. Sufficiently high adsorption capacity and low rate of desorption are the key properties for effective VOC decontamination by adsorptive-catalytic oxidation Influence of CeO 2 and La 2 O 3 addition on CuO catalyst activity in VOC oxidation reactions The preliminary tests started performing a feasibility study on three primarily synthesized catalysts. Activity tests of methanol, methyl acetate and toluene oxidation over CuO/γ-Al 2 O 3, 9CuO-2CeO 2 /γ-al 2 O 3 and CuO-La 2 O 3 /γ-al 2 O 3 catalysts were carried out in order to compare their ability to convert VOC into desired and nonhazardous products: carbon dioxide and water vapor. Results of methanol, methyl acetate and toluene conversion are represented in Fig

12 Fig Methanol (a), methyl acetate (b) and toluene (c) oxidation over supported catalysts: 1 - CuO/γ-Al 2 O 3 ; 2-9CuO- 2CeO 2 /γ-al 2 O 3 ; 3 - CuO-La 2 O 3 /γ-al 2 O 3 Comparing temperatures of 95% (T 95 ) methanol conversion in activity test, 9CuO-2CeO 2 /γ-al 2 O 3 catalyst showed the highest activity, reaching the mentioned conversion value at 192 C. T 95 values for CuO/γ-Al 2 O 3 and CuO- La 2 O 3 /γ-al 2 O 3 were 205 C and 227 C, respectively. Thus, temperature shift of T 95 between ceria and lanthanum(iii) oxide promoted catalyst was sufficiently high (35 C). The results of methyl acetate oxidation over supported catalysts revealed that activity of all three primarily tested catalysts is almost the same above 180 C. However, copper(ii) oxide catalyst with addition of ceria performed better at lower temperatures. Toluene oxidation reaction starts at higher temperatures as compared to methanol and methyl acetate. At 225 C toluene conversion over CuO/γ-Al 2 O 3 catalyst was 3.9 %. Though, using 9CuO-2CeO 2 /γ-al 2 O 3 catalyst, toluene conversion was 29.5 % at the same temperature. 95% (T 95 ) conversion of toluene is obtained at 353 C over CuO/γ-Al 2 O 3 catalyst and 349 C over 9CuO- 2CeO 2 /γ-al 2 O 3 catalyst. At lower temperatures activity of CuO-La 2 O 3 /γ-al 2 O 3 catalyst in toluene oxidation reaction was higher compared to CuO/γ-Al 2 O 3. 11

13 Though, at higher temperatures activity of 9CuO-2CeO 2 /γ-al 2 O 3 catalyst is insufficient. At 370 C toluene conversion over 9CuO-2CeO 2 /γ-al 2 O 3 and CuO/γ-Al 2 O 3 catalysts equals nearly 100 %. While using CuO-La 2 O 3 /γ-al 2 O 3 catalyst the mentioned value was only 82.5 %. Fig Methanol (a) methyl acetate (b) and toluene (c) oxidation specific activity over supported catalysts: 1 - CuO/γ-Al 2 O 3 ; 2-9CuO-2CeO 2 /γ-al 2 O 3 ; 3 - CuO- La 2 O 3 /γ-al 2 O 3 The sum of the active components of three firstly tested catalysts is different (Table 3.4). Therefore it was reasonable to compare specific activity of VOC oxidation over CuO/γ-Al 2 O 3, 9CuO-2CeO 2 /γ-al 2 O 3 and CuO-La 2 O 3 /γ-al 2 O 3 catalysts (Fig. 3.4). Specific activity in VOC oxidation reactions is expressed as a quantity of VOC in µmol reacted per 1 s over 1 g of catalyst active components (µmol/(g AC s)). As results show, CuO-La 2 O 3 /γ-al 2 O 3 catalyst alters the previous sequence of catalytic activity in methanol and toluene oxidation (interchanges with CuO/γ-Al 2 O 3 catalyst) and performs better in methyl acetate oxidation. CuO/γ-Al 2 O 3 catalyst contained wt. % of CuO, though the integral quantity of active components was 11.3 wt. % in CuO-La 2 O 3 /γ-al 2 O 3. Therefore specific activity in oxidation reactions of all VOC was higher in the analyzed 12

14 temperature range. However 9CuO-2CeO 2 /γ-al 2 O 3 catalyst exhibited the highest specific activity and outperformed other tested catalysts. Fig Carbon monoxide yield during methanol (a), methyl acetate (b) and toluene oxidation and methanol yield during methyl acetate (d) oxidation over supported catalysts: 1 - CuO/γ-Al 2 O 3 ; 2-9CuO-2CeO 2 /γ-al 2 O 3 ; 3 - CuO-La 2 O 3 /γ-al 2 O 3 Since desired products of complete oxidation are CO 2 and H 2 O, it was necessary to verify whether by-products were formed or not during catalytic oxidation of VOC. Results of analysis show (Fig. 3.5) the formation of CO during methanol, methyl acetate and toluene oxidation and methanol formation during methyl acetate oxidation. CuO/γ-Al 2 O 3 catalyst produced three times, CuO-La 2 O 3 /γ-al 2 O 3 - two times more of CO per mg of methanol and toluene compared to 9CuO-2CeO 2 /γ-al 2 O 3 catalyst. The peak concentration of CO was observed in the range of C during methanol oxidation and in the range of C during toluene oxidation. Yield of CO was almost the same over CuO/γ-Al 2 O 3 and CuO-La 2 O 3 /γ-al 2 O 3 catalysts during methyl acetate oxidation. However 9CuO-2CeO 2 /γ-al 2 O 3 produced the lowest amount of carbon 13

15 monoxide. According to the results obtained with GC/MS methanol formation has occurred during methyl acetate oxidation. The highest amounts of methanol were formed in the temperature range of C. Though, in accordance with amounts of methanol formed during methyl acetate oxidation, catalysts may be arranged in the same sequence as it was presented in the comparison of their specific activity in VOC oxidation or CO yield Influence of CeO 2 and CuO ratio on catalyst activity in VOC oxidation reactions Since CuO catalyst promoted with CeO 2 was the most active and yielded the lowest amounts of by-products among tested catalysts, it was reasonable to determine optimal ratio of active components. Fig Dependence of CeO 2 /CuO ratio on the specific activity in methanol (a), methyl acetate (b) and toluene (c) oxidation reactions at different temperatures Therefore, VOC oxidation specific activity over three catalysts with different loadings of CuO and CeO 2 was compared with specific activity of CuO/γ-Al 2 O 3 at two constant temperatures (Fig. 3.6). Composition of CuO/γ-Al 2 O 3 and xcuoyceo 2 /γ-al 2 O 3 catalysts was expressed as a ratio of CeO 2 and CuO in the 14

16 horizontal axis. The obtained parabolic shape curves determine the optimal ratio of CeO 2 and CuO in the catalyst which equals Influence of CeO 2 and La 2 O 3 addition on CuO catalyst structure and specific surface properties According to the results of XRD analysis (Fig. 3.7), characteristic peaks confirm the formation of active components (CuO, CeO 2 and La 2 O 3 ) in the supported catalysts. As the aim was to obtain catalysts with different CeO 2 /CuO ratio, three solutions of different Ce 3+ concentration (5, 10 and 20 g/dm 3 ) and constant Cu 2+ concentration of 50 g/dm 3 were used to impregnate γ-al 2 O 3. Fig XRD patterns of synthesized supported catalysts and γ-al 2 O 3 support In this manner 11CuO-1CeO 2 /γ-al 2 O 3, 9CuO-2CeO 2 /γ-al 2 O 3 and 9CuO- 4CeO 2 /γ-al 2 O 3 catalysts with CeO 2 /CuO ratio of 0.10, 0.21 and 0.42 were synthesized, respectively. Relative intensities of CuO (2θ=35.46 ) and CeO 2 (2θ=28.49 ) characteristic peaks in the XRD patterns of xcuo-yceo 2 /γ-al 2 O 3 catalysts correlate with the quantities of active components obtained by AAS and AES methods (Table 3.4). Table 3.4. Content of active components and catalyst surface characteristics Content, wt. % Average Catalyst sample S BET, m 2 size of CuO /g CuO CeO 2 La 2 O 3 crystallites, nm CuO/γ-Al 2 O CuO-La 2 O 3 /γ-al 2 O CuO-1CeO 2 /γ-al 2 O CuO-2CeO 2 /γ-al 2 O CuO-4CeO 2 /γ-al 2 O

17 Minimal average size of CuO crystallites was obtained in CuO-La 2 O 3 / γ-al 2 O 3. Though, the addition of CeO 2 leads to the increase of the average size of CuO crystallites. The specific surface areas (S BET ) of synthesized CuO/γ- Al 2 O 3 and xcuo-yceo 2 /γ-al 2 O 3 catalysts were about the same (~ m 2 /g). However, as was expected, CuO-La 2 O 3 /γ-al 2 O 3 catalyst had the lowest value of S BET, which can be attributed to thermal aging of support during calcination procedure at 600 C. 3.5 BTX oxidation over CuO-CeO 2 /NaX adsorbent-catalyst Toluene oxidation. Results in Fig. 3.8 represent adsorptive-catalytic oxidation of toluene over CuO-CeO 2 /NaX dual function catalyst carried out at different saturation levels (2, 6, 7 and 10 mg/g) and constant regenerative air flow (3 dm 3 /min). First stage of cyclic process was adsorption of toluene from simulated effluent gas flow under room temperature. Fig Temperature profiles of reactor and heating chamber (a), evolution of CO (b), thermal runaway of toluene (c) and evolution of benzene (d) during oxidation process at different initial saturation levels: 1 2 mg/g; 2 6 mg/g; 3 7 mg/g; 4 10 mg/g; 5 heating chamber temperature 16

18 Concentration of toluene (800 mg/m 3 ) and flow rate (3 dm 3 /min) were kept constant during saturation of the adsorbent-catalyst. When desired saturation level was reached, toluene feed was turned off, only leaving the feed of regenerative air. After the adsorption of toluene, second stage of cyclic process was temperature swing oxidation induced by turning on heating in the chamber with built-in adsorptive reactor and air preheater. According to obtained internal reactor temperature profile curves (Fig. 3.8a), it is clearly seen that the lightoff occurs at ~250 C (9 th min), leading to highly exothermic oxidation reaction. As an incomplete oxidation product of carbohydrates, CO evolution (Fig. 3.8b) indicates that reaction starts at C inside the adsorptive reactor. Though, the peak of CO evolution corresponds to the light-off temperature. The main criterion in temperature swing oxidation that determines process efficiency in terms of VOC conversion into non-harmful products (CO 2 and H 2 O) is thermal runaway of desorbed and unreacted contaminants. Thermal runaway concentration curves (Fig. 3.8c) show that the highest amount of toluene at all saturation levels is desorbed unreacted in first 10 minutes, until the light-off occurs. According to results, the higher saturation level of the adsorbent catalyst the lower conversion of toluene is reached. Since conversion of toluene is dependent on saturation level, results were sufficiently high %. Benzene formation was observed during temperature swing oxidation of toluene (Fig. 3.8d). Since peak of benzene concentration corresponds to the light-off temperature, benzene is originated either from toluene thermal decomposition or is a product of disproportionation reaction. However, one of toluene disproportionation products xylene was not observed. Carbon monoxide and benzene formation is undesired during catalytic oxidation and lowers overall VOC conversion into CO 2 and H 2 O from 96.9 to 98.6 % in the range of investigated adsorbent-catalyst saturation levels. The formation of incomplete products is a drawback, though observed quantities of carbon monoxide and benzene were very small compared to estimated conversion of toluene into total oxidation products. To have a clearer view about the operating parameters of dual function adsorbent-catalyst, adsorptive-catalytic oxidation of toluene was also conducted at different flow rates of regenerative air (Fig. 3.9). In this case saturation level of CuO-CeO 2 /NaX was constant (6 mg/g of toluene) and was achieved by method described earlier. Flow rate of regenerative air was in range of 1-5 dm 3 /min. Higher temperatures were obtained at higher flow rates of regenerative air (Fig. 3.9a). Though, in terms of CO (Fig. 3.9b) and benzene (Fig. 3.9d) evolution and thermal runaway of toluene (Fig. 3.9d), best efficiency of temperature swing oxidation was achieved at flow rate of 1 dm 3 /min, which is referred to longer contact time of the regenerating air flow oxygen with the adsorbent-catalyst. Depending on the flow rate of regenerative air, toluene 17

19 conversion and overall VOC conversion into CO 2 and H 2 O was in range of % and %, respectively. Fig Temperature profiles of reactor and heating chamber (a), evolution of CO (b), thermal runaway of toluene (c) and evolution of benzene (d) during oxidation process at different flow rates of regenerative air: 1 1 dm 3 /min; 2 2 dm 3 /min; 3 3 dm 3 /min; 4 4 dm 3 /min; 5 5 dm 3 /min; 6 heating chamber temperature o-xylene oxidation. Temperature swing oxidation of o-xylene over dual function adsorbent-catalyst CuO-CeO 2 /NaX was conducted at both operating conditions - different saturation levels of the adsorbent-catalyst and different flow rates of regenerative air. Fig represents results obtained at different saturation levels and constant flow rate (3 dm 3 /min). As seen from temperature profile curves of the reactor (Fig 3.10a), the light-off temperature shifts in time compared to temperature swing oxidation of toluene. Though, it remains the same as before (250 C). This phenomenon can be explained by the fact that the adsorption heat of o-xylene is higher than adsorption heat of toluene. The 18

20 adsorptive reactor bed temperature increased up to C and up to C (room temperature C) during toluene and o-xylene adsorption stage, respectively, obviously meaning that more energy is required to desorb o-xylene. Fig Temperature profiles of reactor and heating chamber (a), evolution of CO (b) and thermal runaway of o-xylene (c) during oxidation process at different initial saturation levels: 1 3 mg/g; mg/g; 3 6 mg/g; 4 9 mg/g; 5 heating chamber temperature Benzene was not obtained during temperature swing oxidation of o-xylene. CO evolution (Fig 3.10b) and thermal runaway curves of o-xylene (Fig 3.10c) show that formation of CO and desorption of o-xylene is also dependent on saturation level as in case of toluene temperature swing oxidation. Total oxidation of o-xylene was in range of % during temperature swing oxidation at different saturation levels of adsorbent-catalyst (3-9 mg/g). It is important to mention that two peaks of maximum CO concentration were obtained during o-xylene oxidation. This phenomenon is associated with probable formation of intermediate products on the surface of the adsorbent catalyst. The first peak of CO is attributed to oxidation of o-xylene and the second peak is attributed to oxidation of formed intermediate products on the surface of CuO-CeO 2 /NaX. Though, no other compounds were found in the gas phase at the outlet of the adsorptive reactor. 19

21 Fig Temperature profiles of reactor and heating chamber (a), evolution of CO (b), thermal runaway of o-xylene (c) during oxidation process at different flow rates of regenerative air: 1 1 dm 3 /min; 2 2 dm 3 /min; 3 3 dm 3 /min; 4 4 dm 3 /min; 5 heating chamber temperature Conducted temperature swing oxidation of o-xylene at different flow rates of regenerative air (Fig. 3.11) confirmed that thermal runaway of o-xylene (Fig. 3.11c) is lower and efficiency in terms of conversion is higher at lower flow rates of regenerative air. Total oxidation of o-xylene was in the range of %. Only one peak of CO maximum concentration was obtained during oxidation reaction at 1 dm 3 /min. Lower flow rate of regenerative air results in higher contact time with the oxygen consequently leading to simultaneous oxidation of o-xylene and formed intermediate products on the surface of CuO-CeO 2 /NaX under these conditions. Benzene oxidation. Since earlier results revealed that temperature swing oxidation of VOC is more efficient at lower flow rates of regenerative air, benzene oxidation was conducted at constant saturation level of 6 mg/g and constant flow rate of 1 dm 3 /min (Fig. 6). 20

22 Fig Temperature profiles (a) of reactor (1) and heating chamber (2) and evolution (b) of CO (3) and thermal runaway of benzene (4) during oxidation process Though, obtained results of temperature swing oxidation of benzene were worse comparing to previously described experiments. High amount of desorbed benzene during initial period of the process resulted in considerably lower conversion equal to 77.5 %. Peak concentration and integral amount of formed CO were also higher compared to toluene and o-xylene oxidation results and reached values of up to 1600 mg/m 3 and mg/g, respectively. Table 3.5. Total oxidation of BTX at different operating conditions Inlet Flow rate of Saturation Saturation VOC concentration, regenerative time, min level, mg/g mg/m air, dm 3 /min 33 2 Toluene 800 Total oxidation, % o-xylene Benzene

23 3.6 Mechanism of benzene, toluene and o-xylene oxidation over CuO- CeO 2 /NaX adsorbent-catalyst In order to explain the oxidation mechanism of toluene and o-xylene over CuO-CeO 2 /NaX, experiments were conducted to identify the possible formation of intermediate oxygenated products on the surface of the adsorbent-catalyst. The flow containing benzene, toluene or o-xylene was passed through the adsorbent-catalyst at three different temperatures (200, 300 and 400 C) for total time on stream (TOS) of 1 h. Table 3.6. Composition of unreacted VOC and formed intermediate products on the surface of CuO-CeO 2 /NaX at different temperatures Composition of VOC used for oxidation Name (IUPAC) Formula unreacted VOC and surface intermediate products, % 200 C 300 C Methylbenzene Benzaldehyde Toluene Methoxymethylbenzene methyl-4- (phenylmethyl)-benzene methyl-3- (phenylmethyl)-benzene ,2-dimethylbenzene methylbenzaldehyde o-xylene o-phtalaldehyde benzofuran-1(3H)- one

24 Then CuO-CeO 2 /NaX was purged with nitrogen flow to block the on-going oxidation reaction and cooled down to the room temperature. The sample of 0.5 g CuO-CeO 2 /NaX was extracted with 3 ml of methanol and analyzed with GC/MS. The obtained results of unreacted VOC and formed intermediate products on the surface of CuO-CeO 2 /NaX are given in Table 3.6. The relative amounts of found components on the surface of CuO-CeO 2 /NaX mainly consisted of unreacted toluene, o-xylene and aldehydes of investigated VOC. Small amounts of 1-methyl-4-(phenylmethyl)-benzene and 1-methyl-3- (phenylmethyl)-benzene were found during toluene oxidation. The mentioned compounds are derivatives of two toluene molecules. Relatively high amount of 2-benzofuran-1(3H)-one (phthalide) was found during o-xylene oxidation at 300 C, which is derivative of o-phtalaldehyde. Though, none of these intermediate products were observed at 400 C. This means that the efficiency of toluene and o-xylene oxidation is attributed to parallel-sequential mechanism, when simultaneous toluene or o-xylene oxidation and formation of lower volatility intermediate oxygenated compounds, which are oxidized at higher temperature, take place in the overall temperature swing oxidation process. Formation of o-xylene oxygenated compounds was observed at higher temperatures compared to toluene. This explains the origin of two CO peaks during o-xylene oxidation. According to the described method, no intermediate products were found on the surface of CuO-CeO 2 /NaX during benzene oxidation. 3.7 Characteristics of CuO-CeO 2 /NaX adsorbent-catalyst The XRD patterns of dual function adsorbent-catalyst CuO-CeO 2 /NaX and initial zeolite NaX are shown in Fig Though, NaX zeolite was impregnated with solution of Ce(NO 3 ) 3 6H 2 O, thermal decomposition of Ce(NO 3 ) 3 6H 2 O is considered to be a redox reaction resulting in formation of CeO 2 after calcination at 450 C. The results in the XRD pattern indicate that monoclinic CuO and cubic CeO 2 are deposited on the surface of NaX zeolite. Phase composition and the properties of the most intense diffraction peaks of zeolite NaX and dual function adsorbent-catalyst CuO-CeO 2 /NaX are described in Table 3.7. Intensities of characteristic NaX peaks in the CuO-CeO 2 /NaX XRD pattern were observed to be lower than in the XRD pattern of initial zeolite NaX. This fact can be explained by two reasons: partial substitution of NaX with CuO and CeO 2 in terms of overall mass and structural changes (dehydration) of NaX phase at 450 C. These changes correspond well to the obtained composition results by AAS and AES methods and standard XRD patterns of hydrated and dehydrated NaX zeolite found in literature. 23

25 Fig XRD patterns of zeolite NaX (1) and adsorbent-catalyst CuO-CeO 2 /NaX (2) Table 3.7. Phase composition and diffraction peaks properties of zeolite NaX and dual function adsorbent-catalyst CuO-CeO 2 /NaX Phase Miller CuO- 2θ, d-spacing, Zeolite NaX indices CeO degree nm 2 /NaX h k l Intensity Intensity NaX (Faujasite synthetic) CuO monoclinic CeO 2 cubic Experimental ratio of copper and cerium in the initial impregnating solution was 5:1 w/w. The pellets of NaX (50 g) were soaked in 250 ml of prepared solution for 2 h at room temperature. After the drying and calcining procedures quantitative analysis was conducted, according to which, CuO-CeO 2 /NaX contained % of copper and 2.87 % of cerium. The observed ratio of copper and cerium slightly deviates from experimental due to distinctive adsorption of different cations. 24

26 CuO and CeO 2 active components are deposited at the outer layer of NaX pellet and has an eggshell form with a sharp boundary. Thickness of the layer is ~ μm. Such adsorbent-catalyst acts as a micro-container. At higher temperatures concentrated VOC desorbs and cross the eggshell layer of the active components and are oxidized into non-harmful products. Fig Cross-sectional images of CuO-CeO 2 /NaX adsorbent-catalyst pellet at different magnifications: a 50x; b 100x; c 200x; d 400x According to the results of specific surface measurements, specific surface area S BET and pore volume V p of used catalyst decreased from to m 2 /g and from to cm 3 /g, respectively, compared to new CuO-CeO 2 /NaX. This may be attributed to thermal aging of the adsorbentcatalyst under cyclic heating-cooling conditions. Table 3.8. Results of BET measurements CuO-CeO 2 /NaX Slope Intercept Pore volume S 10 2 I 10 4 V p, cm 3 /g C BET S BET, m 2 /g R 2 New Used

27 3.8 Technological recommendations for BTX oxidation According to the obtained results, adsorptive-catalytic oxidation of toluene and o-xylene may be implemented by two parallel adsorptive reactors which operate under cyclic adsorption-regeneration regime (Fig. 3.14). For decontamination of m 3 /h air flow containing 100 mg/m 3 of toluene or o-xylene, one adsorptive reactor has to be loaded with 1000 kg of CuO-CeO 2 /NaX adsorbent-catalyst. Adsorption stage would last 6 hours and the initial saturation of the adsorbent-catalyst would be equal to 6 g/kg. To initiate autothermic oxidation reaction and to achieve total oxidation degrees higher than 99 % adsorbent-catalyst bed has to be preheated up to 250 C in less than 10 minutes during the regeneration stage. Fig Process flow diagram of BTX oxidation: 1, 2 contaminated and regenerative air fans; 3, 4 flow direction changing units; 5, 6 adsorptive reactors; 7 flow distribution device; 8 catalyst bed; 9 start-up heat exchanger; 10 additional heater; 11 recuperative heat exchanger; 12 flow divider; 13 optional heat exchanger; 14 open flow control valve; 15 closed flow control valve Conventional catalytic oxidation technology is more suitable for decontamination of air containing benzene, since effectivity of adsorptivecatalytic oxidation was insufficient. Though, toluene and o-xylene may be decontaminated using conventional catalytic oxidation too. Contaminated air has to be supplied to the reactors through a by-pass which is indicated by a dashed line in the process flow diagram. Complete oxidation of benzene would be obtained above 370 C at a gas hourly space velocity of 3100 h -1. As the area of surface in the recuperative heat exchanger is equal to m 2, autothermic oxidation of benzene, toluene and o-xylene is available at the concentration levels above 4.48, 4.40 and 4.37 g/m 3, respectively. Otherwise additional heat is required. 26

28 4. CONCLUSIONS 1. Zeolites NaX and NaA are the best supports for dual function catalyst: the ratio of benzene vapor adsorption capacity and benzene desorption rate coefficient (X m /k t ) is equal to and 791.1, respectively. All tested γ- Al 2 O 3 supports are suitable for production of conventional oxidation catalysts. The aforementioned ratio (X m /k t ) varied between 1.6 and 9.2 for the latter supports and was significantly lower compared to zeolites NaX and NaA. 2. CuO (9.32 %) catalyst modified with CeO 2 (1.94 %) additive was the most active among tested catalysts in methanol, methyl acetate and toluene oxidation reactions at temperatures of C. The highest degree of conversion was obtained and the lowest amounts of intermediate or incomplete oxidation products were formed using this catalyst. The activity of CuO (9.56 %) catalyst modified with La 2 O 3 (1.74 %) additive and single supported CuO (13.51 %) catalyst was lower under the same conditions. 3. It was established that the optimal ratio of CeO 2 /CuO amounts in the supported catalyst is equal to The degree of VOC conversion and specific activity was the highest using catalyst matching such composition at temperatures of C. 4. It was established that the addition of СeO 2 (1.94 %), maintaining a similar cumulative amount of the active components (~11-13 %), slightly changes the surface characteristics of CuO catalyst by using γ-al 2 O 3 as a support: specific surface area S BET decreases from to m 2 /g, pore volume V p decreases from to cm 3 /g, the predominant pore diameters in the catalysts: 3-5 and nm. 5. The optimal conditions for toluene and o-xylene vapor decontamination over CuO-CeO 2 /NaX adsorbent-catalyst are: oxidation temperature C, catalyst bed heating rate 25 C/min, initial saturation up to 6 mg/g and gas hourly space velocity of regenerative air 1033 h -1. The degree of complete oxidation of adsorbed toluene and o-xylene vapor is equal to 99.3 and 99.8 %, respectively, under these conditions. The degree of complete oxidation of benzene is lower and equals 77.5 %. The degree of toluene conversion was 88.6 % over CuO-CeO 2 /NaA catalyst at 377 C. Insufficient activity of this catalyst is attributed to the diffusion of toluene which is limited by small pores of zeolite NaA. 6. The oxidation of toluene and o-xylene undergoes according to parallelsequential mechanism. Toluene and o-xylene are oxidized into CO 2, water vapor and a small amount of CO, along with formation of surface intermediate oxidation compounds at temperatures below 340 C. The latter compounds are completely oxidized at higher temperatures ( C). The surface intermediate compounds were not observed during benzene oxidation. Benzene is oxidized directly into products of complete oxidation 27

29 (CO 2 and H 2 O vapor) along with small amount of CO over CuO-CeO 2 /NaX adsorbent-catalyst. The apparent rate constants (k t 10 3 ) of reactions of toluene and o-xylene adsorptive-catalytic oxidation were equal to 1,243 and 1,136 s -1, respectively. The apparent rate constants of benzene oxidation reaction varies between 1.72 and 7.09 s -1 at temperatures of ºC and the activation energy is equal to 49.3 kj/mol. 7. Two adsorptive reactors have to be equipped with 2000 kg of CuO-CeO 2 /NaX adsorbent-catalyst. Such technology allows decontaminating m 3 /h air flow containing 100 mg/m 3 of toluene or o-xylene. CuO-CeO 2 /NaX adsorbent-catalyst is saturated with 6 g/kg of toluene or o-xylene after 6 hours of adsorption stage. The degree of complete oxidation of toluene and o-xylene is higher than 99 % as the initial heating rate of the catalyst bed equals 25 C/min. Autothermic decontamination of benzene, toluene and o-xylene by conventional catalytic oxidation is possible when concentrations of these compounds are higher than 4.48, 4.40 and 4.37 g/m 3, respectively. LIST OF PUBLICATIONS AND PROCEEDINGS ON THE THEME OF DISSERTATION Publications corresponding to the list of Thompson Reuters Web of Science database 1. Urbutis, Aurimas; Kitrys, Saulius. Structure and activity of CuO catalysts promoted with CeO 2 and La 2 O 3 for complete oxidation of VOC // Chemija: mokslo darbai / Lietuvos mokslų akademija. Vilnius: Lietuvos mokslų akademija. ISSN , T. 24, nr. 2, p Urbutis, Aurimas; Kitrys, Saulius. Dual function adsorbent-catalyst CuO- CeO 2 /NaX for temperature swing oxidation of benzene, toluene and xylene // Central European Journal of Chemistry. Warsaw: Versita. ISSN , Vol. 12, iss. 4, p Publications referred in International databases 1. Urbutis, Aurimas; Kitrys, Saulius. Metanolio ir metilacetato garų desorbcija nuo katalizatoriaus CuO/NaX paviršiaus // Cheminė technologija / Kauno technologijos universitetas. Kaunas: Technologija. ISSN , nr. 2(55), p Urbutis, Aurimas; Dabrilaitė-Kudžmienė, Gitana; Kitrys, Saulius. Metanolio ir metilacetato sąveika su CuO ceolito NaX paviršiuje // Cheminė technologija / Kauno technologijos universitetas. Kaunas: Technologija. ISSN , nr. 1-2(57), p Publications in Proceedings of Conferences 1. Urbutis, Aurimas; Kitrys, Saulius. Adsorbatų reakcijos su CuO ceolito NaX paviršiuje // Neorganinių medžiagų chemija ir technologija = Chemistry and 28

30 technology of inorganic materials: konferencijos pranešimų medžiaga / Kauno technologijos universitetas. Kaunas: Technologija, ISBN p Urbutis, Aurimas; Kitrys, Saulius. Support selection for synthesis of adsorbent-catalyst for VOC oxidation // Chemistry and chemical technology of inorganic materials: proceedings of scientific conference chemistry and chemical technology / Kaunas university of technology. Kaunas: Technologija. ISSN , p Urbutis, Aurimas; Kitrys, Saulius. Activity of CuO, CuO-CeO 2, CuO- La 2 O 3 /Al 2 O 3 systems for total catalytic oxidation of VOC // Riga Technical University 53rd International Scientific Conference, dedicated to the 150th Anniversary and The 1st Congress of World Engineers and Riga Polytechnical Institute / RTU Alumni, October 2012, Riga, Latvia / Riga Technical University. Riga: Riga Publishing House, ISBN p Urbutis, Aurimas; Kitrys, Saulius. Activity of supported CuO, CuO-CeO 2, CuO-La 2 O 3 /Al 2 O 3 catalysts for total oxidation of toluene // Chemistry and chemical technology of inorganic materials: proceedings of scientific conference chemistry and chemical technology / Kaunas university of technology. Kaunas: Technologija. ISSN , p Urbutis, Aurimas; Kitrys, Saulius. Total oxidation of toluene over CuO- CeO 2 /NaX adsorbent-catalyst // EcoBalt 2013: 18th international scientific conference, October 25-27, 2013 Vilnius, Lithuania: book of abstracts. Vilnius: Vilniaus universiteto leidykla, ISBN p CURRICULUM VITAE Surname, name Urbutis, Aurimas Date of birth Place of birth Klaipėda Education Secondary school of Žalgiriai, Tauragė Kaunas University of Technology Faculty of Chemical technology: BSc in Chemical Engineering Kaunas University of Technology Faculty of Chemical technology: MSc in Chemical Engineering Doctoral degree studies at Kaunas University of Technology Faculty of Chemical technology Employment Employee at Kaunas University of Technology Department of Physical Chemistry (later Department of Physical and Inorganic Chemistry): From to senior laboratory assistant From assistant 29

31 REZIUMĖ Temos aktualumas. Antropogeninės kilmės lakūs organiniai junginiai (LOJ) yra žalingi ir, patekę į atmosferą, sukelia neigiamą poveikį aplinkos oro kokybei, žmonių sveikatai ir supančiai aplinkai. Todėl šių medžiagų koncentracijos įvairių gamybų išlakose yra griežtai kontroliuojamos, o LOJ šalinimo technologijos yra aktuali aplinkosaugos ir chemijos inžinerijos problema. Vieni iš sunkiausiai nukenksminamų teršalų yra lakūs aromatiniai junginiai, tokie kaip benzenas, toluenas, ksilenai (BTK). Dėl savo bendros cheminės savybės aromatiškumo, benzeno, tolueno ir ksilenų molekulės yra labai stabilios, o jų šalinimo technologijos dar labiau komplikuojasi, kai šių medžiagų koncentracijos dujinėse išlakose yra mažos. Vienas iš geriausiai prieinamų ir plačiausiai tyrinėjamų LOJ nukenksminimo būdų yra jų heterogeninis katalizinis oksidavimas oro deguonimi. Tokie katalizatoriai yra naudojami tiek stacionarių, tiek ir mobilių (automobiliai) taršos šaltinių išlakų nukenksminimui. Geriausi žinomi oksidavimo katalizatorių aktyvieji komponentai yra taurieji metalai (Pt, Pd), disperguoti ant įvairių nešiklių. Tačiau šie katalizatoriai yra brangūs. Kaip alternatyva tauriesiems metalams plačiausiai tyrinėjami pereinamųjų metalų (Cu, Co, Mn, Cr ir kt.) oksidai ir kiti junginiai. Remiantis literatūros šaltinių duomenimis ir ankstesniais KTU Fizikinės chemijos katedroje atliktais darbais, vienas iš aktyviausių pavienių metalų oksidų LOJ oksidavimo reakcijose yra CuO. Lyginant su kitais metalais, CuO katalizatoriai veikia žemesnėje temperatūroje. Alkoholių oksidavimo reakcijose šių katalizatorių aktyvumas prilygsta platinos grupės metalų katalizatoriams. Tačiau aromatinių junginių oksidavimo technologijose CuO katalizatorių aktyvumas yra mažesnis. Remiantis literatūros šaltiniais, lantanoidų ar jų oksidų priedai gali padidinti CuO katalizatorių aktyvumą. Todėl pastaruoju metu labai susidomėta La ir, ypatingai, Ce bei kitų retųjų žemės metalų ir jų oksidų savybėmis žematemperatūriuose heterogeniniuose kataliziniuose procesuose. CeO 2 ir La 2 O 3 turi daug teigiamų savybių, kurios daugiakomponentėse sistemose gali būti racionaliai išnaudotos: CeO 2 yra stiprus oksidatorius, turi didelę deguonies talpą ir didesnį nei kitų metalų oksidų kristalinės gardelės deguonies mobilumą, La 2 O 3 padidina nešiklių terminį stabilumą ir pagerina kitų metalų oksidų dispersiškumą katalizatorių paviršiuje. Dispergavus CuO-CeO 2 ir CuO-La 2 O 3 ant didelį savitąjį paviršiaus plotą turinčių nešiklių, galima gauti dvigubo veikimo medžiagą adsorbentą-katalizatorių. Tokia medžiaga sukoncentruoja LOJ iš įkrovą pratekančio valomo oro srauto. Vėliau adsorbuotos medžiagos, padidinus įkrovos temperatūrą, suoksiduojamos į CO 2 ir vandens garus. Sukoncentravus medžiagas adsorbento-katalizatoriaus paviršiuje, dirbtinai sudaromos sąlygos oksidavimo reakcijai pagreitinti, kadangi yra žinoma, kad reakcijos greitis priklauso nuo pradinių medžiagų koncentracijos. Taip pat prasidėjusios oksidavimo reakcijos metu išsiskirianti šiluma tampa varomąja jėga toliau vykstančiai oksidavimo reakcijai. Esant 30