The application of modified cenospheres in DeNOx process

Cenospheres were modified with iron, manganese and/or copper ions by the hydrotalcite method. The obtained catalysts were characterized by FTIR, XRD and low-temperature nitrogen sorption. The best catalyst at low temperature (200 oC) was CBFe-Mn while at the highest measured temperature of 500 oC both CBFe-Mn and CBMn-Cu showed similar performance.


Introduction
The application of hard coal as an energy source, in addition to a significant amount of particulate matter, also results in introducing other pollutants into the environment, mainly SO2 and NOx. The latter are particularly harmful for both people and environment, and the amount of emitted NOx are regulated by the Directive of the European Parliament and of the Council of 14 December 2016 on the reduction of national emissions of certain atmospheric pollutants [1]. In order to reduce NOx emissions from power plants and industrial boilers, apart from primary methods, whose effectiveness is not sufficient, additional technologies are used [2][3][4][5][6]: SCR (selective catalytic reduction), SNCR (selective non-catalytic reduction) and hybrid technologies SNCR-SCR. SCR is the most effective method of removing NOx from flue gases, but requires a suitable catalyst. The catalyst currently used in power industry is V2O5 supported on TiO2 and promoted with WO3, placed on a monolith. However, this catalyst has a number of disadvantages, among others high activity is achieved only in the medium-temperature range (about 300-400 o C), and at higher temperatures the structure of the support is changed [7]. The catalyst also contains environmentally harmful vanadium. Because of these disadvantages, new catalysts for the SCR process are still searched. Those investigated in the literature are most often oxides of transition metals applied on an oxide support, usually with the addition of promoters increasing their thermal and / or chemical stability. Numerous systems have been proposed as catalyst carriers, e.g. activated carbons [8][9][10][11][12][13][14][15][16][17], aluminosilicates [18][19][20][21][22], zeolites [23][24][25][26], hydrotalcites [27][28][29], etc. Most often, delectron metals, such as Fe [20,22,[30][31][32][33], Cu [5,18,[34][35][36][37][38], Mn [8,[39][40][41][42][43], etc. were studied as active materials. The catalysts based on activated carbons showed appropriate of NO conversion in the low to medium-temperature range (150-300 o C), but are prone to oxidation [3,9,10,44]. Promoted aluminosilicates have good catalytic properties at the high temperature range [18,22,[45][46][47][48]. Numerous hydrotalcite-like materials or their derivatives have been reported as catalysts for a wide variety of chemical processes including selective reduction of nitrogen oxides with ammonia (SCR, DeNOx). Due to the high content of silicate and aluminosilicate, cenospheres obtained from fly ashes may be an interesting carrier.
The global production of fly ash is estimated at around 4.2 billion tons per year, the largest of which is produced in China and the United States [49]. Annually, about 20 million tonnes of energy waste are generated in Poland [50], with only 4-9% being subject to use [49]. One of the components of fly ash are cenospheres whose content is in the range of 0.3 -2 wt.%. They are formed at high temperatures and are therefore thermally stable. The size of the cenospheres is in the range of 0.01-350 μm, dependent on the SiO2/Al2O3 ratio [37,[51][52][53][54]. The high SiO2 content results in the formation of small-scale cenospheres. The sintering temperature also has a large influence on the size of the microspheres, the higher the temperature, the larger the diameters. The composition and structure of the cenospheres is important and influences their use. The outer part has a glassy and crystalline structure. The glassy phase is dominant and consists of silicon and aluminium oxides, and smaller amounts of iron and calcium oxides. The crystalline phase consists mainly of quartz and mullite, as well as hematite and magnetite [54]. The typical composition of the solid phase cenospheres is [55]: SiO2 (50% -65%), Al2O3 (19% -42%), Fe2O3 (0.7% -6.5%). There are also: CaO, MgO, K2O, Na2O, TiO2, SO3 and P2O5 in negligible amounts [55]. The interior of the cenosphere is mainly filled with CO2 and N2 gas, but also CO, O2 and H2O in smaller quantities [53,55]. Cenospheres also have low density (0.2-0.8 g/cm 3 ) and very good mechanical strength (210-350 kg/cm 2 ). They exhibit good corrosion resistance, including an oxidizing environment, as well as high resistance to thermal shock. These properties result from high mullite content in the cenospheres. Microspheres tend to sinter only at temperatures of 950 o C to 1200 o C and melt at a temperature of 1250 o C to 1450 o C [56].
The main aim of this work was to study the possibility of using cenospheres modified with copper or iron as catalysts in the selective catalytic reduction of NO with ammonia SCR-NH3.So-promoted materials were characterized by FTIR, X-ray diffraction, and the lowtemperature nitrogen sorption.

Experimental part 2.1 Synthesis of catalysts
White cenospheres (Cenospheres Trade & Engenering), further denoted CB, studied here were used as supports. The cenospheres were modified with Cu, Mn and/or Cu, using a method similar to that applied for hydrotalcites formation (precipitation at constant pH). The resulting materials were then calcined at 500 °C for 12 h.

Characterization
The crystalline structure of the catalysts was determined by X-ray diffraction XRD using PANalytical Empyrean diffractometer (CuK,= 0.15406nm). Diffractograms were recorded in the range of 2θ angles from 5 to 90 o , with a measurement step of 0.02 o /min. The measurements were taken at room temperature.
The FTIR spectroscopy (Perkin Elmer Frontier FT-IR) was used to determine the functional groups. Spectra were recorded in the 4000-400 cm -1 range, with a resolution of 4 cm -1 .
The specific surface area was determined using low-temperature nitrogen sorption (ASAP 2060 by Micrometrics). Before the measurement, the catalysts were degassed at 500 °C. All physicochemical tests were performed for the samples after calcination.

Catalytic tests
The reaction of selective catalytic reduction NO with ammonia was carried out in a fixed bed reactor in a setup shown in Fig. 1, with the following parameters: catalyst mass 200 mg; composition of reaction gases 800 ppm NO, 800 ppm NH3 and 3% O2; temperature range 200-500 o C with measurement every 100 o C; flow 100 ml/min.
The concentration of NO and N2O was determined at the outlet of the reactor using an ABB IR analyzer. 3 Results and discussion Fig. 2 shows the diffractograms for the modified cenospheres. The resulting XRD patterns are typical for cenospheres [46], [47] and indicate the presence of mullite (Al2O3) (2θ = 41; 43; 61; 65 o ), quartz (SiO2) (2θ = 26; 47; 56 o ), hematite (Fe2O3) (2θ = 35 o ), and magnetite (Fe3O4) (2θ = 33 o ). After modification, no additional phases of iron, manganese and/or copper were found, which indicates either the formation of amorphous forms of active materials introduced and/or a high degree of dispersion of the deposited compounds [57]. The infrared absorption spectra were typical for cenospheres, showing bounds of Si-O-Si, Si-O-Al and Al-O. The introduction of active material did not affect either the intensity, or did not result in the formation of additional peaks [58][59][60].
The textural tests have shown that the obtained systems were characterized by small BET surface area (about 10 m 2 /g) and small pore volume values (Table 1).   For the CBFe-Cu catalyst, the NO conversion increased with temperature, and reached the value close to 100% at the highest tested temperature (500 o C). Other tested catalysts obtained somewhat lower x(NO). In the case of manganese-modified catalysts, higher amounts of nitrous oxide were observed than for CBFe-Cu. This in good agreement with the performance of MnOx-containing activated carbons [8] or hydrotalcites [61]. On the other hand, the lower amount of N2O formed for CBFe-Cu agrees well with other Fe-containing types of catalysts [4,22]. It should be mentioned, however, that the values for CBFe-Mn and CBFe-Cu are only slightly over the experimental error of the used analyser (ca. 30 ppm). Taking into account both NO conversion and N2O formation, the best catalysts at low temperature (200 o C) was CBFe-Mn while at the highest measured temperature of 500 o C both CBFe-Mn and CBMn-Cu had similar efficiency. At 300 o C, NO conversion was similar for all studied samples but because of the lowest formation of N2O, CBFe-Cu was the best.
Taking into account the registered results and additionally the fact that cenospheres are obtained from waste (fly ash), it makes them promising materials for new DeNOx catalysts. This work was supported by the AGH Research Grant No. 16.16.210.476