Copper exchanged ultrastable zeolite Y – A catalyst for NH3-SCR of NOx from stationary biogas engines

Catalysis Today 191 (2012) 6–11

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Copper exchanged ultrastable zeolite Y – A catalyst for NH3 -SCR of NOx from stationary biogas engines a,b ´ J. Ochonska , D. McClymont d,e , P.J. Jodłowski a , A. Knapik a , B. Gil a , W. Makowski a , a W. Łasocha , A. Kołodziej b,c , S.T. Kolaczkowski e , J. Łojewska a,∗ a

Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Kraków, Poland Institute of Chemical Engineering of the Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland c Faculty of Civil Engineering, Opole University of Technology, ul. Katowicka 48, 45-061 Opole, Poland d Doctoral Training Centre in Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath BA2 7AY, UK e Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK b

a r t i c l e

i n f o

Article history: Received 3 October 2011 Received in revised form 31 May 2012 Accepted 4 June 2012 Available online 12 July 2012 Keywords: NH3 -SCR Ultrastabilized zeolite Y ZSM-5 Cu-exchanged zeolites Structured reactors Biogas engines

a b s t r a c t NOx emissions will need to be controlled from gas engines, which in the future could be using a syngas/producer gas as a fuel (generated from the gasification of biomass). In anticipation of this need, the activity of a copper-exchanged ultrastabilized zeolite Y was investigated, for the selective catalytic reduction of NOx . The catalyst performance and properties were compared with reference samples of Cu-exchanged un-stabilized Y and Cu exchanged ZSM-5 zeolites. Activity was related to the number and nature of active centers, which were determined by in situ FTIR and TPD experiments using CO, NO, and NH3 as probe molecules. In the test reaction the ultrastabilized Cu-USY catalyst showed high activity at low temperatures and offered high hydrothermal resistance, whilst the Cu-ZSM-5 catalyst demonstrated the highest overall activity. The observed low temperature activity of the ultrastabilized Cu-USY catalyst was linked to the high abundance of Cu+ sites, which were detected by in situ FTIR analyses using CO adsorption. © 2012 Elsevier B.V. All rights reserved.

1. Introduction At present, technologies are being developed which enable a feed of biomass to be gasified, producing a gaseous fuel that has a high CO and H2 content. Depending on the process, this type of gas may be known as a syngas (low nitrogen content) or a producer gas (high nitrogen content). The gas may then be used as an alternative fuel in stationary gas engines to supply energy in the form of electricity and heat at a local level. The development of these technologies creates an opportunity to explore more effective and economic solutions for the clean-up of emissions of CO, hydrocarbons and NOx , in the exhaust gas from such engines. To reduce NOx emissions, the well established process of selective catalytic reduction (SCR) with NH3 could be used (e.g. [1,2]) and in this paper the development of a catalyst based on zeolites is explored. In catalytic clean-up systems, although the use of multi-channel monolith supports for catalysts is well established, catalyst support structures based on “short channel structures” such as wire meshes, offers interesting possibilities. For example in [3], the heat and mass transfer characteristics of wire gauzes were studied, and

∗ Corresponding author. Tel.: +48 12 6632245; fax: +48 12 6340515. E-mail address: [email protected] (J. Łojewska). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.06.010

displayed mass transfer characteristics comparable with those in packed beds, whilst being substantially higher than those obtained in monolith supports. However, to exploit the enhanced rates of mass transfer in such structures, active catalysts also need to be employed. On the other hand, the highest flow resistance per unit bed length was ranked in the following order: packed bed > wire gauze > monolith. This does not necessarily mean that the overall pressure drop would be higher in the same ranking, as that would depend on the length of each bed to achieve the necessary conversion. In the literature, there are many papers (e.g. [4–7]), which describe NOx abatement options, how different catalysts work, and possible reaction mechanisms. It has also been shown [8] that Cu2+ -exchanged ZSM-5 zeolites are active catalysts for the reduction of NO with NH3 , in the presence of oxygen. The formation of nitrogen was found to proceed almost selectively, except that at higher temperatures (>573 K) the oxidation of ammonia with oxygen occurred at the same time. To clarify the effect of the zeolite structure on the specific activity of Cu2+ sites, the catalytic properties of Cu2+ -exchanged mordenite and Y-Zeolite were also studied in [8]. Interestingly, they found that the activity of Cu2+ -Y-zeolite was low above 573 K, yet it exhibited a significant activity at 373 K, at which temperature the Cu2+ -exchanged mordenite and ZSM-5 were almost inactive.

J. Ocho´ nska et al. / Catalysis Today 191 (2012) 6–11

Unfortunately, as described in a review article [9], one of the main challenges for the practical application of SCR catalysts is their durability under hydrothermal conditions. In the petroleum industry, Y zeolites are used in catalytic cracking and hydrocracking processes and the problem of hydrothermal stability has been overcome by a process known as ultrastabilization. In summary, this entails following a repeated sequence of partial exchanges of zeolite Y crystals with ammonium nitrate and subsequent steaming and/or calcinations. These treatments reduce the effective Al content in the framework and redistribute Al atoms producing ultra-stabilized zeolite Y (USY) with enhanced resistance toward steam degradation. Further information on the ultrastabilization process may be obtained in references [10–13]. Based on this review of the literature, ultrastabilized zeolite Y was selected for further exploratory work. A key objective in this paper was to start to develop a zeolite based catalyst system for the SCR of NOx emissions from the exhaust of stationary engines, which may use biomass derived fuels (syngas/producer gas). NH3 was selected as a reducing agent for the SCR reaction, and the catalyst system would need to exhibit both high activity and high hydrothermal stability. This was explored by comparing the properties and performance of three different types of Cu-exchanged zeolites. The zeolites (as supplied) were of the following form: (a) Ultra-stabilized Y-type zeolites (ammonium form) supplied by Union Carbide. (b) Y-type zeolite (ammonium form), supplied by Zeolyst Int. (c) ZSM-5 (sodium form), supplied by the Industrial Chemistry Research Institute (Warsaw, Poland), which was used as a reference. As another objective in this project was to explore methods of supporting the chosen catalyst on a metal support (e.g. wire gauzes, plate), such catalyst systems were also prepared on metal surfaces. 2. Experimental techniques 2.1. Sample preparation The three Cu-exchanged zeolite samples were made from their supplied form, by ion-exchange, using an excess amount of 0.5 M Cu(NO3 )2 solution (acidic, pH = 2.99) at 85 ◦ C. The three Cu exchanged zeolites, were designated the labels Cu-USY, Cu-Y and Cu-ZSM5, and their Si/Al ratios are shown in Table 1. After Cu exchange, the sodium content was below the detection limit (<2.8 Na atoms per unit cell of faujasite zeolite) as determined by inductively coupled plasma mass spectrometry (ICP-MS). Samples of zeolites were pressed into pellet form, and then the pellets were crushed and sieved. Their particle size varied from 400 to 500 ␮m, and a 20 wt% suspension was formed in water, which would then be used to coat the metal plates. The Cu content (determined by XRF) of the zeolites is illustrated in Table 1.

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The metal ‘kanthal’ plates (consisting of Cr/Al, supplied by Baildon Poland), were first pre-calcined at 1000 ◦ C for 48 h, and then allowed to cool. Then following the procedure described in [14], they were immersed in a 20 wt% suspension of zeolite in water, and a layer of zeolite coated each plate. The plates were then dried for 2 h at room temperature. Prior to the catalytic tests, the coated samples were further activated in air at a slow temperature ramp of 1 ◦ C/min and then maintained at 605 ◦ C for 4 h. The amount of deposited material (catalyst loading) was established by measuring the increase in mass of the coated plate, see Table 1. 2.2. Sample characterization The crystal structure of the zeolites (as supplied) and the Cuexchanged zeolites was characterized using XRD (X’Pert Pro). The porosity (micro and mesopores) of the samples was also measured, using temperature programmed sorption–desorption with n-nonane (following the procedure described in [15]). The XRD patterns confirm the expected zeolite structure patterns for all of the samples tested. Also, the thermo-desorption profiles with n-nonane were as expected for these samples. The values of the micropore volume calculated for samples of ZSM-5 and Y-type zeolite, were 0.13 and 0.25 cm3 g−1 , respectively, and there was very little difference following ion exchange (Cu-exchanged form). However, for sample USY, a larger decrease in the micropore volume was observed, i.e. from 0.22 to 0.16 cm3 g−1 . Thermo-desorption of nonane performed under quasi-equilibrium conditions revealed the existence of mesopores (about 4 nm in diameter) in the three samples. As pure crystals were used (without binder) then this can most probably be attributed to the presence of inter-crystalline ‘mesopores’ in agglomerates of the zeolite crystals in the sample tested. 2.3. Catalytic activity The catalytic activity of the samples was then tested in a fixedbed quartz reactor. This was loaded either with 25 mg of sieved (400–500 ␮m) powder sample, or with a catalyst impregnated ‘kanthal’ plate (1 cm × 0.3 cm). The composition of the gas (NH3 , NO, NO2 , N2 O, N2 and H2 O) from the reactor, was measured with a quadropole mass spectrometer (QMS). Prior to each experiment the catalyst was conditioned in 5% O2 in helium at 500 ◦ C. Once the maximum temperature was reached the sample was cooled to the required experimental temperature. In all experiments, helium was used as an inert (balance) gas and a total flow rate of 25 ml/min was kept constant. The catalytic activity experiments were carried out (a) under atmospheric pressure and over a temperature range of 40–500 ◦ C, and (b) with 2000 ppm NO, 2000 ppm NH3 and 5 vol.% O2 in He. The amount of water vapour in the gases fed into the reactor was also measured and found to be about 300 ppm. At the flow conditions in this reactor, for the powdered catalysts the space time was 1.28 × 10−5 h, and the space velocity

Table 1 The catalyst composition and kinetic parameters. Sample name

Powder Cu-USY Cu-Y Cu-ZSM5 Deposited plates Cu-USY/CrAl Cu-Y/CrAl Cu-ZSM5/CrAl

Si/Al

Cu/Al

Cu content (in zeolite), wt.%

Catalyst loading (on support), wt.%

Sorption capacity, mol g−1 cat. (TPD) NH3

NO

Preexponential factor, k∞ , m3 kg−1 s−1

Activation energy, Ea , kJ mol−1

4.5 2.6 37

0.59 0.25 1.06

2.4 6.2 1.6

– – –

1.4 × 10−5 1.4 × 10−5 3.5 × 10−6

8 × 10−8 9 × 10−8 5 × 10−7

2.3 × 103 1.9 × 104 6.1 × 103

41 48 49

4.5 2.6 37

0.25 0.59 1.06

2.4 6.2 1.6

0.9 1.5 1.6

– – –

– – –

6.7 9.1 3.8

10 11 10

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J. Ocho´ nska et al. / Catalysis Today 191 (2012) 6–11

Fig. 1. Catalytic performance of zeolite samples for the reduction of NO (0.2 vol.%) with NH3 (0.2 vol.%) in excess oxygen: (A) NO conversion; reaction rate in Arrhenius coordinates (k, m3 s−1 g−1 ), (B) for powder catalyst, and (C) for coated metal plates.

was 3.1 × 106 h−1 gcat −1 , whereas for the catalyst coated plates it was 3.3 × 10−6 h, and 1.2 × 109 h−1 gcat −1 . 3. Results and discussion 3.1. Catalyst performance The performance of the ultrastabilzed catalyst, Cu-USY, was compared with the performance of the two reference catalysts, Cu-Y and Cu-ZSM5. For all of these the selectivity toward N2 was almost 100%. The catalytic activity was then compared by looking at the conversion (as a function of temperature), the pre-exponential factor, k∞ , and the activation energy, Ea . As illustrated in Fig. 1A, for the powder form of catalyst, 100% conversion of NO was achieved at different temperatures for all of the 3 samples. However, only the ultastabilized catalyst, Cu-USY, showed stable activity across the temperature range of 300–450 ◦ C. At lower temperatures (<200 ◦ C) the Cu-USY and Y zeolites were more active than the Cu-ZSM5. The temperature necessary to achieve 50% conversion was compared, and found to have the following values: Cu-Y ≈ 120 ◦ C, Cu-USY ≈ 170 ◦ C and Cu-ZSM5 ≈ 200 ◦ C. The conversion of NO over the zeolites deposited (coated) on the metal plates was much lower and at 500 ◦ C it did not exceed 55%. Experiments were also performed with uncoated precalcined metal plates, and these were found to exhibit some catalytic activity. Up to a temperature of 400 ◦ C, their performance was comparable with the zeolite coated plates, see Fig. 1A (symbol: calcined CrAl). Above 400 ◦ C the uncoated plate samples tend to mainly catalyze the oxidation of NH3 . Their catalytic activity most probably arose as a result of the presence of trace levels of metal oxides (e.g. FeO(OH), Cr2 O3 , Co3 O4 , which had been detected in earlier work [16]) on ‘kanthal’. The reaction rate (expressed in terms of mass of zeolite) across the reactor was calculated, and assuming isothermal conditions along the reactor and that the rate was first order with respect to NO [8,9], the integral method of analysis was used to evaluate the apparent rate constant. The results of that analysis are presented in Fig. 1B. There is a clear change in the slope of the plots for the powdered samples. This indicates a possible transition between kinetic and diffusional regimes, which occurs at around 250 ◦ C. Above that temperature the reaction rate starts to decrease. Since the presence of diffusion limitations is very difficult to exclude (even at the lower temperature range < 250 ◦ C), all values of the kinetic parameters including the reaction rate, rate constant, and the activation energy should be treated as ‘apparent’ or complex in the sense that they bear both diffusional limitation of reaction rate. Thus in Fig. 1B, apparent rate constants have been plotted across the temperature range studied. For the powdered samples, in terms of reaction rates per mass of zeolite, above a temperature of 250 ◦ C the Cu-USY catalyst shows the highest activity. Next, for the coated plates (Fig. 1C),

the slope of the plots remains approximately constant across the temperature range, and these apparent rate constants (expressed in terms of mass of zeolite), show a high level of activity. This exceeds the activity of the powder counterpart samples, and this observation can be interpreted as an indication of a high level of exploitation of the catalyst material deposited in low amounts on the plate samples. To look more closely at the slope in the lower temperature region, a separate set of kinetic experiments was performed over a temperature range of 50–150 ◦ C, under these conditions, conversion did not exceed 10%, and the rate constant was evaluated using the differential method of analysis. As the temperature was gradually increased, the reaction rate was measured every 10 ◦ C. Then, assuming that the reaction is first order with respect to NO [8,9], the rate constant was calculated, then the activation energy was determined using the Arrhenius equation. From this set of preliminary kinetic experiments, the results are presented in Table 1. The results for the powdered catalysts are encouraging, as the apparent activation energy for the Cu-ZSM5 was Ea = 49 kJ mol−1 , which was comparable with 35–50 kJ mol−1 reported in [8] and that depended on the degree of Cu exchange. For Cu-Y, the apparent activation energy was 48 kJ mol−1 , compared with 29 kJ mol−1 reported in [17]. These comparative measurements provide a degree of confidence in the experimental technique used. The activation energy for the ultrastabilized Cu-USY sample, is lower than that for Cu-Y. However, for Cu-USY the number of collisions expressed by the k∞ coefficient, is comparable to the Cu-ZSM5 sample. For the coated metal plates (data in Table 1), even if the activation energies are low (∼10 kJ mol−1 ), the k∞ coefficients are very high and the rate of reaction per mass of catalyst is also very high (relative to the powdered form). This clearly looks very promising, but further work is necessary to explore these findings in more detail. Some of the challenges of performing kinetic experiments on metal supports are discussed in [18]. In further work it is planned to improve the coating method, and then to explore the reaction kinetics, using some of the techniques described in [19]. 3.2. Active sites In an attempt to explore the relationship between the structure of a sample and its catalytic activity, the type of active sites and their concentration was determined by sorption of probe molecules using in situ FTIR (Fourier Transform Infrared) and their desorption by TPD (Temperature-Programmed Desorption) experiments. For the in situ FTIR measurements (Fig. 2A–C), NH3 , NO and CO were used as probe molecules, following the same procedure as described in [20] for NO. Self-supported wafers were activated under vacuum conditions at 450 ◦ C for 1 h before the adsorption of probe molecules. The FTIR spectra were recorded on a Bruker Tensor 27 (resolution 2 cm−1 ). Absorption coefficients were:

J. Ocho´ nska et al. / Catalysis Today 191 (2012) 6–11

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Fig. 2. FTIR spectra of the zeolite samples with adsorbed probe molecules: (A) CO, (B) NO, and (C) NH3 .

PyH+ = 0.078 cm ␮mol−1 , and PyL = 0.165 cm ␮mol−1 (further information on the technique is available in [21]). The TPD experiments were performed using a Hiden Analytical CATLAB-PCS microreactor integrated with a Hiden Quadropole Mass Spectrometer (QIC-20). Prior to the TPD experiments the samples were exposed to probe molecules (1 vol.% NH3 at 100 ◦ C, or 0.2 vol.% NO at 50 ◦ C) for 1 h. The TPD experiments (Fig. 3 A and B) were carried out by heating the sample to 500 ◦ C at a rate of 10 ◦ C/min in a flow of He (25 ml/min). Qualitative information from the TPD results is limited because of the relatively high heating rates which were used. These were intentionally selected to reduce uncertainty over peak areas in the output signal from the QMS detector. The TPD results were used mainly to determine the absorption capacities of the samples (Fig. 3 and Table 1). There is a reverse tendency in the sorption capacities of Cu-Y and Cu-ZSM5 zeolite samples for NH3 and NO molecules. The Cu-ZSM5 zeolite adsorbs more NO (mol g−1 cat.) than the Y zeolites, and the opposite occurs for NH3 sorption. The NH3 sorption capacity follows the trend in the Al content of the samples (Table 1). In a literature review by [6], it was reported [7] that the sorption of NO per ion, on different Cu-exchanged zeolites decreases in a similar manner (ZSM-5 > offretite/erionite > mordenite > L > ferrierite > Y). In the in situ FTIR experiments (Fig. 2 A and B), CO and NO probe molecules were used to distinguish Cu oxidation states in the exchanged zeolite samples. At room temperature CO molecules interact exclusively with Cu+ [22]. The appearance of two maxima at 2146 cm−1 and 2160 cm−1 (from CO vibrations) on the FTIR spectra (Fig. 2 A), is evidently caused by the presence of two non-equivalent Cu+ sites. In the Cu-USY and Cu-Y catalysts, their contribution is relatively even, while for the Cu-ZSM5 catalyst, the CO molecule is bound at one site. The NO adsorb on both Cu2+ and Cu+ sites (at room temperature) in the zeolites samples, revealing a complex pattern in the FTIR spectra (Fig. 2B). The bench mark to distinguish these two

groups of active sites is the frequency of a free NO molecule which occurs at 1876 cm−1 . In this way, “blue shifted” NO frequencies provide evidence of the presence of Cu2+ sites, whereas the “redshifted” ones indicate the presence of Cu+ sites [23]. Although the NO vibration bands emerge at similar frequencies for both the Cu-Y and Cu-USY samples (revealing both oxidation states of Cu), their contribution is different. As there is a huge difference in the intensities of the “blue shifted” and “red-shifted” bands, this may imply a higher abundance of the Cu2+ over the Cu+ sites (the extinction coefficients are unknown so the data cannot be treated quantitatively). At least four different types of Cu2+ sites interacting with the NO probe molecule can be inferred from the adsorption patterns of both Y samples. The NO probe cannot distinguish clearly between the two types of Cu+ sites (which could be identified with the CO probe). The presence of Cu+ of more than just one type can only be inferred from the broadening of the bands at 1819 cm−1 , which is characteristic of the Cu-USY sample [23], and then at around 1730 cm−1 for the Cu-Y sample. The binding energy of NO with the Cu2+ sites in the Cu-ZSM5 sample is much lower in comparison with Cu-USY and Cu-Y. The desorption maxima of the NO molecules in Fig. 3B suggest that both higher and lower binding energy sites are present; the number of desorption maxima corroborates with the number of sites detected in IR (compare Figs. 3B and 2B). Exclusively for the Cu-ZSM5 sample, the evolution of N2 was noted, demonstrating its ability to directly decompose the NO molecules. Overall, the results show that activation of Cu under vacuum conditions (or inert gas flow), leads to the partial auto-reduction of Cu2+ to Cu+ , with the extent of reduction dependent on the type of zeolite (both topology and Si/Al ratio), which was also observed in [23]. The NH3 probe molecules are able to identify all Lewis (Cu and Al-based types) and Brønsted (residual acidic OH groups) active sites, from the characteristic bands above and below 1550 cm−1 on the FTIR spectra, respectively (Fig. 2C). This information can

Fig. 3. Temperature-programmed desorption of probe molecules (0.2 vol.% NO; 1 vol.% NH3 ) on zeolite samples: (A) NO desorption (pre-adsorbed at 50 ◦ C) and (B) NH3 desorption (pre-adsorbed at 100 ◦ C).

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J. Ocho´ nska et al. / Catalysis Today 191 (2012) 6–11

help to explain the reverse tendency in the sorption capacities of Cu-Y and Cu-ZSM5 zeolite samples for NH3 and NO molecules. The number of Brønsted acidic sites is similar for both the CuY and Cu-USY samples and much lower for the Cu-ZSM5. This is in agreement with the Si/Al content for these samples (Table 1). The spectra demonstrate again that there are various Lewis sites present in the zeolite samples especially in the Y samples. In this case they may originate from the Cu+ and Cu2+ detected by CO and NO probe molecules, as well as from aluminum cations in the zeolite framework. For both Cu-USY and Cu-Y samples, the two bands with maxima around 1610 cm−1 can be attributed to NH3 bonded to Cu+ (presence of two CO–Cu+ bands, Fig. 2A). For the Cu-USY sample, the high frequency band at 1674 cm−1 can be assigned to NH3 bonded to Cu2+ sites (comparable with the presence of the NO–Cu2+ bands presented in Fig. 2B), which has also been confirmed by the results of CO adsorption at low temperature (spectra not presented here). A common feature of all NH3 TPD profiles (Fig. 3A) is a low temperature maximum at around 170 ◦ C. This shows a weakly adsorbed form of NH3 , most likely on the weak Lewis sites detected by IR (Fig. 2B and C) and overlapped high temperature maxima. For the latter, desorption of NH3 both from Lewis and Brønsted sites was confirmed by ammonia desorption followed by IR (spectra not shown). For the stabilized Cu-USY sample, several desorption maxima overlapping with one another are seen (Fig. 3B), and this may indicate multilevel organization of material porosity (as the ultrastablized sample contains a considerable amount of mesopores [24,25]). As described in a literature review [26], there is no established opinion concerning the role of Cu+ and Cu2+ active sites in the NH3 -SCR process and their final arrangement within the zeolite structure. Although there are some authors who claim that only the reduced form of Cu cations is active in the NH3 -SCR process, many results show that the Cu2+ sites are necessary for the oxidation of NO to a NO2 intermediate – the prerequisite and the slowest step [9] in the NH3 -SCR mechanism. Under typical SCR conditions for zeolites (with an excess of O2 ), then Cu+ and Cu2+ convert into one another [4]. This works like an electron pump (between NO and NH3 molecules), enabling a redox cycle to be completed. In order to launch such a cycle both forms seem to be necessary, which is the case in all samples studied. The general conclusion from the sorption and desorption experiments is that the ultrastabilization of the Y zeolites gives rise to a different distribution of copper-based Lewis active sites (of the same type) in comparison with the non-stabilized sample. This is an important observation, which may help to elucidate their performance at different reaction temperatures in the catalytic reaction experiments (Fig. 1). A closer look at the distribution and energy of the Cu2+ sites present in the USY and Y zeolites may account for their high performance in the lower temperature range (Fig. 1A). It has been shown by many authors that the NH3 -SCR mechanism proceeds via oxidation of NO to NO2 . According to [17], the mechanism is of the Mars-van Krevelen type and changes with temperature. At low temperatures (<250 ◦ C), a redox cycle starts with the oxidation of Cu+ to Cu2+ with O2 , which has been shown to be the rate determining step. This is followed by the adsorption and oxidation of NO on the Cu2+ sites, during the reduction of NO2 with NH3 . Therefore based on this information it seems likely that the presence of the low energy Cu+ in the Cu-Y and the Cu-USY samples (in comparison with the Cu-ZSM5 sample), is necessary to create low temperature activity. The difference in the level of low energy Cu+ contributions in these two samples may explain the differences observed in their reaction rates. The low sorption capacity of NO found by TPD experiments for the Cu-Y samples (when compared with Cu-ZSM5) is consistent with the assumed Mars-van Krevelen mechanism and the rate determining step.

4. Concluding remarks This preliminary study demonstrates that for the ‘biomass to energy’ process described at the start of this paper, copper exchanged zeolite catalysts are possible candidates for the SCR of NOx with ammonia. The ultrastabilized catalyst, Cu-USY, demonstrated high activity at low temperature and also offered good hydrothermal resistance. The zeolite catalysts when deposited onto the surface of metallic plates also exhibited a very high level of activity, which is very encouraging. However, before the Cu-USY catalyst (developed in this paper) may be tested in a wire gauze reactor, the method of coating/depositing this catalyst on the support structure still needs to be improved, after which, further work is planned to determine the kinetic parameters. The catalytic activity of the Cu-USY catalyst (and the reference samples tested) is connected to the abundance of the Lewis (Cu+ to Cu2+ ) sites and their distribution, which were determined by in situ FTIR experiments with probe molecules. The activity of the Cu-Y and Cu-USY catalyst samples at low temperatures (in comparison with Cu-ZSM5) has been linked to the presence of the Cu+ active sites, which have a low binding energy with NO molecules. Acknowledgements This work was supported by BRIDGE Programme grant (No 2010-1/4) within the Foundation for Polish Science, co-financed by the EU Structured Funds. The IR studies were carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund, in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). We also thank the EPSRC for funding (EP/G03768X/1) the DTC PhD Studentship for David McClymont. References [1] I. Nova, A. Beretta, G. Groppi, L. Lietti, E. Tronconi, P. Forzatti, Monolithic catalysts for NOx removal from stationary sources, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, 2nd ed., Taylor & Francis, 2006, pp. 171–214. [2] R.M. Heck, Catalytic abatement of nitrogen oxides – stationary applications, Catalysis Today 53 (1999) 519–523. [3] A. Kołodziej, J. Łojewska, Mass transfer for woven and knitted wire gauze substrates: experiments and modelling, Catalysis Today 147 (2009) S120–S124. [4] S. Roy, M.S. Hegde, G. Madras, Catalysts for NOx abatement, Applied Energy 86 (2009) 2283–2297. [5] M. Iwamoto, S. Yokoo, K. Sakai, S. Kagawa, Catalytic decomposition of nitric oxide over copper(II)-exchanged Y-type zeolites, Journal of the Chemical Society, Faraday Transactions 1 (77) (1981) 1629–1638. [6] M.A. Gomez-Garcia, V. Pitchon, A. Kiennemann, Pollution by nitrogen oxides: an approach to NOx abatement by using sorbing catalytic materials, Environment International 31 (2005) 445–467. [7] H. Arai, M. Machida, Removal of NOx through sorption-desorption cycles over metal oxides and zeolites, Catalysis Today 22 (1994) 97–109. [8] T. Komatsu, M. Nunokawa, Il S. Moon, T. Takahara, S. Namba, T. Yashima, Kinetic studies of reduction of nitric oxide with ammonia on Cu2+ exchanged zeolites, Journal of Catalysis 148 (1994) 427–437. [9] S. Brandenberger, O. Kröcher, Oliver, A. Tissler, R. Althoff, The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts, Catalysis Reviews-Science and Engineering 50 (2008) 492–531. [10] J. Klinowski, J.M. Thomas, C.A. Fyfe, G.C. Gobbi, Monitoring of structural changes accompanying ultrastabilization of faujasitic zeolite catalysts, Nature 296 (1982) 533–536. [11] C.A. Fyfe, J.L. Bretherton, L.Y. Lam, Solid-state NMR detection, characterization, and quantification of the multiple aluminium environments in US-Y catalysts by 27 Al MAS and MQMAS experiments at very high field, Journal of the American Chemical Society 123 (2001) 5285. [12] J. Sanz, V. Fornes, A. Corma, Extraframework aluminium in steam – and SiC4 – dealuminated Y zeolite, Journal of the Chemical Society, Faraday Transactions I 84 (9) (1988) 3113–3119. [13] D.M. Ruthven, Principles of Adsorption Processes, John Willey & Sons, 1984, p. 23.

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