NOx reduction behaviour in copper zeolite catalysts for ammonia SCR systems: A review

Journal Pre-proofs Review NOx reduction behaviour in copper zeolite catalysts for ammonia SCR systems: A review Sooraj Mohan, P. Dinesha, Shiva Kumar PII: DOI: Reference:

S1385-8947(19)32665-8 https://doi.org/10.1016/j.cej.2019.123253 CEJ 123253

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 August 2019 1 October 2019 19 October 2019

Please cite this article as: S. Mohan, P. Dinesha, S. Kumar, NOx reduction behaviour in copper zeolite catalysts for ammonia SCR systems: A review, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123253

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NOx reduction behaviour in copper zeolite catalysts for ammonia SCR systems: A review Sooraj Mohan, P. Dinesha* and Shiva Kumar Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal – 576104, India *Corresponding author: [email protected]

Abstract This paper reviews the current copper substituted zeolite catalysts, their operating temperature ranges with NOx reduction efficiencies and their deactivation mechanisms. Different zeolite candidates suitable for NH3-SCR are large and medium pore zeolites like ZSM-5, MOR, FER, BEA and small pore zeolites like SSZ-13 and SAPO-34, both having CHA topology. Cu substituted small pore zeolites have demonstrated low temperature NOx reduction with stability at higher temperatures. Cu/SSZ-13 and Cu/SAPO-34 are by far the best catalysts suitable for a broader temperature window and also operate well below 350 C with good hydrothermal stability. Optimum values of Si/Al ratio (SAR) play a crucial role and hence more studies on variation in SAR and adsorbed monodentate nitrates are required to get a fundamental understanding on their impact on low temperature SCR activity of CHA based catalysts.

Keywords: ammonia-SCR, catalyst deactivation, copper-zeolite, engine emissions, NOx reduction.

1

Nomenclature BEA BET CHA CI Cu-Z CVD DOC DPF EPR FER Fe-Z GHSV IR IZA LNT MOR NOx PM SAPO-34 SAR SCR SOx SSIE SSZ-13 TPD TPR UBHC UV-Vis AIE XPS XRD ZSM-5

large pore beta zeolite Brunauer-Emmett-Teller chabazite compression ignition copper substituted zeolite chemical vapour deposition diesel oxidation catalyst diesel particulate filter electron paramagnetic resonance ferrierite; medium pore zeolite iron substituted zeolite gas hourly space velocity infrared international zeolite association lean NOx trap mordenite; large pore zeolite oxides of nitrogen particulate matter silicoaluminophosphate; CHA based small pore zeolite silicon to aluminium ratio selective catalytic reduction oxides of sulphur solid-state ion exchange CHA based small pore zeolite temperature programmed desorption temperature programmed reduction unburnt hydrocarbons ultraviolet visible aqueous ion exchange X-ray spectroscopy X-ray diffraction MFI based medium pore zeolite

2

1. Introduction Compression ignition (CI) engines running on diesel fuels and biodiesel blends are of important concern for future energy sustainability. The associated issues with the environment lead to a need for research in the field of CI engine emissions. Due to its increased use in road transport and standalone systems, diesel engines become an important segment to be focused upon in terms of reducing exhaust emissions. The main constituents of exhaust gas from CI engines are particulate matter (PM), unburnt hydrocarbons (UBHC), carbon dioxide (CO2), carbon monoxide (CO), oxides of sulphur (SOx) and oxides of nitrogen (NOx) [1]. The major carbon containing emissions can be reduced by adopting biodiesel as a fuel. However, NOx emissions increase due to higher oxygen (O2) content of biodiesel [2,3]. NOx is a combination of nitric oxide (NO) and nitrogen dioxide (NO2) which are formed at high temperatures by combining nitrogen (N2) and oxygen (O2) present in fuelair mixture. NO2 combines with OH ― of water vapour in the atmosphere to form nitric acid which is the major cause for acid rain [4]. Major respiratory symptoms like bronchitis, asthma lung infections are caused due to the inhalation of NO2 [5]. There is evidence which state that premature deaths from cardiovascular and respiratory diseases may be due to the presence of NO2 [6]. Smog is another issue resulting from NOx emissions in urban areas due to increased vehicle usage and power plants among which about 40% of the smog comes from vehicular transport [7]. With the stringent emission norms being in place, the old two way catalyst involving platinum (Pt), palladium (Pd), rhodium (Rh) on metal oxides can no longer comply with the regulations [8]. The exhaust gas after treatment is the best suitable option for emission reduction without altering the engine, operating conditions, fuel and has

3

the potential to meet the stringent regulatory norms. The major after gas treatment technologies for CI engines are: 

Diesel Oxidation Catalyst (DOC) for oxidizing UBHC and CO into CO2 and water (H2O)



Diesel Particulate Filter (DPF) for removal of PM



Selective Catalytic Reduction (SCR) and Lean NOx Traps (LNT) for removal of NOx

A conventional configuration of these catalysts installed in a typical heavy duty CI engine is represented in Figure 1.

Figure 1: Conventional configuration of different catalysts in a heavy duty CI engine.

A key development that can easily reduce the major emissions from CI engines is to use biodiesel as a fuel substitute for conventional diesel. Majority of the biodiesel come from biological and agro-based sources. The amount of CO2 the plants absorb during its growth is equal to the amount of CO2 released during biodiesel combustion. This fact makes the system carbon-neutral or in other words, the additional release of CO2 can easily be reduced. The additional presence of O2 in biodiesel facilitates complete combustion, thereby greatly reducing CO and UBHC. On the other hand, due to the presence of fuel bounded O2 present in biodiesel, at high temperatures, there is increased production of NOx. The O2 content in biodiesel may vary from 10 to 45% while the conventional diesel has none [9]. It is very important to note that, production of NOx and other emissions are inversely proportional to 4

each other when using biodiesel as fuel. Without modifying any engine design, selective catalytic reduction (SCR) at the exhaust of biodiesel/biodiesel blended engines can be attached thereby reducing the NOx [10].

SCR as the name implies reduces a specific component under catalytic reaction environment. The SCR technology has been developed to reduce the NOx gases to pure nitrogen (N2). In this technology, ammonia (NH3) vapours reduce the harmful NOx gases into environmentally friendly N2 and water (H2O) on a catalyst site. Major constituents of NOx are NO and NO2 in proportions of about 90% and 10% respectively. The reduction of NO to N2 is termed as a slow reaction and NO2 to N2 is termed as fast reaction. The standard SCR reduction is shown in Eq. (1) where the reduction of NO to N2 proceeds. Due to the presence of DOC upstream of SCR catalyst, the oxidation of NO to NO2 would have been formed. This leads to a faster reaction as shown in Eq. (2) where moles of NO equal to NO2 participate in the reduction. However, in leaner conditions (O2/NO2 = 0), a much slower reaction as shown in Eq. (3) is proceeded [11].

(1)

4𝑁𝐻3 +4𝑁𝑂 + 𝑂2→4𝑁2 +6𝐻2𝑂

4𝑁𝐻3 +2𝑁𝑂 + 2𝑁𝑂2→4𝑁2 +6𝐻2𝑂

8𝑁𝐻3 +6𝑁𝑂2→7𝑁2 +12𝐻2𝑂

(2)

(3)

5

Catalyst substrate

Catalyst washcoat Figure 2: Cross sectional view of a cell in a typical SCR Catalyst. A typical catalyst consists of a base substrate on whose surface the required catalyst material is washcoated. The catalyst substrate can be made of either ceramic (cordierite) or metallic (aluminium/stainless steel) materials. Since the porosity of ceramic substrate is higher, the ceramic substrate finds a more suitable application in SCR. Most of the ceramic substrate is commercially available in monolith form whereas the metal substrate is usually available in interchanged flat and sinusoidal corrugated sheets. Metallic substrates are used for standalone systems however, for mobile engine applications, ceramic are the most widely used catalyst substrate. A typical view of the catalyst cell is shown in Figure 2.

One of the commonly available substrates is of cordierite ceramic material in square crosssection monolith form. For application in SCR, a catalyst must be coated on the substrate to form active areas for ammonia adsorption. The catalyst may be of inorganic oxides or precious metals having high surface area. The catalyst coating is carried out in two stages, 6

washcoating and impregnation with noble metals. The different washcoat materials that can be used are Pt, aluminium oxide (Al2O3), titanium dioxide (TiO2), cerium oxide (CeO2), zirconium dioxide (ZrO2), silicon dioxide (SiO2), vanadium oxide (V2O5), Lanthanum oxide (La2O3) and zeolites [12]. Among these, the prominent catalyst carriers are platinum, vanadia/titania, and zeolites. Regeneration of DPF upstream of the SCR catalyst rises the temperature of the exhaust stream in excess of 600 C. Hence, the catalyst requires to be stable hydrothermally beyond this temperature also. zeolite based catalysts have better stability at these temperatures as compared to the vanadium based catalysts that are used in power plants. However, the temperature does not reach these limits during stationery applications [13]. Due to the applicability at a wide operating temperature range, zeolite catalysts are best suited for automotive SCR [14]. Zeolites are crystalline aluminosilicates consisting of SiO4 and AlO4 tetrahedral units to form porous three dimensional networks. An ion exchange function as well as an acid function is obtained in the zeolite lattice due to the replacement of Al ion with Si ions. Regular shaped pores are formed in the zeolite structure. These pores are interconnected to each other through openings (or windows) of the framework. Currently, there are more than two hundred distinctive structures of zeolite that have been recognised [15]. Each structure can be identified with a code having three letters. International Zeolite Association (IZA) has the details of each of these structures and related materials [16]. The differentiation of the type of zeolite can be made based on the largest pore size opening of the framework structure. These pores can be small (8-membered ring), medium (10-membered ring), large (12-membered ring), and extra-large (>12-membered ring). Medium and large pore zeolites 7

are less stable under severe hydrothermal conditions as compared to the smaller pores [17– 20]. In the recent past, the smaller pore zeolites augmented with Cu have demonstrated better selectivity to N2 and reduced N2O formation. The different candidates of zeolite species on which research has been carried out are mordenite (MOR), beta (BEA) having large pores and ZSM-5 (MFI), ferrierite (FER) having medium pores. The research focus has now shifted towards small-pore chabazite (CHA) based SAPO-34 and SSZ-13, both having microporous topologies [21]. Table 1 shows the comparison of different zeolite species suitable for NH3SCR with regards to its topology, BET (Brunauer-Emmett-Teller) areas and micro-pore volumes. It can be observed that SSZ-13 and SAPO-34 have a high surface area and micrpore volume and that SAPO-34, BEA, MOR and FER have lower Si/Al ratio. It becomes evident that chabazite framework zeolite can be a good candidate for NH3-SCR considering its stability to dealumination and other deactivation mechanisms.

Table 1: Topologies, BET areas and pore volumes of different zeolite species [22–26]. Zeolite

Topology

BET area

Micro pore

(m2/g)

volume (cm3/g)

Si/Al ratio

ZSM-5

MFI

45

430

0.154

SSZ-13

CHA

16

830

0.304

SAPO-34

CHA

0.2

730

0.25

Beta

BEA

13

570

0.163

Mordenite

MOR

6-10

NA*

0.2

Ferrierite

FER

10

330

0.11-0.13

*NA: Not Available

8

2. Metal Substituted catalysts Copper and iron exchanged zeolites have found to be extremely successful and have demonstrated excellent NH3-SCR activity. These catalyst can be operated with a larger temperature window, have shown better stability to sulphur poisoning and more importantly it does not require rare metals like Pt, Pd etc. [8]. In comparison with the vanadia based catalysts, Cu- and Fe- exchanged zeolites have a better hydrothermal stability at higher temperatures. The detailed SCR mechanism is not completely known and the durability of the catalyst is a matter of ongoing research due to many other existing deactivation mechanisms apart from hydrothermal aging. Since the SCR is subjected to higher temperatures due to the regeneration of DPF upstream, the hydrothermal stability of these catalysts is still a concern. Uncontrolled reactions in the DOC and DPF regeneration can lead to performance deterioration and loss of SCR activity. Fe- based zeolites are generally active at higher temperatures as compared to Cu- based zeolites which operate better at low temperatures. The most investigated catalysts among copper substituted zeolites (Cu-Z) are FMI framework based Cu/ZSM-5 and BEA framework based Cu/BEA. Superior hydrothermal stability is obtained with Cu/BEA. Small pore zeolites like Cu/SSZ-13 and Cu/SAPO-34 are of interest in the recent past that has demonstrated NOx reduction activities at lower temperatures with adequate hydrothermal stability. An aluminosilicate zeolite contains a tetrahedral co-ordination arrangement. Si and Al atoms are connected and linked to O2 atoms through oxygen bonds. A charge imbalance in the structure is effected due to the valence nature of Al ions. Due to this factor an additional positive ion is attracted towards the Al site. This site is called a Brønsted acid site if it contains a proton. If the site contains a metal ion, it is called an ion exchanged site because 9

other cat ion can get exchanged with the metal ion. For an optimum working of SCR, the structure, pore size, the available Brønsted acid and ion exchanged sites play crucial roles [21]. Gases can easily enter in to the pore openings of the zeolite. These structures allows to have a higher surface area on which NH3 and NOx can be adsorbed and contained to assist the SCR reaction to facilitate. The major step in this mechanism is the adsorption of ammonia as NH4+ on the acid sites. The other exchange site hold the copper ions. Catalyst preparation method becomes an important factor that determines the level of CuO formation within the zeolite and its surface usually at higher temperatures. The formation of CuO has a negative effect on SCR activity rather it increases the oxidation of NH3, thereby reducing the effective concentration of NH3 for SCR reactions. Cu-Z catalyst basically provide two important functions: 

An acid site for ammonia adsorption



An exchange redox site to hold Cu ion that forms complex to react with ammonia for catalytic reduction to N2

Both these functions are of utmost concern as it has to be well balanced within the zeolite catalyst to achieve the SCR activity at wider temperature window. An increase in the Cu sites will retard the adsorption of NH3 and hence starve the SCR reaction of NH4+. Also, oxidation of NH3 to N2O may proceed at higher temperatures. On the other hand, lower Cu may itself be insufficient to have adequate redox sites for SCR. Thus loading of Cu need to be optimised for the best SCR activity [27]. The aim of this paper is to review the recent developments regards to NOx reduction performance and deactivation of copper zeolite species suitable for NH3-SCR. The focus of 10

this article is to ascertain the performance and behaviour of Cu-Z for low temperature NH3SCR.

The various Cu-Z under consideration are Cu/SSZ-13, Cu/SAPO-34, Cu/BEA,

Cu/FER and Cu/MOR.

3. Catalyst synthesis and characterisation 3.1. Catalyst synthesis Cu-Z can be synthesized using various methods that yield varying results. The major techniques are as follows: 

Aqueous Ion Exchange (AIE)



Direct synthesis



Chemical vapour deposition (CVD)



Solid-state ion exchange (SSIE)

AIE can be used to yield different types of metal exchanged zeolites. An ammonia zeolite (Hzeolite) will be immersed in to a continuously stirred solution containing the copper ions. Varying temperature conditions results in varying distribution of Cu on the zeolite. For CHA topology, higher temperatures are desired to increase the diffusion rate [21]. Filtration and washing processes are to be carried after the copper ion exchange process is completed. One limitation of this procedure is excess amount of copper being exchanged in the zeolite sites. Operating conditions that need to be considered for optimum active catalysts are solution concentration containing Cu and the synthesis timing [28]. AIE is by far the easiest method to synthesize Cu-Z. CVD method can also be followed to effectively deposit Cu even inside the zeolite structures. However, the right salts, flow and temperature conditions are crucial factors for the best results. It is usually used for depositing on a flatter surfaces. Chlorine salts are conventionally 11

the preferred salts for this technique. Washing also need to be carefully carried out in order to remove all the chlorine content from the zeolite [29]. In Solid-state ion exchange method, Cu containing carrier and the zeolite are required to be mixed and consequently heated to high temperatures. This will result in Cu to move around and get deposited on surface and sub surfaces of the zeolite. Direct synthesis method is predominantly used for developing Cu-Z. A Cu containing carrier is used to develop the zeolite surrounding it that immediately results in Cu-Z. The main benefit of this method is that the amount of wastage is greatly reduced because no further treatment is necessary after the synthesis. The resulting synthesized Cu-Z have shown to have hydrothermal stability and also good SCR activity [30]. Other synthesis processes like mechanochemical method and Feng-Hall methods have also been tried [21] .

3.2. Catalyst Characterisation There are many characterisation methods currently used to ascertain the catalyst behaviours like surface topology, particle size and area, temperature behaviour etc. An X-ray diffraction (XRD) technique is used to determine particle sizes of long-range order that uses averaging technique. It cannot show potential for disordered structures [31]. An X-ray photoelectron spectroscopy (XPS) analysis can reveal atomic concentrations, oxidation states in near surface layer [32]. An infrared (IR) technique can be used to characterise acid, base, redox sites and other structural information can be obtained [33]. An ultraviolet-visible (UV-Vis) spectroscopy can be employed for analysis of oxidation states in particular for Cu-Z catalysts. Concentration analysis of paramagnetic sites is possible using electron paramagnetic resonance (EPR), however its accuracy is limited and not all oxidation states are accessible 12

[34]. Temperature programmed desorption (TPD) is used for differentiation of adsorption sites on a surface, energetics of desorption [35]. A temperature programmed reduction (TPR) is used to ascertain reduction properties of redox phases in samples. It can easily detect interactions between phases [36].

4. NOx reduction activity NOx reduction activity of a catalyst is dependent on various characteristics like NH3 storage capacities, catalyst structure, temperature behaviour, deactivations etc. At lower temperatures (<350 C), a significant increase in NOx reduction is observed for Cu-Z whereas the SCR activity is more for iron zeolite (Fe-Z) beyond this temperature. A standard SCR reaction having equi-molar concentrations of NH3 and NO at 200 ppm for both iron and copper aged zeolites is shown in Figure 3 [37]. It is clear that Cu-Z will be preferred to Fe-Z at low temperatures. Cu-Z has higher capacity to store larger quantities of NH3 as compared to Fe-Z for a given volume. This is a consistent finding reported in literature where Cu-Z has higher ammonia adsorption sites as compared to Fe-Z [38]. In addition to the available acid site, it is also possible that ammonia can easily be stored on a copper site but not on an iron site. In the absence of oxygen (rich condition) both Fe-Z and Cu-Z store higher amounts of NH3. However in the presence of oxygen (lean condition), due to the capacity of NH3 oxidation in the presence of Cu site, there tends to be lower storage of NH3 as compared to a Fe-Z at lower temperatures [37].

13

Iwasak

Figure 3: Fe-Z and Cu-Z NOx performance behaviour for NH3/NOx = 1 [37].

i and Shinjoh [39] used Fe/ZSM-5 and showed that Fe-Z shows better NH3-SCR activity for equi-molar mixtures of NO and NO2 (fast reaction) while the NOx conversion drops when NO2/NOx is more than 50 % as shown in Figure 4 (a). Once the fast reaction is complete, with no more NO present in the system, the NO2/NOx ratio is independent of the NO2 SCR activity as shown in Figure 4 (b). However, beyond 350 C, only NO2 SCR proceeds although the conversion remains a constant 80 % irrespective of the NO2/NOx ratio. Similarly for a 100% fast reaction, the standard SCR activity behaviour is independent of any NO2/NOx ratios less than 0.5. The reaction conditions contained gas stream having 380 ppm of NOx, 400 ppm of NH3, O2 (8%), H2O (8%), CO2 (10%) and the remaining was N2.

14

Figure 4: (a) NOx conversion of Fe-Z for varying NO2/NOx ratios and temperatures (b) NO2 SCR conversion [39].

An increase with NH3 concentration decreases the NOx reduction capability below 300 C for Fe-Z. The reverse of the same is true for temperatures beyond this limit. NH3 oxidation is also measured beyond 300 C. This NH3 conversion is reported to be only 15 % (550 C) when compared to 90% for Cu-Z [40]. This also explains the reason for high and low SCR activities for Fe-Z and Cu-Z respectively at higher temperatures.

15

Copper zeolite has better performance at low temperatures, however, at the ideal ratio of NO2/NOx = 0.5, the low-temperature efficiency of Fe-Z outperforms Cu-Z [41]. The operating temperature range of employing these catalysts can be extended from 50 to 550 C [14]. Mathematical modelling of SCR catalyst has also been carried out and the result supports the NOx conversion efficiency increasing with increase in NO2/NOx ratio from 0 to 0.5 [42]. The results also revealed that the NOx conversion efficiency increases from 50 to 100% with an increase in NH3/NOx ratio from 0.5 to 1. Increase in efficiency was also observed for longer lengths of catalyst. Cu/SSZ-13 has been by far the best small pore zeolite suitable for low-temperature NOx reduction reported in literature [31,33,43]. The reaction conditions for the standard SCR reactions are given in Table 2. An optimum Si/Al ratio (SAR) of 15 provides the balance between best SCR activity and hydrothermal stability [43]. A near 100% reduction is reported in the temperature window of 200 to 500 C as shown in Figure 5. Additional substitution of MnO2 and CeO2 to Cu/SSZ-13 also have been studied. A 100% NOx removal efficiency is reported between temperature ranges of 150 to 350 C. The efficiency further drops to 80% when the temperature reaches 500 C. The catalyst had 4% CeO2 and 6% MnO2. BET surface area reduced by 28.8 % in the Mn-Ce-Cu-Z sample as compared to the Cu-Z catalyst. The study also revealed that higher NO was desorbed as compared to NO2 and the more active monodentate nitrates were formed on the surface that could have been the reason for higher SCR activity at low temperatures as compared to Cu/SSZ-13 alone. The authors concluded that there was reaction of bridging nitrates with NH3 over Mn-Ce-Cu-Z catalyst, by converting into monodentate nitrate. In comparison with 16

Cu/SSZ-13, the amount of monodentate nitrate increased on the surface leading to higher SCR activity at low temperatures [31]. Similar work carried out by Guan and Huang ascertains the effect of Si/Al ratio (SAR) along with hydrothermal aging on Cu/SSZ-13 catalysts. However, it is found that the increase in SAR decreases the amount of monodentate nitrates adsorbed on the surface [44]. These results suggests that SAR of 12 possess less active sites for NH3 adsorption leading to lower SCR activity which slightly deviate with the results obtained by others in literature [36,43]. This necessitates more corroboration studies on effect of SAR on monodentate nitrates and NOx reduction performance.

17

Silic oalu

Figure 5: NOx reduction activity of Cu/SSZ-13 reported in literature [31,33,43].

minophosphate (SAPO) is another type of zeolite having chabazite topology and SAPO-34 is the optimum choice of zeolite. Cu/SAPO-34 with a copper content varying from 1.6 to 3.9% have been investigated in literature [33,45,46]. The reaction conditions for the standard SCR reactions are given in Table 2. Similar trends of NOx conversion characteristics were reported that showed conversion in excess of 90% between temperatures of 200 and 350 C. A comparative illustration of the same is shown in Figure 6.

18

Figure 6: NOx conversion of Cu/SAPO-34 reported in literature [33,45,46].

Additional doping of yttrium [35] and neodymium (Nd) [45] have also marginally improved the NOx conversion activity of Cu/SAPO-34. An average increase of 11% was found with Yttrium dosing having highest improvement at temperatures below 170 C and more than 500 C. However Nd doping found benefit only in the temperature range of 400 C and beyond. A maximum of 4.8% was found at temperature of 500 C. Below 400 C, the addition of Nd depreciated the NOx reduction performance. What must be appreciated is that the hydrothermal stability increases with the addition of Nd at all the temperature ranges. Investigations on a mixture of copper and iron zeolites have been carried out in recent past. A mechanical mixture of Cu/SAPO-34 (50%) and Fe/MOR (50%) have also been investigated for the NOx reduction performance. Copper catalyst and iron catalyst were active at lower (below 300 C) and higher temperatures (up to 450 C) respectively. This mixture resulted in the operating range of the catalyst to extend from 150 to 450 C. The NOx reduction activity increased with the NH3/NOx ratio of 1.3 and above [22]. Copper with beta zeolite (Cu/BEA) has a relatively higher NO oxidation potential as compared to its chabazite counterparts [33] when investigated with a 1:1 ratio of NH3:NO (400 ppm) at a GSHV of 30,300 h-1 with Ar as carrier gas. The catalyst was degreened at 500 C for 1 hr. The NH3 oxidation remained low at low temperatures (<350 C) and Cu/BEA responded with a predominantly higher oxidation over chabazite catalysts beyond 400 C. However, below 200 C, the NOx conversion is lower than that of SAPO-34 and SSZ-13. Higher NO conversions have been reported with increase in the copper loading [47]. But at higher temperatures (500 C), higher oxidation of NH3 due to higher Cu sites resulted in a lower SCR activity. Hybrid Cu-Fe catalysts on different types of zeolites including Mordenite (MOR), beta zeolite (BEA) and ferrierite 19

(FER) have been investigated by preparing the catalyst using the SSIE method [26]. All the catalysts were subjected to identical testing conditions of 1:1 ration of NH3 :NO (200 ppm) with Helium as carrier gas. A total gas hourly space velocity (GHSV) of 250,000 h-1 was maintained through the catalyst. NOx reduction performance in the increasing order was FeCu/FER, Fe-Cu/MOR, and Fe-Cu/BEA respectively. It is due to the fact that BEA has a more selective transformation of N2 due to its large pore sizes as compared to its counterparts. This finding has led to the fact that the structure of zeolite has an influence on selective NOx reduction performance along with the distribution of Cu and Fe species. Among Cu/SAPO34, Cu/SSZ-13 and Cu/BEA, the SSZ-13 catalyst returns with a better NOx reduction efficiency over a wide temperature range as shown in Figure 7. The reaction test conditions for all the three catalysts are given in Table 2. Cu/SAPO-34 and Cu/BEA shows similar results, although the latter has a marginally higher conversions beyond 200 C.

20

Figure 7: Comparison of Cu exchanged SSZ-13, SAPO-34 and BEA catalysts on their NOx reduction performance [31,33,46]. Nishiyama et al. [48] also have carried out experimental investigations on Fe and Cu combinations on zeolite. At a ratio of NO2/NOx=0.5 and a GHSV of 39,000 h-1, iron-based catalyst gives the best performance. The optimum NOx reduction performance for the combination of Cu and Fe is obtained at a ratio of Cu: Fe = 2. When ammonia concentration is made rich inside the catalyst with ammonia equivalence ratio of 1.5, 25% NOx is reduced per kWh. However the production of N2O increases. A comparison of some important literature with regards to NOx reduction (standard SCR reaction) at different temperature ranges and their testing conditions of Cu-Z catalysts is shown in Table 2. All the authors have followed ion exchange method for impregnation of Cu on to the respective species of zeolite barring Fan et al. [43] and Tang et al. [46] for which one pot synthesis method was adopted as described by Wang et al. [49]. The reactor used by most of the authors is fixed bed quartz tube type inside which the catalyst is placed. The table also 21

includes the degreening/pre-heating temperature and time of the catalysts before its use in each experiment.

22

Table 2: Comparison of NOx reduction performance (standard SCR reaction) of various metal substituted catalysts reported in literature

Species

Author

Leistner et al. [33] Cu/SSZ-13

Fan et al. [43] Liu et al. [31] Feng et al. [45]

Cu/SAPO34

Leistner et al. [33] Tang et al. [46] Leistner et al. [33]

Cu/BEA Mihai et al. [47] Cu/ZSM-5

Vennestrøm et al. [50]

Cu/FER

Tarot et al. [51]

* cm3/min

Maximum NOx reduction % (temperature in C) <350 C

> 350 C

99 % (200-250) 99 % (250-350) 99 % (175-350) 98 % (300-350) 95 % (300-350) 98 % (250-300) 98 % (300) 99 % (250-300) 98 % (240-300) 95 % (300)

95 % (380-490) 99 % (350-500) 99 % (400-500) 87 % (400-450) 91 % (400) 90 % (400-450) 93 % (400) 94 % (350-400) 88 % (400) 92 % (400-450)

Test conditions Flow rate

Cu loadin g (wt. %)

NO (ppm)

NH3 (ppm)

O2 (%)

H2O (%)

CO2 (%)

Medium

3.1

400

400

8

5

-

2.8

1000

1100

5

10

4

500

500

3

1.6

200

200

2.6

400

3.9

Degreening conditions Temper Time ature (hrs) (C)

GHSV (h-1)

ml/ min

Ar

30,300

3,500

650

4

-

N2

30,000

1,045*

550

1

-

-

N2

NA

300

300

1

10

5

-

N2

40,000

NA

550

0.5

400

8

5

-

Ar

30,300

3,500

700

2

500

500

5

3

8

N2

NA

1,000

500

0.5

2.5

400

400

8

5

-

Ar

30,300

3,500

500

1.5

4

400

400

8

5

-

Ar

30,330

3,500

500

2

0.62

500

530

10

5

-

N2

NA

285

550

1

2.8

500

500

10

9

10

N2

NA

250

600

0.5

NA – Not Available

23

5. Catalyst Deactivation Major deactivation mechanisms of Cu-Z are hydrothermal deactivation, chemical poisoning, hydrocarbon poisoning, sulphur poisoning which are discussed in the following sections. 5.1. Hydrothermal Deactivation Zeolite structures can collapse to form denser crystalline structures at high temperatures [52]. In the presence of water at higher temperatures, the water molecules is attracted to Al sites resulting in a dealumination process where the Al atoms get detached from their respective sites. This will lead to defects in the required zeolite [53,54]. Copper oxide cluster or aluminates of Cu may form in those defect sites that will lead to structural changes to the existing zeolite framework [55–57]. The change in structure is not significant in the early stages of dealumination; however, the reduction of acid sites is substantial. Typical temperature range under normal operating conditions at the SCR catalyst is from 200 to 300 C. However, these temperatures can shoot to 600 C due to the thermal regeneration activities of DPF upstream of the catalyst. Water composition in a typical exhaust is less than 10%. At these instances of high temperature contact, the water content can attack the Al sites leading to dealumination and deactivation of the catalyst. In addition to the temperature, the number of Al sites in the zeolite structure (or Si/Al ratio) also plays an important role [52]. ZSM-5 and beta forms of zeolite have demonstrated good hydrothermal stability among large and medium pore topologies. Also, the presence of Cu ion inhibits the dealumination up to a certain limit [56,58,59]. The Cu ions at the redox sites restricts the water molecules to come in close contact to the adjacent Al sites. But aluminates of Cu can form due to the reaction of Cu and Al that can also lower the SCR activity by increasing the rate of dealumination. Hence an optimum Cu loading (or Cu/Al ratio) is necessary to balance the hydrothermal 24

stability and formation of undesired aluminates. Dealumination due to loss of acid sites is the major cause at a temperature of 670 C. The undesired aluminate interaction is rather a slow process at that temperature. Different Cu/SSZ-13 samples aged between 550 to 850 C for 4 hours show extensive hydrothermal stability up to a temperature of 800 C [60]. However a sudden drop was observed on the sample aged at 850 C due to the complete disintegration of zeolite structure. Even the NOx conversion remains to be in excess of 90% for all samples aged up to 800 C. Two peaks of N2O formation resulting due to NH3 oxidation were observed at 300 C and 500 C. Cu/SAPO-34 aged at 750 C shows a slight decrease in standard NOx reduction activity over the temperature range of 150 to 550 C [32]. A comparative analysis of NOx conversion of fresh and aged samples (at 750 C) of Cu/SSZ-13 and Cu/SAPO-34 is shown in Figure 8 and Figure 9 respectively. Both the catalysts were exposed to a gas concentration of 500 ppm NO, 500 ppm NH3, 10% O2, 10% H2O and the medium was N2. The GHSV maintained was 240,000 h-1. Both the catalysts were hydrothermally aged at 750 C and 800 C for 16 h. It can be clearly seen that there is a lower deactivation observed in SAPO-34 as compared to SSZ-13 suggesting a higher stability of Cu/SAPO-34 catalyst [61]. An increase in NOx reduction performance was observed for Cu/SAPO-34 after hydrothermal aging at low temperatures, which is a consistent finding that has been found in other literatures as well [32,62]. This may be due to the reason that the residual Cu species getting migrated into the pores of SAPO-34 catalyst after high temperature treatment increasing the availability of Cu sites [61].

25

Figure 8: NOx conversion efficiencies of Cu/SSZ-13 after hydrothermal aging at 750 and 800 C [61]

Figure 9: NOx conversion efficiencies of Cu/SAPO-34 after hydrothermal aging at 750 and 800 C [61]

26

Comparison of Fe/CHA and Cu/CHA cannot be made at this extent since Fe-Z based on a CHA topology have not been effectively developed and addressed in literature [63]. The current technology of Fe-Z can withstand hydrothermal conditions only up to 670 C, but with advancements on Fe/CHA catalyst, possibilities to venture SCR activity at extreme low and high temperatures can be achieved [64].

5.2. Chemical Poisoning Chemicals containing P, Zn and Ca may be found in the exhaust stream due to interactions with lubricant oil [65]. These chemicals may come in contact with the SCR catalyst which can lead to deactivation. However, due to the presence of these quantities in minute amounts, the poisoning effect is usually very small. Also, the effect of these poisons are noted at the entry point of the catalyst. A suitable sizing factor will have to be accommodated in the Cu-Z catalyst design to overcome the deactivations from these chemicals [66,67]. However, alkali metals have the capability to displace Cu ions in Cu-Z catalyst further forming CuO, leading to severe deactivation, even at low concentrations [68]. Higher CuO leads to NH3 oxidation leading to loss of SCR activity. When biodiesel blends are used as fuels, a check on input quality of the fuel is required to reduce the amounts of alkali coming in the exhaust stream. Presence of Pt has demonstrated to have adverse effect on NH3-SCR activity of Cu/SAPO34[69]. At low temperatures, N2 selectivity will get reduced although the conversion of NO is favoured. NH3 gets oxidised to N2O at higher temperatures reducing the SCR activity. Pt sintering and oxides of Pt are formed when the catalyst is subjected to hydrothermal treatment at 850 C. It also increases the formation of Cu2+ species and Cu-Pt oxo-complexes.

27

These oxo-complexes have shown to have improved SCR activity at higher temperatures and is a topic that needs much understanding. 5.3. Sulphur poisoning Reaction of sulphur with Cu in a Cu-Z catalyst forms stable compounds like copper sulphates because of higher oxidation conditions and chemical binding strength [70–76]. After undergoing hydrothermal aging beyond 800 C, additional sulphates are formed at lower temperatures as compared to a fresh catalyst. This reveals the formation of CuO on the surface [76]. NO oxidation to NO2 is inhibited due to SO2 poisoning resulting in a lower SCR activity below 350 C. The sulphate compounds beyond this temperature are unstable and the available Cu sites may be sufficient enough for SCR activity. Therefore, Sulphur poisoning beyond 350 C is insignificant. In order to be relieved from Sulphur poisoning, a temperature of 500 C needs to be achieved that can thermally decompose the sulphate compound. This usually will be achieved during the DPF regeneration upstream of the SCR. 5.4. Hydrocarbon poisoning In addition to NH3 storage, the zeolite cannot inhibit the storage of UBHC. Hence, at lower temperatures, the adsorbed UBHC blocks the active sites required for SCR activity. However, this condition is reversible whenever the hydrocarbon is removed from the catalyst. There is a possibility of some UBHC to react with the acid and redox sites leading to carbon containing deposits on the surface. This creates a poisoning of the catalyst which demands high temperatures to regenerate the catalyst where carbon deposit is oxidised and removed from the zeolite [77,78]. During normal engine operation, a DPF regeneration process or a DOC essentially ensures the complete oxidation of UBHC thereby limiting its effect on SCR catalyst. But during 28

conditions of low exhaust temperatures as in idling or engine cold start periods, the temperatures may not be sufficient enough to burn off the UBHC that may slip through the DOC and comes in contact with the SCR. At these undesired conditions, hydrocarbon poisoning can decrease the SCR activity of Cu-Z catalyst that may lead to extreme deactivation [79]. Hydrothermal deactivation is the most crucial challenging type of deactivation mechanism for Cu-Z catalysts. In order to be applicable for automotive applications, especially in association with DPF, a significant amount of technical know-how needs to be established to protect the acid/redox sites of the catalyst from deactivation. A list of major deactivation mechanisms reported in literature is shown in Table 3.

5.5. Effect of Si/Al ratio (SAR) More number of Al sites attract water that can lead to dealumination of zeolite, particularly at high temperatures. Hence lower Al or higher SAR is desirable for better hydrothermal stability at high temperatures. On the other hand, higher SAR leads to lower acid sites (due to lesser Al sites) that may not be sufficient for Cu loading or SCR activity leading to a loss in performance. SAR of lower values have better NOx reduction potential due to better adsorption of NH3 species [36]. In order to balance both these conditions, a maximum SAR value of 50 can support good SCR activity [54,80]. An SAR of 12.5, 15, and 45 for Cu/BEA, Cu/SSZ-13 and Cu/ZSM-5 respectively

have been consistently suggested in

literature for best balance between hydrothermal stability and SCR activity [25,36,43].

29

Table 3: Deactivation mechanisms of various catalysts reported in literature. Catalyst type

Major deactivation

References

mechanisms Cu/SSZ-13

Hydrothermal

deactivation, [60,73,77,81,82]

phosphorous poisoning Cu/ZSM-5

Hydrothermal deactivation

Cu/SAPO-34

Hydrothermal

[50]

deactivation, [32,74,75,83–85]

Chemical poisoning Cu/MOR

Hydrothermal deactivation

[26]

Cu/BEA

Hydrothermal deactivation

[25]

Cu/FER

Hydrothermal

deactivation, [51]

Chemical poisoning

6. Conclusion This paper reviews the recent developments in NOx reduction activity of Copper zeolite catalyst for NH3-SCR reaction. Cu-Z catalysts show better SCR activity at low temperatures as compared to Fe-Z. The different Cu-Z candidates suitable for low temperature NH3-SCR are Cu/ZSM-5, Cu/BEA, Cu/MOR, Cu/FER, Cu-SSZ-13 and Cu/SAPO-34. Small pore chabazite topologies are found to have good NOx reduction characteristics for a wide temperature window between which Cu/SAPO-34 appears to be superior in withstanding severe hydrothermal conditions. Considering the hydrothermal stability, selecting an optimum value of SAR for each species of catalysts plays a major role in NOx reduction behaviour. Increase in copper loading has a positive impact in reducing NOx wherein a Cu loading of 4-6% wt. is found to be an optimum through experimentation. Additional metal 30

substitutions to Cu/SSZ-13 like MnOx, CeO2, Y etc. further improves the NOx reduction activity in the temperature window of 150 to 350 C. However, further investigations are required to obtain adequate substantial results that can improve the NOx reduction behaviour for low temperature automotive applications. A few gaps exists in the field of Cu-Z for NH3-SCR activity. An evaluation void exists wherein experimental results need to be augmented with empirical evidences to better understand the relationship between SCR activity and other independent factors like Cu loading, SAR values, zeolite topology etc. The mechanism of nitrite formation and inhibition of nitrates on the catalyst, causes and solutions to formation of complexes of metal ions are still a research challenge that needs to be addressed. More studies are required in ascertaining fundamental understanding of low temperature SCR activity in Cu/CHA for a combination of variations in SAR, adsorbed monodentate nitrates and the Brønsted acid sites.

31

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Graphical Abstract

44

N2

NH3 Exhaust Temperature < 350 C

SCR Catalyst H2O

NOx

45

Highlights



NOx reduction performance of various metal substituted catalysts are discussed.



Copper zeolite catalysts are best suited for NH3-SCR activity at low temperature.



Major deactivation mechanisms of copper zeolite catalysts are discussed.

From, P. Dinesha Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. To, Dr. Carolina Belver, Associate Editor, Chemical Engineering Journal

Dear Professor, Sub: Declaration of interest statement– reg. The authors of the manuscript entitled “NOx reduction behaviour in copper zeolite catalysts for ammonia SCR systems: A review” have no conflict of interest. All authors have agreed for the submission. With thanks, 46

Yours sincerely, P. Dinesha Corresponding Author 01 October 2019

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