1 Introduction

Diesel engines have attracted much attention around the world because of their higher power output and better fuel efficiency compared with their gasoline counterparts. However, the in-cylinder lean-burn combustion of diesel engines also generates significant NO x emissions (Heywood, 1988). Substantial NO x emissions can result in acid rain, photochemical smog, and ozone depletion. To meet the increasingly stringent NO x emission regulations, diesel engines have started to be required to have exhaust NO x after-treatment systems (Johnson, 2015). The selective catalytic reduction of NO x with NH3 (NH3-SCR) is one of the most effective ways to abate NO x emissions from diesel engine exhaust (Naseri et al., 2014; 2015).

Diesel NH3-SCR systems use a 32.5% (in weight) aqueous urea, known as diesel emission fluid (DEF), as a reducing agent to reduce NO x selectively with SCR catalysts (Wurzenberger and Wanker, 2005). As DEF is injected into the hot diesel exhaust gas, it undergoes water evaporation Eq. (1), urea thermolysis Eq. (2), and hydrolysis Eq. (3) to form the real reducing agent of NH3, which is then involved in the subsequent SCR reactions inside the SCR catalyst.

$${\rm{CO}}{({\rm{N}}{{\rm{H}}_2})_2}\;({\rm{aq}}) \rightarrow {\rm{CO}}{({\rm{N}}{{\rm{H}}_2})_2}({\rm{s}}/{\rm{l}}) + 6{.}9{{\rm{H}}_2}{\rm{O},}$$
(1)
$${\rm{CO}}{({\rm{N}}{{\rm{H}}_2})_2} ({\rm{l}}) \rightarrow ({\rm{N}}{{\rm{H}}_3})\;({\rm{g}}) + {\rm{HNCO}}\;({\rm{g}}),$$
(2)
$${\rm{HNCO}}\;({\rm{g}}) + {{\rm{H}}_2}{\rm{O}} \rightarrow {\rm{N}}{{\rm{H}}_3}({\rm{g}}) + {\rm{C}}{{\rm{O}}_2}({\rm{g}}){.}$$
(3)

However, DEF injection and decomposition in real diesel NH3-SCR systems are more complicated. Because of the low diesel exhaust gas temperature and high exhaust flow velocity, it is usually difficult for the injected DEF to completely decompose to NH3 before entering the SCR catalyst (Munnannur et al., 2012; Weeks et al., 2015). The unevaporated DEF can cause liquid-wall interaction on the exhaust wall, the exhaust mixer, and even the inlet face of SCR catalyst. Part of the wall-interaction DEF may form DEF liquid films along the exhaust pipe in extreme cases. This phenomenon causes the SCR catalyst to have a high risk of being soaked by DEF, causing urea deposits and catalyst deactivation issues. Previous studies have shown that the unevaporated and undecomposed DEF has a great tendency to generate high molecular solid byproducts, such as biuret, cyanuric acid, ammelide, and ammeline (Smith et al., 2014; Weeks et al., 2015). Most of them need a higher temperature to decompose again, leaving urea deposits in the diesel exhaust system. The urea deposits, if occurring on the SCR catalyst surface because of DEF soaking, will change the catalyst performance and its durability.

Numerous studies have concerned the potential reaction pathways of urea decomposition as well as the impact of SCR catalysts on them (Bernhard et al., 2012; Ebrahimian et al., 2012; Brack et al., 2014; Ku and Hong, 2015). Several studies have also focused on the deactivations of SCR catalysts caused by steam, sulfur, phosphorus, hydrocarbon, and other impurities or additives in commercial diesel and lubricating oils (Kern et al., 2010; Klimczak et al., 2010; Lezcano-Gonzalez et al., 2014; Ummel and Price, 2014). However, only a few publications have considered the impact of DEF and urea deposits on the performance of SCR catalysts. Among them, Cheng et al. (2008) investigated the effect of hydrothermal aging on the reactivity of Cu-zeolite SCR catalysts with DEF exposure, and found that DEF caused obvious deteriorations of the performance of the SCR catalyst. Eichelbaum et al. (2010a; 2010b) impregnated Fe-beta zeolite powder with DEF followed by hydrothermal treatments at different temperatures, and found that DEF and urea deposits reduced the catalyst performance drastically but the high temperature helped to restore the performance after the removal of urea deposits. Therefore, DEF has been proved to be another cause of SCR catalyst deactivation beyond traditional knowledge. However, most recent studies simply ascribe the DEF-induced catalyst deactivation to the urea deposits on the catalyst, and almost no one has noticed the impact of DEF solution itself; the detailed catalyst deactivating mechanism during DEF soaking is still unclear, especially for those Cu-zeolite SCR catalysts like Cu-SAPO-34. Due to the inevitable interaction between DEF and SCR catalysts in real diesel SCR design, the impact of DEF soaking and urea deposits on the SCR catalyst performance needs to be considered during the whole SCR lifetime.

This study was mainly to investigate the impact of DEF soaking and urea deposits on the performance of Cu-zeolite SCR catalysts. To simulate DEF soaking and urea deposits on real SCR catalysts during diesel engine operations, several fresh SCR catalyst samples were impregnated with DEF for certain times and then blown dry with heated indoor lab air. Subsequently, all the impregnated samples were hydrothermally treated on a flow reactor with typical feeding gas in diesel exhaust to simulate the behavior of DEF and urea deposits in real applications. Finally, NO x conversion, N2O selectivity, NH3 oxidation, NO oxidation, and NH3 storage of each SCR catalyst sample were evaluated on the flow reactor with a four-step test protocol (Kamasamudram et al., 2010). Test results and data analysis have been illustrated to clarify the impact of DEF soaking and urea deposits on the performance of Cu-zeolite SCR catalysts.

2 Experimental

2.1 SCR catalyst samples

An integrated diesel after-treatment system from a Champaign-Urbana Mass Transit District (CUMTD) bus at Urbana, USA had attracted our attention, because the Cu-zeolite SCR catalyst in the system had encountered DEF soaking and urea deposit issues in the extremely cold winter of 2014. All the Cu-zeolite SCR catalyst samples in this study were drilled from a new full-size SCR catalyst with the same formulation of Cu-SAPO-34 as used on the bus. Each sample had a diameter of 2.54 cm, a length of 5.08 cm and a cell density of 400 CPSI. The entire experimental procedure for those samples in this study is shown in Fig. 1. At the beginning of the experiments, all the catalyst samples were hydrothermally treated in a flow reactor at 700 °C for 4 h with the feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), and a balance gas of N2, to ensure their stable performance in the subsequent tests.

Fig. 1
figure 1

Schematic diagram of the entire experimental procedure (1 ppm=1×10−6)

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2.2 DEF soaking and hydrothermal treatment

After hydrothermal pretreatment, two catalyst samples were impregnated with DEF solutions for 1 h and 2 h separately to simulate DEF soaking and urea deposits formation on the catalyst surface during diesel engine operations. Moreover, to investigate the potential physical and chemical interactions between the soaking liquid and the catalysts, one additional sample was also impregnated with distilled water as a comparison. All the DEF and water impregnated samples were then blown dry with heated indoor lab air. The DEF-impregnated samples were subsequently treated hydrothermally in a flow reactor at temperatures of up to 550 °C for 2 h, with a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), and a balance gas of N2, to simulate the behaviors of DEF and urea deposits on the catalyst in real applications. The water-impregnated sample was also hydrothermally treated in the same flow reactor with the same conditions as above as a comparative reference.

2.3 SCR performance evaluation tests

All the performance tests of catalyst samples were conducted on the same flow reactor above. The flow reactor has a synthetic exhaust gas mixture delivery device, in which different gas species from high-pressure gas bottles can be regulated and mixed by their mass flow controllers. A high-pressure pump is used to deliver a desired amount of water stream into the mixtures. The synthetic gas mixtures are then fed directly to a tube resistance furnace based reactor, in which the testing catalyst sample is placed in a quartz tube inside the furnace and the catalyst temperature can be controlled. All the stainless steel gas lines in the flow reactor are heated up to at least 190 °C to avoid potential water condensation and NH3 adsorption. The gas species and their concentrations both upstream and downstream of the catalyst sample are measured and recorded by an MKS MultiGas 2030 FTIR gas analyzer (USA).

A four-step integrated test protocol was used to conduct the performance tests for those SCR catalyst samples based on the flow reactor above (Kamasamudram et al., 2010). During each four-step test cycle with a certain temperature, the basic feeding gas species were kept fixed as 7% H2O, 8% CO2, and 10% O2 (in volume), which represented the typical diesel exhaust environment. However, the NO x and NH3 concentrations in the feeding gas were switched periodically between 0 ppm and 200 ppm, as shown in Fig. 2. Four individual steps with different combinations of inlet feeding NO and NH3 concentrations existed in each isothermal cycle, including NO oxidation, standard SCR, NH3 saturation, and clean. Each step could be used to summarize different performances of the test catalyst sample, such as NO x conversion, N2O selectivity, NO and NH3 oxidations, and NH3 storage amount. The isothermal four-step test cycle was conducted at different temperatures to evaluate the entire SCR performance under different diesel exhaust conditions. In this study, the experimental temperature was ramped down from 550 °C to 200 °C with a decrement of 50 °C for each four-step test cycle, and the gaseous hourly space velocity (GHSV) was kept at 60 000 h−1 all the time.

Fig. 2
figure 2

Schematic diagram of the four-step based SCR catalyst performance test protocol

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3 Results and discussion

3.1 Catalyst DEF soaking behavior

To simulate DEF soaking and urea deposits formation on the SCR catalyst during diesel engine operations, two SCR catalyst samples were impregnated with DEF for 1 h and 2 h separately. It was observed that the original clear DEF solutions were all found to have a blue hue after impregnating the SCR sample each time. It seems that some Cu was dissolved and leached down from the catalyst samples, and the largest change in blue color occurred during the first hour. To exclude the DEF’s physical dissolution effect, one additional catalyst sample was also impregnated in the same volume of distilled water as a comparison. However, no obvious blue hue in the water was found at all. To be clear, it is the urea in the DEF solution that caused some Cu to leach out from those SCR catalyst samples through potential chemical processes.

For further verification of the loss of Cu from catalyst samples during liquid impregnation, DEF and water that had impregnated catalyst samples were collected and used for inductively coupled plasma (ICP) analysis. During ICP analysis, Cu concentration was clearly detected in the DEF solution, but none was found in water. From the ICP results, it can also be inferred that the Cu detected in the DEF solution probably came from the structure of zeolite rather than the free Cu in the catalyst pores. There must be potential chemical reactions, in which parts of Cu ions on the zeolite structure were replaced and dissolved into the DEF solution.

In fact, urea can decompose easily in aqueous solutions such as DEF, yielding ammonium (NH4+) and cyanate (OCN−) ions (Okazaki and Funazukuri, 2008). Therefore, when the Cu-zeolite SCR catalyst samples were impregnated into such an ion solution, the ammonium ions could replace some Cu2+ ions from the catalyst zeolite structure and leave copper ammonia complex ions ([Cu(NH3)4]2+) in the DEF solution. The copper ammonia complex ions would show an obvious blue hue even at a low ion concentration. Thus, besides there being urea deposits, DEF soaking will also definitely slightly change catalyst Cu loading. Each can drastically influence the activity and durability of SCR catalysts. In the following sections, the SCR catalytic performance of those DEF-impregnated samples will be illustrated and analyzed to investigate the impact of DEF soaking and urea deposits.

3.2 Catalyst NO x conversions

Fig. 3 shows the NO x conversions of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), 200 ppm NO, 200 ppm NH3, and N2 as a balance gas. All the NO x conversion data were obtained from the standard SCR segment of the four-step test at each temperature point (Fig. 2). For different SCR catalyst samples, the NO x conversions show almost the same trend of single-peak curves. The NO x conversions of each sample can exceed 90% in a wide medium temperature range from 250 °C to 400 °C and begin to decrease at low and high temperatures. The low NO x conversions at low temperatures (<250 °C) are caused by the low catalytic activity, whereas at high temperatures (>400 °C) it is mainly ascribed to the strong NH3 oxidation process.

Fig. 3
figure 3

NO x conversions of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H 2 O, 8% CO 2 , 10% O 2 (in volume), 200 ppm NO, 200 ppm NH 3 , and N 2 as a balance gas

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From the NO x conversions over different DEF-impregnated SCR catalyst samples, the DEF impregnation has almost no impact on NO x conversions at low and medium temperatures, but it can drastically increase high-temperature NO x conversions above 400 °C, as illustrated in Fig. 3. The catalyst sample impregnated by DEF for 2 h has the highest NO x conversions in the high temperature range, almost doubling the value of the non-soaked sample at 550 °C. DEF impregnation was thought to have a negative effect on the catalyst NO x conversions because of the potential urea deposits on the catalyst surface (Eichelbaum et al., 2010a; 2010b). However, in this study, those DEF-impregnated SCR catalyst samples have nearly the same level or even higher NO x conversions in the high temperature range. There are probably two reasons. First, the high-temperature hydrothermal treatment (up to 550 °C) after DEF impregnation helped to remove the urea deposits from the catalyst surface and recovered their performance according to the urea decomposition properties (Brack et al., 2014). Second, the catalyst Cu loading change during DEF impregnation also altered their catalytic activities. A more detailed influence mechanism will be given in the following sections.

Fig. 3 also shows the NO x conversions of a distilled-water-impregnated SCR catalyst sample as a comparison. The water impregnation also has almost no impact on the NO x conversions at low and medium temperatures, but it can slightly increase the high-temperature NO x conversions as well. That is because physical effects such as water flushing and dissolution can remove isolated impurities inside the catalyst samples and increase their reaction surface area. However, compared with those DEF-impregnated SCR catalyst samples, the NO x conversions of the water-impregnated sample at high temperatures are much lower. Therefore, besides the physical effects, there must be additional reasons that could affect the NO x conversions of SCR catalyst samples during DEF impregnation. From the ICP analysis above, the catalyst Cu loading change during DEF impregnation is probably the root cause of the improvement in NO x conversations.

3.3 Catalyst NO oxidations

During the reduction process of NO x with NH3, the oxidation of NO to NO2 is a rate-determining step. If the catalyst can oxidize a significant amount of NO to NO2, the fast SCR reaction will be dominant over the standard SCR reaction, which helps to improve the entire SCR performance over a wide temperature range (Wurzenberger and Wanker, 2005). Fig. 4 shows the NO oxidations of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), 200 ppm NO, and N2 as a balance gas. All the NO oxidation data were obtained from the NH3-free NO oxidation segment of the four-step test at each temperature point (Fig. 2). The NO oxidation capability of the non-soaked SCR catalyst sample shows one single-peak curve with the maximum value at approximately 450 °C. However, for those DEF-impregnated samples, their NO oxidation performance decreases drastically, and the longer the impregnation time is, the greater the degradation of NO oxidation becomes.

Fig. 4
figure 4

NH 3 -free NO oxidations of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H 2 O, 8% CO 2 , 10% O 2 (in volume), 200 ppm NO, and N 2 as a balance gas

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In fact, for metal-exchanged zeolites SCR catalysts like Cu-SAPO-34, previous studies have found that the oxidation reaction of NO to NO2 always takes place on the metal sites, which are essential for SCR reactions (Ma et al., 2012). Therefore, the decrease of NO oxidations of those DEF-impregnated SCR catalyst samples may be ascribed to the Cu loss during DEF impregnation, as discussed in Section 3.1. Some researchers also believe that the metal sites as well as the active sites on the catalyst surface may be blocked by urea deposits after DEF impregnation, which can decrease NO oxidations (Eichelbaum et al., 2010a; 2010b). However, from the relatively high NO x conversion results in Fig. 3, the active sites responsible for SCR reactions were not blocked by urea deposits at all after high-temperature hydrothermal treatment. Therefore, Cu loss during DEF impregnation is the main reason for the decrease in catalyst NO oxidations.

3.4 Catalyst NH3 oxidations

Fig. 5 shows the NO x -free NH3 oxidations of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), 200 ppm NH3, and N2 as a balance gas. All the NH3 oxidation data were obtained from the NO x -free NH3 oxidation segment of the four-step test at each temperature point (Fig. 2). For the non-soaked SCR catalyst sample, the NH3 oxidations are almost less than 20% below 300 °C, gradually increasing through the medium temperatures and reaching nearly 100% above 450 °C. The NO x -free NH3 oxidation results above indicate that NH3 can effectively participate in SCR reactions below 300 °C, but as the temperature increases above 350 °C, the NH3 oxidation process gradually becomes dominant, decreasing the NO x conversions in the high temperature range, as shown in Fig. 3.

Fig. 5
figure 5

NO x -free NH 3 oxidations of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H 2 O, 8% CO 2 , 10% O 2 (in volume), 200 ppm NH 3 , and N 2 as a balance gas

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For those DEF-impregnated SCR catalyst samples, the NO x -free NH3 oxidations have the same trend as for the non-soaked sample, almost all less than 20% below 300 °C, and begin to increase gradually above 300 °C. However, the absolute NH3 oxidations of those DEF-impregnated samples are much lower than the non-soaked sample at each temperature point, especially at high temperatures, and the longer the impregnation time, the larger the NH3 oxidation decreases. The above results indicate that the oxidation abilities of catalyst samples were weakened after DEF impregnation, and more NH3 could be involved in SCR reactions, which helped to increase the NO x conversions in the high temperature range.

Fig. 6 shows the NO and NH3 oxidations of SCR catalyst samples at 400 °C with different DEF impregnation times. It is clear that both NO and NH3 oxidations exponentially decrease with DEF impregnation time, because the Cu loss during DEF impregnation reduced the metal sites and weakened the catalyst’s oxidation ability. Since NO is the main product in the NH3 oxidation reaction (Ma et al., 2014), the Cu loss-induced degradation of NO oxidation also inhibits the oxidation process of NH3.

Fig. 6
figure 6

NO and NH 3 oxidations of SCR catalyst samples with different DEF impregnation time at the testing temperature of 400 °C

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3.5 Catalyst NH3 storage level

It is well established that NH3 should be adsorbed on the SCR catalyst surface before it reduces the gaseous NO x (Wurzenberger and Wanker, 2005). Therefore, the SCR performance is greatly affected by the catalyst NH3 storage capability. Fig. 7 shows the NH3 storage level of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), 200 ppm NH3, and N2 as a balance gas. All the NH3 storage data were obtained from the NO x -free NH3 oxidation segment of the four-step test at each temperature point (Fig. 2), by integrating the difference of inlet and outlet NH3 concentrations. Although the total NH3 storage should also contain the dynamic NH3 storage in the standard SCR segment of the four-step test, the NO x -free NH3 storage, to some extent, can also reflect the NH3 storage capacities of those test SCR catalyst samples.

Fig. 7
figure 7

NO x -free NH 3 storage level of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H 2 O, 8% CO 2 , 10% O 2 (in volume), 200 ppm NH 3 , and N 2 as a balance gas

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As illustrated in Fig. 7, the NH3 storage capacities of all test SCR catalyst samples decrease drastically with increasing temperature. However, compared with the non-soaked sample, those DEF-impregnated SCR catalyst samples appear to have a slightly higher NH3 storage capacity, especially in the medium and high temperature ranges. In fact, for metal-exchanged zeolite SCR catalysts, NH3 can be adsorbed on either metal sites or acid sites, such as Lewis/Bronsted sites. NH3-TPD (temperature programmed desorption) tests on Cu-zeolite catalysts in many studies have demonstrated that the storage NH3 is mainly adsorbed on acid sites above 200 °C, whereas the NH3 adsorption on Cu sites usually happens at low temperatures (Ma et al., 2013; 2014). Thus, the NH3 storage capacity at a normal SCR reaction temperature range (above 200 °C) is mainly determined by the number of acid sites on the catalyst surface. The Cu loss during DEF impregnation tests released more acid sites for NH3 adsorption on the catalyst surface. Therefore, DEF impregnation actually slightly improved the catalyst acidity and NH3 storage capability (Fig. 7). The higher NH3 storage capability at medium and high temperatures is another reason for the higher high-temperature NO x conversions of those DEF-impregnated SCR catalyst samples.

3.6 Catalyst N2O selectivity

N2O selectivity is another important practical issue for SCR catalysts. For metal-exchanged zeolite SCR catalysts, N2O is usually produced in two ways, low-temperature ammonium nitrates decomposition and high-temperature NO/NH3 oxidation (Kamasamudram et al., 2012; Kumar et al., 2015). Fig. 8 shows the N2O selectivity of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H2O, 8% CO2, 10% O2 (in volume), 200 ppm NO, 200 ppm NH3, and N2 as a balance gas. All the N2O selectivity data were obtained from the standard SCR segment of the four-step test at each temperature point (Fig. 2). As expected, all the SCR catalyst samples have a similar qualitative behavior of N2O selectivity with two N2O production peaks. One is located at approximately 250 °C, and the other at around 500 °C. The two N2O peaks correspond to the two processes of N2O generation as mentioned above.

Fig. 8
figure 8

N 2 O selectivity of SCR catalyst samples at a space velocity of 60 000 h−1 and a feeding gas of 7% H 2 O, 8% CO 2 , 10% O 2 (in volume), 200 ppm NO, 200 ppm NH 3 , and N 2 as a balance gas

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However, compared with the non-soaked SCR catalyst sample, those DEF-impregnated samples have a relatively low N2O selectivity across the entire temperature range. As mentioned above, DEF impregnation slightly reduced the Cu loading and Cu metal concentration on the catalyst surface. The decrease of Cu loading made those DEF-impregnated SCR catalyst samples less active to the decomposition of surface ammonium nitrates during SCR reactions and reduced N2O formation in the low temperature range (Kamasamudram et al., 2012; Kumar et al., 2015). Moreover, it also has been proved that the Cu oxocations, such as [CuOCu]2+, act as the catalytic active sites for the NO/NH3 oxidation and N2O formation at high temperatures (Beutel et al., 1996). Therefore, the Cu loss during DEF impregnation also gave those SCR catalyst samples a lower N2O selectivity in the high temperature range.

From the results and discussion above, it is clear that DEF impregnation will definitely slightly change the catalyst Cu loading besides the urea deposit issue. The negative effect of urea deposits on the performance of SCR catalysts was not clearly observed, because the high-temperature hydrothermal treatment (up to 550 °C) helped to remove the urea deposits. In contrast, the Cu loading change of those catalyst samples due to DEF impregnation had caused a great effect on the SCR performance, as shown in Figs. 3–8. The Cu loss of catalyst reduced the metal active sites on the catalyst surface and weakened the catalytic oxidation ability, which caused lower NO/NH3 oxidation and lower high-temperature N2O formation. Lower Cu loading also slowed down the decomposition of surface ammonium nitrates during SCR reactions, leading to lower N2O selectivity at low temperatures. Furthermore, the replaced Cu during DEF impregnation also released more acid sites on the catalyst surface and increased the catalyst acidity, and more NH3 was able to be adsorbed at medium and high temperatures for SCR reactions. Both the higher NH3 storage and lower NH3 oxidation helped to involve more NH3 into SCR reactions for NO x reduction, especially at high temperatures, leading to higher NO x conversions at high temperatures as shown in Fig. 3.

4 Conclusions

DEF soaking and urea deposits on SCR catalysts are critical issues in real diesel engine NH3-SCR systems. To investigate the impact of DEF soaking and urea deposits on the SCR performance of Cu-zeolite catalysts, several fresh catalyst samples were drilled from a new full-size SCR catalyst with a formulation of Cu-SAPO-34. Those catalyst samples were impregnated with DEF solutions for certain times and dried, and subsequently hydrothermally treated for the simulation of DEF soaking and urea deposits on real SCR catalysts during diesel engine operations. Their SCR performance was finally evaluated in a flow reactor with a four-step test protocol.

Test results show that DEF impregnation caused some Cu to leach out from the SCR catalyst samples and slightly reduced their Cu loadings. The Cu loss of catalysts reduced the metal active sites on the catalyst surface and weakened the catalytic oxidation abilities, which caused lower NO/NH3 oxidation and lower high-temperature N2O formation. Lower Cu loading also slowed down the decomposition of surface ammonium nitrates during SCR reactions, thus decreasing N2O selectivity at low temperatures. Cu loss during DEF impregnation released more acid sites on the catalyst surface and increased the acidity of the catalyst, which caused more NH3 to be adsorbed and involved in SCR reactions at medium and high temperatures. The lower NH3 oxidation and higher NH3 storage gave those DEF-impregnated SCR catalyst samples higher NO x conversions, especially in the high temperature range. Urea deposits due to DEF impregnation should have a negative impact on SCR performance because of the potential blocking effect on the micro pores and catalytic active sites (Eichelbaum et al., 2010a; 2010b). However, in this study, the urea deposits were probably removed during the high-temperature hydrothermal treatment after DEF impregnation; thus, their negative impact was not clearly observed.