Generation of multi-valence CuxO by reduction with activated semi-coke and their collaboration in the selective reduction of NO with NH3

Multi-valence CuxO has been demonstrated to have high activity in the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). Here, CuxO was loaded onto activated semi-coke (ASC) for SCR, which has shown satisfactory low-temperature SCR activity. By virtue of the reduction property of carbon, the valence of Cu was regulated by simply adjusting the calcination temperature. The high concentration of Cu+ generated from the reduction of CuO by ASC during calcination can collaborate to form Cu2+/Cu+ circulation. After systematic characterization by XPS, H2-TPD, and NH3-TPR, it is revealed that abundant acidic sites and surface reactive oxygen species are formed on the surface of the catalysts. Further investigation with in situ DRIFTS confirms that the NH3-SCR over the as-prepared CuO|Cu2O-ASC catalysts simultaneously follows the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) pathways, attributed to the synergistic effects of Cu2+ and Cu+.


Introduction
As a major pollutant, the emission control of NO x has attracted much attention. Selective catalytic reduction with NH 3 (NH 3 -SCR) is considered the most effective technology and has been commercialized for decades. [1][2][3] In recent years, many efforts have been devoted to low-temperature SCR (LT SCR, <200 C) since it can widen the utilization of SCR and avoid severe ue gas conditions. The development of catalysts that can be utilized at a low temperature range has presented great prospects. 4,5 Various investigations have demonstrated that transition metal oxides are ideal candidates for low-temperature SCR, such as Cu x O, Mn x O y , and Fe x O y . [5][6][7][8] Among the potential metal oxides that have been employed as active ingredients for low-temperature SCR catalysts, Cu x O can offer high redox capacity and suitable acidic sites, which are signicant for LT SCR performance. It was found that the valence of Cu directly affected the surface acidity and redox capacity of the catalysts, which are vital for their catalytic performance. 9 Redox cycles of Cu 2+ /Cu + and Cu + /Cu 0 are considered to play a governing role during LT SCR. 10 Although it has been well demonstrated that the appearance of a large number of Cu + merits superior catalytic performance, pure Cu 2 O or CuO oxide catalysts show poor activity in a narrow temperature window. 11 Hence, Cu x O has usually been loaded onto a support, such as carbon, 10,[12][13][14] alumina, 15 and molecular sieve. 16 Cu x O can be highly dispersed on the surface of the supports to expose more active sites. Meanwhile, the defects on the substrate are favorable during the creation of multi-valence Cu x O. In comparison with alumina and other supports, carbonaceous materials are not only able to provide high surface area for the active sites but are also equipped with a large amount of functional groups. 10,13 Close interactions have been observed during the loading of Cu x O onto carbonaceous materials, which can regulate the valence distribution of copper species by virtue of their surface oxygen vacancy storage/release capacities and facilitate electron transfer among different Cu species. [17][18][19] Li et al. prepared Cu x O-carbon nanotubes (CNTs), Cu x Oactivated carbon (AC) and Cu x O-graphite catalysts by wet impregnation methods to probe the effect of carbonaceous material supports and the nature of Cu x O species. It was found that the good de-NO x of the Cu-CNTs catalyst originated from the good dispersion of Cu x O, the existence of Cu + and the strong acid sites on the catalyst surface. However, the interaction between Cu 2+ and Cu + was not discussed in detail. 20 Zhu et al. loaded CuO on activated carbon and the catalysts showed high activities for NO reduction with NH 3 at temperatures above 180 C. They found that calcination temperature and Cu loading of the catalyst strongly inuence the activity and structure of the catalyst. During the NO-NH 3 -O 2 reaction, Cu 2 O can be easily oxidized into active CuO and results in increased activity. 14 10 This study presents the inuence of the surface properties of the substrate, but the correlation between Cu 2+ /Cu + and the reaction pathways is still ambiguous.
Based on previous research ideas, we employed activated semi-coke (an economical industrial carbonaceous material 22 ), as the substrate to load Cu x O, followed by regulating the calcination condition to obtain multi-valence Cu oxides. The abundant oxygen-containing groups on the surface of activated semicoke can enhance the production of multi-valence Cu. The inuence of the calcination temperature and atmosphere on the physical and chemical properties was further examined to probe the role of Cu 2+ /Cu + circulation and the oxygen vacancies during LT SCR. Reaction mechanisms were further investigated with in situ diffuse reectance infrared Fourier transform spectroscopy (in situ DRIFTS).

Preparation of activated semi-coke
The raw semi-coke was purchased from Shanxi Shenmu Group Limited Liability Company. Firstly, the semi-coke was mechanically crushed and sieved into 10-20 mesh, which was denoted as SC. Then, the SC sample was activated with 40 wt% HNO 3 solution at 60 C accompanied by magnetic stirring (600 rpm) for 2 h. Aer the acid activation, the sample was washed with de-ionized water until neutral and dried at 110 C for 6 h. The obtained sample was then calcinated at 700 C for 2 h with the protection of Ar ow, named as ASC.

Preparation of Cu x O loaded catalysts
A certain amount of copper nitrate trihydrate (analytical purity) was measured and fully dissolved in 20 mL of deionized water as the precursor solution. Excessive impregnation was employed to load the active component by adding 5 g of activated semi-coke into the precursor solution. Aer ultrasonic treatment (60 Hz) for 1 h, the sample was dried for 12 h at 110 C, and calcinated at 350 C for 2 h with the protection of Ar, denoted as CuX, in which X presents the molar concentration of the copper nitrate in precursor solution (0.3, 0.6, 1.0, 1.5 and 2.0 mol L À1 ).
The calcination temperature was varied in the range of 250-450 C, and the samples were denoted as Cu1.0-Y, in which Y stands for the calcination temperature (250, 300, 400, and 450 C).
The calcination atmosphere was regulated as well, including pure Ar and the Ar ow containing 10% O 2 .

Characterization methods
The loading amount of metal oxides supporting on the sorbents was analyzed by inductive coupled plasma emission spectrometer (ICP, AFS-2000). The surface morphologies and microstructure of the samples were observed by eld emission scanning electron microscope (FESEM, Zeiss supra 55) installed with an EDS system, operating at 10 kV. X-ray diffraction (XRD) analysis was investigated by X-ray powder diffractometer (Rigaku Dmax-2500 diffractometer using Cu Ka radiation) to understand the crystal phase structure of the samples. Nitrogen adsorption-desorption isotherms were measured on a Micromeritics ASAP2020 instrument. The Brunauer-Emmett-Teller   (BET) surface area measurements were evaluated from the adsorption data. X-ray photoelectron spectroscopy (XPS) analysis was carried out on AXIS ULTRA (Kratos, Japan) with the Al Ka radiation for the surface binding and elemental speciation. Further nanoscale morphology characterization of the sorbents was performed by high resolution transmission electron microscopy (HRTEM, Tecnai F20) installed with EDS system, operating at 200 kV. TG-DSC analysis of the precursor and other samples was carried out under N 2 ow from room temperature to 800 C (heat rate: 10 C min À1 ). Moreover, hydrogen temperature programmed reduction (H 2 -TPR) and NH 3 temperature programmed reduction (NH 3 -TPD) experiments were separately performed on Micromeritics AutoChem 2920 equipment with these conditions: 10 vol% H 2 /Ar and 10% NH 3 /He ow, 10 C min À1 . The thermal gravity-differential scanning calorimetry (TG-DSC) performance of samples were recorded with a Simultaneous Thermal Analyzer (LABSYS EVO, SETARAM).

Activity test
The activity test was carried out on a self-made equipment. The de-NO x activity measurements were performed in the range of 90-210 C in a xed-bed continuous ow quartz reactor (with an inner diameter of 25 mm and equipped with a thermocouple). The ow rate of the reaction path was 200 mL min À1 with the space velocity (GHSV) of 10 000 h À1 . The simulated ue gas contained 500 ppm NO, 500 ppm NH 3 , 3% O 2 and N 2 as the equilibrium gas. The inlet NO concentration was collected ahead of each measurement. Aer the reaction system reached a stable state for 30 min, the NO X concentration in the outlet gas was measured by a NO/NO 2 ue gas analyzer (Testo Pro350). By comparison of the NO X concentration in the inlet and outlet gas, the NO conversion rate is calculated according to the following formula.

Chemical composition and textural properties of CuX catalysts
Cu x O was loaded onto the activated semi-coke (ASC) by impregnation method with Cu(NO 3 ) 2 . By varying the concentration of the Cu precursor from 0.3 to 2.0 mol L À1 , the loading amount was manipulated to obtain CuX catalysts. The XRD patterns in Fig. 1(a) illustrate the chemical composition of as-prepared catalysts. By activation with nitric acid, CaCO 3 in the original raw material SC was successfully removed, giving rise to the production of pores to accommodate the subsequent loading of active components. With the increase of loading amount, the diffraction proles of Cu x O, including CuO and Cu 2 O, began to emerge and the intensities increased gradually. The peaks assigned to Cu is also observed and strengthened with the rise in loading amount. The XRD patterns in Fig. 1(a) not only demonstrate the successful loading of Cu-containing active sites on ASC, but also exhibit a multi-valence state of Cu, which is commonly regarded to be convenient for the transfer of electrons during the redox cycle in SCR reactions. For clarity, the actual loading amount of Cu with respect to the Cu precursor concentration is quantied with ICP-AES, as depicted in Fig. 1(b). It can be seen that by increasing the precursor concentration, the loading amount increases as well, presenting a nearly linear increase. When the precursor concentration is higher than 1.5 mol L À1 , the growth rate becomes slow. At a concentration of 2.0 mol L À1 , the loading amount (8.19%) only exhibits a slight increase compared to that of Cu1.5 (7.75%), which indicates that the loading process has approached saturation.   The morphologies of CuX catalysts are shown in Fig. 2. As illustrated in Fig. 2(a), the surface of the sample ASC presents a porous structure, which can provide high surface area and is suitable for the subsequent loading of active composites. When the Cu precursor concentration is 0.3 mol L À1 , large Cu x O sheets with a size of $10 mm are observed. With the rising of Cu precursor concentration, the sheets begin to splinter into small pieces (Fig. 2(c)) and then form rod morphologies (Fig. 2(d-f)). At a concentration of 2.0 mol L À1 , the rods accumulate and the surface of ASC is completely covered, which coincides with the variation of loading amount exhibited in Fig. 1(b). In addition, the EDS characterization results show that the element distribution in the catalyst surface is very uniform (Fig. S1 †).
The textural properties of as-prepared catalysts are illustrated in Table 1. The pore size distributions and N 2 adsorption isotherms are shown in Fig. S2. † The treatment with HNO 3 greatly improved the specic surface area from 27 m 2 g À1 to 357 m 2 g À1 , which can expose more effective surface. Aer loading Cu x O, the surface areas of catalysts present an initial increase and then a decreasing trend in the range of 260-290 m 2 g À1 , among which Cu1.0 achieves the largest surface area (289.5 m 2 g À1 ). Meanwhile, the pore diameters of all CuX samples decreases to $2 nm. Therefore, the formation of Cu x O can inuence the textural properties, which is also consistent with the morphology analyses. Nevertheless, overdose of active components usually causes obvious accumulation and block the pores. Hence, the precursor concentration has a great inuence on the catalytic activity.

Morphologies and SCR activities of CuX catalysts
It has been well demonstrated that the performances of catalysts are affected by several different factors, such as loading amount, textural properties, and chemical composition. 5,23 Fig. 3 reveals the SCR activities of samples at a low temperature range (90-210 C). As a comparison, we can nd that the unloaded ASC shows only about 20% catalytic conversion in the 90-210 C window (Fig. S3 †).
It can be observed from Fig. 3 that the activities of asprepared samples increase along with the rising temperature. Taking the loading amount into account, the catalysts present an initial increase and then decreasing tendency, in which sample Cu1.0 has the highest efficiency in the whole conducted range. At 150 C, sample Cu1.0 presents about 90% NO x removal efficiency and achieves 100% efficiency at 180-210 C. Furthermore, at a higher temperature range (180 and 210 C), the advantages that originated from the high loading amount are reduced. Combined with the characterization of textural properties in Table 1 and the morphologies in Fig. 2, we proposed that when the precursor concentration is lower than 1.0 mol L À1 , the main limitation is the absence of abundant active sites, but a higher loading amount may result in the loss of surface area. Therefore, we choose Cu1.0 as the best load condition for the next exploration. The N 2 selectivity of these catalysts can be seen in Fig. S4, † which indicates high values close to 100%.

Catalysts calcinated at different atmosphere
Nevertheless, the mechanism for the superior activities of the catalysts is still ambiguous, especially taking the valence distribution into account. It should be noted that due to the carbonaceous substrate, the abundant oxygen functional groups on the surface of ASC may react with carbon to generate CO during calcination with Ar, which can cause partial reduction of CuO. Herein, the reduction atmosphere during calcination was adjusted to further detect the formation process of multi-valence Cu x O. The samples were treated with inert (Ar) and limited oxygen (10%) atmosphere at a heating rate of 10 C min À1 .
The catalytic activities of the sample calcinated at 350 C were compared as shown in Fig. 4(a). Activities of the sample treated in Ar atmosphere is more than the one treated with O 2 , particularly at a temperature lower than 150 C. The XRD patterns were thus examined and are illustrated in Fig. 4(b). For the sample treated with protection of Ar, the main phases of Cu x O are Cu 2 O, CuO, and even elemental Cu, resulted from the reduction by C aer decomposition of Cu(NO 3 ) 2 . However,  when limited oxygen (10%) was introduced (Ar as the balancing gas), only CuO species can be found. Therefore, it can be deduced that the existence of O 2 can effectively restrain the appearance of high content of Cu + and Cu, which is not bene-cial for the production of multi-valence Cu. From the morphologies demonstrated in Fig. 4(c), we can observe that with Ar atmosphere, the CuO grown on the surface of ASC mainly presents a sheet morphology with a size of 20-50 mm. When O 2 is involved, the Cu x O presents occulent morphology, resulting from the erosion reaction by O 2 , indicating the transformation of Cu + and Cu 0 to high valence Cu 2+ . Combined with the de-NO x performance in Fig. 4(a) and the XRD patterns in Fig. 4(b), we can further draw the conclusion that the appearance of multi-valence Cu is favorable for low temperature SCR, owing to the synergistic effect of CuO/Cu 2 O. 18 Considering that the precursors of samples with different loads are consistent, we choose the samples with better performance to study the thermal stability of precursors.
Thermal analyses of sample Cu1.0 with Ar were performed to probe the production process of multi-valence Cu and the results are shown in Fig. 5. The weight loss is slow due to the inert atmosphere. The initial weight loss stage (to 2.1%) takes place at a temperature of $100 C, resulting from the evaporation of water from the sample. Then, a rapid loss of weight observed below 200 C is usually ascribed to the formation of Cu 2 (OH) 3 NO 3 , originating from Cu(NO 3 ) 2 $3H 2 O. The decomposition of Cu 2 (OH) 3 NO 3 over 200 C results in the mixture of CuO and Cu 2 O. On further increasing the temperature, the weight loss rate becomes slow again, and the wide endothermic peak centered around 330 C is assigned to the transformation of Cu 2 O to Cu. 24 Herein, the decomposition reactions of Cu(NO 3 ) 2 $3H 2 O on ASC can be described as Cu(NO 3 ) 2 $5H 2 O / Cu 2 (OH) 3 Another important proposal that should be also noted is the properties of the carbonaceous support, which not only provides a high surface area to load the active sites, but also affects the loading process of the metal oxides since the carbon element can reduce the high valence metal ion, and oxygen functional groups usually display synergistic effect accompanied with the active sites. 18,21,22,25

Comparison of properties before and aer SCR Ar protection
It is widely demonstrated that Cu 2+ /Cu + pairs are the catalytic active species for the reduction of NO X , and the distribution of Cu 2+ and Cu + is a key factor that inuences the performance of catalysts. 26 To further investigate the roles of CuO and Cu 2 O, characterizations for sample Cu1.0 before and aer the NH 3 -SCR were carried out, and the results are shown in Fig. 6. As shown in Fig. 6(a), the XPS spectra of sample Cu1.0 before and aer SCR have Cu 2p peaks. In detail, Fig. 6(b) exhibits the spectra of deconvoluted Cu 2p peaks. The peak located at 934.4 eV is assigned to the binding energy of Cu 2+ and the one centered around 932.6 eV is attributed to Cu + . 27 It can be seen that the intensity of Cu 2+ increases aer the SCR reactions, and meanwhile, the peak presenting Cu + is weaker, indicating the transformation of Cu + to Cu 2+ during SCR reactions. In Fig. 6(c), one can observe a stronger peak of O 1s aer SCR reactions, which conrms the existence of higher valence of Cu x O. The XRD patterns in Fig. 6(d) furthermore evidences that the intensity of peaks ascribed to CuO increases and Cu 2 O decreases aer SCR    18 Herein, it is of great importance to regulate the distribution of Cu 2+ and Cu + with an easy and convenient approach.

Properties and SCR performance of Cu1.0-Y catalysts
On the basis of the above investigation above, we found that the decomposition temperature of Cu(NO 3 ) 2 $3H 2 O greatly inuences the chemical construction of Cu x O, in which the activated semi-coke not only plays the role of a support, but also provides a reductive atmosphere. 29 Hence, the following study focuses on the distribution of Cu with different valences by simply changing the calcination temperature. The as-prepared catalysts were denoted as Cu1.0-Y, in which Y presents the calcination temperature. Fig. 7 reveals the variations of the morphologies and XRD patterns. When the calcination temperature was 250 C, the sheet-structure of Cu x O can be seen on the surface of activated semi-coke. The XRD patterns in Fig. 7(f) shows the main chemical pattern of Cu x O is CuO at lower temperatures. With increasing calcination temperature, the morphologies gradually transfer into needle-like cluster structures, of which the XRD intensity of CuO decreases and the main components turn into Cu 2 O and even Cu can be noticed. This transformation agrees well with the TG-DSC results in Fig. 5. The de-NO X performances of the above samples were conducted to further verify the relationships between the distribution of Cu valence and the reaction mechanisms ( Fig. 8 and S6 †). With the increase in calcination temperature, activities of catalysts reveal an initial increase and then a decreasing trend, and the one treated at 350 C (Cu1.0-350) shows the best performance, which implies an optimal proportion of Cu 2+ and Cu + . Hence, several different characterization methods are employed to systematically discuss the inuence of Cu valence distribution.
X-ray photoelectron spectroscopy (XPS) was used to probe the surface states and atomic concentration of Cu, as illustrated in Fig. 9(a). The XPS spectrum of Cu 2p was deconvoluted into several peaks by tting the Gaussian peaks. The peaks detected at 934.3 eV and 932.6 eV can be assigned to Cu 2+ and Cu + respectively, evidencing the co-existence of multi-valence Cu ions. The peak area ratios of Cu 2+ and Cu + are listed in Table 2. When the calcination temperature was 250 C, the peak was totally composed of Cu 2+ . By increasing the calcination temperature, the value of Cu 2+ /Cu + remarkably decreases from 6.9 (300 C) to 0.8 (450 C), further evidencing that the composition of Cu ions can be manipulated via the calcination process. Meanwhile, the collaboration of Cu 2+ and Cu + facilitates the oxidation-reduction process during NH 3 -SCR reactions. 17 The synergistic effect of Cu 2+ and Cu + can also result in changes in the active oxygen. The O 1s peaks are displayed in Fig. 9(b). The peak around 530.37 eV corresponds to lattice oxygen (O a ), whereas the one at 531.96 eV is attributed to the surface adsorbed oxygen species O 2 À or O À (O b ). The peak that originates from the hydroxyl-like groups, including chemisorbed water, is denoted as O g . It has been well demonstrated that O b species can exibly adsorb and release oxygen and transfer the lattice oxygen atoms to the surface of the catalysts. 18 Thus, a higher relative concentration of O b is benecial to the formation of adsorbed NO 2 and favors the fast SCR reaction. Meanwhile, with the increasing temperature, the binding energies become higher, demonstrating a better transfer capability of lattice oxygen atoms to the surface of the catalysts. 21,30 The relative content of O a and O b of each sample are compared in Table 2. It can be seen that O b is the dominant of the O species, and the ratio of O b increases along with the increased Cu + content, consistent with the study conducted by Wu et al. 21 Combined with the value of Cu 2+ /Cu + , it is found that a certain ratio of Cu + is benecial to the SCR activity, and the oxygen vacancies are closely related to the Cu 2+ /Cu + circulation. Consequently, surface active oxygen species with better mobility account for one of the key factors that permits a high SCR activity. 31 Nevertheless, the SCR activities are not completely in conformity with the O b variation, which implies acidic sites that inuence the adsorption of NH 3 might also play a signicant role during NH 3 -SCR. 32 Fig. 11 In situ DRIFTS spectra of the catalyst pre-treated in different flowing gas at 180 C for various times: (a) 500 ppm NO/N 2 + 3% O 2 and (b) 500 ppm NH 3 /N 2 . Fig. 12 In situ DRIFTS spectra of the catalyst with different flowing gas at 180 C for various times: (a) pretreated in 500 ppm NH 3 /N 2 for 30 min followed by exposure to 500 ppm NO/N 2 + 3% O 2 ; (b) pretreated in 500 ppm NO/N 2 + 3% O 2 for 30 min followed by exposure to 500 ppm NH 3 /N 2 .

Redox properties and surface acidity analyses
The transformation between Cu 2+ and Cu + is accompanied by the redox reactions during NH 3 -SCR of NO X . Herein, H 2 -Temperature-Programmed-Reduction (H 2 -TPR) can be used to assess the reducibility of the catalysts, as shown in Fig. 10(a). Two distinct peaks can be observed, including the one located around 200 C and another at 260 C. According to the XPS and XRD results, we assume the higher temperature peak can be assigned to the transformation of isolated Cu 2+ to Cu + , and the one at a lower temperature is attributed to the reduction of Cu + to Cu 0 . Comparatively, the reduction temperature in the process is greatly decreased since the active components are in a high dispersion state instead of being in a crystalline state. 33 Particularly, the Cu 2+ / Cu + reduction peak of the sample calcinated at 350 C is much lower than other samples, indicating its better reduction properties. Hence, the better NH 3 -SCR properties of sample Cu1.0-350 might have originated from the optimized distribution of Cu 2+ and Cu + . In other words, the synergetic effect of Cu 2+ and Cu + can be achieved and optimized by adjusting the calcination temperature.
Another factor that triggers the SCR reactions is the adsorption of NH 3 on acidic sites. 34 Herein, NH 3 -TPD technology is used to determine the surface acidity of the catalysts. Trends of NH 3 -TPD patterns along with the variation of calcination temperature are shown in Fig. 10(b). Two obvious sections can be observed in the desorption temperature range for all samples. One section corresponded to the desorption of weakly bound NH 3 around 100 C, which is derived from the breakage of fairly weak hydrogen bonds between adsorbed NH 3 and surface acidic groups. 35 The other part in a wide range of 600-900 C is attributed to the desorption of NH 3 from the strong acid sites, specically Lewis acid sites. One small peak around 385 C, owing to the existence of Brønsted acid sites can be found for the sample calcinated at 250 and 350 C. It has been demonstrated that Brønsted and Lewis acid sites on catalysts can greatly affect the adsorption states of NH 3 and the reaction pathway. It can be seen that the sample treated at 350 C has abundant acidic sites. Thus, we propose that the NH 3 -SCR activities of as-prepared catalysts should take the integrated effect of oxygen vacancies and acidic sites into consideration. 36 Herein, it is of great importance to systematically conduct the NH 3 -SCR process for better understanding.

In situ DRIFTS analyses
In order to further investigate the reaction mechanism for the high activity by catalyst Cu1.0-350 at a low temperature, in situ DRIFTS experiments were performed at 180 C to detect the surface adsorption reaction of NH 3 and NO + O 2 .
The adsorptions of NH 3 were conducted with the inlet gas of 500 ppm NH 3 /N 2 , which occurred aer pretreatment in an N 2 atmosphere at 300 C and background collection at 180 C. As shown in Fig. 11(a), the NH 3 adsorption on Cu x O/ASC was performed. As shown in Fig. 11(a), the NH 3 adsorption on the catalyst resulted in the formation of several peaks originating from the NH 4 + bound to Brønsted acid sites (1681-1900 cm À1 , 1452 cm À1 ), NH 3 coordinated on Lewis acid sites (1630-1070 cm À1 ), and weakly adsorbed NH 3 (970 and 927 cm À1 ). 21,37 In detail, the peak at 1681 and 1452 cm À1 present symmetric and asymmetric bending vibrations of NH 4 + species aer reaction with Brønsted acid sites. 25,38 Peaks at 1627 and 1241 reveal the existence of NH 3 coordinated to the Lewis acid sites. A strong peak is centered around 1525 cm À1 , which can be assigned to the amide species (-NH 2 ) and intermediates from the partial oxidation of NH 3 . 39 Peaks at 970 and 927 cm À1 can be assigned to either gas phase or weakly adsorbed NH 3 . 40 It has been demonstrated from NH 3 -TPD that Lewis acid sites are in the majority among acid sites, which is consistent with the in situ DRIFTS analyses. The adsorptions of NO and O 2 were conducted with the inlet gas of 500 ppm NO/N 2 + 3% O 2 , which occurred aer pretreatment in an N 2 atmosphere at 300 C and background collection at 180 C. The absorption spectrum is shown in Fig. 11(b). Aer NO + O 2 was purged in, the peak at 973 cm À1 assigned to C-O vibration in primary C-OH appears, owing to the ASC support. 25 Due to gaseous or weakly adsorbed NO, one peak at 1882 cm À1 was found. The peak surrounding 1697 cm À1 reveals the weakly adsorbed NO 2 molecules. 41 The peak at 1523 cm À1 reveals the appearance of bidentate nitrate, and the one at 1427 cm À1 can be attributed to monodentate nitrate. The band observed at 1168 cm À1 can be assigned to cis-N 2 O 2 2À , suggesting the improved oxidation of NO to NO 2 . 42 In situ DRIFTS transient reactions were further performed. First, NH 3 /N 2 ow was purged for 30 min and then shut down. Aerwards, NO + O 2 /N 2 was switched on over the NH 3 -adsorbed samples to further probe the reaction mechanisms. The spectra are shown in Fig. 12 The order of purged ow was reversed to further evaluate the reactivity of the adsorbed nitrogen oxide species. The catalysts were rst treated with NO + O 2 for 30 min, and then NH 3 was introduced for 30 min to observe the reactions among NO and the NH 3 -originated intermediates. The results are shown in Fig. 12(b). The pretreatment with NO + O 2 results in the form of adsorbed NO 2 (1697 cm À1 ), nitrate (1523 and 1427 cm À1 ), and nitrite (1168 cm À1 ) on the catalyst surface. Aer NH 3 is switched in, most peaks of the NO-related species vanish immediately, suggesting that adsorbed NO x species on the catalyst surface participate in the SCR reaction. Meanwhile, new peaks at 1621, 1357, 1064, and 943 cm À1 can be observed, which represent the NH 3 coordinated on Lewis acid sites, and weakly adsorbed NH 3 , respectively. Therefore, one can conclude that the adsorbed NO 2 , bridge nitrates, and nitrite are quite active during the SCR reactions of the prepared catalysts.

Catalytic mechanism discussion
The chemical mechanism and critical role species of the catalytic process were comprehensively explained by in situ DRIFTS and active adsorption site characterization of the bestperforming catalyst Cu1.0-350. In combination with the characterizations by XPS and H 2 -TPR, one can observe that the transformation between Cu 2+ and Cu + can induce the variation of O b /(O a +O b ). The existence of metal ions can also affect the amount of acidic sites. In accordance with the in situ DRIFTS results, it is found that both the oxygen species and the acidic sites on the surface of catalysts play a signicant role during the low temperature NH 3 -SCR reactions. 43 The high content of O b and mobility of O a guarantee the adsorption and activation site for NO into NO 2 , which promotes the proceeding of "fast SCR" reaction (NO + NO 2 + 2NH 3 / 2N 2 + 3H 2 O). 44 Consequently, the SCR reactions follow both the Langmuir-Hinshelwood (reaction between adsorbed NH 3 and adsorbed NO x ) and Eley-Rideal (reaction between adsorbed NH 3 and gaseous NO).

Conclusions
In summary, the reduction feature of semi-coke provides a new approach for regulating the valence of Cu. Multi-valence Cu x O was loaded onto activated semi-coke through a simple impregnation-calcination method. By varying the concentration of the Cu(NO 3 ) 2 precursor, calcination atmosphere, and calcination temperature, the CuOjCu 2 O/ASC heterostructure catalysts showed above 80% NH 3 -SCR activity in a temperature range of 100-200 C (Cu1.0-350). The synergistic catalysis mechanism between Cu 2 O and CuO species was investigated based on the dual function of oxygen vacancies and acidic sites. The SCR reactions simultaneously follow both L-H and E-R mechanisms.

Author contributions
Mei Zhang and Bo Peng conceived the idea and designed the experiments; Bo Peng and Zhao Meng conducted the experiments; Bo Peng analyzed the data and wrote the manuscript; Mei Zhang and Zheng Yan provided guidance on the manuscript preparation, modied the manuscript, and supervised the whole work; Shuoyang Liang and Hao Wang discussed and modied the manuscript.

Conflicts of interest
There are no conicts to declare.