Selective Catalytic Reduction of NO by NH₃ over Mn–Cu Oxide Catalysts Supported by Highly Porous Silica Gel Powder: Comparative Investigation of Six Different Preparation Methods

Abstract

In this study, Mn-based catalysts supported by highly porous silica gel powder (SSA up to 470 m²·g⁻¹ and total pore volume up to 0.8 cm³·g⁻¹) were prepared by six different methods in liquid solutions (electroless metal deposition, stepwise addition of a reducing agent, wet impregnation, incipient wetness impregnation, urea hydrolysis, and ammonia evaporation) and tested for selective catalytic reduction of NO_x with ammonia (NH₃-SCR de-NO_x). Prior to the activity test all the catalysts prepared were characterized by ICP-OES, SEM, EDX mapping, XPS, XRD and N₂ adsorption techniques to provide the comprehensive information about their composition and morphology, investigate the dispersion of active components on the carrier surface, identify the chemical forms and structural properties of the catalytically active species of the catalysts prepared. The results revealed that all the methods applied for preparation of SCR de-NO_x catalysts can ensure the uniform distribution of Mn species on the carrier surface, however as it is typical for preparation techniques in a liquid phase the significant reduction in SSA and pore volume along with increasing the loading was observed. Considering both the physicochemical properties and the catalytic performance of the catalysts the least effective preparation method was shown to be ammonia evaporation, while the most attractive techniques are incipient wetness impregnation and electroless metal deposition.

1. Introduction

Nitrogen oxides (NO_x) are harmful and dangerous air pollutants for both the environment and human health. These compounds are generated as a result of fuel combustion and cause serious environmental problems, such as eutrophication, photochemical smog and acid rains [1,2]. The latter can lead to secondary negative changes in ecosystems such as the acidification of soil, rivers, and lakes. Atmospheric air with NO₂ concentrations above the limited value can irritate the human respiratory system and with the short-term exposure may increase the symptoms of respiratory diseases. The long-term exposure of the elevated concentrations of NO₂ in the ambient air can cause asthma and increased sensitivity to respiratory infections [3].

Catalytic removal of NO_x from the flue gas, known as selective catalytic reduction with ammonia (NH₃-SCR de-NO_x), is considered to be the most efficient and attractive technique to reduce NO_x emissions [4,5]. This technique provides the highest NO_x conversion efficiency and does not cause secondary pollution or transfer of pollutants from one medium (air) to another (adsorbent/absorbent), since as a result of the catalytic reaction new harmless substances (N₂ and H₂O) are generated.

Considering that the energy production and distribution sector is a source of NO_x emissions, significant interest has been focused on the development of low-temperature catalysts which can provide effective NO_x conversion at the temperature range of 100–300 °C [6,7]. This is highly important for thermal power plants, where the tail-end SCR configuration is applied. In this case, the SCR reactor is placed downstream of all air pollution control equipment installed on a plant (i.e., particulate control and desulfurization equipment) [8].

Mn-based catalysts are known as the low-temperature catalysts for NO_x conversion by ammonia [9,10,11,12,13,14,15,16,17,18,19,20,21]. Additionally, it is well-known that bimetallic and polymetallic oxide catalysts are more effective compared to the single-metal oxide catalysts [13,15,22,23,24]. Among the different transition metals, Cu was found to be an effective and economically feasible doping element of Mn-based SCR de-NO_x catalysts [15,25,26]. In this study Mn–Cu active species were deposited on a highly porous support—silica gel 60 (powder with the size of 0.063–0.1 mm), possessing a high specific surface area of 470 m²·g⁻¹ and high pore volume of 0.8 cm³·g⁻¹, and the catalysts prepared were investigated for NH₃-SCR de-NO_x. The use of a porous support is highly important, as such a support provide the required surface area for a proper distribution of active species to be deposited. Higher NO_x conversion efficiencies can be achieved via a larger distribution of the catalytically active species, as the agglomeration causes the decrease in the number of active sites available for the catalytic reaction. Another way to provide the required surface area for the deposition of active species is via the use of nanoparticles as a support. However, the application of nanoparticles, in the fluidized bed catalysts in particular, is highly problematic due to the significant agglomeration. The catalysts being investigated were prepared by six different methods: electroless metal deposition, stepwise addition of a reducing agent, wet impregnation, incipient wetness impregnation, urea hydrolysis, and ammonia evaporation. The aim of the study was to compare the abovementioned preparation methods by conducting the comprehensive investigation of both the physicochemical properties and the SCR performance of the catalysts prepared. Silica gel itself is not a favorable carrier for NH₃-SCR due to its low acidity, however it is suitable for the comparison of different techniques for the deposition of the catalytically active species on a highly porous support.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. ICP-OES, SEM, EDX Elemental Mapping

Scanning electron microscopy (SEM) revealed that the silica gel powder is not stable in liquid solutions as it broke up into smaller pieces during the deposition process. The exception was the incipient wetness impregnation method, where the limited volume (equal to the total pore volume of the carrier) of the precursor solution was used. In all the catalysts prepared catalytically active species can be clearly observed from the SEM images (see Figure 1) excluding the sample prepared by the incipient wetness impregnation (IWI), since in the latter the active Mn–Cu species were mostly deposited inside the pores. The Mn_89.9Cu_10.1/SiO₂–AE catalyst was characterized by the obviously increased agglomeration, which was also proved by N₂ adsorption method demonstrated the significant reduction in the specific surface area and pore volume due to the pore clogging (see Section 2.1.2. Textural Properties). All the catalysts prepared were characterized by the same composition (ca. 90% of Mn and ca. 10% of Cu), however the loadings provided by the active species deposited were different for the different deposition methods applied (see

Table 1. Composition of the active catalytic layer deposited on the silica gel powder by different chemical methods in a liquid phase and the loading provided by the active components (based on ICP-OES analysis).

Active Component	Share in the Catalytic Layer, wt%	Loading in the Supported Catalyst, wt%
Mn90.8Cu9.2/SiO2–EMD
Mn	90.8	3.5
Cu	9.2	0.4
Mn90.6Cu9.4/SiO2–SARA
Mn	90.6	4.8
Cu	9.4	0.5
Mn90.9Cu9.1/SiO2–WI
Mn	90.9	8.7
Cu	9.1	0.9
Mn91.1Cu8.9/SiO2–IWI
Mn	91.1	1.1
Cu	8.9	0.1
Mn91.1Cu8.9/SiO2–UH
Mn	91.1	11.1
Cu	8.9	1.1
Mn89.9Cu10.1/SiO2–AE
Mn	89.9	13.3
Cu	10.1	1.5

). According to the ICP-OES results, the similar and the highest loadings were observed for the Mn_89.9Cu_10.1/SiO₂–AE and the Mn_91.1Cu_8.9/SiO₂–UH catalysts, however the urea hydrolysis method led to preparation of the catalyst characterized by better textural properties, in particular the specific surface area, total pore volume and average pore size, which all are important for the effective NO_x catalytic conversion.

The dispersion of the catalytically active species (Mn and Cu) on the surface of the silica gel powder is presented in Figure 2. As it can be seen, the main active component (Mn) was well-distributed throughout the surface, while some nonuniformity can be observed for the Cu species. Copper was used as a doping element, and it was deposited on the bare silica gel powder prior to the manganese species. The reason of slightly nonuniform distribution of Cu remained unclear and needs further understanding.

2.1.2. Textural Properties

Textural properties of the catalysts prepared are presented in

Table 2. Textural properties and the total metal loading of the Mn–Cu oxide catalyst supported by the silica gel powder (based on the N₂ adsorption analysis).

Sample	Textural Properties			Total (Mn + Cu) Loading, %
	BET SSA, m2·g−1	Pore Volume, cm3·g−1	Average Pore Size, nm
Silica gel powder	474	0.8	6.5	0
Mn91.1Cu8.9/SiO2–IWI	251	0.62	8.6	1.2
Mn90.8Cu9.2/SiO2–EMD	196	0.63	11.4	3.9
Mn90.6Cu9.4/SiO2–SARA	61	0.44	35.3	5.3
Mn91.1Cu8.9/SiO2–UH	50	0.27	22	12.2
Mn90.9Cu9.1/SiO2–WI	32	0.22	43.6	9.6
Mn89.9Cu10.1/SiO2–AE	0.9	0.01	82.6	14.8

. As it mentioned before, different preparation methods applied resulted in different loadings of the active species deposited on the silica gel powder. This in turn led to different textural properties since such properties as SSA, pore volume and pore size strongly depend on loading. However, it was observed that the method applied can also impact the textural properties of the catalysts. As it can be seen from the Table 2, BET specific surface area and pore volume are in high correlation with Mn and Cu loading (correlation coefficient was equal to −0.84 and −0.96 for SSA and pore volume, respectively). At first, this can be explained by the fact that deposited materials fill the pores and reduce the SSA. However, from the pore size distribution (see Figure 4) it can be supposed that the textural properties of all the catalysts prepared, except for the Mn_91.1Cu_8.9/SiO₂–IWI and Mn_90.8Cu_9.2/SiO₂–EMD catalysts, are mainly provided by the active species deposited. In this case the decrease in SSA and pore volume along with increase in loading can by assigned to agglomeration: the higher the loading, the higher the agglomeration of active species. However, the catalyst prepared by the urea hydrolysis method (Mn_91.1Cu_8.9/SiO₂–UH) was out of this trend, since with loading 1.3-times higher than that of the Mn_90.9Cu_9.1/SiO₂–WI catalyst it showed 1.6-times higher SSA. This can be explained by both the pore size and the pore volume of the Mn_91.1Cu_8.9/SiO₂–UH catalyst, which were 2-times and 1.2-times smaller, respectively, than that of the Mn_90.9Cu_9.1/SiO₂–WI catalyst. The results revealed that based on the textural properties wet impregnation method is definitely not the optimal method for preparation of catalysts. The only obvious advantage of this method is its simplicity. However, to provide the final conclusions, the other parameters such as structural and chemical properties, as well as catalytic performance were further investigated.

The catalyst prepared by the ammonia evaporation method (Mn_89.9Cu_10.1/SiO₂–AE) was characterized by extremely low SSA (less than 1 m²·g⁻¹), pore volume and large average pore size. Although the loading of this catalyst was the highest among all the catalysts prepared, it was only 1.2-times higher compared to the Mn_91.1Cu_8.9/SiO₂–UH catalyst being characterized by 50-times higher SSA and 27-times higher pore volume. Therefore, the ammonia evaporation method cannot be considered as the optimal preparation technique as well. The extremely low SSA and pore volume of the catalyst prepared by this method can be explained by clogging the pores of the carrier and significant agglomeration. The enhanced agglomeration was also shown by SEM images (see Figure 1). Li et al. [27] showed the opposite results, where the catalyst prepared by the ammonia evaporation method possessed even higher SSA compared to the support, and exhibited better textural properties than the catalysts prepared by impregnation and deposition precipitation techniques. At the same time, the predominance of the ammonia evaporation technique over the urea hydrolysis in terms of dispersion of active species on the carrier surface can be found in [28]. However, it is difficult to compare the results obtained in the present work with that existing in the literature as there are too many differences, such as different active species to be deposited and their precursors, different carriers used for the deposition, and different additional chemical reagents used during the deposition process. All these parameters can affect the textural properties of the catalysts prepared.

Considering the textural properties, the catalyst prepared by urea hydrolysis (Mn_91.1Cu_8.9/SiO₂–UH) can be considered as more attractive than the catalyst prepared by stepwise addition of a reducing agent (Mn_90.6Cu_9.4/SiO₂–SARA) since it has 2.3-times higher loading, however its SSA is only 1.2-times lower. This can be explained by the average pore size of the Mn_91.1Cu_8.9/SiO₂–UH catalyst, which is much lower compared to that of the Mn_90.6Cu_9.4/SiO₂–SARA catalyst (see Table 2). However, to provide final conclusions on that which catalyst is more suitable for SCR the other properties and especially catalytic performance should be investigated.

According to the average pore size all the catalysts prepared can be assigned to mesoporous materials as the silica gel powder itself, except for the Mn_89.9Cu_10.1/SiO₂–AE catalyst, which was related to macroporous materials based on the IUPAC classification [29]. The adsorption/desorption isotherms of the catalysts prepared are depicted in Figure 3. According to the IUPAC classification the adsorption/desorption isotherms of the Mn_91.1Cu_8.9/SiO₂–IWI and the Mn_90.8Cu_9.2/SiO₂–EMD catalysts can be assigned to the type V with the hysteresis loop of H1 type, which is typical for mesoporous materials possessing well-defined cylindrical-like pores. The adsorption/desorption isotherms of the other catalysts more likely can be attributed to the type II, although the desorption isotherm does not coincide with the adsorption isotherm. The type II isotherms are typical for adsorption on macroporous materials with strong adsorbent-adsorbate interactions [30].

Although according to the average pore size all the catalysts excluding the Mn_89.9Cu_10.1/SiO₂–AE were assigned to the mesoporous materials, the amount of macropores was significant for Mn_90.6Cu_9.4/SiO₂–SARA, Mn_90.9Cu_9.1/SiO₂–WI and Mn_91.1Cu_8.9/SiO₂–UH catalysts (Figure 4), which can explain the shape of the adsorption/desorption isotherms. The hysteresis loop of the type II isotherms can be defined as H3, meaning that the catalysts assigned to this isotherm type can be characterized by the slit-shaped pores [29,31]. From the Figure 4 it can be seen that the catalysts prepared by EMD and IWI methods have more or less similar pore size distribution to that of the silica gel powder. However, other catalysts are characterized by the significant changes in pore distribution after the deposition of active species and such catalysts possess much lower volume occupied by pores with the size lower than 25 nm, and huge amount of much larger pores. This is obviously related to the loading provided by the Mn and Cu species deposited, however the method applied also has the significant impact on the pore distribution. The pore size distribution data allow the assumption that pores of the Mn_90.6Cu_9.4/SiO₂–SARA, Mn_90.9Cu_9.1/SiO₂–WI Mn_91.1Cu_8.9/SiO₂–UH, and Mn_89.9Cu_10.1/SiO₂–AE catalysts are mainly the pores of the active species deposited on the carrier surface, rather than the pores of the carrier itself. The smaller pores mean higher SSA at the same total pore volume. Although the SSA of the Mn_90.6Cu_9.4/SiO₂–SARA catalyst is slightly higher than that of the Mn_91.1Cu_8.9/SiO₂–UH catalyst, this is only due to the lower loading and consequently higher total pore volume, since in the latter there is much higher volume occupied by the pores with the size lower than 25 nm (see Figure 4).

Therefore, the data of pore size distribution additionally demonstrated the predominance of the textural properties of the catalyst prepared by urea hydrolysis technique compared to that of catalysts prepared by stepwise addition of a reducing agent. It can also be clearly seen that the Mn_90.9Cu_9.1/SiO₂–WI catalyst possesses much higher volume occupied by the larger pores compared to the Mn_91.1Cu_8.9/SiO₂–UH catalyst, and therefore, although its loading is lower, it still possesses lower SSA. Thus, considering the textural properties, the urea hydrolysis technique is obviously more preferable compared to both wet impregnation and stepwise addition of a reducing agent. However, it was difficult to provide any reliable comparison between such preparation techniques as urea hydrolysis, electroless metal deposition and incipient wetness impregnation based on the textural properties due to the huge difference in loading.

2.1.3. XPS Analysis

Mn2p and O1s XPS spectra for all the Mn–Cu oxide catalysts supported by the silica gel powder are presented in Figure 5 and Figure 6. The oxidation state of Cu was not under consideration since this element was used as the doping element and its loading was quite low for precise identification in most of the catalysts. Based on the XPS results it was supposed that Mn in all the catalysts prepared exists in Mn³⁺ and Mn⁴⁺ oxidation states with the predominance of the latter (see Figure 5 and Figure 6 and Table 3).

Table 3. Binding energies and the corresponding oxidation states of Mn2p XPS spectra for the prepared Mn-based catalysts.

Catalyst	Mn2p3/2, eV Peak		${\mathrm{Mn}}^{4+}/{\displaystyle {{\displaystyle \sum }}_{n=0}^4}{\mathrm{Mn}}^{\mathrm{n}+}$	Reference
	Mn4+ (Mn4+ Satellite)	Mn3+
Mn91.1Cu8.9/SiO2–IWI	642.4, (643.8)	641.1	0.70	[32,33,34]
Mn90.8Cu9.2/SiO2–EMD	642.5, (644.3)	640.8	0.78	[32,33,34,35]
Mn90.6Cu9.4/SiO2–SARA	642.9, (644.7)	641.3	0.79	[33,34,36]
Mn91.1Cu8.9/SiO2–UH	642.7, (644.4)	641.2	0.69	[33,34]
Mn90.9Cu9.1/SiO2–WI	642.6, (644.5)	641.5	0.72	[33,34]
Mn89.9Cu10.1/SiO2–AE	642.4, (643.9)	641.1	0.81	[33,34]

Table 3. Binding energies and the corresponding oxidation states of Mn2p XPS spectra for the prepared Mn-based catalysts.

Catalyst	Mn2p3/2, eV Peak		${\mathrm{Mn}}^{4+}/{\displaystyle {{\displaystyle \sum }}_{n=0}^4}{\mathrm{Mn}}^{\mathrm{n}+}$	Reference
	Mn4+ (Mn4+ Satellite)	Mn3+
Mn91.1Cu8.9/SiO2–IWI	642.4, (643.8)	641.1	0.70	[32,33,34]
Mn90.8Cu9.2/SiO2–EMD	642.5, (644.3)	640.8	0.78	[32,33,34,35]
Mn90.6Cu9.4/SiO2–SARA	642.9, (644.7)	641.3	0.79	[33,34,36]
Mn91.1Cu8.9/SiO2–UH	642.7, (644.4)	641.2	0.69	[33,34]
Mn90.9Cu9.1/SiO2–WI	642.6, (644.5)	641.5	0.72	[33,34]
Mn89.9Cu10.1/SiO2–AE	642.4, (643.9)	641.1	0.81	[33,34]

As it can be found in the literature, the higher the oxidation state of Mn in catalysts, the higher NO conversion efficiency can be achieved, and the Mn⁴⁺ is considered to be most effective oxidation state of manganese among Mn²⁺, Mn³⁺, and Mn⁴⁺ [9,36]. Table 3 demonstrates that all the catalysts prepared possess relatively high ${\mathrm{Mn}}^{4+}/{\displaystyle \sum }_{\mathrm{n}=0}^4{\mathrm{Mn}}^{\mathrm{n}+}$ ratio (from 0.7 to 0.8). For instance, this value in [36] varies in the range of 0.23–0.42. Based on the XPS results the highest ${\mathrm{Mn}}^{4+}/{\displaystyle \sum }_{n=0}^4{\mathrm{Mn}}^{\mathrm{n}+}$ ratio was observed for the catalysts prepared by electroless metal deposition, stepwise addition of a reducing agent and ammonia evaporation techniques, however the difference observed between all the catalysts investigated was not significant.

Considering that the ${\mathrm{Mn}}^{4+}/{\displaystyle \sum }_{n=0}^4{\mathrm{Mn}}^{\mathrm{n}+}$ ratio is not the only parameter determining the NO_x conversion efficiency, both chemical and physical properties should be investigated to provide the proper and comprehensive explanation of the efficiency of the catalysts prepared.

Considering the O1s spectra (Figure 5 and Figure 6) and the survey XPS spectra (not shown) describing all the elements existing in the catalysts, oxygen was found as a part of silicon dioxide (carrier), adventitious carbon (formed as a result of catalysts being exposed to ambient air), and oxides of Mn and Cu existing as lattice oxide/hydrated or defective oxide/adsorbed water and organic oxygen. The existence of Cu hydroxides and MnOOH can be excluded because of the calcination conducted at the temperature of 350 °C for 3.5 h, since according to [37] all MnOOH species should be converted to MnO₂ at the temperature of 300 °C, while Cu(OH)₂ species would be transformed to CuO in the temperature range of 155–170 °C [38].

2.1.4. XRD Analysis

Based on the XRD measurements it was determined that the active Mn species in the catalysts prepared by stepwise addition of a reducing agent, wet impregnation, urea hydrolysis and ammonia evaporation are rather amorphous than crystalline, since no sharp Bragg peaks were observed from the XRD patterns (see Figure 7).

The characteristic MnO₂ peak was observed in the catalysts prepared by electroless metal deposition (Mn_90.8Cu_9.2/SiO₂–EMD) and incipient wetness impregnation (Mn_91.1Cu_8.9/SiO₂–IWI) methods. However, these peaks are quite broad, which can be related to stacking faults and other defects in a crystal structure. This means that even in these two catalysts there was no well-defined and organized crystal structure formed. Considering the Cu species, some crystalline phase was observed only for the Mn_90.9Cu_9.1/SiO₂–WI and the Mn_91.1Cu_8.9/SiO₂–UH catalysts, and again the observed peaks are quite broad meaning that there was no well-defined crystal structure even in these two mentioned catalysts. Therefore, generally all the Mn–Cu oxide catalysts prepared more likely can be assigned to the amorphous phase which is in agreement with the results of Tang et al. [39], where the unsupported MnO_x catalysts were prepared by the rheological phase reaction method, low temperature solid phase reaction method and co-precipitation method and calcined at the temperature of 350 °C. In the same article [39], Tang et al. demonstrated that the amorphous phase is more active in regard to NO_x conversion compared to the crystalline phase.

2.2. Catalytic Activity Test

Since the loading provided by the active species deposited on the silica gel powder was different for different catalysts, the performance of catalysts in NO_x conversion with ammonia was determined calculating both the absolute conversion efficiency obtained during the experiments and the conversion rate provided by 1 g of the active species deposited.

The catalyst prepared by the incipient wetness impregnation method (Mn_91.1Cu_8.9/SiO₂–IWI) characterized by the highest SSA and at the same time the lowest loading yielded the highest conversion rate related to 1 g of the active species deposited (Figure 8).

Followed by this catalyst the highest rate of NO_x conversion was provided by the Mn_90.8Cu_9.2/SiO₂–EMD and the Mn_90.6Cu_9.4/SiO₂–SARA catalysts, respectively, which can be correlated to the loading and SSA (see Table 2). The lower the loading, the lower the agglomeration, and consequently the higher the SSA. At the same time, at lower agglomeration and higher SSA, the higher NO_x conversion provided by 1 g of the active species deposited can be supposed (the correlation coefficient between the loading and the NO_x conversion provided by 1 g of the active species was −0.83). However, the SSA is not the only reason to explain the better NO_x conversion provided by the Mn_90.8Cu_9.2/SiO₂–EMD catalyst compared to the Mn_90.6Cu_9.4/SiO₂–SARA catalyst. Both the catalysts showed almost the same rate of NO_x reduction at the temperature range of 250–300 °C (see Figure 8), however the predominance of the former was obvious at lower temperatures. The Mn_90.8Cu_9.2/SiO₂–EMD catalyst was more effective than the Mn_90.6Cu_9.4/SiO₂–SARA at the temperatures of 100–200 °C even considering the absolute values of NO_x reduction (see Figure 9), however the loading of the latter was higher. Therefore, considering both the physical properties and the catalytic performance, the Mn_90.8Cu_9.2/SiO₂–EMD catalyst can be considered as more attractive one. It can be supposed that the electroless metal deposition method provides better textural properties, which can be the reason to explain the predominance of the Mn_90.8Cu_9.2/SiO₂–EMD catalyst over the Mn_90.6Cu_9.4/SiO₂–SARA catalyst, since the difference in loading between these two catalysts is quite small (1.4% only) and the ${\mathrm{Mn}}^{4+}/{\displaystyle \sum }_{n=0}^4{\mathrm{Mn}}^{\mathrm{n}+}$ ratio was almost the same for both catalysts.

The catalyst prepared by the incipient wetness impregnation (Mn_91.1Cu_8.9/SiO₂–IWI) was shown to be more effective than that prepared by the electroless metal deposition (Mn_90.8Cu_9.2/SiO₂–EMD) based on both the rate of NO_x conversion related to 1 g of the active species deposited and the absolute values of NO_x conversion. Although the absolute values of NO_x conversion efficiency of the latter catalyst were higher in the temperature range of 200–300 °C, the difference is insignificant considering the loading of Mn and Cu species.

The lowest rate of NO_x conversion related to 1 g of the active species was observed for the Mn_89.9Cu_10.1/SiO₂–AE catalyst characterized by the highest Mn and Cu loading and the lowest SSA. It can be concluded that the dispersion of catalytically active species plays the important role in the NO_x conversion. With increasing the loading, the distribution of active species decreases, while the agglomeration and pores clogging increases causing the reduction in SSA and pore volume. Moreover, as a result of clogging the pores and agglomeration the amount of active species available for the catalytic reaction decreases significantly. Thus, as it has been assumed considering the textural properties and SEM images of the Mn_89.9Cu_10.1/SiO₂–AE catalyst, the ammonia evaporation method was shown to be ineffective, and it definitely cannot be considered as the optimal technique for preparation of NH₃-SCR de-NO_x catalysts due to the agglomeration and extremely intensive clogging the pores.

The obvious advantage of the urea hydrolysis method for preparation of the NH₃-SCR de-NO_x catalysts is the possibility to rich the high enough loading (up to 12 wt %) maintaining the relatively high SSA (up to 50 m²·g⁻¹). However, considering both the absolute NO_x conversion efficiency and the rate of NO_x conversion related to 1 g of the active species deposited, it can be seen that the Mn_90.9Cu_9.1/SiO₂–WI catalyst was shown to be more effective than the Mn_91.1Cu_8.9/SiO₂–UH catalyst at all the temperatures investigated except for the temperature of 150 °C, although the latter possesses better textural properties, higher loading and almost the same ${\mathrm{Mn}}^{4+}/{\displaystyle \sum }_{n=0}^4{\mathrm{Mn}}^{\mathrm{n}+}$ ratio. The high efficiency of the catalyst prepared by wet impregnation requires deeper investigation and further understanding. Considering all the results obtained such methods as incipient wetness impregnation and wet impregnation can be considered as the most attractive techniques for preparation of NH₃-SCR de-NO_x catalysts from all investigated in the present study.

3. Materials and Methods

3.1. Catalyst Preparation

Mn–Cu–O_x/SiO₂ catalysts were prepared by six different methods: electroless metal deposition, stepwise addition of a reducing agent, wet impregnation, incipient wetness impregnation, urea hydrolysis, and ammonia evaporation. Mesoporous silica gel powder (silica gel 60, 0.063–0.1 mm, Merck, Darmstadt, Germany) was used as a support for deposition of Mn and Cu oxides.

3.1.1. Electroless Metal Deposition

The electroless metal deposition technique is described in [40]. Prior to the active layer deposition, silica gel powder (30 mL) was activated in the 2.0 g·L⁻¹ PdCl₂ solution at the temperature of 25 °C for 1 min. This allows the formation of active sites on the carrier surface required for improved deposition of the active species and avoids or reduces bulk deposition. After the activation procedure, the support was rinsed with deionized water to remove all physisorbed chemical species. Cu and Mn catalytically active species were deposited on the activated support by sequential treatment of the latter in the Cu-plating and Mn-plating solutions, respectively. The composition of the Cu-plating solution was as follows: CuSO₄—0.01 mol·L⁻¹, CH₂O (formaldehyde)—0.45 mol·L⁻¹, (ethylenedinitrilo)tetra-2-propanol—0.1 mol·L⁻¹, while the Mn-plating solution contained 0.25 mol·L⁻¹ KMnO₄ only. The residence time of the support in the Cu-plating and Mn-plating solutions was 1 h and 40 h, respectively. During the whole deposition process the support was vigorously stirred with a magnetic mixer. After the deposition was finished the support coated with the catalytically active Cu and Mn species was extracted from the metal-plating solution by a centrifugation, rinsed with the deionized water, dried under ambient conditions, and calcined at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–EMD.

3.1.2. Stepwise Addition of a Reducing Agent

The principle of deposition of active species by stepwise addition of a reducing agent can be found elsewhere in the literature [27]. Sodium borohydride (NaBH₄) was applied as a reducing agent for deposition of the catalytically active Cu and Mn species. As in the electroless metal deposition the support was activated in the 2.0 g·L⁻¹ PdCl₂ solution prior to the deposition process. After the activation, 50 mL of the silica gel powder were sequentially treated in the Cu-plating (1 h) and the Mn-plating (24 h) solutions with the same composition as it has been mentioned previously. Vigorously stirring was maintained throughout the deposition process. The total amount of the sodium borohydride added to the metal-plating solutions of 500 mL each was 2 g. During the deposition of Cu species 0.5 g of NaBH₄ was divided by three equal portions and added to the solution in the beginning of the deposition procedure, after the 15 min and after the 30 min of deposition, while 1.5 g of NaBH₄ was added to the Mn-plating solution during the deposition of Mn species by ca. 0.2 g every 30 min. After getting the deposition completed the active Cu and Mn species supported be the silica gel powder were extracted from the metal-plating solution by a centrifugation, rinsed with the deionized water, dried under ambient conditions, and calcined at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–SARA.

3.1.3. Wet Impregnation

The basis of the wet impregnation method can be found elsewhere in the literature [27]. The silica gel powder in the amount of 50 mL was sequentially treated in 0.01M CuSO₄ and 0.25M KMnO₄ solutions of 500 mL each for 3 h and 40 h, respectively under the vigorous stirring with a magnetic mixer. After the deposition, the suspension was centrifuged to separate the catalyst from the liquid phase, followed by rinsing the former with the deionized water, drying under ambient conditions and calcination at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–WI.

3.1.4. Incipient Wetness Impregnation

The general description of the incipient wetness impregnation technique can be found in [41]. Prior to the deposition process the silica gel powder in the amount of 45 g was dried in the oven at the temperature of 120 °C for 1 h. For the deposition of active species, the solution containing 0.02 mol·L⁻¹ CuSO₄ and 0.3 mol·L⁻¹ KMnO₄ was used. Based on the pore volume of the support determined by N₂ adsorption (0.8 cm³·g⁻¹) the volume of the solution required for deposition was calculated to be 36 mL. The deposition process was conducted under intensive mixing and as a result the solution was completely adsorbed by the support. After that, the support with the deposited active species was dried under ambient conditions and calcined at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–IWI.

3.1.5. Urea Hydrolysis

The principle deposition by urea hydrolysis can be found in [28]. For the deposition of active species 500 mL of the solution containing 0.01 mol·L⁻¹ CuSO₄ and 0.25 mol·L⁻¹ KMnO₄ were prepared, and the pH was adjusted to 3 by means of HNO₃. After that 100 mL of the silica gel powder and 45 g of the urea (CH₄N₂O) were added to the solution with the following heating the latter to 90 °C and vigorous stirring. Under such conditions the deposition process was conducted for 12 h (until the pH 7 was achieved). The support with the deposited Cu and Mn species was extracted from the solution by a centrifugation, rinsed with the deionized water, dried under ambient conditions, and calcined at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–UH.

3.1.6. Ammonia Evaporation

The principle of ammonia evaporation method can be found in [28]. For the deposition of active species 500 mL of the solution containing 0.01 mol·L⁻¹ CuSO₄, 0.25 mol·L⁻¹ KMnO₄ and 0.1 mol·L⁻¹ K₂CO₃ were prepared, and the pH was adjusted to 11 by adding 150 mL of the NH₄OH. After that 100 mL of the silica gel powder were added to the solution followed by heating the latter to 90 °C under vigorous stirring. Under such conditions the deposition process was conducted for 12 h (until the pH value was changed from 11 to 8). The silica gel powder with the deposited Cu and Mn species was extracted from the solution by a centrifugation, rinsed with the deionized water, dried under ambient conditions, and calcined at the temperature of 350 °C for 3.5 h. The prepared catalyst was denoted as MnCu/SiO₂–AE.

3.2. Catalyst Characterization

Composition of the catalytic layer deposited on the surface of the silica gel powder and the loading (wt%) of the active metals (Mn and Cu) being part of the catalytic layer were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) with the ICP optical emission spectrometer Optima 7000DV (Perkin Elmer, Waltham, MA, USA). To investigate the morphology of the Mn–Cu–O_x catalytic layer supported by the silica gel powder and the dispersion of the active species on the surface of the catalysts prepared SEM images and EDX elemental mapping were provided by means of the scanning electron microscope TM4000Plus (Hitachi). The oxidation states of the main active species being part of the catalytic layer were determined by X-ray photoelectron spectroscopy (XPS) using ESCALAB MK II spectrometer (VG Scientific, East Grinstead, UK) equipped with a Mg Kα X-ray radiation source (1253.6 eV) operated at 300 W with the fixed pass energy of 20 eV. The pressure of 10⁻⁷ Pa was kept in an UHV analysis chamber. X-ray diffraction (XRD) measurements were conducted using X-ray diffractometer D2 Phaser (Bruker AXS, Billerica, MA, USA) to provide comprehensive information about the structure of the active species deposited on the surface of the amorphous silica gel powder. XRD patterns were recorded using Cu Kα radiation. A step-scan mode was used in the 2θ range from 10° to 70° with the step length of 0.04° and the counting time of 1 s per step. Additionally, the specific surface area (SSA), total pore volume, pore distribution and the average pore size of both the carrier used and the supported catalysts were determined by means of nitrogen adsorption at −196 °C using the Micromeritics TriStar II 3020 analyzer. The Brunauer–Emmett–Teller model was used to calculate the specific surface area. Before measurements, all the samples were degassed in a nitrogen atmosphere at 100 °C for 2 h.

3.3. Catalytic Activity Test (NH₃-SCR de-NO_x)

The prepared Mn–Cu oxide catalysts supported by the silica gel powder (0.063–0.1 mm) were tested for the SCR de-NO_x by ammonia using the experimental setup presented in Figure 10. Three separate gas mixtures NO(1500 ppm)/N₂, NH₃(1500 ppm)/N₂ and O₂ (vol. 9%)/N₂ were supplied from three gas cylinders into the setup through mass flow controllers, creating the resulting gas mixture with NO/NH₃ ratio of ~1.0 and concentrations of the main gas components of ca. 300–500 ppm, ca. 300–500 ppm and ca. 3–5% for NO, NH₃, and O₂, respectively. The catalysts were investigated as a fixed bed at GHSV of 450 h⁻¹ and the NO_x conversion efficiency was expressed as both the absolute conversion rate achieved and the rate of NO_x conversion provided by 1 g of the catalytically active species deposited. The volumetric flow rate of the resulting gas mixture passing through the catalysts under such operating conditions was about 0.105 L·min⁻¹. Additionally, the catalyst provided the highest SCR performance was tested as a fluidized bed under bubbling fluidization conditions at GHSV of 940 h⁻¹ (volumetric gas flow rate of 0.219 L·min⁻¹). Catalytic activity tests were conducted in the temperature range of 100–300 °C (low-temperature conditions). NO_x (NO and NO₂) concentrations were measured prior to and after the catalysts using the TESTO 350XL gas analyzer (Testo, Germany).

4. Conclusions

It was shown that a preparation method has a significant impact on both the physicochemical properties and the catalytic performance of the catalysts. A method applied for preparation of SCR de-NO_x catalysts affects the morphology, oxidation state of the active species, and textural properties, in particular specific surface area, total pore volume and pore size distribution. Using the silica gel powder as a carrier it was shown that the incipient wetness impregnation is the only method that ensures the remaining of initial size of the support, while other techniques where the significant amount of liquid solutions was used to prepare the catalysts resulted in segmentation of the silica gel. Although the carrier used for the deposition of the active species is a highly porous material, all the methods applied for preparation of catalysts provided the uniform distribution of the main active component—Mn, while some nonuniformity was observed for the doping element—Cu. Considering both the rate of NO_x conversion related to 1 g of the active species deposited and the absolute values of NO_x conversion, and taking into account the loading provided by the active catalytic species the most attractive method for preparation of SCR de-NO_x catalysts is supposed to be incipient wetness impregnation. Additional advantages of this preparation method are its simplicity and low amount of precursor solution required for deposition of active species. Moreover, this technique is fast, there is no need for additional chemicals apart from Mn and Cu precursors and rinsing the catalyst after the deposition is not required.