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Article

Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3

by
Michael Liebau
,
Wolodymyr Suprun
,
Marcus Kasprick
and
Roger Gläser
*
Institute of Chemical Technology, Universität Leipzig, Linnéstraße 3, 04103 Leipzig, Germany
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(1), 22; https://doi.org/10.3390/catal15010022
Submission received: 2 December 2024 / Revised: 23 December 2024 / Accepted: 28 December 2024 / Published: 30 December 2024
(This article belongs to the Special Issue Catalytic Reactions in Hydrogen and Ammonia Economy)
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Abstract

:
The conversion of NOx and the yield of N2O during NH3-SCR-DeNOx below 473 K over TiO2-supported transition metal oxide catalysts with equal loading of 20 wt.-% decreases in the following order of the supported oxides: MnOx > CuOx > CoOx > FeOx > NiOx > CeOx. The storage capacity for NH3, characterized by the acid site density of the catalyst, is not directly correlated with the catalytic activity. Rather, the temperature range for the reduction of the supported transition metal oxides as determined by TPR-H2 is the main governing factor for high NH3-SCR-DeNOx activity, especially in the temperature range below 473 K. At the same time, oxidation temperature range and the density of Lewis acid sites govern the formation of N2O. The decomposition of NH4NO3 as an intermediate in the NH3-SCR-DeNOx reaction is determined by the redox property of TMO-based catalysts, which further influences both the windows of the decomposition temperature and the yield of N2O. The correlation between the redox properties and the activity for NH3-SCR-DeNOx was confirmed for a series of MnOx-CeOx/TiO2-SiO2 mixed transition metal oxide catalysts as a promising combination of the less active and more selective CeOx with less selective and highly active MnOx. The linear correlation between reduction temperature range and the NH3-SCR-DeNOx activity indicates that the found relation can be transferred to other supported transition metal-containing catalysts for low-temperature NH3-SCR-DeNOx.

Graphical Abstract

1. Introduction

The selective catalytic reduction of NOx with NH3 as a reducing agent (NH3-SCR-DeNOx) is one of the most widespread methods for the depletion of NOx emissions from exhaust gases [1,2,3]. For both mobile and stationary emission sources, V2O5, modified with MoO3 or WO3, supported on TiO2 has been employed as catalyst in the state-of-the-art technology [3,4,5,6]. However, to achieve the necessary conversion of NOx in order to adhere to the increasingly stringent regulation regarding NOx emissions, operating temperatures of 573–673 K are required [7]. For small-scale industrial plants, these temperatures often cannot be reached in the exhaust treatment system without additional heating of the gas to be treated. Alternatively, in order to retain temperatures above 573 K in the exhaust gas, the NH3-SCR-DeNOx unit can be installed in an upstream position, i.e., prior to the removal of dust or SOx from the gas stream, which leads to a reduced lifetime of the catalyst [7]. To avoid deactivation of the catalyst, the NH3-SCR-DeNOx unit can be installed at a downstream position, with lower prevalent concentration of potential catalysts poisons. Flue gases usually exhibit temperatures below 573 K at this point, requiring re-heating of the gas stream before DeNOx as an additional cost [8].
In order to avoid additional expenses and prolong the catalyst lifetime, multiple catalytic systems with lower required operation temperatures, such as MnOx, FeOx, CoOx, NiOx or CuOx, were investigated for NH3-SCR-DeNOx [9,10,11,12,13,14]. Among the investigated catalysts, MnOx has been repeatedly shown to possess high activity for the NH3-SCR-DeNOx [2,15,16,17,18]. However, MnOx-based catalysts have also been shown to exhibit a low selectivity for N2, due to the pronounced formation of undesired N2O [19,20,21,22]. Both Cu- and Fe-oxides supported on zeolites and Cu- or Fe-ion containing zeolites were investigated as catalysts for NH3-SCR-DeNOx, with the latter showing a high conversion of NOx only at temperatures above 523 K [23,24,25,26]. Different support materials, such as Al2O3 [27], ZrO2 [14], titanate species [11] or TiO2 [9,28] have been also investigated. Especially catalysts based on Cu-ion-containing zeolites have been shown to be promising catalysts for the NH3-SCR-DeNOx at low temperatures, although with limited hydrothermal stability depending on the type of zeolite used [26,27,29,30,31]. While small-pore zeolites such as Cu-SSZ-13 exhibit sufficient hydrothermal stability for the application as NH3-SCR-DeNOx catalysts [32], tolerance for chemical catalyst poisons, such as SO2 alkali metals or P, remains an issue [33,34,35,36]. Other transition metal oxides (TMO) have been mainly employed as bi- and tri-metal oxidic catalyst systems. CeOx does not show a significant conversion of NOx at temperatures below 573–623 K, but has been well investigated as a support or dopant for NH3-SCR-DeNOx catalysts due to its ability for oxygen storage [12,37,38,39,40,41]. Smirniotis et al. [9] investigated NiOx and CoOx supported on TiO2 and achieved only low to average conversion of NOx at low temperature. Still, both TMOs are frequently investigated as dopants for NH3-SCR-DeNOx catalysts, promoting the conversions of NOx and the N2-selectivity of Mn-based catalysts [10,42,43,44,45].
Apart from the active component, the support can be modified to adjust the properties of the catalyst. Hao et al. [46] demonstrated increased resistance against H2O and SO2 for CeO2 catalysts supported on TiO2 modified with SiO2. The increased water resistance of catalysts by addition of SiO2 was confirmed by Dong et al. [47] for Mn-Ti-Si mixed oxide catalysts. Shi et al. [48] further established enhanced SO2 resistance for CeO2/TiO2 modified with SiO2 and reported a decreased reduction temperature as well as increased NH3 adsorption due to formation of additional Lewis acid sites after modification with SiO2. Accordingly, modification of the TiO2 support with SiO2 is a promising approach for the design of NH3-SCR-DeNOx catalysts.
It is generally accepted that during NH3-SCR-DeNOx the adsorbed NO2 and NH4+ species can interact to form ammonium nitrates (AN) and deactivate the NH3-SCR-DeNOx catalyst by blocking the active sites, especially at temperatures below 523 K [49,50,51]. Additionally, AN can react with NO to generate unstable nitrites, which then decompose to N2, NO, NO2, N2O and H2O.
Many of the AN-decomposition studies reported earlier were carried out by in-situ diffuse reflectance infrared Fourier transform spectroscopy and DSC/DTG methods [52,53]. Skordilis et al. [53] studied the decomposition of AN in mixtures thermogravimetrically with varying amounts of metal nitrate (Al, Fe, Mn, Co, Cu and Cr). Those results show that Co and Cu oxides stabilized AN and lowered the exothermic decomposition temperature of AN. Contrarily, Cr, Al, and Fe nitrate strongly promotes the thermal AN-decomposition and has a strong destabilizing effect on AN. Especially CrCl3, CrBr3 and Cr2O3 strongly promoted the thermal decomposition of AN. However, no details about the impact of TMOs on the formation of decomposition products of AN were given [52,54]. Additionally, AN can play an important part in elevating the overall performance of NH3-SCR-DeNOx catalysts as “enhanced NH3-SCR”, according to Tronconi et al. [55]. Therefore, whether AN poses a useful intermediate during the low temperature NH3-SCR-DeNOx or a catalyst poison for solid catalysts is still under discussion [49,50,54].
The activity of a catalyst for NH3-SCR-DeNOx is usually considered to be governed by the acidity and the redox properties of the catalyst [56]. Liu et al. [11] studied the impact of different molar ratios of Fe to Ti in iron titanate catalysts with respect to redox and adsorption properties. Correlations between the catalytic activity and the sorption ability for NOx and NH3 were found [11]. The relationship of structure and activity for Co-Mn oxide catalysts on lamellar-like supports was investigated by Shi et al. [57]. Dong et al. [58] studied the influence of different loadings of CeO2 on the structure, strength and density of acid sites, surface oxygen content and reduction temperature of CeO2 supported on TiO2. The study revealed an increasing conversion of NOx with increasing CeO2 loading up to the dispersion capacity of the support and promotion of the fast NH3-SCR-DeNOx due to increased storage capacity for oxygen and therefore increased oxidation rate of NO to NO2 [58]. Gao et al. [59] investigated the effect of different Cu loading for Cu-SSZ-13 catalysts and found a correlation of the location of Cu2+ ions and the reaction rate and mass-transfer limitations for oxidation of NH3 at low temperatures.
Despite extensive investigations, the relationship between the physical and chemical properties and the reactivity/selectivity for the NH3-SCR-DeNOx over supported transition metal oxide catalysts still remains under debate.
In this study, a series of bimetallic (mixed metal oxide) MOx/TiO2 (M: Fe, Mn, Cu, Ce, Co, and Ni) catalysts supported on TiO2 with the same nM/nTi molar ratio of 0.4 are synthesized and studied in the NH3-SCR-DeNOx in the presence of oxygen excess. The redox and adsorption properties of the prepared catalyst were studied by TPR- H2, TPD-NOx and TPD-NH3 to evaluate the relationship between their physico-chemical characteristics, i.e., reduction behavior, reactant adsorption capability and surface acid site density and strength (distribution).
The potential of the TiO2-supported mixed TMO-containing catalysts was explored for a MnOx-CeOx/TiO2-SiO2 with different MnOx to CeOx ratios, with the goal to enhance the catalytic performance especially below 523 K. Finally, the thermal decomposition of AN as a source of N2O in the presence of the catalysts mentioned above was investigated by temperature-programmed methods. The systematic investigation of the physico-chemical properties of the catalysts and their impact on the conversion of NOx and formation of N2O aims to provide guidelines for effective design of NH3-SCR-DeNOx catalyst.

2. Results and Discussion

2.1. Catalytic Activity During NH3-SCR

Multiple catalysts with MnOx, CuOx, CoOx, FeOx, NiOx and CeOx as the active component supported on TiO2 were synthesized and investigated for their catalytic activity for the NH3-SCR-DeNOx in a temperature range from 393 to 673 K. N2-sorption revealed a specific surface area of 45 m2 g−1 and an average pore width of 20.5 nm for the support. After impregnation with the active component, a comparable reduction of the specific surface by ~8 m2 g−1 and an average pore width of 22.5 nm was observed for the catalysts compared to the support. Differences between the catalysts was within the uncertainty of the measurement. Accordingly, comparable textural properties can be assumed for the investigated catalyst. The reduction in specific surface area and increase in average pore width can be explained by blockage of smaller pores of the support by the active component. XRD of the investigated catalysts are presented in Figure 1.
The diffractograms reveal that the TiO2 support primarily consists of anatase (reflexes at 2θ = 23.3°, 37.7°, 48°, 62.6° etc.) and, to a lower degree, TiO2 in the rutile phase, observable as a reflex at 2θ = 27.3°. The reflexes for the support are present for all investigated catalysts, although the reflex allocated to the rutile phase is overlapped for CeOx/TiO2 and exhibits a significantly lower intensity for MnOx/TiO2. A phase transition in the support is unlikely under the preparation conditions for the MnOx/TiO2 catalysts. Furthermore, the preparation conditions were identical for all TMO catalysts. The lower intensity of the reflex assigned to the rutile phase could be explained either by difference in the sample preparation during the XRD measurement, or a higher coverage of the rutile phase by MnOx species compared to the other TMOs. Zhuang et al. [60] investigated MnOx catalyst supported on both rutile and anatase. Their results show that the reaction rate for the conversion of NOx during NH3-SCR-DeNOx was mainly influenced by the MnOx loading, with no significant differences between MnOx located on rutile or anatase. Accordingly, it can be expected that the different surface distribution of MnOx on the TiO2 phases does not have a significant influence on the NH3-SCR-DeNOx activity. For CeOx/TiO2, broad reflexes at 2θ = 28.5° and 33° indicate the presence of CeO2. Reflexes observed for NiOx/TiO2 and FeOx/TiO2 revealed NiO (reflexes at 2θ = 43.2° and 62.8°) and Fe2O3 (2θ = 33.4°, 35.8°) as TMO species present on the catalysts, respectively. Furthermore, Co3O4 (2θ = 36.8°) and CuO (2θ = 35.4°, 38.6° and 49.1°) were identified as TMO species on CoOx/TiO2 and CuOx/TiO2. For MnOx/TiO2, only a broad reflex with relatively low intensity (2θ = 32.9°) was observed, indicating a Mn2O3 phase. Other characteristic reflexes for Mn2O3 at 2θ = 55.2° and 65.8° are overlapped by reflexes assigned to the anatase phase of the support. Compared to the other investigated TMO catalysts, the lack of sharp reflexes observed for MnOx/TiO2 might indicate a lower crystallinity of the deposited MnOx species in contrast to the other investigated TMOs. While the analysis of the effect of the species and distribution of each individual TMO on the activity and selectivity during NH3-SCR-DeNOx would warrant further investigation, it is beyond the scope of this work. The following discussion will focus on the overall reduction and adsorption properties in relation to the catalytic properties during NH3-SCR-DeNOx over TMO catalysts prepared under identical condition, with the same support and comparable textural properties. NH3-SCR-DeNOx experiments revealed no significant conversion of NOx (less than 3%) for the TiO2 support itself even at temperatures above 673 K. The catalytic activity in the low temperature range below 523 K was highest for the catalyst MnOx/TiO2 by far, reaching a conversion of NOx of 100% at 473 K, followed by the CuOx/TiO2 with about 90% conversion at the same temperature, as seen if Figure 2. A maximum conversion of NOx of around 55% could be achieved for CoOx/TiO2. FeOx/TiO2, NiOx/TiO2 and CeOx/TiO2 allow high conversion of NO of 80% and more, but only at temperatures of 573 K, 603 K and above 700 K, respectively. Overall, the trend of catalytic activity for MnOx, CuOx, CoOx, FeOx and NiOx in the low temperature range qualitatively matches the results of investigations done by Smirniotis et al. [9]. Low-temperature activity during the NH3-SCR-DeNOx decreased in the order: MnOx/TiO2 > CuOx/TiO2 > CoOx/TiO2 > FeOx/TiO2 > NiOx/TiO2 > CeOx/TiO2. In the same order that the conversion of NOx was shifted to higher temperatures, a decrease of the yield of N2O of 80% for MnOx/TiO2 down to nearly no observed formation of N2O for CeOx/TiO2 at a temperature of 473 K was established, as seen in Figure 3.
Overall, a decrease in the conversion of NOx and simultaneously of the formation of N2O was observed in the following order: MnOx/TiO2 > CuOx/TiO2 > CoOx/TiO2 > FeOx/TiO2 > NiOx/TiO2 > CeOx/TiO2 at a temperature of 473 K, representative for low-temperature applications. The influence of the physico-chemical properties on the conversion of NOx and yield of N2O is discussed in the following chapters.

2.2. Impact of Adsorption Properties

The adsorption capacity and the temperature range of desorption of NOx was investigated by TPD-NOx experiments. The profiles of desorption of NOx as well as an overview of the desorption temperature range from the experiments are depicted in Figure 4. The NOx-evolution profiles were deconvoluted into low (<473 K), moderate (472–573 K) and high (>573 K) temperature range respectively, to obtain qualitative and quantitative information on the active species.
For the catalysts that showed high activity for the conversion of NOx at low temperatures, i.e., MnOx/TiO2 and CuOx/TiO2, significant desorption of NOx was observed at temperatures lower than 473 K. Weakly bonded NOx in this temperature range was absent or was not observed in significant quantities for the catalysts that show activity only at higher temperatures. This implies that the presence of weakly bonded NOx on the catalyst surface is beneficial for low-temperature activity. According to Cen et al. [61] and Shi et al. [62], the NH3-SCR-DeNOx can proceed according to the Langmuir-Hinshelwood mechanism over iron titanate or CeOx-NbOx based catalysts. The contribution of the Langmuir-Hinshelwood mechanism to the reaction decreased at temperature above 523 K [61]. Similar observations were published by Liu et al. [63] for Mn-Ce catalysts on different zeolite supports. Accordingly, it can be assumed that the Langmuir-Hinshelwood mechanism also contributes to the low-temperature NH3-SCR-DeNOx over the catalysts in the present investigation. NOx species weakly bonded to the catalyst could therefore possess a higher mobility than strongly bonded NOx, allowing migration to active sites on the surface. Additionally, adsorbed NO with higher surface mobility might also be more readily available for the oxidation to NO2. This NO2 would allow for an increased contribution of the fast SCR to the overall conversion of NOx. First described by Kato et al. [64], the fast NH3-SCR-DeNOx involves an equimolar ratio of NO and NO2. The reaction rate of the fast NH3-SCR-DeNOx is about 10 times higher than the rate for the standard NH3-SCR-DeNOx, improving the conversion of NOx [64].
In order to gain deeper insights into the chemistry of surface acidity of mixed metal oxide catalysts the TPD-NH3 method with online mass spectrometry was used not only for monitoring of the desorbed NH3 during the temperature program, but also for the simultaneous detection of NO, NO2 and N2O. NOx and N2O could be formed as a product of the partial oxidation of the adsorbed NH3 by active surface and lattice oxygen species [19,65]. The profiles of the NH3 evolution during TPD experiments as well as the acid site density of the catalysts is presented in Figure 5.
A shift of desorption of NH3 to lower temperatures compared to the support and the other investigated catalysts was observed especially for MnOx/TiO2 and CuOx/TiO2. A lower adsorption strength is usually attributed to Lewis acid sites [45,66], which are active for the NH3-SCR-DeNOx as well as the oxidation of NH3. The higher content of Lewis than Brønsted acid sites could therefore be one of the causes for the increased activity of those catalysts for the NH3-SCR-DeNOx. Additionally, the oxidation of NH3 to N2O is also increased with increasing number of Lewis acid sites, reflecting the experimental results for the formation of N2O during the NH3-SCR-DeNOx reaction. Interestingly, the highest acid site density, i.e., NH3 storage capacity on the catalyst surface, was found for CoOx/TiO2, despite only limited conversions of NOx with a maximum of only 50–60% at 500 K during the NH3-SCR-DeNOx. Although a high storage capacity for NH3 is beneficially for the NH3-SCR-DeNOx activity, no direct correlation of higher storage capacity for NH3 alone with NH3-SCR-DeNOx activity at low temperatures was found. The acid site density of the highly active MnOx/TiO2 and CuOx/TiO2 was similar to that of the support material. This suggests that the nature of the acid sites, i.e., Lewis acidity, has a significantly higher impact on the activity than the overall NH3 storage capacity [45].
The oxidation of NH3 to NOx, mainly NO, and N2O, in presence of oxygen in the gas phase or active surface oxygen from the catalyst is caused by adsorbed NH3 on Lewis acid sites [19,20]. In our previous paper, we established that Brønsted acid sites are inactive for the oxidation of NH3 [67]. The evolution of NO, NO2 and N2O as oxidation products was monitored during the TPD experiments by online mass spectroscopy, allowing for further insight into the presence of Brønsted and Lewis acid sites. Profiles of the desorption of N2O during the TPD experiments and the calculated selectivity of the desorption products is presented in Figure 6.
For highly active catalysts at low temperatures, e.g., MnOx/TiO2 and CuOx/TiO2, evolution/desorption of N2O occurred already at temperatures below 573 K, while for CoOx/TiO2, FeOx/TiO2 and NiOx/TiO2, desorption of N2O is shifted to higher temperature range. This is consistent with the formation of N2O already at low temperatures during the NH3-SCR-DeNOx reaction over MnOx/TiO2 and CuOx/TiO2. The selectivity to oxidation products of NH3 increases in the following order: TiO2 support < CeOx/TiO2 < FeOx/TiO2 < NiOx/TiO2 < CoOx/TiO2 < CuOx/TiO2 < MnOx/TiO2. Qualitatively, the formation of N2O during the NH3-SCR-DeNOx at 473 K increases in the same order, with highest yield of N2O obtained for MnOx/TiO2 as the active component. The observation suggests that the oxidation of NH3 is one of the main pathways for the formation of N2O during the NH3-SCR-DeNOx over the TMO catalysts, as supported by the literature [19,65,67,68,69,70]. Other potential pathways for N2O formation include the decomposition of NH4NO3 or the reduction of NOx. In our previous publication [67], the contribution of the individual pathways for N2O formation over a MnOx catalyst was investigated in more details. For both the oxidation of NO and during TPD-NOx experiments with the catalyst, no significant formation of N2O as a side product above the uncertainty of the detector were observed [67]. Accordingly, the reduction of NOx over TMO catalysts supported on TiO2 can be neglected as a major pathway for N2O formation during NH3-SCR-DeNOx, also in accordance with literature [19,65,67,68,69,70]. To investigate the relation between catalytic activity for the oxidation of NH3 observed during the TPD experiments and the formation of N2O during the NH3-SCR-DeNOx, the temperature for a N2O yield of 10% during the NH3-SCR-DeNOx was plotted over the temperature maximum of the desorption of N2O during the TPD experiments in Figure 7. In case of multiple maxima, the peak with the highest intensity was chosen. A yield of 10% was chosen as a value for comparison that could be both observed over all catalysts in the investigated temperature range and is well above the uncertainty of the experiment (~1.5%).
As represented in Figure 7, a nearly linear correlation between the temperatures for a yield of N2O of 10% during the NH3-SCR-DeNOx and the maximum of the N2O desorption during the TPD-NH3 experiments was found. Combined with the previous observations, this indicates that the formation of N2O is at least partially caused by the oxidation of NH3 and governed by the nature and oxidation ability of the active components, i.e., the redox properties and number of available Lewis acid sites for the oxidation.

2.3. Decomposition of Ammonium Nitrates

The formation and decomposition of AN during the NH3-SCR-DeNOx reaction has an impact on the long time stability of the catalyst and selectivity of the whole process especially at operation temperature range below 473 K [66,68,69,70] The thermal decomposition of AN could prevent blocking of the catalyst surface area in the respective temperature range. Furthermore, since N2O is one of the possible decomposition products from AN, the overall selectivity of the NH3-SCR-DeNOx reaction is influenced by the decomposition process [52,71,72,73]. At temperatures between 478 K to 623 K, AN can also cause the so called “enhanced” NH3-SCR-DeNOx, according to Tronconi et al. [55,74]. In this temperature range, AN can act as an oxidizing co-reactant, allowing for superior efficiency for the reduction of NO [55]. Hence, the impact of the support and the catalytically active TMOs on the AN-decomposition was investigated.
Each of the catalyst samples and the support were loaded with 5 wt.-% of AN. After drying, the samples were then heated up to 773 K with 5 K min−1 and the evolution of the decomposition and oxidation products N2O, NO and NO2 were monitored by on-line NDIR detection. For all investigated samples, the total yield of NOx products was 26% or less based on the NH4NO3-loading, as demonstrated in Figure 8 (right). Since no IR band assignable to AN was observed after treatment at 773 K during subsequent DRIFTS experiments, this indicates that the decomposition products were mainly N2, NH3 and H2O. Among the investigated catalysts, MnOx/TiO2 exhibited the highest yield of N2O from AN with 15%. TPD-NH3 experiments (see Section 2.2) have shown that oxidation of NH3 over MnOx/TiO2 occurs already at temperatures as low as 410 K. It can therefore be assumed that formed N2O not only originates directly from the decomposition of AN, but also from the oxidation of NH3 (released from AN in this temperature range above 400 K).
From the profiles of the desorbing NOx products depicted in Figure 8 (left), an obvious effect of the active component on the AN-decomposition is visible. For the TiO2 support, a broad desorption range of decomposition products starting at around 473 K was observed. Remarkably, this temperature range largely corresponds to the temperature range for AN-decomposition [52]. For MnOx/TiO2 and CuOx/TiO2, the investigated catalysts with the highest oxidation ability and simultaneously highest activity for the NOx-conversion during NH3-SCR-DeNOx, the initial decomposition temperature was significantly reduced to 400 K and 413 K, respectively. Contrary to the broad desorption of N2O, NO and NO2 from the TiO2 support, two different regions for the AN-decomposition were observed for both catalysts: one at lower temperatures, up to roughly 70 K higher than the initial decomposition temperature and a second one at higher temperature range between 500 and 620 K. The first region is characterized by a sharp peak of both N2O and NOx, indicating a fast decomposition already below or close to 473 K. This suggests that the active metal oxide catalyzes the AN-decomposition and decreases its thermal stability. In the second temperature range, a much broader desorption peak occurs consisting mainly of NO and NO2.
Over CoOx/TiO2, FeOx/TiO2 and NiOx/TiO2 catalysts, AN-decomposition occurs at temperatures higher than 450 K, similar to the TiO2 support, while two temperature regions are also observable. However, decomposition behavior of AN over FeOx/TiO2 and NiOx/TiO2 catalysts was not as clearly pronounced as for MnOx/TiO2 and CuOx/TiO2 due to the broader desorption peaks. The lack of sharp peaks at the low-temperature region of 420–500 K in the NOx profile suggests a weak AN-decomposition, mainly due to different redox properties of the active component. For CoOx/TiO2, a sharp peak of the concentration of NO and NO2 occurs, but only at higher temperatures around 530 K. The desorption of NOx from CeOx/TiO2 occurs over a broad temperature range starting at 450 K up to 720 K. Among the investigated catalysts, the profile obtained from CeOx/TiO2 is most similar to the support, which indicates that the weak redox properties of CeOx/TiO2 do not significantly impact the AN-decomposition. The desorption of NO and NO2 up to 720 K, roughly 40 K higher than from the support material, could suggest a stabilizing effect of CeOx, increasing the thermal stability of AN on the surface. However, strongly bonded NO and NO2 desorb only above 550 K up to 710 K. These results are in agreement with the TPD-NOx experiments performed with CeOx/TiO2. Therefore, the apparently stabilizing effect of CeOx can be also explained by a stronger interaction or adsorption of NO and NO2 with the catalyst surface and therefore desorption at higher temperatures.
Overall, the different impact of the investigated catalysts on the thermal stability of AN was mainly influenced by the redox properties of the active component. While this effect decreases the yield of N2 of the NH3-SCR-DeNOx process by formation of N2O, the reduced thermal stability of AN in presence of MnOx and CuOx is also beneficial for the NH3-SCR-DeNOx at the temperature range below 500 K. The reduced decomposition temperature of AN allows the utilization of the NH3-SCR-DeNOx unit at the low operating temperature without blockage of active catalyst sites by AN, which can be formed during such operation conditions.

2.4. Reduction Properties

Additional to the density and nature of acid sites on the surface of the catalysts, the redox properties of the active component are one of the governing factors for high NO conversion during the NH3-SCR-DeNOx. The profile of the consumption of hydrogen during TPR-H2 experiments is presented in Figure 9 (left). TiO2 support only exhibited minor reducibility in the investigated temperature range, with a maximum at 843 K. For the TiO2-supported catalysts, the main area of the reduction was shifted to higher temperatures in the following order: MnOx/TiO2 > CuOx/TiO2 > CoOx/TiO2 > FeOx/TiO2 > NiOx/TiO2 > CeOx/TiO2. Comparing the results to the maxima of the NO conversion during NH3-SCR-DeNOx, it becomes apparent that the NH3-SCR-DeNOx activity was shifted to higher temperatures in the same order. To further analyze the correlation between the activity and the redox properties, the temperature necessary for a NO conversion of 50% was plotted over the temperature of the main reduction window, i.e., the maximum of the peak with highest area after deconvolution for each catalyst in Figure 9 (right). The temperature for a conversion of 50%, also known as light off temperature, was chosen as a value for comparison. The maximum of the conversion over CeOx/TiO2 was not obtained within the investigated temperature range and different maximum conversions and broadness of the operating temperature ranges of the catalysts inhibit a direct comparison of these parameters.
As can be seen in Figure 9 (right), the correlation between the reduction temperature and temperature for NOx conversion of 50% is linear for the investigated catalysts, revealing a direct dependency of the NH3-SCR-DeNOx activity on the redox properties. As this correlation not only occurs for MnOx/TiO2 and CuOx/TiO2 in the low-temperature range for the NH3-SCR-DeNOx below 473 K but also at higher temperatures, this indicates that the observation is valid over the whole temperature range of potential NH3-SCR-DeNOx applications and that the redox properties of a catalyst are one of the main governing factors for NH3-SCR-DeNOx activity.

2.5. Application of Mixed-Oxide Catalysts

While the redox properties are one of the main governing factors for single metal oxide catalysts in the previous section, mixed oxide catalysts have been a focus of research to overcome the limitations of mono-oxidic catalysts for the NH3-SCR-DeNOx [2,7,10,11,43,75,76,77,78].
Among the catalysts prepared and investigated for the NH3-SCR-DeNOx reaction, MnOx/TiO2 showed the highest activity at low temperatures with low yield of N2. Contrary, CeOx/TiO2 as a counterpart catalyst showed lower NH3-SCR-DeNOx activity but the lowest yield of N2O. To verify the validity of the correlation between redox properties and NH3-SCR-DeNOx activity and to develop a more efficient low-temperature catalyst, a series of MnOx-CeOx mixed oxide catalysts supported on TiO2-SiO2 have been prepared as a combination of opposite ends of the activity-selectivity-range of the investigated catalysts. Furthermore, the TiO2 support was modified with the addition of 4.4 wt.-% SiO2 to improve the H2O and SO2 resistance of the catalysts [46,47]. The respective mono-oxidic catalysts, MnOx and CeOx, represent the investigated materials with the highest and lowest activity for low-temperature NH3-SCR-DeNOx, respectively, as well as the strongest and weakest redox properties. The total loading of the active component for each catalyst was kept constant and the molar ratio of MnOx to CeOx was varied. The conversion of NOx and yield of N2O during the NH3-SCR-DeNOx in a temperature range from 348 to 673 K is presented in Figure 10.
High conversions of NOx at 473 K of nearly 100% were observed for the catalysts with a CeOx-content of up to 50 mol.-% of the overall active component. The highest conversion of NOx already at low temperatures was observed for (Mn0.8Ce0.2)Ox/TiO2-SiO2. The high low-temperature activity can be attributed to the dual redox cycle of MnOx and CeOx [39]. This dual redox cycle enhances the electron transfer between the TMOs by reducing the required energy, thus augmenting the re-oxidation [39,79]. The re-oxidation is considered to be the rate-determining step during NH3-SCR-DeNOx at low temperatures [80]. A higher content of CeOx of more than 50 mol.-% of the active component leads to a shift of the maximum conversion to higher temperatures as the redox properties of CeOx become more dominant.
As seen in Figure 11, the shift of the reduction temperatures to higher temperatures was concurrent with the observations made for the mono-metal oxidic catalysts. With increasing content of CeOx in the active component of the catalyst, the reduction temperature was shifted from 600 to 700 K. It must be noted that the catalyst composed of pure MnOx as the active component exhibits a much sharper peak for the consumption of H2 at 620 K, that was not observed for the previously investigated catalyst in Figure 9. The differences in the reduction behavior can be explained by the different composition of the support material. The presence of SiO2 has a strong influence on the reduction properties of the active component.
The addition 50 mol.-% of CeOx in the active component leads to a shift in the reduction temperature similar to (Mn1.0Ce0.0.)Ox/TiO2-SiO2 that is in accordance with similar catalytic activity in the low-temperature range during the NH3-SCR-DeNOx as can be seen in Figure 11 left for both catalysts. With increasing content of CeOx, a stronger shift of the temperature in both the conversion of NO and the reduction of the temperature could be observed. Notably, the main reduction temperature for (Mn0.8Ce0.2)Ox/TiO2-SiO2 is shifted to lower temperatures by 5–10 K compared to (Mn1.0Ce0.0)Ox/TiO2-SiO2. This confirms the synergetic effect of the dual redox cycle on the redox properties observed during NH3-SCR-DeNOx [39]. Therefore, it can be established that the aforementioned correlations between the redox properties of the catalysts and their catalytic activity at low temperatures during the NH3-SCR-DeNOx are not only valid for a catalyst with a single metal oxide as the active component but also applicable for bi-metal oxidic catalysts.

3. Materials and Methods

3.1. Catalyst Preparation

Catalysts were prepared by excess solution impregnation. The respective metal nitrates (Mn(NO3)2 × 4H2O (Sigma Aldrich (St. Louis, MO, USA), 97 wt.-%), Cu(NO3)2 × 3 H2O (Merck (Darmstadt, Germany), 99,5 wt.-%), Fe(NO3)3 × 9 H2O (Merck (Darmstadt, Germany), 99 wt.-%), Co(NO3)2 × 6 H2O (Merck (Darmstadt, Germany), 99 wt.-%), Ni(NO3)2 × 6 H2O (Merck (Darmstadt, Germany), 99 wt.-%) or Ce(NO3)3 × 6H2O (Sigma Aldrich (St. Louis, MO, USA), 99.5 wt.-%)) were dissolved in deionized water and added to 5 g TiO2 (P25, Evonik (Essen Germany)), for a target metal oxide loading of 20 wt.-%. For the MnOx-CeOx mixed oxide catalysts, separate solutions of the respective nitrates were prepared. The solutions were than simultaneously added to the 3 g of the TiO2 support doped with 4.4 wt.-% SiO2 (TiO2-SiO2) (provided by Venator Materials PLC (Duisburg, Germany)) in a volume ratio according to the desired loading. For all mixed oxide catalysts, the overall loading with the active component were kept constant at 20 wt.-%. Water was evaporated at 343 K under stirring and the collected materials were dried for 10 h at 373 K. After that, the catalysts were calcined at 523 K for 1 h and subsequently at 623 K for additional 4 h. The TMO catalysts supported on TiO2 are denoted as TMO/TiO2, e.g., MnOx/TiO2 for MnO2 supported on TiO2. For the mixed TMOs supported on TiO2-SiO2, the normalized molar ration of MnOx and CeOx are given as indices, e.g., (Mn0.8-Ce0.2)Ox/TiO2-SiO2 describing the Mn-Ce-oxide catalyst with a molar ratio of MnOx to CeOx of 0.8:0.2.

Loading with Ammonium Nitrate

Catalyst samples (1 g) were loaded with 5 wt.-% NH4NO3 (Carl Roth (Karlsruhe, Germany), purity: ≥98 wt.-%) in deionized water using excess solution impregnation. After evaporation of liquid at 343 K for 3 h, samples were dried at 363 K over 6 h.

3.2. Catalyst Characterization

An ASAP 2010 instrument (Micrometrics (Norcross, GA, USA)) was used to record N2-sorption isotherms at 77 K. Before the measurement, samples were degassed for 14 h at 523 K at 10−5 bar. X-ray diffraction measurements were performed using a XRD-7 (Seifert (Hürth, Germany)) with a Cu-anode and monochromatic Cu-Kα (λ = 0.1541 nm). For analysis of the X-ray powder diffractograms, the software Match! (Crystal Impact (Bonn, Germany)) version 3.6.2.121 was used. Reflexes observable in all diffractograms at 2θ = 17.7° and 21.3° were identified as measurement artifacts from the spectrometer.

Temperature-Programmed Methods

50 mg of catalyst (particle size 100–315 µm) were placed in a glass tube and pre-treated at 573 K for 0.5 h in He-flow (40 mL min−1) for TPD-NH3 experiments. Samples were saturated with 6 consecutive pulses of 1 mL NH3 at 363 K. Sample were then purged with He (40 mL min−1) for 1 h to remove physisorbed NH3. The TPD was started by heating the sample with a heating rate of 9 K min−1 up to 833 Kin in He-flow (40 mL min−1). The desorbed gases were analyzed using an online mass spectrometer (MS; QME 200, Pfeiffer Vacuum (Aßlar, Germany)). For detection of NH3 the mass fragment with m/z = 15 (NH) was recorded. For the observation of the NH3-oxidation products N2O and NO, the mass fragments with m/z = 44 and 30 were used, respectively.
Catalyst samples (200 mg, particle size 100–315 µm) were pretreated in a fixed-bed quartz glass reactor (inner diameter = 6 mm) in He/O2 flow (6 vol.-% O2; 100 mL min−1) at 573 K for 1 h for TPD-NOx experiments. The sample was saturated with NO for 60 min in NO/He-flow (900 ppm NO in He; 100 mL min−1) at 363 K. Physisorbed NOx was removed by purging with He (200 mL min−1) for 60 min. The gas mixture was switched to 6 vol.-% O2 in He (150 mL min−1) and the temperature was increased from 363 to 793 K with a heating rate of 7 K min−1. The NOx (NO and NO2) concentrations in the effluent gas were monitored using an online chemiluminescence NOx analyzer CLD 70S (Eco Physics (Dürnten, Switzerland)) to obtain the total adsorption capacity of NOx (ACNOx in μmol g−1).
Temperature-programmed reduction with hydrogen (TPR-H2) was carried out in an AMI-101 (Altamira Instruments (Cumming, GA, USA)) with a moisture trap connected to a thermal conductivity detector. Prior to the TPR experiment, the sample (50 mg, particle size 100–315 µm) was pretreated in air flow (50 mL min−1) at 573 K for 30 min and then cooled down to 343 K. The sample was then heated to 973 K using a heating rate of 10 K min−1 in a gas mixture of hydrogen and argon (20 mL min−1; 5 vol.-% H2).
For temperature programmed decomposition of AN, 100 mg sample (particle size 100–315 µm) impregnated with NH4NO3 (see Section 3.1, Loading with Ammonium Nitrate were pretreated in a vertical quartz-tube reactor (inner diameter of 8 mm) on a bed of quartz wool place inside a furnace in He-flow (100 mL min−1) at 453 K. The sample was than heated to 723 K with a heating rate of 7 K min−1 in He-flow (100 mL min−1). NO and N2O formed during the decomposition of AN were monitored by an online NDIR detector (URAS 10E, Hartmann und Braun (Frankfurt am Main, Germany)). The effluent was passed through a CGO-K NOx-converter (Hartmann und Braun (Frankfurt am Main, Germany)), in which NO2 was selectively reduced to NO. The yield of N2O and NO were calculated based on the amount of AN on the catalyst after impregnation. Uncertainty of the measurement of concentrations of N2O and NO was determined as ±10 ppm.

3.3. Catalytic Experiments

200 mg of catalyst (particle size 100–315 µm) were placed in a fixed reactor (inner diameter of 8 mm). Pretreatment of the catalyst was done in He-flow containing 4 vol.-% O2 at 603 K for 1 h. The NH3-SCR-DeNOx started by passing the reactant gas (500 ppm NO, 575 ppm NH3, 4 vol.-% O2 and He balance) through the reactor with 120 mL min−1 total gas flow rate corresponding to a GHSV of 30,000 h−1. The concentrations of NO and N2O in the reactor effluent were analyzed by an online NDIR detector (URAS 10E, Hartmann und Braun (Frankfurt am Main, Germany)). The effluent was then passed through a NOx-converter (CGO-K, Hartmann und Braun (Frankfurt am Main, Germany)) which was used for the reduction of NO2 to NO to obtain the total concentration of NOx after passing through 75 wt.-% aqueous H3PO4 to absorb unconverted NH3. The reaction temperature was increased in steps of 50 K from 373 to 673 K with a duration of 90 min for every step to ensure steady-state conditions.
Yield of N2O was calculated according to following Equation (1)
Y N 2 O = 2 ∗ C N 2 O C N O , i n + C N H 3 , i n
with C N 2 O being the concentration of N2O measured in the reactor effluent, cNO,in being the inlet concentration of NO and cNH3,in being the inlet concentration of NH3 determined by the flow rates.

4. Conclusions

MnOx, CuOx, CoOx, FeOx, NiOx and CeOx supported on a TiO2 were studied as catalysts for NH3-SCR-DeNOx at temperatures below 473 K as a systematic investigation of the nature and density of acid sites and the redox properties of transition metal oxide catalyst as the most relevant properties for NH3-SCR-DeNOx. Both DeNOx activity and formation of undesired N2O decrease in the same order as a result of decreasing reduction temperature of the catalyst observed by TPR-H2. A linear correlation between the main reduction temperature of the transition metal oxide by TPR-H2 and the temperature required for the conversion of NOx of 50% during NH3-SCR-DeNOx was established. The correlation indicates the reduction temperature is a governing factor for the NOx-Conversion of NH3-SCR-DeNOx over supported catalysts. Moreover, this correlation between the reduction temperature and conversion of NOx during NH3-SCR-DeNOx was also confirmed for a binary metal oxide catalysts with different compositions of MnOx and CeOx supported on TiO2-SiO2 with different molar ratios of nMnOx: nCeOx. MnOx and CeOx was selected as a combination of the highest activity for the conversion of NOx below 473 K and lowest yield of N2O.
Oxidation of NH3 and ammonium nitrate-decomposition over the catalyst are recognized as the main factors for the formation of N2O during NH3-SCR-DeNOx. TPD-NH3 investigations revealed no direct correlation between the total acid site density, i.e., storage capacity for NH3, and the activity for NH3-SCR-DeNOx was observed. However, the desorption of NH3 was shifted to lower temperatures for the catalysts exhibiting higher conversion of NOx at low temperatures, indicating a higher content of Lewis acid sites. The formation of oxidation products, mainly NO and N2O from NH3 by oxidation at Lewis acid sites in the absence of additional O2 followed a trend correlated to the formation of N2O during NH3-SCR-DeNOx. Therefore, it must be assumed that the formation of N2O is strongly correlated to the density of Lewis acid sites and the temperature range where oxidation of NH3 over the catalyst can occur. For the ammonium nitrate-decomposition as a model reaction for the depletion of ammonium nitrate formed during NH3-SCR-DeNOx, the highest selectivity for N2O with 15% during the decomposition was again observed over MnOx/TiO2, with the trend of decreasing decomposition temperature and increasing yield of N2O following the trend of the increasing formation of N2O during NH3-SCR-DeNOx over the transition metal oxide catalysts. Transition metal oxide with high redox activity at lower temperatures, e.g., MnOx or CuOx, reduce the decomposition temperature and increase the N2O-yield during ammonium nitrate-decomposition.
The established correlation between reduction temperature of transition metal oxides and the conversion of NOx as well as between the oxidation temperature range, available Lewis acid sites and the N2O formation provide a basis for the design of new NH3-SCR-DeNOx catalysts. The correlation is not only valid for supported monometallic TMO catalysts for NH3-SCR-DeNOx below 473 K, but also for supported mixed transition metal oxide catalysts over a broad temperature range. The active transition metal oxide catalyst component can be selected or combined based on the reduction temperature in the range of the desired operation temperature range. The high redox activity necessary for high NOx-Conversion at low temperatures was also shown to cause strong formation of undesired N2O. Modification of the support, e.g., by variation of the composition or post-treatment, to tailor the density of Lewis acid sites of the catalyst can be further investigated to counteract the increased N2O formation.

Author Contributions

Conceptualization, R.G., W.S. and M.L.; methodology, W.S., M.K. and M.L.; validation, M.L. and M.K.; formal analysis, M.K., M.L. and W.S.; investigation, M.L. and M.K.; resources, R.G.; data curation, M.L. and M.K.; writing—original draft preparation, M.L.; writing—review and editing, W.S., R.G., M.K. and M.L.; visualization, M.L.; supervision, R.G.; project administration, W.S.; funding acquisition, R.G. and W.S. All authors have read and agreed to the published version of the manuscript.

Funding

Funding from the national research foundation BMWI (AIF Project: 19650 BG, Germany) is gratefully acknowledged. Publication funded by the Open Access Publishing Fund of Leipzig University supported by the German Research Foundation within the program Open Access Publication Funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank Venator Materials PLC (Duisburg, Germany), especially Ralf Becker, for provision of the support material. Furthermore, the authors would like to thank Valerij Teleskij and Szymon Ratowicz who contributed to the experimental work as part of their practical laboratory course and master thesis, respectively.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-Ray diffractograms of investigated catalysts.
Figure 1. X-Ray diffractograms of investigated catalysts.
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Figure 2. Conversion of NOx (left) and yield of N2O (right) during the NH3-SCR-DeNOx over TMO-containing catalysts (reaction conditions: see Section 3.3).
Figure 2. Conversion of NOx (left) and yield of N2O (right) during the NH3-SCR-DeNOx over TMO-containing catalysts (reaction conditions: see Section 3.3).
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Figure 3. Conversion of NOx and yield of N2O for TMO-containing catalysts during NH3-SCR-DeNOx at 453 K (reaction conditions: see Section 3.3).
Figure 3. Conversion of NOx and yield of N2O for TMO-containing catalysts during NH3-SCR-DeNOx at 453 K (reaction conditions: see Section 3.3).
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Figure 4. Profile of desorption of NOx from TMO-containing catalysts during TPD-NOx experiments (left) and sorption capacity for NOx of selected catalysts below 473 and above 573 K (right) (after saturation at 363 K with 500 ppm NO for 30 min, heating rate during desorption: 7 K min−1). Peak deconvolution was done using OriginPro 2017 (Version SR2 9.41038).
Figure 4. Profile of desorption of NOx from TMO-containing catalysts during TPD-NOx experiments (left) and sorption capacity for NOx of selected catalysts below 473 and above 573 K (right) (after saturation at 363 K with 500 ppm NO for 30 min, heating rate during desorption: 7 K min−1). Peak deconvolution was done using OriginPro 2017 (Version SR2 9.41038).
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Figure 5. Profile of the desorption of NH3 (m/z = 15) from TMO-containing catalysts during TPD-NH3 experiments (left) and acid site density of the catalysts determined from TPD-NH3 (right).
Figure 5. Profile of the desorption of NH3 (m/z = 15) from TMO-containing catalysts during TPD-NH3 experiments (left) and acid site density of the catalysts determined from TPD-NH3 (right).
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Figure 6. Profile of desorption of N2O (m/z = 44) from TMO-containing catalysts during TPD-NH3 experiments (left) and selectivity to desorbing species (NH3 and NO + N2O) for selected catalysts during the TPD-NH3 experiments (right).
Figure 6. Profile of desorption of N2O (m/z = 44) from TMO-containing catalysts during TPD-NH3 experiments (left) and selectivity to desorbing species (NH3 and NO + N2O) for selected catalysts during the TPD-NH3 experiments (right).
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Figure 7. Temperature required for a yield of N2O of 10% during NH3-SCR-DeNOx experiments over temperature for the maximum of the formation of N2O during TPD-NH3 experiments.
Figure 7. Temperature required for a yield of N2O of 10% during NH3-SCR-DeNOx experiments over temperature for the maximum of the formation of N2O during TPD-NH3 experiments.
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Figure 8. Profiles of the formation of N2O as well as NO + NO2 during the decomposition of AN loaded on TMO-containing catalysts (left) and yield of decomposition products related to loading of AN (right).
Figure 8. Profiles of the formation of N2O as well as NO + NO2 during the decomposition of AN loaded on TMO-containing catalysts (left) and yield of decomposition products related to loading of AN (right).
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Figure 9. Profile of the consumption of H2 over temperature during TPR-H2 experiments for selected catalysts (left) and relation between the temperature required for a conversion of NOx of 50% during NH3-SCR-DeNOx and the main reduction window during TPR-H2 experiments (right). Peak deconvolution was done using OriginPro 2017 (Version SR2 9.41038).
Figure 9. Profile of the consumption of H2 over temperature during TPR-H2 experiments for selected catalysts (left) and relation between the temperature required for a conversion of NOx of 50% during NH3-SCR-DeNOx and the main reduction window during TPR-H2 experiments (right). Peak deconvolution was done using OriginPro 2017 (Version SR2 9.41038).
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Figure 10. Conversion of NOx and yield of N2O during the NH3-SCR-DeNOx over MnOx-CeOx mixed oxides catalysts as well as mono-oxidic MnOx and CeOx (Reaction conditions: see Section 3.3).
Figure 10. Conversion of NOx and yield of N2O during the NH3-SCR-DeNOx over MnOx-CeOx mixed oxides catalysts as well as mono-oxidic MnOx and CeOx (Reaction conditions: see Section 3.3).
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Figure 11. Profiles of the consumption of H2 during TPR-H2 experiments on MnOx-CeOx mono- and mixed oxide catalysts over temperature (left) and temperature required for conversion of NOx of 50% during NH3-SCR-DeNOx over the temperature of main reduction window during TPR-H2 experiments (right).
Figure 11. Profiles of the consumption of H2 during TPR-H2 experiments on MnOx-CeOx mono- and mixed oxide catalysts over temperature (left) and temperature required for conversion of NOx of 50% during NH3-SCR-DeNOx over the temperature of main reduction window during TPR-H2 experiments (right).
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Liebau, M.; Suprun, W.; Kasprick, M.; Gläser, R. Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3. Catalysts 2025, 15, 22. https://doi.org/10.3390/catal15010022

AMA Style

Liebau M, Suprun W, Kasprick M, Gläser R. Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3. Catalysts. 2025; 15(1):22. https://doi.org/10.3390/catal15010022

Chicago/Turabian Style

Liebau, Michael, Wolodymyr Suprun, Marcus Kasprick, and Roger Gläser. 2025. "Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3" Catalysts 15, no. 1: 22. https://doi.org/10.3390/catal15010022

APA Style

Liebau, M., Suprun, W., Kasprick, M., & Gläser, R. (2025). Transition Metal Oxides Supported on TiO2 as Catalysts for the Low-Temperature Selective Catalytic Reduction of NOx by NH3. Catalysts, 15(1), 22. https://doi.org/10.3390/catal15010022

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