Catalytic Removal of NOx on Ceramic Foam-Supported ZnO and TiO₂ Nanorods Ornamented with W and V Oxides

Abstract

Energy consumption steadily increases and energy production is associated with many environmental risks, e.g., generating the largest share of greenhouse gas emissions. The primary gas pollution concern is CO₂, CH₄, and nitrogen oxides (NOx). Environmental catalysis plays a pivotal role in NOx mitigation (DeNOx). This study investigated, for the first time, a collection of ceramic foams as potential catalyst support for selective catalytic NOx reduction (SCR). Ceramic foams could be an attractive support option for NOx removal. However, we should functionalize the surface of raw foams for such applications. A library of ceramic SiC, Al₂O₃, and ZrO₂ foams ornamented with nanorod ZnO and TiO₂ as W and V oxide support was obtained for the first time. We characterized the surface layer coating structure using the XPS, XRF and SEM, and TEM microscopy to optimize the W to V molar ratio and examine NO₂ mitigation as the SCR model, which was tested only very rarely. Comparing TiO₂ and ZnO systems reveals that the SCR conversion on ZnO appeared superior vs. the conversion on TiO₂, while the SiC-supported catalysts were less efficient than Al₂O₃ and ZrO₂-supported catalysts. The energy bands in optical spectra correlate with the observed activity rank.

1. Introduction

Energy demand growth is a fundamental problem of civilization in the Anthropocene. The production of energy is, however, associated with many environmental risks. Mainly, NOx formation is a consequence of energy production. One of the targets of environmental catalysis is the mitigation of NOx from the air. The design and development of porous functional materials is an essential environmental catalysis domain [1]. In particular, porous ceramics are essential catalyst supports in this area [2]. Ceramic foams are monolithic three-dimensional structures with an 80–90% void spaces fraction. However, these materials were developed initially to filter out molten metal impurities [3], which means that their surface area is generally too low for catalytic applications. Therefore, ceramic foams are modified to increase their surface area [4]. A variety of novel catalytic applications of ceramic foams involve, for example, the catalytic pyrolysis of waste oils to renewable fuels [5], the water gas shift reaction [6], and the solar photocatalytic ozonation in water treatment using supported TiO₂ [7] methane steam reforming [8]. The advantages of the innovatively structured foam catalysts involve fluid dynamics and heat transfer phenomena, which can positively influence catalyst performance.

This study investigated a collection of ceramic foams as potential catalysts for selective catalytic NOx reduction (SCR) reactions. Despite potential advantages, the literature rarely describes the application of ceramic foams in SCR catalysis [9,10,11]. The availability of the commercial honeycomb or plate SCR catalysts may be one reason for this fact. NOx generation, pollution, and reduction are complex problems. First, combustion in air yields two forms of NOx, namely NO and NO₂, in the ratio NO/NOx of 0.90 to 0.95 [12]. However, the dominating NOx form in the atmosphere is NO₂ resulting from NO oxidation. Accordingly, the main issue in the environmental catalysis of NOx refers to NO, while the main topics of NOx ecotoxicology refer to NO₂. The current technology routinely uses Selective Catalytic Reduction (SCR) for NOx removal from flue gases [12,13]. SCR is the reaction between the NOx in exhaust gases and the reducing agent (NH₃ as ammonia water or urea solution) at the so-called deNOx catalyst at temperatures below 400 °C to produce N₂ and water vapor. The reaction formulas for NO and NOx are as follows [12,14]:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

4NH₃ + 6NO → 5N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

8NH₃ + 6NO₂ → 7N₂ + 6H₂O

Accordingly, SCR needs twice as much NH₃ for NO than for NO₂ reduction.

The design of the supporting material of the SCR catalysts (TiO₂, ZrO₂, Al₂O₃, and ZnO) and the synthesis method of the system affect a final catalyst structure not only in the direct titania phase character, e.g., surface area and porosity structure, but also indirectly by deciding the structure of the W and V surface deposits, which form the acid catalytic centers controlling the mechanisms of the SCR reaction with NH₃ [14]. Specifically, NOx needs to be removed from the flue gases, particularly in electric power stations where generally TiO₂-SCR catalysts are used. Regenerating the spent TiO₂-SCR catalysts is a significant problem that needs further improvement despite many available options [15,16]. Currently, industrial TiO₂-based SCR systems suffer from surface deposits. Moreover, the exploitation deteriorates surface texture. As SCR catalysts are expensive, they need to be regenerated. The management of deactivated SCR catalysts should minimize the adverse environmental effects of these materials. On the other hand, it could also be a valuable resource of rare chemical elements such as vanadium and tungsten [17].

In particular, we focused this study on nitrogen dioxide (NO₂) mitigation. NO₂ is the most toxic NOx form in the atmosphere. Both indoor and outdoor NO₂ pollution exposure to humans was extensively studied [18]. At the same time, there are only several publications reporting the catalytic decomposition of NO₂, even though a complex NO/NO₂ SCR should support a standard NO SCR [19,20]. Recently, catalytic decomposition of NO₂ over a copper-decorated metal-organic framework by non-thermal plasma was studied [21].

The industrial SCR installations are TiO₂ layers with surface-engineered coatings by metal oxides. Zinc oxide is a low-cost TiO₂ potential alternative for catalytic SCR applications in environmental catalysis. However, the reported applications in this area, particularly NOx removal, are rare. The low ZnO stability is a reason why titania is much more popular. For example, the surface of polar ZnO nanoparticles undergoes significant change during storage at room temperature in the presence of moisture, oxygen, or light. During two-month storage, the specific surface area of the ZnO decreases from 115 to 35 m² g by room temperature sintering of polar ZnO nanosheets [22]. Recent efforts involve the potential enhancement of ZnO stability by structural modifications, such as doping [23] and core-shell nanoparticle formation [24], e.g., using hybrid ZnO-TiO₂ systems [25]. Surface deposits, e.g., Cu₂O/MoS₂/ZnO composites on Cu mesh, can upgrade the ZnO system, reducing N₂ to NH₃ (with water as the proton source) in the liquid membrane reactor under simulated visible light [26]. The room temperature sintering of polar ZnO nanosheets was prevented by forming the surface layer of silica (2 atom %) [27]. The engineering of ZnO-structured surfaces is an issue of general interest. In particular, nanostructures (nanowires, nanotubes, nanobelts, nanorings, flower-like morphologies, multipods, tetrapods, and sponge-like structures) were broadly investigated [28]. Chemical and Physical Vapor Deposition (CVD and PVD) sputtering, as well as evaporation approaches and epitaxial growth, are the methods for forming the continuous functional thin layers on ZnO. Wet chemical and template-assisted methods are alternatives [28].

For the first time, here, we investigated the influence of the ornamentation of Al₂O₃, SiC, and ZrO₂ ceramic foams by nanorod ZnO and TiO₂ coatings as W and V support on DeNOx catalysis, in particular on the NO₂ SCR process (gas flow rate of 3 dm³/h and temperature of 400 °C at atmospheric pressure). In particular, we compared the broad library of the VOx or WOx supported on the SiC, Al₂O₃, ZnO, CeO₂, MgO, SiO₂, TiO₂, and ZrO₂ in the DeNOx within the temperature range of 200–400 °C. Physical and chemical analyses of the functionalized foam surface confronted with the catalyst performance indicate that these materials can be a new efficient SCR reaction platform.

2. Materials and Methods

2.1. Materials

Commercially available chemical reagents were used in the study: Tungsten powder <12 µm, 99.9% trace metal basis (Sigma-Aldrich, St. Louis, MO, USA); Vanadium powder 100 mesh, 99.9% trace metal basis (Sigma-Aldrich); and 30% hydrogen peroxide (Avantor Performance Materials., Gliwice, Poland). All chemicals were used without further purification.

2.2. Preparation of TiO₂ or ZnO Nanofilaments at Ceramic Foams

ZnO nanorods were prepared according to the following procedure. First, we used the ALD method to deposit the zinc oxide nanoseeds on Al₂O₃, SiC, and ZrO₂ substrates. We performed 12 ALD cycles using diethylzinc (DEZ; Sigma-Aldrich) as a zinc precursor and deionized water as an oxygen precursor at the temperature of 100 °C in the ALD Savannah 100 reactor from Cambridge NanoTech, Waltham, MA, USA (now called the Vecco Savanah^® Series). The hydrothermal process of zinc oxide nanorods’ growth consists of two steps, as described previously [29]. First, the reaction mixture with a Zn concentration of 1 mM was prepared. Zinc acetate dihydrate (Roth, 99% pure) was dissolved in 60 mL of deionized water. Afterward, the pH of the solution was adjusted to 7.5 using a 1 M aqueous solution of sodium hydroxide (Sigma-Aldrich). The prepared mixture was heated using an induction heater at 95 °C with nucleated substrates inside and kept at this temperature for approximately 2 min. Then, the samples were removed from the reactor, rinsed with isopropanol, and dried in air.

Part of the received substrates with ZnO nanorods were used as a scaffold for the growth of the nanostructured TiO₂ surface. These substrates covered by ZnO nanorods were placed in the ALD reactor at 100 °C. The 90 ALD cycles deposited approximately 50 nm thick layers of TiO₂. We used tetrakis (dimethylamino) titanium (IV; TDMAT) from STREM Chemicals as a titanium precursor and deionized water as the oxygen precursor. After TiO₂ deposition, nanorods were etched using 1% HCl acid for 1 min, rinsed in water, and dried.

2.3. General Preparation of W and V Nanoparticles at Ceramic Foams and Other Carriers

To a weight of tungsten and/or vanadium powders (Table S1), 2 mL of 30% hydrogen peroxide solution was added and mixed for 3 h until the metal wholly dissolved. Ceramic foams described in paragraph 2.2 were powdered in mortar and sieved, and 990 mg of the selected material (Al₂O₃, ZrO₂, or SiC carrier with TiO₂ or ZnO₂ nanofilaments) was suspended in a solution containing tungsten and/or vanadium. The mixture was stirred until the solvent evaporated, yielding a powder catalyst. The catalysts containing tungsten and vanadium oxides deposited on powders ZnO, CeO₂, MgO, SiO₂, or TiO₂ (Sigma-Aldrich) were prepared using the same method (Table S2).

2.4. Methods of Catalyst Characterization

XPS measurements were completed using the PHI 5700 photoelectron spectrometer (Physical Electronics Inc., Chanhassen, MN, USA). Photoelectrons were excited by a monochromatic X-ray beam (Al Kα of energy of 1486.6 eV) from the sample surface. The resulting photoelectron spectra obtained for each element constituting the sample were analyzed using PHI MultiPak (v.9.6.0.15, ULVAC-PHI, Chigasaki, Japan) software. The obtained high-resolution spectra were calibrated using the C1s peak (284.8 eV), the occurrence of which is related to the presence of adsorbed carbon on the sample surface. Analyzed core levels were fitted using a combination Gauss–Lorentz shape of the photoemission line and Shirley background.

We used an energy-dispersive X-ray fluorescence (EDXRF) spectrometer—Epsilon 3 (Panalytical, Almelo, The Netherlands) to perform chemical analysis. The spectrometer was equipped with a thermoelectrically cooled silicon drift detector (SDD) and Rh target X-ray tube. It was operated at a maximum voltage of 30 keV and maximum power of 9 W. We applied the Omnian software with the fundamental parameter method for quantitative analysis. Measurement conditions were as follows: counting time 5 kV, 300 s, and helium atmosphere for Si and Al determination; counting time 12 kV, 300 s, air atmosphere, and 50 μm Al primary beam filter for V; 20 kV, counting time 120 s, air atmosphere, and 200 μm Al primary beam filter for Fe; and 30 kV, counting time 120 s, air atmosphere, and 100 μm Ag primary beam filter for W, Zn, Zr, Y, and Hf. We fixed the current of the X-ray tube not to exceed the dead-time loss of ca. 50%.

We used Hitachi SU-70 equipment (15 kV of accelerating voltage using a secondary electron detector) for Scanning Electron Microscopy (SEM) and the transmission electron microscopy (TEM) was performed in the JEOL high-resolution (HRTEM) JEM 3010 microscope operating at a 300 kV accelerating voltage with a Gatan 2k × 2k OriusTM 833SC200D CCD camera and an EDS detector from IXRF Systems. The samples were suspended in isopropanol and deposited on a Cu grid with an amorphous carbon film standardized for TEM observations. Selected Area Electron Diffraction (SEAD) patterns were indexed using dedicated ElDyf software (Institute of Material Science, University of Silesia., Katowice, Poland).

The X-ray powder diffraction (XRD) measurements were carried out using a Malvern Panalytical Empyrean diffractometer. Cu anodes operated at a wavelength of 1.54056 Å, at an electric current of 30 mA and voltage of 40 kV, and equipped with a PIXcell3D solid-state hybrid pixel detector. The XRD was registered in the angular range of 2θ = 15–145° with 0.02° steps. The phase analysis involved reference standards from the International Centre for Diffraction Data (ICDD) PDF-4 database. Rietveld refinement was performed using FullProf computer software (available at www.ill.eu/sites/fullprof/ (accessed on 10 February 2022)).

2.5. NOx Decomposition in a Flow Reactor

We used a fixed-bed quartz flow reactor using a 200 mg sample of the catalysts at 200–400 °C under atmospheric pressure to test the SCR catalysis performance. We crushed the catalyst to a fine powder before SCR tests. The feed gas was composed as follows I: 0.2% NO₂ + 5% O₂ + 94.8% He and inlet II: 0.2% NH₃ + 5% O₂ + 94.8% He (volume ratio 3:4). The tests were performed at the flow rate of 3 dm³/h. We used the GC-FID method to monitor the gas composition. The NOx conversion amounted to:

NOx conversion = [(NOx inlet − NOx outlet)/NOx inlet] × 100%

3. Results and Discussion

3.1. The Catalysts Design, Preparation, and Structure

The exemplary EDXRF spectra of the selected catalysts collected using the Rh X-ray tube operating at 30 and 5 kV are given in Figure 1. Spectra of ZnO/Al₂O₃ show peaks at 6.40, 7.06, 8.64, 9.57, 15.77, 20.21, 22.72, 2.70, 1.49, and 1.74 keV, corresponding to Fe Kα, Fe Kβ, Zn Kα, Zn Kβ, Zr Kα, Rh Kα, Rh Kβ, Rh Lα (X-ray tube), Al Kα, and Si Kα, respectively. Spectra of ZnO/ZrO₂ show peaks at 2.04, 8.64, 9.57, 14.96, 15.77, and 17.67 keV, corresponding to Zr Kα, Zn Kα, Zn Kβ, Y Kα, Zr Kα, and Zr Kβ, respectively. The spectra also reveal the presence of Hf as common impurities of zirconium compounds (Lα, Lβ, and Lγ lines at 7.90, 9.02, and 10.52 keV, respectively). The EDXRF spectra of the 1.0% W,V/ZnO/Al₂O₃ and 1.0% W,V/ZnO/ZrO₂ systems reveal the presence of tungsten (Lα, Lβ, and Lγ lines at 8.40, 9.67, and 11.29 keV, respectively) and vanadium (Kα and Kβ lines at 4.95 and 5.43 keV, respectively). Table S3 presents the results of the quantitative EDXRF analysis of ZnO/Al₂O₃, 1.0% W/ZnO/Al₂O₃, 1.0% W,V/ZnO/Al₂O₃, ZnO/ZrO₂, 1.0% W/ZnO/ZrO₂, and 1.0% W,V/ZnO/ZrO₂.

We performed X-ray powder diffraction measurements to examine the phase composition of the material samples. Compared with reference standards from the ICDD PDF4+ database of the Al₂O₃/ZnO sample, this analysis revealed four phases. The Al₂O₃ (PDF 04-006-9730) and Al₂SiO₅ (PDF 01-088-0892) phases were dominating, while we also observed the ZnO (PDF 01-078-4603) and SiO₂ (PDF 04-013-9484) phases. The additional nano-size SiO₂ phase (PDF 01-073-3436) was detected for the samples containing W or the combined W and V oxides. No additional phases nor impurities were observed. The performed full pattern Rietveld refinement allowed for the determination of the crystallographic parameters of the phases formed. Accordingly, the mean crystallite size based on the peak broadening was calculated. The obtained results of the Rietveld refinement are shown in Table S4. The X-ray powder diffraction measurements were also conducted for ZrO₂/ZnO samples. The phase analysis revealed the presence of two ZrO₂ phases. The dominant, monoclinic one (PDF 04-010-6452) was accompanied by the cubic phase (PDF 01-078-3193). Additionally, ZnO (PDF 01-078-4603) and Zn₂SiO₄ (PDF 01-076-8176) phases were also detected. Table S4 and Figure 2 show the results of the Rietveld refinement. We should remember that metallic oxides could not be observed with the X-ray powder diffraction method due to the X-ray diffraction detection limit. Accordingly, the XRF, XPS, and TEM techniques confirmed the presence of W and V oxides.

The TEM with an energy-dispersive X-ray detector confirmed the occurrence of metallic particles. The metallic nanoparticles occurred on the ceramic Al₂O₃ microparticles as indicated by the bright-field images (Figure 3). These structures were also proved with other spectroscopic measurements using XRF or XPS techniques that map the presence of metallic elements.

The XPS analysis determined the chemical states of the elements composing the samples. The main focus of the analysis was to determine the chemical states of vanadium and tungsten. We performed deconvolution of the V2p, W4f, and W4d photoemission lines. Additionally, lines O1s and C1s, and those associated with the Al₂O₃, ZrO₂, and ZnO matrix were analyzed.

The surface XPS spectra of the V2p core level, as shown in Figure 4a, indicate vanadium oxide in the studied samples. A slight shift in the position of the V2p_3/2 line was observed between examined systems; for the 1.0% W,V/ZnO/ZrO₂ system, the binding energy of the V2p_3/2 line is 516.96 eV, while for the 1.0% W,V/ZnO/Al₂O₃ system, it is 516.64 eV. The reference V2p_3/2 binding energies of the V₂O₅, as given in the NIST database [30], range from 516.6 eV to 517.7 eV. However, literature data specify that the 516.94 eV peak can also be assigned to vanadium 4+ [31]. The vanadium line is presented in Figure 4a together with the oxygen line. The location of the deconvoluted oxygen lines was assigned to the metal oxides or C=O bond, as also seen with the carbon C1s lines (not shown here).

The chemical state analysis of tungsten was based on two lines, namely W4f and W4d (see Figure 4b,c). For the W4f line, a superposition of the core levels originating from other elements detected on the sample surface was observed. Therefore, the tungsten chemical states, as identified through the W4f line analysis, were further examined, inspecting the shape of the W4d line.

The deconvoluted peaks of the W4f XPS spectra showed two oxides for both examined systems; the binding energy of the W4f_7/2 line at 33.79 eV can be assigned to WO₂ [32], whereas the peak at 35.76 eV can be assigned to the WO₃ [33]. Other peaks seen in the W4f line are associated with various elements detected on the sample’s surface (e.g., Na and F were visible in the overview spectra, while Zr and V are components of the studied systems), as marked in Figure 4b. The presence of WO₂ and WO₃ oxides was confirmed by analysis of line W4d, as illustrated in Figure 4c. Analysis of the W4d line allows us to more accurately see the differences in the ratio of each oxide’s contribution to a given system. XPS chemical analysis showed no significant differences in the spectra of the base compounds. The 1.0% W,V/ZnO/Al₂O₃ system and the corresponding reference sample showed the presence of the Al₂O₃ (Al2p line at 74.62 eV [34]). The binding energy of Zn2p_3/2 can be ascribed to the Zn²⁺ state [35] and ZnO oxide [36]; those chemical states were observed at 1020 eV and 1022.3 eV, respectively. Similarly, for the 1.0% W,V/ZnO/ZrO₂ system, the presence of zinc and zirconium oxides (ZnO assigned for Zn2p_3/2 at 1022.3 eV [36], and clusters of ZrO₂ for Zr3d_5/2 at 182.44 eV [37]) were detected. Additionally, a relatively small amount of zirconium oxide nanocrystallites for the Zr3d_5/3 at 181.2 eV [38]) was present on the sample surface. The positions of the photoemission lines were consistent with those observed for the reference sample. A detailed comparison of the XPS spectra for the 1.0% W,V/ZnO/ZrO₂ vs. 1.0% W,V/ZnO/Al₂O₃ vs. 1.0% W,V/ZnO/SiC systems is shown in Figure 4 and Figure S1 Supplementary Materials.

3.2. Catalyst Performance in SCR Reaction

We designed a broad library of tested catalyst systems by combining SiC, Al₂O₃, and ZrO₂ (foams) with ZnO, CeO₂, MgO, and SiO₂ or TiO₂ (coatings) with W and V oxides’ loads. In these initial experiments, we decided to use the 1:1 W to V molar ratio due to the synergistic effect of W and V oxides in SCR catalysis [39,40]. A library of potential supports and coatings were pretested at five operating temperatures, assuming the maximum process temperature of 400 °C. Table S2 summarizes the results.

The NOx conversion was 22.5% vs. 19.7% vs. 18.1% at 250 °C for the most active systems Al₂O₃, MgO, and ZrO₂ (Table S2, entries 2, 5, and 8). The increase in temperature to 300 °C resulted in a slight increase in the conversion to 45.3% vs. 36.4% vs. 31.8%. However, it was not but at the temperature of 350 °C where a significant increase in the NOx conversion was observed, especially for ZrO₂ and Al₂O₃, where conversion takes a value of 85.2% and 80.6%, respectively (Table S2, entry 8 vs. 2). Increasing the temperature to 400 °C in the tested systems resulted in a slight decrease (ZnO, MgO, and ZrO₂) or increase (SiC, Al₂O₃, CeO₂, SiO₂, and TiO₂) of the NOx conversion rate (Table S2, entries 3, 5, and 8 vs. 1, 2, 4, 6, and 7). In turn, the W and V oxides supported by CeO₂, SiO₂, MgO, TiO₂, and SiC allowed for a relatively low conversion of NOx (60–79%) at 400 °C. Interestingly, the literature often describes these systems as an attractive alternative for commercial SCR catalysts [40,41,42,43,44]. In the context of the potential supporting foams (Al₂O₃, ZrO₂, and SiC), the Al₂O₃ and ZrO₂ outperform the SiC one. The catalytic performance of both Al₂O₃ and ZrO₂ is higher than 80% at 400 °C, which locates these supports just after the superior ZnO support (entries 2 vs. 8 vs. 3, Table S2).

As for Al₂O₃, considering the ZrO₂ foam supports appeared comparable in the catalytic performance tests (Table S2), we selected the Al₂O₃ support coated with nanorod ZnO for extensive and thorough testing. TiO₂ was selected as a comparison, providing illustrative insight into the current SCR systems. In particular, we tested the influence of the VOx to WOx ratio on the NOx conversion. This ratio was set at 1:0; 7:3; 1:1; 3:7; and 0:1, respectively. The nominal content of the supported metal oxides in all catalyst systems was 1%. We used the TiO₂ or ZnO nanorods at the Al₂O₃ carrier as the comparison standards (entries 1 and 7,

Table 1. The V and W load optimization on TiO₂ and ZnO nanorod-coated Al₂O₃ foam.

Entry	Catalyst	NOx Conversion [%] a	TON $[\frac{\mathit{m}\mathit{m}\mathit{o}\mathit{l}}{\mathit{k}\mathit{g}\cdot \mathit{h}}]$
1	TiO2/Al2O3	78.7	527
2	1% V/TiO2/Al2O3	85.5	573
3	1% V,W(7:3)/TiO2/Al2O3	89.4	599
4	1% V,W(1:1)/TiO2/Al2O3	88.3	591
5	1% V,W(3:7)/TiO2/Al2O3	90.2	604
6	1% W/TiO2/Al2O3	91.5	613
7	ZnO/Al2O3	81.1	543
8	1% V/ZnO/Al2O3	82.9	555
9	1% V,W(7:3)/ZnO/Al2O3	87.6	587
10	1% V,W(1:1)/ZnO/Al2O3	90.8	608
11	1% V,W(3:7)/ZnO/Al2O3	93.0	623
12	1% W/ZnO/Al2O3	92.4	619

^a Gas flow rate of 3 dm³/h, temperature 400 °C, and atmospheric pressure.

).

For TiO₂-based systems with a higher VOx content vs. WO₃ (1:0 and 7:3, respectively), NOx conversion is slightly higher by 1.8–2.4% than the analogous ZnO-based systems (Table 1, entries 2 and 3 vs. 8 and 9). The opposite tendency was observed for the systems with VOx to WO₃ ratios of 1:1, 3:7, and 0:1 (Table 1, entries 4–6 vs. 10–12). The screen of the systems in Table 1 allowed us to optimize the molar ratio of the WOx to VOx. The best catalysts were selected based on the TON parameter. For ZnO/Al₂O₃ nanorods, the highest TON of 623 mmol/kg^.h was observed for 1% V,W(3:7)/ZnO/Al₂O₃, while for TiO₂/Al₂O₃ nanorods, the highest TON of 623 mmol/kg^.h was observed for the 1% W/TiO₂/Al₂O₃ (TON = 613 mmol/kg^.h). Both for the TiO₂/Al₂O₃ nanorods and ZnO/Al₂O_3, we observed a systematic increase in TON when the VOx content decreased in favor of the WOx content (Table 1: 543 vs. 619 mmol/kg^.h or 527 vs. 613 mmol/kg^.h). Accordingly, in the next step, we prepared the ceramic foams with nanorod TiO₂ or ZnO coating with a VOx to WO₃ load at the optimal molar ratio of 3:7 (Table 1).

Table 2. Catalytic performance of the Al₂O₃, SiC, and ZrO₂ foam coated with zinc oxide nanorods with surface loadings of V and W oxides.

Entry	Catalyst	Sample Code	NOx Conversion (%) a
1	ZnO/Al2O3	ZA	88.6
2	1% W/ZnO/Al2O3	W/ZA	90.3
3	1% V/ZnO/Al2O3	V/ZA	88.2
4	1% W,V/ZnO/Al2O3	W,V/ZA	94.8
5	ZnO/SiC	ZS	81.7
6	1% W/ZnO/SiC	W/ZS	89.9
7	1% V/ZnO/SiC	V/ZS	83.2
8	1% W,V/ZnO/SiC	W,V/ZS	89.1
9	ZnO/ZrO2	ZZ	91.3
10	1% W/ZnO/ZrO2	W/ZZ	92.4
11	1% V/ZnO/ZrO2	V/ZZ	85.4
12	1% W,V/ZnO/ZrO2	W,V/ZZ	93.0

^a Gas flow rate of 3 dm³/h, temperature 400 °C, and atmospheric pressure.

shows a detailed comparison of NOx conversions for ZnO nanofilaments with different surface coatings on ZrO₂, Al₂O₃, and SiC foam supports measured in SCR reaction. ZnO nanofilaments in the presence of VOx and WOx on the Al₂O₃ carrier turned out to be the most active (NOx conversion of 94.8%), slightly outperforming other carriers (ZrO₂ and SiC) with ZnO nanofilaments decorated with surface VOx and WOx (having a NOx conversion of 93.0% and 89.1%, respectively). Specifically, ZnO nanofilaments with VOx and/or WOx increase the deNOx activity of all crude foam supports. High activity of these systems may result from expanding the surface of the support by the nanorod coating.

Figure 5 illustrates the catalytic performance of these systems in the temperature range of 200–400 °C. For the catalytic systems with TiO₂ nanorods, the highest degree of NOx conversion at 400 °C (88–96%) was noted when the carrier was ZrO₂ (Figure 5A), while the lowest was noted for SiC support (55–63%). ZnO nanorods deposited on Al₂O₃ made it possible to obtain the 90–95% degree of NOx conversion at 400 °C, while the worst carrier for ZnO nanorods was again SiC, with an 83–90% NOx conversion (Figure 5B). Generally, the comparison of TiO₂ and ZnO nanorods in Figure 5A,B reveals that the SCR conversions on ZnO appeared better than on the TiO₂ supports in the range of the tested temperatures.

A comparison of the foam supports indicates that for the oxide-supported (Al₂O₃ and ZrO₂) catalysts, SCR reactivity slightly outperforms the SiC supported systems. Formally, all supports are semiconductors of the wide gap of ca. 3.2 ÷ 3.37 eV, but in nano ZnO or TiO₂, the gap energy can be modulated by nanostructure organization and interactions with supported metals (W or V) present at the surface in the form of metal oxides. Optical spectra of the ZnO and TiO₂ supported on Al₂O₃ and SiC (Figure S2, Supplementary Materials) indicate a correlation between deNOx behavior and the energy band gap. In particular, unlike the Al₂O₃, the SiC-supported ZnO and TiO₂ systems indicate an additional low systems energy absorption band. It is, however, not clear if this can affect the thermal (dark) SCR reaction.

Interestingly, material structure and coatings can influence the band gap value. For example, the tungsten oxides’ band gap can be reduced to 2.47 through the interactions with other semiconductors, e.g., CdTe [45]. Similarly, the contacts of Ti and V₂O₅, even as large crystallites, modified the optical band gap (1.96 eV vs. 2.2 eV for undoped V₂O₅). This effect is attributed to lattice expansion by the Ti ion and oxygen vacancies formation [46]. The specific interactions of the individual nanorod coating and synergistic interaction of the V and W oxide coatings can also play a role. For example, the doping of the W oxide with V can result in synergic interactions downshifting the material band gap [47]. In turn, the band-gap energies of the SiC nanowires are higher than the corresponding SiC bulk values [48].

4. Conclusions

Energy production is associated with many environmental risks; e.g., generating greenhouse gas emissions and nitrogen oxides (NOx) are among the primary gas pollution concerns. Environmental catalysis plays a pivotal role in NOx mitigation (DeNOx). This study investigated a collection of ceramic foams as a potential catalyst support for selective catalytic NOx reduction (SCR). To be an attractive support, we should functionalize the surface of raw foams. A library of ceramic SiC, Al₂O₃, and ZrO₂ foams ornamented with nanorod ZnO and TiO₂ as W and V oxide coatings was obtained for the first time. We characterized the surface layer structure using XPS, XRF, and SEM and TEM microscopy to optimize the W to V molar ratio.

NOx generation, pollution, and reduction are complex problems. First, combustion in air yields two forms of NOx, namely NO and NO₂, in the ratio NO/NOx of 0.90 to 0.95. However, the dominating NOx form in the atmosphere is NO₂ resulting from NO oxidation. Accordingly, the main issue in the environmental catalysis of NOx refers to NO, while the main topics of NOx ecotoxicology refer to NO₂. Low NO₂ content in flue gases also decides NO₂. SCR is tested only very rarely, even though a complex NO/NO₂ SCR should support a standard NO SCR; therefore, here, we used the NO₂ SCR model.

Comparing TiO₂ and ZnO systems reveals that the SCR conversion on ZnO appeared superior vs. conversion on TiO₂, while the SiC-supported catalysts were less efficient than Al₂O₃ and ZrO₂-supported catalysts. The energy bands in optical spectra correlate with the observed activity rank. However, a more detailed study is needed to assess whether this effect is coincidental only.