**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 threedimensional 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

**Citation:** Kapkowski, M.; Siudyga, T.; Bartczak, P.; Zubko, M.; Sitko, R.; Szade, J.; Balin, K.; Witkowski, B.S.; Ozga, M.; Pietruszka, R.; et al. ˙ Catalytic Removal of NOx on Ceramic Foam-Supported ZnO and TiO2 Nanorods Ornamented with W and V Oxides. *Energies* **2022**, *15*, 1798. https://doi.org/10.3390/en15051798

Academic Editor: Wasim Khan

Received: 11 February 2022 Accepted: 26 February 2022 Published: 28 February 2022

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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 [9–11]. 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 agen<sup>t</sup> (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]:

$$4\text{NO} + 4\text{NH}\_3 + \text{O}\_2 \to 4\text{N}\_2 + 6\text{H}\_2\text{O}$$

$$4\text{NH}\_3 + 6\text{NO} \to 5\text{N}\_2 + 6\text{H}\_2\text{O}$$

$$2\text{NO}\_2 + 4\text{NH}\_3 + \text{O}\_2 \to 3\text{N}\_2 + 6\text{H}\_2\text{O}$$

$$8\text{NH}\_3 + 6\text{NO}\_2 \to 7\text{N}\_2 + 6\text{H}\_2\text{O}$$

 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 managemen<sup>t</sup> 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 m<sup>2</sup> 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 ZnOstructured 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**
