**Preface to "Environmental Friendly Catalysts for Energy and Pollution Control Applications"**

Catalysts are extensively used in various technologies, playing a fundamental role in the efficient generation of energy and in controlling industrial emissions. The use of catalysts has great importance in terms of its productive, economic and environmental implications. Catalysts not only improve production systems but also contribute controlling the emissions of pollutants. Catalysts are mainly applied in the gaseous phase (to control emissions from either fixed or mobile sources) and in the liquid phase (mostly for water and wastewater treatment). In the gas phase, catalysts are used to reduce the emissions of pollutants from power generation systems, chemical and manufacturing industries, and vehicle exhaust gases, whereas in the liquid phase, catalysts enable the catalytic oxidation of refractory organic pollutants, converting industrial wastewaters into admissible discharges.

Nevertheless, catalysts in some cases have a negative environmental impact due to their intensive production strategies. Therefore, the search for new materials such as environmentally friendly catalysts (EFCs) is urgently needed. Hence, the development of novel EFCs for sustainable energy production, to face climate change problems, and to abate industrial emissions has become a challenge in the current research fields. A great variety of catalytic materials, including single metals as well as mixed metals (and their oxides), are currently being used, either supported over alumina, silica, titania, platinum, ceria, strontium, cobalt, sodium citrate, activated carbons, and zeolites, or directly attached to the reactor itself, allowing their continuous use and avoiding waste emissions.

This Special Issue addresses applications of catalysts as an effective solution for the treatment of industrial emissions, focusing on catalytic ozonation, heterogeneous photocatalysis and Fenton catalysis, as well as the conversion into EFCs through optimized, heterogeneous Fenton-like variants and catalytic combustion. It also includes other applications involving biocatalysts for the design of bioanodes and new catalysts for energy production in fluid catalytic cracking (FCC) units. In addition to the above-mentioned catalysts, a significant part of the new research is focused on catalyst designs that allow recycling through a metal–organic framework (MOF). Thus, there is a marked tendency to develop materials that can be used in multiple operating cycles with high efficiency and selectivity.

This Special Issue of *Catalysts* comprises contributions from 57 authors distributed in 10 countries: Spain (11 authors), Chile (10 authors), Malaysia (10 authors), China (6 authors), Poland (6 authors), United States of America (6 authors), Ecuador (5 authors), Turkey (1 author), Bangladesh (1 author), and Nigeria (1 author). It is intended for academics and practitioners working in the energy and environmental sectors searching for environmentally sustainable catalysts. To gain a better insight into the essence of this Special Issue, summaries of the published papers are presented below. Please note that the first six contributions are about catalysts' design and their application and the others are related to the improvement of catalytic processes.

1. *Design of Co3O4@SiO<sup>2</sup> Nanorattles for Catalytic Toluene Combustion Based on Bottom-Up Strategy Involving Spherical Poly(styrene-co-acrylic Acid) Template*.

Co3O4@SiO<sup>2</sup> nanorattles were synthesized in this study, bearing in mind the need to develop optimal transition-metal-oxide-based catalysts for the combustion of volatile organic compounds (VOCs). The introduction of Co3O<sup>4</sup> nanoparticles into empty SiO<sup>2</sup> spheres resulted in their loose distribution, facilitating the access of reagents to active sites and, on the other hand, promoting the involvement of lattice oxygen in the catalytic process. As a result, the catalysts obtained in this way showed a very high activity in the combustion of toluene, which significantly exceeded that achieved over a standard silica gel-supported Co3O<sup>4</sup> catalyst.

2. *Catalytic Ozonation of Toluene over Acidic Surface Transformed Natural Zeolite: A Dual-Site Reaction Mechanism and Kinetic Approach*.

Volatile organic compounds (VOCs) can damage human health due to their carcinogenic effects. Catalytic ozonation using zeolite appears to be a valuable process to eliminate VOCs from industrial emissions at room temperature. Results obtained here provide a mechanistic approach during the initial stage of catalytic ozonation of toluene using an acidic surface on modified natural zeolite. Experimental evidence suggests that ozone is adsorbed and decomposed at Lewis acid sites, forming active atomic oxygen that leads to the oxidation of adsorbed toluene at Brønsted acid sites.

3. *Photocatalytic Study of Cyanide Oxidation Using Titanium Dioxide (TiO2)-Activated Carbon Composites in a Continuous Flow Photo-Reactor*.

In this study, the photocatalytic oxidation of cyanide by titanium dioxide (TiO2) supported on activated carbon (AC) was evaluated in a continuous flow UV photo-reactor. The continuous photo-reactor was made of glass and covered with a wood box to isolate the fluid of external conditions. These results showed that photocatalysis and the continuous photo-reactor's design enhanced the photocatalytic cyanide oxidation performance compared to an agitated batch system. Therefore, the use of TiO2-AC composites in a continuous-flow photo-reactor is a promising process for the photocatalytic degradation of cyanide in aqueous solutions.

4. *Enhanced SO<sup>2</sup> Absorption Capacity of Sodium Citrate Using Sodium Humate*.

A novel method of improving the SO<sup>2</sup> absorption performance of sodium citrate (Ci-Na) using sodium humate (HA–Na) as an additive was put forward in this study. The influences of different Ci-Na concentration, inlet SO<sup>2</sup> concentration and gas flow rate on desulfurization performance were studied. The synergistic mechanism of SO<sup>2</sup> absorption by HA–Na and Ci-Na was also analyzed.

5. *Characterization of Anaerobic Biofilms Growing on Carbon Felt Bioanodes Exposed to Air*.

This research article elucidates the structure and performance of an electrogenic biofilm that develops on air-exposed, carbon-felt electrodes that are commonly used in bioelectrochemical systems. The research demonstrates the influence of a protective aerobic layer present in the biofilm (mainly formed by *Pseudomonas* genus bacteria) that prevents electrogenic bacteria (such as *Geobacter* sp.) from hazardous exposure to oxygen during its normal operation.

6. *Metal–Organic Frameworks (MOFs) and Materials Derived from MOFs as Catalysts for the Development of Green Processes*.

This review paper focused on the development of metal–organic frameworks (MOFs) serving as catalysts for the conversion of carbon dioxide into short-chain hydrocarbons and the generation of clean energies starting from biomass. The common patterns in the performance of the catalysts, such as the acidity of MOFs, metal nodes, and surface area and the dispersion of the active sites, are discussed in this review.

7. *Practical Approaches towards NO<sup>x</sup> Emission Mitigation from Fluid Catalytic Cracking (FCC) Units*.

This research article reviews options for cost-effective and emissions mitigation, using optimal amounts of precious metals while evaluating the potential benefits of current promoter dopant packages. Thus, the refinery is no longer forced to make a promoter selection based on preconceived notions regarding only precious metal activity but can rather make decisions based on the best financial strategy without measurable loss of CO/NO<sup>x</sup> emission selectivity.

8. *Application of a Combined Adsorption-Ozonation Process for Phenolic Wastewater Treatment in a Continuous Fixed-Bed Reactor*.

This research article studies the removal of phenol from industrial effluents through catalytic ozonation using granular activated carbon in a continuous fixed-bed reactor. Based on the evolution of total organic carbon (TOC) and phenol concentration, a kinetic model was proposed to study the effect of the operational variables on the combined adsorption–oxidation (Ad/Ox) process. The interpretation of the constants allows for the study of the benefits and behaviour of the activated carbon during the ozonization process under different conditions affecting adsorption, oxidation, and mass transfer.

9. *Turbidity Changes during Carbamazepine Oxidation by Photo-Fenton*.

This research article studies the turbidity generated during the Fenton photoreaction applied to the oxidation of waters containing carbamazepine as a function of factors such as pH, H2O<sup>2</sup> concentration and catalyst dosage. The results allow for establishing the degradation pathways and the main decomposition byproducts.

10. *Pragmatic Approach toward Catalytic CO Emission Mitigation in Fluid Catalytic Cracking (FCC) Units*.

This work reveals how CO promoter design strategies can afford a tangible and immediate CO conversion efficiency increase without the need for additional loading of precious metals. The key lies in the support material architecture that is essential to boost the CO conversion and reduce the NO<sup>x</sup> generation in the FCC unit. It was demonstrated that the suppression of Pt sintering, as well as the enhancement of the oxygen mobility on the catalyst surface, leads to a lower amount and cost of Pt and a higher usage rate compared to current industry-standard designs.

11. *Optimization of Fenton Technology for Recalcitrant Compounds and Bacteria Inactivation*.

In this research article, Fenton technology was applied to decolorize methylene blue (MB) and to inactivate *E. coli* K12, used as recalcitrant compound and bacteria models, respectively, in order to provide an approach into single and combinative effects of the main process variables influencing the Fenton technology. Box–Behnken design (BBD) was applied to evaluate and optimize the individual and interactive effects of three process parameters, namely ferrous ion concentration, molar ratio between H2O<sup>2</sup> and ferrous ion, and pH for Fenton technology.

12. *Photocatalysis for Organic Wastewater Treatment: From the Basis to Current Challenges for Society*.

This paper reviews details on the fundamentals, the common photocatalyst preparation for coupling heterojunction, the morphological effect and photocatalyst-characterization techniques. The important variables that potentially affect the process efficiency, namely catalyst dosage, pH, the initial concentration of sample pollution, irradiation time by light, temperature, durability and stability of the catalyst, are also discussed. Overall, this paper offers an in-depth perspective of photocatalytic degradation, according to pollution cases, and its future direction.
