Effect of the Modification of Catalysts on the Catalytic Performance

1. Introduction

Changing the composition and structure of a catalyst to obtain a positive impact on its performance is challenging. Therefore, the optimization of the surface and the bulk properties (electronic or physical structure) offers a strategy for the development of advanced catalysts used to be used in the global challenges of energy conversion and environmental protection [1,2,3] (Figure 1).

Catalyst performance plays a significant role in the catalytic processes and is expressed in terms of selectivity, activity, and stability (resistance to deactivation and regeneration capacity). Successful catalyst development depends on several factors, including the preparation method, the interaction between the active phase and support, the structural and physicochemical properties of the active metal or support, and the metal precursor used in the preparation [4].

Starting from the premises shown above that underline the special importance of this essential field in catalysis and catalytic materials, we felt honored to receive the invitation to be guest editors of this Special Issue. Following the excellent collaboration with Ms. Assistant Editor Mia Zhang in 2021 and with Ms. Assistant Editor Maeve Yue, starting in 2022, as well as following the special work undertaken by the entire editing team, 10 articles could be successfully published, including two reviews. Lastly, we are grateful to all of the authors of these publications for their excellent research work and for their contribution to this Special Issue.

2. This Special Issue

The purpose of the Special Issue, “Effect of Catalyst Modification on Catalytic Performance”, was to gather a collection of papers that present new strategies for modifying catalysts that facilitate the establishment of composition–performance and structure–performance relationships. In what follows, we present some conclusions from the works published in this volume.

Raciulete et al. [5] developed a multi-step ion-exchange methodology for the fabrication of Cu(LaTa₂O₇)₂ lamellar architectures by exchanging Rb⁺ with a much smaller Cu²⁺ spacer in the RbLaTa₂O₇ host to achieve photocatalysts capable of wastewater depollution. Cu-modified layered perovskites exhibited enhanced photocatalytic activity compared to the RbLaTa₂O₇ host. The superior photocatalytic activity of CuLTO-800R was attributed to its narrow band gap and photogenerated–carriers separation. Petcu et al. [6] presented the synthesis and characterization of new photocatalysts obtained through the immobilization of titanium and gold on the supports with various porous structures (micropores—zeolite Y, micro and mesopores—hierarchical zeolite Y, smaller mesopores—MCM-48 and larger mesopores—KIT-6). The effects of porous structure and surface properties on TiO₂ dispersion, crystal structure, the nature of the interaction between support–titanium species and Au-TiO₂, respectively, were studied. The authors further investigated the influence of the support properties, the presence of TiO₂ and Au species, and their interaction with amoxicillin photodegradation under UV and visible light irradiation. In another paper [7] pristine titanate nanorods were modified with iron to improve the light absorption and separation of photogenerated charges and to favor ammonia photodegradation under solar light irradiation. The morphological and structural characterizations (SEM, XRD, XRF, UV–Vis, H₂-TPR, NH₃-TPD, PL, PZC) of the studied catalysts were correlated with their activity on ammonia degradation with ozone- and photo-assisted oxidation. Preda et al. [7] optimized the aqueous ammonia oxidation process to obtain a high ammonia conversion and increase selectivity to gaseous nitrogen-containing products. In reference [8], the authors focused on the preparation of a ZnO/CuO/g-C₃N₄ (ZCG) nanocomposite through an efficient co-crystallization method, performed a photocatalytic test for the treatment of dye-containing wastewater followed by visible light irradiation and evaluated the material’s durability and recycling capability. The outstanding photocatalytic performance was observed due to heterojunction formation among the g-C₃N₄, CuO-NPs, and ZnO-NPs compounds, which minimized the photogenerated e⁻-h⁺ pair recombination and increased the electron flow rate. Li et al. [9] modified polyacrylonitrile hollow nanospheres (HPAN) derived from the polymerization of acrylonitrile in the presence of polystyrene emulsion (as template) using surface amination with ethylenediamine (EDA), using them as a support for loading Pd or PdCo nanoparticles. The authors demonstrated that the prepared PdCo nanoparticles supported on the surface of aminated polyacrylonitrile hollow nanospheres (EDA-HPAN) could be used as a highly active and stable catalyst for the dehydrogenation of formic acid. Zhang et al. [10] investigated the promotion of the Keggin structure to the sulfur and water resistance of Pt/CeTi catalysts for CO oxidation. They prepared Pt catalysts using cerium titanium composite oxide (CeTi), ammonium molybdophosphate with Keggin structure-modified CeTi (Keg-CeTi), and molybdophosphate without Keggin structure-modified CeTi (MoP-CeTi) as supports. Their research revealed that the high SO₂ and H₂O resistance of Pt/Keg-CeTi in CO oxidation was related to its stronger surface acidity, surface cerium and molybdenum species reduction, and lower SO₂ adsorption and transformation compared to Pt/CeTi and Pt/MoP-CeTi. In reference [11], an easy synthetic process for H₂O₂ generation was described, and a new comprehension of the conception and mechanistic examination of metal-free N- and O-doped carbon materials were also provided. The content and type of surface functional groups were improved by treating the PAN-based ACF with concentrated mixed acid in different volume ratios. The selectivity and yield of H₂O₂ in the reaction on modified activated carbon fiber (ACF) catalysts were well interconnected with the amounts of pyrrolic/pyridone nitrogen (N5) and desorbed carboxyl–anhydride groups from the ACF surface. The possible reaction pathway over the ACF catalysts promoted by N5 was also shown. In order to improve the alkali metal resistance of commercial catalyst Cu/SSZ-13 for ammonia-selective catalytic reduction (NH₃-SCR) reaction, Chen et al. [12] developed a simple method to synthesize Cu/SSZ-13 with a core–shell-like structure. Cu/SSZ-13, with a crystal size of 2.3 μm, exhibited excellent resistance to Na poisoning. They investigated physical structure characterization (XRD, BET, SEM, NMR) and chemical acidic distribution (H₂-TPR, UV-Vis, Diethylamine-TPD, pyridine-DRIFTs, EDS). Bratan et al. [13] presented in their review the main factors affecting the catalytic performances of CoOx and MnOx metal-oxide catalysts concerning the total oxidation of hydrocarbons. The catalytic behavior of the studied oxides was discussed and could be closely related to their redox properties, nonstoichiometric, defective structure, and lattice oxygen mobility. It was emphasized that controlling the structural and textural properties of the studied metal oxides, such as the specific surface area and specific morphology, plays an important role in catalytic applications. Dobrescu et al. [14] reviewed scientific results related to oxide catalysts, such as lanthanum cobaltites and ferrites with perovskite structure, and nanoparticle catalysts (such as Pt, Rh, Pt-Cu, etc.), emphasizing their fractal properties and the influence of their fractal modification on both the fractal and catalytic properties. They discussed some methods used to compute the fractal dimensions of the catalysts (micrograph fractal analysis and the adsorption isotherm method) and computed catalysts’ fractal dimensions, underlining that increasing the fractal dimensions of the catalysts is of significant demand in heterogeneous catalysis.

In conclusion, the Special Issue, “Effect of the Modification of Catalysts on the Catalytic Performance”, should be of great interest to all researchers involved in this scientific area regarding the synthesis and characterization of various catalysts or catalytic materials; catalytic performance (activity and selectivity); synergetic effect; the modification of catalysts or suitable promoters that are added to modify the catalyst structure, improve stability, or enhance the catalytic reactions, enabling better activity or selectivity; and the reaction mechanism and kinetic parameters (reaction rate and activation energy).