Editorial for the Special Issue “Design and Application of Combined Catalysis”

The global community is currently confronting the dual challenges of energy shortages and environmental degradation, underscoring an urgent need for green and sustainable technological solutions [1,2]. To break away from dependence on fossil resources and build a resilient energy system, green catalytic processes are regarded as a transformative pathway [3]. These processes can meet energy demands while simultaneously alleviating environmental pressures [4]. Green catalytic technologies have been widely applied in the chemical industry, pharmaceuticals, the energy sector, and environmental protection. However, conventional single-catalytic modes—such as photocatalytic, electrocatalytic, enzymatic, or chemocatalytic approaches—though promising, often suffer from inherent limitations, including low efficiency, poor stability, unsatisfactory selectivity, or high costs, which severely hinder their large-scale implementation [5,6].

To overcome these barriers, the field is increasingly turning to combinatorial catalysis. This innovative approach integrates distinct catalytic modalities, such as enzymatic, chemical, photocatalytic, electrocatalytic, and others, to create synergistic effects that are unattainable by any single system alone [7,8]. By uniting, for example, photocatalysis with electrocatalysis, enzymatic with heterogeneous catalysis, or whole-cell systems with synthetic catalysts, it becomes possible to orchestrate reaction networks that leverage the unique advantages of each component [9,10]. These strategies not only enhance reaction activity, selectivity, and durability but also create novel reaction pathways. Furthermore, integrated catalysis contributes to reduced energy consumption and minimized waste generation, aligning perfectly with the core objectives of sustainable chemistry [11].

This Special Issue, titled “Design and Application of Combined Catalysis”, highlights the expanding role of combinatorial catalysis in addressing the inefficiencies of single-catalytic modes and reducing catalysis costs across diverse fields. Bringing together advances in catalyst design (e.g., magnetic immobilization and doping modification), innovative synthesis strategies, and mechanistic insights into multi-catalysis synergy, this Special Issue exhibits the dynamic evolution of combinatorial catalysis systems. Beyond showcasing technical progress, it underscores the importance of combinatorial catalysis as a key tool for achieving efficient compound production through sustainable approaches. The research presented in this Special Issue vividly demonstrates how the synergistic interplay between different catalysts can lead to remarkable enhancements in activity, stability, and selectivity, thereby paving the way for more sustainable and economically viable chemical processes.

In Contribution 1, a systematic study of two glycosidase immobilization methods—cross-linked enzyme aggregates (CLEAs) and magnetic cross-linked enzyme aggregates (MCLEAs)—revealed that MCLEAs eliminate the need for ultrafiltration, enhance immobilization efficiency and catalytic activity (approximately 30% higher than CLEAs), and maintain over 70% activity after 10 reuse cycles, providing an efficient and stable biocatalytic strategy for icaritin production.

Contribution 2 employed ion beam sputtering as a post-synthesis surface modification technique to enhance the low-temperature activity of commercial Pt/CeO₂–ZrO₂ catalysts. Using low-energy Ar⁺ ions, the authors selectively generated oxygen vacancies and improved Pt dispersion without altering the bulk structure. The treated catalysts showed a progressive reduction in light-off temperatures for CO and C₃H₆ oxidation, with the most extensively irradiated samples achieving 13 and 22 K decreases, respectively, highlighting the potential of ion beam processing to address the persistent challenge of cold-start emissions in high-efficiency diesel and lean-burn engines.

In Contribution 3, Aleithan et al. synthesized MoS₂ nanohybrids via chemical vapor deposition and combined them with Au nanoparticles to construct MoS₂/Au hybrid materials for enhancing the performance of the electrocatalytic hydrogen evolution reaction (HER). They found that Au incorporation significantly enhances the conductivity and active sites of composites, enabling outstanding HER activity in neutral media. The onset potential reaches as low as −0.152 V with an exchange current density of 0.22 mA/cm², accompanied by excellent long-term stability. This work provides a viable strategy for developing efficient, stable non-precious metal-assisted electrocatalysts.

Contribution 4 investigated Mg-doped β-Bi₂O₃ nanoparticles prepared using sol–gel synthesis. The sol–gel-synthesized Mg-doped Bi₂O₃ samples showed a reduction in crystallite size from 79 to 13 nm and bandgap narrowing from 3.8 to 3.08–3.3 eV. The optimized Mg_0.075Bi_1.925O₃ catalyst achieved a high photodegradation rate constant of 0.0217 min⁻¹ for methylene blue and exhibited enhanced hydrogen evolution due to the optimal band alignment structure. This study highlights the role of alkaline earth metal doping in designing efficient photocatalysts for environmental and energy applications.

Contribution 5 employed density functional theory (DFT) to explore the oxygen evolution reaction (OER) mechanism on NiP surfaces, revealing synergistic catalysis. P atoms stabilized O* intermediates, while Ni-induced electron donation enhanced the O*-P interaction. Moreover, Lewis acidic sites (from electron loss at adsorption sites) reduced overpotential by up to 0.61 V, with the Ni₄P₂ surface showing the lowest overpotential (0.78 V) via the single-site pathway. This study establishes a valuable framework for guiding base metal catalyst design.

Contribution 6 reported reduced graphene oxide (RGO)-coated ZrO₂ heterostructures synthesized by sol–gel combustion and ultrasonic mixing, where synergistic catalysis between RGO and ZrO₂ led to a reduced ZrO₂ bandgap from 5.6 to 3.87 eV and ZrO₂@4%RGO achieved 80% methylene blue (MB) degradation in 120 min under sunlight, while the reaction rate constant was nearly twice that of pure ZrO₂ (65%).

In Contribution 7, TiO₂/(MA)₂SnCl₄ nanocomposite films were synthesized via sunlight-driven drop-casting, where the synergistic effects of structural modification, optimized charge dynamics, and defect-mediated electronic interactions between TiO₂ and (MA)₂SnCl₄ led to a narrowed bandgap (from 3.1 to 2.6 eV) and enhanced visible-light absorption. The composite achieved 90% MB degradation in 60 min, outperforming pure TiO₂ (75%) due to Sn-O-Ti interactions and oxygen vacancies.

Contribution 8 demonstrated that Gd-doped ZnO nanocomposites, synthesized via sol–gel combustion, exhibit enhanced photocatalytic activity. Gd doping (0.025–0.075) modulated the bandgap (with a minimum of 3.07 eV for the 0.075 sample) and inhibited electron–hole recombination, with 0.075 Gd-doped ZnO degrading 89% MB in 120 min under UV light (40% for pure ZnO) and maintaining stability over five reuse cycles.

The final contribution (Contribution 9) established the optimal conditions for the laccase-catalyzed degradation of 2-MIB (pH 4.0, 25 °C, 150 rpm, 0.1 U/mL laccase, and 200 ng/L 2-MIB), achieving a degradation efficiency of approximately 78% within 4 h. The degradation efficiency was further elevated to 90.78% upon the application of a micro-electric field (0.04 A), demonstrating a notable synergistic effect between the enzymatic and electrocatalytic processes.

Taken together, the contributions in this Special Issue provide mechanistic insights, practical knowledge, and technological innovations that advance the field of combinatorial catalysis. They demonstrate how multi-catalysis synergy, material modification, and process optimization can improve catalytic efficiency and reduce costs.

I wish to extend my deepest and most sincere gratitude to all the authors for their invaluable contributions, without which this Special Issue would not have been possible. I hope that the original articles included in this issue will help advance the understanding of and resolve the current challenges in the field. I also sincerely thank MDPI’s journal Catalysts for the opportunity to serve as a Guest Editor, and I appreciate the dedicated efforts of the assistant editors and reviewers in enhancing the quality of this Special Issue.