Catalytic Reforming and Hydrogen Production: From the Past to the Future

1. Introduction

Continuously increasing energy demands and the intense environmental pollution caused by the increasing global population and modern lifestyles have driven research interest toward finding alternative sustainable energy sources [1]. These renewable energy sources can be divided into primary and secondary energy sources depending on whether the produced energy comes directly from a natural source, such as solar or wind energy, or requires conversion into another form [2]. One of the most characteristic secondary energy sources is hydrogen (H₂), typically produced via water electrolysis or the reforming of biomass [3,4,5,6]. Because of hydrogen’s high energy content, the reforming processes used for various hydrocarbons obtained from non-renewable sources, such as natural gas, have been extensively investigated over recent decades [7,8,9,10]. The significant development of high-yield H₂-fueled low-temperature fuel cells has contributed toward this research direction [11]. The development of innovative catalytic systems for the aforementioned hydrogen-related processes and their optimization has attracted the interest of the research community. This Special Issue includes eight research articles, and two review papers related to catalytic reforming processes for H₂ production, highlighting the significance of this secondary energy source of H₂ in a fossil-fuel-free future. The detailed research articles introduce potential readers to new insights and perspectives concerning catalytic processes for hydrogen production, whereas the reviews summarize recent developments in catalytic systems for reforming methanol and the synthesis and catalytic properties of cobalt manganese oxide spinels typically applied in processes such as the production of chemicals and fuels using the Fischer–Tropsch process.

2. Overview of Published Articles

The articles published in this Special Issue focus on advanced catalytic processes for H₂ production, including the dry reforming of methane (DRM), methanol steam reforming (MSR), methanol aqueous-phase reforming (APR), and photocatalytic processes.

DRM is among the most promising technologies, utilizing both CH₄ and CO₂, i.e., the two primary greenhouse gases, for syngas (CO+H₂) production. One of the most significant challenges hampering DRM applications is the severe carbon formation on the catalytic surface, thus resulting in rapid catalytic deactivation. Henni et al. (contribution 8) studied DRM over Ni-Ag catalytic systems and found that the addition of Ag stabilizes the Ni phase, thus reducing carbon accumulation on the catalytic surface. A catalytic system with a Ni/Ag ratio equal to 1 exhibited the highest performance for DRM reaction for temperatures up to 650 °C. In similar work related to the development of active catalytic systems for DRM, Elnour et al. (contribution 6) investigated the addition of Ga in Ni-Ga/(Mg, Al)O_x catalytic systems. The catalyst with a Ga/Ni ratio equal to 0.3 exhibited the highest catalytic activity and stability, suppressing carbon formation and, thus, facilitating CO₂ utilization. Trishch et al. (contribution 4) focused on the study of the conservatively perturbed equilibrium (CPE) phenomenon, which can result in an increase in the produced H₂ and CO concentrations up to 39% and 49%, respectively, while reducing the required reactor length to achieve equilibrium.

Regarding methanol reforming, some of the included research focuses on the APR process. Sousa et al. (contribution 5) examined the performance of APR over Pt/Al₂O₃ catalytic systems. It was observed that an increase in pressure and methanol concentration reduced catalytic activity, whereas the gradual deactivation of the applied catalyst was attributed to the sintering and leaching of Pt crystallites as well as the phase change of the alumina support to boehmite. Nguyen et al. (contribution 7) studied the effect of the size of Pt nanoparticles and the nature of carbon support (various commercial carbon supports, such as Vulcan XC72) on APR activity and stability. Platinum nanoparticles with an average diameter of 1.5 nm, which were well dispersed on Ketjenblack carbon, were found to exhibit a high hydrogen site time yield (8.92 min⁻¹ at 220 °C) along with excellent stability under high-temperature treatment conditions and multiple recycling cycles.

Photocatalytic hydrogen production is one of the most promising technologies for the production of green energy through the utilization of solar radiation. Chang et al. (contribution 1) studied the effect of the calcination-induced oxidation method for the synthesis of cubic Cu₂O/CuO nanomaterials on photocatalytic H₂ production. The researchers found that the formation of a Cu₂O/CuO heterostructure improved light absorption while increasing the separation capacity between electrons and holes, thus enhancing H₂ production (11.888 μmol h⁻¹g⁻¹). In another study, Chaudhary et al. (contribution 2) studied the development and photocatalytic performance of La-doped MoS₂ (La-MoS₂) for the hydrogen evolution reaction (HER). Doping with La resulted in a decrease in the band gap of the material from 1.80 eV (MoS₂) to 1.68 eV (La-MoS₂), thus enhancing H₂ production (1670 μmol g⁻¹).

A unique contribution to this Special Issue is the work of Valecillos et al. (contribution 3) investigating bio-oil reformation via a combined steam and CO₂ reforming (CSDR) process that uses a Rh/ZDC catalyst, which initially exhibited high activity (yield of syngas equal to 77% and H₂/CO ratio equal to 1.2). However, the deactivation of the catalyst during the reaction mainly affected methane conversion, while regeneration through carbon black combustion was not able to fully restore activity due to structural changes in the support (CeO₂-ZrO₂) and the formation of amorphous carbon black. The results indicated that Rh/ZDC is an efficient catalyst for the conversion of bio-oil to syngas, although its limited stability during regeneration is a critical obstacle for long-term applications.

The publication of the two review papers in this Special Issue significantly increases its scientific impact. These reviews provide thorough investigations into recent developments in catalytic systems for MSR (contribution 10) and Co- and Mn-based spinels applied in various applications, such as volatile organic compound (VOC) degradation and Fischer–Tropsch processes (contributor 9).

Béres et al.’s (contribution 9) review focuses on cobalt manganese oxide spinels (Co_xMn_3−xO₄, 0 < x < 3), typically applied in industrial applications, i.e., the CO oxidation process, the removal of VOCs, and Fischer–Tropsch synthesis. The distribution of Co/Mn ions in tetrahedral and octahedral sites of the spinel framework was found to directly affect the catalytic properties, whereas the nature of the structure phases and the synthesis conditions were strongly related to their chemical and redox properties.

Zhang et al. (contribution 10), studying MSR, found that Cu-based catalysts, such as Cu/ZnO/Al₂O₃, exhibited high activity but reduced stability due to Cu particle agglomeration. On the contrary, noble-metal-based catalysts (e.g., Pd/ZnO or Pt/In₂O₃/CeO₂) demonstrated lower CO production and higher resistance to poisoning, whereas novel single-atom alloy catalysts (e.g., Pd-Cu system) appeared to be promising materials for MSR.

3. Conclusions

The research and reviews published in this Special Issue entitled “Catalytic Reforming and Hydrogen Production: From the Past to the Future” highlight significant advances in the development of innovative catalytic systems and provide an in-depth understanding of the insights into the various mechanisms of reaction.

Improvements in the catalytic stability of DRM catalysts, with a focus on the addition of Ag and Ga in Ni-based systems to reduce the carbon accumulation on catalytic surfaces and the utilization of the CPE phenomenon to optimize H₂ and CO production, were among the main challenges investigated in DRM-related work. The significance of Pt nanoparticle size and the type of support for catalytic stability in methanol reforming was underlined; moreover, the review highlighted the need for developing resilient Cu-based and Pd-Cu single-atom alloy catalytic systems to increase the conversion of methanol to H₂ while reducing byproducts.

Extremely intriguing were the results presented in the articles investigating the photocatalytic production of H₂. Cu₂O/CuO heterostructures and La-MoS₂-doped materials improve the separation capacity of electron–hole pairs and enhance photocatalytic performance toward H₂. Finally, despite the initial enhanced catalytic activity of Rh-based catalysts for the production H₂ via bio-oil reforming processes, the irreversible catalytic deactivation caused by structural changes in the support, as well as intense carbon accumulation on the surface, hampers its potential for larger-scale applications.

The papers included in this Special Issue contribute significantly to promoting innovative solutions for sustainable H₂ production. Novel catalytic systems with advanced properties that consider resistance to carbon deposition or catalytic poisoning are thoroughly investigated, and important issues requiring immediate attention such as catalytic deactivation are underscored. Future research efforts for H₂ production technologies should focus on (a) the development of nanostructured, multi-metallic, hybrid catalytic systems that may be fine-tuned with respect to their surface structure and chemistry, characterized by high activity, stability, and low cost; and (b) the detailed investigation of reaction kinetics and mechanisms.