A review study on methanol steam reforming catalysts: Evaluation of the catalytic performance, characterizations, and operational parameters

Conventional fossil-based energy sources have numerous environmental demerits; sustainable and renewable sources are attracting the undivided attention of researchers owing to their valuable physical and chemical features. Several industrial-scale technologies are employing hydrogen as a green energy source as the most preferential source. Not only is hydrogen a potent energy carrier but also it is not detrimental to the environment. Among many other hydrogen production processes, steam reforming of methanol (SRM) is deemed a practical method due to its low energy consumption. Cu, Ni, noble metals, etc., are the salient catalysts in SRM. Many researchers have conducted thorough studies incorporating improvement of the catalysts’ activity, mechanism predictions, and the impacts of operational parameters and reformers. This review concentrates on the SRM catalysts, supports, promoters, and the effect of the operational parameters on the process efficiency and H2 production yield. In this regard, the methanol conversion, H2 and CO selectivity, and operating parameters are notably contingent on the surface characterization and chemistry of the catalysts. Herein, Cu-, Ni-, and noble metal-based catalysts on various metal oxide supports, such as Al2O3 and ZnO, are assessed meticulously in the SRM process from the standpoint of mechanism and catalyst characterization. Most of the peer-reviewed studies had encountered agglomeration, metal particle sintering at high temperatures, coke formation, and deactivation of catalysts as the prevalent barriers. Hence, the novel methods of conquering the above-mentioned obstacles are evaluated in this review. Employment of diverse synthetic methods, bimetallic catalysts, distinct catalyst promoters, and unconventional supports, such as metal–organic frameworks, carbon nanotubes, and zeolites, are the salient routes to overcome the metal dispersion and thermal stability issues. In addition, the influence of operational parameters (temperature of the process, steam/carbon ratio, and feed flow rate) has been weighed painstakingly, along with introducing the research gap and future perspectives in the territory of SRM catalysts.

The Critical Role of βPdZn Alloy in Pd/ZnO Catalysts for the Hydrogenation of Carbon Dioxide to Methanol

The rise in atmospheric CO₂ concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO₂ as a feedstock. Prime among these is the hydrogenation of CO₂ to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO₂ hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m²g^–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO₂ conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO₂ hydrogenation and will aid future catalyst discovery.

Highly Efficient Hierarchical Porous Carbon Supported Pd-Based Catalysts for Additive-Free Dehydrogenation of Formic Acid

Formic acid (FA) is one of the most prospective hydrogen carriers for renewable energy transformation. In this context, the addition of extra-amine is always required for promoting the reactivity of FA, which is still a key challenge. Herein, we report a simple but effective strategy to synthesize Pd nanoparticles, supported on NH2-functionalized, phosphorous-doped glucose-based porous carbon (NH2-P-GC). The introduction of NH2- groups on the support acts as an immobilized amine-additive for FA dehydrogenation, while phosphorus not only serves as an electronic promoter to keep Pd in the electronic deficient state for FA dehydrogenation, but also as an enlarger of the aperture size of the carbon. As a result, the Pd/NH2-P-GC has exceptional catalytic activity, 100% H2 selectivity, CO generation that is undetectable, and good reusability for hydrogen production from FA. In the additive-free dehydrogenation of aqueous FA solution, the initial turnover frequency (TOF) can reach 5126 h−1 at room temperature, which is substantially higher than the best heterogeneous catalyst so far recorded. Overall, the system’s high activity, selectivity, stability, and simplicity in producing CO-free H2/CO2 gas from FA, without the need for any additive, makes it attractive for practical deployment.

Multifunctional Catalyst Combination for the Direct Conversion of CO2 to Propane

The production of carbon-rich hydrocarbons via CO₂ valorization is essential for the transition to renewable, non-fossil-fuel-based energy sources. However, most of the recent works in the state of the art are devoted to the formation of olefins and aromatics, ignoring the rest of the hydrocarbon commodities that, like propane, are essential to our economy. Hence, in this work, we have developed a highly active and selective PdZn/ZrO₂+SAPO-34 multifunctional catalyst for the direct conversion of CO₂ to propane. Our multifunctional system displays a total selectivity to propane higher than 50% (with 20% CO, 6% C₁, 13% C₂, 10% C₄, and 1% C₅) and a CO₂ conversion close to 40% at 350 °C, 50 bar, and 1500 mL g^-1 h^-1. We attribute these results to the synergy between the intimately mixed PdZn/ZrO₂ and SAPO-34 components that shifts the overall reaction equilibrium, boosting CO₂ conversion and minimizing CO selectivity. Comparison to a PdZn/ZrO₂+ZSM-5 system showed that propane selectivity is further boosted by the topology of SAPO-34. The presence of Pd in the catalyst drives paraffin production via hydrogenation, with more than 99.9% of the products being saturated hydrocarbons, offering very important advantages for the purification of the products.

Performance Study on Methanol Steam Reforming Rib Micro-Reactor with Waste Heat Recovery

Automobile exhaust heat recovery is considered to be an effective means to enhance fuel utilization. The catalytic production of hydrogen by methanol steam reforming is an attractive option for onboard mobile applications, due to its many advantages. However, the reformers of conventional packed bed type suffer from axial temperature gradients and cold spots resulting from severe limitations of mass and heat transfer. These disadvantages limit reformers to a low efficiency of catalyst utilization. A novel rib microreactor was designed for the hydrogen production from methanol steam reforming heated by automobile exhaust, and the effect of inlet exhaust and methanol steam on reactor performance was numerically analyzed in detail, with computational fluid dynamics. The results showed that the best operating parameters were the counter flow, water-to-alcohol (W/A) of 1.3, exhaust inlet velocity of 1.1 m/s, and exhaust inlet temperature of 773 K, when the inlet velocity and inlet temperature of the reactant were 0.1 m/s and 493 K, respectively. At this condition, a methanol conversion of 99.4% and thermal efficiency of 28% were achieved, together with a hydrogen content of 69.6%.

Benzyl alcohol oxidation with Pd-Zn/TiO2: computational and experimental studies

Pd-Zn/TiO2 catalysts containing 1 wt% total metal loading, but with different Pd to Zn ratios, were prepared using a modified impregnation method and tested in the solvent-free aerobic oxidation of benzyl alcohol. The catalyst with the higher Pd content exhibited an enhanced activity for benzyl alcohol oxidation. However, the selectivity to benzaldehyde was significantly improved with increasing presence of Zn. The effect of reduction temperature on catalyst activity was investigated for the catalyst having a Pd to Zn metal molar ratio of 9:1. It was found that lower reduction temperature leads to the formation of PdZn nanoparticles with a wide particle size distribution. In contrast, smaller PdZn particles were formed upon catalyst reduction at higher temperatures. Computational studies were performed to compare the adsorption energies of benzyl alcohol and the reaction products (benzaldehyde and toluene) on PdZn surfaces to understand the oxidation mechanism and further explain the correlation between the catalyst composition and its activity.