Catalytic Hydrogen Production from Methanol – Fuel for the Future
The diminishing supply of fossil fuel sources and environmental degradation have emerged as major discussion topics in the global discourse on sustainable development. Fossil fuels, including coal, crude oil, and natural gas, are the main resources used by the energy sector to supply power for different aspects of human life. Nowadays, the primary uses of coal and natural gas are in the production of power, whereas the primary use of refined crude oil is in the transportation industry. Fossil fuel combustion significantly increases atmospheric carbon and aggravates climate change. At present, renewable energy sources including solar, wind, hydro, tidal, geothermal, and biofuels contribute to worldwide energy production.
Many nations have highlighted hydrogen as a crucial route for deep decarbonization in the energy and transportation sectors. Hydrogen is a diverse energy carrier that has a higher energy-to-movement conversion efficiency than gasoline. Hydrogen may be produced from a variety of sustainable (biomass and water) and non-renewable (natural gas, coal, and hydrocarbons) sources. Hydrogen is more efficient than traditional fossil fuels, in addition, it also emits fewer air pollutants. As a result, several methods of hydrogen production such as photocatalysis, electrocatalysis, thermocatalytic reforming, pyrolysis of hydrocarbons (directly employing solar energy or renewable electricity), and biological methods are commonly adopted.
Methanol – A source for H2 production
Recently there has been a growing interest in synthesizing hydrogen from methanol. Methanol has a higher H/C ratio since it only has one carbon atom. Due to the absence of a strong C-C bond, it can be easily transformed into hydrogen at low temperatures (200-300°C). Compared to other typical hydrocarbons like ethanol, which reforms at a temperature above 600°C, and methane, which reforms at a temperature between 800 and 1000°C, this temperature range is comparatively smaller. Additionally, biomass and other renewable resources can be used to produce methanol. Because of its many advantages, methanol has emerged as a suitable source for hydrogen production.
H2 production technologies from methanol
Steam reforming of methanol and aqueous phase reforming of methanol are the main types of methanol-reforming processes that are commonly utilized to produce hydrogen. The above process is typically utilized to produce pure hydrogen with varying feed composition and operation temperature.
Steam reforming of methanol
In recent years, methanol steam reforming has been employed in to synthesize hydrogen. Technology has advanced significantly in the direction of industrial mass manufacturing since then. Even with the higher output and lower cost, the product still contains contaminants, especially carbon monoxide, which calls for further purifying equipment. Many new catalysts with high activity and selectivity at lower temperatures were used to produce hydrogen from methanol.
The catalyst employed in steam reforming of methanol
Group VIII metals such as Pt, Pd, Ru, and Ni are commonly used in the steam reforming process, particularly at low temperatures. Considering its typically high performance, zinc oxide-supported palladium (Pd/ZnO) was the first catalyst in this category to be utilized. It was initially reported that Pd supported on ZnO, which reduced at >300 °C, had extraordinarily high activity and selectivity to CO2 and H2. The production of Pd-Zn alloy significantly alters Pd’s catalytic function; this alloy has been shown to be stable across a broad temperature range. Ru-based catalysts are frequently employed in homogeneous catalysis to produce H2 in a long-lasting and stable manner. Even though the reaction temperatures are already around 100 °C, the majority of catalysts that have been described require significant amounts of base typically KOH in order to effectively catalyze the reaction.
Aqueous phase reforming of methanol
Despite the relatively mature development of the methanol-reforming sector, the current technology mostly operates at temperatures above 200°C and pressures between 25 and 50 bar, which restricts the synthesis of H2 on a large scale. Therefore, a different low-temperature aqueous phase reforming of methanol technique is thought to be a possible route to effective on-board H2 synthesis.
Notably, there are two primary categories of catalysts employed in aqueous phase reforming of methanol. There are two types of catalysts: atomically scattered heterogeneous catalysts and homogeneous catalysts like Ru, Rh, Ir, and Fe-based pincer complexes. Large-scale uses of typical homogeneous catalysts are limited by their inability to be readily separated from the reaction mixture while having clearly defined structures. Conversely, heterogeneous catalysts can be highly stable and easily separated from reactants and products even if they may have uneven active sites that result in inefficient use of metal atoms.
Though creating an efficient and reliable catalytic system for on-board use is challenging, recent discussions have shed light on key aspects. First and foremost, the development of carefully crafted catalysts with high activity at low temperatures and long-term stability, alongside effective management of by-products, is crucial for efficient hydrogen production from methanol. In conclusion, forthcoming research on hydrogen production and utilization, especially for on-board applications, should focus on ensuring consistent system output and immediate hydrogen yield.
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