Catalysis for efficient low-temperature hydrogen production and storage

Abstract

Hydrogen, an energy-intensive and clean power source, has been treated as an ideal energy carrier to replace the coal and oil based global energy system for the more sustainable economic development. Especially, with the increasing finical support from worldwide in the researching of renewable energy both for the public and military using. Generally, hydrogen cycling includes three steps: (1) the continuously large scale hydrogen generation; (2) the safe and efficient hydrogen storage and transportation; (3) the efficient conversion of hydrogen to electricity or other forms of energy. For the using of hydrogen, a representative example is polymer electrolyte membrane fuel cells (PEMFCs) which has been widely studied. However, the bottle-neck for the bursting of hydrogen economy besides the effective using of hydrogen lies in the efficient generation and safe storage of hydrogen. Recently, our research group developed α-molybdenum carbide (α-MoC) supported Au and Pt catalysts for the low temperature water-gas shift (WGS) reaction and aqueous-phase reforming of methanol, respectively, both were essential reactions for industrial hydrogen generation. The WGS reaction (where carbon monoxide plus water yields hydrogen and carbon dioxide) is an important process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. However, the reported catalysts by now always show unsatisfied performance under low operating temperatures and no catalysts ever reported reaching the activity of 0.1 molCO molmetal−1 s−1 below 150°C. Meanwhile, potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient to match the working temperature of on-site hydrogen generation and consumption units. Concerning this problem, we synthesized layered gold clusters on a molybdenum carbide (Au/α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoC at 30°C, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity reaching 1.05 molCO molmetal−1 s−1 under 120°C which is one order of magnitude higher than the early reported catalysts. Beside the Au/α-MoC for robust WGS reaction, we also developed highly distributed single atom Pt supported by molybdenum carbide (Pt/α-MoC) for aqueous-phase reforming of methanol. With the reforming of methanol and water, hydrogen with a high gravimetric density of 18.8% by weight can be in situ released. The average turnover frequency of Pt/α-MoC can reach 18046 mol of hydrogen per mole of platinum per hour at 190°C, which is two order of magnitude higher than the traditional catalysts. Based on the X-ray absorption fine structure (XAFS) and single atom resolution electron microscopy characterization results, Pt was determined to be atomic dispersed over α-MoC support at the low metal loadings, which maximized the exposure of Pt atom. Based on reaction mechanism investigation and DFT calculation, Pt/α-MoC was proved to be a bifunctional catalyst, with α-MoC support dissociating the O−H bond of H2O and methanol at low temperature, atomic dispersion of Pt scissoring C−H bond of CH3OH. The effectively reforming of intermediates with surface hydroxyls generate CO2 at the interface of Pt/α-MoC. The excellent performance of our catalytic system provide a new strategy for the efficient low-temperature hydrogen production and storage. In this short perspective, we introduce the new findings by our group and also summarize the recently developed representative catalysts involving the low temperature water-gas shift reaction and aqueous-phase reforming of methanol. We hope it can provide a reference for the future designing of catalysts in the field of hydrogen production and storage.