适用于氢气低温制备与高效存储的催化新体系

Catalysis for efficient low-temperature hydrogen production and storage

氢气是一种具有极高能量密度的二次清洁能源,被认为最有可能替代现有的煤炭和石油等化石燃料作为未来人类社会赖以生存和发展的能源基础.以清洁、高效、无污染的氢循环代替目前对环境有严重威胁且日益枯竭的碳循环.在可预见的未来,全球主要国家将会加大氢能开发和利用的投入.尤其是伴随着我国能源体系的升级和新能源产业的快速发展,氢气作为高效的能量载体势必会成为未来清洁能源发展的主要方向之一.氢能应用循环主要包括3个环节,即(1)氢燃料的制备;(2)氢燃料的存储和输运;(3)氢燃料化学能到电能或其他形式能量的高效转变.结合国家能源战略及基础研究的需求,本研究团队近期在氢气的低温制备和存储方面取得了一定的研究成果.尤其是以α-MoC作为强相互作用载体制备的Au和Pt纳米催化剂,分别在低温水汽变换反应和液相甲醇水重整产氢反应方面取得了较为突出的研究成果.该研究成果为氢燃料的低温原位制备,氢燃料安全、高效的存储运输及大规模工业制氢过程的优化提供了新的思路.本文结合该领域近年来的国内外研究进展和本实验室的研究成果,简单介绍适用于工业化制氢过程的低温水汽变换过程和液相储氢新体系,并对未来该领域的发展提出一定的展望.

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 mol_（CO）mol_（metal）^（-1） s^（-1） below 150℃. 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℃, 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 mol（CO）mol（metal）^（-1） s^（-1） under 120℃ 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℃, 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.