China team develops highly efficient catalyst for low-temperature aqueous phase refoming of methanol to produce hydrogen
Researchers in China, along with colleagues in the US, have developed a new catalyst that shows outstanding hydrogen-production activity and stability in the low-temperature aqueous phase reforming of methanol (APRM).
In a paper in the journal Nature, the team reports that platinum (Pt) atomically dispersed on α-molybdenum carbide (α-MoC) enables low-temperature (150–190 ˚C), base-free hydrogen production through APRM, with an average turnover frequency reaching 18,046 moles of hydrogen per mole of platinum per hour. The new catalyst, the researchers suggest, paves a way towards a commercially achievable hydrogen-storage strategy.
Polymer electrolyte membrane fuel cells (PEMFCs) running on hydrogen are attractive alternative power supplies for a range of applications, with in situ release of the required hydrogen from a stable liquid offering one way of ensuring its safe storage and transportation before use. The use of methanol is particularly interesting in this regard, because it is inexpensive and can reform itself with water to release hydrogen with a high gravimetric density of 18.8 per cent by weight.
But traditional reforming of methanol steam operates at relatively high temperatures (200–350 degrees Celsius), so the focus for vehicle and portable PEMFC applications has been on aqueous-phase reforming of methanol (APRM). This method requires less energy, and the simpler and more compact device design allows direct integration into PEMFC stacks. There remains, however, the need for an efficient APRM catalyst.—Lin et al.
Earlier work has suggested that to achieve a high rate of hydrogen production from the reaction of methanol and water at low temperatures, both the water and the methanol must be activated effectively. This, the team said, can be difficult to achieve with a homogeneous catalyst that contains only isolated noble-metal sites; as a result, they reasoned that a bifunctional structure might be important.
A suitable material would not only act as a support for confined metal atoms, but would also modulate their electronic structure.
Because the electronic structure of metal catalysts can be tuned by their supports or promoters, and because electron-deficient platinum nanoparticles have been proposed to be responsible for the high activity of the low-temperature water–gas shift reaction1, careful choice of the support material for platinum should in principle make it possible to obtain bifunctional constructs with atomically dispersed noble-metal sites that catalyse low-temperature APRM.—Lin et al.
The research team had earlier found that α-MoC exhibits stronger interactions with platinum than do common oxide supports or β-Mo2C. The strong interactions drive an atomic dispersion of platinum (Pt1) over α-MoC during a high-temperature activation process. This results in an exceptionally high density of electron-deficient surface Pt1 sites for the adsorption/activation of methanol.
The α-MoC substrate shows high water-dissociation activity, producing abundant surface hydroxyls that accelerate the reforming of reaction intermediates at the interface between platinum and α -MoC. These two effects combine to confer the platinum/α-MoC catalyst with its very high catalytic and good stability in the base-free APRM process at 150–190 °C.
China is the global leader in methanol use and has recently expanded its methanol production capacity. A study commissioned by the US Energy Information Administration (EIA) recently found that since the early 2000s, China’s consumption of methanol in fuel products has risen sharply. The report estimates consumption to have been more than 500,000 barrels per day (b/d) in 2016.
Lili Lin, Wu Zhou, Rui Gao, Siyu Yao, Xiao Zhang, Wenqian Xu, Shijian Zheng, Zheng Jiang, Qiaolin Yu, Yong-Wang Li, Chuan Shi, Xiao-Dong Wen & Ding Ma (2017) “Low-temperature hydrogen production from water and methanol using Pt/α-MoC catalysts” Nature doi: 10.1038/nature21672