Hans Geerlings

A multifaceted approach to hydrogen storage

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

Sorption enhanced CO2 methanation

The transformation from the fatuous consumption of fossil energy towards a sustainable energy circle is most easily marketable by not changing the underlying energy carrier but generating it from renewable energy. Hydrocarbons can be principally produced from renewable hydrogen and carbon dioxide collected by biomass. However, research is needed to increase the energetic and economic efficiency of the process. We demonstrate the enhancement of CO2 methanation by sorption enhanced catalysis. The preparation and catalytic activity of sorption catalysts based on Ni particles in zeolites is reported. The functioning of the sorption catalysis is discussed together with the determination of the reaction mechanism, providing implications for new ways in catalysis.

CO2 Mineralization—Bridge Between Storage and Utilization of CO2

CO2 mineralization comprises a chemical reaction between suitable minerals and the greenhouse gas carbon dioxide. The CO2 is effectively sequestered as a carbonate, which is stable on geological timescales. In addition, the variety of materials that can be produced through mineralization could find applications in the marketplace, which makes implementation of the technology more attractive. In this article, we review recent developments and assess the current status of the CO2 mineralization field. In an outlook, we briefly describe a few mineralization routes, which upon further development have the potential to be implemented on a large scale.

Post-synthetic cation exchange in the robust metal–organic framework MIL-101(Cr)

In this work an unambiguous proof of post-synthetic solvent-assisted cation exchange in the robust metal–organic framework MIL-101(Cr) is reported. Such substitution can alter directly the secondary building unit, which often defines the properties of the material.

Probing hydrogen spillover in Pd@MIL-101(Cr) with a focus on hydrogen chemisorption

Palladium nanoparticles can split the dihydrogen bond and produce atomic hydrogen. When the metal nanoparticles are in intimate contact with a hydrogen-atom host, chemisorption of H-atoms by the host has been suggested to occur via the hydrogen spillover mechanism. Metal-organic frameworks were predicted to be able to act as effective chemisorption sites, and increased ambient-temperature hydrogen adsorption was reported on several occasions. The intimate contact was supposedly ensured by the use of a carbon bridge. In this work, we show that it is possible to introduce catalyst palladium particles into MOFs pores and simultaneously ensuring good contact, making the employment of the carbon bridge redundant. The addition of Pd nanoparticles indeed increases the ambient-temperature hydrogen uptake of the framework, but this is found to be solely due to palladium hydride formation. In addition, we show that the hydrogen atoms do not chemisorb on the host framework, which excludes the possibility of hydrogen spillover.

Manipulating the reaction path of the CO2 hydrogenation reaction in molecular sieves

We demonstrate that the kinetics of the Sabatier reaction catalysed by sorption catalysts depends on the nanostructure of the catalyst–sorbent system. The catalysts are prepared by ion exchange of a nickel nitrate solution in two zeolites with different pore sizes. Besides their different pore sizes — which enables or hinders the adsorption of the reactants, intermediates and products in the inner of the crystallites — the catalyst systems have slightly different size distributions of the Ni-particles. By studying various catalysts with different Ni-contents we can attribute different catalytic activity and in particular the shape selectivity of the zeolite support. Therefore we focus on the microstructural characterization of the catalyst. We observe that the selectivity for methane is greatly enhanced if the pore size of the support is larger than 5 A, while pore sizes of less than 3 A reduce the overall conversion rate and the selectivity for methane. Thus, Ni on 3A zeolites can be used as low temperature catalysts for the reversed water-gas shift reaction to produce carbon monoxide.

Aluminium hydride nanoparticles nested in the porous zeolitic imidazolate framework-8

Aluminium hydride particles were loaded for the first time in a ZIF-8 structure by the solution infiltration method using dimethylethylamine alane [AlH3:NMe2Et] as a precursor. The infiltrated aluminium hydride is present within ZIF-8 pores preserving the zeolitic structure, as shown by various advanced analytical techniques.

The Impact of Post-Synthetic Linker Functionalization of MOFs on Methane Storage: The Role of Defects

Natural gas is increasingly being viewed as one of the most viable alternatives to gasoline. Its vehicular application however will only be widespread if safe and high-capacity methane stores are developed. In this work report an over 33% increase in methane uptake on a post-synthetically modified metal-organic framework. The underlying mechanism for this dramatic increase is due to lattice defects formed upon post-synthetic modification. This method may open new approaches to natural gas storage.

Pathways to electrochemical solar-hydrogen technologies

Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

Functionalised metal-organic frameworks: a novel approach to stabilising single metal atoms

We have investigated the potential of metal–organic frameworks for immobilising single atoms of transition metals using a model system of Pd supported on NH2-MIL-101(Cr). Our transmission electron microscopy and in situ Raman spectroscopy results give evidence for the first time that functionalised metal–organic frameworks may support, isolate and stabilise single atoms of palladium. Using thermal desorption spectroscopy we were able to evaluate the proportion of single Pd atoms. Furthermore, in a combined theoretical-experimental approach, we show that the H–H bonds in a H2 molecule elongate by over 15% through the formation of a complex with single atoms of Pd. Such deformation would affect any hydrogenation reaction and thus the single atoms supported on metal–organic frameworks may become promising single atom catalysts in future.