New research on palladium membrane technology clarifies hydrogen transfer pathways

Researchers at the University of British Columbia have conducted a new detailed study of the hydrogen transfer pathways using a palladium membrane reactor. The study looks at the differences between homolytic and heterolytic pathways during hydrogenation reactions, noting how the behaviour of hydrogen changes based on different levels of current density. In a homolytic transfer, two Pd–H bonds generate two hydrogen radicals (H•), while in a heterolytic transfer they create a proton (H+) and a hydride (H-).

The palladium membrane reactor consists of two chambers – an electrochemical chamber and a hydrogenation chamber. This allows the equipment to separate hydrogen adsorption and hydrogenation processes, supporting distinct reaction pathways and thereby avoiding complications related to exclusive thermochemical catalysis. The palladium membrane reactor offers both thermochemical and electrochemical functionalities, with the membrane acting variously as a cathode, a hydrogen-selective membrane, and a heterogeneous hydrogenation catalyst.

With thermochemical processes, where dissociation and hydrogenation occur at the same surface, small changes in temperature or pressure can result in significant changes to the PdHx ratio. Additionally, hydrogen is most often transferred to the reactant through a homolytic mechanism, given the minimal electronegativity difference between the two atoms. Heterolytic mechanisms, on the other hand, usually require catalyst surface modification such as oxides, the isolation of single atoms or a catalyst support.

Electrochemical hydrogenation reactions support direct electron transfer from an electrode surface to a reactant in solution. This electron transfer is followed by a proton transfer or vice versa. The pathway depends on the reduction potential of the reactant, the electrode material, and the potential applied to the cathode.

The results of the research showed that palladium membrane reactors enable hydrogenation in ambient conditions, without the need for hydrogen. The protons generated by the electrolysis of water were reduced to Pd–H on one side of a palladium foil layer, and the adsorbed H atoms then migrate through the palladium from the electrolysis chamber to the hydrogenation chamber on the other side of the foil. The hydrogen reacts with unsaturated species in the hydrogenation chamber.

Palladium has unique properties that make it ideal for processes involving hydrogen. Firstly, in a standard temperature and pressure environment the platinum group metal (PGM) can absorb as much as 900 times its own volume of hydrogen. More importantly for hydrogen transfer, it is permeable only to hydrogen, allowing very pure hydrogen to be obtained in experimental conditions.

The researchers were able to electrochemically control the amount of hydrogen loaded into the palladium, achieving a well-defined and stable PdHx ratio so as to properly study the chemical hydrogenation process. As a result of the study, it was shown that hydrogen atoms, protons and hydrides all display reactivity on heterogeneous palladium surfaces, while different reaction conditions favour different outcomes.

Though the hydrogenation of unsaturated species is a common process carried out at a large scale across many industries, it is difficult to define the identity of reactive hydrogen species, and therefore scientific understanding about whether hydrogenation proceeds through a homolytic or heterolytic pathway has been lacking.

The latest study found that a low current density, i.e. a low level of electrode polarisation, favoured homolytic dissociation, whereas high currents precipitated heterolytic dissociation. With each increase of current density – 40, 80, 120, 160 and 200 mA cm-2 were all tested – the rate of H• transfer decreased and the rate of H+/H- transfer increased.

The research was carried out by Mia Stankovic, Bowen Ge, Jessica Sperryn and Curtis Berlinguette, all of whom are based at the University of British Columbia’s Department of Chemistry. The study was published by the Journal of the American Chemical Society, and received funding from a number of Canadian bodies including the Canada First Research Excellence Fund.

Source: https://pubs.acs.org/doi/10.1021/jacs.4c14671