Published 14 April 2020
Hydrogen is of more interest than ever as a green power source in transportation. However, the chemical reaction for its industrial production was poorly understood. Scientists have now analysed the reaction in detail to gain insights into a more efficient and cost-effective production.
The vast potential of hydrogen as an alternative fuel source was identified nearly two centuries ago: The first hydrogen-oxygen fuel cell was constructed in 1839. In 1966, the first car running on a hydrogen fuel cell was built and by the late 1980s, Russian engineers had even constructed a hydrogen-powered aircraft. In the modern-day race to green energy, hydrogen is of more interest than ever, because the only by-product of this power source is water vapor.
Despite the fact that it is frequently produced for industrial purposes, mainly for the production of fertiliser or for the desulphurisation of fossil fuels, hydrogen is still waiting for its breakthrough moment in everyday transportation. “Although hydrogen gas is widely used, the chemical reaction of its production process is not yet fully understood at an atomic level”, says Philip Hoggan, professor in chemistry and surface physics at the Institute Pascal, Clermont University, France. With the help of PRACE supercomputing power, Hoggan and his co-workers have now analysed the chemical reaction of the hydrogen production process using quantum-theory-based methods in order to gain a better understanding of its limiting steps — and, ultimately, to gain insights about how to render the production of the gas more efficient and cost effective.
One industrial hydrogen production process uses the so-called water-gas-shift reaction. It follows the overall equation: CO + H2O -> CO2 + H2. This means that carbon monoxide (CO), which is abundantly available as a waste gas from many industrial processes, and water serve as chemical precursors for forming carbon dioxide and the sought hydrogen gas. For the reaction to occur, a catalyst is also needed, often a metal like platinum. However, several facets of the reaction were unknown — for instance, the rate limiting step of the reaction, its exact progression, and the nature of the intermediates and the transition state, which corresponds to the highest-energy state on the reaction path — until now.
To investigate the mysterious reaction, Hoggan and his co-workers employed a newly developed Quantum Monte Carlo method. “This method is computationally very expensive, but it provides the most accurate calculation possible today based on quantum theory”, explains Hoggan. In fact, the generated statistical data can accurately represent the structure and properties of the electrons of the participating atoms and simulate the changes occurring during the reaction.
An unexpected assembly
With their new calculations, Hoggan and his team were able to pinpoint the sequence and geometry of the reaction. Interestingly, the results confirm a different reaction mechanism than previously assumed. It was thought that the rate limiting reaction step was the dissociation of the first O-H bond of water (H2O) to give an OH radical — meaning an OH molecule carrying a single unpaired electron that renders it extremely reactive — and a hydrogen atom (H) bound to the platinum catalyst. However, according to Hoggan’s findings, the energy barrier for this to occur is much higher than assumed: more than 75 kJ/mol instead of the previously calculated 68 kJ/mol. In comparison, the overall activation energy for the reaction to occur is lower, at around 71 kJ/mol.
Based on this and further results of the new simulations, Hoggan proposes a different reaction progression and mechanism: First, the carbon monoxide (CO) binds to the platinum catalyst, which renders the CO molecules ready to react with the oxygen in water molecules. Subsequently, the intermediate state of the reaction is formed, an additive complex between CO and water. This is again a radical featuring an unpaired electron. To form this intermediate species, the molecules have to pass through the transition state — the highest energy maximum of the reaction. This transition state is key to determining the overall activation energy, and as the calculations showed, the transition state is rate limiting. Its nature and geometry can only be determined by calculations, as there is no experimental method of detecting a transition state. Finally, the intermediate species dissociates into the end products: CO2 and H2.
A better look into dynamics
Thanks to their novel findings, Hoggan and his team were selected for the cover of the International Journal of Quantum Chemistry. Their research is far from finished, though. As a next step, the scientists are already planning to enhance their method to better include the dynamics of the atoms in the chemical reaction. The existing system already represents movement to a certain degree, showing for instance that out of the two precursors, water is more mobile than carbon monoxide, but the current model does not yet include the atoms’ movements during the reaction.
In addition, the modelling of the platinum catalyst has been limited for now. The model worked under the assumption that the platinum surface is planar, while in reality the surface possesses irregularities, such as ripples. Future modelling of a more realistic, imperfect catalyst surface would enable the researchers to assess how the platinum surface affects the overall activation energy of the reaction — and thus provide even more valuable input for the improvement of the industrial production of hydrogen gas.
Reference: Sharma, R.O., Rantala, T.T. and Hoggan, P.E.: Selective hydrogen production at Pt(111) investigated by Quantum Monte Carlo methods for metal catalysis. Int. J. Quantum Chem. (2020). See here.
Project title: Platinum Reaction Observed by Monte-Carlo for Insight into Sustainable Energy (PROMISE)
Supercomputing resources: 41 600 000 core hours on Joliot-Curie hosted (KNL) hosted by GENCI at CEA, France.
Research field: Chemical Sciences and Materials
Team: Sharma, R.O., Monier, G., Travert A., Rantala, T.T. and Hoggan, P.E.