Hydrogen gas (H2), which is currently used in world-wide production of ammonia, is also being considered as an alternative fuel. But how is hydrogen gas made? Carbon monoxide (CO) and water (H2O) can be combined to form hydrogen gas (H2) and carbon dioxide (CO2) in a process known as the water-gas shift reaction. The water-gas shift reaction is one of our primary sources of hydrogen. Several other processes of producing hydrogen exist and are being developed, such as electrolysis, direct solar water splitting, and microbial biomass conversion.
For efficient production of hydrogen using the water-gas shift reaction, a catalyst is necessary. A catalyst is a substance that will increase the rate of the reaction. For example, the catalytic converters in cars use platinum in order to catalyze the conversion of carbon monoxide to carbon dioxide. Scientists have developed catalysts for the water-gas shift reaction containing a range of metals such as iron, platinum, gold, copper, and nickel. More recently, it has been found that heterogeneous catalysts (catalysts which contain more than one metal) can catalyze the reaction at lower temperatures than some which only contain one metal. These lower temperatures translate to less energy required to complete the reaction. In particular, a family of heterogeneous catalysts which involve ceria (CeO2) and another metal such as platinum, gold, nickel, or copper have gained much attention.
Ceria (CeO2) has become a common denominator in many of these newly developed catalysts. Cerium (Ce) is able to easily capture and release oxygen which plays an important role in the catalytic mechanism. With this new family of catalysts along comes our need to optimize their catalytic efficiency. The C. C. Jarrold Group at Indiana University is able to model potential catalysts using their in-house built instrument and Big Red II, the university’s supercomputer.
The in-house built instrument is a combined mass and anion photoelectron spectrometer. A mass spectrometer separates different chemical species according to their mass and charge. Within the C. C. Jarrold group, it is used to observe the reactivity of the metal catalysts with water or carbon monoxide which are the reactants in the water-gas shift reaction. An anion photoelectron spectrometer uses a laser to remove an electron from a negatively charged chemical species to form a neutral one. An electron from a species which is very stable requires more energy to be removed than an electron from a species which is not. Additionally, the energy of the detached electron provides insight into the types of bonds present in the species. The spectrum which results thus supplies information about the stability and structure of the chemical species. Finally, using computational chemistry, the lowest energy geometric structure can be calculated. The combination of these three techniques elucidates the chemical, electronic, and physical properties of the metal catalysts.
Most recently, the C. C. Jarrold Group has published a study on cerium and platinum containing species. These studies were able to show how cerium and platinum interact with one another. Overall, it was seen that oxygen vacancies on the ceria surface possibly lead to an accumulation of electrons on platinum resulting in a more positive cerium and negative platinum. This charge disparity between the metals is believed to aid the formation of hydrogen gas. Using this new knowledge, the C. C. Jarrold Group hopes to design new heterogeneous catalysts for hydrogen production. After all, if we are considering increasing the world’s consumption of hydrogen we must first optimize our production.