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The Role of Platinum in Catalysts

Source: Stephen B Streater, Public Domain

Platinum group metals are among the most widely used catalysts in catalytic reactions, including, for example, water gas shift reactions or gas phase oxidation. All six of the platinum group elements have good catalytic properties; however, platinum, palladium, rhodium, and ruthenium are the most active in reactions of this kind. Platinum group metals are more expensive in initial cost than base metal ones, but often prove to be more selective and require less severe reaction conditions. Nevertheless, the cost factor is one of the reasons that scientists have been seeking ways to create catalysts that use less of this precious metal. Understanding exactly what the platinum does is an essential step in this process.

Now (2021), scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have analysed how a platinum-based catalyst functions in the water gas shift reaction, and revealed astonishing details. The water gas shift reaction converts carbon monoxide (CO) and water (H2O) into carbon dioxide (CO2) and hydrogen gas (H2). This is an important step in producing and purifying hydrogen for multiple applications, including for use as a clean fuel in fuel-cell vehicles and in the production of hydrocarbons. The scientists suspected that only certain platinum atoms play an important role in the chemical conversion.Thus, an experiment was conducted where the catalyst was made of platinum nanoparticles on a cerium oxide (ceria) surface. Some of those platinum atoms were on the surface of the nanoparticle, some – in the core, some - in the interface with ceria, and some - at the perimeter the interface. The scientists witnessed an interesting effect when the catalysts reached their active state in reaction conditions - the platinum atoms at the perimeter of the particles were moving in and out of focus in an electron microscopy experiment while the rest of the atoms were quite stable. This movement proved to be responsible for the catalytic activity as the dynamic properties at these perimeter sites enabled the CO to get oxygen from the water so it could become CO2, and the water (H2O) released oxygen to become hydrogen. Infrared (IR) spectroscopy studies showed that the appearance of the perimeter sites went hand in hand with “oxygen vacancies” forming on the cerium oxide surface. They also discovered that CO tended to migrate across the platinum nanoparticle surface toward the perimeter atoms, and that hydroxy (OH) groups stayed on the ceria support near the perimeter platinum atoms.X-ray photoelectron spectroscopy studies revealed that perimeter platinum atoms also became activated, changing from a nonmetallic to a metallic state that could capture oxygen atoms from the OH groups and deliver that oxygen to CO. A final set of experiments looked at the dynamic structural changes of the catalyst.

A lot of research has been carried out in the last 20 years concerning the mechanisms of platinum catalysts. In 2010, a study found that alkali ions (sodium or potassium) added in small amounts activated platinum adsorbed on alumina or silica in the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) for producing H2. The alkali ion–associated surface OH groups were activated by CO at low temperatures (~100°C) with the help of atomically-dispersed platinum. Both experimental evidence and density functional theory calculations suggested that a partially oxidized Pt-alkali-Ox(OH)y species was the active site for the low-temperature Pt-catalysed WGS reaction. The aim of the study was the design of highly active and stable WGS catalysts that contained only small amounts of a precious metal without the need for a reducible oxide support such as ceria.

In 2015, an experiment demonstrated that infrared spectroscopy could be a fast and convenient means of directly distinguishing and quantifying Pt single atoms from nanoparticles. In addition, it was observed that only Pt nanoparticles exhibited activity for carbon monoxide (CO) oxidation and water-gas shift at low temperatures, whereas Pt single atoms behaved as spectators. The strong binding of CO molecules was partly responsible for the lack of catalytic activity of Pt single atoms.

Understanding the exact role platinum atoms play in a catalytic reaction can play an important role in future catalyst design, as it may allow catalysts to be created that contain only those active platinum atoms. The scientists are convinced that for a catalyst to work properly not all atoms are needed, just the active ones. This could help make the catalyst less expensive by removing the atoms that are not involved in the reaction. They also believe that this mechanism can be transferred to other catalytic systems and reactions.

Until then, more related research will have to be carried out as some details concerning the long bond between the platinum atoms and the oxygen on the ceria support have yet to be fully understood. The bond suggests that something invisible to the x-rays was occupying the space between the two. The scientists believe that atomic hydrogen might be responsible for this, as light atoms like hydrogen do not become visible under x-rays. “We think there is some atomic hydrogen between the nanoparticle and the support. X-rays can’t see light atoms like hydrogen. Under reaction conditions, those atomic hydrogens will recombine to form H2.”