Gold, atomic number 79, often overshadows its less illustrious sounding neighbor, Platinum, atomic number 78. Platinum sounds like nothing special. Even its name is an ode to another precious metal, silver. However, platinum is just as valuable and perhaps has more ubiquitous applications than gold and silver. In fact, platinum is currently trading at $1,178.50 per ounce in the US, just as much, or more, than the better renowned silver and gold. So what makes platinum so valuable?
Platinum is one of the most valuable of the precious metals due to unique chemical and material properties. Silvery-white and difficult to oxidize or corrode, platinum has extensive applications in jewelry, wires, and high temperature systems. Most of us use 1-2 grams of platinum on a regular basis; cars and trucks often use platinum in catalytic converts that convert carbon monoxide to carbon dioxide. Eliminating carbon dioxide production can also be achieved using platinum catalysts. The most important use of platinum today is as a catalyst for fuel cells, which produce hydrogen gas and hopefully eliminating sources of carbon dioxide emission. Typically, splitting water into hydrogen gas and oxygen is sluggish and inefficient, however platinum can catalyze, or speed up this reaction due to its lack of reactivity in most environments and its unique electron orbital energy. But if platinum is such a good catalyst for fuel cells, why aren’t we splitting water and generating hydrogen fuel in our homes and cars already?
Remember, platinum is trading at $1,178.50 per ounce, equivalent to the weight of a slice of bread. Simply put, platinum is too expensive to use in large quantities- and there just isn’t enough on this planet. Platinum makes up 0.00000037% of the Earth’s crust, which comes out to be about 7.8 billion tons. Billion tons? That seems like a lot, but global production of platinum is a mere 130 tons a year, due to difficulties mining and finding platinum. Despite the cost, scientists continue to push for platinum as a catalyst. Non-corrosive in water and acidic solutions, platinum can bind and release oxygen molecules, an essential part in water splitting for hydrogen evolution. Researchers at Caltech have explored other earth abundant catalysts, such as nickel, but platinum constantly outperforms all other metals.
While Caltech and other institutions continue to search for economical alternatives to platinum, several scientists are working to reduce the amount of platinum required. Platinum alloys, or platinum combined with other metals, were tested as catalysts and preformed well, but not great. However, creating a layer of a platinum alloy, specifically platinum-copper (Pt-Cu), then removing copper from the layer enhances the capability of electrodes in fuel cells beyond what a pure platinum catalyst can do. These electrodes are more efficient, and more importantly, cheaper because they use less platinum! Scientists in Germany reduced the amount of platinum in their electrodes by 80% by dealloying Pt-Cu surfaces. While the researchers continue to search for an explanation to this phenomenon, they currently believe that the increased compressive strain of dealloyed Pt surfaces increases catalytic capabilities. Removing copper from the Pt-Cu alloy creates vacancies in upper layers, but not lower layers of the alloy. The lower layers still include copper, so the copper and platinum are closer together than the platinum atoms in the upper layer. This forces the upper layer of platinum atoms to be closer together, changing the electron orbital energy. Compressing the Pt atoms increased strain which increased catalytic activity.
Platinum is an extremely valuable catalyst if we want to reduce our greenhouse gas emissions and move towards sustainable energy. However, the limited amount of platinum makes the element inherently unsustainable. Until we learn to harvest meteors for platinum, we need to continue to reduce the amount of platinum required for processes or find alternatives.
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