Chemical engineers have found a cheaper and possibly better material than platinum for cleaning up the exhaust streams of diesel vehicles. Many automobiles use platinum-containing catalytic converters to help clean their exhaust streams of various pollutants, notably the nitrogen oxide compounds that can contribute to smog. But the high and volatile price of the precious metal makes it difficult to build an economical catalyst. An alternative material, known as perovskite, is far less expensive than platinum and may do the job more effectively, engineers from the research arm of General Motors report in the March 26 Science. “It’s excellent work,...