Studies at the Swiss Federal Technical Institute (ETH), Zurich, have produced results that, if confirmed by further research, could pose problems for the developers of catalytic converters that reduce emissions from diesel and lean-burn gasoline engines. The Swiss researchers have found that catalytic reduction of nitrogen oxides with ethene and propene over the zeolite catalyst Cu-ZSM-5, in the temperature range of 450 to 600 K, produces significant quantities of hydrogen cyanide. In addition, significant amounts of undesirable nitrous oxide are formed below 700 K. Auto catalytic converters operate at a wide range of temperatures, including these. At present, there is a major effort by diesel engine makers and diesel fuel suppliers to develop a catalytic converter that will meet forthcoming Environmental Protection Agency limits on nitrogen oxides in exhaust. The HCN concentration in the reactor effluent reaches a maximum of 20 or 30 ppm, depending on whether ethene or propene is used as a reductant. These values both exceed the maximum value of 10 ppm recommended by an agency of the Swiss government. No ammonia was detected in the exhaust gases. HCN formation appears to peak at about 500 K with propene and at about 600 K with ethene. Othermore » catalysts that have been suggested for similar catalytic systems include H-zeolites and metal oxides with and without transition-metal additives. At present, it is not known whether they have the same effects with respect to HCN.« less