The reaction gas entered the reactor via the bottom. A four-way valve was used to select if a stream of pure nitrogen or a mixture of nitrogen and air entered the reactor. Brooks E 5850 TR mass flow controllers were used to control the flow of the pure nitrogen stream (which could be varied between 60 and 540Nml min−1) and the flow rate and composition
of the mixture of nitrogen and air (the flow rate of the mixture was chosen to be identical to the flow rate of the nitrogen stream, while the oxygen content could be varied between 2.0 and 21.0 vol.%). The sulfur dioxide content of the gas that left the reactor was continuously monitored using a MAIHAK UNOR 610 infrared sulfur dioxide analyzer. Unless stated otherwise, the following experimental procedure was applied. The selected quartz glass cup was filled with a sample of copper sulfide of a precisely known weight varying between 1 and 32 mg.