Redox chemistry of Cu/ZSM-5

Motivated by the unique catalytic activity of Cu/ZSM-5 in the decomposition of NO to N-2 + O-2, the redox chemistry of Cu in zeolite ZSM-5 has been studied using FTIR, TPR, EPR, and EXAFS. Isolated ions, CU2+, oxocations, [Cu-O-Cu](2+), and oxide particles have been identified. Their relative abundances depend on the overall Cu loading, the pH during ion exchange, and the gas atmosphere. Oxocations are only detected at Cu exchange levels exceeding 40%; their concentration is higher in high-pH preparations favoring hydrolysis. Oxocations are reduced by CO and NO at room temperature; they act as catalytic sites for the disproportionation of NO into N2O + NO2. CuO particles are detected in all samples; at elevated temperature they decompose in He to Cu2O. In FTIR Cu+ is detected using CO or NO as a probe. Flowing H-2 reduces CU2+ ions; the first detectable product is Cu+ because Cu-0 is thermodynamically unstable in the presence of CU2+. After all CU2+ is used up Cu-0 is detected, In CO only oxide particles and oxocations are reduced, but when CU2+ ions are present, they react with Cu-0 to form Cu+ ions. Zeolite protons oxidize CU0 to Cu+; this process is accelerated by CO, which forms a stable Cu+-CO complex. Protons also react with CuO to CU2+ + H2O.