SCR of NOx with CH3OH on H-mordenite: mechanism and reaction intermediates

The selective reduction of NOx over H-mordenite (H-m) was studied using CH3OH as reducing agent. Results are compared with those obtained with other conventional reducing agents (ethylene and methane), with gas-phase reactions, and with other metal-exchanged mordenites (Cu-mordenite (Cu-m) and Co-mordenite (Co-m)). H-m was found to be an effective catalyst for the SCR of NOx with CH3OH. When different reducing agents were compared over H-m, CH3OH>C2H4>CH4 was the order according to the maximum NO conversion obtained using 1% of oxygen in the feed. Instead, if selectivity is considered, the order results CH4>CH3OH>C2H4. In reaction experiments, two distinct zones defined by two maxima with NO to N2 conversion are obtained at two different temperatures. A correlation exists between the said zones and the CO:CO2 ratio. At low temperatures, CO prevails whereas at high temperatures CO2 prevails. These results indicate that there exist different reaction intermediates. Evidence from reaction experiments, FTIR results, and transient experiments suggest that the reaction mechanism involves formaldehyde and dimethyl ether (DME) as intermediates in the 200–500°C temperature range. The surface interaction between CH3OH (or its decomposition products) and NO is negligible if compared with NO2, indicating that the oxidation of NO to NO2 on acid sites is a fundamental path in this system. Different from other non-oxygenated reductants (methane and ethylene), a gas-phase NOx initiation effect on hydrocarbon combustion was not observed.