Conditions in which Cu-ZSM-5 outperforms supported vanadia catalysts in SCR of NOx by NH3

Continuous hows of a standard reactant mixture, featuring 0.6% nitric oxide, 0.6% ammonia and 3.3% oxygen at moderate space velocities over 4 different catalysts, have been used to compare relative activities for selective catalytic reduction of NOx at 373-773 K. The catalysts tested were: Cu-ZSM-5 featuring >100% ion-exchange; a conventionally prepared vanadia-titania-tungstate (VTT) material and two unconventional catalysts prepared by vanadia deposition onto ex-sol-gel WO3-TiO2 supports. At catalytic temperature 473 K, higher conversion to N-2 was achieved over Cu-ZSM-5 than over the other three materials. Tests without NO at 473 K showed insignificant contributions to N-2 formation from ammonia oxidation over any of the catalysts, whereas tests at 573, 623, 673 and 773 K revealed larger progressive increases in such contributions over Cu-ZSM-5 than over the other catalysts. Values for SCR activities corrected for such contributions demonstrated that activity of Cu-ZSM-5 for SCR conversion of the standard NO+NH3+O-2 reactant mixture to N-2 at 473 K was ca. twice as great as the other three catalysts at that temperature, but that increasing the reaction temperature to 573 K caused only a slight further increase. 'Corrected' SCR activities in the standard reactant mixtures were rather similar for all four materials at 573 K, but with Cu-ZSM-5 marginally out-performed by one of two unconventional catalysts featuring vanadia upon an ex-sol-gel WO3-TiO2 support having tungsten incorporated into the TiO2 anatase structure. Both of these unconventional catalysts outperformed a conventional 'VTT' catalyst. Observations upon variations in conversion to N, with variation in the oxygen content of the reactant gas mixture from 1 to 6% established another unique feature of the Cu-ZSM-5 catalyst at 473 K, viz. the need for ca. 4.5% O-2 to raise conversion to the maximum attainable over that catalyst at this temperature. No deactivation was observed after short-term runs at temperatures up to 823 K. Introduction of water vapour into the standard reactant mixture slightly enhanced the activity of Cu-ZSM-5 at 473 K.