The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N
2, increased as Cu loading increased. N
2 selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H
2 (H
2-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu
2+ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H
2-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu
+ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO
2 suggested that CuO could form N
2, but, independent of the CuO dispersion, nitrogen oxides (NO
x) were formed above 350 °C. Cu
+ and the square-pyramidal and distorted square-pyramidal forms of Cu
2+ showed low activity for AN decomposition. Temperature-programmed desorption of NH
3 suggested that N
2 formation from NH
3 proceeded on Cu
2+, resulting in the formation of Cu
+. The Cu
+ ions were oxidized to Cu
2+ at around 300 °C. Thus, high N
2 selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu
2+, which can be reversibly reduced and oxidized.
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