FT-IR study of partial reduction of over-exchanged, O2-calcined CuII-ZSM-5 using flowing CO/He at stepwise increasing temperatures. Part 1

Over-exchanged, O2-calcined 3.1 wt% CuII-ZSM-5 (nSi/nAl=20, nCu/nAl=0.75) was treated with flowing CO/He mixture, with stepwise increases of the temperature from 293 to 773 K. The (partial) reduction of ionic CuII species was followed by the changes in the spectral features of intrazeolitic Cu+–CO carbonyls and skeletal vibrations of ZSM-5, related mainly to ν(CO) and (locally easily perturbable internal asymmetric) νasint(TOT) bands. Somewhat surprisingly, a relatively large amount of Cu+(12CO) formed immediately already at 293 K [absorbance, ACO(2157) = 0.5892]. The significant decrease in the intensity of νasint(TOT) band, widened towards higher frequencies, but centered at 918 cm−1, along with the formation of a new band at 968 cm−1, also suggested transformation of Cu2+ (ionic CuII) to Cu+ species, i.e., Cu+–CO carbonyls. In proportion to the natural 13CO content of CO, a similar (but weak) band was also obtained at 2108 cm−1 [due to Cu+(13CO)]. With increasing temperature up to 773 K, the intensities of both ν(12CO) and ν(13CO) bands gradually increased [ACO(2157) = 1.4624], in good accordance with the growing related νasint(TOT) band at 968–966 cm−1. The analysis of the BCO integrated intensities of the two carbonyl bands, the half-width (Δν1/2) and band asymmetry (As1/2) of the developing ν(12CO) band at half ACO(2157) values showed some unexpected changes.