AN IR STUDY OF NO ADSORPTION ON A CROX/ZRO2 CATALYST

The adsorption of NO at room temperature (RT) on a CrO(x)/ZrO2 catalyst has been investigated by IR spectroscopy and compared with data previously obtained by ESR spectroscopy and chemical titrations. Prior to NO adsorption, the sample was heated in O2 at 923 K (average oxidation number of chromium, nBAR = 5.5) and reduced in CO from 373 to 623 K (nBAR varying from 5.5 to 2.5). The adsorption of NO on samples heated in O2 yields mainly dinitrosyls of Cr(III), arising from the reduction of Cr(V). The adsorption of NO on samples reduced at 373 K yields mainly dinitrosyls of Cr(III), in addition to N2O, nitrites and nitrates. The admission of NO on fully reduced samples (nBAR = 2.5) yields dinitrosyls of Cr(III) and mono- and dinitrosyls of Cr(II), in addition to N2O, nitrites and nitrates. Upon evacuation of the latter sample at 423 K, the adsorbed NO oxidized most of the Cr(II) to Cr(VI) (nBAR = 4.2) and, in fact, the nitrosyls of Cr(III) are the only species detected by IR on NO readmission after evacuation treatment, in addition to N2O(ads), nitrites and nitrates. Blank experiments on pure ZrO2 show that the dismutation of NO leading to N2O and nitrites (or nitrates) takes place on sites of the ZrO2 support. When a fully reduced sample is further reacted with water at 853 K, mainly dinitrosyls and mononitrosyls of Cr(III) are detected upon exposure to NO, since reaction with water selectively oxidized Cr(II) to chromia-like species.