THE ADSORPTION OF O2 AND NO ON CROX/ZRO2 CATALYSTS, AS INVESTIGATED BY IR AND ESR SPECTROSCOPIES

The adsorption of O2 at 195 K and that of NO at 298 K on CrO(x)/ZrO2 samples have been investigated by the double isotherm method. The nature of the surface species formed before and after adsorption has been studied by means of IR and ESR spectroscopies. Prior to O2 or NO adsorption, CrO(x)/ZrO2 Samples were heated in O2 at 773 K (average oxidation number of Cr, nBAR = 5.5) and reduced with CO at 423 to 623 K to controlled extents (nBAR = 5.5 to 2.5). The results show that the adsorption of O2 at 195 K oxidizes to Cr(VI) the Cr(II) present in the sample after the reduction. The oxidation of Cr(II) to Cr(VI) is quantitative and nearly selective. Only a small fraction of Cr(III), also present in the reduced sample, is oxidized to Cr(V) (ESR, gamma-signal). The adsorption of NO on the sample heated in O2 yields mono- and dinitrosyls of Cr(III), arising from the reduction of Cr(V). The reduction of Cr(V) to Cr(III) is evidenced by both ESR and IR data. The adsorption of NO on the reduced sample (nBAR = 2.5) leads, on average, to 1.7 to 2.0 NO molecules adsorbed per Cr atom. In the same conditions, dinitrosyls and mononitrosyls of Cr(II) and Cr(III) are formed, in addition to N2O and nitrites (and/or nitrates). Upon evacuation of the latter sample at 423 K, the adsorbed NO oxidizes 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 re-admission after the evacuation treatment. Blank experiments on pure ZrO2 show that the dismutation of NO leading to N2O and nitrites (and/or nitrates) takes place on sites of the ZrO2 support. When the reduced sample is further reacted with H2O at 853 K, only the dinitrosyls and mononitrosyls of Cr(III) are detected upon exposure to NO, since the reaction with H2O at 853 K selectively oxidizes the Cr(II) to chromia-like species. It is suggested that coordinatively unsaturated Cr(III) ions, two vacancies at least on IR evidence, are the active sites for the hydrogenation of propene and H-2-D2 equilibration reactions, previously investigated on the same CrO(x)/ZrO2 Catalysts.