Quantifying accessible Cu(I) ions in Cu-zeolites by CO titration

Cu-exchanged zeolites are the most studied materials for the direct conversion of methane to methanol (DMTM), a “dream reaction” with important consequences in the fields of environment, energy and resource management. Although the mechanism of DMTM is debated, Cu(I)/(II) redox speciation, and especially Cu reducibility, plays a key role in this reaction. Currently, the main technique used to quantitatively assess Cu oxidation state in these materials is XANES spectroscopy. We have been currently developing a new technique to titrate Cu(I) on such systems, based on the specific interaction of CO with this species, that exploits isothermal volumetric adsorption as a quantitative tool. This methodology, however, could be affected by the lack of accessibility of Cu(I) ions by CO within the zeolitic framework. A comparison with a well-affirmed technique for the same quantification, as Cu K-edge XAS, is thus desirable.