Solid-State Ion-Exchange of Copper into Zeolites Facilitated
by Ammonia at Low Temperature
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Abstract
The effect of the gas phase during
solid-state ion-exchange of
copper into zeolites was studied by exposing physical mixtures of
copper oxides (Cu<sup>I</sup><sub>2</sub>O and Cu<sup>II</sup>O) and
zeolites (MFI, *BEA, and CHA) to various combinations of NO, NH<sub>3</sub>, O<sub>2</sub>, and H<sub>2</sub>O. It is shown that heating
these mixtures to 250 °C results in active catalysts for the
selective catalytic reduction of NO with NH<sub>3</sub> (NH<sub>3</sub>-SCR), indicating that the Cu has become mobile at that temperature.
Such treatment allows for a fast (<5–10 h) preparation of
copper-exchanged zeolites. Scanning transmission electron microscopy
analysis of Cu-CHA prepared using this method shows homogeneous distribution
of the Cu in the primary particles of the zeolite. In situ XRD reveals
that the Cu ion-exchange is related to the formation of Cu<sup>I</sup><sub>2</sub>O. When the zeolite is mixed with Cu<sup>II</sup>O, addition
of NO to the NH<sub>3</sub>-containing gas phase enhances the formation
of Cu<sup>I</sup><sub>2</sub>O and the Cu ion-exchange. The mobility
of Cu at low temperatures is proposed to be related to the formation
of [Cu<sup>I</sup>(NH<sub>3</sub>)<sub><i>x</i></sub>]<sup>+</sup> (<i>x</i> ≥ 2) complexes
