(126d) Determination of Active Sites for NO Oxidation On Cu–ZSM5

Catalytic NO oxidation to NO₂ is considered to be an important step in lean burn diesel engine NOx abatement techniques, such as Selective Catalytic Reduction (SCR) and NOx storage/Reduction (NSR). It is known that an equimolar feed mixture of NO and NO₂ increases the SCR activity in comparison to just NO in the feed. In fact, even for the standard SCR reaction (NO + O₂ + NH₃) on Cu-Zeolites, NO oxidation is proposed as the rate limiting step¹. The objective of this work was to obtain a fundamental understanding of the nature of the active sites on Cu-ZSM5 for the catalytic NO oxidation to NO₂that can be used to aid the unraveling of the mechanism of SCR on these catalysts.

In the work reported here, steady state kinetics for catalytic NO oxidation were studied on a series of Cu-ZSM5 catalysts ranging from Cu/Al = 0.15 to Cu/Al = 2.1 under differential conditions. The variation of the rate per gram of catalyst versus the Cu/Al ratio led to the identification of 3 regimes of catalyst performance. The catalysts in regime 1 (Cu/Al = 0 – 0.2) did not display any NO oxidation activity. In regime 2, for Cu/Al = 0.2 – 0.45, the rate per gram of catalyst was sensitive to the copper loading and increased monotonically to a maximum of 2.7 x10^-7 (mol NO) (g. cat. s)^-1 at Cu/Al = 0.45. For catalysts in regime 3 with Cu/Al = 0.5 – 2.1, no further enhancement in the NO oxidation rate was realized. The NO, O₂ and NO₂ orders were 1.9 ± 0.08, 0.9 ± 0.04, and -1.2 ± 0.1, respectively, with an apparent activation energy between 90 – 113 kJ mol^-1 for all the catalysts in regimes 2 and 3. Inactivity of Cu-ZSM5 samples in regime 1 was attributed to the presence of a majority of isolated Cu²⁺ ions at the ion exchange positions of the zeolite. We postulate that further increase of the Cu loading (Regime 2) produced dimeric (paired) or oligomeric Cu species within the zeolite. The rate per gram of catalyst in regime 2 scaled with the literature value of the 22700 cm^-1 band intensity of the UV-Vis spectra of oxygen activated Cu-ZSM5 and was attributed to mono-(μ-oxo) dicupric species². We assume that increasing the copper loading in regime 3 led to the formation of unpaired copper species or bulk copper oxide clusters which did not enhance the rate of NO oxidation further. Since the kinetic parameters (reaction orders and apparent activation energy) remained unchanged, the nature of the active site also remained unaltered and the increase in the rate with copper loading in regime 2 was only due to the increase in the number of active sites which appear to be mono-(μ-oxo) dicupric species.

1. Metkar, P.S., Balakotaiah, V., Harold M., Catal. Today, 2012, 184,115-128
2. Beznis, N.V., Wekhuysen, B.M., Bitter J.H. Catal. Lett., 2010, 138, 14-22.