(351g) Spectroscopic Properties of Cu in Cu-SSZ-13 and Cu-SAPO-34 during the Selective Catalytic Reduction of NOx with NH3: Atomic and Electronic Study from First Principles

Nitrogen oxides (NO_x) are one of the main air pollutants present
in the exhaust from diesel engines, which are popular for vehicle transportation
due to their efficiency and durability [1].
However, NO_x emission control is a
challenge in these "lean-burn" engines [2]. As is well known, the selective catalytic reduction (SCR) of NO_x with NH₃ is a reaction between
the NO, NO₂, and O₂ oxidants and the NH₃reductant to form N₂ and H₂O. Copper-exchanged
small-pore micro-crystalline materials with the chabazite
structure (Cu/CHA), such as Cu-SSZ-13 and Cu-SAPO-34, display excellent catalytic
activity and hydrothermal stability in SCR of NO_x,
as has been shown in a number of recent studies [3,
4]. Although Cu-SSZ-13 and Cu-SAPO-34 have been commercialized as diesel
after-treatment catalysts, the fundamental chemical and physical properties
need to be characterized in order to aid in the design of new and better
catalysts [5].X-ray absorption
spectroscopy (XAS) is a versatile tool to determine the oxidation state and the
local structure of Cu in Cu/CHA. For example, the appearance of a pre-edge
signal in the X-ray absorption near edge spectrum (XANES) has been associated
with a reduced Cu(I) ion through a comparison to a
reference sample [6]. In this work, X-ray
absorption near edge spectroscopy (XANES) of Cu ions under different conditions
are modeled from first principles. We will look at
several structures and assign the several peaks in the experimental XANES by
examining various Cu species. The relationship between the edge position of Cu
K-edge XANES and the oxidizing power of several species will also be
investigated. The partial density of state (PDOS) of Cu with a 1s core-hole, as
well as the orbital distribution, were used to understand the results of Cu
K-edge XANES.

We find that a small
peak feature at around 8979.5 eV in the Cu K-edge
XANES of clean Cu/CHA is associated with Cu in an 8MR site while there is no such
feature peak when Cu is in a 6MR site. We seek to understand this by analyzing
the corresponding PDOS of Cu with the core-hole effect. Molecular adsorption
onto Cu in a 6MR or an 8MR site results in the Cu K-edge XANES being
independent of its location. We also compared our computational results with the  experimental
results [7], as shown in Figure 1. It is
found that the small peak feature A in our calculated XANES for Cu⁺-SSZ-13
under different CO conditions is consistent with the experimental small peak
feature A. By analyzing the corresponding PDOS, we find that the origin of the small
peak feature in the K-edge XANES is induced by the splitting of the Cu 4p state,
which is generated by the 2¹ orbital of adsorbed CO. More in-depth study of the
molecular co-adsorption on ZCu shows that the edge
position of Cu K-edge XANES follows this order (from low to high energy): clean
< M < M+H₂O < 2M < M+ OH^- (M denotes NO or
CO). By studying Cu K-edge XANES of ZCu with various adsorption
species with different oxidizing power, it is concluded that our computational
XANES results capture the real trend of the edge shift with oxidation state,
which is that a higher oxidation state of Cu results in a higher edge position
in the XANES. These results are expected to greatly aid our understanding of
the properties of Cu-SSZ-13 and Cu-SAPO-34 as SCR catalysts.

References

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