(148j) Sensitivity of XANES and XES to the local environment: Multiple adsorption of H2O and NH3 on Cu-SSZ-13

Sensitivity of
XANES and XES to the local environment: Multiple adsorption of H₂O
and NH₃ on Cu-SSZ-13

Renqin
Zhang¹, Hui Li¹, Christopher Paolucci², Atish
A. Parekh³, Janos Szanyi⁴, Feng Gao⁴,
Trunojoyo Anggara², Hui Li², William F. Schneider²,
Fabio Ribeiro³ and Jean-Sabin McEwen^1*

1.
Washington State University, Pullman, WA; 2. University of Notre Dame, Notre
Dame, IN; 3. Purdue University, West Lafayette, IN; 4. PNNL, Richland, WA

*js.mcewen@wsu.edu

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 NH₃ reductant to form N₂ and H₂O. Copper-exchanged
small-pore micro-crystalline materials with the chabazite structure (Cu/CHA),
such as Cu-SSZ-13, 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 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. Cu K-edge X-ray absorption near
edge spectra (XANES) has been widely used to study the properties of Cu-SSZ-13
because of its high activity and selectivity in the NH₃ SCR of NO_x.
It is demonstrated that XANES is extremely sensitive to the local structure of
Cu ions [6, 7]. However, the exact relationship between XANES features and
local structure is still unclear. In this work, we examine how the geometry of a Cu⁺
ion with a linear configuration and Cu²⁺ ion with a square planar
configuration in Cu-SSZ-13 correlate with the XANES features. When water and
ammonia binds to a Cu⁺
ion with a linear configuration, a strong intensity peak around 8983 eV in Cu
K-edge XANES appears. When two ammonia molecules bind at a Cu⁺ ion,
it is also found that the intensity of the peak around 8983 eV decreases as one
decreases the N-Cu-N bond angle from 180 to 100 degree (see Figure 1a). By analyzing the
corresponding PDOS, it is found that the splitting of 4p state correlates with
the N-Cu-N bond angle and the corresponding decrease in the K-edge intensity.
Concerning the case of Cu²⁺ ions with a square planar configuration
bounded to four ammonia molecules, it is found that there are two feature peaks
around 8986 and 8993 eV in their Cu K-edge XANES. By studying this simple
model, we find that changing the N-Cu-N angle between one of the bonded NH₃
and the Cu-centered ion from 180 to 100 degree results in a decrease in the intensity of both
peaks around 8986 and 8993 eV. It is also found that the broken of square
planar configuration results in the delocalization of 4p state.

An
important limitation of XANES is that it is difficult to distinguish the
nitrogen and oxygen atoms in the proximity of the metal center. X-ray emission
spectroscopy (XES) could overcome this limitation. It is reported that the kbeta'' peak in
XES of a Cu-SSZ-13 sample presents a blue shift with the formation of Cu-N bond
[8], as shown in Figure 1b. This can be understood by the fact that the kbeta'' peak
originates from the ligand 2s to a metal 1s crossover transition and the
nitrogen 2s level lies higher in energy than the one for oxygen. By comparing
the XES results between H₂O and NH₃ adsorbed on
Cu-SSZ-13, it is found that the kbeta'' peak of Cu-O bond appears at lower
energy than that of Cu-N bond. In addition, the coordination number of Cu ions
significantly affects the energy difference between the position of the kbeta'' peak. By
performing a density of states (DOS) analysis, these results can be understood
at the electronic level. Overall,
our careful elucidation of the spectroscopic properties of Cu ions in Cu-SSZ-13
via theoretical techniques can assist us in interpreting the XANES and XES
experimental results so as to gain a deeper understanding of the SCR of NO_x
with NH₃ throughout its catalytic cycle.

Figure 1. (a) Variation of Cu K-edge XANES for 2 NH₃
adsorbed on Cu⁺-SSZ-13 while changing the N-Cu-N angle (as shown
insert in the right bottom) from 180 degrees (black line) to 100
degrees (blue line). (b) Computational XES of Cu ions in H₂O
and NH₃ adsorbed on Cu⁺-SSZ-13. Two emission lines, kbeta" and kbeta_2,5, are presented.

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