434646 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

Tuesday, November 10, 2015: 2:30 PM
355E (Salt Palace Convention Center)
Renqin Zhang1, Kathy Helling1, Janos Szanyi2, Feng Gao2 and Jean-Sabin McEwen1, (1)Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, (2)Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA

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Figure 1. Comparison between (a) experimental XANES [7] and (b) our computational XANES. In our calculated XANES, there is a small feature peak A for the CO, CO+H2O and CO+OH adsorbate systems on Cu+-SSZ-13 (ZCu).

Nitrogen oxides (NOx) 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, NOx emission control is a challenge in these "lean-burn" engines [2]. As is well known, the selective catalytic reduction (SCR) of NOx 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 and Cu-SAPO-34, display excellent catalytic activity and hydrothermal stability in SCR of NOx, 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 2 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

[1] U. Deka, I. Lezcano-Gonzalez, B.M. Weckhuysen, A.M. Beale, Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx, ACS Catal., 3 (2013) 413-427.

[2] S.i. Matsumoto, Catalytic Reduction of Nitrogen Oxides in Automotive Exhaust Containing Excess Oxygen by NOx Storage-Reduction Catalyst, CATTECH, 4 (2000) 102-109.

[3] J.H. Kwak, R.G. Tonkyn, D.H. Kim, J. Szanyi, C.H. Peden, Excellent activity and selectivity of Cu-SSZ-13 in the selective catalytic reduction of NOx with NH3, J. Catal., (2010) 1-4.

[4] L. Ma, Y. Cheng, G. Cavataio, R.W. McCabe, L. Fu, J. Li, Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust, Chem. Engin. J., 225 (2013) 323-330.

[5] F. Gao, J. Kwak, J. Szanyi, C.F. Peden, Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts, Top. Catal., 56 (2013) 1441-1459.

[6] J.-S. McEwen, T. Anggara, W.F. Schneider, V.F. Kispersky, J.T. Miller, W.N. Delgass, F.H. Ribeiro, Integrated operando X-ray absorption and DFT characterization of Cu–SSZ-13 exchange sites during the selective catalytic reduction of NOx with NH3, Catal. Today, 184 (2012) 129-144.

[7] J.H. Kwak, T. Varga, C.H.F. Peden, F. Gao, J.C. Hanson, J. Szanyi, Following the movement of Cu ions in a SSZ-13 zeolite during dehydration, reduction and adsorption: A combined in situ TP-XRD, XANES/DRIFTS study, J. Catal., 314 (2014) 83-93.


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