Activation of oxygen on (NH3CuNH3)+ in NH3-SCR over Cu-CHA
Graphical abstract
Introduction
Nitrogen oxides (NOx) are formed during combustion in diesel engines and a main challenge in automotive emission control is the conversion of NOx to N2. The development of NOx–aftertreatment systems with high activity and durability is enforced by increasingly stricter legislations. The current approach to reduce NOx to N2 and H2O in emissions from diesel engines is selective catalytic reduction with NH3 as reducing agent (NH3–SCR) [1], [2], [3]. The NH3–SCR reaction proceeds according to:Zeolites exchanged with Cu are efficient catalysts for the NH3-SCR reaction. In particular, Cu-exchanged chabazites (Cu-CHA) have over the past decade emerged as a promising candidate thanks to good performance over a wide temperature window (473–773 K) and high hydrothermal stability [3], [4], [5], [6].
The underlying reason for the catalytic activity of Cu-CHA for NH3-SCR has recently been investigated extensively. The NH3–SCR reaction cycle is a combination of a reduction part, in which Cu(II) is reduced to Cu(I), and an oxidation part, where Cu(I) is oxidized to Cu(II) [7], [8], [9], [10], [11], [12]. The NH3–SCR activity of Cu-zeolites is a consequence of the ability of Cu-ions in the zeolite to change oxidation state between Cu(II) and Cu(I). The reduction of Cu(II) to Cu(I) requires both NH3 and NO for the release of the reaction products N2 and H2O. In the oxidation part, NO reacts with oxygen on Cu(I) and the oxidation state is changed to Cu(II). This has been confirmed experimentally by infrared spectroscopy (FTIR), X-ray absorption spectroscopy (EXAFS), and electron paramagnetic resonance (EPR) [8], [12], [13], [14], [15].
The structure and location of the active Cu sites in the NH3–SCR reaction is not yet established. The reaction cycle proposed in Ref. [8], indicates that the NH3–SCR reaction could proceed on isolated Cu ions, however, without a further specification of the structure of the Cu-sites. For low Cu loading and low temperatures (below 523 K), the NH3–SCR activity shows a second order dependence on the Cu loading. At high temperatures (above 623 K), the activity has instead been measured to have a first order dependence on copper loading [16]. The second order dependence at low temperatures indicates that the NH3–SCR reaction requires the formation of Cu-pairs, whereas the first order at high temperatures indicates that the reaction can proceed on single Cu sites. The formation of the pairs should depend on both Cu loading and the Si/Al ratio [9].
The NH3–SCR reaction, as given in Eq. (1), requires dissociation of an oxygen molecule. Density Functional Theory (DFT) calculations indicate that the dissociation should occur on Cu(I) species, as oxygen interacts weakly with Cu(II) [9], [12], [17]. This result together with the existence of Cu-pairs implies that the pairs consist of Cu(I) ions. DFT calculations have shown that dissociation of O2 in the presence of NO on a pair of framework-coordinated Cu(I) ions proceeds with a considerably lower barrier than on a single framework-coordinated Cu(I) ion [17]. Note that the importance of Cu-pairs for O2 activation is well established within the field of homogeneous catalysis [18].
Recently, Paolucci et al. [9] reported that Cu ions respond sensitively to the reaction environment and that an NH3 atmosphere results in mobile Cu species. Cu K-edge XANES spectroscopy in combination with DFT calculations suggest that Cu+ diffuses as diamine complexes, [8], [19], [20]. The identification of NH3–solvated Cu-ions is important as it suggests that the character of the active sites depends on the operating conditions. By use of operando X-ray spectroscopy, Lomachenko et al. [21] identified two distinct regimes for the copper ions. At low temperatures (473 K), the samples were dominated by NH3-solvated Cu-ions, whereas framework-coordinated Cu-ions were found to be abundant at higher temperatures (above 523 K). The presence of mobile complexes at low temperatures could have implications for the formation of Cu(I) pairs and the dissociation of oxygen.
Given the emerging picture that copper ions at low temperatures are solvated by NH3 during typical SCR-conditions, it becomes important to explore whether can act as sites for O2 activation. It should be noted that O2 dissociation generally is assumed to be a key step in the SCR reaction [8], [9], [12], [22], [23]. In a recent experimental and computational study, Gao et al. proposed that NO may assist the dissociation of O2 on transient [Cu(NH3)2]+O2[Cu(NH3)2]+ species [12].
Herein, we use DFT calculations to establish as the thermodynamically stable copper phase during low temperature conditions. Thereafter, we study O2 dissociation together with subsequent nitrite and nitrate formation over . It is demonstrated that NO promotes O2 dissociation and that direct dissociation proceeds preferably over pairs of complexes.
Section snippets
Electronic structural calculations and systems
Spin-polarised Density Functional Theory (DFT) calculations are performed with the Vienna Ab-Initio Simulation Package (VASP) [24], [25], [26], [27], [28]. The Kohn-Sham orbitals are expanded with plane waves using an energy cut-off of 480 eV and the interaction between the valence electrons and the core is described with the plane augmented wave (PAW) method [29], [30]. The number of electrons treated in the valence are Cu(11), Si(4), Al(3), O(6), N(5) and H(1). The BEEF-vdW functional [31] is
Cu(NH3)x+ phase diagram
Before presenting the activation of oxygen on complexes, it is important to establish the thermodynamic stability of the complexes with respect to NH3 partial pressure and temperature. complexes are formed without barriers upon ammonia adsorption on a framework-coordinated Cu(I) ion [20]. We find that the first two NH3 are strongly bonded to Cu+ in the zeolite with sequential binding energies of −1.58 eV and −1.50 eV. The third and fourth NH3-ligands are, instead, weakly
Discussion
We find that the entropy differences are small for molecular adsorption on the Cu-complexes. This is in agreement with previous studies for other zeolite systems [43], [44], [51]. The small entropy differences motivate the use of only the energy contribution when representing the SCR-reaction profile. Moreover, the result suggests that absorption of reactants from the gas phase to the fluid phase in zeolites should be treated as a separate step in kinetic modeling.
Our calculations show that the
Conclusions
By use of density functional theory calculations we have investigated the reactivity of complexes for O2 dissociation and subsequent nitrite and nitrate formation within the context of selective catalytic reduction of NOx with NH3 in CHA. Several conclusions can be made on the basis of the calculations. (i) Cu+ ions are preferably solvated by ammonia forming linear [NH3CuNH3]+ complexes under low-temperature operation conditions. (ii) Direct O2 dissociation is feasible on a pair of
Acknowledgement
The Competence Centre for Catalysis (KCK) is hosted by Chalmers University of Technology and is financially supported by the Swedish Energy Agency and the member companies AB Volvo, ECAPS AB, Haldor Topsøe A/S, Scania CV AB, Volvo Car Corporation AB, and Wärtsilä Finland Oy. Additional financial support from the Swedish Research Council and the Chalmers Areas of Advance Nano and Transport is acknowledged. The calculations have been performed at C3SE (Göteborg) and PDC (Stockholm) through a SNIC
References (56)
- et al.
Excellent activity and selectivity of Cu-SSZ-13 in the selective catalytic reduction of NOx with NH3
J. Catal.
(2010) - et al.
Thermal durability of Cu-CHA NH3-SCR catalysts for diesel NOx reduction
Catal. Today
(2012) - et al.
Structure-activity relationships in NH3-SCR over Cu-SSZ-13 as probed by reaction kinetics and EPR studies
J. Catal.
(2013) - et al.
Selective catalytic reduction of nitric oxide by ammonia over Cu-FAU catalysts in oxygen-rich atmosphere
J. Catal.
(1999) - et al.
Understanding ammonia selective catalytic reduction kinetics over Cu/SSZ-13 from motion of the Cu ions
J. Catal.
(2014) - et al.
Selective catalytic reduction of NOx with NH3 over zeolite H-ZSM-5: influence of transient ammonia supply
J. Catal.
(2003) - et al.
Experimental and kinetic modeling study of NO oxidation: comparison of Fe and Cu-zeolite catalysts
Catal. Today
(2012) - et al.
Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
J. Comput. Mater. Sci.
(1996) - et al.
Guest-host interactions of arenes in H-ZSM-5 and their impact on methanol-to-hydrocarbons deactivation processes
J. Catal.
(2013) - et al.
Reversible work transition state theory: application to dissociative adsorption of hydrogen
Surf. Sci.
(1995)
Synthesis of high silica CHA zeolites with controlled Si/Al ratio
Stud. Surf. Sci. Catal.
Monomolecular cracking of propane over acidic chabazite: an ab initio molecular dynamics and transition path sampling study
J. Catal.
Coordination chemistry and reactivity of copper in zeolites
A. Coord. Chem. Rev.
The role of spatial constraints and entropy in the adsorption and transformation of hydrocarbons catalyzed by zeolites
J. Catal.
Controlling NOx emission from industrial sources
Environ. Prog.
Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials
Chem. Soc. Rev.
Current understanding of Cu-exchanged chabazite molecular sieves for use as commercial diesel engine DeNOx catalysts
Top. Catal.
A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia
ACS Catal.
Catalysis in a cage: condition-dependent speciation and dynamics of exchanged Cu cations in SSZ-13 zeolites
J. Am. Chem. Soc.
Operando spatially-and time-resolved XAS study on zeolite catalysts for selective catalytic reduction of NOx by NH3
J. Phys. Chem. C
What makes copper-exchanged SSZ-13 zeolite efficient at cleaning car exhaust gases?
J. Phys. Chem. Lett.
Selective catalytic reduction over Cu/SSZ-13: linking homo-and heterogeneous catalysis
J. Am. Chem. Soc.
Isolation of the copper redox steps in the standard selective catalytic reduction on Cu-SSZ-13
Angew. Chem. Int. Ed.
Nitrate-nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH3-SCR
Catal. Sci. Tech.
Coordination environment of copper sites in Cu-CHA zeolite investigated by electron paramagnetic resonance
J. Phys. Chem. C
Activation of oxygen and NO in NH3-SCR over Cu-CHA catalysts evaluated by density functional theory
Catal. Today
Copper active sites in biology
Chem. Rev
Interaction of NH3 with Cu-SSZ-13 catalyst: a complementary FTIR, XANES, and XES study
J. Phys. Chem. Lett.
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