Elsevier

Journal of Catalysis

Volume 358, February 2018, Pages 179-186
Journal of Catalysis

Activation of oxygen on (NH3single bondCusingle bondNH3)+ in NH3-SCR over Cu-CHA

https://doi.org/10.1016/j.jcat.2017.12.009Get rights and content

Highlights

  • A phase diagram for NH3 solvated Cu-ions is presented.

  • Direct O2 dissociation is possible on pairs of Cu(NH3)2+ but not on single Cu(NH3)2+.

  • NO promotes O2 dissociation.

  • Nitrates may be a part of the NH3-SCR reaction.

  • Entropy changes during reactions in CHA are small.

Abstract

Cu-CHA materials are efficient catalysts for NH3–SCR of NOx in oxygen excess. A crucial step in the reaction is oxygen (O2) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O2 dissociation on Cu(NH3)2+ species, which are present under NH3–SCR conditions. Direct dissociation of O2 is found to be facile over a pair of Cu(NH3)2+ complexes whereas dissociation on a single Cu(NH3)2+ species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH3)2+ complexes. Nitrites and nitrates are easily formed as O2 dissociates, and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3–SCR.

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:4NH3+4NO+O24N2+6H2OZeolites 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, Cu(NH3)2+ [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 Cu(NH3)2+ 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 Cu(NH3)2+ 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]+single bondO2single bond[Cu(NH3)2]+ species [12].

Herein, we use DFT calculations to establish Cu(NH3)2+ as the thermodynamically stable copper phase during low temperature conditions. Thereafter, we study O2 dissociation together with subsequent nitrite and nitrate formation over Cu(NH3)2+. It is demonstrated that NO promotes O2 dissociation and that direct dissociation proceeds preferably over pairs of Cu(NH3)2+ 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 Cu(NH3)2+ complexes, it is important to establish the thermodynamic stability of the complexes with respect to NH3 partial pressure and temperature. Cu(NH3)2+ 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 Cu(NH3)2+ 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 [NH3single bondCusingle bondNH3]+ 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

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