The spin significance in the capture and activation of N2O by small Rh nanoparticles

doi:10.1016/j.molcata.2013.03.025

Journal of Molecular Catalysis A: Chemical

Volume 376, September 2013, Pages 22-33

https://doi.org/10.1016/j.molcata.2013.03.025 Get rights and content

Highlights

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Rh_n + N₂O reaction study must consider the geometry of the reactants and spin.
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N₂O reduction mechanism catalysed by Rh_n clusters ends up breaking the N₂O bond.
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The N₂O activation never occurs through the NN bond rupture.
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Most of Rh clusters capture N₂O and there are some spin states that also activate it.
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The spin states transfer enough charge from Rh to N₂O to break the N₂O bond.

Abstract

This work presents many possible theoretical reaction pathways of N₂O reduction to N₂ and O on Rh_n nanoparticles (n = 1–4) using density functional theory (DFT) method and the zero order regular approximation (ZORA), which explicitly considers the scalar relativistic corrections. The Rh spin multiplicity is an essential condition to dissociate N₂O, because can promote or inhibit the electron back donation from the metal. Rh activates N₂O by exothermic and spontaneous reactions. For each case presented, the optimized geometry adsorption site, reaction energy, spin multiplicity and Voronoi charges are calculated. On a single Rh atom in the ground and low-lying excited states, the N₂O is captured only. On the Rh₂ quintet ground state, N₂O is also chemisorbed and dissociation occurs for the next two excited states (triplet and septet). In the Rh₃ case, there are N₂O adsorption and dissociation for Rh₃ quartet ground state and for most excited states. Several dissociation cases take place when N₂O is parallel to the plane and parallel to a Rh₃ bond. On the Rh₄, however, there are two optimal geometries: tetrahedral and square; for both cases there are N₂O adsorption and dissociation.

Graphical abstract

Introduction

One of the real problems of the last decade is how to reduce atmospheric pollution caused by nitrogen oxides (NO_x), specially the nitrous oxide (N₂O). This pollutant is a major cause of ozone formation at the biosphere, and simultaneously, is the dominant substance that is depleting the ozone layer in the stratosphere [1]. At the present, the N₂O amount in the atmosphere is measured and has been considered by the Montreal Protocol, but in earlier times it was considered harmless to atmosphere. For this reason, only recently has it become considered harmful and the study interest has grown. Industry has had to fabricate catalytic devices to reduce NO_x emissions of stationary sources and automotive vehicles. Pt, Pd and Rh metals are commonly used in three-way catalyst (TWC) systems; Rh metal is recognized as the most appropriate metal to reduce NO_x to N₂, since Rh readily provides electrons for nitrogen oxide reduction. In fact, Rh has the lowest ionization energy of the three metals in the TWC (7.46 [2], 8.34 [2] and 8.96 eV [3] ionization energies for Rh, Pd and Pt atoms, respectively). N₂O activation is complex, at certain temperatures N₂O is an intermediate and its contents increases at temperatures below ~250 °C, i.e., up to 80% of NO is reduced to N₂O when engine is warming up [4].

The need to reduce pollutant emissions has increased the interest in best experimental and theoretical knowledge of NO_x reductions, especially the N₂O intermediate. Many experimental and theoretical studies have been reported [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] about the mechanisms of NO_x activation reaction considering the best reduction conditions and catalytic systems.

Theoretical studies on these processes are suitable for understanding the NO_x reduction produced by industries and motor vehicles, because these processes are quite complex and difficult to reproduce in laboratories using normal chemical devices. Due to current state development of the quantum chemistry methods it is also complicated to carry out some theoretical studies, but it is still possible to perform partial ab initio studies of the processes, although the transition metal atoms number is restricted. The density functional theory (DFT) represents a practical method to study systems with transition metal atoms. A further difficulty in theoretical studies is related to Rh nanoparticles, since they have many degenerate states with several structural isomers [17], [18], [19], [20], [21], [22]. Ghosh et al. [20] found that NO binds more strongly to Rh clusters than to Rh surfaces. Bae et al. [21] showed that open structures are more stable than icosahedral structures, which are normally employed. Moreover, many of these isomers are very close in energy and likely to be present in experiments, unless this are carried out at very low temperatures. In other metals, such as Au, the ground state is unable to bind and decompose N₂O, whereas excited states capture and activate it [22].

This article is organized as follows: In Section 2, the computational method is described summarily; density functional theory has been used in all calculations taking into account relativistic effects included explicitly in the ZORA method, which are absolutely necessary to study transition metal clusters. In Section 3, results and discussion are shown. First, Rh clusters and their isomers with the lowest energy are shown. Subsequently, the N₂O with Rh_n interactions yielding to Rh_nN₂O adduct are discussed. This is followed by the analysis of N₂O adsorption and decomposition catalysed by Rh_n particles with n = 1–4 on gas phase, considering the most stable Rh nanoclusters and some Rh_n low-lying excited states, considering various N₂O approaches to Rh_n clusters with different spin multiplicities. In the cases where Rh_n catalysed the N₂O dissociation, the reduction mechanism and the role played by charge transfer processes were analysed through Voronoi charges. Finally, the conclusions are presented in Section 4.

Section snippets

Computational method

All calculations were performed using DFT method [23], [24] within the generalized gradient approximation (GGA) by means of the exchange-correlation functional free of empirical parameters proposed by Perdew–Burke–Ernzerhof (PBE) [25] using the ADF 2009.01 package [26]. A triple zeta basis set with two polarization functions (TZ2P) was used with the frozen core approximation for Rh atom, in which only 18 valence electrons are considered explicitly in the self-consistent-field cycles. For oxygen

Rh clusters

To explain the N₂O activation catalysed by Rh small clusters, the first step is to define the catalyst geometrical factors and spin multiplicity. For small clusters, there are few experimental data due to the difficulty to determine the geometrical properties and energy states directly from small metal clusters [30]. Moreover, Rh has more degenerate states than any other transition metal; hence there is a significant dispersion in the available theoretical and experimental results.

Table 1

Conclusions

The quantum chemical theoretical PBE/ZORA method is adequate for studying Rh_n + N₂O reaction, because it describes quite well the reaction electronic states order considering different geometrical approaches and spin multiplicity.

The Rh_n clusters calculations performed are in good agreement with calculations reported by other authors, most included in this article, particularly in spin multiplicity, optimal geometry, binding energies, charge distribution and Rh single bond Rh distances.

According to our

Acknowledgements

The authors are indebted to the UAM-Iztapalapa Computer Centre by the computer time granted to accomplish this work in the Aitzaloa Supercomputer and with UAM-Azcapotzalco FAMA area by the computing time in the Molphys Supercomputer. R.A. is grateful to Instituto de Educación Media Superior of México City by the leave with pay grant. O.O.-N. thanks the PROMEP – México financial support project No. 22310793.

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The spin significance in the capture and activation of N2O by small Rh nanoparticles

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Computational method

Rh clusters

Conclusions

Acknowledgements

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The spin significance in the capture and activation of N₂O by small Rh nanoparticles