Gas-phase reactivities of charged platinum dimers with ammonia: A combined experimental/theoretical study

doi:10.1016/j.cplett.2007.11.059

Chemical Physics Letters

Volume 450, Issues 4–6, 4 January 2008, Pages 268-273

https://doi.org/10.1016/j.cplett.2007.11.059 Get rights and content

Abstract

Computational and experimental studies of ${Pt}_{2}^{\pm}$ with ammonia are reported. While ammonia is dehydrogenated with either cluster type, the reaction efficiencies ϕ are quite different with ϕ = 0.27 for ${Pt}_{2}^{+} / {NH}_{3}$ [K. Koszinowski, D. Schröder, H. Schwarz, J. Phys. Chem. A 107 (2003) 4999.] and ϕ = 0.0033 for ${Pt}_{2}^{-} / {NH}_{3}$ . DFT-based relativistic calculations are consistent with this distinct behavior, with maximum energies along the reaction path of −13.3 kcal/mol for the cationic and +1.4 kcal/mol for the anionic clusters relative to the reactants. The recently proposed mechanism for the ${Pt}_{2}^{+} / {NH}_{3}$ system [D. Xu. X.-Y. Chen, S.-G. Wang, Int, J. Quant. Chem. 107 (2007) 1985.] needs to be modified to account for the experimental findings.

Graphical abstract

The reactivity of the charged platinum dimers towards ammonia is investigated by the FT-ICR mass spectroscopy and DFT calculations.

Introduction

Mass-spectrometry based experiments provide valuable insight into the nature of fundamental chemical processes, e.g. elementary steps in bond activation of catalytic processes mediated by cluster-metal ions (for a recent review, see [1]). Among them, studies of charged platinum clusters have received quite some attention as model systems for numerous catalytic reactions such as the large-scale synthesis of HCN from methane and ammonia (DEGUSSA process) [2], [3], the low-temperature oxidation of CO by N₂O [4], or the dehydrogenation of alkanes [5], [6], [7], just to mention a few. Detailed computational/experimental studies suggest that relativistic effects play a pivotal role in these – and other – reactions (for a review, see [8]). The dependence of the gas-phase reactivity on the platinum cluster size was also investigated both experimentally [5], [6], [7], [9], [10] and computationally [11], [12], [13], [14], [15], [16].

In this Letter, we will address the thermal dehydrogenation of ammonia by charged platinum dimers, Eq. (1). ${Pt}_{2}^{\pm} + {NH}_{3} \to [{Pt}_{2} NH]^{\pm} + H_{2}$

While the chemistry of the cationic ${Pt}_{x}^{+}$ clusters (x = 1–5) with ammonia has already been studied experimentally [10], to the best of our knowledge the ${Pt}_{2}^{-} / {NH}_{3}$ system has not yet been examined. Among the cationic ${Pt}_{x}^{+}$ clusters examined, the dimer x = 2 exhibits an exceptionally high reactivity in comparison to the other cluster sizes with a rate constant k = 5.4 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, corresponding to an efficiency of ϕ = 0.27. Mechanistic aspects of this process were recently addressed theoretically by means of relativistic density functional calculations [17]. While this study reveals many interesting facets, e.g. the operation of a two-state reactivity scenario (for reviews, see [18], [19]), the highest barrier along the multi-step reaction pathway of dehydrogenation was located only −0.2 kcal/mol below the entrance channel of the separated reactants ${Pt}_{2}^{+}$ and NH₃; this cannot explain the high reactivity of the system.

Here, this problem will be resolved by a new calculation of the ${Pt}_{2}^{+} / {NH}_{3}$ potential-energy surface, and an experimental/computational study of the corresponding ${Pt}_{2}^{-} / {NH}_{3}$ couple will be reported for comparison.

Section snippets

Experimental and computational methods

The experiments were performed on a modified Bruker/Spectrospin CMS47X mass spectrometer, equipped with an Apex III data station and an external, home-built laser vaporization ion source [20], [21], [22], [23]. Platinum clusters were produced by laser vaporization/ionization of a solid platinum target with the 5 ns pulse of a frequency-doubled Nd:YAG laser (Continuum Surelite II, 10 Hz, 5 mJ pulse energy), followed by supersonic expansion of the hot plasma entrained in a 50 μs helium pulse. The ${Pt}_{x}^{-}$

Reaction of ${Pt}_{2}^{-}$ with ammonia

Fig. 1 depicts the time dependence for the reactant and product ion intensities in the thermal reaction of the ${Pt}_{2}^{-} / {NH}_{3}$ system, along with a kinetic fit. The only chemical process corresponds to N–H bond activation resulting in the dehydrogenation of NH₃ according to Eq. (1).

The rate constant for this reaction amounts to k = 2.3 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, which translates to an efficiency of ϕ = 0.0033. Compared with earlier experiments of the cationic ${Pt}_{2}^{+}$ cluster, with ϕ = 0.27 [10], the considerably

Conclusions

The thermal dehydrogenation of NH₃ by Pt₂ clusters depends crucially on the charge state. While ${Pt}_{2}^{+}$ is rather reactive (ϕ = 0.27), the efficiency for the anionic system is much smaller (ϕ = 0.0033). DFT calculations reveal not only interesting mechanistic details but can also explain the origin of the different efficiencies. For the anionic cluster the most favorable pathway proceeds via a transition structure that is nearly isoenergetic with the entrance channel, thus accounting for the small

Acknowledgements

This project was financially supported by generous grants from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. M.K.B. gratefully acknowledges a Heisenberg Fellowship from the Deutsche Forschungsgemeinschaft. Insightful discussions with Maria Schlangen and Christoph van Wüllen are appreciated.

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Gas-phase reactivities of charged platinum dimers with ammonia: A combined experimental/theoretical study

Abstract

Graphical abstract

Introduction

Section snippets

Experimental and computational methods

Reaction of Pt2- with ammonia

Conclusions

Acknowledgements

Int. J. Mass Spectrom.

J. Mol. Struct. Theochem.

J. Mol. Struct. Theochem.

Chem. Phys. Lett.

Int. J. Mass Spectrom.

Int. J. Mass Spectrom. Ion Process.

Vacuum

Coord. Chem. Rev.

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Chem. Commun.

Chem. Eur. J.

Angew. Chem. Int. Ed.

Phys. Chem. Chem. Phys.

J. Phys. Chem. A

J. Chem. Phys.

J. Phys. Chem. A

J. Phys. Chem. B

Int J. Quant. Chem.

Acc. Chem. Res.

J. Chem. Phys.

Reaction of ${Pt}_{2}^{-}$ with ammonia