Gas-phase reactivities of charged platinum dimers with ammonia: A combined experimental/theoretical study
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).
While the chemistry of the cationic clusters (x = 1–5) with ammonia has already been studied experimentally [10], to the best of our knowledge the system has not yet been examined. Among the cationic 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 and NH3; this cannot explain the high reactivity of the system.
Here, this problem will be resolved by a new calculation of the potential-energy surface, and an experimental/computational study of the corresponding 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
Reaction of with ammonia
Fig. 1 depicts the time dependence for the reactant and product ion intensities in the thermal reaction of the 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 cluster, with ϕ = 0.27 [10], the considerably
Conclusions
The thermal dehydrogenation of NH3 by Pt2 clusters depends crucially on the charge state. While 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.
References (46)
- et al.
Int. J. Mass Spectrom.
(2006) J. Mol. Struct. Theochem.
(1999)- et al.
J. Mol. Struct. Theochem.
(2004) - et al.
Chem. Phys. Lett.
(2006) Int. J. Mass Spectrom.
(2004)- et al.
Int. J. Mass Spectrom. Ion Process.
(1997) - et al.
Vacuum
(1983) - et al.
Coord. Chem. Rev.
(2003) - et al.
Angew. Chem. Int. Ed.
(2005) - et al.
Angew. Chem. Int. Ed.
(1998)
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.
Cited by (17)
Insight into the multicomponent reaction mechanisms of prop-2-en-1-amine and ethyl propiolate with alloxan derivative: A density functional theory study
2010, Chemical Physics LettersCitation Excerpt :However, as shown in Scheme 1, it is difficult to understand essentially how the reaction happens and what the role of water molecule plays in the reaction process unless we can obtain more details at the molecular level. In the present work, the compounds R1 (prop-2-en-1-amine), R2 (ethyl propiolate) and R3 (alloxan) were selected as the objects of investigation, and the reaction mechanisms for the title reaction in different pathways were studied using density functional theory, which has been widely used in the study of the mechanism [23–33]. All structures of the reactants, transition states, intermediates and products were fully optimized at the B3LYP/6-311++G(d,p) level of theory [34–36] using polarized continuum model (PCM) [37–39] to simulate the solvation effects (water chosen from the available experiment [22]).
Metal Clusters and Their Reactivity
2020, Metal Clusters and their ReactivityAmmonia Dehydrogenation on Cobalt Cluster Cations Doped with Niobium
2018, Topics in CatalysisReactivity of Metal Clusters
2016, Chemical ReviewsPlatinum group metal clusters: From gas-phase structures and reactivities towards model catalysts
2014, Chemistry - A European JournalReactions of neutral platinum clusters with N<inf>2</inf>O and CO
2013, Journal of Physical Chemistry A