Abstract

We investigate the gas phase proton transfer reactivity of the dianionic molybdenum oxysulfide clusters [Mo2O2S6]2– and [Mo2O2S5]2– in binary collisions in an FT-ICR mass spectrometer in combination with quantum chemical calculations. The proton transfer reactions are probed with a series of acids with increasing proton affinity, namely trifluoroacetic acid, difluoroacetic acid, pyruvic acid and formic acid. Proton transfer followed by Coulomb explosion is observed for both dianions with all acids except for formic acid. The calculations predict a proton affinity of 16.1(1) and 16.5(1) eV for [Mo2O2S6]2– and [Mo2O2S5]2–, respectively, significantly higher than the proton affinities of tri- and difluoroacetate as well as pyruvate and formate, which range from 14.00(18)–14.98(10) eV. Calculated reaction potential energy surfaces explain the observations. A Coulomb barrier separating the collision complex from the products is present in all cases. The barrier lies well below the entrance channel for trifluoroacetic acid, difluoroacetic acid and pyruvic acid. For formic acid, however, the barrier lies near the entrance channel.

Highlights

  • We live in an energy transition era, where the hydrogen evolution reaction (HER) is about to become a key process in a sustainable hydrogen economy [1e4]

  • A necessary condition for a proton transfer reaction is that the proton affinity of the proton acceptor, in this case a dianion B2À, must be higher than that of the proton donor HA

  • We used a set of four organic acids to cover gas phase proton affinities in the range of 14.00(18)e14.98(10) eV to bracket the height of the Coulomb barrier for proton transfer to [Mo2O2S6]2e and [Mo2O2S5]2e

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Summary

Introduction

We live in an energy transition era, where the hydrogen evolution reaction (HER) is about to become a key process in a sustainable hydrogen economy [1e4]. The best electrolyzer efficiency is achieved under acidic conditions, where HER is typically catalyzed by Pt [5,6]. The low abundance and high price of Pt gave rise to a search for cheap, earth-abundant materials, which could efficiently catalyze HER [7]. In 2007, Jaramillo et al showed that the edge, rather than the plane of MoS2, is catalytically active [9]. This led to increased research on molybdenum sulfide based nanomaterials, which expose a high number of edge-like sites [10]. The [Mo3S13]2e and [Mo2S12]2e nanoclusters represent molecular analogues of these edge sites and have been shown to exhibit excellent activity and stability under acidic electrocatalysis [11e13].

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