Abstract
Large oriented electric fields spontaneously arise at all solid–liquid interfaces via the exchange of ions and/or electrons with the solution. Although intrinsic electric fields are known to play an important role in molecular and biological catalysis, the role of spontaneous polarization in heterogeneous thermocatalysis remains unclear because the catalysts employed are typically disconnected from an external circuit, which makes it difficult to monitor or control the degree of electrical polarization of the surface. Here, we address this knowledge gap by developing general methods for wirelessly monitoring and controlling spontaneous electrical polarization at conductive catalysts dispersed in liquid media. By combining electrochemical and spectroscopic measurements, we demonstrate that proton and electron transfer from solution controllably, spontaneously, and wirelessly polarize Pt surfaces during thermochemical catalysis. We employ liquid-phase ethylene hydrogenation on a Pt/C catalyst as a thermochemical probe reaction and observe that the rate of this nonpolar hydrogenation reaction is significantly influenced by spontaneous electric fields generated by both interfacial proton transfer in water and interfacial electron transfer from organometallic redox buffers in a polar aprotic ortho-difluorobenzene solvent. Across these vastly disparate reaction media, we observe quantitatively similar scaling of ethylene hydrogenation rates with the Pt open-circuit electrochemical potential (EOCP). These results isolate the role of interfacial electrostatic effects from medium-specific chemical interactions and establish that spontaneous interfacial electric fields play a critical role in liquid-phase heterogeneous catalysis. Consequently, EOCP—a generally overlooked parameter in heterogeneous catalysis—warrants consideration in mechanistic studies of thermochemical reactions at solid–liquid interfaces, alongside chemical factors such as temperature, reactant activities, and catalyst structure. Indeed, this work establishes the experimental and conceptual foundation for harnessing electric fields to both elucidate surface chemistry and manipulate preparative thermochemical catalysis.
Highlights
Electric fields are known to play an important role in many catalytic transformations throughout chemistry[1−20] and biology.[21,22] For example, in molecular catalysis, charged moieties in the secondary sphere of a coordination compound can impart oriented internal electric fields,[11,13−15] which have been used to accelerate CO2 activation (Figure 1, left).[11]
Investigating the influence of spontaneous electrical polarization on thermochemical catalysis requires, first, a method for tracking order to arrive at the following expression for the equilibrium the magnitude of interfacial polarization at distributed metal−
Despite the disparate mechanisms of polarization and the disparate reaction media, we find that both modes of polarization influence the catalytic rate of an explicitly nonpolar probe reaction liquid-phase ethylene hydrogenation on Pt/C with similar scale factors
Summary
Electric fields are known to play an important role in many catalytic transformations throughout chemistry[1−20] and biology.[21,22] For example, in molecular catalysis, charged moieties in the secondary sphere of a coordination compound can impart oriented internal electric fields,[11,13−15] which have been used to accelerate CO2 activation (Figure 1, left).[11]. Electrical polarization arises spontaneously at all solid−liquid interfaces, even those entirely disconnected from any wires or electrical circuits This phenomenon exists because differences in the chemical composition of the two phases can drive the spontaneous transfer of charged species (i.e., ions and/or electrons) across the interface. Upon contacting the catalyst surface with solution, any difference between the chemical potentials of the charged species at the surface and in solution will drive spontaneous, incremental transfer of that charged species until an electrostatic potential difference, Δφsurface−solution = φsurface − φsolution, is generated that counteracts the chemical potential difference;[31] in this manner, electrochemical equilibrium is established Such equilibration processes result in spontaneous electrical polarization of the solid−liquid interface, with attendant interfacial electric fields spanning molecular length-scales. Http://pubs.acs.org/journal/acscii composition of the Pt, and relates to the Pt work function
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