Elucidating Electric Field-Induced Rate Promotion of Brønsted Acid-Catalyzed Alcohol Dehydration

Applied potentials have been demonstrated as a powerful
tool to
promote heterogeneous Brønsted acid catalysis by orders of magnitude,
leveraging interfacial electric fields to stabilize protonated intermediates.
However, the use of flat two-dimensional electrodes with inherently
low active site densities limits the application of conventional thermochemical
characterization techniques that can probe the nature of catalytic
active sites. Here, we use kinetic analyses with an electrostatics-based
model to elucidate the intricacies of potential-induced rate promotion,
employing liquid-phase dehydration of 1-methylcyclopentanol catalyzed
by carboxylic acid groups on carbon nanotubes as a probe system. Using
a basket electrode to directly polarize catalyst powder, we demonstrate
that thermocatalytic reaction rates can be promoted by 100,000-fold,
exhibiting a log–linear dependence on applied potential with
rate-potential scalings as high as 125 ± 4 mV per 10-fold rate
increase. In agreement with model predictions, we show that lower
ionic strengths attenuate potential sensitivity, resulting from a
weakening of the interfacial electric field that interacts with the
acidic proton. Furthermore, we experimentally confirm the model-predicted
“isokinetic potential” (at ∼0.6 V vs Ag/AgCl)the
potential at which all rate scaling lines at various ionic strengths
intersect, making the rate independent of ionic strength. Base titrations
reveal that only ∼8% of the carboxylic acid sites are catalytically
active, yet these same active sites are operational at the highest
and lowest potentials. Collectively, our results provide a key methodology
for modeling catalytic effects of electric fields, quantifying active
sites under applied potential, and demonstrating fundamental principles
of electric field-induced rate promotion.