(197d) Comparisons of Molecular and Extended Tungsten Oxides for Nitrobenzene Electroreduction

Homogeneous molecular catalysts are advantageous candidates for fundamental studies of their reactivity because their composition is highly uniform and tunable. However, molecules are not often used in large-scale industrial processes due in large part to the difficulty in separating them from the reaction mixture. To preserve the key advantages of molecular control, researchers have used the approach of dispersing these catalysts on a heterogenous support^[1]. This raises the question of how much the support affects the catalytic activity of these materials and how their inherent reactivity varies from compositionally similar extended solids.

To this end, we have devised a series of experiments intended to compare the redox reactivity of carbon-supported polyoxotungstate clusters with that of extended tungsten oxide nanoparticles toward industrially relevant hydrogenation reactions. Specifically, Symes et al. recently demonstrated highly selective aniline (PhNH₂) production from nitrobenzene (PhNO₂) with faradaic efficiency greater than 90% using phosphotungstic acid (H₃PW₁₂O₄₀) in the solution-phase^[2]. Thus, we are extending these measurements to carbon-supported [TBA]₃PW₁₂O₄₀ before and after thermal oxidation using methods we had developed previously to study oxygen electroreduction^[3].

The experimental design comprises the use of rotating disk electrode (RDE) voltammetry for electro-analytical measurements alongside preparative hydrogenation reactions using a continuous flow reactor setup with product characterization via H-NMR. Our observations for both solution-phase chemistry and carbon-supported clusters were compared to careful controls in absence of polyoxometalates. We observed an increased rate of PhNO₂ to PhNH₂ when H₃PW₁₂O₄₀ was added to our electrolyte solution (0.5 M sulfuric acid) in the flow reactor. In an analogous study on the carbon-supported cluster using RDE voltammetry also showed similar behavior. Ongoing efforts are focused on quantifying the selectivity and conversion for the solution-phase and carbon-supported clusters along with equivalent studies of reaction rates for carbon-supported tungsten oxides synthesized from the clusters via thermal treatments.

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