Metals that are active catalysts for methane (Ni, Pt, Pd), when dissolved in inactive low-melting temperature metals (In, Ga, Sn, Pb), produce stable molten metal alloy catalysts for pyrolysis of methane into hydrogen and carbon. All solid catalysts previously used for this reaction have been deactivated by carbon deposition. In the molten alloy system, the insoluble carbon floats to the surface where it can be skimmed off. A 27% Ni-73% Bi alloy achieved 95% methane conversion at 1065°C in a 1.1-meter bubble column and produced pure hydrogen without CO or other by-products. Calculations show that the active metals in the molten alloys are atomically dispersed and negatively charged. There is a correlation between the amount of charge on the atoms and their catalytic activity.

We report the non-Faradaic electrochemical promotion of a Brønsted acid-catalyzed reaction over a metal oxide surface. Isopropanol dehydration to propylene was used as a probe reaction to study the in situ modification of a molybdenum catalyst film deposited on a yttria-stabilized zirconia solid electrolyte. Upon polarizing the Mo film by +1.5 V, the rate of isopropanol dehydration (1.2 kPa IPA, 3.3 kPa O 2 , 673 K, 135 kPa total pressure) was enhanced by 2.5×. Smaller rate enhancements of c.a. 1.3× were also observed for 2-butanol dehydration to butenes over the same catalyst. Although electrochemical dehydration pathways for this chemistry are implausible, by postulating a hypothetical Faradaic dehydration route, we calculate Faradaic efficiencies greater than 100 for IPA dehydration, confirming the non-Faradaic nature of the promotional effect. This effect is reversible and does not appear to permanently alter the chemistry of the Mo film, based on XPS analysis. We hypothesize that this promotion originates from generation of Brønsted acid sites localized to the three-phase boundary at the catalyst/gas/electrolyte interface and/or acid site strengthening due to electrical polarization. This work demonstrates an alternative handle to promote catalytic turnover, which with further understanding, could be applied toward other Brønsted acid-catalyzed chemistries.

Solvent identity and pore polarity are known to influence Lewis acidic catalysis in zeolite pores for a variety of liquid-phase chemistries. We investigated how these parameters alter the rates of self-aldol addition of ethyl pyruvate (EP), a model biomass-derived compound, over hydrophobic and hydrophilic Hf-BEA zeolites in both toluene and acetonitrile solvents. Aldol addition rates are of first order across the entire EP activity range (0.02−0.4) for all four systems, consistent with the nucleophilic attack by the enolate as the rate-determining step and a single adsorbed EP as the most abundant reactive intermediate. Apparent first-order rate constants span 2 orders of magnitude across the four systems; at 363 K, the highest rates were observed over hydrophobic Hf-BEA-F in toluene (k app = 0.36 (mmol) (mmol closed Hf) −1 (s) −1 ), while the lowest rates were observed in hydrophilic Hf-BEA-OH in an acetonitrile solvent (k app = 0.0026 (mmol) (mmol closed Hf) −1 (s) −1 ). Apparent reaction enthalpies and entropies for each system, estimated using non-ideal transition-state theory, revealed that despite the substantial rate constant variation across the four systems, apparent enthalpies for Hf-BEA-F in both solvents and Hf-BEA-OH in acetonitrile were within the error of each other (∼70 kJ mol −1 ). Reactions performed using Hf-BEA-OH with toluene featured a higher apparent enthalpic barrier of 83.8 kJ mol −1 . The differences between the systems are attributed to hydrogen-bonding interactions between the EP molecules and polar silanol nests during catalysis in toluene using Hf-BEA-OH, which hinder EP adsorption to the active site in the hydrophilic framework. These hydrogen-bonding interactions are not present when acetonitrile is used as the solvent, as acetonitrile itself binds to and blocks silanol groups. Equilibrium EP absorption measurements indicate that while both toluene and acetonitrile are present in pores during catalysis, neither solvent forms a tight solvation shell around EP in the pores that must be disrupted prior to EP adsorption. These findings show that aldol addition kinetics are not significantly modified by solvent polarity in hydrophobic frameworks beyond site-blocking effects; however, silanol nests in hydrophilic frameworks significantly alter substrate adsorption to the active site.