Nonidealities in CO2 Electroreduction Mechanisms Revealed by Automation-Assisted Kinetic Analysis

In electrocatalysis, mechanistic analysis of reaction rate data often relies on the linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to 10 electrochemical cells, where each cell can have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We used this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms that are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.

Catalytic Activation of Polyethylene Model Compounds Over Metal-Exchanged Beta Zeolites

Decomposition of polymers by heterogeneous catalysts presents a promising approach for reuse of waste plastics. We demonstrated non-hydrogenative decomposition of model polyolefins over proton-form and metal (Cu, Ni) ion-exchanged beta (BEA) zeolites at moderate temperatures (around 300 °C). Near complete polyolefin decomposition was observed in batch reactions monitored by thermogravimetric analysis, while decomposition at partial conversion was studied in flow reactions. Ni-exchanged zeolites produced H₂ at substantially higher rates (>10x) than other catalysts while also uniquely resisting deactivation over time. Application of the delplot formalism offered insights into the reaction network for polyolefin decomposition over Ni/BEA most notably that H₂ is solely a primary product. We deduce that H₂ production is catalyzed by activation of C-H bonds at ionic Ni sites, and H₂ prevents buildup of polyaromatic coke species in Ni-exchanged zeolites that deactivate Cu-exchanged and protonic zeolites.

Oxidative dehydrogenation of ethane: catalytic and mechanistic aspects and future trends

Ethane oxidative dehydrogenation (ODH) is an attractive, low energy, alternative route to reduce the carbon footprint for ethene production, however, the commercial implementation of ODH processes requires catalysts with improved selectivity.

Multi-element (C, H, Cl, Br) stable isotope fractionation as a tool to investigate transformation processes for halogenated hydrocarbons

Compound-specific isotope analysis (CSIA) is a powerful tool to evaluate transformation processes of halogenated compounds. Many halogenated hydrocarbons allow for multiple stable isotopic systems (C, H, Cl, Br) to be measured for a single compound. This has led to a large body of literature describing abiotic and biotic transformation pathways and reaction mechanisms for contaminants such as chlorinated alkenes and alkanes as well as brominated hydrocarbons. Here, the current literature is reviewed and a new compilation of Λ values for multi-isotopic systems for halogenated hydrocarbons is presented. Case studies of each compound class are discussed and thereby the current strengths of multi-element isotope analysis, continuing challenges, and gaps in our current knowledge are identified for practitioners of multi-element CSIA to address in the near future.

Ethanol-to-butadiene: the reaction and its catalysts

Catalytic conversion of ethanol is a promising technology for producing sustainable butadiene. This paper reviews the reaction and its catalysts, and discusses the challenges their development faces.

Kinetics and reactor modeling of the conversion of n-pentane using HZSM-5 catalysts with different Si/Al ratios

The production of olefins and aromatics from n-pentane using two ZSM-5 catalysts is faithfully predicted by a versatile kinetic model.