Uncertainty in Composite Membranes: From Defect Engineering to Film Processing.

Composite membranes featuring metal-organic framework (MOF)-dispersed polymers have attracted tremendous attention in recent years. However, evaluating commercial viability is oftentimes obscured by the irreproducibility in both MOF synthesis and film manufacturing protocols. Variability in MOF property sets are typically ascribed to crystal defects resulting from subtle variations in synthesis, but quantitative studies investigating the role of defects on transport properties are exceedingly rare. Likewise, controlled film formation protocols are rarely reported in the open literature, making it difficult to provide substantial and informative structure-property correlations. This study aims to address these uncertainties. To this end, two samples of a prototypical MOF, UiO-66-NH2, were synthesized to feature similar particle size, morphology, and colloidal stability. However, defect engineering protocols coupled with careful screening experiments were developed to synthesize the two MOFs with maximally different porosities. Composite membranes were prepared for each MOF and a high-performance polymer, 6FDA-Durene, and then tested for light gas permeation measurements, revealing a small and unexpected enhancement in CO2/CH4 performance for samples containing low-porosity UiO-66-NH2. Mechanistic studies on sorption revealed a surprising 50% decrease in sorption capacity for high-porosity UiO-66-NH2, completely offsetting enhancements from increased gas diffusion. By using multiple replicate experiments, the sample-to-sample variation was large enough to obscure any differences in permeability and selectivity between the two types of MOF composites at low volume fractions. Application of the Maxwell model to extrapolate pure-MOF performance led to significant variations in predicted values, demonstrating the importance of collecting and reporting replicate experiments for membrane preparation and testing.

Universal Kinetic Mechanism Describing CO2 Photoreductive Yield and Selectivity for Semiconducting Nanoparticle Photocatalysts.

Photocatalytic conversion of CO2 to generate high-value and renewable chemical fuels and feedstock presents a sustainable and renewable alternative to fossil fuels and petrochemicals. Currently, there is a dearth of kinetic understanding to inform better catalyst design, especially at uniform reaction conditions across diverse catalytic species. In this work, we investigate 12 active, stable, and unique but common nanoparticle photocatalysts for CO2 reduction at room temperature and low partial pressure in aqueous phase: TiO2, SnO2, and SiC deposited with silver, gold, and platinum. Our analysis reveals a single consistent chemical kinetic mechanism, which accurately describes the yield and selectivity of all single-carbon containing (C1) products obtained in spite of the diverse catalysts employed. Formaldehyde is predicted as the first product in the reaction network and we report, to the best of our knowledge, the highest selectivity to date toward formaldehyde during CO2 photoreduction when compared against all other C1 products (∼80%) albeit at low CO2 conversion (<0.5 µmol gcat-1 h-1, <16.8 nmol m-2 h-1). Further, we observe a volcano-like relationship between the electron-transfer rate of a given photocatalyst for CO2 reduction and the net rate at which reduced products are produced in the reaction mixture taking into account unfavorable product oxidation. We establish an empirical upper limit for the maximum rate of production of CO2 reduction products for any nanoparticle photocatalyst in the absence of a hole-scavenging agent. These results form the basis for the design and optimization of the next generation of highly efficiency and active photocatalysts for CO2 reduction.