Engineering topics

Bollini, Praveen

Publications and source records attributed to Bollini, Praveen.

4 records

DOE OSTI2024

Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. Here, we present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over μ m -oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central μ m -oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Recommendations for improving rigor and reproducibility in site specific characterization

Heterogeneous catalysis is driven by the interaction of reactant molecules and the catalyst surface. The locus of this interaction as well as the surrounding ensemble of atoms is referred to as the catalyst active site. Active site characterization attempts to distinguish active catalytic sites from inactive surface sites, to elucidate the structural and chemical nature of active sites, and to quantify active site concentration. Numerous techniques have been demonstrated to provide compositional and structural information about the active sites within a catalyst. However, each technique has its own limitations and experimental pitfalls that can lead to data misinterpretation or irreproducible results. Further, this work aims to provide an overview of the types of data that can be collected, to outline common experimental challenges and how to avoid them, and to assemble relevant references for the most used active site characterization techniques. More broadly, we aim to outline best practices for researchers to collect, interpret, and report active site characterization data in a way that provides the most benefit to the broader catalysis community. Increasing the rigor and reproducibility of active site characterization offers a strategy to better link properties with catalytic performance and to enable the community to develop consensus concerning these relationships.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Kinetic and X-ray Absorption Spectroscopic Analysis of Catalytic Redox Cycles over Highly Uniform Polymetal Oxo Clusters

Metal–organic framework materials (MOFs) offer an opportunity for investigating catalytic properties of polymetal oxo clusters that are highly well defined and uniform in nature, in contrast to other classes of catalysts that may exhibit a propensity toward active site heterogeneity. We report herein a kinetic and X-ray absorption spectroscopy (XAS) analysis of the two-electron oxidation of CO over divalent metal sites in MIL-100(M = Fe, Cr) (MIL = Materials of Institut Lavoisier) materials carrying μ 3 -oxo bridged trimers, and connect observations about the kinetic relevance of redox steps to density functional theory (DFT) predictions published previously. The high degree of uniformity evident from in situ titration measurements leads to a congruence in mechanistic inferences made from steady-state catalytic, transient stoichiometric, isotopic exchange, and isotopic tracer data that all point to a sequential mechanism comprised of separate oxidation and reduction half-cycle steps conjoined by an active oxygen intermediate. In situ XAS data reinforce mechanistic conclusions derived from kinetic analysis, and suggest that the active oxygen intermediate may be more appropriately characterized as an iron-oxyl (Fe 3+ –O – ) rather than an iron-oxo (Fe 4+ =O 2– ) species. The Cr analogue of MIL-100 exhibits contrasting rate features that can be rationalized using an identical sequence of steps as MIL-100(Fe), but with a highly dissimilar set of kinetic parameters that can also be validated using transient stoichiometric experiments. The larger coverages of active oxygen intermediates on MIL-100(Cr) are consistent with predictions from prior DFT studies that suggest more stable and less reactive active oxygen species for metals with lower d-electron counts, and point to metal identity as a lever for precisely controlling the kinetic relevance of oxidation and reduction half-cycles in catalytic redox sequences over polymetal oxo clusters. Importantly, the results presented point to the utility of developing broadly applicable structure–catalytic property relationships over MOF nodes specifically, and highly uniform catalysts more generally.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Plastic Waste Product Captures Carbon Dioxide in Nanometer Pores

We discuss, plastic waste (PW) and increasing atmospheric carbon dioxide (CO 2 ) levels are among the top environmental concerns presently facing humankind. With an ambitious 2050 zero-CO 2 emissions goal, there is a demand for economical CO 2 capture routes. Here we show that the thermal treatment of PW in the presence of potassium acetate yields an effective carbon sorbent with pores width of 0.7–1.4 nm for CO 2 capture. The PW to carbon sorbent process works with single or mixed streams of polyolefin plastics. The CO 2 capacity of the sorbent at 25 °C is 17.0 ± 1.1 wt % (3.80 ± 0.25 mmol g –1 ) at 1 bar and 5.0 ± 0.6 wt % (1.13 ± 0.13 mmol g –1 ) at 0.15 bar, and it regenerates upon reaching 75 ± 5 °C. The CO 2 capture cost from flue gas via this technology is estimated to be <$21 ton –1 CO 2 , much lower than competing CO 2 capture technologies. Hence, this PW-derived carbon material should find utility in the capture of CO 2 from point sources of high CO 2 emissions while providing a use for otherwise deleterious PW.

01 COAL, LIGNITE, AND PEAT↗