More Like this

1. Understanding the role of a novel internal conditioning technique with functionalized montmorillonite in cement hydration kinetics

   https://doi.org/10.1016/j.conbuildmat.2023.131223  

   Luo, Dayou; Wei, Jianqiang (July 2023, Construction and Building Materials)

   A novel internal conditioning (InCon) technique based on saturated sodium montmorillonite (sMT) functionalized with two non-ionic surfactants, polyoxyethylene (9) nonylphenylether and t-octyl phenoxy poly ethoxyethanol, is investigated in this study. With the integration of water for internal curing and pozzolanic reactivity in a single system, the role of InCon in modifying cement hydration kinetics is comprehensively elucidated. The results indicate that, in the presence of InCon, both silicate reaction and secondary aluminate reaction rates are enhanced, and the apparent activation energy (Ea) of cement hydration was decreased from 34.3 KJ/mol to 28.7 KJ/mol indicating a lower temperature sensitivity and threshold of the cement hydration reactions. In addition, decreased CH contents, improved degree of hydration, increased chemical shrinkage, and the formation of additional Csingle bondSsingle bondH and aluminum-containing phases were obtained from the cement with InCon. The autogenous shrinkage of cement and the negative impact of dry sMT on the early age strength of cement can be offset by InCon paving a new path to improve the overall properties of concrete. 

   more » « less
2. H2−D2 Exchange Activity and Electronic Structure of AgxPd1−x Alloy Catalysts Spanning Composition Space

   Golio, Nicholas; Sen, Irem; Yu, Xiaoxiao; Kondratyuk, Petro; Gellman, Andrew J (July 2024, ACS catalysis)

   Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a AgxPd1−x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2 −D2 exchange kinetics were measured at atmospheric pressure on AgxPd1−x catalysts over a temperature range of 333−593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from −6.2 eV for pure Ag to −3.4 eV for pure Pd. At all reaction conditions, the H2 −D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with xPd < 0.58. Measured H2 −D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H′) mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔEads ‡ , associative desorption, ΔEdes ‡ , and the surface-to-subsurface diffusion energy, ΔEss, as a function of alloy composition, xPd. The 2H′ mechanism predicts ΔEads ‡ = 0−10 kJ/mol, ΔEdes ‡ = 30−65 kJ/mol, and ΔEss = 20−30 kJ/mol for all alloy compositions with xPd ≥ 0.64, including for the pure Pd catalyst (i.e., xPd = 1). For these Pd-rich catalysts, ΔEdes ‡ and ΔEss appeared to increase by ∼5 kJ/mol with decreasing xPd. However, due to the coupling of kinetic parameters in the 2H′ mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when xPd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2 −D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing xPd can be explained by a reduction in the availability of surface Pd at high Ag compositions. 

   more » « less
3. On the intrinsic reaction kinetics of polypropylene pyrolysis

   https://doi.org/10.1016/j.matt.2023.07.020  

   Sidhu, Nathan; Mastalski, Isaac; Zolghadr, Ali; Patel, Bryan; Uppili, Sundararajan; Go, Tony; Maduskar, Saurabh; Wang, Ziwei; Neurock, Matthew; Dauenhauer, Paul J (October 2023, Matter)

   The growing global plastic waste challenge requires development of new plastic waste management strategies, such as pyrolysis, that will help to enable a circular plastic economy. Developing optimized, scalable pyrolysis reactors capable of maximizing the yield of desired products requires a fundamental understanding of plastic pyrolysis chemistry. Accordingly, the intrinsic reaction kinetics of polypropylene pyrolysis have been evaluated by the method of pulse-heated analysis of solid reactions (PHASR), which enables time-resolved measurement of pyrolysis kinetics at high temperature absent heat and mass transfer limitations on the millisecond scale. Polypropylene pyrolysis product evolution curves were generated at 525°C–625°C, and the overall reaction kinetics were described by a lumped first-order model with an activation energy of 242.0 ± 2.9 kJ mol−1 and a pre-exponential factor of 35.5 ± 0.6 ln(s−1). Additionally, the production of solid residues formed during polypropylene pyrolysis was investigated, revealing a secondary kinetic regime. 

   more » « less
4. Comparative investigation of Ga- and In-CHA in the non-oxidative ethane dehydrogenation reaction

   https://doi.org/10.1016/j.jcat.2022.07.018  

   Pan, Jian; Lee, Jason; Li, Muyuan; Trump, Benjamin A; Lobo, Raul F (September 2022, Journal of Catalysis)

   Ga-and In-exchanged chabazite (CHA) zeolites with same Si/Al and metal/Al ratios were prepared via the incipient wetness impregnation method, were characterized using N-2 adsorption, electron microscopy, temperature-programed reactions and were evaluated for the ethane dehydrogenation reaction using flow microreactors. Ga-CHA has higher reaction rates and a lower activation energy of 107 kJ/mol than In-CHA (E-a = 175 kJ/mol). Rietveld refinement of the X-ray powder diffraction pattern shows that the In+ cation is predominantly located above the 6-ring of the CHA cage. It is proposed that the reaction proceeds through the alkyl mechanism based on stability of alkyl hydride intermediates as determined using DFT calculations. The oxidative addition of ethane to the metal shows much lower Gibbs free energy for Ga-CHA (+27.95 kJ/mol) vs In-CHA (+124.85 kJ/mol). These results indicate that oxidative addition may be the rate-limiting step of ethane dehydrogenation in these materials. 

   more » « less
5. Plasma-Assisted Abatement of Per- and Polyfluoroalkyl Substances (PFAS): Thermodynamic Analysis and Validation in Gliding Arc Discharge

   https://doi.org/10.3390/plasma6030029  

   Surace, Mikaela J.; Murillo-Gelvez, Jimmy; Shaji, Mobish A.; Fridman, Alexander A.; Rabinovich, Alexander; McKenzie, Erica R.; Fridman, Gregory; Sales, Christopher M. (September 2023, Plasma)

   Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic organofluorine surfactants that are resistant to typical methods of degradation. Thermal techniques along with other novel, less energy-intensive techniques are currently being investigated for the treatment of PFAS-contaminated matrices. Non-equilibrium plasma is one technique that has shown promise for the treatment of PFAS-contaminated water. To better tailor non-equilibrium plasma systems for this application, knowledge of the energy required for mineralization, and in turn the roles that plasma reactive species and heat can play in this process, would be useful. In this study, fundamental thermodynamic equations were used to estimate the enthalpies of reaction (480 kJ/mol) and formation (−4640 kJ/mol) of perfluorooctanoic acid (PFOA, a long-chain legacy PFAS) in water. This enthalpy of reaction estimate indicates that plasma reactive species alone cannot catalyze the reaction; because the reaction is endothermic, energy input (e.g., heat) is required. The estimated enthalpies were used with HSC Chemistry software to produce a model of PFOA defluorination in a 100 mg/L aqueous solution as a function of enthalpy. The model indicated that as enthalpy of the reaction system increased, higher PFOA defluorination, and thus a higher extent of mineralization, was achieved. The model results were validated using experimental results from the gliding arc plasmatron (GAP) treatment of PFOA or PFOS-contaminated water using argon and air, separately, as the plasma gas. It was demonstrated that PFOA and PFOS mineralization in both types of plasma required more energy than predicted by thermodynamics, which was anticipated as the model did not take kinetics into account. However, the observed trends were similar to that of the model, especially when argon was used as the plasma gas. Overall, it was demonstrated that while energy input (e.g., heat) was required for the non-equilibrium plasma degradation of PFOA in water, a lower energy barrier was present with plasma treatment compared to conventional thermal treatments, and therefore mineralization was improved. Plasma reactive species, such as hydroxyl radicals (⋅OH) and/or hydrated electrons (e−(aq)), though unable to accelerate an endothermic reaction alone, likely served as catalysts for PFOA mineralization, helping to lower the energy barrier. In this study, the activation energies (Ea) for these species to react with the alpha C–F bond in PFOA were estimated to be roughly 1 eV for hydroxyl radicals and 2 eV for hydrated electrons. 

   more » « less