Clarifying mechanisms and kinetics of programmable catalysis.

Programmable catalysis-the purposeful oscillation of catalytic potential energy surfaces (PES)-has emerged as a promising method for the acceleration of catalyzed reaction rates. However, theoretical study of programmable catalysis has been limited by onerous computational demands of integrating the stiff differential equations that describe periodic cycling between PESs. This work details methods that reduce the computational cost of finding the limit cycle by ≳108×. These methods produce closed-form analytical solutions for didactic case studies, examination of which provides physical insights of programmable catalysis mechanisms. Generalization of these analyses to more complex reaction networks, including CO oxidation on Pt (111) surfaces, exposes the key catalyst properties required to achieve enhanced rates and conversions via one of two programmable catalysis mechanisms: quasi-static (high frequency) and stepwise (intermediate frequency). Analytical description of each mechanism is critical in understanding the consequences of the Sabatier principle on achievable rate enhancement through programmed catalysis.

A chemo-mechanical model for describing sorption hysteresis in a glassy polyurethane.

Hysteretic sorption and desorption of water is observed from 0 to 95% relative humidity and 298-333 K on a glassy polyurethane foam. It is postulated that sorption-induced swelling of the glassy polyurethane increases the concentration of accessible hydrogen-bonding adsorption sites for water. The accessibility of sites is kinetically controlled due to the restricted thermal motions of chains in the glassy polymer, causing a difference in accessible site concentrations during sorption and desorption. This discrepancy leads to hysteresis in the sorbed concentrations of water. A coupled chemo-mechanical model relating volumetric strain, adsorption site concentration, and sorbed water concentration is employed to describe water sorption hysteresis in the glassy polyurethane. This model not only describes the final mass uptake for each relative humidity step, but also captures the dynamics of water uptake, which exhibit diffusion and relaxation rate-controlled regimes.