(569eb) Tunable Reactivity: Achieving Enhanced Catalytic Turnover with Surface Coverage Modulation

Next-generation reactor concepts, aiming to overcome economies of scale, lower process footprint and enable distributed chemical manufacturing (DCheM) has driven innovation in microreactors, with the opportunity to relax assumptions conventionally applied to large scale reactors – namely steady state, isothermal reactor operation. Such concepts enable modes of operation like dynamic catalysis, involving forced periodic change of process inputs like temperature or reactant concentration to enhance performance beyond steady state thermodynamic limits. One example is the alternance of reaction and catalyst regeneration cycles with oxygen stepping after deactivation in large reactors, at long timescales. In a dynamic microreactor, significantly shorter length- and timescales for surface cleaning can enable regeneration before deactivation with modulation at kinetic timescales and fast transport.

In this work, two microreactor platforms were designed to control surface coverage through temperature and concentration square wave modulation. Dynamic concentration experiments were conducted in a 250-micron diameter wash-coated glass capillary capable of 10-millisecond gas pulses and integrated in a gas chromatograph with fast product analysis. For methane partial oxidation, oxidant/reductant feed switching resulted in 6-fold enhancement at 395^oC as turnover responded to input concentration oscillations with a maximum as methane loading timescale is matched. This optimum reveals a dependence of dynamic enhancement on timely tuning of surface coverage through induced concentration transients. The temperature dynamic microreactor was designed for isothermal and up to 10 Hz dynamic heating/cooling cycles through a resistively-heated platinum wire used as a catalyst for carbon monoxide oxidation. Bi-directional thermal ramping revealed hysteresis attributed to kinetically favorable surface coverage at catalytic light off and less reactive species dominance at light out. This finding, along with reactant pre-treatment experiments, indicated that modulating catalyst temperature between light off and light off tunes surface coverage to optimize reaction and reactant adsorption, respectively, leading to faster time-averaged turnover than isothermally.