Catalysts play a vital role in speeding up reactions by reducing the energy input required for chemical transformations. In fact, approximately 90% of industrially produced chemicals rely on catalysts at some stage of their production. Take ammonia and hydrogen, for example, both crucial for the clean energy future. These two molecules can store intermittent energy sources, like wind or solar energy, in the form of chemical bonds. They can also be conveniently transported as liquids like the fuels used today. Ammonia and hydrogen are both produced with the help of a catalyst in highly optimized industrial processes. However, these existing catalysts are unable to meet a new challenge of the 21st century: producing chemicals from renewable electricity and sustainable chemical feedstocks.
Enter the Center for Programmable Energy Catalysis (CPEC), which aims to revolutionize catalyst design by introducing a new variable: time. Traditionally, catalysts have been developed through the study of the "structure–function" relationship. This method involves synthesizing and testing many unique catalyst materials. However, this is a time-consuming and challenging process. It often relies on serendipity, as exemplified by the early 20th century discovery of the catalyst used to produce ammonia via the Haber-Bosch process. Haber’s lab-scale synthesis of ammonia relied on a catalyst made from uranium and osmium, elements which were too expensive (and rare!) to use at an industrial scale. It was only after some twenty thousand experiments that Bosch’s assistant found a suitable alternative based on iron, an abundant and cheap material.[1]
CPEC proposes a departure from this conventional approach to catalyst design (Figure 1). The center’s goal is to synthesize and study a new class of "programmable" catalyst materials. An external stimulus is used to change the properties of these catalysts on the timescale of a catalytic turnover. This means that a programmable catalyst can be continually optimized throughout a chemical transformation, changing its properties to promote each individual surface reaction or desorption event. These individual reaction and desorption events, called elementary steps, often demand radically different catalyst properties for optimal performance. That means that traditional catalysts, unable to be tuned during reaction, must be designed using a “one size fits all” compromise that balances all elementary steps. Because programmable catalysts can be tuned, they are not limited by this balancing act. CPEC therefore hypothesizes that reaction rates and selectivity can be dramatically improved.[2,3]
CPEC will focus on two types of external stimuli for creating programmable catalysts: light and electricity. For light-stimulated programmable catalysts, different wavelengths and intensities of “pulsed” light are applied to metal catalysts to enhance their activity. In a proof-of-concept study, the rate of methanol decomposition over Pt was increased by over 25% using pulsed light at 3.5 kHz frequency. In contrast, continuous light illumination only increased the rate by 15% over “dark” conditions, demonstrating that pulsing the light yields larger performance enhancement than traditional photocatalysis.[4]
The electricity-based approach involves devices called "catalytic condensers," which are parallel plate capacitors in which the top “plate” is made from a catalytically active material. This catalyst design is distinct from electrochemistry because the electrons are not consumed during the reaction. Rather, when the capacitor is charged, the electronic occupation of the catalyst changes. Initial studies demonstrated that the catalytic activity of both oxides[5] and metals[6] can be tuned using this approach. For example, CPEC demonstrated that the Lewis acidity of alumina can be continuously tuned by changing the amount of charge stored in the catalytic condenser. Changing the amount of charge stored is simple – just increase or decrease the electrical bias applied to the condenser. In contrast, using traditional catalyst design approaches, distinct catalyst materials (e.g., alumina, titania, zirconia) must be synthesized to observe these same changes in Lewis acidity. This is a much more involved (and non-reversible!) process.
In parallel to the aforementioned experimental thrusts, CPEC is working to develop enhanced computational methods for simulating programmable catalysts. This second thrust will include a combination of first-principles computational chemistry[7] and microkinetic modeling[8] to simulate both molecular interactions and overall reaction performance. Because programmable catalysts have additional operating parameters (e.g., how quickly to pulse the light source), computational studies will provide valuable insight into how these materials can be optimized.
In the coming years, CPEC aims to demonstrate that these devices can increase catalyst performance in key energy chemistries. In addition to these experimental demonstrations, CPEC aims to gain a fundamental understanding of the mechanism by which these devices increase catalyst performance, and how they can be optimized for peak performance and minimal energy consumption.[9] With a stroke of luck, catalyst discoveries at CPEC will make substantial contributions to the grand challenge of producing essential chemicals like hydrogen and ammonia using renewable energy and sustainable feedstocks.
