Primary School Student:
You know about cars? The gasoline your car uses was made pure through a process called catalysis. Catalysis is a process which speeds up chemical reactions. Catalysis is mediated by materials called catalysts, which are exposed to the molecules we use like gas in a controlled way. If that process could be done more efficiently (require less heat, pressure, catalyst material), than the gas we use in cars would be cheaper. To do that, I study how changing the shape of very tiny pieces of metal causes changes in catalysis. We can think about chemical reactions as being like climbing over a mountain to get to a spot on the opposite side. Catalysts offer a way to walk around the hill instead, so you don’t have to do the hard work of climbing all the way to the top and back down! If we can understand how the shape of these tiny pieces of metal impacts the chemical reactions, we can make it easier for the molecules to get to the other side of the mountain.
Secondary School Students:
Catalysis is a process that can speed up chemical reactions and produce products, such as gasoline for your car. Catalysis is controlled through materials called catalysts, which are exposed to the reactant chemicals. Chemical reactions require energy to perform a reaction, like how walking up a steep hill requires energy. Catalysts provide a pathway that lets you walk around the hill, instead of over, reducing the energy needed! Metals are typically used as catalysts, specifically in the form of metal nanoparticles. We can call things nanoparticles instead of particles when they are very small. For example, when you compare the sizes of a soccer ball and a marble, you can recognize that although they may have the same shape, there are important differences based on relative size. The metals used in catalytic reactions are usually expensive, and therefore it is advantageous to design more efficient metal catalysts so that less metal can be used in the reaction. One way to better understand these design principles is to change the shape of the metal. Different shapes of metal can behave differently as catalysts, similar to how a football and soccer ball behave differently when kicked. By changing the shape of the metal and examining the difference in the catalytic reactions, a more effective catalyst can be designed.
College student:
Do you remember the concept of unit cells from your classes? Unit cells are the smallest building block of materials and describe the position of all the atoms which repeat throughout the structure and are three-dimensional units which have parallelograms for faces. If you cut a unit cell down in different ways, you can expose different atom configurations. When studying metals, these different atom configurations are referred to as facets. Metals are commonly used as nanoparticles catalysts to mediate catalytic reactions. Catalysts are used to lower the activation energy of a reaction, to make the reaction more efficient. The facet of the metal can influence its behavior as a catalyst; therefore, it is beneficial to study how the exposed facet of the metal changes the reaction products or intermediates so that more effective catalysts can be designed.
There is then the problem of producing these catalytic metal particles. Metal nanoparticles are formed through the joining of many different metal unit cells, consisting of the metal atoms. These unit cells repeat in a periodic pattern, which is referred to as a crystal structure. These unit cells fit together very specifically, that is why metals nanoparticles can have distinct shapes and facets. However, the syntheses of metal nanoparticles are not very reproducible as even minor changes in reaction conditions, like how the reagents are mixed, can lead to a full breakdown in shape selectivity. This is similar to how the titration experiment in your general chemistry lab may tell you a different concentration of your unknown chemical every time you perform the experiment, even though you think you are keeping everything the same. However, you can measure the real-time chemical potential of the solution using electrodes, just like you would measure the voltage of your car battery. By measuring the chemical potential, you can gain insight into what minor changes may occur between your experiments, as the chemical potential would also change, increasing the reproducibility of the synthesis.
Graduate Student:
Understanding the structure-function relationships of nanoparticles is important to the design of more efficient catalysts. The majority of surface studies are performed on bulk metals, with much larger dimensions than their nanoparticle counterparts, in ultra-high vacuum (UHV), but industrially, these materials are generally used as nanoparticles at or near atmospheric pressure. This discrepancy in the fundamental understanding of these systems is referred to as the “pressure-materials” gap. Establishing structure-function relationships of shaped metal nanoparticles across this pressure gap can help inform design principles of more efficient catalysts.
I am studying the impact of facet type on ethanol dehydrogenation mediated by shaped palladium nanoparticles. Differences in reaction products and intermediates are interrogated using mass spectrometry and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), a surface-sensitive infrared spectroscopy technique that allows you to see adsorbates. Additionally, I am studying the synthesis of these nanoparticles are using an electroanalytical technique called open circuit potential (OCP). OCP is an in situ, time-resolved electroanalytical method that provides information about the chemical potential of the colloidal solution during nanoparticle growth. Knowing the chemical potential of the growth solution is useful in benchmarking synthetic conditions as well as identifying factors contributing to synthetic irreducibility, such as mixing effects or impurities in the reagents.
Expert:
There exists a pressure-materials gap between real heterogenous catalysis conditions (nanoparticle catalysts and atmospheric conditions) and surface science studies (bulk material and ultrahigh vacuum conditions). Additionally, the colloidal synthesis conditions of nanoparticles are poorly understood, making synthetic reproducibility challenging. Increasing the structure-function understanding of nanoparticles, from synthesis to application, can help inform design decisions to develop efficient catalysts. To increase the understanding of nanoparticle synthesis, open circuit potential (OCP) can be used as a time-resolved, in situ, electroanalytical tool. This technique provides information on the open circuit potential of the colloidal reaction, providing real-time insight into reaction conditions and kinetics. Here, OCP is used to benchmark the reaction conditions for different faceted palladium nanoparticles, as well as providing information on what reaction handles can be used to optimize the synthesis.
To address the pressure-materials gap, these shaped palladium nanoparticles are interrogated under both atmospheric and ultra-high vacuum (UHV) conditions. In atmosphere, gas flow reactors and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are used to examine potential structure-function relationships of different-faceted palladium nanoparticles using the ethanol dehydrogenation model reaction. By examining the different products and reaction intermediates arising from different faceted palladium nanoparticles, structure-function relationships can be drawn.
UHV techniques, specifically temperature programmed desorption (TPD), are also used to study the structure-function relationship of these shaped palladium nanoparticles. In UHV, surface of the metal stays clean, and the intrinsic behavior of the surface can be analyzed without interference from unwanted atmospheric molecules. Here, TPD is utilized to elucidate the intrinsic impact surface facet has on binding strength between the shaped nanoparticle and ethanol.