A team of researchers from Brookhaven National Lab (BNL) and the Integrated Mesoscale Architecture for Sustainable Catalysis (IMASC) EFRC have probed the innerworkings of a mysterious nanospace. The exotic location, sandwiched between a palladium metal surface and a 2-dimensional silicon dioxide cover, was found to enable new chemistry, and by using advanced techniques, the team discovered how.
A catalyst is a material that is used to push a chemical reaction in a desired direction. Catalysts can be used for many purposes, such as increasing the speed and/or productivity of a reaction, changing the temperature at which a reaction occurs, or increasing the selectivity towards a desired product in reactions with multiple products. For many scientists and engineers, the question is “can we engineer a new catalyst with properties that will push a given reaction in the desired direction?” In order to answer this question, researchers need to study the fundamental interactions between the catalytic materials, reactant, and product molecules at the smallest possible scale, right down to where the atoms interact.
At the atomic level, a typical catalytic material can be thought of as a smooth surface of atoms that is exposed to reactant molecules that form products upon interactions with the surface. An emerging type of catalyst, the so-called “interfacially confined microenvironment,” is a little different. The smooth surface is still there, but a “cover” is added on top of the surface, which is held on by a weak electrostatic interaction (similar to the static charge that holds a balloon on the ceiling after you rub it on your head).
The weak interaction between the cover and the surface is key because it allows the reactant molecules to enter, and the products to leave. The area between the surface and the cover is known as a nanospace. Simply covering a surface can completely change the game. The chemistry works differently under the cover, causing much intrigue in recent years. The team of BNL and IMASC researchers moved to investigate the innerworkings of the nanospace during a specific chemical reaction before and after adding a cover to the surface.
Their findings were published recently in Angewandte Chemie,* a Journal of the German Chemical Society. The team investigated the carbon monoxide (CO) oxidation reaction on a palladium (Pd) surface with and without a 2D silica cover. In the usual CO oxidation reaction catalyzed by a Pd surface, two things need to happen. First, Pd sites break oxygen molecules (O2) to form a surface oxide. Next, gaseous CO molecules collide with the Pd surface oxide, stealing oxygen atoms to form carbon dioxide molecules (CO2). There’s a small complication though, in that CO molecules are quite sticky, so they completely cover the Pd sites at room temperature. This “poisoned” state needs to be overcome by a high temperature, so Pd sites can be free to form surface oxides— setting the oven to about 485 °F should do the trick. The high temperature “active” state requires a delicate balance of CO adsorbates and surface oxides at a given moment, leading to CO2 formation.
To investigate if the story remains the same after adding a 2D silica cover, the researchers used three techniques: mass spectrometry (MS), for figuring out how much CO2 was being produced at a given moment; ambient pressure X-ray photoelectron spectroscopy (APXPS), for looking at the chemical environment around specific elements using X-ray light; and infrared reflection-adsorption spectroscopy (IRRAS), for looking at where the CO adsorbs on the Pd surface using infrared light. If MS detects CO2 production, one can say that the reaction is taking place. The distinct way in which X-ray light interacts with the carbon (in the gases), palladium (on the surface), and silicon atoms (on the cover), can tell the researchers what kind of bonds are formed or broken during the reaction. Infrared light vibrates CO adsorbates in distinct ways depending on how they are attached to the surface. For a Pd surface, CO can adsorb at hexagonal close packed (HCP) and face centered cubic (FCC) sites. HCP and FCC describe the two ways spheres can be stacked up on top of each other. The diagram at the bottom of Figure 1 makes this idea clearer. The red arrow points to an HCP site, and the green arrow points to an FCC site. The differences between the two sites are seemingly very small, but small differences at the nanoscale explain large changes at our scale.
Initially, the team found that the covered Pd surface was poisoned just like the open surface, i.e., the MS did not detect CO2 formation. However, at high temperature, when the catalyst was active, the MS indicated a ~12% increase in CO2 formation as compared to the open Pd surface. How could simply covering the surface with a weakly interacting cover lead to a huge increase in production? The team found the likely explanation thanks to the APXPS and IRRAS tools available at BNL.
The first clue was provided by IRRAS at room temperature, when both catalysts were in the poisoned state. When looking at the open Pd surface, IRRAS revealed characteristic vibrations for CO molecules adsorbed on two distinct positions (FCC and HCP). However, in the nanospace, only HCP vibrations were observed. Therefore, HCP sites are preferentially occupied by CO under the cover. Significantly, the geometry around adsorbates can completely change the chemical reaction.
The second clue was provided by APXPS. In the active state, the team found that the covered Pd surface contained almost no adsorbed CO, in stark contrast to the open surface. As previously mentioned, when active, the open surface is suspended in a delicate balance of surface oxide and CO adsorbates. In addition, APXPS also indicated that, in the active state, more surface oxide develops in the nanospace than on the open surface.
These observations led the team to conclude that the nanospace formed between the surface and cover is directly responsible for the enhanced productivity of the CO oxidation reaction. The cover reduces the number of available adsorption sites in the active state; therefore, there is much less CO poisoning to deal with. With less, and more dispersed CO adsorbates, the formation of surface oxide is facilitated, and since CO happens to desorb from HCP sites at a lower temperature than FCC sites, the surface oxides develop sooner. The increased amount of surface oxide, enabled by the cover, increases the amount of CO2 production by ~12%.
What does this mean for the future? Dr. Calley Eads, first author of the work, believes the next step is investigating nanospaces confined by porous covers. Dr. Eads and her colleagues are interested in taking advantage of pore size and shape to act as a filter for molecules or branches of large molecules before interaction with the catalytic surface. For example, small molecules may infiltrate the pores and become trapped below the cover, whereas large molecules may anchor part of themselves on top of the silica cover, while another part enters the pore to react with the surface. Depending on the identity of the porous cover, it may be possible to engineer selectivity to certain chemical reactions, namely blocking unwanted chemical reactions while facilitating the ones that matter.
