Over the past several years, increased attention has been focused on developing catalysts at the nanometer scale to increase catalytic activity while reducing the amount of expensive precious metals.
Catalytic activity is directly proportional to the surface area of the active material, and small nanoclusters and nanoparticles are attractive for catalysis applications as they offer a high surface area per unit volume by reducing the proportion of material needed to maintain high catalytic activity.
However, there are major barriers that remain before nanoparticle catalysts can be adopted on a wider scale. Generally, catalysts operate at high temperatures, and small nanoparticles can behave differently to bulk materials, sintering and losing surface area at relatively low temperatures. Because of this, thermal stability of nanoparticles continues to be major focus of catalyst research groups worldwide.
In 2012, a new nanoparticle catalyst system with exceptional activity for methane combustion was developed by a group at the University of Pennsylvania. The catalyst contains modular palladium-ceria core-shell subunits on silicon-functionalized alumina (Figure 1).
The ceria shell structure is thermally stable and protects small palladium nanoparticles from sintering at high temperatures and at the same time maintains access to the active palladium surface.
The authors demonstrated how the core-shell structure was successfully synthesized through transmission electron microscopy (TEM), but to understand catalyst behavior under reaction conditions a complete analysis of the catalyst behavior over a wide temperature range was needed.
Until recently, researchers were not able to perform in situ gas reactions within the TEM. Only TEM can provide a way to analyze samples in high vacuum at room temperature, but this does not well represent real-world catalyst reaction conditions. Generally, reactions take place at or near atmospheric pressure, and several hundred degrees Celsius.
With the launch of the Protochips Atmosphere 200 E-cell system, catalyst samples can now be exposed to temperatures and gases that closely match actual reaction conditions - up to 1 atm of pressure and 1000 °C. Atmosphere features an ultra stable heating for atomic resolution imaging at high temperature, and a patented silicon carbide-based heating membrane with best in class temperature accuracy and uniformity.
It provides a way visualize atomic scale processes in real-time, so experiments yield relevant and more meaningful information with minimal additional effort.
To gain a better insight into their catalyst behavior under reaction conditions, researchers at the University of Pennsylvania teamed up with the University of Michigan, who employed advanced electron microscopy techniques, including the Atmosphere 200 system, to image and analyze samples in controlled temperature and gas environments at atomic resolution.
Their in situ analysis demonstrated that the modular palladium-ceria core-shell subunits experience structural evolution over a broad temperature range, creating two distinct forms - a coarse mixture of palladium and ceria particles, and a mixture of cerium, palladium, oxygen and silicon with very high dispersion.
At the University of Michigan’s EMAL facility, both in situ and ex situ experiments were performed on an aberration-corrected JEOL JEM-2100F 200 kV STEM. In situ gas cell experiments were performed using the Protochips Atmosphere 200 system, which consists of a MEMS-based closed cell holder, the Atmosphere Clarity workflow-based software, and fully automated gas manifold.
The closed cell uses two E-chips (MEMS devices) - a window E-chip with a SiN membrane and a heating E-chip with a thin SiC heating membrane. When stacked in the TEM holder and sealed with o-rings, a thin layer of gas is formed between the E-chips and this layer is separated from the high vacuum of the TEM column.
The Atmosphere Clarity software is used to actively control the heater temperature and it automatically maintains the temperature setpoint in different pressures (up to 1 atm) and gas environments. The samples, Pd-CeO2 catalyst on a silicon-functionalized alumina support, was directly deposited on the heating E-chip device from a methanol suspension.
During the experiment, the sample was imaged under 150 Torr of pure oxygen to simulate air calcination between 500 °C and 800 °C.
Figure 1. Left — Pd-CeO2. The core shown in black represents Pd, and the porous shell in orange represents CeO2. The gray support is Al2O3. The right image shows the silicon functionalized support.
To study the thermal stability of samples, the researchers first subjected them to ex situ calcination at 500 °C and 800 °C. The palladium and ceria particles remain stable at 500 °C, without any major change in the physical structure. When temperatures reached 800 °C, there was a distinct change in the structure - smaller atomic-scale clusters appear which coexist with larger ceria and palladium nanoparticles.
To better understand this phenomenon, in situ experiments using Atmosphere were performed step by step after the calcination process. In 150 Torr of oxygen, the temperature was raised to 500 °C, and up to this temperature the sample continued to remain stable.
Beyond 500 °C, changes were observed in the structure, where nanoparticles started to disassociate, the smallest first and then larger particles, into highly dispersed atomic-scale species, as shown in Figure 2. As the temperature increased to 650 °C, a second structural change took place.
Surrounded by atomic-scale species of ceria, ceria particles started to coalesce and formed larger faceted nanoparticles (Figure 3).
Figure 2. The Pd-CeO2 sample held at 500 °C and 150 Torr of oxygen. Over time the CeO2 begins to dissociate into small atomic species.
Figure 3. At temperatures reaching 800 °C, small species of CeO2 begin to coalese on larger, neighboring nanoparticles. As the particle increases in size, it begins to facet, to reduce surface energy.
With these results, the research team could better understand the process and the behavior of the catalyst. Ceria begins to disassociate at 500 °C, forming very small, atomic-scale species. These tiny species are not detected in samples without the silicon functionalized surface, and according to the authors, silicon plays a key role in stabilizing the small clusters of ceria either chemically or physically.
As the temperature increases, the small species no longer remain stable and coalesce into larger nanoparticles, but this only happens if an existing particle is in close proximity, or they will nucleate into very small clusters.
The formation of the two distinct structures - highly dispersed atomic-scale clusters and larger nanoparticles - was confirmed by both in situ and ex situ experiments. The authors conclude that the highly dispersed structures, stabilized by silicon, are the source of the exceptional catalyst activity.
Without in situ observation, the dynamic structural evolution is unlikely envisioned by any ex situ method, and more importantly this finding may open new perspectives about the origin of the activity of this catalyst.
Shuyi Zhang, Lead Author of the Study
Catalyst materials used in a wide range of industry sectors are critical components in producing industrial and consumer materials and products. Nanoparticle catalysts show exceptional potential in these processes and applications if critical barriers are overcome.
Despite the high vacuum conditions found in the TEM column, TEM analysis of nanoparticle catalysts shows exceptional results. Atmosphere builds on those results by providing the means to apply accurate temperatures across a wide pressure range, without reducing the performance of the most powerful TEMs available on the market today.
Atmosphere is a holder-based system that can be used with most modern TEMs, and can be safely added to new and existing instruments without any modification.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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