Catalyst effectiveness: Size does matter

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2010

The size of palladium atom catalyst clusters can affect the conversion of carbon monoxide to carbon dioxide.

 

KEY CONCEPTS
• The size of palladium atom catalyst clusters affects the rate of conversion of carbon monoxide to carbon dioxide.
• Differences in the binding energy of electrons in palladium atoms also affect catalyst performance because more tightly bound valence electrons are less likely to participate in bond-making processes.
• Catalyst activity increases as the size of palladium atom clusters increases up to 20 atoms before dropping when 25 atom clusters are reached.

Catalysts play an important role in a number of different processes that involve lubricants. The first application that comes to mind for many of us is in reduction of automotive emissions. A three-way catalyst based on precious metals (palladium, platinum and rhodium) is used to simultaneously remove carbon monoxide, hydrocarbons and NOx.

In a previous TLT article, a new approach was discussed to maximize the performance of the three-way catalyst (1). The precious metal used is transformed into fine nanoparticles that exhibit comparable performance to conventional three-way catalysts but are effective over a longer operating period and at a lower treat rate.

A second application that we are familiar with is the use of catalysts in the preparation of mineral oil basestocks. Group II and Group II base oils are mainly produced through the use of hydrocracking. In a previous TLT article, a new approach to prepare higher molecular weight hydrocarbons from lower molecular weight hydrocarbons can be carried out through the process of metathesis (2).

The work done showing the increased effectiveness of nanoparticles of a specific catalyst vs. micrometer-sized particles of the same material means that particle size can have an impact on performance. Bill Kaden, a chemistry doctoral student at the University of Utah, in Salt Lake City, Utah, says, “Researchers have taken stabs at determining the size of active catalyst particles currently used in the chemical industry. But typical preparation methods and analytical tools used have been insufficient to provide this information.”

One other factor that has been seen is that after large particles are removed from a catalyst, reasonable activity is still seen with smaller particles. This suggests that work is needed to determine how catalyst particles on the atomic-scale function.

Such work has not been conducted until now.

CLUSTER SIZE
Kaden and his fellow researchers set out to evaluate the differences in catalyst performance as a function of size. He says, “We decided to evaluate the effectiveness of palladium metal deposited in various cluster sizes on a titanium dioxide surface. The process studied is the conversion of carbon monoxide to carbon dioxide.”

The clusters represent palladium atoms in sizes ranging from one atom up to 25 atoms. Kaden adds, “A 30-atom palladium cluster is less than one nanometer in size.”

The catalyst is prepared by laser vaporizing palladium metal in a flow of helium. An electric field and a quadruple mass filter were used to isolate clusters of specific sizes, which were then deposited onto a titanium dioxide crystal of the (110) orientation. In all cases, the number of palladium atoms was maintained at 1.53 x 1014 per square centimeter.

The researchers evaluated catalyst effectiveness by conducting a temperature programmed reaction. Initially, the palladium catalyst was treated with 10 Langmuirs (1L = 1 x 10-6 Torr partial pressure for 1 second) of oxygen at a temperature of 127 C and 5 L of carbon monoxide at -93 C. The temperature of the catalyst was then increased from -143 C to 257 C at a rate of 3 C/second.

Concurrently, a mass spectrometer was used to monitor the concentration of carbon monoxide and carbon dioxide present during the reaction. Kaden says, “We found that the size of the cluster influenced the performance of the catalyst.”

Data produced shows that a single palladium atom does not generate any activity. When the cluster size is increased, catalyst activity increases up to two palladium atoms and then declines as the number of atoms is increased to seven. From there, activity increases linearly up to 20 atoms before dropping when 25 atom clusters were reached.

At the atomic level, Kaden indicates that differences in the binding energy of electrons within the core levels of the palladium atoms significantly impacts catalyst performance. He says, “We measured the shifts in the binding energy of the 3d orbitals of palladium atoms and used the results to indirectly reflect what the valence orbitals are doing. We found that higher-than-expected binding energies led to lower levels of catalytic activity because the valence electrons are more tightly bound and less likely to participate in bond-making processes.”

Kaden further explained that this binding energy is very important because it impacts the activation of the bond palladium atoms have with oxygen atoms during the reaction with carbon monoxide. He adds, “Activation of the palladium-oxygen bond is the limiting step in the conversion of carbon monoxide to carbon dioxide.”

One other consideration evaluated by the researchers is the composition of the palladium atom clusters on the titanium dioxide surface. As the clusters increase in size, helium ion scattering shows that a second layer of palladium layers starts to form above a cluster size of 10. The appearance of this second layer occurs as catalytic activity increases from 10 to 20 palladium atoms but then declines to 25 palladium atoms.

The complex apparatus used to prepare the catalyst, conduct and analyze the reaction results is shown in Figure 3. Kaden says, “The structure of the palladium atom clusters is very important in determining catalytic performance. A particular number of atoms present in specific clusters lead to geometries that dictate catalyst performance.”


Figure 3. The apparatus used to prepare the catalyst, conduct and analyze the reaction results is quite complex as shown above. (Courtesy of the University of Utah)

It is very apparent that the orientation of atoms at this sub-nanometer level influences catalyst performance. Differences in size and orbital binding energies are major considerations in predicting how well a specific number of palladium atoms perform.

Future development of catalysts should benefit from this work, which will hopefully lead to more effective approaches to reduce emissions, produce mineral oil basestocks and generate the wide variety of additives used in the lubricant industry.

Additional information on this research can be found in a recent article (3) or by contacting Kaden at kaden@chem.utah.edu. 

REFERENCES
1. Canter, N. (2009), “Nanoparticle-Based Emission Catalysts,” TLT, 65 (11), pp. 10–11.
2. Canter, N. (2006), “Dual Catalyst System Could Produce Lubricant Basestocks,” TLT, 62 (7), pp. 15–16.
3. Kaden, W., Wu, T., Kunkel, W. and Anderson, S., “Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces,” Science, 326 (5954), pp. 826–829.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.