Part 1: Function and importance of nanoparticles in catalysis

As a science teacher, if you were asked for an example of a catalyst, your car’s catalytic converter may come to mind. Most informed citizens would do the same. What may come as a surprise is that catalytic converters have dramatically changed in the past decade and now use only a small fraction of the precious metals that older models used. This is due to the development of nanoparticles, extremely tiny particles of metal with enormous surface areas relative to their size. Modern catalytic converters contain nanoparticles of platinum, palladium, rhodium and other metals on inert supports. Each metal catalyzes certain desired reactions in the process of treating the exhaust stream. Metal catalysts in elemental form, such as Pt, Pd and Rh, are examples of heterogeneous catalysts and are intrinsically involved in our lives through the myriad of reactions they facilitate to make the products we use on a daily basis. For example, the hydrogenation of alkenes proceeds rapidly at room temperature in the presence of a catalyst but not at all without the catalyst present. One major use of this reaction is to convert vegetable oils into fats, which mix better with flour in making breads and baked goods. The development of nanoparticles is greatly advancing the efficacy of catalysts simply by providing vastly increased surface areas where catalytic reactions take place.

Catalysis as a concept is introduced into the chemistry curriculum in the discussion of kinetics. The catalyst provides a lower energy pathway from reactants to products. The reactions are faster, cleaner and less expensive — a step in the direction of green and sustainable living, with fewer waste byproducts. The nanoparticle version of a catalyst can do the same things but much better.

Why and where should one introduce nanoparticle catalysts into the curriculum? First, learning about nanoparticles without even mentioning the catalytic properties that some possess provides a rewarding opportunity to think about small clusters of atoms that number from a few dozen to many thousands. For example, carbon black nanoparticles (soot) is a topic of current interest because it is now known to be the second most significant contributor to global warming. Nanoparticle calculations combine skills learned regarding mass, moles and geometry to provide truly astonishing results. In the process, new vocabulary will allow one to start becoming conversant in nanoscience, an area that students might explore at university. Many universities offer undergraduate programs in nanoscience and nanotechnology engineering. Even as informed citizens, we hear of nanomaterials as they pertain to new materials that are becoming increasingly prominent in daily life.

In this series, we hope to explain nanoparticles at a level appropriate to students’ chemistry and math skills. As a science teacher, perhaps the best opportunities to incorporate nanoparticles into the curriculum are:

1. when mass and mole calculations are introduced
2. when discussing properties of metallic solids and unit cells, and
3. as part of the kinetics discussion, where catalysts are introduced

In Part 1 of this three-part series we describe:

• how metals facilitate hydrogenation reactions,
• what nanoparticles are,
• how nanoparticles compare to other forms of metal, (d) the various forms of nanoparticles, and
• how supported nanoparticles are made.

In Part 2 we explore the importance of surface area through calculations students can understand and perform themselves. In Part 3 we will describe the atom-level structure of nanoparticles and how one could build a nanoparticle classroom exhibit.

Over half of the transition metals exhibit catalytic properties of some sort, but we will use palladium as our primary example for several reasons:

1. Palladium metal is widely used as a catalyst for gas-phase reactions, particularly those involving hydrogen and hydrogenation,
2. palladium is especially relevant as it is a major component in automotive catalytic converters, and
3. we are most familiar with nanoparticle palladium from our undergraduate research projects with this catalyst.

 How hydrogen interacts with palladium

The hydrogen-hydrogen bond is one of the strongest single bonds in chemistry. Its dissociation energy is 436 kJ/mol. Yet hydrogen readily dissociates into atoms in the presence of palladium and does so at temperatures as low as 37 K. How does this work? One reasonable and simple explanation is summarized in Fig. 1.¹ In Fig. 1(a), molecular hydrogen approaches the palladium surface and undergoes physical adsorption with the release of 10 – 20 kJ/mol. Tethered to palladium by intermolecular forces, dissociative chemisorption occurs: the hydrogen molecules dissociate into atoms possibly through a transition state shown in Fig. 1(b) and with activation energy of less than 10 kJ/mol.² This results in atoms of hydrogen occupying holes between the packed palladium atoms as shown in Fig. 1(c). The overall process is exothermic for palladium. Hydrogen atoms readily migrate from hole-to-hole throughout the entire palladium sample, including below the surface, at temperatures above 50 K.³

Fig. 1. The dissociative addition of hydrogen into palladium

The extraordinary chemistry between hydrogen and palladium just described is unique among the transition metals. Many transition metals interact with hydrogen as a surface phenomenon, but with the possible exception of iridium, only palladium chemisorbs hydrogen atoms below the surface.² And hydrogen is the only species that can migrate below the surface in palladium — even helium is restricted to surface interactions.

The hydrogenation of alkenes by palladium

Details of alkene hydrogenation were proposed by Horiuti and Polanyi in 1934.⁴ Fig. 2 summarizes the process for ethene. During the process, the p-bond of the alkene interacts with one or two surface palladium atoms as shown in Step 1. In Step 2, a hydrogen atom migrates to one of ethene’s carbon atoms while the other carbon forms a s-bond to a surface Pd atom, forming a Pd-bonded ethyl group referred to as ethyl[Pd]. In Step 3 another hydrogen migrates to the Pd-bonded carbon of ethyl[Pd], resulting in free ethane in a process called reductive elimination. Step 2 is reversible and likely, produces ethane.⁵ The newly formed alkane does not interact to any appreciable extent with the palladium atoms and is free to leave.

What are nanoparticles?

Nanoparticles are generally defined as small particles with diameters less than 100 nm.⁷ This definition works reasonably well for metal particles with more-or-less sphere-like structures — and the word diameter makes sense. Other nanomaterials have been produced including nanotubes, nanosheets and nanowires in which only one or two of the dimensions is/are smaller than 100 nm. Accordingly, the journal Nature has defined nanoparticles as small particles that measure below
100 nm in at least one dimension.⁸ Adding an official endorsement, the European Union has settled on a definition of the material itself, in which 50% of the particles must be in the 1 – 100 nm range in one dimension.⁹ The lower limit of the range, 1 nm, is just a few times larger than the diameters of many transition metals.

Nanoparticles of palladium are crystalline in structure and spheroidal or at least glob-shaped like potatoes or asteroids. A spherical nanoparticle of palladium with a diameter of 1 nm contains ~36 atoms, and a particle with a diameter of 100 nm contains ~36 million palladium atoms. (Calculations are described Part 2.) One would naturally expect a number of differences between these two extreme-size particles and the bulk metal. What sort of differences and similarities are there?

Samples of the bulk metal, such as a sheet of palladium or a wire of platinum, exhibit constant physical properties. The physical properties of nanoparticles vary with size: as the particles become smaller, there is a point in which metallic properties are lost. These include magnetic properties, malleability, ductility and conductivity. Electronic properties can refer to the entire metal or to the surface. Nanoparticles with less than ~150 atoms generally do not exhibit the same overall electronic properties as bulk samples, while similar surface electronic properties exist between the bulk metal and nanoparticles containing >25 atoms.¹⁰ Optical properties are sometimes changed in dramatic ways because nanoparticles confine their electrons to the particle, producing quantum effects.^11,12 For example, objects made of “cranberry glass”, such as fancy serving ware, contain gold nanoparticles and appear red.

The most important similarities between the bulk metal and nanoparticles of an element include the unit cell structure and hence density. Bulk palladium exhibits a cubic close packed (ccp) lattice. The ccp structure will be discussed in Part 3, however for now one can think of stack displays of oranges or grapefruits at the grocery or a stack of cannonballs at a military exhibit, all of which are examples of cubic close packing. Nanoparticles of palladium retain the structure of the bulk metal, only on a smaller scale. Surface atoms are constantly moving locations except at very low temperatures. Reactions that take place at the surface of a catalyst are generally exothermic, and the heat released causes surface atoms to move even more. Nevertheless, the close-packed structure remains the lowest energy state and prevails after the reaction. In his book, Bond poetically describes the surface of a nanoparticle: “A small metal particle may comprise a solid core and a semi-fluid surface layer.”¹⁰

Nanoparticles are of significant interest to scientists because of the differences and similarities between nanoparticles and bulk samples of the metal. Nanoparticles provide a link between the properties of individual atoms and the bulk metal.

None of the methods of producing nanoparticles results in particles of completely uniform size and shape. Inevitably there is a range of diameters with a median value. When we present calculations in the next section, it is important to remember that each calculation is for a specific radius and that we are assuming spherical shapes for these estimations. Mindful of the limitations of the calculations, the values we get are interesting and help us understand basic concepts of surface chemistry and nanoparticles.

Nanopowders, suspensions and supported nanoparticles

Nanoparticle palladium is sold in a variety of formulations. Nanopowders consist of the pure metal in nanoparticle form. In the case of palladium, the price is over $1 per milligram. Nanopowders are sold by particle size, so it is possible to buy nanoparticles that are <50 nm in diameter, or <25 nm in diameter or even <10 nm in diameter. Nanoparticles are also sold as an aqueous suspension, for example 0.1%. Suspensions in organic solvents are also available. The suspension helps prevent the particles from aggregating, which is a common problem with nanopowders. The intermolecular forces between the nanoparticles and the solvent are strong enough to counteract gravity and prevent or minimize settling. Nanoparticles tend to aggregate and form partially amorphous larger clumps in a process called sintering. Sintering results in loss of surface area, can occur at low temperatures, and is exothermic — think of it as the opposite of breaking a piece of metal into smaller pieces.

Perhaps the easiest form of palladium in terms of use and handling is supported palladium. The support may be in the form of small pellets of microporous alumina or silica. Most importantly, the support also prevents aggregation of nanoparticles. In our research lab, we buy Pd as 0.5% Pd on 1/8-inch diameter (~3 mm) alumina pellets (Acros Organics CAS 7440-05-3). We place these pellets in a 6 mm diameter (OD) glass U-tube as shown in Fig. 3. Catalyst tubes such as this cost less than $2 in materials and can be used to demonstrate a variety of reactions, including how a catalytic converter works.¹³

How supported nanoparticle materials are made

There are many methods for preparing nanoparticles, and here we briefly describe only one that could have been used to produce the nanoparticle palladium on alumina pictured in Fig. 3. In the first step, pellets of the support are added to an aqueous solution containing a salt of palladium, such as palladium(II) chloride. The support is microporous with a typical surface area of100 m²/g or more, so it functions as a sponge drawing the palladium solution into the support. Next, the water is removed by reducing the pressure and increasing the temperature and thus fixing the location of the salt throughout the support. This leaves the support coated with PdCl2(s), which may have formed nanocrystals of the salt as evaporation occurred. In the final step, hydrogen gas is passed through the material to reduce the salt to elemental palladium, and HCl(g) is formed as a byproduct. The nanoparticles of palladium are firmly attached to the support through strong chemical interactions with the oxides of the support material.¹⁵ In Part 2, we will outline some calculations for nanoparticles that produce some astonishing results. In Part 3 we will look more closely at the structure of close-packed metals and describe how a simple model of a nanoparticle can be constructed.

Student questions

1. Where does palladium come from? What countries are the largest producers? Is it found in elemental form, or does it exist as a mineral or minerals, and what are they? Repeat for platinum.
2. The lower limit for nanoparticles was stated to be 1 nm. Considering the covalent radius of a palladium atom is 138 pm, how many atoms thick is 1 nm? [Answer: ~7]
3. Consider a spherical nanoparticle of soot with a diameter of 20 nm. Assuming it to be mostly carbon with a density of 2.3 g/cm³, estimate the mass of the particle and the number of carbon atoms present. You will need the formula for volume of a sphere. [Answer: 9.6 x10^-21 g; 480 atoms]
4. Look up current prices for palladium and platinum. The amount of precious metals in a catalytic converter varies with size of vehicle and the environmental regulations applicable where the vehicle is operated. Ten years ago, this ranged from 2 – 15 g precious metal per catalytic converter. In today’s dollars, what would this metal be worth?
5. Within the last decade, nanoparticle technology has reduced the amount of precious metal used in catalytic converters by 70 - 90%. How many moles of metal are present in catalytic converter containing 2.0 g palladium and 1.5 g platinum? [0.019 mol Pd and 0.0077 mol Pt]
6. The catalytic beads shown in Fig. 3 contain 0.5% Pd by mass. The mass of the catalyst beads in this particular catalyst tube is 0.23 g. What mass of Pd is present in these beads? How many moles and how many palladium atoms are present in this catalyst tube? [0.0012 g Pd; 1.1 x 10^-5 mol Pd; 6.5 x 10¹⁸ atoms]
7. This article included many new terms, such as chemisorption, dispersion, nanotubes, and even cranberry glass. Look up interesting or unfamiliar terms online as you read the article.

 References

1. (a) G. Bond, Metal Catalysed Reactions of Hydrocarbons, Springer, 2005, page 98; (b) J.E. Lennard-Jones, Transactions Faraday Society, 1932, 28, page 333.

2. G. Bond, Reference 1(a) pages 93–98.

3. T. Mitsui, M.K. Rose, E. Fomin, D.F. Ogletree and M. Salmeron, Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies, Nature 2003, 422, pages 705–707.

4. M. Polanyi, J. Horiuti, Transactions Faraday Society, 1934, 30, page 1164.

5. Scientists have studied the reaction between ethene and deuterium, D₂, and found extensive hydrogen deuterium exchange, resulting in all isotopologues of C₂H_xD_6-x. See: B. Mattson, W. Foster, J. Greimann, T. Hoette, N. Le, A. Mirich,
   S. Wankum, A. Cabri, C. Reichenbacher and E. Schwanke, Journal of Chemical Education, 2013, 90 (5), pages 613–619.

6. Figure modified from Mattson, (Reference 5).

7. For perspective, a nanoparticle of palladium with a diameter of 100 nm would be over 350 Pd atoms across.

8. www.nature.com/subjects/nanoparticles

9. Encyclopaedia Britannia www.britannica.com/science/nanoparticle

10. G. Bond, Reference 1(a), pages 50 and 68

11. www.sciencedaily.com/terms/nanoparticle.htm

12. R. Marta, Quantum dots: Harnessing the nanoscopic rainbow, Chem 13 News, November 2017, pages 6-8.

13. B. Mattson, J. Fujita, R. Catahan, C. Cheng, J. Greimann, T. Hoette, P, Khandhar, A. Mattson, A. Rajani, P. Sullivan, R. Perkins, Demonstrating Heterogeneous Gas Phase Catalysis with the Gas Reaction Catalyst Tube, Journal of Chemical Education, 2003, 80, pages 768-773.

14. Details for the construction of catalyst tubes is provided in the Supplementary Materials to our recent article: Heterogeneous catalysis: deuterium exchange reactions of hydrogen and methane, A. Mirich, T. Hoette Miller, E. Klotz, B. Mattson, Journal of Chemical Education, 2015, 92, pages 2087–2093