From the clothes that we wear and the paints that cover our walls, to the fuel in our cars and the medications we take, catalysts greatly affect our everyday lives. Additionally, catalysts help us to protect the environment, since all the technology for abating air pollution and much of the technology for water pollution abatement stems from catalytic processes. The field of catalysis is an extremely fertile area of research, offering scientists the ability to create new reactions and processes that can have an impact in industry and beyond.

Here in the college, our researchers are at the forefront of this field, designing new catalytic systems to increase our understanding of fundamental principles and furthering both basic and applied research. Novel processes for creating drugs, organizing molecules precisely on solids, and tapping stranded fuel reserves—all are goals being pursued here through the development and optimization of catalysts. Moreover, with the scientific manipulation in the lab comes new knowledge of how catalysts work and how changes to the structure of the catalyst will affect both the reactions that a catalyst accelerates and the speed and selectivity at which they will occur.

However, before we can delve into the world of catalysis at Berkeley, a few terms need to be defined. To begin with, in chemistry, a catalyst is something that speeds up the rate of a chemical reaction while remaining unchanged itself. This acceleration is accomplished because the catalyzed reaction pathway between the reactants and the product requires less energy than the transformation would take without a catalyst. Reactivity refers to the proclivity of a given substrate to interact with a catalyst in a way that yields a product. Selectivity refers to the ability of a catalyst to produce a desired product with little or no other products created.

Additionally, researchers speak of homogeneous and heterogeneous catalysis. In homogeneous catalysis, the reactants and catalyst are all in the same phase (solid, liquid or gas), whereas in heterogeneous catalysis, the reactants and catalysts are in different phases. In the latter systems, the catalysts provide a surface on which the chemical reaction takes place. In order for the reaction to occur in a heterogeneous system, the reactants must absorb onto the surface and interact with the catalytic site, and then the products must detach from the surface and diffuse away. Understanding what controls the transport of the molecules to and from the surface is an important area of heterogeneous catalyst research, as we shall see.

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Transforming Natural Gas

One area in which catalysts have the potential for an enormous impact is that of energy generation. As the oil reserves on the earth are decreasing and global warming is increasing, catalysts are being called upon to meet strategic needs for cleaner fuel. In 2000, members of the Department of Chemical Engineering entered into a ten-year collaboration with BP to find new ways of transforming methane gas, the principal component of natural gas, into a liquid that can be easily transported and used for making such products as fuel and synthetic polymers. Methane, which has the chemical composition of CH₄, is the shortest, lightest hydrocarbon molecule and is far more widely distributed throughout the world than is oil. However, many natural gas reserves are considered stranded and are untapped because they are located either in a remote region or in an area where the demand for energy near the reserves is already being met and exporting the gas to a location where it could be consumed is prohibitively expensive.

Currently, natural gas is converted to useful products via an intermediate known as synthesis gas, a nebulous mixture of carbon monoxide and hydrogen. Seventy percent of the cost of converting methane goes into its transformation into the synthesis gas, which involves numerous processes and intermediates, according to chemical engineering professor Enrique Iglesia, whose group is developing new ways to integrate these processes efficiently. "The idea behind this effort is that methane (CH₄) has more hydrogen and less carbon than oil (CH_1.6), so it should be a cleaner source of fuel," he noted. "We hope these projects, if adopted by industry, will eventually have a significant impact on global warming."

Iglesia and his colleagues have worked on the direct and indirect conversion of methane to higher hydrocarbons, and they have created catalytically active nanostructures as well as isolated single-site catalysts within solids that contain pores of various sizes. Their projects currently include the design of membrane and microchannel reactors as well as catalysts for the synthesis of hydrocarbons and H₂ from natural gas. They are also developing methods to characterize the local structure and atomic connectivity in these inorganic solids and, in many instances, are able to characterize what occurs during the catalytic reactions themselves. By using high-tech kinetic methods as well as isotopically labeled reactants, they can elucidate the mechanism of catalysis reactions on surfaces, at the level of both primary and secondary reaction networks and of elementary surface steps. "These research projects take advantage of the outstanding research tools that we have in the college," said Igelsia.

The goals of his research are to determine and model the principles behind catalytic processes relevant to oil refining and petrochemical synthesis. "We want to know how structure in and transport to the catalyst affect the reaction rate," he added. "The catalysis field is full of unknowns, and we are in the unique position to answer many questions about the materials."

Another approach to decreasing the expense of creating products from methane through intermediates is to skip the intermediates altogether and convert methane into a useful product directly as the natural gas is extracted. This tactic is one being taken by chemical engineering professor and chair Alex Bell.

Bell's group is studying the direct conversion of methane gas to liquid formaldehyde, an important polymer building block used in the manufacturing of such products as Formica, plywood and paints. Formaldehyde can be prepared from the methanol that results from the oxidation of methane, but Bell is working on ways to bypass the methanol intermediate. To achieve this, his group has developed both the catalyst and the processing steps to convert methane and oxygen to acetic acid, a product that can be shipped and eventually transformed into acetate for use in polymers and other goods.

"The reaction occurs with palladium in a sulfuric acid solution," explained Bell. "We have determined the mechanism and found that this process seems to be unique to this combination of materials." His lab is now working toward making this homogeneous reaction more heterogeneous. "We would like to transfer it into solid so that separation of the reactants from the products can easily take place."

Spectroscopic techniques are an essential part of Bell's work, and he is well known for developing innovative tools to monitor reactions. "They give us a set of eyes to see the intermediates and products of catalyzed reactions, as well as the structure of catalysts themselves," he said. The spectroscopic techniques Bell and his colleagues have developed are unusual, probing not just starting materials and end products, but the action in between as well.

"It's the equivalent of watching bread bake and understanding what actually happens during that process," he has said, "versus just seeing the dough before and the bread after."

Bell also has a strong collaboration with Don Tilley. "We are working together on heterogeneous catalysis methods to incorporate iron onto solid supports to perform selective partial oxidation reactions to achieve such transformations as benzene into phenol or alcohols into aldehdyes," he noted.

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Water to Produce Hydrogen

Although natural gas is widely regarded as the current likely successor to oil, it is also a nonrenewable resource. Researchers worldwide are working to find an economical and sustainable source of power. One such scientist researching catalysts that can further the green revolution is chemistry professor Christopher Chang. "A promising solution to growing energy needs is the hydrogen economy," noted Chang. "But currently almost all of the hydrogen that we produce is by steam reformulation of natural gas, which is not sustainable or carbon-neutral." Ideally, researchers would like to produce hydrogen from abundant and renewable sources, and the hydrogen contained in water could be one of the most promising energy sources for the future. Unlike fossil fuels, it is highly efficient, low-polluting and is transportable so it can be used for power generation in far-off regions.

Water is an obvious source for hydrogen, but the only known practical method to split the water molecule, electrolysis, costs much more than both natural gas and gasoline. "My group is studying the underpinnings of how to make catalysts that lower the energy barrier to splitting water," said Chang. This is quite a challenge, since it currently takes more energy to perform this reaction than is economically feasible. "However, by combining catalysts with a solar energy component so that we can harness sunlight, which is essentially free, to drive the reaction, we hope ultimately to produce usable energy from water splitting," Chang explained.

Chang's projects in catalysis involve learning how to control oxygen, electron and proton flow. By synthesizing chemical intermediates of water splitting, his group is working to better understand how to build an effective catalyst for this reaction. "Our goal is to develop a hydrogen-producing catalyst that will be clean-burning with no release of carbon dioxide (a major greenhouse gas)," said Chang. "We are building the conceptual framework for how to put all the pieces of this puzzle together and are making progress, but splitting water cheaply is still in the future."

Chang finds his inspiration for this research in nature. "In the natural world, both electron flow and proton flow are used for energy storage and release. Photosynthesis is nature's paradigm of solar-to-fuel and provides a springboard of ideas for how to make this happen in synthetic systems," he said.