By Dr. Russell Chianelli, University of Texas at El Paso, Materials Research and Technology Institute
Catalytic materials have played a crucial role in providing fuels, commodities, and fine chemicals for more than 100 years. The role of these materials is going to become more important than ever in the future. Catalytic materials will provide fuels for environmentally friendly transportation vehicles, as well as materials for these vehicles, well into the future. Today, more than 60% of all chemical products and 90% of all chemical processes are catalytic processes. We define catalytic materials, such as the TMS (transition metal sulfide) catalysts (described below), as solid-state materials with the recognition that many catalysts are inorganic molecules or biological molecules. A full description of catalysts and catalytic materials and the breadth of the field can be found in the Encyclopedia of Catalysis, which includes an article by the author of this report.1 Catalytic materials are the catalysts, solids in the case of heterogeneous catalytic processes, that are optimized for activity and selectivity in catalytic processes. The process of optimizing the activity and selectivity through synthetic techniques is called structure/function optimization. This process controls the surface chemistry of the catalytic material and has been used since the dawn of catalytic processes. Today, it is called optimization of nanomaterials.
An example of catalytic materials that will continue to be important in the future is the transition metal sulphides (TMSs). These catalysts have been mainstays of hydro-processing fuels (upgrading fuel quality and removing pollutants) and upgrading bitumen and coal for more than 100 years. Every refinery in the world uses them every day to remove sulphur and other pollutants from transportation liquid fuels. As environmental regulations increase the need for improved active and selective TMSs, refineries currently are using Co/Mo/Al2O3 or Ni/W/Al2O3 or combinations of both. The active materials are sulphides, such as WS2, MoS2, Ni3S2, and Co9S8, while Al2O3 is not part of the catalytic system. In addition, Co and Ni sulfides act as promoters of the M and W sulfides, increasing their activities by as much as a factor of 10. These are the so-called “synergic pairs.” These synergic pairs result from the interfacial coordination between the two solid-state phases, for example Co9S8 and MoS2. A cluster of Co/Mo/S atoms exist at the interface, which through d-electron donation cause the Mo to mimic a noble metal catalytic material.
It is also crucial to remember that for a real understanding to develop, we must study the catalytically stabilized materials and not materials that are changing under catalytic conditions. In the case of the TMSs, this means that we must study catalytic materials such as MoS2–xCx and RuS2–xCx. It has been demonstrated that “surface carbides” are the catalytically stabilized state under hydro-treating conditions. This progress has been made by combining synthetic, experimental, and theoretical techniques, and thecombination of these techniques will be required to develop the catalytic materials of the future, which are crucial to transportation energy in the future.2
Finally, a few years ago, I was invited to speak at the Santa Fe Institute on the “Origins of Life.” Why? Because the molecules necessary for life are created on deep sea vents at the high pressures that occur at the deepest part of the oceans. Such a vent is shown below.
The black smoke, the Black Smoker, emerging from the vent is an iron sulfide catalytic material that is producing compounds necessary for life from indigenous gases (methane, CO). It is thought that similar reactions may occur on distant planets. Thus catalysis by the TMS is central to the search for life on other planets. When I presented at the “Origins of Life Conference,” I started by saying, “I don’t know anything about the “Origins of Life,” but I do know that my life began on the transition metal sulfides.
- Encyclopedia of Catalysis, John Wiley and Sons Inc. (2010).
- R.R. Chianelli, G. Berhault, B. Torres, "Unsupported Transition Metal Sulfide Catalysts: 100 years of Science and Application," Catalysis Today 147, 275 (2009).