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Heterogeneous Catalysis

In the chemical and energy industries, heterogeneous catalysis plays an important role and will be driving the transition to their ensuing carbon neutral operation as the central science. The ultimate goal of the world’s scientists is the development of active and robust catalytic processes with new modification which gives a new direction to catalysis. Ideally, for critical catalytic processes, materials can be designed to develop the ability to self-regenerate active sites. The most energy-intensive processes depend on the catalysis, especially on heterogeneous catalysis. Hence, it would be of great help in energy-intensive processes by the development of efficient heterogeneous catalysts which reduces energy consumption and environmental waste. There are various metals based heterogeneous catalysts well known which are used for the synthesis of various compounds. This catalysis has opened a versatile area of research. The catalysis of Gold, Cerium, Platamium, Palladium, Nickel, and so on nanoparticles have been reported in over 5000 research, review articles.

Cerium is the most abundant metal of rare-earth elements. It can be utilized in various fields of application, such as phosphors, alloys, magnetics, catalysis, catalytic converters, and gas mantles. China is the biggest producer of rare-earth elements in the last decades. The price of cerium has consistently dropped and reached the cost of about 6.0 USD/Kg in December 2015. Even the price of cerium oxide is cheaper than lanthanum oxide. It can be extracted from its ores. Cerium shows +4, +3, +2, +1 oxidation states but +4 is the most stable oxidation state. It is biologically inactive as well as non-toxic for humans. Cerium oxide (Ceria or CeO2) and ceria-based material have been explored in various applications in academia and industries such as catalysts, pharmaceuticals, electrochemistry, sensor, and so on. Cerium-based materials are most commonly used in heterogeneous catalysis.

Recently, the world’s scientists gave two major directions to boost the applications of ceria-based materials in catalysis. Firstly, the increase of surface area and the enhancement of its thermal stability was a mandatory target for application in car converters, which happens to be the most relevant industrial application of ceria. Formation of solid solutions with transition and/or rare earth metals, zirconium, in particular, gained this goal, with the additional benefit to boost also redox properties as the introduction of cations with different ionic radii into the ceria lattice induces structural disorder that facilitates oxygen release. These successful approaches allowed for the development of closed coupled catalysts (CCC) that can cope with a working temperature higher than 1000 °C, making modern ceria-zirconia-based TWCs the most relevant example of environmental catalysts. This application is mature, the materials are mostly optimized and widely adopted in current car converters. Improvements are essentially limited to the reduction of the number of active components and to incorporate changes in formulation needed to cope with fluctuation in cost and availability. However, still, intriguing is the ongoing debate on the importance of compositional phase homogeneity of these doped/nanocomposites materials. Palladium-cerium based materials have wide applications in organic syntheses, such as cross-coupling reaction, oxidation, hydrogenation, methane activation, etc.

Secondly, the advent of nanotechnology allowed for the obtainment of well-controlled nanomaterials in shape and size. Despite some limitations due to the thermal instability of nanostructures in general, this opened the perspective of understanding the different reactivity of exposed facets, and in combination with today’s state-of-the-art computational simulations allowed for a clear correlation of structural parameters and reactivity. This approach has proven to be successful in demonstrating how OSC depends on nanostructure, the surface dependence of soot combustion, or the effect of ceria crystal plane on CO and propane oxidation. Impressive catalytic improvements have been achieved by modulating metal-support interaction in ceria-based systems, opening new directions for boosting existing materials. Very sensitive in the case of gold supported on ceria-based systems, in which CO chemisorbed capability of gold was significantly perturbed by mild reduction treatments.

Before I get into the research, I just have to say that I think catalysts are cool.  Catalyst is nothing but everything, it just promotes the reaction without involving. Various thoughts came into my mind about catalysts, their structure, activity, and so on. After a deep literature survey, I found that it is really interesting as well as an important topic in chemistry. Lots of organic syntheses have been reported in the literature using hazards, chemicals, high temperatures, high pressure, homogenous bases. These drawbacks were eliminated by using heterogeneous catalysts. We have a step up laboratory in the Department of Chemistry, SGT university where you can learn about heterogeneous catalysis using various metal-based catalysts and their application in organic transformations. We are working on the synthesis of organic molecules using cerium-based materials as heterogeneous catalysts in the green solvent.

Dr. Ravi Tomar
Assistant Professor
Department of Chemistry
Faculty of Science
SGT University
Gurugram, India