Catalysts are materials that are added to reaction systems to increase the rate of reaction. Because catalysts are not consumed in the reaction, they are utilized in many industrial-scale processes. Catalysts increase reaction rate by providing an alternative mechanistic pathway with a lower activation energy, which is the minimum energy required for a reaction to proceed. Catalytic reactions can be either homogeneous, meaning the catalyst and reactants are in the same phase, or heterogeneous, meaning the catalyst and reactants are in different phases. Typically, heterogeneous catalysts are solid, nano-scale entities dispersed on a support material. The catalytic reaction begins with adsorption of the reactants to active site on the nano-particle surface, followed by the reaction, and then desorption of the products. Reactors using heterogeneous catalysts are central to several industrial-scale processes, such as the production of hydrocarbon fuels. This video illustrates the principles of heterogeneous catalysts and catalytic reactors, demonstrates a pilot-scale catalyzed ethylene hydrogenation process, and discusses some applications.
The most common heterogeneous catalysts for gas-phase reactions are transition metal nanocrystals. They frequently operate by the Langmuir-Hinshelwood Mechanism. This hypothetical mechanism begins with the reactants adsorbing to the catalyst's surface, delocalizing their bonding electrons into the empty orbitals of the transition metal atoms and often dissociating in the process. Several intermediate steps may follow, but the mechanism concludes with a biomolecular elementary reaction that forms the products which then desorb from the catalyst's surface. The overall reaction rate depends on the rate of the slowest elementary step, which may be expressed in terms of the equilibrium constants of the other elementary steps. Importantly the rate depends on the availability of catalytic reactive site, and catalysts are fabricated and supported to maximize surface area. Now that you know how a catalyst works at the atomic level let's see how it is used in a pilot reactor setup.
For this reaction we use a tubular plug flow reactor in which reactants are continuously added and products continuously withdrawn. The reactor is a steel tube packed with catalyst, it is contained in an electrically heated temperature controlled sand bath. The reactants are piped to a constant pressure mixing tee before entering the reactor. The reactor effluent may be vented or input to a gas chromatograph for analysis. Flow rates are monitored using bubble meters and rotameters. For safety, diluted reactants, high pressure relief valves, high temperature shut-down, bypass and venting systems, and a combustible gas leak-detector are installed. This system is appropriately modeled as a plug-flow reactor since high conversions and spatially variable reaction rates are expected. Applying this model to the reactor effluent data yields a power law rate expression for the catalyzed reaction. This expression provides a wealth of information about the catalyst and provides experimental evidence for the Langmuir-Hinshelwood Mechanism. Those are the principles, now let's demonstrate a catalytic reactor in the laboratory.
In this demonstration ethylene is hydrogenated in a catalytic reactor to form ethane. The reactor is packed with nickel catalyst on a silica support, as well as an inert silicone carbide filler to ensure mixing. Start the gas chromatograph with the valve on the sample injection port closed. Load the appropriate methods. Then allow the instrumental parameters to equilibrate to their required set points. Switch on the air flow to the sand bath, then ensure the rotameter consistently reads between five and seven over the course of the procedure. Switch the reactant lines to pass through the bubble meter and begin gas flow. Use the bubble meter to set the initial reactant-flow rate. Subsequent flow rate adjustments do not need to be measured through the bubble meter. Next, check the reactant composition by flowing the reactants through the reactor bypass and into the gas chromatograph inlet port.
The reactant gases may now be input to the catalytic reactor. Turn on the sand bath heater and enter the temperature set-point. Auxiliary heaters may be used with automatic temperature control, but when the set-point is reached they must be switched off manually as they are not governed by the control system. When the temperature stabilizes at the set-point, continue feeding the reactants into the reactor. Monitor product flow-rate and temperature regularly. If product flow stops, or temperature run-away occurs, quickly close the reactant valves and shut off all heaters, but maintain air-flow to the sand bath. Take samples of the gaseous product regularly for gas chromatography. The method generates sequential chromatograms of the sample components including any unreacted gas. Shut down the reactor by pressing the emergency stop button on the sand bath temperature controller. Set the ethylene flow controller to zero percent and close all valves. Wait two minutes before doing the same for the hydrogen. Maintain air-flow to the sand bath until it reaches room temperature.
The reaction is conducted under both ethylene-limiting, and hydrogen-limiting conditions. Ethane production is measured at different reactant concentrations through gas chromatography. This raw data is converted into fractional conversions and reactant space times. The data is regressed to trial power-law rate expressions and the best fit selected. In this case, the best rate expression includes ethylene concentration raised to the first power, and hydrogen concentration to the one-quarter power. This expression indicates that hydrogen adsorbs strongly to the catalyst, while ethylene adsorbs weakly. It is consistent kinetically controlled Langmuir-Hinshelwood Mechanism.
Pilot-scale catalytic reactors are usually industrially to study the effects of catalysts and reaction conditions on chemical synthesis. The Fischer-Tropsch Synthesis produces alkane and alkane fuels from carbon monoxide and hydrogen. Industrially, it is often performed in a fixed-bed reactor using iron, cobalt, or ruthenium catalysts. The reaction is not highly selected and generates multiple products through a variety of reaction pathways that vary based on the choice of catalyst. Basic research continues into its mechanism and product composition. The Haber-Bosch Process uses hydrogen and nitrogen to produce ammonia, most importantly for artificial fertilizers. It is an energy intensive process catalyzed by iron, or less frequently, cobalt molybdenum alloys. Research is proceeding into long life-span, high-selectivity catalysts to increase nitrogen adsorbtion rates without deactivating, thus reducing the necessary pressure. This process has historically spurred the theory and modeling of heterogeneous catalysis, and is now the subject of computational catalyst design.
You've just watched JoVE's introduction to heterogeneous catalyzed reactors. You should now be familiar with basic mechanism of heterogeneous catalysis, a procedure for operating a pilot-scale reactor and some applications. As always, thanks for watching.