The possible existence of multiplicity features, including ignition and extinction phenomena, is examined in ethylene hydrogenation over a very short fixed bed of Pt/boehmite catalysts. Ethylene hydrogenation is compared with CO oxidation for which it has been claimed in several studies that multiple steady states can occur within the kinetic region.
In our experiments an important experimental parameter is the size of the catalyst charge in the reactor. If the multiplicity features were observed above certain size of catalyst charge but not observed below that charge for identical experimental conditions, the observed multiplicity cannot be attributed to a purely kinetic process, but must be explained by invoking other processes. The examination for multiplicity is carried out by increasing and decreasing an inlet gas temperature. The experimental results show that even though multiplicity with anticlockwise hysteresis was found with any catalyst charge, below a limiting catalyst charge no multiple steady states containing ignition and extinction points were obtained, but clockwise hysteresis was obtained probably due to the deactivation of catalysts. In contrast to the case of ethylene hydrogenation the multiplicity existed for CO oxidation regardless of the catalyst quantity. The multiplicity for ethylene hydrogenation requires nonisothermal conditions that result from strong external heat transfer limitations from the catalyst bed to the surrounding, whereas multiplicity for CO oxidation is observed even under nearly isothermal condition. From our observations, it is concluded that the multiplicity in ethylene hydrogenation cannot be attributed to a purely kinetic process involving only adsorption, desorption and surface reaction steps, but it arises from the interaction of the chemical reaction and the heat transfer process. On the other hand, as shown earlier in several studies, it is confirmed once more using our experimental method that the multiplicity for CO oxidation can exist within a kinetic region.
Both theoretical and numerical analysis show that multiplicity for the ethylene hydrogenation with Horiuti-Polanyi mechanism requires conditions for a strong chemisorption of ethylene so that one ethylene molecular occupies less than two active sites of catalyst, whereas multiplicity features is not obtained when an ethylene molecule adsorbes on more than two active sites. From our experiments chemisorption of ethylene requires geometrically 3-4 active sites based on a dissociatively adsorbed hydrogen atom. Therefore theoretical and numerical analysis can well surpport experimental results that the existence of kinetically induced multiplicity is not possible for ethylene hydrogenation. On the other hand, the kinetically induced multiplicity for CO oxidation will arise from the fact that carbon monoxide adsorbs more strongly than oxygen, chemisorption of carbon monoxide requires less than two active sites and CO oxidation proceeds via Langmuir-Hinshelwood mechanism with dissociatively adsorbed oxygen and competitive adsorption of the two reactant species.
From our studies, it is found that the geometrically occupied active sites of adsorbed reactant exert an important influence on the existence of the multiplicity features in the reaction proceeding via Langmuir-Hinshelwood mechanism. Therefore it is believed that existence of multiplicity must be studied by examining the number of geometrically occupied active sites rather than bonding numbers with active sites, especially for the reaction with Langmuir-Hinshelwood type mechanism. It may be worthwhile to apply our experimental and analytical methods to study possible existence of multiplicity in several other olefin hydrogenations.