Oliver Korup

Reactor for in situ measurements of spatially resolved kinetic data in heterogeneous catalysis

The present work describes a reactor that allows in situ measurements of spatially resolved kinetic data in heterogeneous catalysis. The reactor design allows measurements up to temperatures of 1300 degrees C and 45 bar pressure, i.e., conditions of industrial relevance. The reactor involves reactants flowing through a solid catalyst bed containing a sampling capillary with a side sampling orifice through which a small fraction of the reacting fluid (gas or liquid) is transferred into an analytical device (e.g., mass spectrometer, gas chromatograph, high pressure liquid chromatograph) for quantitative analysis. The sampling capillary can be moved with microm resolution in or against flow direction to measure species profiles through the catalyst bed. Rotation of the sampling capillary allows averaging over several scan lines. The position of the sampling orifice is such that the capillary channel through the catalyst bed remains always occupied by the capillary preventing flow disturbance and fluid bypassing. The second function of the sampling capillary is to provide a well which can accommodate temperature probes such as a thermocouple or a pyrometer fiber. If a thermocouple is inserted in the sampling capillary and aligned with the sampling orifice fluid temperature profiles can be measured. A pyrometer fiber can be used to measure the temperature profile of the solid catalyst bed. Spatial profile measurements are demonstrated for methane oxidation on Pt and methane oxidative coupling on Li/MgO, both catalysts supported on reticulated alpha-Al(2)O(3) foam supports.

Resolving kinetics and dynamics of a catalytic reaction inside a fixed bed reactor by combined kinetic and spectroscopic profiling

The oxidative dehydrogenation of ethane to ethylene was studied using a MoO3 based catalyst supported on γ-alumina spheres. The measurement of species and temperature profiles through a fixed bed reactor shows for the first time the reaction pathways inside the catalyst bed directly. Oxidative dehydrogenation of ethane to ethylene and water occurs on the redox sites of MoO3 only in the presence of gas phase oxygen. Further oxidation of the product ethylene to carbon dioxide occurs as a subsequent reaction step by lattice oxygen of MoO3. Deep oxidation of ethylene to CO2 is the only existing reaction in the absence of gas phase oxygen reducing MoO3 to MoO2. Oxidation of CO and C2H6 by lattice oxygen does not occur. The reduction of the catalyst can be followed by in situ fiber Raman spectroscopy as a function of the oxygen partial pressure. The in situ Raman measurements are complemented by ex situ micro-Raman spectroscopy and X-ray diffraction. The combined measurement of kinetic and spectroscopic reactor profiles presents a novel approach in in situ catalysis research to establish catalyst structure–function relationships under technically relevant conditions of temperature and pressure.

Methane Oxidation on Platinum Catalysts Investigated by Spatially Resolved Methods

Catalytic partial oxidation (CPO) of methane is an attractive technology for industrial production of synthesis gas, an important precursor for the production of diverse basic chemicals, e.g. methanol, dimethyl ether, and formaldehyde. The exothermic reaction operates autothermally at temperatures around 1000 ◦C and gas hourly space velocity (GHSV) values up to 500000 h−1. On noble metal catalysts such as rhodium and platinum coated alumina foams, equilibrium synthesis gas yields are reached within millisecond contact time [1]. The CPO reaction proceeds in two steps along the catalyst bed. First a combination of direct methane partial oxidation coupled with methane deep oxidation is observed in the catalyst entrance section, where gas phase oxygen is present. After the oxygen is converted, product formation continues by a change in the reaction mechanism to steam reforming chemistry. Quantitative analysis reveals that the rates of oxidation and steam reforming are much lower on platinum than on rhodium coated foam catalysts. For rhodium catalysts sophisticated microkinetic models are available in literature, which can predict the reactant conversion and product formation with high accuracy [2–4]. These models allow a good understanding of the reaction mechanism and transport properties in rhodium coated foam monoliths. Within the last decade it became possible to validate the microkinetic models, due to the development of high resolution spatial profile measurement techniques, that can measure species and temperature gradients inside the catalyst foams. The pioneering work by Horn et al. [2,3,5–10], mainly focused on rhodium catalysts, is in this work extended to platinum catalysts. In a next generation reactor setup a set of reactor profiles was measured, systematically varying gas feed composition, contact time and reactor pressure. Besides foam monoliths, sphere beds and catalytic wall reactors have been tested. Microkinetic simulations applying a pseudo-2D heterogeneous reactor model that couples heat and mass transport limitations with detailed chemical kinetics of two different state-of-the-art microkinetic models taken from the literature have been used to simulate the experimentally measured reactor profiles through platinum coated foam monoliths. The reaction mechanisms predict species profiles considerably different from the measured profiles. By preand post-catalytic characterization of the catalyst by means of geometric, BET and platinum surface area, as well as metal dispersion and platinum crystallite size in combination with spatially resolved Raman spectroscopy and electron microscopy it was possible to identify significant metal redistribution and carbon formation on the catalyst surface as missing reaction pathways in the existing state-of-the-art microkinetic models. These findings are supported by in-situ Raman experiments on a polycrystalline platinum foil that follow the transition of the carbonaceous deposits with time on stream and reaction temperature [11]. The results presented in this thesis give new impulses for ongoing mechanism development.