Abstract
Atmospheric pressure reactions on model catalysts are typically performed in so-called high-pressure cells, with product analysis performed by gas chromatography (GC) or mass spectrometry (MS). However, in most cases, these cells have a large volume (liters) so that the reactions on catalysts with only cm2 surface area can be carried out only in the (recirculated) batch mode to accumulate sufficient product amounts. Herein, we describe a novel small-volume (milliliters) catalytic reactor that enables kinetic studies under atmospheric pressure flow conditions. The cell is located inside an ultrahigh vacuum chamber that is deliberately limited to basic functions. Model catalyst samples are mounted inside the reactor cell, which is locked to an oven for external heating and closed by using an extendable/retractable gas dosing tube. Reactant and product analyses are performed by both micro-GC and MS. The functionality of the new design is demonstrated by catalytic ethylene (C2H4) hydrogenation on polycrystalline Pt and Pd foils.
Highlights
Much of the fundamental molecular-level understanding of heterogeneous catalysis originates from studies of well-defined model systems in ultrahigh vacuum (UHV).1,2 Starting in the 1960s, investigations of noble metal single crystal surfaces of different crystallographic orientations directly revealed the structure-sensitivity of gas adsorption, co-adsorption, and reactivity
UHV conditions guarantee the cleanliness of the surfaces and are required for many of the typical surface-sensitive methods such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature programmed desorption (TPD), X-ray/UV photoelectron spectroscopy (XPS/UPS), electron energy loss spectroscopy (EELS), and others
The basic principle is that the reactor compartment is located inside a UHV chamber, but is sealed-off for catalytic reactions; this is similar to the original “Somorjai design”, but includes several improvements
Summary
Much of the fundamental molecular-level understanding of heterogeneous catalysis originates from studies of well-defined model systems in ultrahigh vacuum (UHV). Starting in the 1960s, investigations of noble metal single crystal surfaces of different crystallographic orientations directly revealed the structure-sensitivity of gas adsorption, co-adsorption, and reactivity. At high temperatures of catalytic reactions, coverages are typically very low in UHV, whereas technological catalytic reactions are carried out at ∼10 orders of magnitude higher pressure These very different conditions may change the chemical state of the catalysts under reaction conditions (e.g., by oxidation, hydride formation, coking, restructuring, etc.). Apart from pre- and post-reaction analyses, some of these high-pressure cells enable in situ surface spectroscopy ( operando, as catalytic performance is acquired simultaneously).34–39 Instead of “blocking” a complex setup by time-consuming kinetic tests, spectroscopic studies are just performed for the most promising catalysts in other instruments.40,41 All these considerations motivated the development of a small-volume reactor cell for atmospheric pressure reaction studies on model catalysts under flow conditions Instead of “blocking” a complex setup by time-consuming kinetic tests, spectroscopic studies are just performed for the most promising catalysts in other instruments. All these considerations motivated the development of a small-volume reactor cell for atmospheric pressure reaction studies on model catalysts under flow conditions
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