CO hydrogenation over unsupported Ni model catalysts has been studied by chemical transient kinetics (CTK) to provide insight into the time-dependent surface processes leading to hydrocarbon formation at atmospheric pressures. Buildup and backward transients were triggered by stepwise changes of the CO flow into the reactor. CTK data have been evaluated, for the first time, to allow counting of the number of surface carbon, oxygen, and hydrogen atoms from the onset of catalytic reaction conditions to steady state. In this manner, it is shown that the total amount of these atoms may considerably exceed the monolayer limit on Ni metal. Back transients from CO/H 2 to pure H 2 show that the intermediates react in two steps, of which the second occurs with a common first-order decay time for all hydrocarbons (C 1 to C 4 , in this case). This is in agreement with a chain growth mechanism, in which the C 1 most abundant surface intermediate (masi) is always of the same type. Indications have been obtained that a CO insertion mechanism is in operation to form this masi.

Carbon dioxide hydrogenation on support-free nickel model catalysts was investigated by means of a time-resolved quantitative analysis of chemical transients triggered by abrupt changes in the reactant partial pressures. It was found that carbon dioxide adsorption is strongly affected by the presence of hydrogen and by coadsorption effects and thus influences the reaction rate in the buildup and back transients. The observed transients suggest that two reaction mechanisms operate in parallel, which is consistent with previous results obtained using a Ni singlecrystal termination. The initial reaction rate involves fast direct hydrogenation of CO 2 , whereas the rate under steady-state conditions is lower due to a change in the mechanism involving an oxygen-containing intermediate.

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Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) has been employed along with chemical and isotope transients to study the catalytic CO hydrogenation over Co/MgO catalysts in a single fixed-bed reactor at T = 523 K and ambient pressure conditions (H2/CO = 3). According to the operando DRIFTS measurements, the catalyst surface contains hydroxyl groups, adsorbed CO, formate, and methylene groups in the steady-state of the reaction. Transient experiments following fast changes in the feed (chemical transient kinetics, CTK) or isotope composition (steady-state isotopic transient kinetic analysis, SSITKA) have been carried out during DRIFTS and demonstrate that the formate/methylene “seen by DRIFTS” plays no role as imminent intermediates of the ambient pressure Fischer−Tropsch (FT) reaction. The SSITKA experiments (replacing 12CO by 13CO) show that the exchange rate of formate/methylene is significantly lower than that of ethane, which is one of the main reaction products of CO hydrogenation (followed by mass spectrometry). Formate is most probably bound as bidentate μ2-species to MgO or at the Co/MgO interface, while methylene stands for skeleton CH2 in either hydrocarbon or carboxylate.

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The aim of this paper is to demonstrate the importance of providing time-resolved information in catalysis research. Two truly in situ methods will be presented and compared for their merits and drawbacks: chemical transient kinetics (CTK) and pulsed field desorption mass spectrometry (PFDMS). The presentation will be given by way of example choosing the syngas (CO/H2) reaction over cobalt-based catalysts as a catalytic process. Despite numerous efforts in the past, the mechanism of this reaction is still under debate. In CTK the reaction is studied on a metal-supported catalyst under flow conditions in a pressure range extending from atmospheric pressure down to 100 Pa. Sudden changes in the partial pressures of the reactants then allow following the relaxation to either steady-state conditions ("transients") or cleanoff ("back transients"). In PFDMS short field pulses of several volts per nanometer are applied to a model catalyst which resembles a single metal particle grain (a "tip"). These pulses intervene during the ongoing reaction under flow conditions at pressures ranging from 10(-1) to 10(-5) Pa and cause field desorption of adsorbed species. This method is particularly suited to detect reaction intermediates in a time-dependent manner since the repetition frequency of the pulses can be systematically varied. It is shown that both methods lead to complementary results. While CTK allows conclusions on the mechanism of CO hydrogenation by following the time-dependent formation of hydrocarbon species, PFDMS provides insight into the initial steps leading to adsorbed CxHy species. A quantitative assessment of the CTK data allows the demonstration that the catalyst under working conditions is in an oxidized rather than metallic state. The initial steps to oxidation are also traced by PFDMS. Most importantly, however, CTK results allow formulation of a reaction mechanism that is common for both hydrocarbon and oxygenate formation and is based on the occurrence of a formate-type species as the most abundant surface intermediate.

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The hydrogenation of CO over Co model catalysts was studied using relaxation-type methods operating in situ either at atmospheric pressures or under surface science conditions. Emphasis was laid on providing information on the surface composition and on how it changes with time under catalytic reaction conditions. Using pressure forcing in chemical transient kinetics (CTK), the build-up of the steady-state was studied at 503 K and atmospheric pressure to demonstrate that the active catalyst surface is not metallic but covered with carbon, oxygen and hydrogen in excess of a monolayer equivalent. Both buildup and backward transients suggest CO to act as the ''monomer'' which probably inserts into an O-H bond to form the primary surface complex necessary for hydrocarbon and oxygenate formation. Repetitive electric field pulses (pulsed field desorption mass spectrometry, PFDMS) at low pressures have allowed the CO dissociation kinetics on a nano-sized Co 3D crystal (''tip'') to be monitored in the millisecond time range. No evidence for the occurrence of the Boudouard reaction was obtained in either PFDMS or CTK. Adsorbed CH x (x = 1-3) species were detected in small amounts demonstrating that CO dissociation is fast compared to carbon hydrogenation. Adsorbed Co-subcarbonyl species, Co(CO) x were also detected by PFDMS and possibly mediate the necessary surface mobility during the initial restructuring of the catalyst. Surface carbon seems to inhibit Co-subcarbonyl formation.