Can isotopic transient and formal steady-state kinetics lead to a converging surface chemistry analysis? Case study on the selective reduction of NO by CH4 over Co-ZSM-5 catalysts

The surface coverage analysis derived from two formal steady-state kinetic models is compared to values directly obtained from steady-state isotopic transient analysis (SSITKA) for the selective catalytic reduction (SCR) of NO by CH₄ over Co-ZSM-5 catalysts. It is shown that the most abundant reacting intermediates are NO _x adspecies, though no clear differentiation between the various adspecies identified by DRIFT spectroscopy was achieved. Less numerous carbon containing adspecies were identified and quantified in the reacting system, essentially as methoxy species. Nitromethane-like intermediates remained undetectable due to a very rapid transformation into N₂ and CO₂. On the basis of these converging kinetic analyses related to each elementary step of the SCR process, a microkinetic model can be derived, which allows describing transient operation, in view of a non steady-state application.     

Mechanistic aspects of the water–gas shift reaction on alumina-supported noble metal catalysts: In situ DRIFTS and SSITKA-mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g⁻¹) of active intermediate species found in the carbon-path from CO to the CO₂ product gas (use of ¹³CO), and in the hydrogen-path from H₂O to the H₂ product gas (use of D₂O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area (θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g⁻¹) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO₂ and H₂ products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction.     

Steady-state isotopic transient kinetic analysis of steam reforming of methanol over Cu-based catalysts

Mechanistic aspects of steam reforming of methanol were studied via steady-state isotopic transient kinetic analysis over three copper-based catalysts, namely combustion-synthesized Cu-Ce-O and Cu-Mn-O, and commercial Cu-ZnO-Al₂O₃. The “C-path” and “O-path” for the production of CO₂ via steam reforming of methanol was analysed with the following step changes in the feed: ¹²CH₃OH/H₂O/Ar/He → ¹³CH₃OH/H₂O/He and CH₃OH/H₂¹⁶O/Ar/He → CH₃OH/H₂¹⁶O/H₂¹⁸O/He. The presence of CH₃¹⁸OH in the products after the switch to ¹⁸O-labeled water indicates that a major path of the reaction is the one involving a methyl formate intermediate. This appears to be the main path over the Cu-Mn-O catalyst, while parallel paths via dioxomethylene and methyl formate intermediates appear to be operative over Cu-Ce-O and Cu-ZnO-Al₂O₃ catalysts.     

The role of oxygen and hydroxyl support species on the mechanism of H2 production in the steam reforming of phenol over metal oxide-supported-Rh and -Fe catalysts

The aim of this work was to probe for the first time the back-spillover of labile O and OH species during steam reforming of phenol towards H₂ production over Rh and Fe supported on MgO, Mg-Ce-O and Mg-Ce-Zr-O metal oxides. This was made possible through SSITKA and other transient isotopic experiments (use of D₂O and ¹⁸O₂). The size of the active pool of H-containing species (H and/or OH) present under reaction conditions as a function of reaction T and catalyst composition, and deuterium kinetic isotope effects were measured.     

A modified commercial DRIFTS cell for kinetically relevant operando studies of heterogeneous catalytic reactions

This paper discusses a number of checks that should be carried out to ensure that the kinetic and spectroscopic measurements made using a DRIFTS cell are meaningful. The observations reported here demonstrate how an appropriately modified commercial DRIFTS cell can provide pertinent kinetic information about both gaseous products and the related surface intermediates. The oxidation of CO with O₂ was used as a test to assess the catalyst bed bypass by the reaction mixture. Full CO conversion was obtained after the light-off temperature in the case of the modified cell, contrary to the case of the original cell, for which 80% of the reaction mixture bypassed the catalyst bed. The water-gas shift reaction over a Pt/CeO₂ catalyst was used as a model reaction to further characterize the behavior of the cell under reaction conditions. The catalyst bed was shown not to be a dead-zone and was purged in essentially the same time as that needed to purge the cell. The reaction chamber globally operated in a quasi plug-flow mode and the gas composition in the thin catalyst bed appears to be homogeneous when operated under differential conditions. The production of the gas-phase reaction product CO₂ could be simultaneously followed both by mass spectrometry and DRIFTS, both techniques leading to identical results. Various IR bands integration methods were discussed to allow a precise and accurate determination of the surface concentration of adsorbates during isotopic exchange.     

Evidence of strong metal–oxide interactions in promoted Rh/SiO2 on CO hydrogenation: Analysis at the site level using SSITKA

It has been suggested that the behavior of Group VIII metal catalysts supported on transition metal oxides can be significantly affected by pretreatment conditions due to strong metal–oxide interactions (SMOI). However, the origins for the SMOI effect are still in debate. In this research, SMOI of Rh and vanadium oxide (as a promoter) supported on SiO₂ were studied at the site level for the first time, which provides an insight into the modification of surface properties after high temperature reduction. H₂ chemisorption, Fischer–Tropsch synthesis (FTS), and SSITKA (steady-state isotopic transient kinetic analysis) were used to probe the SMOI effects. The catalytic properties of the catalysts for CO hydrogenation were investigated using a differential fixed bed reactor at 230 °C and 1.8 atm, while for SSITKA, a reaction temperature of 280 °C and an excess of H₂ was used to maximize methane production. The addition of V to Rh/SiO₂ suppresses H₂ chemisorption, and high reduction temperature further decreases H₂ chemisorption on Rh/V/SiO₂ but has little effect on Rh/SiO₂. As reduction temperature increases, the activity for CO hydrogenation on Rh/SiO₂ remains essentially unchanged, but the activity of Rh/V/SiO₂ decreases significantly. SSITKA shows that the concentration of surface reaction intermediates decreases on Rh/V/SiO₂ as the reduction temperature increases, but the activities of the reaction sites increase. The results suggest that Rh being covered by VO_x species is probably the main reason for the decreased overall activity induced by high reduction temperature, but more active sites appear to be formed probably at the Rh–VO_x interface.     

Steady State Isotopic Transient Kinetic Analysis of Flameless Methane Combustion over Pd/Al2O3 and Pt/Al2O3 Catalysts

The SSITKA measurements were performed in the steady state of complete methane oxidation on the Pd/Al₂O₃ and Pt/Al₂O₃ catalysts. It was found that the number of intermediates and their average life-time on the catalyst surface changes with the increase of reaction temperature. On the Pd/Al₂O₃ catalyst there is larger number of active centres than on Pt/Al₂O₃ catalyst which permits the course of methane oxidation at lower temperatures.     

Fuel-rich methane combustion: Role of the Pt dispersion and oxygen mobility in a fluorite-like complex oxide support

For catalysts comprised of Pt supported onto dispersed complex fluorite-like oxides (ceria doped by Pr, Gd, Sm, or CeO₂–ZrO₂ doped by La, Gd or Pr), the effects of the oxygen mobility in supports and Pt dispersion on the performance in methane selective oxidation into syngas at short contact times were elucidated using combination of kinetic and spectroscopic methods. While in general any simple universal relation between the oxygen mobility, Pt dispersion and the rate of methane transformation into syngas was not found, for some series, a good correlation was observed agreeing with the bifunctional scheme of the methane selective oxidation into syngas.     

Fischer-Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium

An investigation of the CO hydrogenation of Pt- or Re-promoted 8.7 wt% Co/Al₂O₃ (1.0 wt% Pt or 1.0 wt% Re) has been carried out at two different conditions: 473 K, 5 bar, H₂/CO = 2 and 493 K, 1 bar, H₂/CO = 7.3. The addition of Pt or Re significantly increases the CO hydrogenation rate (based on weight of Co), but the selectivity was not changed by the presence of Pt or Re. The results show that the observed increases in the reaction rates are caused by increased reducibility and increased number of surface exposed Co-atoms. Steadystate isotopic transient kinetic analysis (SSITKA) with carbon tracing was used to decouple the effects of the concentration of active surface intermediates and the average site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concentration of active surface intermediates leading to CH₄ increased as a result of the addition of a noble metal promoter. However, the average site activity was not significantly affected upon Re or Pt addition.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

The effect of O₂ and N₂O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO_x species (0.79 V/nm²) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O₂ with N₂O. This is due to suppressing CO_x formation in the presence of N₂O. The most significant improving effect of N₂O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N₂O were derived from transient experiments using ¹⁸O₂ traces. ¹⁸O¹⁶O species were detected in ¹⁸O₂ and ¹⁸O₂–C₃H₈ transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O₂ strongly increased. Based on the distribution of labeled oxygen in CO_x and in O₂ as well as on the increased CO_x formation in sequential O₂–C₃H₈ experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O₂ are non-selective ones and responsible for CO_x formation. These species are not formed from N₂O.