Isotopic Transient Kinetic Analysis Research Articles

Transient In situ DRIFTS Investigation of CO2 Hydrogenation to Methanol over Unsupported CuGa Catalysts

AbstractGallium oxide‐based catalysts are promising candidates for CO2 hydrogenation to methanol, whilst mechanistic understanding of the catalytic systems is not yet fully understood. Here, transient in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies of CO2 hydrogenation over the unsupported CuGa and CuGaN (with N doping) catalysts were conducted. The findings show that N doping could possibly promote the formation of surface Cu+ sites on the interface between Cu and Ga2O3, and hence increasing the selectivity to methanol. However, N doping prohibited the reduction of Ga2O3 to oxygen‐deficient Ga2O3‐x, which is unfavorable for CO2 conversion. Additionally, during DRIFTS the steady‐state isotope transient kinetic analysis (SSITKA) was performed (under H2/D2 isotopic switching conditions). The SSITKA results confirm that the Cu+‐bound formates were likely the key intermediates for methanol synthesis over the catalyst, and the N doping could also weak the interaction between CO* species and the catalyst surface, and thus facilitating the selectivity towards methanol rather than CO. Findings of the work show that the partial nitridation of the metal oxides could be the strategy to promote methanol synthesis selectivity.

Exploiting the continuity equation for mechanistic understanding through spatially resolved SSITKA-DRIFTS: The role of carbonyls in RWGS over Pt/CeO2

Insight into mechanisms of heterogeneously catalyzed reactions holds importance in the development and optimization of new catalytic materials. Yet, the approaches often used in such investigations heavily rely on assumptions concerning the reactor and kinetics. Herein we report a new kind of kinetic investigation taking CO2 hydrogenation reaction, specifically the reverse water–gas shift (RWGS) reaction over 3 wt% Pt/CeO2, as an exemplifying case. The reported approach is based on spatially resolved steady-state isotopic transient kinetic analysis (SSITKA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identifying gaseous/surface species and their spatial variations along the reactor. This approach allows accurate evaluation of reaction mechanism by identifying correlations among the concentrations of gaseous/surface species and by quantitative description of their spatial variations by a kinetic model. Spatially resolved SSITKA-DRIFTS experiments show carbonate decomposition via a Pt-bound carbonyl to be the main route towards the production of carbon monoxide. Further kinetic modeling of the spatially resolved data confirms this mechanism proposal, and points to the production of water as the rate-limiting step.

Progress through Temporal Analysis of Products and Steady‐state Isotopic Transient Kinetic Analysis to Elucidate Oxidation, CO2 Hydrogenation and Lower Olefins Production Reactions

A prerequisite for the development or optimization of heterogenous catalysts is a comprehensive understanding of the reaction mechanism and kinetics at the level as elementary as possible. In comparison with steady‐state catalytic tests typically used for this purpose, non‐steady‐state (transient) studies provide deeper insights due to higher time resolution. For several decades, temporal analysis of products (TAP) and steady‐state isotopic transient kinetic analysis (SSITKA) have been successfully applied to the analysis of various heterogenous reactions. In this review, we highlight the contributions made by TAP and SSITKA studies between 2017 and 2023 to the understanding of reaction mechanisms and to the derivation of microkinetics. The versatility of these methods is illustrated by the examples of oxidative coupling of methane, CO/CO2 hydrogenation to hydrocarbons or methanol, metathesis of ethylene with 2‐butenes to propene, acrolein oxidation to acrylic acid, methanol conversion to olefins and non‐oxidative propane dehydrogenation to propene. We also provide our personal views on possible developments in this area.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8H0.2 Catalyst for CO2 Hydrogenation to Methanol

AbstractTuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal‐support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3‐fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High‐pressure steady‐state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3. An operando high‐pressure diffuse reflectance infrared spectroscopy (DRIFTS)‐SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate‐limiting. However, the catalytic reactivity of Cu/BaTiO2.8H0.2 towards the formate species is much higher than Cu/BaTiO3, likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8 H0.2 Catalyst for CO2 Hydrogenation to Methanol.

Tuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal-support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3-fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8 H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8 H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High-pressure steady-state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8 H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3 . An operando high-pressure diffuse reflectance infrared spectroscopy (DRIFTS)-SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate-limiting. However, the catalytic reactivity of Cu/BaTiO2.8 H0.2 towards the formate species is much higher than Cu/BaTiO3 , likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Performance Descriptors for Catalysts Based on Molybdenum, Tungsten, or Rhenium Oxides for Metathesis of Ethylene with 2-Butenes to Propene.

The metathesis of ethylene with 2-butenes to propene is an established large-scale process. However, the fundamentals behind in situ transformation of supported WOx , MoOx , or ReOx species into catalytically active metal-carbenes and the intrinsic activity of the latter as well as the role of metathesis-inactive cocatalysts are still unsolved. This is detrimental for catalyst development and process optimization. In this study, we provide the required essentials derived from steady-state isotopic transient kinetic analysis. For the first time, the steady-state concentration, the lifetime, and the intrinsic reactivity of metal carbenes were determined. The obtained results can be directly used for the design and the preparation of metathesis-active catalysts and cocatalysts, thereby opening up possibilities for optimizing propene productivity.

Understanding of the Fate of α-Fe2O3 in CO2 Hydrogenation through Combined Time-Resolved In Situ Characterization and Microkinetic Analysis

CO2 hydrogenation over Fe-based catalysts is a promising pathway to mitigate emissions of this greenhouse gas and provides a possibility for crude-oil-free production of chemicals and fuels. Understanding of (i) the role of crystalline phases or/and surface species in the working catalysts, (ii) the factors affecting their formation under reaction conditions, and (iii) the kind and reactivity of surface precursors of gas-phase products is vital for controlling the efficiency of CO2 hydrogenation. In this study, we applied time-resolved in situ characterization techniques for monitoring phase transformations during Fe2O3 reduction, starting of CO2 hydrogenation, steady-state operation, and finally catalyst deactivation. The obtained structural information after different times on stream was related to kinetic data obtained from temporal analysis of H2 and CO2 activation as well as from steady-state isotopic transient kinetic analysis (SSITKA). Fe2O3 is easily reduced to Fe3O4 and Fe in H2 above 300 °C. Fe5C2 and Fe3C, which are quickly formed from metallic Fe/Fe3O4 under CO2 hydrogenation conditions, do not undergo oxidation with rising time on the reaction stream under ambient-pressure conditions. Nevertheless, the catalyst loses its initial activity and, particularly, the selectivity to hydrocarbons in favor of CO. Thus, we do not confirm the well-recognized deactivation mechanism of CO/CO2 hydrogenation through oxidation of iron carbides. Instead, surface carbonaceous species identified by in situ Raman and pseudo in situ XPS measurements were concluded to cause catalyst deactivation and deselectivation due to hindering the catalyst ability to generate surface species from H2 and CO2. Specifically, the strength of CO2 adsorption and the catalyst activity to dissociate adsorbed CO2 to adsorbed CO decrease in the presence of carbon deposits. Kinetic evaluation of SSITKA tests revealed the presence of (i) only one kind of surface intermediate yielding gas-phase CO after 1.5 h on reaction stream but (ii) at least two kinds (short-lived and long-lived) of surface intermediates participating in CH4 formation in parallel. Carbon deposits seem to block the sites responsible for the formation of short-lived intermediates.

Recent advances in the use of Steady-State Isotopic Transient Kinetic Analysis data in (micro)kinetic modeling for catalyst and process design

Recent advances in the use of Steady-State Isotopic Transient Kinetic Analysis data in (micro)kinetic modeling for catalyst and process design

Insight into the true role of hydrogen‑carbonate species in CO oxidation over Pd/Al2O3 catalyst using SSITKA-transmission IR technique

The powerful combination of steady state isotopic transient kinetic analysis (SSITKA) and operando infrared spectroscopy (IR) was applied to study the CO oxidation reaction on Pd-supported Al2O3 catalyst at low temperature. The aim was to reveal the true role of hydrogen‑carbonate species in the reaction mechanism. The SSITKA-IR experiments, conducted in presence and absence of CO2 in the gas feed, have confirmed that hydrogen‑carbonate species behave as spectator species in the reaction. Its formation was due to re-adsorption of the CO2 product itself on the alumina surface as confirmed by the direct 12CO2/13CO2 exchange observed on the bare support.

Dynamics of Co/Co2C redox cycle and their catalytic consequences in Fischer–Tropsch synthesis on cobalt–manganese catalysts

Fischer–Tropsch synthesis over CoMn oxides is a surface-catalyzed structure-sensitive reaction. Active site changes greatly alter product distribution. The dynamics and essence of the changes in Co species during the reaction have seldom been explored. Herein, multiple characterizations were applied to confirm that two types of active sites, i.e., Co and Co2C, coexist on CoMnOx under the reaction conditions. Apart from the direct participation of carbon species in the reaction, the steady-state isotopic transient kinetic analysis also suggested the mutual transformation of carbon species in Co2C. The ratio of the two paths altered with the temperature, thus leading to Co2C-rich and Co-rich surfaces at intermediate and high temperatures, respectively. Kinetic calculations and microkinetic modeling showed that Co2C-rich surfaces facilitated CO adsorption and suppressed hydrogen adsorption. This gave high carbon-chain growth activity. The Co-rich surfaces showed high vacancy and hydrogen coverages, lowering the CO activation and CH4 formation energies.

CH4 Activation over Perovskite Catalysts: True Density and Reactivity of Active Sites

The high thermal stability of perovskites has drawn attention toward their applications for catalytic CH4 activation and conversion, typically occurring at high temperatures. The reaction rates of perovskite catalysts for CH4 combustion, however, trail behind those of noble metal catalysts. Ways to optimize the performance of perovskite catalysts are destined to trial-and-error approaches unless their complex reconstructed surfaces are correlated with fundamental kinetic parameters. Discerning the intrinsic activity of surface catalytic sites and the density of those sites is crucial to rationally envision complex metal oxides with enhanced catalytic performance. The present work presents a detailed kinetic analysis of catalytic CH4 combustion over a set of seven perovskites (SrTiO3, SrZrO3, SrFeO3, LaFeO3, LaInO3, LaCoO3, LaMnO3) with various surface terminations. Steady-state isotopic transient kinetic analysis was employed to measure turnover frequency (TOF) and density of surface intermediates (N) under operando conditions. Top surface characterization elucidated performance-structure relationships between near-monolayer surface composition and intrinsic reactivity of the catalysts. By using a chemical etching procedure to expose Fe-sites at the top surface of LaFeO3 (LaFeO3,HNO3), its TOF was increased 4-fold, compared with the unmodified sample, although N on the surface of LaFeO3,HNO3 decreased. Density functional theory simulations corroborated that surface Fe-termination and La-Fe termination offer lower energetic barriers for CH4 activation when compared with La-termination. In general, surface reconstruction is shown as a tool to tune TOF and N to improve reaction rates. This work fills a gap in current kinetic studies of perovskites through a careful assessment and discussion of the density and intrinsic reactivity of active sites for methane combustion over well-characterized reconstructed perovskite surfaces.

Engineering electronic platinum–carbon support interaction to tame carbon monoxide activation

CO oxidation has been studied for more than a century; however, molecular-level understanding of its activation protocol and related intermediates remains elusive. Here, we present a unified mechanistic and kinetic picture of various electronic metal–support interactions within platinum–carbon catalysts via in situ spectroscopic/kinetic analyses and multi-scale simulations. Transient kinetic analysis and molecular dynamics simulations with a reactive force field provided a quantitative description of the competition between the oxygen association and oxygen dissociation mechanisms tuned by the interfacial charge distribution and CO coverage. Steady-state isotopic transient kinetic analysis and density functional theory calculations revealed a simultaneous shift in the rate-determining step (RDS) from O2* dissociation to O* and CO* and O2* and CO* association. A de novo strategy from the interfacial charge distribution to the reaction mechanism, kinetics/thermodynamics of RDS, and, ultimately, catalytic performance was developed to quantitatively map the above CO activation mechanism with an order-of-magnitude increase in reactivity. The proposed catalytic picture and de novo strategy are expected to prompt the development of theories and methodologies for heterogeneous catalysis.

Crystallographic dependence of CO2 hydrogenation pathways over HCP-Co and FCC-Co catalysts

Efficient conversion of CO2 is of great significance for sustainable supply of chemicals and fuels. While Co-based catalysts are known to be effective for CO hydrogenation in Fischer-Tropsch synthesis, they work very differently in CO2 hydrogenation. This study reveals a crystallographic dependence of reaction pathways for CO2 hydrogenation on Co catalyst showing a new type of structure sensitivity and structure-activity-selectivity relationship for CO2 conversion to chemicals and fuels. The experimental work on CO2 conversion including steady-state isotopic transient kinetic analysis (SSITKA) using 13C-labeled CO2 shows a preferential CH4 formation over HCP-Co but dominant CO formation over FCC-Co. The density functional theory calculations indicate that CO2 does dissociate directly into chemisorbed CO* and O* on both HCP-Co and FCC-Co, but the CO* intermediates on HCP-Co prefer to be hydrogenated to form CH4 whereas the CO* on FCC-Co preferentially desorb to form CO. The significantly altered adsorption strength of CO* due to the presence of chemisorbed O* and CO2* species on the catalyst surface is responsible for the mechanistic disconnection in product selectivity between the CO2 and CO hydrogenation over Co catalysts. This study also shows that the addition of K to Co diminishes the direct impact of Co crystal structure, but improves the selectivity to C2+ hydrocarbons along with higher CO2 conversion. This seems to result from another pathway originating from HCOO* intermediate from bonding interaction of surface Co atoms with carbon in CO2, leading to the formation of CHx* whose coupling subsequently give rises to C2+ products. The present study sheds new light into the crystallographic structural sensitivity of CO2 hydrogenation towards the rational design of more selective catalysts for CO2 conversion.

Kinetic insights into the effect of promoters on Co/Al2O3 for Fischer-Tropsch synthesis

The fundamentals of promoter effects in the Fischer − Tropsch synthesis over Co-based catalysts were systematically investigated by employing the Rh, Ir, Sb and Ga as promoters of Co/Al2O3. Various characterizations showed the higher dispersion and reducibility over Rh- and Ir- promoted catalysts would cause a promoted adsorption property, while the Sb- and Ga- promoted catalysts showed a contrary result because the formation of alloy suppressed the accessibility of Co species. Based on the experimental results and kinetic calculations, a linear relationship for the activation energies of these catalysts as a function of activation entropies was proposed. Besides, steady state isotopic transient kinetic analysis (SSITKA) revealed that promoters mainly affected the surface CHx concentration to alter the CO reaction rate, and the chain growth rate constants would affect the product distribution. Since CoRhA and CoIrA greatly enhanced the CHx concentration and inhibited the chain growth ability, they owned a higher CO reaction rate and CH4 selectivity. While the CoSbA and CoGaA suppressed the CHx concentration and intrinsic activity, but enhanced the chain growth ability, thereby revealing a lower CO reaction rate and CH4 selectivity. Furthermore, the reactivity and selectivity difference were attributed to the modification of adsorption strength dictated by the enthalpy of activation on cobalt catalysts with different promoters. These findings provide kinetic insights into the promoter effects and pave the way for optimizing catalysts.

Controlling aging processes during bio-syngas methanation: SSITKA experiments over Ni/Al2O3 catalysts

A major barrier in the commercialization of biomass or waste gasification into bio-syngas is the presence of impurities such as NH3, HCl, tars and alkali in the products, being detrimental to downstream processes like the conversion of bio-syngas to substitute natural gas. The exact role and effects of these biomass-derived impurities on methanation catalysts are not clearly documented, especially when poisoning factors can combine with structural aging processes such as metal sintering under operating conditions. In order to unravel these aging phenomena under methanation conditions, the direct observation of Steady-State Isotopic Transient Kinetic Analysis curves semi-log transforms is combined with their modeling in view to evaluate the efficiency and sensitivity of this transient kinetic analysis. Thus, the toxicity of acetonitrile, trichloroethylene and toluene as representative of N-, Cl-, C- based poisons respectively, is shown to be strongly linked to the metal dispersion for the studied case of alumina supported nickel catalysts. A rationale linking such a transient kinetic analysis to methanation mechanisms is proposed in the present paper.

Isotopic transient kinetic analysis of CO2 hydrogenation to methanol on Cu/SiO2 promoted by Ga and Zn

Cu-based catalysts promoted with elements such as Zn or Ga are the most studied for CO2 hydrogenation to methanol due to their low cost and high activity. However, the fundamental causes of promotion are still not fully understood. In this work we assessed the effect that these promoters have on the number and intrinsic activity of surface sites using steady state isotopic transient kinetic analysis (SSITKA). We found an order of magnitude increase in the concentration of surface intermediates leading to CO when Zn and Ga are added. At the same time, it reduced the intrinsic rate of CO formation by a similar magnitude, leading to a similar macroscopic CO formation rate as compared with non-promoted Cu. The quantification of surface intermediates showed that only a small fraction of the surface is occupied by intermediates leading to CO and methanol, experimentally confirming theoretical studies who described the unique configuration of active sites for this reaction.

Microkinetic model validation for Fischer-Tropsch synthesis at methanation conditions based on steady state isotopic transient kinetic analysis

A Single-Event MicroKinetic (SEMK) model has been extended towards the simulation of Steady State Isotopic Transient Kinetic Analysis (SSITKA) data for Co catalyzed Fischer-Tropsch Synthesis (FTS). The extended model considers two types of sites and both direct and H-assisted CO dissociation. Regression of the model parameters to the data obtained from 17 steady state and 11 SSITKA experiments resulted in physicochemically meaningful estimates for the activation energies and atomic chemisorption enthalpies. The application of the phenomenological UBI-QEP method allows to physically interpret the nature of the two site types considered in the model, i.e., terrace and step sites. A reaction path analysis shows that over 80 percent of the CO reacts on the step sites. Furthermore, chain growth exclusively occurs on these sites. The terrace sites are less reactive for CO dissociation and are identified as responsible for methane production. A fraction of the alkenes, produced on the step sites, is hydrogenated to alkanes on the terrace sites. Based on model simulations, the metal particle size effect, i.e., a lower TOF, higher methane selectivity and increasing alkane to alkene ratio with decreasing metal particle size, is attributed to an increasing relative importance of the terrace sites on the reaction kinetics.

Promotional effects of sodium and sulfur on light olefins synthesis from syngas over iron-manganese catalyst

Fischer–Tropsch synthesis of light olefins plays a vital role in the production of major chemical building blocks from non-petroleum resources, having great academic and commercial importance. Herein, Na and S modulated FeMnOx catalysts were employed to systematically investigate the influence of additives, which not only enhanced the CO conversion but also facilitated the olefin formation and suppressed the undesired methane formation. Multiple characterizations confirmed that the existence of promoters would enhance the formation of active species of Fe5C2 at FTO conditions because of the promoted carbon insertion into the iron species from the intermediates dissociation. Combined the steady-state isotopic transient kinetic analysis with dynamic calculation, it confirmed that the promoters indeed had the ability to lower CO activation energy, as well as increase the carbon chain growth activity and the energy barrier to hydrogenation. This study provides a practical strategy for exploring the highly active and stable FTO catalysts.

An Application of Steady‐state Isotopic‐transient Kinetic Analysis (SSITKA) in DeNOx Process

AbstractThis Minireview presents an overview of the advancement and capabilities of the steady‐state isotopic transient kinetic analysis in the selective catalytic reduction of NOx. Firstly, a brief overview of the method and the kinetic parameters of catalyst‐surface reaction intermediates, including concentration and coverage of surface intermediates, surface residence time and intrinsic turnover frequency (TOF), is provided. Furthermore, the focus is on the application of SSITKA or a unique combination of SSITKA‐DRIFTS for the identification of active and/or inactive (spectators) species in NH3−SCR, H2−SCR and HCs−SCR. Different forms of adsorbed species and their formation rates revealing the main elementary steps on the catalyst's surface involving labeled molecules are discussed. The emphasis is laid on the optimization and design of industrial catalysts.

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.