Studies Of Heterogeneous Catalytic Reactions Research Articles

A near-ambient pressure flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy for operando studies of heterogeneous catalytic reactions over model catalysts.

The model catalyst approach is often used for fundamental investigations of complex heterogeneous catalysis, in which operando characterizations are critical. A flow reactor is usually adopted for gas-solid heterogeneous catalytic reactions. Herein, we report a home-designed near-ambient pressure (NAP) flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) and an online quadrupole mass spectrometer for operando studies of heterogeneous catalytic reactions over model catalysts. A unique gas supply system is designed and manufactured to enable a stable gas inlet to the NAP flow reactor at pressures up to ∼100mbar. An ultrahigh vacuum chamber equipped with the facilities for x-ray photoelectron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy, E-beam evaporation source, and ion sputtering gun is connected to the NAP flow reactor via a gate valve for preparations and routine characterizations of model catalysts. The functions of the system are demonstrated by in situ PM-IRAS characterization of CO adsorption on Pt(111) and operando characterizations of CO oxidation on Pt(111) under NAP conditions.

Silicon microfabricated reactor for operando XAS/DRIFTS studies of heterogeneous catalytic reactions

A novel microreactor for operando XAS and DRIFTS studies of catalytic reactions is reported, exhibiting plug-flow, isothermal behaviour and absence of mass transfer resistances and dead volume, enabling time- and spatially-resolved experiments.

On-Line Analysis of the Heterogeneous Pd-Catalyzed Transfer Hydrogenation of p-Nitrophenol in Water with Formic Acid in a Flow Reactor

An versatile experimental setup composed of a fixed-bed reactor and an HPLC apparatus has been developed for the study of heterogeneous catalytic reactions under transient conditions. The transfer hydrogen reaction of formic acid in water to give p-nitrophenol catalyzed by Pd@Al2O3 support was chosen as the model reaction in a flow reactor. On one hand, this setup allowed the first experimental on-line measurements of p-aminophenol in an acidic medium, indicating that it is the only product of the reaction at steady state. On the other hand, the setup evidenced a quite long transient period before stationary conditions were reached, involving a transient component that is quite instable and can transform to p-aminophenol without a catalyst. Results of numerical simulations indicate that only a change in the reaction mechanism, due to a change in the catalyst activity after few minutes of reaction, can explain the transient production of this intermediate and that this occurs when the active hydrogen coverage of the catalyst is low.

Kinetic Study of Waste-derived Solid Hydroxy Sodalite Catalyst during Transesterification of Animal Fat Oil to Biodiesel in a Batch Reactor

Kinetic studies of heterogeneous catalytic reactions form a crucial step necessary for the understanding of catalytic behaviour of a catalyst towards designing, controlling and optimizing a reactor. This study reports kinetics of waste animal fat oil (AFO) transesterification to biodiesel using waste-derived heterogeneous catalyst, hydroxy sodalite (HSOD) in a batch reactor. The catalyst was synthesized from coal fly ash and waste industrial brine via hydrothermal treatment. At a temperature range of 49 - 62 °C and a time range of 30 -120 minutes, the transesterification of animal fat oil to biodiesel was conducted at a fixed methanol/oil mass proportion 9:1, percent mass weight of catalyst 3 (based on the AFO) and stirring intensity of 300-500 rpm. Experimental findings reveal that reaction rate, which is first-order, was anticipated to increase with increasing temperature, resulted in an activation energy and a pre-exponential factor of 58554.65 J mol-1 and 2.83 min-1, respectively. The value of the activation energy suggests that the reaction is endothermic and a minimum energy of 58.55 kJ is required to achieve an effective collision at a frequency of 2.83 min-1. The highest biodiesel yield was 90 % at 62 °C and this corresponds to a highest AFO conversion of 93 % at a reaction time of 120 minutes.

Robust In Situ Magnetic Resonance Imaging of Heterogeneous Catalytic Hydrogenation with and without Hyperpolarization

AbstractMagnetic resonance imaging (MRI) is a powerful technique to characterize reactors during operating catalytic processes. However, MRI studies of heterogeneous catalytic reactions are particularly challenging because the low spin density of reacting and product fluids (for gas phase reactions) as well as magnetic field inhomogeneity, caused by the presence of a solid catalyst inside a reactor, exacerbate already low intrinsic sensitivity of this method. While hyperpolarization techniques such as parahydrogen induced polarization (PHIP) can substantially increase the NMR signal intensity, this general strategy to enable MR imaging of working heterogeneous catalysts to date remains underexplored. Here, we present a new type of model catalytic reactors for MRI that allow the characterization of a heterogeneous hydrogenation reaction aided by the PHIP signal enhancement, but also suitable for the imaging of regular non‐polarized gases. These catalytic systems permit exploring the complex interplay between chemistry and fluid‐dynamics that are typically encountered in practical systems, but mostly absent in simple batch reactors. High stability of the model reactors at catalytic conditions and their fabrication simplicity make this approach compelling for in situ studies of heterogeneous catalytic processes by MRI.

Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy.

The study of heterogeneous catalytic reactions remains a major challenge because it involves a complex network of reaction steps with various intermediates. If the vibrational spectra of individual molecules could be monitored in real time, one could characterize the structures of the intermediates and the time scales of reaction steps without ensemble averaging. Surface-enhanced Raman scattering (SERS) spectroscopy does provide vibrational spectra with single-molecule sensitivity, but typical single-molecule SERS signals exhibit spatial heterogeneities and temporal fluctuations, making them difficult to be used in single-molecule kinetics studies. Here we show that SERS can monitor the single-molecule catalytic reactions in real time. The surface-immobilized reactants placed at the junctions of well-defined nanoparticle-thin film structures produce time-resolved SERS spectra with discrete, step-transitions of photoproducts. We interpret that such SERS-steps correspond to the reaction events of individual molecules occurring at the SERS hotspot. The analyses of the yield, dynamics, and the magnitude of such SERS steps, along with the associated spectral characteristics, fully support our claim. In addition, a model that is based on plasmonic field enhancement and surface photochemistry reproduces the key features of experimental observation. Overall, the result demonstrates that it is possible, under well-controlled conditions, to differentiate the chemical and physical processes contributing to the single-molecule SERS signals, and thus shows the use of single-molecule SERS as a tool for studying the metal-catalyzed organic reactions.

Elucidation of Intermediates and Mechanisms in Heterogeneous Catalysis Using Infrared Spectroscopy

Infrared spectroscopy has a long history as a tool for the identification of chemical compounds. More recently, various implementations of infrared spectroscopy have been successfully applied to studies of heterogeneous catalytic reactions with the objective of identifying intermediates and determining catalytic reaction mechanisms. We discuss selective applications of these techniques with a focus on several heterogeneous catalytic reactions, including hydrogenation, deNOx, water-gas shift, and reverse-water-gas shift. The utility of using isotopic substitutions and other techniques in tandem with infrared spectroscopy is discussed. We comment on the modes of implementation and the advantages and disadvantages of the various infrared techniques. We also note future trends and the role of computational calculations in such studies. The infrared techniques considered are transmission Fourier transform infrared spectroscopy, infrared reflection-absorption spectroscopy, polarization-modulation infrared reflection-absorption spectroscopy, sum-frequency generation, diffuse reflectance infrared Fourier transform spectroscopy, attenuated total reflectance, infrared emission spectroscopy, photoacoustic infrared spectroscopy, and surface-enhanced infrared absorption spectroscopy.

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.

The UBI-QEP method: Mechanistic and kinetic studies of heterogeneous catalytic reactions

Applications of the unity bond index-quadratic exponential potential (UBI-QEP) method in mechanistic and kinetic studies of complex heterogeneous catalytic reactions are discussed. It is shown how UBI-QEP energetics helps to answer specific questions regarding the mechanisms of various reactions on metal surfaces. Examples of the following reactions are considered: elementary reactions of hydrogen transfer (from one carbon atom connected to the metal surface to another such atom in a different surface species), H-X bond breaking with the assistance of coadsorbed oxygen, methanol synthesis, Fischer-Tropsch synthesis, ammonia synthesis and decomposition, and partial methanol oxidation to formaldehyde. Then, examples of kinetic simulations using UBI-QEP energetics are considered for the following processes: ethane hydrogenolysis, watergas shift reaction, selective hydrogenation of acetylene in ethylene-rich mixtures, oxidative conversions of hydrogen and methane on Pt and Rh surfaces, ammonia decomposition, C1–C2 product formation in Fischer-Tropsch synthesis over cobalt, and steam reforming of methane. It is shown how the use of UBI-QEP and Monte Carlo methods makes it possible to calculate reaction parameters that depend on temperature in the equilibrium and kinetic regimes. To increase the accuracy of simulations, we added nonenergetic parameters affecting energetics at nonzero coverages: a spatial constraint on the distance between adsorbed atoms and the distance of hot atom traveling upon oxygen dissociation on metal surfaces. The UBI-QEP/Monte Carlo simulation method is illustrated by the study of molecular oxygen adsorption on single crystalline nickel surfaces. In most UBI-QEP-based mechanistic and kinetic studies of heterogeneous catalytic reactions, good agreement is observed with experimental observations, in many cases on a quantitative level. Taking into account diversity of reactions to which the method has been applied the agreement with experiments supports the efficiency of the method in solving mechanistic and kinetic problems.

Application of 14C Radiotracer for the Study of Heterogeneous Catalytic Reactions

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Characterization of Catalytic Surfaces by Isotopic-Transient Kinetics during Steady-State Reaction

Steady-state isotopic-transient kinetic analysis (SSITKA) is a very useful technique for the kinetic study of heterogeneous catalytic reactions. The technique is based upon the detection of isotopic labels in the reactor effluent species versus time following a switch (step change) in the isotopic labeling of one of the reactant species in the reactor feed. In addition to maintaining isothermal and isobaric reaction conditions, the reactant and product concentrations and flow rates remain undisturbed during the step change. Thus, in the absence of isotopic mass effects, steady-state reaction conditions are maintained under isotopic-transient operation. The reaction intermediates present on the catalyst surface do not change, and unlike for other transient techniques, analysis of the steady-state kinetic behavior of the catalyst surface is possible. An isotopic-step input is typically used so that the transient behavior of the less-active catalytic sites is included in the kinetic determinations, information which may be lost when using an isotopic-pulse technique. Steady-state kinetic information which has been obtained from SSITKA includes concentrations of different types of adsorbed reaction intermediates, coverages, surface lifetimes, site heterogeneity, activity distributions, and identification of possible mechanisms. This overview of SSITKA includes discussions of the technique of steady-state isotopic-transient labeling for kinetic study,more » the mathematical formalisms used in transient analysis, the kinetic parameters which can be obtained, the experimental considerations of the technique, and the reactions to which SSITKA has been applied. 98 refs.« less

Combined elevated pressure reactor and ultrahigh vacuum surface analysis system

A combined elevated pressure reactor/ultrahigh vacuum (UHV) surface analytical system has been designed for studies of heterogeneous catalytic reactions. The reaction cell is separated from the surface analysis chamber by a differentially pumped sliding seal. This feature allows simple in situ sample transfer capabilities from the surface analysis region to the reactor cell and also facilitates the introduction and repair of the sample and/or holder with no significant perturbation of the UHV analysis chamber vacuum. The pumping arrangements provide for rapid evacuation of the reaction cell as well as maximum sensitivity with respect to the detection of reaction products.

Local gas sampling and surface hydrogen detection in catalysis on planar surfaces

A new experimental set-up for in situ studies of heterogeneous catalytic reactions on metal films in the pressure range 10 −8–2000 mbar is described. The reactions take place on the outermost layer of a metal-insulator-semiconductor structure, whose capacitance-voltage characteristics are extremely sensitive to variations of the hydrogen coverage on the metal surface. It is shown how this device can be used in combination with mass spectrometry and work function measurements. Mass spectrometry is performed with high sensitivity and fast response even at atmospheric pressures, owing to local gas sampling with a capillary leak. Using this technique reactant pressure gradients close to a catalyst surface can be quantified, allowing true rates of mass transfer limited reactions to be determined. The performance of the system is illustrated by the H 2 + O 2 reaction on Pt in the mbar region. The maximum efficiency of water production is found for a hydrogen/oxygen pressure ratio of 0.1, and the reaction probabilities are S H 2 = 4 × 10 −3 and S O 2 = 8 × 10 −4 at this optimum point. The behaviour of the experimental set-up is further demonstrated by the CO + O 2 oscillatory reaction on Pd and Pt.

A computerized concentration-controlled recycle reactor for the kinetic studies of heterogeneous catalytic reactions

A computerized concentration-controlled recycle reactor for kinetic studies is described. For concentration control a two step control algorithm is developed (model based feed forward control and integral controller, respectively). Different sequential experimental designs for model discrimination and parameter estimation can be performed automatically by the experimental set-up. This is demonstrated for the CO-oxidation as a simple test reaction. Some drawbacks of the classical sequential designs are pointed out and possibilities for an improvement by modifications of the designs are presented. Using all possibilities of a concentration-controlled recycle reactor, a new experimental procedure for kinetic investigations based on the one-variable-at-a-time-concept is presented. It seems superior to classical sequential designs for special applications.

An Infrared Spectroscopic Flow Reactor for in Situ Studies of Heterogeneous Catalytic Reactions at High Temperature and the Reactions of CO2 and Benzyl Benzoate on Alumina

A simple infrared spectroscopic flow reactor has been fabricated for studying hetereogeneous catalytic reactions at elevated temperatures. Adsorption of CO2 on alumina and modified alumina catalysts shows the formation of several different surface carbonates. The hydrogen carbonate on alumina has a fast equilibrium with the gas phase. During the reduction of benzoic acid esters in this reactor, the formation of surface benzoate and carbonate has been observed. The desorption rate of the surface benzoate depends on the composition of the catalyst.

Flow reactor for kinetic studies with simultaneous X-ray phase analysis of a catalyst

A flow reactor for kinetic studies of heterogeneous catalytic reactions with simultaneous X-ray phase analysis of the catalyst is described. As an example, the correlation between the selectivity of acetylene hydrogenation and the β-Pd hydride concentration in palladium catalysts is shown.

Linear free energy relations in modeling heterogeneous catalytic reactions

Modeling a complex heterogeneous catalytic reaction involves consideration of each elementary rate or equilibrium step. In principle, each step can be modeled with an analog reaction. For example, first order Langmuir-Hinshelwood kinetics can be modeled by combining two analogs, one for adsorption and one for surface rearrangement. The result is the sum of two Hammett-type relations. A linear free energy relation, which must be empirically established, proves the validity of the model and thereby elucidates the details of the catalytic process. The recent work of Mochida and Yoneda is discussed to demonstrate the important utility of linear free energy modeling procedures in the study of heterogeneous catalytic reactions.

Use of the isotope effect in the study of heterogeneous catalytic reactions

The applicability of the isotope effect on heterogeneous reaction rates as a kinetic tool is discussed. The isotope effect on the exchange reaction rate of ammonia and hydrogen on iron catalyst has been measured using deuterium and tritium as labelling isotope. A general rate equation is derived for this exchange reaction, and it is shown that uder certain conditions information on the reaction mechanism can be obtained from the measured isotope effect.