Tracing and elucidating visible-light mediated oxidation and C-H functionalization of amines using mass spectrometry

The co-existing mechanism of visible light mediated direct oxidation and C-H functionalization of amines was investigated by capturing all the intermediates using online mass spectrometry. The two-step dehydrogenation of amine involving a proton coupled electron transfer (PCET) process was revealed for the first time.

New analytical tools for advanced mechanistic studies in catalysis: photoionization and photoelectron photoion coincidence spectroscopy

How can we detect reactive and elusive intermediates in catalysis to unveil reaction mechanisms? In this mini review, we discuss novel photoionization tools to support this quest.

Rate/Concentration Kinetic Petals: A Transient Method to Examine the Interplay of Surface Reaction Processes

Transient pulse response experiments are used to construct rate/concentration kinetic dependencies, RC Petals and provide a new method to distinguish the timing and interplay of adsorption, surface reaction, and product formation on complex (industrial) materials. A petal shape arises as the dynamic "reaction-diffusion" experiment forces the concentration and reaction rate to return to zero. In contrast to the typical steady-state "Langmuir-type" RC dependence, RC petals have two branches, which arise as a result of decoupled gas and surface concentrations in the non-steady-state regime. To demonstrate this approach, the characteristics of petal shapes using ammonia decomposition as a probe reaction are presented. Ammonia, hydrogen, and nitrogen transformation rates are compared on three simple materials: iron, cobalt, and a bimetallic CoFe preparation when ammonia is pulsed at 550 °C in a low-pressure diffusion reactor. All materials demonstrate a two-branch kinetic RC dependence for ammonia adsorption, and rate constants are quantified in the low-coverage regime. We found that H₂ and N₂ product formation was dependent on the concentration of surface intermediates for all materials with one exception: for cobalt, an additional fast hydrogen generation process was observed; the rate of which coincided with ammonia adsorption. Nitrogen generation was only significant for CoFe and cobalt and on the CoFe catalyst, a self-inhibition property was observed. A method for estimating the number of active sites based on the RC petals is presented and was applied to the iron and CoFe samples. The surface coverage and rate of formation/conversion of surface intermediates are interpreted from the examination of shape characteristics of the RC petals for each material.

Fifty Years of Moment Technique for Dynamic Analysis of Chemical Reactor Parameters

Moment technique has been extensively used for the evaluation of the rate and equilibrium parameters in chemical reactors and also in adsorption vessels, for about five decades. Adsorption and reaction rate parameters, as well as axial dispersion constants, effective diffusivities within porous catalysts and heat and mass transfer coefficients were shown to be effectively evaluated by analyzing the moments of the response peaks, which could be obtained from pulse-response experiments performed in a reaction/adsorption vessel. A detailed review of chromatographic processes, involving moment analysis of adsorption equilibrium and rate constants in fixed beds, dynamic analysis of batch adsorbers, moment analysis of fluidized bed, slurry and trickle bed reactors are reported in this manuscript. Applications of the single-pellet moment technique, which was developed for the effective investigation of intrapellet rate and equilibrium processes, by eliminating the contributions of axial dispersion and external transport parameters, are comprehensively discussed. Recent studies for the analysis of reaction rate parameters using the TAP reactor approach, use of single pellet system for the investigation of catalytic and non-catalytic solid-gas reactions and extension of the moment technique to non-linear systems opened new pathways in reaction engineering research.

Perspectives on the design of nanoparticle systems for catalysis

An overview of the Faraday Discussion, "Designing Nanoparticle Systems for Catalysis", is presented. Examples are taken from the papers presented at the meeting and from the literature to illustrate the main discussion points.

Alcohol Synthesis from CO2 , H2 , and Olefins over Alkali-Promoted Au Catalysts-A Catalytic and In situ FTIR Spectroscopic Study

Au/TiO₂ and Au/SiO₂ catalysts containing 2 wt % Au and different amounts of K or Cs were tested for alcohol synthesis from CO₂ , H₂ , and C₂ H₄ /C₃ H₆ . 1-Propanol or 1-butanol/isobutanol were obtained in the presence of C₂ H₄ or C₃ H₆ . Higher yields of the corresponding alcohols were obtained over TiO₂ -based catalysts in comparison with their SiO₂ -based counterparts. This is caused by an enhanced ability of the TiO₂ -based catalysts for CO₂ activation, as concluded from in situ fourier-transform infrared (FTIR) spectroscopy and temporal analysis of products (TAP) studies. The synthesized carbonate and formate species adsorbed on the support do not hamper CO₂ conversion into CO and the hydroformylation reaction. The transformation of Au^δ+ to active Au⁰ sites proceeds during an activation procedure. As reflected by CO adsorption and scanning transmission electron microscopy, the accessible Au⁰ sites are influenced by the amount of alkali dopants and the support. FTIR data and TAP tests reveal a very weak interaction of C₂ H₄ with the catalyst, suggesting its quick reaction with CO and H₂ after activation on Au⁰ sites to form propanol and propane.

Transient Kinetic Experiments within the High Conversion Domain: The Case of Ammonia Decomposition

In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic preparation of the two. These materials presented a wide range of activity and it was not possible to capture transient information in the advecting device for all samples at the same conditions while ensuring uniformity. We push the boundary for the theoretical estimates of uniformity in the TAP device and find reliable kinetic measurement up to 90% conversion. However, what is more advantageous from this technique is the ability to observe the time-dependence of the reaction rate rather than just singular points of conversion and yield. For example, on the iron sample we observed reversible adsorption of ammonia and on cobalt materials we identify two routes for hydrogen production. From the time-dependence of reactants and product, the dynamic accumulation was calculated. This was used to understand the atomic distribution of H and N species regulated by the surface of different materials. When ammonia was pulsed at 550 °C, the surface hydrogen/nitrogen, (H/N), ratios that evolved for Fe, CoFe and Co were 2.4, 0.25 and 0.3 respectively. This indicates that iron will store a mixture of hydrogenated species while materials with cobalt will predominantly store NH and N. While much is already known about iron, cobalt and ammonia decomposition, the goal of this work was to demonstrate new tools for comparing materials over a wider window of conversion and with much greater kinetic detail. As such, this provides an approach for detailed kinetic discrimination of more complex industrial samples beyond conversion and yield.

Controlling the speciation and reactivity of carbon-supported gold nanostructures for catalysed acetylene hydrochlorination

Carbon-supported gold catalysts have the potential to replace the toxic mercuric chloride-based system applied industrially for acetylene hydrochlorination, a key technology for the manufacture of polyvinyl chloride. However, the design of an optimal catalyst is essentially hindered by the difficulties in assessing the nature of the active site. Herein, we present a platform of carbon supported gold nanostructures at a fixed metal loading, ranging from single atoms of tunable oxidation state and coordination to metallic nanoparticles, by varying the structure of functionalised carbons and use of thermal activation. While on activated carbon particle aggregation occurs progressively above 473 K, on nitrogen-doped carbon gold single atoms exhibit outstanding stability up to temperatures of 1073 K and under reaction conditions. By combining steady-state experiments, density functional theory, and transient mechanistic studies, we assess the relation between the metal speciation, electronic properties, and catalytic activity. The results indicate that the activity of gold-based catalysts correlates with the population of Au(i)Cl single atoms and the reaction follows a Langmuir-Hinshelwood mechanism. Strong interaction with HCl and thermodynamically favoured acetylene activation were identified as the key features of the Au(i)Cl sites that endow their superior catalytic performance in comparison to N-stabilised Au(iii) counterparts and gold nanoparticles. Finally, we show that the carrier (activated carbon versus nitrogen-doped carbon) does not affect the catalytic response, but determines the deactivation mechanism (gold particle aggregation and pore blockage, respectively), which opens up different options for the development of stable, high-performance hydrochlorination catalysts.

An autonomous microreactor platform for the rapid identification of kinetic models

Rapid estimation of kinetic parameters with high precision is facilitated by automation combined with online Model-Based Design of Experiments.

Application of modulation excitation-phase sensitive detection-DRIFTS for in situ/operando characterization of heterogeneous catalysts

A new more general method and guidelines for the implementation of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

Unraveling the H2 Promotional Effect on Palladium-Catalyzed CO Oxidation Using a Combination of Temporally and Spatially Resolved Investigations

The promotional effect of H₂ on the oxidation of CO is of topical interest, and there is debate over whether this promotion is due to either thermal or chemical effects. As yet there is no definitive consensus in the literature. Combining spatially resolved mass spectrometry and X-ray absorption spectroscopy (XAS), we observe a specific environment of the active catalyst during CO oxidation, having the same specific local coordination of the Pd in both the absence and presence of H₂. In combination with Temporal Analysis of Products (TAP), performed under isothermal conditions, a mechanistic insight into the promotional effect of H₂ was found, providing clear evidence of nonthermal effects in the hydrogen-promoted oxidation of carbon monoxide. We have identified that H₂ promotes the Langmuir–Hinshelwood mechanism, and we propose this is linked to the increased interaction of O with the Pd surface in the presence of H₂. This combination of spatially resolved MS and XAS and TAP studies has provided previously unobserved insights into the nature of this promotional effect.