Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

Improving time-resolution and sensitivity of in situ X-ray photoelectron spectroscopy of a powder catalyst by modulated excitation

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful tool to characterize the surface structure of heterogeneous catalysts in situ. In order to improve the time resolution and the signal-to-noise (S/N) ratio of photoemission spectra, we collected consecutive APXP spectra during the periodic perturbation of a powder Pd/Al2O3 catalyst away from its equilibrium state according to the modulated excitation approach (ME). Averaging of the spectra along the alternate pulses of O2 and CO improved the S/N ratio demonstrating that the time resolution of the measurement can be limited solely to the acquisition time of one spectrum. Through phase sensitive analysis of the averaged time-resolved spectra, the formation/consumption dynamics of three oxidic species, two metal species, adsorbed CO on Pd0 as well as Pdn+ (n > 2) was followed along the gas switches. Pdn+ and 2-fold surface PdO species were recognised as most reactive to the gas switches. Our approach demonstrates that phase sensitive detection of time-resolved XPS data allows following the dynamics of reactive species at the solid-gas interface under different reaction environments with unprecedented precision.

Emergence of Dynamically-Disordered Phases During Fast Oxygen Deintercalation Reaction of Layered Perovskite

Determination of a reaction pathway is an important issue for the optimization of reactions. However, reactions in solid-state compounds have remained poorly understood because of their complexity and technical limitations. Here, using state-of-the-art high-speed time-resolved synchrotron X-ray techniques, the topochemical solid-gas reduction mechanisms in layered perovskite Sr₃ Fe₂ O_{7- δ} (from δ ∼ 0.4 to δ = 1.0), which is promising for an environmental catalyst material is revealed. Pristine Sr₃ Fe₂ O_{7- δ} shows a gradual single-phase structural evolution during reduction, indicating that the reaction continuously proceeds through thermodynamically stable phases. In contrast, a nonequilibrium dynamically-disordered phase emerges a few seconds before a first-order transition during the reduction of a Pd-loaded sample. This drastic change in the reaction pathway can be explained by a change in the rate-determining step. The synchrotron X-ray technique can be applied to various solid-gas reactions and provides an opportunity for gaining a better understanding and optimizing reactions in solid-state compounds.

CuO/La0.5Sr0.5CoO3: precursor of efficient NO reduction catalyst studied by operando high energy X-ray diffraction under three-way catalytic conditions

Substitution of critical raw materials such as platinum group metals in automotive catalysts is challenging. In this work we prepared a nanocomposite in which CuO nanoparticles are highly dispersed on a La0.5Sr0.5CoO3 perovskite-type oxide. The behaviour and reactivity under three way catalyst conditions was monitored by operando time-resolved high-energy X-ray diffraction under oscillating rich/lean feed. The reducing environment converted CuO into Cu(0) in a two step process: Cu(ii) to Cu(i) and to Cu(0), while the perovskite evolved to an oxygen deficient brownmillerite phase. These structural transformations are shown to be crucial for catalytic activity. The in situ generated Cu(0)/Cu(i)/brownmillerite nanocomposite is active for NO reduction above 300 °C, reaching 90% NO conversion at 450 °C. The effect of feed composition on the diffraction patterns was studied by Rietveld refinement in order to rationalize the experimental observations under TWC conditions.

Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

Supported nanoparticles are commonly applied in heterogeneous catalysis. The catalytic performance of these solid catalysts is, for a given support, dependent on the nanoparticle size, shape, and composition, thus necessitating synthesis techniques that allow for preparing these materials with fine control over those properties. Such control can be exploited to deconvolute their effects on the catalyst's performance, which is the basis for knowledge-driven catalyst design. In this regard, bottom-up synthesis procedures based on colloidal chemistry or atomic layer deposition (ALD) have proven successful in achieving the desired level of control for a variety of fundamental studies. This review aims to give an account of recent progress made in the two aforementioned synthesis techniques for the application of controlled catalytic materials in gas-phase catalysis. For each technique, the focus goes to mono- and bimetallic materials, as well as to recent efforts in enhancing their performance by embedding colloidal templates in porous oxide phases or by the deposition of oxide overlayers via ALD. As a recent extension to the latter, the concept of area-selective ALD for advanced atomic-scale catalyst design is discussed.

Pulsed reactivity on LaCoO3-based perovskites: a comprehensive approach to elucidate the CO oxidation mechanism and the effect of dopants

In this contribution we focus on three lanthanum cobaltate perovskites: undoped, Sr-doped, and Cu-doped to investigate the effect of doping on catalytic activity through pulsed reactivity 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).

Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides

The understanding of the fundamental properties and processes of chemoresistive gas sensors based on semiconducting metal oxides is driven by the available characterization techniques and sophisticated approaches used to identify structure-function-relationships. This article summarizes trends and advances in the characterization of gas sensing materials based on semiconducting metal oxides, giving a unique overview of the state of the art methodology used in this field. The focus is set on spectroscopic techniques, but the presented concepts apply to other characterization methods, such as electronic, imaging or diffraction-based techniques. The presented concepts are relevant for academic research as well as for improving R&D approaches in industry.

Fluorescence-detected XAS with sub-second time resolution reveals new details about the redox activity of Pt/CeO2 catalyst

A setup for fluorescence-detected X-ray absorption spectroscopy (XAS) with sub-second time resolution has been developed. This technique allows chemical speciation of low-concentrated materials embedded in highly absorbing matrices, which cannot be studied using transmission XAS. Using this setup, the reactivity of 1.5 wt% Pt/CeO₂ catalyst was studied with 100 ms resolution during periodic cycling in CO- and oxygen-containing atmospheres in a plug-flow reactor. Measurements were performed at the Pt L₃- and Ce L₃-edges. The reactivity of platinum and cerium demonstrated a strong correlation. The oxidation of the catalyst starts on the ceria support helping the oxidation of platinum nanoparticles. The new time-resolved XAS setup can be applied to various systems, capable of reproducible cycling between different states triggered by gas atmosphere, light, temperature, etc. It opens up new perspectives for mechanistic studies on automotive catalysts, selective oxidation catalysts and photocatalysts.

An operando emission spectroscopy study of Pt/Al2O3 and Pt/CeO2/Al2O3

In situ time-resolved spectroscopic examination of catalysts based on well dispersed nanoparticles on metal oxides under transient conditions significantly facilitates the elucidation of reaction mechanisms. In this contribution, we demonstrate the level of structural information that can be obtained using high-energy resolution off-resonant spectroscopy (HEROS) to study 1.3 wt% Pt/Al₂O₃ and 1.3 wt% Pt/20 wt% CeO₂/Al₂O₃ catalysts subjected to redox pulsing. First, HEROS is compared with XANES in a temperature programmed reduction experiment to demonstrate the increased sensitivity and time resolution of HEROS. Second, modulation excitation spectroscopy is exploited by redox pulsing to enhance the sensitivity of HEROS to structural changes by the application of phase sensitive detection (PSD) to the time-resolved HEROS data set. The HEROS measurements were complemented by resonant X-ray emission (RXES) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements performed under identical conditions and in a single reactor cell in order to probe different aspects of the catalyst materials under the selected experimental conditions.

Principal component analysis for automatic extraction of solid-state kinetics from combined in situ experiments

A new algorithm to extract in an automatic way kinetic parameters from a set of measurements from in situ experiments is presented and applied to X-ray powder diffraction and Raman spectroscopy.

Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption

We present a comprehensive study of a 5 wt% Pd/C catalyst in various environments by using in situ and operando X-ray absorption and diffraction.

X-ray emission spectroscopy: highly sensitive techniques for time-resolved probing of cerium reactivity under catalytic conditions

Oxygen storage materials such as ceria are used in many catalytic applications because they can reversibly bind and release oxygen. Tools are needed to observe and quantify this activity which involves a change in the cerium oxidation state and to understand the involvement of cerium in catalytic processes. To prove that cerium changes its oxidation state in the catalytic cycle the transient rates of Ce³⁺ formation and decay should be compared to the overall reaction rate. For such mechanistic studies the time resolution is essential as the quantification of the Ce³⁺ species should be faster than the reaction rate. However, it is challenging to follow the dynamic changes of the cerium oxidation state under reaction conditions, especially when the concentration of cerium atoms involved in the reaction cycle is low. In this paper, we evaluate the sensitivity of high-resolution X-ray emission-based methods for the in situ time-resolved quantification of small concentrations of Ce³⁺ in ceria-based materials. We demonstrate that resonant X-ray emission spectroscopy (RXES) at optimal excitation energy is more sensitive than high energy resolution off-resonant spectroscopy (HEROS) and non-resonant X-ray emission spectroscopy (non-resonant XES) and that it can track the reactivity of less than 0.3% of cerium atoms in a 1% Pt/CeO₂ catalyst in a plug-flow reactor with sub-second time resolution. These results demonstrate that X-ray emission-based methods can be used as very sensitive tools and provide new insights into dynamic changes of the oxidation state in reducible oxides in a variety of applications.

Frequency analysis for modulation-enhanced powder diffraction

Periodic modulation of external conditions on a crystalline sample with a consequent analysis of periodic diffraction response has been recently proposed as a tool to enhance experimental sensitivity for minor structural changes. Here the intensity distributions for both a linear and nonlinear structural response induced by a symmetric and periodic stimulus are analysed. The analysis is further extended for powder diffraction when an external perturbation changes not only the intensity of Bragg lines but also their positions. The derived results should serve as a basis for a quantitative modelling of modulation-enhanced diffraction data measured in real conditions.

Pitfalls and benefits of in situ and operando diffuse reflectance FT-IR spectroscopy (DRIFTS) applied to catalytic reactions

The procedures and conditions that need to be fulfilled to be able to carry out appropriate in situ and operando diffuse reflectance FT-IR (DRIFTS) analyses are discussed.

Lithographically fabricated silicon microreactor for in situ characterization of heterogeneous catalysts—Enabling correlative characterization techniques

We report on a new modular setup on a silicon-based microreactor designed for correlative spectroscopic, scattering, and analytic on-line gas investigations for in situ studies of heterogeneous catalysts. The silicon microreactor allows a combination of synchrotron radiation based techniques (e.g., X-ray diffraction and X-ray absorption spectroscopy) as well as infrared thermography and Raman spectroscopy. Catalytic performance can be determined simultaneously by on-line product analysis using mass spectrometry. We present the design of the reactor, the experimental setup, and as a first example for an in situ study, the catalytic partial oxidation of methane showing the applicability of this reactor for in situ studies.

Catalyst characterisation techniques and reaction cells operating at realistic conditions; towards acquisition of kinetically relevant information

Catalyst characterisation techniques and reaction cells operating at realistic conditions; towards acquisition of kinetically relevant information.

Chemical selectivity in structure determination by the time dependent analysis of in situ XRPD data: a clear view of Xe thermal behavior inside a MFI zeolite

PSD/PCA analysis of MED data allowed to enhance the chemical selectivity in X-ray powder diffraction and to obtain Xe substructure into MFI zeolite.

Structural evolution of an intermetallic Pd–Zn catalyst selective for propane dehydrogenation

Formation of PdZn intermetallic nanoalloys selective for propane dehydrogenation tracked using in situ synchrotron XRD.

Adding diffuse reflectance infrared Fourier transform spectroscopy capability to extended x-ray-absorption fine structure in a new cell to study solid catalysts in combination with a modulation approach

We describe a novel cell used to combine in situ transmission X-ray absorption spectroscopy (XAS) with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in a single experiment. The novelty of the cell design compared to current examples is that both radiations are passed through an X-ray and IR transparent window in direct contact with the sample. This innovative geometry also offers a wide surface for IR collection. In order to avoid interference from the crystalline IR transparent materials (e.g., CaF2, MgF2, diamond) a 500 μm carbon filled hole is laser drilled in the center of a CaF2 window. The cell is designed to represent a plug flow reactor, has reduced dead volume in order to allow for fast exchange of gases and is therefore suitable for experiments under fast transients, e.g., according to the concentration modulation approach. High quality time-resolved XAS and DRIFTS data of a 2 wt.% Pt/Al2O3 catalyst are obtained in concentration modulation experiments where CO (or H2) pulses are alternated to O2 pulses at 150 °C. We show that additional information can be obtained on the Pt redox dynamic under working conditions thanks to the improved sensitivity given by the modulation approach followed by Phase Sensitive Detection (PSD) analysis. It is anticipated that the design of the novel cell is likely suitable for a number of other in situ spectroscopic and diffraction methods.