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.

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.

New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions

Infrared absorption spectroscopy has proven to be one of the most powerful spectroscopic techniques available for the characterization of catalytic systems. Although the history of IR absorption spectroscopy in catalysis is long, the technique continues to provide key fundamental information about a variety of catalysts and catalytic reactions, and to also offer novel options for the acquisition of new information on both reaction mechanisms and the nature of the solids used as catalysts. In this review, an overview is provided of the main contributions that have been derived from IR absorption spectroscopy studies of catalytic systems, and a discussion is included on new trends and new potential directions of research involving IR in catalysis. We start by briefly describing the power of Fourier-transform IR (FTIR) instruments and the main experimental IR setups available, namely, transmission (TIR), diffuse reflectance (DRIFTS), attenuated total reflection (ATR-IR), and reflection-absorption (RAIRS), for advancing research in catalysis. We then discuss the different environments under which IR characterization of catalysts is carried out, including in situ and operando studies of typical catalytic processes in gas-phase, research with model catalysts in ultrahigh vacuum (UHV) and so-called high-pressure cell instruments, and work involving liquid/solid interfaces. A presentation of the type of information extracted from IR data follows in terms of the identification of adsorbed intermediates, the characterization of the surfaces of the catalysts themselves, the quantitation of IR intensities to extract surface coverages, and the use of probe molecules to identify and titrate specific catalytic sites. Finally, the different options for carrying out kinetic studies with temporal resolution such as rapid-scan FTIR, step-scan FTIR, and the use of tunable lasers or synchrotron sources, and to obtain spatially resolved spectra, by sample rastering or by 2D imaging, are introduced.

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.

Synchrotron high energy X-ray methods coupled to phase sensitive analysis to characterize aging of solid catalysts with enhanced sensitivity

X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information. Here we show in two case studies related to hydrothermal aging and chemical modification of model automotive catalysts that enhanced detailed information of structural changes can be obtained when the two methods are combined with a concentration modulated excitation (cME) approach and phase sensitive detection (PSD). The catalysts are subject to a modulation experiment consisting of the periodic variation of the gas feed composition to the catalyst and the time-resolved data are additionally treated by PSD. In the case of a 2 wt% Rh/Al2O3 catalyst, a very small fraction (ca. 2%) of Rh remaining exposed at the alumina surface after hydrothermal aging at 1273 K can be detected by PSD. This Rh is sensitive to the red-ox oscillations of the experiment and is likely responsible for the observed catalytic activity and selectivity during NO reduction by CO. In the case of a 1.6 wt% Pd/Al2O3-Ce(1-x)Zr(x)O2 catalyst, preliminary results of cME-XRD demonstrate that access to the kinetics of the whole material at work can be obtained. Both the red-ox processes involving the oxygen storage support and the Pd component can be followed with great precision. PSD enables the differentiation between Pd deposited on Al2O3 or on Ce(1-x)Zr(x)O2. Modification of the catalyst by phosphorous clearly induces loss of the structural dynamics required for oxygen storage capacity that is provided by the Ce(4+)/Ce(3+) pair. The two case studies demonstrate that detailed kinetics of subtle changes can be uncovered by the combination of in situ X-ray absorption and high energy diffraction methods with PSD.

IR and X-ray time-resolved simultaneous experiments: an opportunity to investigate the dynamics of complex systems and non-equilibrium phenomena using third-generation synchrotron radiation sources

Third-generation storage rings are modern facilities working with high currents and designed to host powerful radiation sources, like undulators and wigglers, and to deliver high-brilliance beams to users. Many experiments at high spatial resolution, such as spectromicroscopy at the nanometre scale and with high temporal resolution to investigate kinetics down to the picosecond regime, are now possible. The next frontier is certainly the combination of different methods in a unique set-up with the ultimate available spatial and temporal resolutions. In the last decade much synchrotron-based research has exploited the advantage of complementary information provided by time-resolved X-ray techniques and optical methods in the UV/Vis and IR domains. New time-resolved and concurrent approaches are necessary to characterize complex systems where physical-chemical phenomena occur under the same experimental conditions, for example to detect kinetic intermediates via complementary but independent observations. In this contribution we present scientific cases from original works and literature reviews to support the proposed IR/X-ray simultaneous approach, with both probes exploiting synchrotron radiation sources. In addition, simple experimental layouts that may take advantage of the high brilliance and the wide spectral distribution of the synchrotron radiation emission will be given for specific researches or applications to investigate dynamic processes and non-equilibrium phenomena occurring in many condensed matter and biological systems, of great interest for both fundamental research and technological applications.