Special Issue on Operando Heterogeneous Catalysis

In the field of heterogeneous catalysis research, in situ and operando characterization techniques, i.e. characterization techniques which can be applied under realistic reaction conditions and ideally on a working catalyst, become essential in order to generate new knowledge and understanding of structure performance relationships. Only this knowledge will enable researchers to develop or rather design new catalysts for existing and novel processes without relying on an ‘educated guess’ approach. In combination with ever improving theoretical predictions, operando characterization techniques are expected to be the main drivers in catalyst research and associated material science in the foreseeable future.

Fischer–Tropsch (FT) catalyst systems, and specifically the iron based catalysts, are highly dynamic under reaction conditions, making deductions on structure-activity relationships difficult when relying on conventional characterization techniques. In addition, various deactivation mechanisms, such as oxidation, poisoning, sintering, attrition and phase separation have been observed. As such, the FT synthesis encompasses several challenges experienced in some form or other in most catalytic applications and material science studies. The present review therefore aims to provide a comprehensive account of characterization techniques employed under (quasi) in situ and operando conditions for FT catalysts. Together with a description of the respective technique and a critical discussion of the employed reaction cell, the actual research conducted is briefly discussed. We hope that this combination will enable readers not only to get a good impression of the capabilities and limitations of the respective technique and available instrumentation but also to understand their applicability in catalysis research and maybe to be inspired to push current boundaries.

In this work, we present several methods to determine the temperature of a catalyst sample, as well as the gas surrounding it, in a typical flow reactor used for operando research on heterogeneous catalysis. To determine the sample temperature, we present an approach using calibrated IR-camera imagery, as well as thermographic phosphors. For the gas temperature, we present methods to extract temperature information from planar laser induced fluorescence measurements, one of which can be used during operando studies with an active catalyst in place.

We used low energy electron diffraction (LEED) and temperature programmed desorption (TPD) to investigate the structure and reactivity of iridium oxide layers prepared by oxidizing Ir(1 0 0) at 765 K and O₂ pressures ranging from 0.05 to 5 Torr. Our LEED results provide evidence that Ir(1 0 0) oxidation at O₂ pressures up to 0.20 Torr produces a mixture of Ir oxide structures present as small domains, including a commensurate IrO₂(1 0 1) structure that coexists with other structures. Oxidizing from 0.50 to 1 Torr causes formation of a commensurate IrO₂(1 1 0)R27° structure and a sharp rise in the oxygen uptake from ~8 to 20 ML (monolayer) as the films exhibit signs of roughening. Further increasing the O₂ pressure from 1 to 5 Torr causes the IrO₂(1 1 0)R27° structure to be replaced with a so-called IrO₂(1 1 0)-aligned structure, for which the IrO₂(1 1 0) lattice vectors align with those of the Ir(1 0 0) substrate. We find that the oxidized Ir(1 0 0) surfaces become increasingly reactive toward the dissociation and oxidation of CH₄ as IrO₂(1 1 0) develops on the surface, and observe that the IrO₂(1 1 0)-aligned structure is more reactive than the IrO₂(1 1 0)R27° phase. Our findings demonstrate that the oxide phase evolution on Ir(1 0 0) is sensitive to the O₂ pressure in the range from 0.05 to 5 Torr, and that the development of reactive IrO₂(1 1 0) structures requires elevated O₂ pressures (>0.5 Torr) and temperature.

The interaction of water vapor with ZnO/CuO_x/Cu(1 1 1) surfaces was investigated using synchrotron-based ambient pressure x-ray photoelectron spectroscopy (AP-XPS) and density-functional theory (DFT) calculations. Cu(1 1 1) does not dissociate the water molecule. Cleavage of O–H bonds was seen with AP-XPS after depositing ZnO or preparing CuO_x on the copper substrate. The results of DFT calculations show unique behavior for ZnO/CuO_x/Cu(1 1 1), not seen on Cu(1 1 1), CuO_x/Cu(1 1 1) or ZnO(0 0 0). The ZnO/CuO_x/Cu(1 1 1) system binds water quite well and exhibits the lowest energy barrier for O–H bond cleavage. The presence of unsaturated Zn cations in the islands of ZnO led to high chemical reactivity. In order to remove the OH from ZnO/CuO_x/Cu(1 1 1) and ZnO/Cu(1 1 1) surfaces, heating to elevated temperatures was necessary. At 500–600 K, a significant coverage of OH groups was still present on the surfaces and did react with CO during the water–gas shift (WGS) process. The final state of the sample depended strongly on the amount of ZnO present on the catalyst surface. For surfaces with a ZnO coverage below 0.3 ML, the adsorption of water did not change the integrity of the ZnO islands. On the other hand, for surfaces with a ZnO coverage above 0.3 ML, a ZnO  →  Zn_xOH transformation was observed. This transformation led to a decrease in the WGS catalytic activity.

In situ and operando powder x-ray diffraction and x-ray adsorption spectroscopy was used to study the structural and electronic changes occurring in various Co–TiO₂ catalysts used for the dry reforming of methane. The experimental data showed that large structural changes occur in these catalysts under catalyst activation and reaction conditions. The variations observed in the TiO₂ polymorph type, morphology, surface area, particle size as well as Co oxidation state all contribute to the observed catalytic activity as well as catalyst deactivation giving direct evidence for the deactivation mechanisms. The deactivation of the catalysts can be separated into two distinct events, the first resulting from structural changes occurring during catalyst activation and catalytic reaction, and the second due to gradual Co oxidation over the course of the reaction.

High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry (MS) and surface optical reflectance (SOR) have been combined to simultaneously obtain sub-second information on the surface structure and morphology from a Pd(100) model catalyst during in situ oxidation at elevated temperatures and pressures resulting in Pd bulk oxide formation. The results show a strong correlation between the HESXRD and SOR signal intensities during the experiment, enabling phase determination and a time-resolved thickness estimation of the oxide by HESXRD, complemented by XRR measurements. The experiments show a remarkable sensitivity of the SOR to changes in the surface phase and morphology, in particular to the initial stages of oxidation/reduction. The data imply that SOR can detect the formation of an ultrathin PdO surface oxide layer of only 2–3 Å thickness.

Despite the utility and promise of carbon nanotubes (CNTs), their production is generally based on empirical principles. There are only a few CNT formation models that predict the dependence of their growth on various synthesis parameters. Typically, these do not incorporate a detailed mechanistic consideration of the various processes that are involved during CNT synthesis. We address this need and present a model for catalytic CNT growth that integrates various interdependent physical and chemical processes involved in CNT production. We validate the model by comparing its predictions with one set of experimental measurements from a previous study for cobalt (Co) catalyzed growth. A brief parametric study is presented subsequently. From an application perspective, the model is able to predict the growth rate of the CNT length and its dependence on the ambient temperature and gas-phase feedstock partial pressure.

In this review, the importance of nanoparticles (NPs), with emphasis on their general and specific properties, especially the high surface-to-volume ratio (A/V), in many technological and industrial applications is studied. Some physical and chemical preparation methods for growing several metallic and binary alloy NP catalysts are reviewed. The growth and mechanism of catalytic reactions for synthesis of 1D nanostructures such as ZnO nanowires and multiwall carbon nanotubes (MWCNTs) are discussed. Gas-phase production with emphasis on dependence of catalytic activity and selectivity on size, shape and structure of NPs is also investigated. Application of NP catalysts in several technological processes including H₂ production and storage as well as antibacterial effect, gas sensors and fuel cells is discussed. The mechanism of H₂ production from catalytic photoelectrochemical and photocatalytic degradation reactions of some organic dyes is discussed. Finally, the future outlook of NP catalysts in various disciplines is presented.

In the thin film area, it is well-known that deposition rate impacts the morphology but tools to quantify online fast growth processes are scarce. Here we show that surface differential reflectivity spectroscopy (SDRS) can be used in real time to follow silver nanoparticle growth during sputtering deposition. The main experimental challenge was to avoid noise and saturation due to the plasma emission and to obtain a reasonable signal/noise ratio to monitor fast deposition. A specific setup was designed resulting in an acquisition speed in the range of hundreds of milliseconds and used to investigate the growth of silver on alumina as a test example. The evolution of the size, density and aspect ratio of growing silver islands were determined by modelling their plasmonic response and compared with previous results obtained at a much lower growth rate using physical vapour deposition (Lazzari and Jupille 2012 Nanotechnology 23 135707). During room-temperature sputter deposition, coalescence leads to significantly larger and flatter aggregates compared to evaporation at the same coverage. However, both deposition techniques lead to similar nucleation and growth behaviours. Higher substrate temperature (575 K) did not change the trend and a sticking coefficient close to one was found. The observed evolution of the particle aspect ratio is discussed in terms of supersaturation and flux driven hindrance of the coalescence.

Synchrotron radiation-based near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) has recently become a powerful tool for the investigation of interfacial phenomena in electrochemical power sources such as batteries and fuel cells. Here we present an in situ NAP-XPS study of the anode of a high-temperature direct methanol fuel cell with a phosphoric acid-doped hydrocarbon membrane, which reveals an enhanced flooding of the Pt₃Ru anode with phosphoric acid in the presence of methanol. An analysis of the electrode surface composition depending on the cell voltage and on the presence of methanol reveals the strong influence of the latter on the extent of Pt oxidation and on the transformation of Ru into Ru (IV) hydroxide.

X-ray absorption spectroscopy is a synchrotron radiation based technique that is able to provide information on both local structure and electronic properties in a chemically selective manner. It can be used to characterize the dynamic processes that govern the electrochemical energy storage in batteries, and to shed light on the redox chemistry and changes in structure during galvanostatic cycling to design cathode materials with improved properties. Operando XAS studies have been performed at beamline XAFS at Elettra on different systems. For Li-ion batteries, a multiedge approach revealed the role of the different cathode components during the charge and discharge of the battery. In addition, Li-S batteries for automotive applications were studied. Operando sulfur K-edge XANES and EXAFS analysis was used to characterize the redox chemistry of sulfur, and to relate the electrochemical mechanism to its local structure.