Comparison of AP-XPS and PLIF Measurements During CO Oxidation Over Pd Single Crystals

Time-resolved surface reaction kinetics in the pressure gap

We extend the use of our recently developed Near-Ambient Pressure Velocity Map Imaging (NAP-VMI) technique to study the kinetics and dynamics of catalytic reactions in the pressure gap. As an example, we show that NAP-VMI combined with molecular beam surface scattering allows the direct measurement of time- and velocity-resolved kinetics of the scattering and oxidation of CO on the Pd(110) surface with oxygen pressures at the surface up to 1 × 10-5 mbar, where different metastable surface structures form. Our results show that the c(2 × 4) oxide structure formed at low O2 pressure is highly active for CO oxidation. The velocity distribution of the CO2 products shows the presence of two reaction channels, which we attribute to reactions starting from two distinct but rapidly interconverting CO binding sites. The effective CO oxidation reaction activation energy is Er = (1.0 ± 0.13) eV. The CO2 production is suppressed at higher O2 pressure due to the number of antiphase domain boundaries increasing, and the missing row sites are filled by O-atoms at O2 pressures approaching 1 × 10-6 mbar. Filling of these sites by O-atoms reduces the CO surface lifetime, meaning the surface oxide is inactive for CO oxidation. We briefly outline further developments planned for the NAP-VMI and its application to other types of experiments.

Near-ambient pressure velocity map imaging

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the “pressure gap” between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 × 10−3 mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

A highly sensitive gas chromatograph for in situ and operando experiments on catalytic reactions

We describe an automated gas sampling and injection unit for a gas chromatograph (GC). It has specially been designed for low concentrations of products formed in catalytic in situ and operando experiments when slow reactions on single crystal models are investigated. The unit makes use of a buffer volume that is filled with gas samples from the reactor at a reduced pressure. The gas samples are then compressed by He to the injection pressure of 1000 mbar and pushed into two sample loops of the GC, without major intermixing with He. With an additional cryo trap at one of the GC column heads, the design aims at concentrating the gas samples and focusing the peaks. The performance is characterized by experiments on the Fischer-Tropsch synthesis, using H₂/CO mixtures (syngas) at 200 and 950 mbar and a Co(0001) single crystal sample as model catalyst. Chromatograms recorded during the reaction display sharp, well separated peaks of saturated and unsaturated C₁ to C₄ hydrocarbons formed by the reaction, whereas the syngas matrix only gives moderate signals that can be well separated from the product peaks. Detection and quantification limits of 0.4 and 1.3 ppb, respectively, have been achieved and turnover numbers as low as 10^-5 s^-1 could be measured. The system can be combined with all known analysis techniques used in in situ and operando experiments.

High energy surface x-ray diffraction applied to model catalyst surfaces at work

Catalysts are materials that accelerate the rate of a desired chemical reaction. As such, they constitute an integral part in many applications ranging from the production of fine chemicals in chemical industry to exhaust gas treatment in vehicles. Accordingly, it is of utmost economic interest to improve catalyst efficiency and performance, which requires an understanding of the interplay between the catalyst structure, the gas phase and the catalytic activity under realistic reaction conditions at ambient pressures and elevated temperatures. In recent years efforts have been made to increasingly develop techniques that allow for investigating model catalyst samples under conditions closer to those of real technical catalysts. One of these techniques is high energy surface x-ray diffraction (HESXRD), which uses x-rays with photon energies typically in the range of 70-80 keV. HESXRD allows a fast data collection of three dimensional reciprocal space for the structure determination of model catalyst samples under operando conditions and has since been used for the investigation of an increasing number of different model catalysts. In this article we will review general considerations of HESXRD including its working principle for different model catalyst samples and the experimental equipment required. An overview over HESXRD investigations performed in recent years will be given, and the advantages of HESXRD with respect to its application to different model catalyst samples will be presented. Moreover, the combination of HESXRD with other operando techniques such as in situ mass spectrometry, planar laser-induced fluorescence and surface optical reflectance will be discussed. The article will close with an outlook on future perspectives and applications of HESXRD.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Novel in Situ Techniques for Studies of Model Catalysts

Motivated mainly by catalysis, gas-surface interaction between single crystal surfaces and molecules has been studied for decades. Most of these studies have been performed in well-controlled environments and have been instrumental for the present day understanding of catalysis, providing information on surface structures, adsorption sites, and adsorption and desorption energies relevant for catalysis. However, the approach has been criticized for being too far from a catalyst operating under industrial conditions at high temperatures and pressures. To this end, a significant amount of effort over the years has been used to develop methods to investigate catalysts at more realistic conditions under operating conditions. One result from this effort is a vivid and sometimes heated discussion concerning the active phase for the seemingly simple CO oxidation reaction over the Pt-group metals in the literature. In recent years, we have explored the possibilities to perform experiments at conditions closer to those of a technical catalyst, in particular at increased pressures and temperatures. In this contribution, results from catalytic CO oxidation over a Pd(100) single crystal surface using Near Ambient Pressure X-ray Photo emission Spectroscopy (NAPXPS), Planar Laser-Induced Fluorescence (PLIF), and High Energy Surface X-ray Diffraction (HESXRD) are presented, and the strengths and weaknesses of the experimental techniques are discussed. Armed with structural knowledge from ultrahigh vacuum experiments, the presence of adsorbed molecules and gas-phase induced surface structures can be identified and related to changes in the reactivity or to reaction induced gas-flow limitations. In particular, the application of PLIF to catalysis allows one to visualize how the catalyst itself changes the gas composition close to the model catalyst surface upon ignition, and relate this to the observed surface structures. The effect obscures a straightforward relation between the active phase and the activity, since in the case of CO oxidation, the gas-phase close to the model catalyst surface is shown to be significantly more oxidizing than far away from the catalyst. We show that surface structural knowledge from UHV experiments and the composition of the gas phase close to the catalyst surface are crucial to understand structure-function relationships at semirealistic conditions. In the particular case of Pd, we argue that the surface structure of the PdO(101) has a significant influence on the activity, due to the presence of Coordinatively Unsaturated Sites (CUS) Pd atoms, similar to undercoordinated Ru and Ir atoms found for RuO₂(110) and IrO₂(110), respectively.

In Situ Optical Reflectance Difference Observations of CO Oxidation over Pd(100)

Using a home-built reflectometer, we have investigated the changes in the optical reflectivity of a Pd(100) model catalyst during CO oxidation under high-pressure, high-temperature conditions. We observe changes in optical contrast when exposing the surface to CO oxidation conditions at 200 mbar from room temperature up to 400 °C. These changes in reflectivity are a result both of the formation of a surface oxide layer and of a change in surface roughness because of gas exposure. However, the reflectivity is more sensitive to the presence of a thin, flat oxide layer than to surface roughness. CO oxidation plays an important role in the decrease of the reflectivity. Since adding a reducing agent to the gas mixture renders it unlikely that the oxide thickness increases, we conclude that the observed decrease in reflectivity is dominated by increased surface roughness because of the catalytic reaction. We contribute this observed surface roughening to a Mars-van Krevelen-type reaction mechanism.

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

Application of surface-science techniques, such as XPS, SXRD, STM, and IR spectroscopy under catalytic reactions conditions yield new structural and chemical information. Recent experiments focusing on CO oxidation over Pt and Pd model catalysts were reviewed.

2D and 3D imaging of the gas phase close to an operating model catalyst by planar laser induced fluorescence

In recent years, efforts have been made in catalysis related surface science studies to explore the possibilities to perform experiments at conditions closer to those of a technical catalyst, in particular at increased pressures. Techniques such as high pressure scanning tunneling/atomic force microscopy (HPSTM/AFM), near ambient pressure x-ray photoemission spectroscopy (NAPXPS), surface x-ray diffraction (SXRD) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) at semi-realistic conditions have been used to study the surface structure of model catalysts under reaction conditions, combined with simultaneous mass spectrometry (MS). These studies have provided an increased understanding of the surface dynamics and the structure of the active phase of surfaces and nano particles as a reaction occurs, providing novel information on the structure/activity relationship. However, the surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface. Therefore, the catalytic activity of the sample itself will act as a gas-source or gas-sink, and will affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, we have applied planar laser induced fluorescence (PLIF) to the gas phase in the vicinity of an active model catalysts. Our measurements demonstrate that the gas composition differs significantly close to the catalyst and at the position of the MS, which indeed should have a profound effect on the surface structure. However, PLIF applied to catalytic reactions presents several beneficial properties in addition to investigate the effect of the catalyst on the effective gas composition close to the model catalyst. The high spatial and temporal resolution of PLIF provides a unique tool to visualize the on-set of catalytic reactions and to compare different model catalysts in the same reactive environment. The technique can be applied to a large number of molecules thanks to the technical development of lasers and detectors over the last decades, and is a complementary and visual alternative to traditional MS to be used in environments difficult to asses with MS. In this article we will review general considerations when performing PLIF experiments, our experimental set-up for PLIF and discuss relevant examples of PLIF applied to catalysis.