Per-Anders Carlsson

The Active Phase of Palladium during Methane Oxidation

The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

High-Energy Surface X-ray Diffraction for Fast Surface Structure Determination

High-energy x-rays incident at grazing angles allow for rapid collection of surface diffraction beams. [Also see Perspective by Nicklin] Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields, such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction–based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a palladium surface in situ during catalytic carbon monoxide oxidation and follow dynamic restructuring of the surface with subsecond time resolution. Speeding Up Surface Diffraction Surface diffraction methods can determine the atomic structure of the topmost layer of a crystal and also subsurface structures. However, many surface diffraction methods either require ultrahigh vacuum conditions, which limits the reaction conditions that can be studied, or require long data acquisition times, which limits temporal resolution. Using high x-ray energies, Gustafson et al. (p. 758, published online 30 January; see the Perspective by Nicklin) were able to measure the intensities of surface-diffracted beams to follow the surface oxidation that accompanies the changes in a palladium surface during the catalytic oxidation of CO with O2.

Synthesis and functionalization of SSZ-13 as an NH3-SCR catalyst

Hydrothermal synthesis of Na-SSZ-13 using a low concentration of the structure-directing agent N,N,N-trimethyl-1-adamantanimonium hydroxide and functionalization of the SSZ-13 framework through the introduction of Cu and Fe via ion exchange have been carried out. The prepared samples were characterized by XRD, SEM, and UV-vis spectroscopy, N2 sorption, XRF and NH3-TPD. It was found that relatively large iron oxide particles and well-dispersed Cu2+ species are formed as a result of ion exchange with iron and copper, respectively. Furthermore, the catalytic activity for NH3 oxidation, NO oxidation and selective catalytic reduction of NOx with ammonia (NH3-SCR) was investigated both in the absence and in the presence of water. The functionalization of the SSZ-13 structure enhances the catalytic activity for NH3-SCR significantly. While the NH3-SCR activity is negligible for the Na-SSZ-13 sample, the Cu-SSZ-13 sample is highly active especially at low temperatures, and the Fe-SSZ-13 sample shows the highest activity at elevated temperatures.

Electrochemically deposited polycrystalline n-CdSe thin films studied with laterally resolved photoelectrochemistry and SEM

Semiconducting thin films of n-CdSe were electrochemically deposited on Ti substrates with the potential difference as the driving force. By using a photomasking technique different film thicknesses could be obtained on the same substrate. The film electrodes were characterized with photoelectrochemical (PEC) imaging, optical microscopy and scanning electron microscopy (SEM) /energy-dispersive X-ray analysis. PEC imaging is done with a fibre-optical laser scan microscope that allows a laterally resolved mapping of the photoelectrochemical response. The results show that substrate preparation is crucial for the photoelectrochemical performance of the film. At scratches in the substrate the film shows a lower photoresponse. Very thin films give low photoresponse. These films also show a different morphology in the SEM pictures. The films are not homogeneous and have a less ideal composition when they are too thin. We have found PEC imaging a good complement to other methods for characterization of semiconducting thin films.

Mechanisms behind sulfur promoted oxidation of methane.

The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied using a flow-reactor, in situ infrared spectroscopy and in situ high-energy X-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in the absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SO(x) species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporarily enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed.

Novel application of scanning tunneling microscopy — tip current voltammetry of n-GaAs and p-GaP in electrolyte solution

Abstract Scanning tunneling microscopy (STM) has been modified for use on semiconductors in an electrolyte solution providing a new technique, tip current voltammetry (TCV), for characterization of semiconductor/electrolyte interfaces. It provides information about chemical reactions on the surface, the interface energetics, and the current transport mechanisms. The TCV technique has been applied to the study of n-GaAs (with two different doping levels) and p-GaP in 2 mM NaOH. The tip current is measured, while scanning the potential on the semiconductor electrode versus a reference electrode, keeping the tip at constant potential versus the reference electrode. With the semiconductor potential in the accumulation region, tunneling is obtained in the normal way. When changing the potential into the depletion region, the distance between tip and electrode surface goes to zero, the instrument goes into “contact mode”. In the depletion region the p-GaP and n-GaAs samples behaved differently. At the p-GaP sample the tip current drastically dropped at the flat-band potential, while at the n-GaAs sample the “tunneling mode” is maintained even at depletion, + 700 mV from the flat-band potential. At the very highly doped n-GaAs sample, a new tunneling mode is obtained again when the semiconductor potential reaches the inversion region. The tip current voltammograms are affected by surface reactions and by illumination. The transition potential between tunneling and contact modes is shifted negatively at an oxidized surface and returns to its original value if the electrode is held at negative potentials.

Characterization of particulate emissions and methodology for oxidation of particulates from non-diesel combustion systems

Tailpipe particulate emissions, i.e., particle number, size distribution and total mass, from a series of four-cylinder engines with 2L displacement and power output of approximately 150 hp have been measured. The engines were in their respective vehicle installation, all midsize vehicles from various manufacturers, and represented different combustion concepts, i.e., port- and direct-injected vehicles and E5 and E85 fuels. The results are compared to post-Euro V emission standards for gasoline and biofuels using diesel as reference. The results show that the type of combustion and fuel significantly affect the particulate formation. In general, direct-injected engines show high particle numbers and mass compared to port-injected engines. The particulate number and total mass can be reduced by using biofuels, e.g., ethanol mixes, instead of gasoline. Moreover, an experimental procedure and setup facilitating precise studies of oxidation of particulates in realistic filter structures by well-controlled gas flow (composition and temperature) and sample (particulate load and temperature) conditions has been developed. The results from this method have been verified by using commercial soot as reference.

Induced low temperature catalytic ignition by transient changes in the gas composition

The effect of gas composition changes on the low temperature activity for supported platinum model catalysts has been studied. By introducing well-controlled periodic O2 pulses to a simple diluted gas mixture of CO and O2, a substantial improvement of the low temperature oxidation activity was observed. The reason for low activity on noble metals at low temperatures is often attributed to self-poisoning by CO. The improved catalytic performance observed is proposed to origin from the transients causing a surface reactant composition that is favourable for the reaction rate, i.e. lower degree of self-poisoning. This was also confirmed by in situ Fourier transform infrared spectroscopy in combination with mass spectrometry measurements, which gave evidence for the existence of a strong interplay between the gas phase concentration and the adsorbate composition for these catalysts.

In-situ STM imaging of n-GaAs during anodic photocorrosion

Abstract Scanning tunnelling microscopy was used to make a real-time, in-situ study of the changes of the surface topography during the anodic photocorrosion of n-GaAs (100) surfaces. To find suitable conditions for such studies, the semiconductor potential was scanned with the tip held at a fixed potential and at a fixed position. The recorded images show a clear photocorrosion process, where the surface roughness increases with time.

Effect of Preparation Procedure on the Catalytic Properties of Fe-ZSM-5 as SCR Catalyst

Fe-ZSM-5 catalysts, prepared by different methods, have been characterized by BET, ICP-AES, XRD, XPS, NH3-TPD and NO-TPD and evaluated for NOx reduction according to standard NH3-SCR, NH3 oxidation and NO oxidation, in absence and presence of water. The presence of water has a significant influence on both the SCR and oxidation reactions. The most active catalyst for NH3-SCR is prepared by ion exchange using FeCl2 as iron precursor. The XPS results indicate that Fe2+ ions are the main active sites for the SCR reactions, while Fe3+ ions are the primarily active sites for oxidation of ammonia.