Anastasios Kambolis

Subsecond and in Situ Chemical Speciation of Pt/Al2O3 during Oxidation Reduction Cycles Monitored by High-Energy Resolution Off-Resonant X-ray Spectroscopy

We report an in situ time-resolved high-energy resolution off-resonant spectroscopy study with subsecond resolution providing insight into the oxidation and reduction steps of a Pt catalyst during CO oxidation. The study shows that the slow oxidation step is composed of two characteristic stages, namely, dissociative adsorption of oxygen followed by partial oxidation of Pt subsurface. By comparing the experimental spectra with theoretical calculations, we found that the intermediate chemisorbed O on Pt is adsorbed on atop position, which suggests surface poisoning by CO or surface reconstruction.

Structural Modification of Ni/γ‐Al2O3 with Boron for Enhanced Carbon Resistance during CO Methanation

Carbon deposition during CO methanation from biomass‐derived gas is a significant challenge in terms of catalyst lifetime. It results from the severe reaction conditions imposed by the presence of unsaturated hydrocarbons in the gasified feedstock. This work investigated the structure of boron‐modified Ni/Al2O3 catalysts exhibiting enhanced carbon resistance. As a consequence of B promoting the growth of Ni crystallites, the structure of the B‐modified catalyst was demonstrated to be different at the nanoscale, especially after calcination. The modified catalyst possesses larger Ni particles with porous regions in which B is present. The absence of carbidic and amorphous carbon species, which are considered critical for catalyst deactivation in low‐temperature processes, confirms that B effectively prevents carbon diffusion into Ni, which thus enhances the durability of the catalyst for CO methanation. These results may reveal a strategy of wider significance for developing catalysts with improved stability.

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.

CO Methanation for Synthetic Natural Gas Production

Energy from woody biomass could supplement renewable energy production towards the replacement of fossil fuels. A multi-stage process involving gasification of wood and then catalytic transformation of the producer gas to synthetic natural gas (SNG) represents progress in this direction. SNG can be transported and distributed through the existing pipeline grid, which is advantageous from an economical point of view. Therefore, CO methanation is attracting a great deal of attention and much research effort is focusing on the understanding of the process steps and its further development. This short review summarizes recent efforts at Paul Scherrer Institute on the understanding of the reaction mechanism, the catalyst deactivation, and the development of catalytic materials with benign properties for CO methanation.

High energy X-ray diffraction and IR spectroscopy of Pt/Al2O3 during CO oxidation in a novel catalytic reactor cell

Abstract The operando methodology dictates not only that catalysts are analysed during reaction while activity and selectivity are monitored by analytical methods, but also that the cell in which the measurement is carried out performs as a catalytic reactor. A cell (Chiarello et al., Rev. Sci. Inst. 85 (2014) 074102) used to conduct spectroscopy and diffraction measurements under operando conditions was tested for CO oxidation on a 2 wt% Pt/Al2O3 catalyst and compared with measurements in a conventional quartz catalytic reactor to demonstrate its suitability to derive kinetic data. High energy X-ray diffraction data were collected during alternate CO and O2 pulses under differential conditions to demonstrate the extent of loss of order of the Pt particles upon exposure to the O2 pulse. The presence of a surface oxide species places the catalyst in a higher activity regime compared to the fully reduced one.

Electrochemical promotion of propane oxidation on Pt deposited on a dense β″-Al2O3 ceramic Ag(+) conductor.

A new kind of electrochemical catalyst based on a Pt porous catalyst film deposited on a β″-Al2O3 ceramic Ag+ conductor was developed and evaluated during propane oxidation. It was observed that, upon anodic polarization, the rate of propane combustion was significantly electropromoted up to 400%. Moreover, for the first time, exponential increase of the catalytic rate was evidenced during galvanostatic transient experiment in excellent agreement with EPOC equation.

Mitigation of secondary organic aerosol formation of log wood burning emissions by catalytic removal of aromatic hydrocarbons

Log wood burning is a significant source of volatile organic compounds including aromatic hydrocarbons (ArHC). ArHC are harmful, are reactive in the ambient atmosphere, and are important secondary organic aerosol (SOA) precursors. Consequently, SOA represents a major fraction of the sub-micron organic aerosol pollution from log wood burning. ArHC reduction is thus critical in the mitigation of adverse health and environmental effects of log wood burning. In this study, two Pt-based catalytic converters were prepared and tested for the mitigation of real-world log wood burning emissions, including ArHC and SOA formation, as well as toxic carbon monoxide and methane, a greenhouse gas. Substantial removal of mono- and polycyclic ArHC and phenolic compounds was achieved with both catalysts operated at realistic chimney temperatures (50% conversion was achieved at 200 and 300 °C for non-methane hydrocarbons in our experiments for Pt/Al2O3 and Pt/CeO2-Al2O3, respectively). The catalytically cleaned emissions exhibited a substantially reduced SOA formation already at temperatures as low as 185-310 °C. This reduces the sub-micron PM burden of log wood burning significantly. Thus, catalytic converters can effectively reduce primary and secondary log wood burning pollutants and, thereby, their adverse health impacts and environmental effects.

Electrochemical Promotion of Propane Combustion on Highly Dispersed Pt Nanoparticles

Pt nanoparticles, in a size range which varies from 3 to 20 nm, were dispersed in the porosity of a 7.5 µm-thick layer of LSCF-GDC electrode interfaced on a dense GDC membrane. Small positive polarizations (200 µA / 0.8V - 1.2V) can strongly increase the Pt nanoparticles catalytic performance for propane deep oxidation at low temperatures (267°C-338°C). 40%-enhancement of the propane conversion was achieved with apparent Faraday efficiency values up to 85. These results clearly demonstrate that metallic nanoparticles dispersed in the porosity of a mixed ionic electronic conducting electrode can be electropromoted.