CO2 methanation over Rh/Al2O3, Rh/CeO2 and Ni/CeO2 at 350 °C highlighting the different surface speciation during reaction.

In situ diffuse reflectance infrared Fourier transform spectroscopy has been used to distinguish surface hydroxyl groups on Al 2 O 3 and PdO/Al 2 O 3 model catalysts calcined at 500−900 °C. Employing the operando approach, the formation of surface hydroxyl groups has been correlated to the methane oxidation activity for PdO/Al 2 O 3 catalysts using a PdO powder sample as reference. The results show that the alumina support stabilizes active PdO particles leading to enhanced apparent methane turnover frequency (TOF), which decreases slowly in dry conditions due to alumina hydroxylation. Wet conditions cause severe hydroxylation that is detrimental for the methane TOF. The hydroxylation follows two different routes, i.e., spillover of hydrogen-containing species to the PdO-Al 2 O 3 boundary and/or the close proximity of the supported PdO particles and under wet conditions also dissociation of gas phase water on the entire alumina surface. Both hydroxylation routes obey varying kinetics such that near saturation is reached quickly (minutes) followed by a continuous slow growth for prolonged exposure times (hours). At low temperatures, inhibition of palladium active sites on the rim of the PdO particles close to alumina seems to be of particular importance for the observed detrimental effect of water, whereas water induced morphological changes (no sintering observed) of the PdO particles play a minor role.

Structural and morphological characterisation of bimetallic Pd-Pt/Al 2 O 3 model catalysts are performed using X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy and CO chemisorption. Further, the catalysts were studied under oxidising and reducing conditions using both X-ray absorption spectroscopy and low-energy ion scattering spectroscopy. For the as-prepared catalysts, the existence of alloyed bimetallic Pd-Pt particles and of (tetragonal) PdO were found for the samples calcined at 800 C. PdO is present in form of crystals at the surface of the Pd-Pt particles or as isolated PdO crystals on the support oxide. Bimetallic Pd-Pt nanoparticles were only formed on the Pd-Pt catalysts after calcination at 800 C. The results show that the Pd-Pt nanoparticles undergo reversible changes in surface structure composition and chemical state in response to oxidising or reducing conditions. Under oxidising conditions Pd segregates to the shell and oxidises forming PdO, while under reducing conditions regions with metallic Pd and Pd-Pt alloys were observed at the surface.No bimetallic Pd-Pt nanoparticles were observed for the sample initially calcined at 500 C, but instead isolated monometallic particles, where small Pt particles are easily oxidised under O 2 treatment. In the monometallic catalysts, the Pd is found to be completely oxidised already after calcination and to consist of metallic Pd after reductive treatment.

No abstract

The catalytic activity for dry and wet methane oxidation over a series of palladium-alumina catalysts with palladium loadings from 0.23 to 3.6 wt.-% Pd and systematically varied PdO dispersions from...

In situ Fourier transform infrared spectroscopy was used to study transient CO oxidation over a series of bimetallic Pd−Pt catalysts with different Pd:Pt molar ratios. The catalysts were found to contain both alloyed PdPt nanoparticles (particle sizes 25−35 nm) and monometallic Pd nanoparticles (sizes below 10 nm). For oxygen-free conditions, CO reduces the surface while simultaneously function as a chemical probe molecule whereby the CO adsorption sites can be characterized. Under these conditions, it is shown that adsorbed carbonyl species form both on the Pd and Pt. On platinum, CO adsorbs predominantly linearly on top, whereas on palladium it adsorbes in bridged configurations. This behavior is used for site speciation of the catalysts. The spectra from the bimetallic Pd−Pt catalysts are more complicated than a direct superposition of the spectra for the monometallic catalysts as a consequence of alloy formation and enrichment of Pd at the surface of the reduced catalysts. The temperatureprogrammed CO oxidation results show that the addition of Pd to the Pt catalyst supported on alumina shifts the CO-poisoned state to lower temperatures, therefore increasing the temperature range for the CO oxidation at low temperatures.

By use of operando spectroscopies under cycling reaction conditions, water is shown to hamper the redox dynamics of realistic palladium oxide nanoparticles dispersed onto alumina and hydrophobic zeolite supports thereby lowering the activity for total oxidation of methane. Water adsorption forms hydroxyl ad‐species that block the methane and oxygen dissociation and seem to prevent lattice oxygen to take part in the methane oxidation. The main catalytic action is thus proposed to shift from the Mars‐van Krevelen mechanism in dry conditions to a slower route that relies on Langmuir‐Hinshelwood type of steps in wet conditions. This key finding has clear implications on catalyst design for low‐temperature gas combustion emission control.

A portable device for the generation of co-feeds of water vapor has been designed, constructed, and evaluated for flexible use as an add-on component to laboratory chemical reactors. The vapor is formed by catalytic oxidation of hydrogen, which benefits the formation of well-controlled minute concentrations of ultra-pure water. Analysis of the effluent stream by on-line mass spectrometry and Fourier transform infrared spectroscopy confirms that water vapor can be, with high precision, generated both rapidly and steadily over extended periods in the range of 100 ppm to 3 vol. % (limited by safety considerations) using a total flow of 100 to 1500 ml/min at normal temperature and pressure. Further, the device has been used complementary to a commercial water evaporator and mixing system to span water concentrations up to 12 vol. %. Finally, an operando diffuse reflective infrared Fourier transform spectroscopic measurement of palladium catalysed methane oxidation in the absence and presence of up to 1.0 vol. % water has been carried out to demonstrate the applicability of the device for co-feeding well-controlled low concentrations of water vapor to a common type of spectroscopic experiment. The possibilities of creating isotopically labeled water vapor as well as using tracer gases for dynamic experiments are discussed.