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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.

Methane conversion during cooling ramp: water inhibits methane conversion more severely in high oxygen concentration.

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Methane oxidation over Pd-Pt/Al 2 O 3 model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd-Pt nanoparticles are present only on catalysts subjected to elevated temperature (800 • C) whereas calcination at lower temperature (500 • C) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd-Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd-Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.