Surface and Subsurface Dynamics of the Intermetallic Compound ZnNi in Methanol Steam Reforming

The intermetallic compound ZnNi has been tested as unsupported powder catalyst in methanol steam reforming, showing severe performance changes during the process. Results from <i>in situ</i> XPS experiments proved the decomposition of the surface of ZnNi by Zn oxidation and complete desegregation of Ni. The decomposition of ZnNi even extends toward the bulk, as proven by <i>in situ</i> DTA/TG and XRD. On the basis of these findings, the catalytic properties initially ascribed to ZnNi can not be attributed to the intermetallic compound but have to be assigned to a mixture of the decomposition products

Restructuring of silica supported vanadia during propane oxidative dehydrogenation studied by combined synchrotron radiation based in situ soft X-ray absorption and photoemission

<p>A series of vanadia catalysts supported on mesoporous silica SBA-15 has been prepared with a loading in the range of 2–14 wt-% V and characterized under oxygen and propane oxidative dehydrogenation reaction conditions at elevated temperature up to 550 °C. <i>In situ</i> soft X-ray absorption spectra at the vanadium L- and oxygen K-edges and <i>in situ</i> synchrotron based X-ray photoemission spectra reveal a restructuring of vanadium species that results in an enhanced degree of dispersion of molecular vanadia species on the silica support. The impact of the X-ray beam on the XAS spectra of dispersed V_<i>x</i>O_<i>y</i> species has been studied and a brief perspective of X-ray based electron spectroscopy as a probe in catalyst characterization is given.</p

Evidence for the Bifunctional Nature of Pt–Re Catalysts for Selective Glycerol Hydrogenolysis

Rhenium substantially promotes the rate of Pt-catalyzed glycerol hydrogenolysis to propanediols and shifts the product selectivity from 1,2-propanediol to a mixture of 1,2 and 1,3-propanediols. This work presents experimental evidence for a tandem dehydration–hydrogenation mechanism that occurs over a bifunctional Pt–Re catalyst. Infrared spectroscopy of adsorbed pyridine and the rate of aqueous-phase hydrolysis of propyl acetate were used to identify and quantify Brønsted acid sites associated with the Re component. Near-ambient-pressure XPS revealed a range of Re oxidation states on the Pt–Re catalysts after reduction in H₂ at 393 and 493 K, which accounts for the presence of Brønsted acidity. A mechanism involving acid-catalyzed dehydration followed by Pt-catalyzed hydrogenation was consistent with the negative influence of added base, a primary kinetic isotope effect with deuterated glycerol, an inverse isotope effect with dideuterium gas, and the observed orders of reaction

Structure–Activity Studies on Highly Active Palladium Hydrogenation Catalysts by X‑ray Absorption Spectroscopy

Functionalized carbon nanotubes were used to produce Pd-based hydrogenation catalysts. Pd/CNT with small (1–2 nm) Pd particles showed classical catalytic behavior in propyne hydrogenation, with high propene selectivity at moderate conversion levels and propane formation near full conversion. Pd/CNT with larger (∼15 nm) nanoparticles, however, was selective (88%) toward propene even at practically full propyne conversion. An additionally prepared Pd₂Ga/CNT catalyst exhibited even higher propene selectivity at full conversion. All of these materials were studied in situ by X-ray absorption spectroscopy at the Pd K-edge. Pd₂Ga/CNT was stable under all conditions examined without variation in XANES or in the derived EXAFS parameters. Both Pd/CNT samples formed β-hydride under hydrogen, as assessed from the calculated lattice expansion and the characteristic red shift of the XANES maxima. The minor spectroscopic difference between the monometallic catalysts observed at high propyne conversion suggests the decisive role of a Pd–C (subsurface C) contribution in the structure of larger Pd particles, being absent with ultrasmall nanoparticles. In general, all factors (intermetallic phase formation, subsurface C, etc.) that reduce the surface H coverage will give rise to enhanced partial hydrogenation selectivity of palladium when secondary alkene hydrogenation at late bed segments or diffusion issues in the pores are avoided

<i>In Situ</i> NAP-XPS and Mass Spectrometry Study of the Oxidation of Propylene over Palladium

The oxidation of propylene over a Pd(551) single crystal has been studied in the millibar pressure range using near-ambient pressure X-ray photoelectron spectroscopy and mass spectrometry. It has been shown that, irrespective of the O₂/C₃H₆ molar ratio in the range 1–100, the total oxidation of propylene to CO₂ and water and the partial oxidation of propylene to CO and H₂ occur when the catalyst is heated above the light-off temperature; increasing the partial pressure of O₂ leads to decreasing the catalytic activity. The selectivity toward CO₂ is at least two times higher than the selectivity toward CO, indicating that the total oxidation is the main reaction route. The normal hysteresis with a light-off temperature higher than the extinction temperature is observed in the oxidation of propylene between 100 and 300 °C. According to NAP-XPS, the main reason for the hysteresis appearing is a competition between two surface processes: carbonization and oxidation of palladium. At low temperatures, the adsorption and following decomposition of propylene dominate, which results in accumulation of carbonaceous deposits blocking the palladium surface. Increasing the catalyst temperature leads to burning the carbonaceous deposits which initiates the following oxidation of propylene. The highest conversion of propylene is observed when both free surface sites and adsorbed oxygen atoms exist in a large amount on the catalyst surface. As the partial pressure of O₂ increases, the catalyst surface gets covered by clusters of surface 2D palladium oxide, which is accompanied by a decrease in the catalytic activity. The mechanism of the oxidation of propylene over palladium is discussed

Mixing Patterns and Redox Properties of Iron-Based Alloy Nanoparticles under Oxidation and Reduction Conditions

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated <i>in situ</i> under H₂ and O₂ atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with <i>ex situ</i> transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined Fe-Me nanoalloys with an initial size of 5 nm was achieved by using the mass-selected low energy cluster beam deposition (LECBD) technique. The spectroscopic methods permit the direct observation of the surface segregation and composition under different gas atmospheres and annealing temperatures. The ambient conditions were found to have a significant influence on the mixing pattern and oxidation state of the nanoparticles. In an oxidative atmosphere, iron oxidizes and segregates to the surface, leading to the formation of core–shell nanoparticles. This structure persists upon mild reduction conditions, while phase separation and formation of heterostructured bimetallic particles is observed upon H₂ annealing at a higher temperature (400 °C). Depending on the noble metal core, the iron oxide shell might be partially distorted from its bulk structure, while the reduction in H₂ is also significantly influenced. These insights can be of a great importance in understanding the activity and stability of Fe-based bimetallic nanoparticles under reactive environments

Ambient-Pressure Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate

In order to close the pressure gap in the investigation of catalyst surfaces under real operation conditions we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis system to study the pressure dependency of the electronic and catalytic properties of catalyst surfaces in reactive atmospheres at elevated temperatures. With this setup we investigated the vanadium L₃-edge and catalytic performance of polycrystalline vanadyl pyrophosphate in the selective oxidation of <i>n</i>-butane to maleic anhydride between 10 and 1000 mbar at 400 °C. As a result, major gas phase and pressure dependent spectral changes are observed at energies attributed to V 2p-3d_<i>z</i>² excitations assigned to vanadium atoms square-pyramidally coordinated to oxygen atoms. This can be interpreted in terms of a shortened vanadyl bond (VO) and an increased vanadium oxidation state with higher pressures. Since this is accompanied by an increasing catalytic activity and selectivity, it indicates that vanadyl oxygen is actively involved in the selective oxidation of the alkane

Adjusting the Chemical Reactivity of Oxygen for Propylene Epoxidation on Silver by Rational Design: The Use of an Oxyanion and Cl

The development of catalysts for propylene oxide production from direct epoxidation using propylene and oxygen remains a challenge. Compared to ethylene epoxidation, where selectivity on silver catalysts is high, the low selectivity to produce propylene oxide over silver is partially attributed to the lack of electrophilic oxygen under propylene epoxidation reaction conditions. Here, we investigate how to mediate the chemical reactivity of oxygen by theory-inspired experiments for propylene epoxidation. We show how adding electrophilic-O via SO4 oxyanions to the surface of silver increases epoxide selectivity. Moreover, we show how the addition of Cl to the SO4-modified catalyst activates the oxyanion, giving a more than 4-fold increase in selectivity to propylene oxide. Finally, we explore different systems using DFT and draw a picture on how the next catalyst/co-catalyst systems should be tuned to design a catalyst with high selectivity for direct propylene oxidation

When a Metastable Oxide Stabilizes at the Nanoscale: Wurtzite CoO Formation upon Dealloying of PtCo Nanoparticles

Ambient pressure photoelectron and absorption spectroscopies were applied under 0.2 mbar of O₂ and H₂ to establish an unequivocal correlation between the surface oxidation state of extended and nanosized PtCo alloys and the gas-phase environment. Fundamental differences in the electronic structure and reactivity of segregated cobalt oxides were associated with surface stabilization of metastable wurtzite-CoO. In addition, the promotion effect of Pt in the reduction of cobalt oxides was pronounced at the nanosized particles but not at the extended foil

Methanol Steam Reforming over Indium-Promoted Pt/Al₂O₃ Catalyst: Nature of the Active Surface

The surface state of the Pt/In₂O₃/Al₂O₃ catalyst coated onto a microchannel stainless steel reactor was investigated under working conditions using synchrotron-based ambient pressure photoelectron (APPES) and X-ray absorption near-edge structure (XANES) spectroscopies, combined with online mass spectrometry. The surface of the fresh catalyst consists of metallic Pt, In₂O₃, and Al₂O₃. Reduction under 0.2 mbar of H₂ at 250 °C leads to surface enhancement of Pt and partial reduction of In₂O₃, while Al₂O₃ remains unchanged. Reoxidation in O₂ atmosphere stimulates surface segregation of In₂O₃ over Pt, accompanied by partial oxidation of Pt to PtO_<i>x</i>. Based on these results a dynamic, gas-phase-dependent surface state is demonstrated. Under methanol steam reforming conditions, the surface state rapidly adapts under the reaction stream regardless of the pretreatment. However, correlation of gas phase with spectroscopic results under working conditions pointed out the beneficial effect of surface indium to reduce the CO selectivity. Finally, evidence of a distorted symmetry of Al sites on Pt/In₂O₃/Al₂O₃ catalyst compared to that of γ-Al₂O₃ is given. The findings obtained in the present study are of fundamental significance in understanding the relation between the surface state and the catalytic performance of a functional methanol reforming catalyst