In situ NAP-XPS spectroscopy during methane dry reforming on ZrO2/Pt(1 1 1) inverse model catalyst

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO₂) in H₂ oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO₂) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO₂, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO₂ support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Partial Oxidation of Bio-methane over Nickel Supported on MgO-ZrO2 Solid Solutions

Syngas can be produced from biomethane via Partial Oxidation of Methane (POM), being an attractive route since it is ecofriendly and sustainable. In this work, catalysts of Ni supported on MgO-ZrO2 solid solutions, prepared by a one-step polymerization method, were characterized by HRTEM/EDX, XRD, XPS, H2-TPR, and in situ XRD. All catalysts, including Ni/ZrO2 and Ni/MgO as reference, were tested for POM (CH4:O2 molar ratio 2, 750 ºC, 1 atm). NiO/MgO/ZrO2 contained two solid-solutions, MgO-ZrO2 and NiO-MgO, as revealed by XRD and XPS. Ni (30 wt%) supported on MgO-ZrO2 solid solution exhibited high methane conversion and hydrogen selectivity. However, depending on the MgO amount (0, 4, 20, 40, 100 molar percent) major differences in NiO reducibility, growth of Ni0 crystallite size during H2 reduction and POM, and in carbon deposition rates were observed. Interestingly, catalysts with lower MgO content achieved the highest CH4 conversion (~ 95%), high selectivity to H2 (1.7) and CO (0.8), and low carbon deposition rates (0.024 g carbon.gcat-1 h-1) with Ni4MgZr (4 mol% MgO) turning out to be the best catalyst. In situ XRD during POM indicated metallic Ni nanoparticles (average crystallite size of 31 nm), supported by MgO-ZrO2 solid solution, with small amounts of NiO-MgO being present as well. The presence of MgO also influenced the morphology of the carbon deposits, leading to filaments instead of amorphous carbon. A combustion-reforming mechanism is suggested and using a MgO-ZrO2 solid solution support strongly improves catalytic performance, which is attributed to effective O2, CO2 and H2O activation at the Ni/MgO-ZrO2 interface.

MgO-ZrO2 Ceramic Composites for Silicomanganese Production

The deterioration of the refractory lining represents a significant problem for the smooth operation in the ferroalloys industry, particularly in the production of silicomanganese, due to the periodic requirements of substitution of the damaged refractory. Within this context, magnesia refractories are commonly employed in the critical zones of the furnaces used in silicomanganese production since the slag involved in the process has a basic character. The behavior of MgO–ZrO₂ ceramic composites with different ZrO₂ nanoparticles (0, 1, 3, and 5 wt.%) contents in the presence of silicomanganese slags is proposed in this manuscript. XPS, XRD and SEM–EDX were used to evaluate the properties of the ceramic composite against the silicomanganese slag. The static corrosion test was used to evaluate the corrosion of the refractory. Results suggest that corrosion is controlled by the change in slag viscosity due to the reaction between CaZrO₃ and the melted slag. Besides, ZrO₂ nanoparticles located at both triple points and grain boundaries act as a barrier for the slag advance within the refractory. The utilization of MgO refractories with ZrO₂ nanoparticles can extend the life of furnaces used to produce silicomanganese.

In Situ Growth of Exsolved Nanoparticles under Varying rWGS Reaction Conditions—A Catalysis and Near Ambient Pressure-XPS Study

Perovskite-type oxides are highly flexible materials that show properties that are beneficial for application in reverse water-gas shift processes (rWGS). Due to their stable nature, the ability to incorporate catalytically active dopants in their lattice structure, and the corresponding feature of nanoparticle exsolution, they are promising candidates for a materials design approach. On an industrial level, the rWGS has proven to be an excellent choice for the efficient utilisation of CO2 as an abundant and renewable carbon source, reflected by the current research on novel and improved catalyst materials. In the current study, a correlation between rWGS reaction environments (CO2 to H2 ratios and temperature), surface morphology, and catalytic activity of three perovskite catalysts (Nd0.6Ca0.4Fe0.9Co0.1O3-δ, Nd0.6Ca0.4Fe0.97Co0.03O3-δ, and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ) is investigated, combining catalytic measurements with SEM and NAP-XPS. The materials were found to react dynamically to the conditions showing both activation due to in situ nanoparticle exsolution and deactivation via CaCO3 formation. This phenomenon could be influenced by choice of material and conditions: less reductive conditions (larger CO2 to H2 or lower temperature) lead to smaller exsolved particles and reduced carbonate formation. However, the B-site doping was also important; only with 10% Co-doping, a predominant activation could be achieved.

Improving the Comprehensive Performance of Na0.7MnO2 for Sodium Ion Batteries by ZrO2 Atomic Layer Deposition

Sodium ion batteries with Na-Mn-O compounds as cathodes have been widely studied as substitutes for lithium ion batteries due to their abundant resources. However, the relatively poor cycling stability and low capacity of Na-Mn-O compounds significantly limit their applications. Different approaches, including element substitution and surface modification, have been applied to improve the electrochemical performance of those cathode materials. Herein, element doping and coating of ZrO₂ on Na_0.7MnO₂ particles have been achieved by atomic layer deposition followed by post-annealing. The rate capability and cycling stability of the modified materials were significantly improved, and the mechanism of performance enhancement was revealed. The ZrO₂ coatings acted as a stable interfacial layer to enhance the cycling stability of Na_0.7MnO₂ by suppressing side reactions between the electrode and electrolyte. The doping of transition metal ions reduced energy barriers for sodium ion insertion and deintercalation during charge/discharge cycling, further improving the charge/discharge capacity and rate performance of Na_0.7MnO₂.

Metal-Support Interactions and C1 Chemistry: Transforming Pt-CeO2 into a Highly Active and Stable Catalyst for the Conversion of Carbon Dioxide and Methane

There is an ongoing search for materials which can accomplish the activation of two dangerous greenhouse gases like carbon dioxide and methane. In the area of C1 chemistry, the reaction between CO₂ and CH₄ to produce syngas (CO/H₂), known as methane dry reforming (MDR), is attracting a lot of interest due to its green nature. On Pt(111), high temperatures must be used to activate the reactants, leading to a substantial deposition of carbon which makes this metal surface useless for the MDR process. In this study, we show that strong metal–support interactions present in Pt/CeO₂(111) and Pt/CeO₂ powders lead to systems which can bind CO₂ and CH₄ well at room temperature and are excellent and stable catalysts for the MDR process at moderate temperature (500 °C). The behavior of these systems was studied using a combination of in situ/operando methods (AP-XPS, XRD, and XAFS) which pointed to an active Pt-CeO_2-x interface. In this interface, the oxide is far from being a passive spectator. It modifies the chemical properties of Pt, facilitating improved methane dissociation, and is directly involved in the adsorption and dissociation of CO₂ making the MDR catalytic cycle possible. A comparison of the benefits gained by the use of an effective metal-oxide interface and those obtained by plain bimetallic bonding indicates that the former is much more important when optimizing the C1 chemistry associated with CO₂ and CH₄ conversion. The presence of elements with a different chemical nature at the metal-oxide interface opens the possibility for truly cooperative interactions in the activation of C–O and C–H bonds.

Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts

Pt/ZrO₂ model catalysts were prepared by atomic layer deposition (ALD) and examined at mbar pressure by operando sum frequency generation (SFG) spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with differentially pumped mass spectrometry (MS). ALD enables creating model systems ranging from Pt nanoparticles to bulk-like thin films. Polarization-dependent SFG of CO adsorption reveals both the adsorption configuration and the Pt particle morphology. By combining experimental data with ab initio density functional theory (DFT) calculations, we show that the CO reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. CO disproportionation occurs on low-coordinated Pt sites, but only at high CO coverages and when the remaining C atom is stabilized by a favorable coordination. Thus, under the current conditions, initial CO oxidation is found to be strongly influenced by the removal of carbon deposits formed through disproportionation mechanisms rather than being determined by the CO and oxygen inherent activity. Accordingly, at variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films. The applied approach enables bridging both the “materials and pressure gaps”.

An ultrahigh vacuum-compatible reaction cell for model catalysis under atmospheric pressure flow conditions

Atmospheric pressure reactions on model catalysts are typically performed in so-called high-pressure cells, with product analysis performed by gas chromatography (GC) or mass spectrometry (MS). However, in most cases, these cells have a large volume (liters) so that the reactions on catalysts with only cm² surface area can be carried out only in the (recirculated) batch mode to accumulate sufficient product amounts. Herein, we describe a novel small-volume (milliliters) catalytic reactor that enables kinetic studies under atmospheric pressure flow conditions. The cell is located inside an ultrahigh vacuum chamber that is deliberately limited to basic functions. Model catalyst samples are mounted inside the reactor cell, which is locked to an oven for external heating and closed by using an extendable/retractable gas dosing tube. Reactant and product analyses are performed by both micro-GC and MS. The functionality of the new design is demonstrated by catalytic ethylene (C₂H₄) hydrogenation on polycrystalline Pt and Pd foils.

Carbide-Modified Pd on ZrO2 as Active Phase for CO2-Reforming of Methane—A Model Phase Boundary Approach

Starting from subsurface Zr0-doped “inverse” Pd and bulk-intermetallic Pd0Zr0 model catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS), in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch reactor cell. Both intermetallic precursors develop a Pd0–ZrO2 phase boundary under realistic DRM conditions, whereby the oxidative segregation of ZrO2 from bulk intermetallic PdxZry leads to a highly active composite layer of carbide-modified Pd0 metal nanoparticles in contact with tetragonal ZrO2. This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO2 reference catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive carbidic/dissolved C-species inside Pd0 toward CO at the Pd/ZrO2 phase boundary, which serves the role of providing efficient CO2 activation sites. In contrast, the near-surface intermetallic precursor decomposes toward ZrO2 islands at the surface of a quasi-infinite Pd0 metal bulk. Strongly delayed Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much less active interfacial ZrO2–Pd0 state.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Growth and structural studies of In/Au(111) alloys and InOx/Au(111) inverse oxide/metal model catalysts

Indium oxide has received attention as an exciting candidate for catalyzing the CO₂ hydrogenation to methanol due to its high selectivity (>80%). Compared to the extent of research on the activity of indium oxide-based powder catalysts, very little is known about the phenomena associated with the formation of surface alloys involving indium or the growth mechanism for indium oxide nanoparticles. In this report, scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS) were employed to elucidate the growth mode, structure, and chemical state of In/Au(111) alloys and InO_x/Au(111) inverse model catalysts. Our study reveals distinct morphological differences between In/Au(111) and InO_x/Au(111), and the InO_x structure also depends strongly on the preparation conditions. In/Au surface alloy systems with extremely low coverage (0.02 ML) form islands preferentially on the elbow sites of reconstructed Au(111) herringbone, regardless of hexagonally closed packed and face centered cubic stacking. At higher coverage (0.1 ML), the In islands expand over the herringbone in the ⟨110⟩ direction and create two dimensional domain structures over the entire surfaces. Moreover, this 2D domain structure is disturbed by temperature with high dispersion of indium atoms observed during the annealing process. Oxidation of the In/Au(111) surface alloys with O₂ at 550 K produces InO_x/Au(111) systems which contain various sizes of InO_x aggregates (from 0.7 nm to 10 nm). On the other hand, InO_x/Au(111) surfaces prepared by vapor deposition of In at 550 K in an O₂ background exhibit highly dispersed and uniformly small InO_x particles (∼1 nm). Both InO_x systems were confirmed to be partially oxidized by XPS.

Zirconium-Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming

C-saturated Pd⁰ nanoparticles with an extended phase boundary to ZrO₂ evolve from a Pd⁰ Zr⁰ precatalyst under CH₄ dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO₂ is efficiently activated at oxidic phase boundary sites and Pd_x C provides fast supply of C-atoms toward the latter.