Near-ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Understanding the Decomposition of Dimethyl Methyl Phosphonate on Metal-Modified TiO2(110) Surfaces Using Ensembles of Product Configurations.

The decomposition of dimethyl methyl phosphonate (DMMP), a simulant for the nerve agent sarin, was investigated on Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces using a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory calculations (DFT). Mass-selected Cu4 clusters and potassium (K) atoms were deposited onto TiO2(110) as a metal catalyst and alkali promoter to improve the reactivity and recyclability of the TiO2 surface after exposure to DMMP. Surface reaction products resulting from decomposition of DMMP were probed by NAP-XPS measurements of phosphorus (P) 2p and carbon 1s core-level spectra. The Cu4/TiO2(110) surface is found to be very active for DMMP decomposition with highly reduced P-species observed even at room temperature (RT). The codeposition of K atoms and Cu4 clusters further improves the reactivity with no intact DMMP detectable. Temperature-dependent measurements show that the presence of K atoms promotes the removal of residual P-species at temperatures > 600 K. Detailed DFT calculations were performed to determine the surface structures and energetically accessible pathways for DMMP decomposition on Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces. The calculations show that DMMP and P-containing reaction products preferentially bind to the TiO2 surface, while the molecular fragments, i.e., methoxy and methyl, bind to both the Cu4 clusters and TiO2. The Cu4 clusters make the P-O, O-C, and P-C bond cleavages of DMMP markedly more exothermic. The Cu4 clusters are highly fluxional with atomic structures that depend on the configuration of fragments bound to them. Finally, the manifold of P 2p chemical shifts calculated for a large number of energetically favorable configurations of decomposition products is in good agreement with the observed XPS spectra and provides an alternative way of interpreting incompletely resolved core-level spectra using an ensemble of observed structures.

Importance of Metal-Support Interactions for CO2 Hydrogenation: An Operando Near-Ambient Pressure X-ray Photoelectron Spectroscopy Study on Gold-Loaded In2O3 and CeO2 Catalysts.

Metal-support interactions, which are essential for the design of supported metal catalysts, used, e.g., for CO2 activation, are still only partially understood. In this study of gold-loaded In2O3 and CeO2 catalysts during CO2 hydrogenation using near-ambient pressure X-ray photoelectron spectroscopy, supported by near edge X-ray absorption fine structure, we demonstrate that the role of the noble metal strongly depends upon the choice of the support material. Temperature-dependent analyses of X-ray photoelectron spectra under reaction conditions reveal that gold is reduced on CeO2, enabling direct H2 activation, but oxidized on In2O3, leading to decreased activity of Au/In2O3 compared to bare In2O3. At elevated temperatures, the catalytic activity of the In2O3 catalysts strongly increases as a result of facilitated CO2 and (In2O3-based) H2 activation, while the catalytic activity of Au/CeO2 is limited by reoxidation by CO2. Our results underline the importance of operando studies for understanding metal-support interactions to enable a rational support selection in the future.

Low-temperature gas sensing mechanism in β-Ga2O3 nanostructures revealed by near-ambient pressure XPS

Ga2O3 is a promising gas-sensing material that can operate over a wide temperature range. However, despite its relevance for practical applications, there is a lack of understanding regarding its sensing mechanism at temperatures below 800 °C. To reveal the sensing mechanism, GaOOH nanorods were grown on β-Ga2O3 seed layers by chemical bath deposition, then converted to β-Ga2O3 by annealing in the air at 500 °C, and the surface properties were investigated by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in the presence of oxidizing/reducing gases. Time-stable shifts in the XPS spectra were used to estimate the relative changes in conductivity in compliance with the ionosorption model. Our results indicate that the sensing mechanism at lower temperatures is governed by redox reactions, leading to an increase/decrease in the conductivity for reducing/oxidizing gases. Furthermore, we provide a detailed description of the ethanol-sensing mechanism at different temperatures.

Investigating methanol decomposition at low temperatures on polycrystalline Pt using near ambient pressure XPS (NAP-XPS)

In this work, methanol was frozen on the surface of polycrystalline Pt and the decomposition of methanol was monitored from its frozen state to 195 K. This works shows that methanol spontaneously decomposes to CO on Pt even at 160 K. CO was observed to adsorb onto the top and bridge site of Pt based on the C 1s and O 1s binding energy assignment from the NAP-XPS measurement. At low temperatures, CO is preferred to bind onto the top site with a bridge to top coverage ratio of 0.4, while CO is found to be predominantly on the bridge site at room temperature, with a CO bridge to top ratio of 4. The difference in the energy of the CO binding sites were found to be 6.4 (±1.0) kJ/mol, which is in good agreement with literature.

The Effect of Mn Doping and Ti3+ Defects at TiO2 Surfaces in NO and SO2 Gas Capture Investigated Using Near-Ambient Pressure X-ray Photoelectron Spectroscopy

The removal of air pollutants is an important research topic in order to improve the environment. In addition, many common pollutants can affect human health to varying degrees. In this work, we investigate NO and SO2 conversion by reaction with a commonly used metal oxide catalyst, TiO2. Rutile TiO2(110) single crystals and industrial powder samples used in sunscreen are studied using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) as a main tool. This allows in situ monitoring of the gas conversion process. We find Ti3+ defects (oxygen vacancies) or Mn oxides/cations (MnO) at the TiO2 surfaces can improve the conversion of NO and SO2 to surface-bound species. MnO and Ti3+ defects at the surface of rutile TiO2(110) exhibit a synergistic effect on the conversion of NO and SO2 that is significantly improved by nearly an order of magnitude. The by-products are mainly in the form of NO3−, SO32−, and SO42−. We find the main oxidation products formed on the single crystals are subtly different from those on the industrial powder samples. For TiO2 nanopowders (undoped and Mndoped), the presence of Mn also shows improvement in toxic gas adsorption capacity. Overall, it is believed that the outcome obtained from NAP-XPS in this research provides useful insights for the future use of TiO2 in pollutant gas capture.

Confining isolated Pt atoms into the porous SnO2 for efficient gas sensing

Highly dispersed isolated sites offer opportunities to rationally design sensing materials with desired performance, but their stability under harsh reaction conditions is still challenging. Herein, we deposited the highly dispersed atomic layer deposition (ALD) platinum (Pt) species into optimized porous SnO2 nanorods derived from the Sn-based metal organic frameworks (MOFs). With confining isolated Pt into porous SnO2, the optimum operating temperature of the sensor based on SnO2–Pt2 nanorods was decreased from 275 °C to 190 °C. And the response (Ra/Rg) to 5 ppm acetone was improved from 2.63 to 6.33, along with the decrease of response time from 23 s to 9 s. Combined with X-ray adsorption, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) simulation, the highly dispersed Pt confined within SnO2 nanorods are confirmed to heighten the adsorption of acetone at working temperature and lower the reaction activation energy. The confined structure enriches the acetone in hole, further enhancing its adsorption on Pt. The highly dispersed isolated Pt sites and confined effects dramatically promote the sensing performance. The understanding of structure-activity relationships offers a novel strategy to design the ultrasensitive and stable acetone sensing materials, and it shows potential applications in design of other gas sensing or catalytic materials.

Operando observation of the dynamic SEI formation on a carbonaceous electrode by near-ambient pressure XPS

Operando near-ambient pressure XPS was carried out on a glassy carbon electrode. For the first time, the dynamic formation of the SEI in contact with the liquid electrolyte was detected by XPS.

Enhancing the performance of a novel CoRu/CeO2 bimetallic catalyst for the dry reforming of methane via a mechanochemical process

A mechanochemical synthesis method has been used to synthesize CoRu nanoparticles supported on CeO2 for methane dry reforming. In this work, we study the effect of Ru addition to Co/CeO2-based catalysts and of the synthesis method by screening their catalytic activity, using synchrotron X-ray diffraction (XRD), and operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Ruthenium addition directly impacts the reducibility of cobalt species and results in smaller particle sizes, as demonstrated by H2-temperature programmed reduction and XRD. NAP-XPS shows that Ru modifies the metal-support interaction, as evidenced by the higher Ce3+/Ce ratios for the bimetallic samples and tuning the oxidation state of Ru. The synthesis method also influences the dispersion of Co and Ru on the surface. Mechanochemically-prepared samples (mono- and bimetallic) outperformed the conventionally-synthesized counterparts by reaching higher CH4 and CO2 conversions, resulting in a stable CoRu/CeO2 catalyst for 24 h at 700 °C and yielding an H2/CO ratio close to 1.

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

Pathways of Water-Induced Lead-Halide Perovskite Surface Degradation: Insights from In Situ Atomic-Scale Analysis.

While organic-inorganic hybrid perovskites are emerging as promising materials for next-generation photovoltaic applications, the origins and pathways of perovskite instability remain speculative. In particular, the degradation of perovskite surfaces by ambient water is a crucial subject for determining the long-term viability of perovskite-based solar cells. Here, we conducted surface characterization and atomic-scale analysis of the reaction mechanisms for methylammonium lead bromide (MA(CH3NH3)PbBr3) single crystals using ambient-pressure atomic force microscopy (AP-AFM) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) in environments ranging from ultrahigh vacuum to 0.01 mbar of water vapor. MAPbBr3 single crystals, grown by a solution process, were mechanically cleaved under UHV conditions to obtain an atomically clean surface. Consecutive topography and friction force measurements in low-pressure water (pwater ≈ 10-5 mbar) revealed the formation of degraded patches, one atomic layer deep, gradually increasing their coverage until the surface was entirely covered at a water exposure of 4.7 × 104 langmuir (L). At the perimeters of these degraded patches, a higher friction coefficient was observed, along with an interstitial step height, which we attribute to a structure equivalent to that of the MA-Br terminated surface. Combined with NAP-XPS analysis, our results demonstrate that water vapor induces the dissociation of surface methylammonium ligands, eventually resulting in the depletion of the surface MA and the full coverage of hydrocarbon species after exposure to 0.01 mbar of water vapor.

Surface-Phosphorylated Ceria for Chlorine-Tolerance Catalysis.

An improved fundamental understanding of active site structures can unlock opportunities for catalysis from conceptual design to industrial practice. Herein, we present the computational discovery and experimental demonstration of a highly active surface-phosphorylated ceria catalyst that exhibits robust chlorine tolerance for catalysis. Ab initio molecular dynamics (AIMD) calculations and in situ near-ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS) identified a predominantly HPO4 active structure on CeO2(110) and CeO2(111) facets at room temperature. Importantly, further elevating the temperature led to a unique hydrogen (H) atom hopping between coordinatively unsaturated oxygen and the adjacent P═O group of HPO4. Such a mobile H on the catalyst surface can effectively quench the chlorine radicals (Cl•) via an orientated reaction analogous to hydrogen atom transfer (HAT), enabling the surface-phosphorylated CeO2-supported monolithic catalyst to exhibit both expected activity and stability for over 68 days during a pilot test, catalyzing the destruction of a complex chlorinated volatile organic compound industrial off-gas.

Revealing the effect of metal-support interactions at the Ni/In2O3(111) interface on the selective CO2 hydrogenation

In2O3-supported Ni catalysts exhibit remarkable catalytic activity and selectivity in CO2 hydrogenation to methanol, but the underlying mechanisms and metal-oxide interactions during the reaction remain elusive. Herein, we investigate the Ni-In2O3 interaction by physical vapor deposition of Ni onto well-defined In2O3(111) thin films. In-situ near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to probe the CO2 hydrogenation processes on these Ni/In2O3(111) model systems. Our results reveal that the small Ni clusters supported on the In2O3(111) surface at low Ni coverages exhibit cationic states. The chemical bonding and associated electron transfer at the Ni/In2O3(111) interface play crucial roles in the activation of H2 and CO2. Importantly, reaction intermediates (CO3*, OH, and HCOO*) are readily formed and desorbed under CO2 hydrogenation conditions. Our study highlights the significance of metal-support interactions on the selectivity of CO2 hydrogenation. These findings provide valuable insights into the rational design of advanced In2O3-based catalysts for CO2 hydrogenation.

Electrocatalytic Enhancement of CO Methanation at the Metal–Electrolyte Interface Studied Using In Situ X-ray Photoelectron Spectroscopy

For the direct reduction of CO2 and H2O in solid oxide electrolysis cells (SOECs) with cermet electrodes toward methane, a fundamental understanding of the role of elemental carbon as a key intermediate within the reaction pathway is of eminent interest. The present synchrotron-based in situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) study shows that alloying of Ni/yttria-stabilized-zirconia (YSZ) cermet electrodes with Cu can be used to control the electrochemical accumulation of interfacial carbon and to optimize its reactivity toward CO2. In the presence of syngas, sufficiently high cathodic potentials induce excess methane on the studied Ni/yttria-stabilized-zirconia (YSZ)-, NiCu/YSZ- and Pt/gadolinium-doped-ceria (GDC) cermet systems. The hydrogenation of carbon, resulting from CO activation at the triple-phase boundary of Pt/GDC, is most efficient.

Studying Surface Chemistry of Mixed Conducting Perovskite Oxide Electrodes with Synchrotron-Based Soft X-rays.

A fundamental understanding of the electrochemical reactions and surface chemistry at the solid-gas interface in situ and operando is critical for electrode materials applied in electrochemical and catalytic applications. Here, the surface reactions and surface composition of a model of mixed ionic and electronic conducting (MIEC) perovskite oxide, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255), were investigated in situ using synchrotron-based near-ambient pressure (AP) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The measurements were conducted with a surface temperature of 500 °C under 1 mbar of dry oxygen and water vapor, to reflect the implementation of the materials for oxygen reduction/evolution and H2O electrolysis in the applications such as solid oxide fuel cell (SOFC) and electrolyzers. Our direct experimental results demonstrate that, rather than the transition metal (TM) cations, the surface lattice oxygen is the significant redox active species under both dry oxygen and water vapor environments. It was proven that the electron holes formed in dry oxygen have a strong oxygen character. Meanwhile, a relatively higher concentration of surface oxygen vacancies was observed on the sample measured in water vapor. We further showed that in water vapor, the adsorption and dissociation of H2O onto the perovskite surface were through forming hydroxyl groups. In addition, the concentration of Sr surface species was found to increase over time in dry oxygen due to Sr surface segregation, with the presence of oxygen holes on the surface serving as an additional driving force. Comparatively, less Sr contents were observed on the sample in water vapor, which could be due to the volatility of Sr(OH)2. A secondary phase was also observed, which exhibited an enrichment in B-site cations, particularly in Fe and relatively in Cr, and a deficiency in A-site cation, notably in La and relatively in Sr. The findings and methodology of this study allow for the quantification of surface defect chemistry and surface composition evolution, providing crucial understanding and design guidelines in the electrocatalytic activity and durability of electrodes for efficient conversions of energy and fuels.

Molecular elucidation of CO2 methanation over a highly active, selective and stable LaNiO3/CeO2-derived catalyst by in situ FTIR and NAP-XPS

The CO2 methanation mechanism over the highly active (TOF=75.1 h−1), selective (>92%) and stable 10% LaNiO3/CeO2-derived catalyst is still unresolved. The surface of the catalyst is monitored under hydrogenation (H2), oxidizing (CO2) and CO2 methanation (H2 +CO2) conditions by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation. Meanwhile, the main reaction intermediates are identified by in situ FTIR analysis. NAP-XPS experiments confirm that LaNiO3 perovskite reduction leads to the ex-solution of Ni0 nanoparticles and Ni2+CeO2−x and Ni2+La2O3 interfaces conformation, favouring the CO2 adsorption and the H2 dissociation/transfer. In situ FTIR experiments combined with the C1s spectra (NAP-XPS) suggest that the CO2 activation occurs on CeO2−x (oxygen vacancies and OH–) at low temperatures, in the form of bicarbonates; whereas, mono-/bidentate carbonates are formed on different strength La2O3 sites at increasing temperatures. These species are consecutively reduced to formates, as the main reaction intermediate, and methane by the H spilled from Ni0 nanoparticles near to NiOCeO2−x and NiOLa2O3 interfaces.

In situ XPS of competitive CO2/H2O absorption in an ionic liquid

Superbasic ionic liquids (SBILs) are being investigated as potential carbon dioxide (CO2) gas capture agents, however, the presence of H2O in the flue stream can inhibit the uptake of CO2. In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P66614][124Triz]) was deposited onto rutile TiO2 (110) using in situ electrospray deposition and studied upon exposure to CO2 and H2O using in situ near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS). The molar uptake ratio of gas in the electrosprayed SBIL (n gas :n IL) was calculated to be 0.3:1 for CO2, 0.7:1 for H2O, and 0.9:1 for a CO2/H2O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO2 and H2O in [P66614][124Triz] varies with sampling depth. A greater concentration of CO2 absorbs in the bulk layers, while more H2O adsorbs/absorbs at the surface. The presence of H2O in the gas mixture does not inhibit the absorption of CO2. Measurements taken during exposure and after the removal of gas indicate that CO2 absorbed in the bulk does so reversibly, whilst CO2 adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO2 reaction shown for bulk ionic liquids (ILs) in literature and suggests that irreversible absorption of CO2 in our highly-structured thin films is largely attributed to reactions at the surface. This has potential implications on IL gas capture and thin film IL catalysis applications.

Direct Evidence of Dynamic Metal Support Interactions in Co/TiO2 Catalysts by Near-Ambient Pressure X-ray Photoelectron Spectroscopy.

The interaction between metal particles and the oxide support, the so-called metal-support interaction, plays a critical role in the performance of heterogenous catalysts. Probing the dynamic evolution of these interactions under reactive gas atmospheres is crucial to comprehending the structure-performance relationship and eventually designing new catalysts with enhanced properties. Cobalt supported on TiO2 (Co/TiO2) is an industrially relevant catalyst applied in Fischer-Tropsch synthesis. Although it is widely acknowledged that Co/TiO2 is restructured during the reaction process, little is known about the impact of the specific gas phase environment at the material's surface. The combination of soft and hard X-ray photoemission spectroscopies are used to investigate in situ Co particles supported on pure and NaBH4-modified TiO2 under H2, O2, and CO2:H2 gas atmospheres. The combination of soft and hard X-ray photoemission methods, which allows for simultaneous probing of the chemical composition of surface and subsurface layers, is one of the study's unique features. It is shown that under H2, cobalt particles are encapsulated below a stoichiometric TiO2 layer. This arrangement is preserved under CO2 hydrogenation conditions (i.e., CO2:H2), but changes rapidly upon exposure to O2. The pretreatment of the TiO2 support with NaBH4 affects the surface mobility and prevents TiO2 spillover onto Co particles.

NAP-XPS study of surface chemistry of CO and ethanol sensing with WO3 nanowires-based gas sensor

The nanowires prepared on a commercial Al2O3-based sensor platform were studied by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) upon detecting CO and ethanol. Our findings indicated that the ex-situ prepared WO3 NWs-based gas sensor might not be suitable for NAP-XPS studies, as the surfaces tend to accumulate significant carbon contamination, which remains on the surface even after long-term annealing in mbar range oxygen atmosphere at 593 K. To address this challenge, we developed a methodology involving exposure of the sensor to NOx, which effectively cleans the surface and enables the detection of subtle chemical changes upon exposure to various carbon-containing analytes. Using NAP-XPS analysis, we investigated the gas sensing mechanisms of WO3 NWs for CO and ethanol at different temperatures, revealing that it might combine several mechanisms commonly observed in metal-oxide chemiresistors. It is shown that the response to CO rather arises from the quick interaction of CO molecules with the ionosorbed oxygen. In the case of ethanol sensing, the main driving force for the sensor macroscopic electrical response at low temperatures is the adsorption of ethanol molecules. In contrast, at high temperatures, it combines the adsorption of ethoxy groups with the strong reducing effect of EtOH on the WO3 surface.

Dynamic Structural Evolution of CeO2 in CuO−CeO2 Catalyst Revealed by In Situ Spectroscopy

AbstractThe oxides and active metals at the interface synergistically activate reactants and thus promote the reaction, but the interface structure often changes dynamically during the reaction. In the conventional supported catalysts, the metals at the interface have been extensively studied, while the structural evolution of oxides is often overlooked due to the interference of the bulk phase signal. In this work, CeO2−CuO inverse catalysts are designed to reveal the dynamic structure evolution of CeO2 in the CeO2−CuO system during the water gas shift (WGS) reaction by in situ Raman, in situ XRD, quasi in situ XPS, and near ambient pressure XPS (NAP‐XPS). CeO2 is partially concealed in the CuO phase in the un‐pretreated catalyst and gradually exposed to the surface, forming an inverse CeOx/Cu structure during the reducing process. This structure exhibits a high catalytic activity in the WGS reaction and remains durable under the reductive conditions. When the inverse CeOx/Cu structure is exposed to the non‐redox conditions, the reconfiguration of the reduced oxide is observed which is caused by the oxygen migration of CeO2. This work explores the structure evolution of CeO2 in CeO2−CuO inverse catalyst under different conditions by in situ characterization technique and provides a reference for monitoring the dynamic changes of oxide structure.