Enhancing catalytic performance and hot electron generation through engineering metal-oxide and oxide-oxide interfaces

Despite the increasing economic incentives and environmental advantages associated to their substitution, carbon-rich fossil fuels are expected to remain as the dominant worldwide source of energy through at least the next two decades and perhaps later. Therefore, both the control and reduction of CO2 emissions have become environmental issues of major concern and big challenges for the international scientific community. Among the proposed strategies to achieve these goals, conversion of CO2 by its reduction into high added value products, such as methane or syngas, has been widely agreed to be the most attractive from the environmental and economic points of view. In the present work, thermocatalytic reduction of CO2 with H2 was studied over a nanostructured ceria-supported nickel catalyst. Ceria nanocubes were employed as support, while the nickel phase was supported by means a surfactant-free controlled chemical precipitation method. The resulting nanocatalyst was characterized in terms of its physicochemical properties, with special attention paid to both surface basicity and reducibility. The nanocatalyst was studied during CO2 reduction by means of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). Two different catalytic behaviors were observed depending on the reaction temperature. At low temperature, with both Ce and Ni in an oxidized state, CH4 formation was observed, whereas at high temperature above 500 °C, the reverse water gas shift reaction became dominant, with CO and H2O being the main products. NAP-XPS was revealed as a powerful tool to study the behavior of this nanostructured catalyst under reaction conditions.

Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at ca. 2500 Pa, or higher in some cases. With NAP-XPS, XPS can be used to analyze moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show survey, O 1s, O KLL, and valence band NAP-XPS spectra from liquid water, a material that could not be analyzed at moderate pressures by conventional approaches. The O 1s signal was fit to two components attributed to liquid and vapor phase water. The carbon in the survey spectrum is attributed to contaminants in the water and/or adventitious carbon.

This Special Issue of Topics in Catalysis focuses on the application of Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) for the investigation of heterogeneous interfaces. The topics of the papers included in this issue range from heterogeneous catalysis, including electrocatalysis, to the chemistry of interfaces in the environment, to solid/liquid and liquid/vapor interfaces. This broad array of applications reflects the utility of APXPS for investigations in a wide range of disciplines. The first APXPS instrument was conceived and installed by the Siegbahn group in Uppsala in the early 1970s and applied in particular to the investigation of concentration gradients across liquid/vapor interfaces, a topic that is as important nowadays as it was then. Several other APXPS instruments were installed over the following decades: At Cardiff at the end of the 1970s, at Novosibirsk in the mid-1980s, and at Orono/ME at the end of the 1980s. All of these instruments used laboratory sources for the incident X-rays, and a set of differentially pumped apertures to separate the high pressure in the sample compartment from the vacuum in the electron analyzer, to minimize the path length of the electrons through the high pressure region and prevent arcing due to large potential differences in the electron analyzer at elevated pressure. The design of a differentially-pumped lens system at Berkeley Lab in the late 1990s helped to overcome the trade-off between efficient differential pumping using small apertures and highthroughput electron detection by placing electrostatic lenses between the differentially-pumped apertures, which increased the solid angle of electron collection in APXPS and thus the pressure limit from previously 1 mbar by at least an order of magnitude. The first two instruments at the Advanced Light Source in Berkeley and at BESSY in Berlin that used this design produced the vast majority of APXPS publications that appeared in the first decade of the twentieth century. At the end of that decade commercial APXPS instruments became available, many of which were installed at other 3rd generation synchrotron sources around the world. At the time of writing 11 synchrotrons either have already operating instruments or are in the process of installing them. The availability of instruments at synchrotrons has dramatically increased access to APXPS measurements for researchers from a wide range of disciplines, including heterogeneous catalysis, geosciences, atmospheric science and electrochemistry. Recently, a growing number of laboratory-based instruments have been installed; these instruments greatly benefit from the availability of small-spot, monochromatized X-ray anode sources. Future growth in the number of APXPS instruments is expected to be mainly due to the installation of additional laboratory-based instruments as opposed to those at synchrotrons. While the growth of the APXPS community and the breadth in the application of the technique over the past 15 years has been an important contribution to the development of new tools for the operando investigation of interfaces, there are many open problems that need to be addressed by the APXPS community over the coming years. Chief among these is the preparation of surfaces and interfaces with the same control over impurity levels and crystallography as one is used to from ultra-high vacuumbased surface science investigations. Sample preparation is in particular challenging for liquids with high vapor pressures, where exact control of the thermodynamic & Hendrik Bluhm hbluhm@lbl.gov

Supported bimetallic nanoparticles (BNPs) are promising catalysts, but study on their compositional and structural changes under reaction conditions remains a challenge. In this work, the structure of PtNi BNPs supported on UiO-67 metal–organic framework (MOF) catalyst (i.e., PtNi@UiO-67) was investigated by in situ by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The results showed differences in the reduction behaviour of Ni species in PtNi BNPs and monometallic Ni supported on UiO-67 catalysts (i.e., PtNi@UiO-67 and Ni@UiO-67), suggesting charge transfer between metallic Pt and Ni oxides in PtNi@UiO-67. Under CO oxidation conditions, Ni oxides segregated to the outer surface of the BNPs forming a thin layer of NiOx on top of the metallic Pt (i.e., a NiOx-on-Pt structure). This resulted in a core-shell structure which was confirmed by high-resolution scanning transmission electron microscopy (HR-STEM). Accordingly, the layer of NiOx on PtNi BNPs, which is stabilised by charge transfer from metallic Pt, was proposed as the possible active phase for CO oxidation, being responsible for the enhanced catalytic activity observed in the bimetallic PtNi@UiO-67 catalyst.

The reducibility of iron oxide nanoparticles (NPs) supported over few-layer thick graphite upon annealing in hydrogen is investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), and high-resolution transmission electron microscopy (HR-TEM). It is found that the stability of the iron oxide NPs toward reduction is enhanced by the interaction with the graphene nanosheets as compared to the bulk iron oxide. Postannealing TEM micrographs reveal the existence of both core/shell and homogeneous iron oxide NPs with the latter forming irregular trenches into the graphene sheets. EELS analysis and TEM images clearly demonstrate that the reducibility of iron oxide particles depends on the specific graphene site on which they are attached. Furthermore, we show that graphene etching can be mediated by iron oxide NPs at relatively mild reduction conditions.

In this submission, we show survey, O 1s, and C 1s near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) spectra from a hard Italian cheese, a material that could not be analyzed by conventional approaches without special sample preparation. The C 1s spectrum is fit under the assumption that the surface of the cheese is primarily fat (triglyceride), which is expected to be the lowest free energy component of the material. The O 1s envelope corresponding to the cheese was well fit to two components of equal area.

This study presents an investigation of the sensing properties of SnO2 and Cu2O/CuO based nanostructured chemiresistors, n-type and p-type semiconductors, respectively. The chemiresistors were used for the detection of ethanol and nitrogen dioxide, reducing and oxidizing agents, respectively. To unravel the chemical processes connected with gas detection, an in-situ approach was applied. This approach was based on near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with simultaneous monitoring of sensor responses. The in-situ measurements were performed during exposure to the analytes at a total pressure of 0.05 – 1.05 mbar and 200 – 300 °C, and were correlated with chemiresistor response measurements carried out at a standard pressure and under an ambient atmosphere. The study enabled assessment of the relationship between the chemistry of surface processes on the sensitive layers and the "integral" response of the sensor devices and presented a complex view on the gas sensing mechanism and chemiresistors functionality. Figure 1

Electrochemical conversion of carbon dioxide into eligible fuels has become the subject of particular fundamental interest. Recently, we have reported electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface [1]. In this study Carbon dioxide electroreduction has been studied at ultrathin palladium layers deposited on copper electrode. The Pd deposits have been prepared by using chemical methods. The spontaneous galvanic displacement and resulting uncontrolled deposition of the palladium on copper has been entirely eliminated by the use of palladium(II). This method allowed us for controlled and precise deposition of palladium layers by electrochemical methods. The deposits coverage was changed within the range 0.5 to 5 atomic monolayers. The effectiveness of the method and the durability of the obtained ultrathin palladium layers have been confirmed by X-ray photoelectron spectroscopy (XPS). XPS has been also used for detailed analysis of the electronic properties of the Pd/Cu systems. Significant chemical interactions of both elements resulted in a partial shift of the electron density from palladium to copper and in the changes in the symmetry of 3d palladium signals characteristic for two-dimensional palladium structures (Pd monolayers). A detailed analysis of the valence band structure of Pd/Cu was correlated to the catalytic activity of the studied systems. The unique properties of the valence band structure cannot be attributed to the simple superposition of pure metal bands (Pd and Cu). It has been shown that the increase in the electron density near the Fermi level results in a significant decrease in the overpotential of the carbon dioxide electroreduction. Analysis of the gaseous products obtained in the CO2 electroreduction was performed by gas chromatography. Adsorption of carbon dioxide on Pd/Cu layers was demonstrated by XPS measurements in the presence of 1 mbar CO2 in XPS measurements under near ambient pressure XPS (NAP-XPS). [1] A. Januszewska, R. Jurczakowski, P. Kulesza, Langmuir 30(2014)14314-14321

X-ray photoelectron spectroscopy meets electrochemistry: From UHV to operando conditions

The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported catalysts were investigated in order to understand the role of charge transfer in catalysis. Pt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10-4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules. The same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes. In the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode. Similar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate was found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it. Charge flow in an oxide supported metal catalyst can be modified even without designing the catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface. The same set of catalysts was used for methanol oxidation. The electronic properties of the TiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect. Ambient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping effect in reaction conditions. In O2 alone, and in CO oxidation conditions, the O1s spectrum showed a high binding energy peak that correlated in intensity with the activity of the different films: for stoichiometric films, the peak decreased in intensity with F-doping, while for nonstoichiometric films, the opposite was observed. No such changes were visible in the C1s spectrum, confirming the role of O activation in the reaction. This thesis adds to the body of knowledge on the importance of charge transfer at the metal-oxide interface in shaping the reactivity of heterogeneous catalysts, and provides examples of how this can be the basis for new methods to tune reactivity.

Enhanced dissociation activation of CO2 on the Bi/Cu(1 1 1) interface by the synergistic effect

Pt-free cathode catalysts are attracting increased interest to replace platinum in polymer electrolyte membrane fuel cells (PEFCs). Heat-treated nitrogen-doped carbons decorated with iron have been the most widely studied alternative. However, lack of understanding of the formation of active sites and degradation mechanisms in such catalysts is hindering their progress. Here, we use a nitrogen-doped carbon foam support with optimised porosity, surface area, and conductivity which is then infiltrated with three different iron precursors: iron (II) acetate (FeAc); iron (III) chloride (FeCl3); and iron phthalocyanine (FePc). These are then pyrolyzed to form Fe-N-C-based ORR electrocatalysts. This approach takes advantage of an already optimized conductive, porous, high surface area and temperature-stable support. The stability of the support helps to separate the role of the different iron precursors from other factors in the generation of catalytic active sites. High temperature X-ray absorption fine structure (HT-XAFS) and high temperature X-ray photoelectron spectroscopy (HT-XPS) were employed to study changes in the iron precursors during the synthesis of Fe-N-C electrocatalysts. Furthermore, near ambient pressure XPS (NAP-XPS) and in-situ electrochemical XAFS were performed to study the interaction between the synthesized catalysts and oxygen. These results are then related to the electrochemical activity.

The effect of Mo(CO) 6 as a co-catalyst in the carbonylation of methanol to methyl formate catalyzed by potassium methoxide under CO, syngas and H 2 atmospheres. HP-IR observation of the methoxycarbonyl intermediate of Mo(CO) 6

Structure sensitivity of oscillating partial oxidation of propene on widely dispersed platinum catalysts

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5/TiO2 catalyst