Strain dependent light-off temperature in catalysis revealed by planar laser-induced fluorescence

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.The authors thank the Knut and Alice Wallenberg Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Crafoord Foundation. Support by the MAX IV staff is gratefully acknowledged. The calculations were performed at C3SE through a SNIC grant. J.E.O. acknowledges support from the Spanish Ministry of Economy (MAT2013-46593-C6-4-P) and the Basque Government (IT621-13).Peer Reviewe

Surface structure and catalytic activity of Pd and Fe oxide surfaces and thin films

The present work is devoted to atomic scale structural studies of the surfaces of model heterogeneous catalysts relevant to oxidation reactions. A novel approach using high-energy surface X-ray diffraction combined with mass-spectrometry measurements is employed to perform in situ structural characterization of Pd(100) and Pd(553) single crystal surfaces acting as catalysts in the process of CO oxidation under semirealistic conditions. The experimental approach greatly facilitates the understanding of surface X-ray diffraction and improves significantly the data collection speed. The phases forming on the surfaces in gas mixtures with different relative concentrations of CO and O2 are determined and are associated to the catalytic activity. The corresponding structural models are proposed. A combination of complementary experimental techniques, including conventional surface X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy, temperature programmed desorption spectroscopy and reflection absorption infrared spectroscopy as well as theoretical calculations, is employed to study in detail the structural and NO adsorption properties of iron oxide ultrathin films grown on Ag(100) and Ag(111) single crystal substrates. Structural models of different phases growing on the surfaces under different preparation conditions are presented. The atomic structural model of a one-layer thick FeO(111) film grown on Ag(100) is proposed. The NO adsorption properties of one-layer thick FeO(111) films on both substrates are investigated and compared to the NO adsorption properties of FeO(111)/Pt(111) reported in the literature. The observed differences are discussed in detail. The results obtained for CO oxidation over Pd model catalysts allow for an increased understanding of the processes occurring on the surface of a working catalyst and the connection between the catalytic properties and the surface structure. The performed studies of iron oxide ultrathin films grown on silver substrates provide insight into how the structural properties are related to the adsorption properties of such systems and knowledge important for the design of novel catalytic materials with improved qualities

Step dynamics and oxide formation during CO oxidation over a vicinal Pd surface.

In an attempt to bridge the material and pressure gaps - two major challenges for an atomic scale understanding of heterogeneous catalysis - we employed high-energy surface X-ray diffraction as a tool to study the Pd(553) surface in situ under changing reaction conditions during CO oxidation. The diffraction patterns recorded under CO rich reaction conditions are characteristic for the metallic state of the surface. In an environment with low excess of O2 over the reaction stoichiometry, the surface seems to accommodate oxygen atoms along the steps forming one or several subsequent adsorbate structures and rapidly transforms into a combination of (332), (111) and (331) facets likely providing the room for the formation of a surface oxide. For the case of large excess of O2, the diffraction data show the presence of a multilayer PdO with the [101] crystallographic direction parallel to the [111] and the [331] directions of the substrate. The reconstructions in O2 excess are to a large extent similar to those previously reported for pure O2 exposures by Westerström et al. [R. Westerström et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 155410]

Redox behavior of iron at the surface of an Fe0.01Mg0.99O(100)\mathrm{Fe_{0.01}Mg_{0.99}O(100)} single crystal studied by ambient-pressure photoelectron spectroscopy

We have studied the oxidation and reduction of iron in an Fe-doped MgO single crystal by $O_2$, $H_2$ and $CO_2$ using ambient-pressure XPS and NEXAFS. Surface charging of the crystal was rendered manageable by the elevated temperatures and the gas atmospheres. The oxidation state of iron was found to shift reversibly between the $Fe^{2+}$ and $Fe^{3+}$ states, with a strong asymmetry in the rates; while oxidation by $O_2$ or $CO_2$ was nearly complete at $100°C$, reduction by $H_2$began at $\sim 300°C$, and was still incomplete at $600°C$ . Grazing-incidence XRD characterization of the crystal indicated the presence of octahedral, nanoscale inclusions assigned to the magnesioferrite spinel $(MgFe_{2}O_{4})$. It is proposed that the redox behavior observed involves interconversion between the rock-salt $(Fe_{x}Mg_{1-x}O)$ and spinel phases, with the more open lattice containing $Fe^{3+}$enabling more rapid ion diffusion and thus more facile oxidation compared to reduction

High energy surface x-ray diffraction applied to model catalyst surfaces at work

Catalysts are materials that accelerate the rate of a desired chemical reaction. As such, they constitute an integral part in many applications ranging from the production of fine chemicals in chemical industry to exhaust gas treatment in vehicles. Accordingly, it is of utmost economic interest to improve catalyst efficiency and performance, which requires an understanding of the interplay between the catalyst structure, the gas phase and the catalytic activity under realistic reaction conditions at ambient pressures and elevated temperatures. In recent years efforts have been made to increasingly develop techniques that allow for investigating model catalyst samples under conditions closer to those of real technical catalysts. One of these techniques is high energy surface x-ray diffraction (HESXRD), which uses x-rays with photon energies typically in the range of 70-80 keV. HESXRD allows a fast data collection of three dimensional reciprocal space for the structure determination of model catalyst samples under operando conditions and has since been used for the investigation of an increasing number of different model catalysts. In this article we will review general considerations of HESXRD including its working principle for different model catalyst samples and the experimental equipment required. An overview over HESXRD investigations performed in recent years will be given, and the advantages of HESXRD with respect to its application to different model catalyst samples will be presented. Moreover, the combination of HESXRD with other operando techniques such as in situ mass spectrometry, planar laser-induced fluorescence and surface optical reflectance will be discussed. The article will close with an outlook on future perspectives and applications of HESXRD

A Novel Method to Maintain the Sample Position and Pressure in Differentially Pumped Systems Below the Resolution Limit of Optical Microscopy Techniques

We present a new method to maintain constant gas pressure over a sample during in situ measurements. The example shown here is a differentially pumped high-pressure X-ray photoelectron spectroscopy system, but this technique could be applied to many in situ instruments. By using the pressure of the differential stage as a feedback source to change the sample position, a new level of consistency has been achieved. Depending on the absolute value of the sample-to-aperture distance, this technique allows one to maintain the distance within several hundred nanometers, which is below the limit of typical optical microscopy systems. We show that this method is well suited to compensate for thermal drift. Thus, X-ray photoelectron spectroscopy data can be acquired continuously while the sample is heated and maintaining constant pressure over the sample. By implementing a precise manipulator feedback system, pressure variations of less than 5% were reached while the temperature was varied by 400 ℃. The system is also shown to be highly stable under significant changes in gas flow. After changing the flow by a factor of two, the pressure returned to the set value within 60 s

Adsorption of NO on FeOx films grown on Ag(111)

We used temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe3O4(111) layer, an FeO(111) monolayer, and an intermediate FeOx multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeOx surfaces, and provides evidence that NO binds more strongly on Fe2+ sites than Fe3+ sites. The NO TPD spectra obtained from the Fe3O4(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe2+ sites, as well as a broad feature centered at ∼250 K that is consistent with NO bound on Fe3+ sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ∼200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeOx islands expose Fe3+ sites that bind NO more weakly than the Fe2+ sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeOx surfaces exhibit an N-O stretch band that blueshifts over a range from about 1800 to 1840 cm-1 with increasing NO coverage. The measured N-O stretching frequency is only slightly red-shifted from the gas-phase value, and lies in a range that is consistent with atop, linearly bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to the NO binding site on the FeOx surfaces. This behavior suggests that π-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site

Methanol Adsorption and Oxidation on Reduced and Oxidized TbOx(111) Surfaces

We used temperature programmed reaction spectroscopy (TPRS) and synchrotron-radiation based photoelectron spectroscopy (PES) to investigate the adsorption and oxidation of methanol (CH3OH) on Tb2O3(111) and TbO2(111) thin films grown on Pt(111). We find that methanol mainly desorbs from the Tb2O3 surface through both molecular and recombinative processes, and that a relatively small amount of adsorbed methanol (< 20%) dehydrogenates to CH2O and H2O with these species desorbing between about 160 and 300 K. Oxidation of the terbia film enhances the surface reactivity as ~50% of the adsorbed methanol on TbO2 oxidizes to mainly CH2O and water as well as CO2 that desorbs near 600 K during TPRS. Quantification of the product yields suggests that all of the excess surface O-atoms, resulting from oxidation of Tb2O3 to TbO2, are removed during TPRS by reaction with adsorbed CH3OH. We did not detect CO or H2 production under any conditions. PES measurements show that several adsorbed intermediates form on the TbOx surfaces at temperatures as low as 140 K, including mainly methoxy (CH3O) as well as smaller quantities of a more oxidized species, thought to be either CH2O2 or CHO2. XPS spectra collected as a function of the surface temperature provide evidence that the adsorbed CH3O groups serve as the main intermediate for both CH2O and CH3OH formation at temperatures below 400 K, while the more oxidized species is a spectator to CH2O formation but undergoes complete oxidation on TbO2 at temperatures above 450 K. The high reactivity of the TbO2 surface correlates with the presence of labile oxygen atoms that are generated during oxidation of the Tb2O3 film