Surface structure and reactivity of
multi-component oxides at the atomic scale

@article{Rupprechter2021,

title = {Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso-Scale Aggregates},

author = {Günther Rupprechter},

doi = {10.1002/smll.202004289},

year  = {2021},

date = {2021-03-10},

urldate = {2021-03-10},

journal = {Small},

publisher = {Wiley},

abstract = {Operando characterization of working catalysts, requiring per definitionem the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure, composition and adsorbed species. Frequently applied operando techniques are discussed, including X-ray absorption spectroscopy, near ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy. In contrast to these area-averaging spectroscopies, operando surface microscopy by photoemission electron microscopy delivers spatially-resolved data, directly visualizing catalyst heterogeneity. For thorough interpretation, the experimental results should be complemented by density functional theory. The operando approach enables to identify changes of cluster/nanoparticle structure and composition during ongoing catalytic reactions and reveal how molecules interact with surfaces and interfaces. The case studies cover the length-scales from clusters via nanoparticles to meso-scale aggregates, and demonstrate the benefits of specific operando methods. Restructuring, ligand/atom mobility, and surface composition alterations during the reaction may have pronounced effects on activity and selectivity. The nanoscale metal/oxide interface steers catalytic performance via a long ranging effect. Combining operando spectroscopy with switching gas feeds or concentration-modulation provides further mechanistic insights. The obtained fundamental understanding is a prerequisite for improving catalytic performance and for rational design.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Winkler2021,

title = {How the anisotropy of surface oxide formation influences the transient activity of a surface reaction},

author = {Philipp Winkler and Johannes Zeininger and Yuri Suchorski and Michael Stöger-Pollach and Patrick Zeller and Matteo Amati and Luca Gregoratti and Günther Rupprechter},

doi = {10.1038/s41467-020-20377-9},

year  = {2021},

date = {2021-01-04},

urldate = {2021-01-04},

journal = {Nature Communications},

volume = {12},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) allow local surface analysis and visualising ongoing reactions on a µm-scale. These two spatio-temporal imaging methods are applied to polycrystalline Rh, representing a library of well-defined high-Miller-index surface structures. The combination of these techniques enables revealing the anisotropy of surface oxidation, as well as its effect on catalytic hydrogen oxidation. In the present work we observe, using locally-resolved SPEM, structure-sensitive surface oxide formation, which is summarised in an oxidation map and quantitatively explained by the novel step density (SDP) and step edge (SEP) parameters. In situ PEEM imaging of ongoing H_{2} oxidation allows a direct comparison of the local reactivity of metallic and oxidised Rh surfaces for the very same different stepped surface structures, demonstrating the effect of Rh surface oxides. Employing the velocity of propagating reaction fronts as indicator of surface reactivity, we observe a high transient activity of Rh surface oxide in H2 oxidation. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty of well-defined surface structures within one sample.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Li2021,

title = {Sum frequency generation spectroscopy in heterogeneous model catalysis: a minireview of CO-related processes},

author = {Xia Li and Günther Rupprechter},

doi = {10.1039/d0cy01736a},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Catalysis Science & Technology},

volume = {11},

number = {1},

pages = {12--26},

publisher = {Royal Society of Chemistry (RSC)},

abstract = {Sum frequency generation (SFG) vibrational spectroscopy is a unique surface/interface-sensitive method, enabling the identification of chemical species and molecular structures, densities and orientations. SFG has been proven to be a powerful probe to examine adsorbates and reactions at solid–gas interfaces related to heterogeneous catalysis, employing well-defined ultra-high vacuum (UHV) grown model catalysts and UHV-compatible high-pressure reaction cells, enabling bridging both the materials and pressure gaps. SFG was thus among the first methods for ambient pressure surface science, enabling the characterization of “high pressure adsorbates”. In this mini-review, we provide an overview of SFG studies of CO-related processes in heterogeneous model catalysis. This includes pressure- and/or temperature-dependent CO adsorption on single crystals (platinum, palladium, rhodium, iridium, copper, nickel) and oxide/graphene-supported (palladium, platinum) nanoparticles, as well as CO reactions (oxidation/hydrogenation) simultaneously monitored by SFG and mass spectrometry. The adsorption of isotopic CO mixtures on single crystals and nanoparticles provides information on the individual contributions of vibrational coupling and chemical interactions to the adsorbate–adsorbate interactions. Altogether, SFG helps to identify various adsorption sites, adsorbate structures, molecular orientations and CO reactions on prototypical catalyst surfaces of increasing complexity. Specifically, the analysis of molecular orientation (tilt angles) can be carried out by polarization-dependent SFG.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Pramhaas2020,

title = {Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO_{2} Model Catalysts},

author = {Verena Pramhaas and Matteo Roiaz and Noemi Bosio and Manuel Corva and Christoph Rameshan and Erik Vesselli and Henrik Grönbeck and Günther Rupprechter},

doi = {10.1021/acscatal.0c03974},

year  = {2020},

date = {2020-12-17},

urldate = {2020-12-17},

journal = {ACS Catalysis},

volume = {11},

number = {1},

pages = {208--214},

publisher = {American Chemical Society (ACS)},

abstract = {Pt/ZrO_{2} 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”.},

keywords = {P08, P10, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Haunold2020,

title = {An ultrahigh vacuum-compatible reaction cell for model catalysis under atmospheric pressure flow conditions},

author = {Thomas Haunold and Christoph Rameshan and Andrey V Bukhtiyarov and Günther Rupprechter},

doi = {10.1063/5.0026171},

year  = {2020},

date = {2020-12-01},

urldate = {2020-12-01},

journal = {Review of Scientific Instruments},

volume = {91},

number = {12},

pages = {125101},

publisher = {AIP Publishing},

abstract = {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 cm^{2} 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 (C_{2}H_{4}) hydrogenation on polycrystalline Pt and Pd foils.},

keywords = {P08, P10, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2020,

title = {Energy-Guided Shape Control Towards Highly Active CeO_{2}},

author = {Jingxia Yang and Huihui Ding and Jinjie Wang and Nevzat Yigit and Jingli Xu and Günther Rupprechter and Min Zhang and Zhiquan Li},

doi = {10.1007/s11244-020-01357-1},

year  = {2020},

date = {2020-08-18},

journal = {Topics in Catalysis},

volume = {63},

number = {19-20},

pages = {1743--1753},

publisher = {Springer Science and Business Media LLC},

abstract = {The shape of nanosized CeO_{2}, obtained via polyvinylpyrrolidone (PVP) micelles, was controlled by microwave (MW)-aided synthesis combined with different combinations of energy input/transfer, including ultrasound (US), ultraviolet (UV) and pressure (P). Whereas ceria nanoflakes resulted from standard solvothermal synthesis, CeO_{2} nanoparticles were obtained from MW, MW + US and MW + UV. New CeO_{2} “nanospindles” (with aspect ratio of 2) resulted from MW + US + UV, and nanorods (with aspect ratio of 11) emerged from MW + P. All ceria morphologies, even nanospindles and nanorods, were mesoporous agglomerates of small CeO_{2} nanocrystals (6–8 nm size), but they still exhibited different specific surface area (SSA) and Ce^{3+}/Ce^{4+} ratio. Underlying reasons of how the different synthesis routes affect the ceria morphology are discussed. Among the six types, CeO_{2} nanorods (MW + P) exhibited the highest SSA (196 m^{2}  g^{−1}) and the most surface defects (Ce^{3+}: 26.4%), resulting in excellent catalytic performance in imine synthesis and CO oxidation.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Suchorski2020,

title = {Catalysis by Imaging: From Meso- to Nano-scale},

author = {Yuri Suchorski and Günther Rupprechter},

doi = {10.1007/s11244-020-01302-2},

year  = {2020},

date = {2020-07-02},

urldate = {2020-07-02},

journal = {Topics in Catalysis},

volume = {63},

number = {15-18},

pages = {1532--1544},

publisher = {Springer Science and Business Media LLC},

abstract = {In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10^{–5}–10^{–6} mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Pollitt2020,

title = {The Dynamic Structure of Au_{38}(SR)_{24} Nanoclusters Supported on CeO_{2} upon Pretreatment and CO Oxidation},

author = {Stephan Pollitt and Vera Truttmann and Thomas Haunold and Clara Garcia and Wojciech Olszewski and Jordi Llorca and Noelia Barrab é and Günther Rupprechter},

doi = {10.1021/acscatal.0c01621},

year  = {2020},

date = {2020-05-08},

urldate = {2020-05-08},

journal = {ACS Catalysis},

volume = {10},

number = {11},

pages = {6144--6148},

publisher = {American Chemical Society (ACS)},

abstract = {Atomically precise thiolate protected Au nanoclusters Au38}(SC_{2}H_{4}Ph)_{24} on CeO_{2} were used for in-situ (operando) extended X-ray absorption fine structure/diffuse reflectance infrared fourier transform spectroscopy and ex situ scanning transmission electron microscopy–high-angle annular dark-field imaging/X-ray photoelectron spectroscopy studies monitoring cluster structure changes induced by activation (ligand removal) and CO oxidation. Oxidative pretreatment at 150 °C “collapsed” the clusters’ ligand shell, oxidizing the hydrocarbon backbone, but the S remaining on Au acted as poison. Oxidation at 250 °C produced bare Au surfaces by removing S which migrated to the support (forming Au^{+}-S), leading to highest activity. During reaction, structural changes occurred via CO-induced Au and O-induced S migration to the support. The results reveal the dynamics of nanocluster catalysts and the underlying cluster chemistry.},

keywords = {P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Diebold2019,

title = {Preface: Surface Science of functional oxides},

author = {Ulrike Diebold and Günther Rupprechter},

doi = {10.1016/j.susc.2018.11.017},

year  = {2019},

date = {2019-03-01},

journal = {Surface Science},

volume = {681},

pages = {A1},

publisher = {Elsevier BV},

keywords = {P02, P08, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Lukashuk2018,

title = {Operando Insights into CO Oxidation on Cobalt Oxide Catalysts by NAP-XPS, FTIR, and XRD},

author = {Liliana Lukashuk and Nevzat Yigit and Raffael Rameshan and Elisabeth Kolar and Detre Teschner and Michael Hävecker and Axel Knop-Gericke and Robert Schlögl and Karin Föttinger and Günther Rupprechter},

doi = {10.1021/acscatal.8b01237},

year  = {2018},

date = {2018-08-07},

urldate = {2018-08-07},

journal = {ACS Catalysis},

volume = {8},

number = {9},

pages = {8630--8641},

publisher = {American Chemical Society (ACS)},

abstract = {Cobalt oxide Co_{3}O_{4} has recently emerged as promising, noble metal-free catalyst for oxidation reactions but a better understanding of the active catalyst under working conditions is required for further development and potential commercialization. An operando approach has been applied, combining near ambient (atmospheric) pressure X-ray photoelectron spectroscopy (NAP-XPS), Fourier transform infrared spectroscopy (FTIR), or X-ray diffraction (XRD) with simultaneous catalytic tests of CO oxidation on Co_{3}O_{4}, enabling one to monitor surface and bulk states under various reaction conditions (steady-state and dynamic conditions switching between CO and O_{2}). On the basis of the surface-specific chemical information a complex network of different reaction pathways unfolded: Mars-van-Krevelen (MvK), CO dissociation followed by carbon oxidation, and formation of carbonates. A possible Langmuir–Hinshelwood (LH) pathway cannot be excluded because of the good activity when no oxygen vacancies were detected. The combined NAP-XPS/FTIR results are in line with a MvK mechanism above 100 °C, involving the Co^{3+}/Co^{2+} redox couple and oxygen vacancy formation. Under steady state, the Co_{3}O_{4} surface appeared oxidized and the amount of reduced Co^{2+} species at/near the surface remained low up to 200 °C. Only in pure CO, about 15% of surface reduction were detected, suggesting that the active sites are a minority species. The operando spectroscopic studies also revealed additional reaction pathways: CO dissociation followed by carbon reoxidation and carbonate formation and its decomposition. However, due to their thermal stability in various atmospheres, the carbonates are rather spectators and also CO dissociation seems a minor route. This study thus highlights the benefits of combining operando surface sensitive techniques to gain insight into catalytically active surfaces.},

keywords = {P08, P10, pre-TACO},

pubstate = {published},

tppubtype = {article}

}