Catalysis by ultrathin
LaBO₃ (B=Co, Fe) perovskite films

@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},

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},

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},

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},

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}

}

@article{Lukashuk2016,

title = {Operando XAS and NAP-XPS studies of preferential CO oxidation on Co_{3}O_{4} and CeO_{2}-Co_{3}O_{4 }catalysts},

author = {Liliana Lukashuk and Karin Föttinger and Elisabeth Kolar and Christoph Rameshan and Detre Teschner and Michael Hävecker and Axel Knop-Gericke and Nevzat Yigit and Hao Li and Eamon McDermott and Michael Stöger-Pollach and Günther Rupprechter},

doi = {10.1016/j.jcat.2016.09.002},

year  = {2016},

date = {2016-12-01},

journal = {Journal of Catalysis},

volume = {344},

pages = {1--15},

publisher = {Elsevier BV},

abstract = {Co_{3}O_{4} is a promising catalyst for removing CO from H_{2} streams via the preferential CO oxidation (PROX). A Mars-van-Krevelen redox mechanism is often suggested but a detailed knowledge especially of the oxidation state of the catalytically active surface under reaction conditions is typically missing. We have thus utilized operando X-ray absorption spectroscopy to examine structure and oxidation state during PROX, and near atmospheric pressure-XPS at low photoelectron kinetic energies and thus high surface sensitivity to monitor surface composition changes. The rather easy surface reduction in pure CO (starting already at ∼100 °C) and the easy reoxidation by O_{2} suggest that molecularly adsorbed CO reacts with lattice oxygen, which is replenished by gas phase O_{2}. Nevertheless, the steady state concentration of oxygen vacancies under reaction conditions is too low even for XPS detection so that both the bulk and surface of Co_{3}O_{4} appear fully oxidized during PROX. Furthermore, the effect of adding CeO_{2} (a less active material) to Co_{3}O_{4} was studied. Promotion of Co_{3}O_{4} with 10 wt% CeO_{2} increases the reduction temperatures in CO and H_{2} and enhances the PROX activity. Since CeO_{2} is a less active material, this can only be explained by a higher activity of the Co-O-Ce interface.},

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

pubstate = {published},

tppubtype = {article}

}

@article{Wolfbeisser2016,

title = {Methane dry reforming over ceria-zirconia supported Ni catalysts},

author = {Astrid Wolfbeisser and Onsulang Sophiphun and Johannes Bernardi and Jatuporn Wittayakun and Karin Föttinger and Günther Rupprechter},

doi = {10.1016/j.cattod.2016.04.025},

year  = {2016},

date = {2016-11-15},

journal = {Catalysis Today},

volume = {277},

pages = {234--245},

publisher = {Elsevier BV},

abstract = {Nickel nanoparticles supported on Ce_{1-x}Zr_{x}O_{2} mixed oxides prepared by different synthesis methods, as well as Ni-ZrO_{2} and Ni-CeO_{2}, were evaluated for their catalytic performance in methane dry reforming (MDR). MDR is an interesting model reaction to evaluate the reactivity and surface chemistry of mixed oxides. Textural and structural properties were studied by N_{2} adsorption and XRD. Mixed oxide preparation by co-precipitation resulted in catalysts with higher surface area than that of pure ZrO_{2} or CeO_{2}. XRD analysis showed the formation of different Ce_{1-x}Zr_{x}O_{2} solid solutions depending on using a surfactant or not. The catalyst prepared by surfactant assisted co-precipitation was not active for methane dry reforming most likely because of the encapsulation of Ni particles by ceria-zirconia particles, as revealed by TEM and H_{2} chemisorption. The catalytic activity of the catalyst prepared by co-precipitation without surfactant was comparable to Ni-ZrO_{2}. Clearly, catalyst activity strongly depends on preparation and on the resulting phase composition rather than on nominal composition. Compared to Ni-ZrO_{2} the ceria-zirconia supported Ni catalyst did not achieve higher activity or stability for methane dry reforming but, nevertheless, the formation of filamentous carbon was strongly reduced (100 times less carbonaceous species). Consequently, using ceria-zirconia as a support material decreases the risk of reactor tube blocking.},

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

pubstate = {published},

tppubtype = {article}

}

@article{Foettinger2014,

title = {In Situ Spectroscopy of Complex Surface Reactions on Supported Pd–Zn, Pd–Ga, and Pd(Pt)–Cu Nanoparticles},

author = {Karin Föttinger and Günther Rupprechter},

doi = {10.1021/ar500220v},

year  = {2014},

date = {2014-09-23},

journal = {Accounts of Chemical Research},

volume = {47},

number = {10},

pages = {3071--3079},

publisher = {American Chemical Society (ACS)},

abstract = {It is well accepted that catalytically active surfaces frequently adapt to the reaction environment (gas composition, temperature) and that relevant “active phases” may only be created and observed during the ongoing reaction. Clearly, this requires the application of in situ spectroscopy to monitor catalysts at work. While changes in structure and composition may already occur for monometallic single crystal surfaces, such changes are typically more severe for oxide supported nanoparticles, in particular when they are composed of two metals. The metals may form ordered intermetallic compounds (e.g. PdZn on ZnO, Pd_{2}Ga on Ga_{2}O_{3}) or disordered substitutional alloys (e.g. PdCu, PtCu on hydrotalcite). We discuss the formation and stability of bimetallic nanoparticles, focusing on the effect of atomic and electronic structure on catalytic selectivity for methanol steam reforming (MSR) and hydrodechlorination of trichloroethylene. Emphasis is placed on the in situ characterization of functioning catalysts, mainly by (polarization modulated) infrared spectroscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and X-ray diffraction. In the present contribution, we pursue a two-fold, fundamental and applied, approach investigating technologically applied catalysts as well as model catalysts, which provides comprehensive and complementary information of the relevant surface processes at the atomic or molecular level. Comparison to results of theoretical simulations yields further insight.



Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd_{2}Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal–oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by).



The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.},

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

pubstate = {published},

tppubtype = {article}

}