Publications

@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}

}

@article{Reticcioli2017,

title = {Polaron-Driven Surface Reconstructions},

author = {Michele Reticcioli and Martin Setvin and Xianfeng Hao and Peter Flauger and Georg Kresse and Michael Schmid and Ulrike Diebold and Cesare Franchini},

doi = {10.1103/physrevx.7.031053},

year  = {2017},

date = {2017-09-25},

urldate = {2017-09-25},

journal = {Physical Review X},

volume = {7},

number = {3},

pages = {031053},

publisher = {American Physical Society (APS)},

abstract = {Geometric and electronic surface reconstructions determine the physical and chemical properties of surfaces and, consequently, their functionality in applications. The reconstruction of a surface minimizes its surface free energy in otherwise thermodynamically unstable situations, typically caused by dangling bonds, lattice stress, or a divergent surface potential, and it is achieved by a cooperative modification of the atomic and electronic structure. Here, we combined first-principles calculations and surface techniques (scanning tunneling microscopy, non-contact atomic force microscopy, scanning tunneling spectroscopy) to report that the repulsion between negatively charged polaronic quasiparticles, formed by the interaction between excess electrons and the lattice phonon field, plays a key role in surface reconstructions. As a paradigmatic example, we explain the (1×1) to (1×2) transition in rutile TiO_{2}(110).},

keywords = {P02, P03, P07, pre-TACO},

pubstate = {published},

tppubtype = {article}

}

@article{Legrain2017,

title = {How Chemical Composition Alone Can Predict Vibrational Free Energies and Entropies of Solids},

author = {Fleur Legrain and Jesús Carrete and Ambroise van Roekeghem and Stefano Curtarolo and Natalio Mingo},

doi = {10.1021/acs.chemmater.7b00789},

year  = {2017},

date = {2017-06-22},

journal = {Chemistry of Materials},

volume = {29},

number = {15},

pages = {6220--6227},

publisher = {American Chemical Society (ACS)},

abstract = {Computing vibrational free energies (F_{vib}) and entropies (S_{vib}) has posed a long-standing challenge to the high-throughput ab initio investigation of finite temperature properties of solids. Here, we use machine-learning techniques to efficiently predict F_{vib} and S_{vib} of crystalline compounds in the Inorganic Crystal Structure Database. Using descriptors based simply on the chemical formula and using a training set of only 300 compounds, mean absolute errors of less than 0.04 meV/K/atom (15 meV/atom) are achieved for S_{vib} (F_{vib}), whose values are distributed within a range of 0.9 meV/K/atom (300 meV/atom.) In addition, for training sets containing fewer than 2000 compounds, the chemical formula alone is shown to perform as well as, if not better than, four other more complex descriptors previously used in the literature. The accuracy and simplicity of the approach means that it can be advantageously used for fast screening of chemical reactions at finite temperatures.},

keywords = {P09, 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},

urldate = {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},

urldate = {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{Roekeghem2016,

title = {High-Throughput Computation of Thermal Conductivity of High-Temperature Solid Phases: The Case of Oxide and Fluoride Perovskites},

author = {Ambroise van Roekeghem and Jesús Carrete and Corey Oses and Stefano Curtarolo and Natalio Mingo},

doi = {10.1103/physrevx.6.041061},

year  = {2016},

date = {2016-06-13},

urldate = {2016-06-13},

journal = {Physical Review X},

volume = {6},

number = {4},

pages = {041061},

publisher = {American Physical Society (APS)},

abstract = {Using finite-temperature phonon calculations and machine-learning methods, we assess the mechanical stability of about 400 semiconducting oxides and fluorides with cubic perovskite structures at 0, 300, and 1000 K. We find 92 mechanically stable compounds at high temperatures—including 36 not mentioned in the literature so far—for which we calculate the thermal conductivity. We show that the thermal conductivity is generally smaller in fluorides than in oxides, largely due to a lower ionic charge, and describe simple structural descriptors that are correlated with its magnitude. Furthermore, we show that the thermal conductivities of most cubic perovskites decrease more slowly than the usual T^{−1} behavior. Within this set, we also screen for materials exhibiting negative thermal expansion. Finally, we describe a strategy to accelerate the discovery of mechanically stable compounds at high temperatures.},

keywords = {P09, 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}

}