Max-Planck-Forschungsgruppe - Nanobased Heterogeneous Catalysts

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Dr. Simon Ristig - Nanobased Heterogeneous Catalysts

Dr. Simon Ristig
Leiter der Gruppe Nanobased Heterogeneous Catalysts

Vita

Chemie Studium
Universität Duisburg-Essen (2004-2010)
PhD
Universität Duisburg-Essen, Inorganic Chemistry (Prof. M. Epple) (2011-2014)
PostdocMPI CEC (Prof. J. Strunk) (2015-2016)
Forschungsgruppenleiter (Vertretung)
MPI CEC 'Nanobased Heterogeneous Catalysts' (seit 2016)
ForschungsgruppenleiterMPI CEC 'Nanobased Heterogeneous Catalysts' (seit 2017)

Publikationen

  • D. Mahl, J. Diendorf, S. Ristig, C. Greulich, Zi-An Li, M. Farle, M. Köller, M. Epple, “Silver, gold, and alloyed silver-gold nanoparticles: characterization and comparative cell-biologic action", Journal of Nanoparticle Research, 2012, 14, 1153.
  • O. Prymak, S. Ristig, W. Meyer-Zaika, A. Rostek, L. Ruiz, J.M. Gonzalez-Calbet, M. Vallet-Regi, M. Epple, "X-ray powder diffraction as a tool to investigate the ultrastructure of nanoparticles", Izvestiya Vuzov, Fizika (in Russian), 2013, 10, pp. 5-9; cover-to-cover translation: Russian Physics Journal, 2014, 56, 1111.
  • H. Fissan, S. Ristig, H. Kaminski, C. Asbach, M. Epple, "Characterization of nanoparticles in dispersions by using aerosol sizing measurement techniques", Analytical Methods, 2014, 6, 7324.
  • S. Ristig, D. Kozlova, W. Meyer-Zaika, M. Epple, "An easy synthesis of autofluorescent alloyed silver-gold nanoparticles", Journal of Materials Chemistry B, 2014, 2, 7887.
  • S. Ristig, S. Chernousova, W. Meyer-Zaika, M. Epple, "Synthesis, characterization and in vitro effects of 7 nm alloyed silver-gold nanoparticles", Beilstein Journal of Nanotechnology, 2015, 6, 1212.
  • S. Ristig, O. Prymak, K. Loza, M. Gocyla, W. Meyer-Zaika, M. Heggen, D. Raabe, M. Epple, " Nanostructure of wet-chemically prepared, polymer-stabilized silver-gold nanoalloys (6 nm) over the entire composition range", Journal of Materials Chemistry B, 2015, 3, 4654.
  • S. Ristig, N. Cibura, J. Strunk, "Manganese Oxides in Heterogeneous (Photo)Catalysis: Possibilities and Challenges", Green, 2015, 5, 23.

Gruppenmitglieder

Postdocs

  • Dr. Ahmet Esat Becerikli

PhD Studenten

  • Martin Dilla

IMPRS-RECHARGE Studierende

  • Niklas Cibura

Nanobased Heterogeneous Catalysts

The research group "Nanobased heterogeneous catalysts" aims at unraveling structure-function relationships of photocatalysts based on nano-sized components for energy applications. This comprises the controlled synthesis of the materials, in situ studies of working photocatalysts, and structural/electronic characterization.

Photocatalysis and structure-function relationships

The surfaces of real catalysts are usually very complex and feature many structurally different sites. Even on pure metals or pure oxides, different structural arrangements exist, such as various extended crystal facets, steps, kinks or defect sites. If such a material is of nano-size, the disorder and strain in the particles likely increase. Active catalysts often feature both: metallic or metal oxide nanoparticles supported on extended surfaces of a support oxide. Then, the interaction between the nanoparticle and the support, and the specific (electronic) structure at the interface or the perimeter of the nanoparticles also needs to be taken into account. In classical heterogeneous catalysis, it is well known that any of these structural arrangements might potentially be the active site of a reaction. In many cases minority sites (e.g. defects, exposed atoms at steps) are so much better in catalyzing the reaction that they dominate the kinetics. To develop a structure-function relationship, it needs to be known exactly which site is the dominating active site. In classical heterogeneous catalysis the understanding of some catalysts and reactions has proceeded to a level of detail that allows the formulation of structure-function relationships. This is the case, for example, in methanol synthesis on Cu/ZnO, or ammonia synthesis over K-promoted Fe catalysts.

In photocatalysis, research has so far been much more application-oriented, in which different (classes of) materials have been tested and further modified, but without much insight into their mode of action or their active site(s). Only for the most studied system, pure TiO2, research has proceeded into the direction of a detailed understanding of the underlying photophysical, electronic and structural properties. However, pure TiO2 is not efficient enough in reactions such as water splitting and the reduction of CO2. Based upon the knowledge of pure TiO2, we aim at understanding the mode of action of nano-scale photocatalysts based on TiO2 and alternative oxide materials. Our research requires the synthesis of well-defined materials, their structural and electronic characterization both in situ and ex situ, and reliable equipment for the testing of the photocatalytic activity. Some details of our research are outlined below.

Photocatalysis under conditions of high purity

In both photocatalytic CO2 reduction and water splitting, it is crucial to assure that hydrocarbon impurities are completely absent. Both reactions are highly endergonic in nature, whereas the photocatalytic oxidation of hydrocarbon impurities is exergonic and will thus be the thermodynamically preferred reaction. The products of the photocatalytic destruction of hydrocarbons (H2, CO, CxHyOz) might then be misinterpreted as products of the desired reaction, leading to false assignments of photocatalytic activity.

To solve this problem, we constructed a high-purity gas-phase photoreactor system (Fig. 1) based on a system reported in literature [1]. It consists entirely of parts for vacuum set-ups, and all sealing is entirely grease-free. The whole reactor can be evacuated to high vacuum. Back diffusion of oil from the pumping stage is prevented by multiple trapping stages. The gas supply contains only gases of highest available purity. Gaseous water is dosed by a saturator unit. Gas analysis is performed by means of a gas chromatograph fit for trace gas analysis. The gas chromatography system is directly attached by an injection loop, so the reactor system is never open to ambient atmosphere during sampling. Irradiation is provided by a 200 W Hg/Xe lamp [2].

In addition to optimizing the reactor system, we take great care to pretreat all samples until they are entirely hydrocarbon-free. We adopt a procedure originally designed by the group of Guido Mul [4]. The samples are irradiated in a gas phase consisting only of water and inert gas for several hours. Then, the impurities that were photocatalytically decomposed are removed by evacuation (Fig. 2a). This cleaning cycle is repeated until no more hydrocarbons can be found in the gas phase, e.g. seven times in the example in Fig. 2b. When CO2 is introduced now to start the photocatalytic reduction reaction, all hydrocarbons that are formed must originate from CO2, because the presence of any other source of hydrocarbons has been excluded.

Mechanistic studies of Photocatalytic CO2 reduction

The mechanism of photocatalytic reduction of CO2 to CH4 is still not fully understood. Although different reaction models are proposed in the literature [5-7], neither of them has been supported with sufficient evidence to be generally accepted by the scientific community. The most prominent potential mechanisms are the carbene pathway, the formaldehyde and the glyoxal pathway (Fig. 3).

By careful design of reaction conditions and reactants, we try to get a deeper insight into the product formation and different intermediates to gain detailed comprehension of the fundamental reaction steps during the photocatalytic reduction of CO2. Under highest purity reaction conditions in batch mode we identified CH4 and CO to be the main products of the photocatalytic CO2 reduction on TiO2 P25. Yet, reference measurements with CO instead of CO2 in the gas phase showed that there is no consumption of CO, ruling out CO as an intermediate of a consecutive reaction to CH4 on TiO2 and thus the carbene pathway [8]. Equally, formaldehyde was found to be implausible as an intermediate due to its decomposition to CO2, CO and H2 under photocatalytic conditions on TiO2. However, recent results from reacting acetaldehyde and acetic acid on TiO2 indicate a possible C2 mechanism, as the product distributions are rather similar to those obtained from photocatalytic CO2 reduction under standard reaction conditions (Fig. 4). This might be an indicator for a prevailing glyoxal pathway.

Photocatalytic CO2 reduction under continuous flow conditions

Furthermore, we investigate the photocatalytic CO2 reduction on TiO2 P25 under high-purity continuous flow conditions with gas chromatographic online detection of the main product CH4. Conducting the thorough photocatalytic cleaning procedure in continuous flow enables us to clearly monitor the removal of carbonaceous contaminants (Fig. 5a). Upon addition of CO2 to the feed under illumination, an increase in CH4 concentration is observed which scales with CO2 concentration in the reactor (Fig. 5a) [9]. It is also demonstrated that CH4 formation ceases as soon as the lamp is switched off, providing clear evidence for the formation of CH4 from CO2 in a photoinduced process (Fig. 5a) [9].

A variation of CO2 concentration shows that higher CO2 concentration accelerates CH4 formation, but only up to a certain optimum. For higher CO2 concentrations, CH4 formation does not increase further or even decreases (Fig. 5b). This observation is tentatively assigned to a limited availability of photogenerated charge carriers at the TiO2 surface, or a lack of suitable catalytically active sites [9].

Fundamental studies on manganese oxide-based materials in photocatalysis

Manganese oxides are already a major research focus at the MPI-CEC. We complement the existing research initiatives by unraveling the fundamental photophysical and (electro)chemical properties of a variety of stable bulk manganese oxides (MnO, Mn3O4, Mn2O3, MnO2) as a function of oxide stoichiometry and nanosize. Due to the wide range of reported physicochemical parameters (e.g. band gap, band positions) [10] we created our own reference database using bulk oxides in highest available purity. It is our aim to evaluate whether a certain (modified) Mn oxide can be used as light absorber, as water splitting catalyst, or to take over both functionalities. Potential benefit of scaling particle sizes of those oxides down to the nanometer range is currently under investigation in cooperation with members of CENIDE (Center for Nanointegration, University Duisburg-Essen). Additionally, isolated Mn cations and small agglomerates of Mn oxides are prepared by different synthetic routes on mesoporous silica to study their photophysical properties in detail and to test them as potential (single-site) photocatalysts. Since nanometer-sized supported and nanocrystalline or amorphous oxide phases are difficult to analyze, we furthermore evaluate the applicability of temperature programmed reactions in a gas phase plug flow reactor for phase identification of nanoscopic manganese oxides.

Photoelectrochemical analyses

In addition to photocatalytic gas-phase and liquid-phase experiments we aim to establish reliable and reproducible photoelectrochemical measurements. An improved homemade photoelectrochemical cell (PEC) was built which is fast equilibrating, prevents formation of air bubbles and allows the use of a double junction electrode (Fig. 6). Comparative photocurrent measurements with Mo:BiVO4 (obtained from Dr. Bastian Mei, University of Twente) proved unrestricted usability of the homemade cell. A general validation of the setup was carried out by conducting a range of cyclic voltammetry (CV), electrochemical impedance (EIS) and photocurrent measurements with suitable standards. The photoelectrochemical properties of heterogeneous photocatalyst materials can only be determined correctly after a reproducible fabrication of stabile photoelectrodes. The reproducibility of photocatalytic/photoelectrochemical measurements and thus the comparability of result from different scientific groups is still an issue to be solved. In the common preparation methods for electrodes, physical binder substances are involved in order to improve the mechanical stability. Binders may alter the (photo)electrochemical properties of photocatalysts, thus the initial properties of freshly prepared materials cannot be measured. Currently, it is the aim to prepare a photoelectrode with the least chemical or physical modification of the material under study.

Consequently, photoelectrodes were prepared by the doctor blading method (adapted from [11]) of semiconducting materials such as Cu2O, Fe2O3, Mn3O4, WO3 and TiO2 on a variety of conducting supports. Alternative preparation methods to doctor-blading are also going to be explored, including the preparation of photoelectrodes via the atomic layer deposition and dip-coating.

References

[1] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731.

[2] B. Mei, A. Pougin, J. Strunk, J. Catal. 306 (2013) 184.

[3] J. Strunk, A. Pougin, Bunsen-Magazin, Dec. 2014.

[4] C.-C. Yang, J. Vernimmen, V. Meynen, P. Cool, G. Mul, J. Catal. 284 (2011) 1.

[5] S. N. Habisreutinger, L. Schmidt-Mende, J. K. Stolarczyk, Angew. Chem. Int. Ed. 52 (2013) 7372.

[6] I. A. Shkrob, T. W. Marin, H. He, P. Zapol, J. Phys. Chem. C 116 (2012) 9450.

[7] C. Amatore, J.-M. Savéant, J. Am. Chem. Soc. 103 (1981) 5021.

[8] A. Pougin, M. Dilla, J. Strunk, J. Phys. Phys. Chem., 18 (2016) 10809.

[9] M. Dilla, R. Schlögl, J. Strunk, ChemCatChem, 10.1002/cctc.201601218.

[10] S. Ristig, N. Cibura, J. Strunk, GREEN 5 (2015) 1.

[11] R. Beránek, Dissertation, Universität Erlangen-Nürnberg, 2007.