Max Planck Research Group - Nanobased Heterogeneous Catalysts
Max Planck Research Groups offer junior scientists who hold a doctoral degree an excellent opportunity to qualify for a further career at a high level. Leaders of these groups are appointed by the President of the Max Planck Society and – similar to directors at Max Planck Institutes – enjoy an independent status within the Institute.
Max Planck Research Groups (MPRG) are limited to five years; extension is only possible for groups that exhibit convincing research performance. Tenure is granted in exceptional cases only. The groups draw on infrastructure and administration of a Max Planck Institute (MPI) but are also provided considerable funds for personnel, equipment and running costs, negotiable funds that enable the group leaders to flexibly engage in their research project. more
Dr. Simon Ristig - Nanobased Heterogeneous Catalysts
|B.Sc./M.Sc.||University of Duisburg-Essen (2004-2010)|
|PhD||University of Duisburg-Essen, Inorganic Chemistry (Prof. M. Epple) (2011-2014)|
|Postdoc||MPI CEC (Prof. J. Strunk) (2015-2016)|
|Research Group Leader (Deputy)||MPI CEC 'Nanobased Heterogeneous Catalysts' (since 2016)|
|Research Group Leader ||MPI CEC 'Nanobased Heterogeneous Catalysts' (since 2017)|
- 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.
Nanobased Heterogeneous Catalysts
The research group "Nanobased heterogeneouscatalysts" aims at unraveling structure-function relationships ofphotocatalysts based on nano-sized components for energy applications. Thiscomprises the controlled synthesis of the materials, in situ studies of workingphotocatalysts, 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 reductionand water splitting, it is crucial to assure that hydrocarbon impurities arecompletely absent. Both reactions are highly endergonic in nature, whereas thephotocatalytic oxidation of hydrocarbon impurities is exergonic and will thusbe the thermodynamically preferred reaction. The products of the photocatalyticdestruction of hydrocarbons (H2, CO, CxHyOz)might then be misinterpreted as products of the desired reaction, leading tofalse assignments of photocatalytic activity.
To solve this problem, we constructed ahigh-purity gas-phase photoreactor system (Fig. 1) based on a system reportedin literature . It consists entirely of parts for vacuum set-ups, and allsealing is entirely grease-free. The whole reactor can be evacuated to highvacuum. Back diffusion of oil from the pumping stage is prevented by multipletrapping stages. The gas supply contains only gases of highest availablepurity. Gaseous water is dosed by a saturator unit. Gas analysis is performedby means of a gas chromatograph fit for trace gas analysis. The gaschromatography system is directly attached by an injection loop, so the reactorsystem is never open to ambient atmosphere during sampling. Irradiation isprovided by a 200 W Hg/Xe lamp .
In addition to optimizing the reactor system,we take great care to pretreat all samples until they are entirelyhydrocarbon-free. We adopt a procedure originally designed by the group ofGuido Mul . The samples are irradiated in a gas phase consisting only ofwater and inert gas for several hours. Then, the impurities that werephotocatalytically decomposed are removed by evacuation (Fig. 2a). Thiscleaning cycle is repeated until no more hydrocarbons can be found in the gasphase, e.g. seven times in the example in Fig. 2b. When CO2 isintroduced now to start the photocatalytic reduction reaction, all hydrocarbonsthat are formed must originate from CO2, because the presence of anyother source of hydrocarbons has been excluded.
Mechanistic studies of Photocatalytic CO2 reduction
The mechanism of photocatalytic reduction of CO2to CH4 is still not fully understood. Although different reactionmodels are proposed in the literature [5-7], neither of them has been supportedwith sufficient evidence to be generally accepted by the scientific community. Themost prominent potential mechanisms are the carbene pathway, the formaldehydeand the glyoxal pathway (Fig. 3).
By careful design of reaction conditions andreactants, we try to get a deeper insight into the product formation anddifferent intermediates to gain detailed comprehension of the fundamentalreaction steps during the photocatalytic reduction of CO2. Underhighest purity reaction conditions in batch mode we identified CH4and CO to be the main products of the photocatalytic CO2 reductionon TiO2 P25. Yet, reference measurements with CO instead of CO2in the gas phase showed that there is no consumption of CO, ruling out CO as anintermediate of a consecutive reaction to CH4 on TiO2 andthus the carbene pathway . Equally, formaldehyde was found to be implausibleas an intermediate due to its decomposition to CO2, CO and H2under photocatalytic conditions on TiO2. However, recent resultsfrom reacting acetaldehyde and acetic acid on TiO2 indicate apossible C2 mechanism, as the product distributions are rathersimilar to those obtained from photocatalytic CO2 reduction understandard reaction conditions (Fig. 4). This might be an indicator for aprevailing glyoxal pathway.
Photocatalytic CO2 reduction under continuous flow conditions
Furthermore, we investigate the photocatalyticCO2 reduction on TiO2 P25 under high-purity continuousflow conditions with gas chromatographic online detection of the main productCH4. Conducting the thorough photocatalytic cleaning procedure in continuousflow enables us to clearly monitor the removal of carbonaceous contaminants (Fig.5a). Upon addition of CO2 to the feed under illumination, anincrease in CH4 concentration is observed which scales with CO2concentration in the reactor (Fig. 5a) . It is also demonstrated that CH4formation ceases as soon as the lamp is switched off, providing clear evidencefor the formation of CH4 from CO2 in a photoinducedprocess (Fig. 5a) .
A variation of CO2 concentrationshows that higher CO2 concentration accelerates CH4formation, but only up to a certain optimum. For higher CO2concentrations, CH4 formation does not increase further or evendecreases (Fig. 5b). This observation is tentatively assigned to a limitedavailability of photogenerated charge carriers at the TiO2 surface,or a lack of suitable catalytically active sites .
Fundamentalstudies on manganese oxide-based materials in photocatalysis
Manganese oxides arealready a major research focus at the MPI-CEC. We complement the existingresearch initiatives by unraveling the fundamental photophysical and(electro)chemical properties of a variety of stable bulk manganese oxides (MnO,Mn3O4, Mn2O3, MnO2) as afunction of oxide stoichiometry and nanosize. Due to the wide range of reportedphysicochemical parameters (e.g. band gap, band positions)  we created ourown reference database using bulk oxides in highest available purity. It is ouraim to evaluate whether a certain (modified) Mn oxide can be used as lightabsorber, as water splitting catalyst, or to take over both functionalities.Potential benefit of scaling particle sizes of those oxides down to thenanometer range is currently under investigation in cooperation with members ofCENIDE (Center for Nanointegration, University Duisburg-Essen). Additionally,isolated Mn cations and small agglomerates of Mn oxides are prepared bydifferent synthetic routes on mesoporous silica to study their photophysicalproperties in detail and to test them as potential (single-site)photocatalysts. Since nanometer-sized supported and nanocrystalline or amorphousoxide phases are difficult to analyze, we furthermore evaluate theapplicability of temperature programmed reactions in a gas phase plug flowreactor for phase identification of nanoscopic manganese oxides.
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 ) 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.
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