Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron-nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal-nitrogen-carbon (MNC) catalysts.

Based on global warming our society need to change towards CO 2 free or neutral options of energy supply. The hydrogen economy plays an important role in providing and utilizing green hydrogen. While platinum is the best-known catalysts for the hydrogen evolution reaction (HER), alternatives are required that are sustainable and free of platinum group metals (PGMs). Metal-nitrogen-carbon (MNC) catalysts have shown promising activity for the HER [1], however, often beside catalytically active MN 4 centers they contain side phases that hinder mechanistic insights. The inhomogeneity in composition is based on the usual preparation approaches, that involve in minimum one pyrolysis or heat-treatment step. To avoid side phases, we focused on immobilized iron porphyrin model complexes and explored there the HER. In this work, I will present our results associated with a substituent variation [2] and in-depth characterization by post-mortem and in situ approaches using different nuclear resonance techniques [3]. On the basis of this, we identified different species associated with the reaction cycle and the rate limiting step. The knowledge on this will help further improvement of the porphyrinic system for HER and validation to what extent a similar mechanism applies after the pyrolysis. References [1] A. Shaharei, A. Moradabi, I. Martinaiou et al., Elucidating the origin of hydrogen evolution reaction activity in mono and bimetallic Metal- and Nitrogen-doped Carbon Catalysts (MeNC), ACS Appl. Mater. Interfaces 2017 , 9, 25184−25193. [2] N. Heppe, C. Gallenkamp, S. Paul et al., Substituent effect in iron porphyrin catalysts for the hydrogen evolution reaction , Chem. Europ. J. 2023 , 29(19) e202202465. [3] N. Heppe, C. Gallenkamp, R. Snitkoff-Sol et al., Applying Nuclear Forward Scattering as in situ and operando tool for the characterization of FeN4 moieties in the hydrogen evolution reaction, ChemRxiv, 2023, 10.26434/chemrxiv-2023-sk5sr.

For a future hydrogen economy, non precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. MNC catalysts with MN4 active sites show promising performance, but an optimization rooted in structure property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphin substituents on the electronic and electrocatalytic properties of MN4 centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [FeII(P▪)]- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Photoelectrochemical (PEC) cells promise to combine the benefits of photovoltaics and electrolysis in one device. They consist of a photoabsorber functionalized with an electrocatalyst to harvest faradaic currents under reduced overpotentials. To protect the absorber from the harsh reaction conditions, a protective buffer layer (e. g. TiO2) is added between absorber and catalyst. In this work, we investigate the influence of the catalyst support systems Ti/TiOx and Ti/TiOx/M (M=Au, Ni, Fe) on the overall activity and stability of nickel and iron mixed layered double hydroxides for the alkaline oxygen evolution reaction (OER). The catalyst performance on the bare Ti/TiOx substrate is very poor, but the incorporation of a metallic interlayer leads to two orders of magnitude higher OER current densities. While a similar effect has been observed for M=gold supported systems, we show that the same effect can be achieved with M=nickel/iron, already contained in the catalyst. This proprietary metal interlayer promises a cheap OER performance increase for PEC cells protected with titania buffer layers. Detailed XPS show an improved transformation of the starting catalyst material into the highly active (oxy)hydroxide phase, when using metallic interlayers. From these experiments a pure conductivity enhancement was excluded as possible explanation, but instead an additional change in the local atomic and electronic structure at the metal-support and metal-catalyst interfaces is proposed.

Ir modification of FeNC catalysts improves the durability of the catalysts, but causes electronic changes that are disadvantageous for the activity.

Abstract FeNC catalysts are promising substitutes of platinum‐type catalysts for the oxygen reduction reaction (ORR). While previous research disclosed that high pyrolysis temperatures are required to achieve good stability, it was identified that a trade‐off needs to be made regarding the active site density. The central question is, if a good stability can also be reached at milder pyrolysis conditions but longer duration retaining more active sites, while enabling the defect‐rich carbon to heal during a long residence time? To address this, a variation of pyrolysis temperatures and durations is used in FeNC fabrication. Carbon morphology and iron species are characterized by Raman spectroscopy and Mössbauer spectroscopy, respectively. Fuel cell (FC) activity and stability data are acquired. The results are compared to ORR activity and selectivity data from rotating ring disc electrode experiments and resulting durability in accelerated stress tests mimicking the load cycle and start‐up and shut‐down cycle conditions. It is discussed how pyrolysis temperature and duration affect FC activity and stability. But, more important, the results connect the pyrolysis conditions to the required accelerated stress test protocol combination to enable a prediction of the catalyst stability in fuel cells.

We show that gaseous nitric oxide (NO) and oxygen (O-2) are useful molecular probes to uncover complex surface processes in Fe-N/C catalysts. We unravel the difference between using gaseous NO in a temperature programmed desorption experiment and using NO (and progenitors) in an electrochemical experiment. Gas phase O-2 adsorption is almost exclusively desorbed as CO2, and continued exposure to oxygen increases the amount of chemisorbed oxygen species on the surface. The oxidation state of the carbon surface is an important activity determining factor, and under normal "electrochemical" conditions many of the active sites are blocked. Only by treatment at 600 degrees C in Ar can we free those sites for oxygen adsorption, however under atmospheric storage, and especially during the oxygen reduction reaction (ORR), the surface quickly becomes deactivated with chemisorbed oxygen species and water. We demonstrate that the material can be super-activated by reductive electrochemical treatment, both in an electrochemical three electrode cell and in a fuel cell. The energy gained following the treatment is significantly larger than the energetic cost.

In this work, the influences of various transition metal ions as active sites in high purity metal- and nitrogen-doped carbon catalysts (in short M-N-C), where M: Mn3+, Fe3+, Co2+ , Ni2+ , Cu2+ , Zn2+, or Sn4+ in the catalyst powders, were systematically investigated for the electrochemical reduction of CO2 in the aqueous electrolyte. The almost exclusive presence of isolated M-N-4 centers as catalytic sites was determined by X-ray photoelectron spectroscopy (XPS). The catalysts were electrochemically investigated in a gas diffusion electrode arrangement in bypass mode coupled in-line to a mass spectrometer. This allowed for the nearly simultaneous detection of products and current densities in linear sweep voltammetry experiments, from which potential-dependent specific production rates and faradaic efficiencies could be derived. Postmortem XPS analyses were performed after various stages of operation on the Cu-N-C catalyst, which was the only catalyst to produce hydrocarbons (CH4 and C2H4) in significant amounts. The data provided insights into the potential-induced electronic changes of the Cu-N-C catalyst occurring under operating conditions. Our work further experimentally revealed the high affinity of M-N-C catalysts to convert CO2 to industrially relevant carbonaceous raw materials, while effectively suppressing the competing hydrogen evolution reaction. These results led to a better understanding of the role of the active sites, especially the central metal ion, in M-N-C and could contribute significantly to the improvement of selectivities and activities for the CO2RR in this catalyst class through tailor-made optimization strategies.

FeNC catalysts are the most promising substitutes for Pt‐based catalysts for the oxygen reduction reaction in proton exchange fuel cells. However, it remains unclear which FeN4moieties contribute to the reaction mechanism and in which way. The origin of this debate could lie in various preparation routes, and therefore the aim of this work is to identify whether the active site species differ in different preparation routes or not. To answer this question, three FeNC catalysts, related to the three main preparation routes, are prepared and thoroughly characterized. Three transitions A–C that are distinguished by a variation in the local environment of the deoxygenated state are defined. By in situ57Fe Mössbauer spectroscopy, it can be shown that all three catalysts exhibit a common spectral change assigned to one of the transitions that constitutes the dominant contribution to the direct electroreduction of oxygen. Moreover, the change in selectivity can be attributed to the presence of a variation within additional species. Density functional theory calculations help to explain the observed trends and enable concrete suggestions on the nature of nitrogen coordination in the two FeN4moieties involved in the oxygen reduction reaction of FeNC catalysts.

FeNC catalysts are important substitutes for the oxygen reduction reaction (ORR) in fuel cells. This work reports on an in situ Mössbauer spectroelectrochemical study of a porphyrin-based catalyst. Activity and selectivity towards ORR were determined from rotating ring disc electrode (RRDE) experiments at different loadings in acidic electrolyte and accompanied by H2O2 oxidation reduction measurements in order to identify the contributions to the different ORR pathways as function of potential. The comparison to in situ 57Fe Mössbauer spectra enables an assignment of these contributions to the iron signatures. The results indicate that two different “onset potentials” for obtaining the deoxygenated state associated with two different iron environments can be identified and being associated with the selectivity data. Moreover, the in situ data enable the determination of mass-based site density and turn-over frequency data for ORR relevant conditions. As a consequence, this work sheds light on the oxygen reduction reaction mechanism involved in FeNC catalysts.

Pyrolysed transition metal porphyrins (MeNxC) were originally developed as alternative catalysts for the electrochemical reduction of oxygen (ORR). The catalytic centers were identified as base transition metal ions, integrated into graphene layers of a carbon matrix via nitrogen atoms. The carbon serves as a conductive support and influences the entire catalytic process with its variable electronic and chemical properties. Thus, this class of materials can be classified as hybrid-material between inorganic and molecular catalysts. Meanwhile it is known that this type of catalysts are also active towards the CO2RR. Our presentation will show results of electrochemical studies on pyrolysed transition metal porphyrins (MeNxC) as catalysts for the CO2RR under variation of the metal ions (Me: Fe, Co, Ni, Sn, Zn, Cu , Mn). Their activities are determined in gas diffusion electrode (GDE) configuration using CO2, CO or Ar gas feeds. The product composition of the gas phase at the outlet of the cell is analyzed simultaneously by mass spectroscopy. Soluble products are analysed via ion-chromatography. It is shown that the product composition of CO2RR, as well as the respective onset potentials, partial current densities, formation rates, and Faraday efficiencies vary strongly with the type of metal ions investigated in the catalytic center. Fe-, Co- and Ni-based catalysts form predominantly CO and suppress the competing hydrogen evolution reaction. CO-formation rates of 4 mmol/(hcm2)at -2V NHE and 200 mA/cm2 (Faraday efficiency close to 100%) were achieved on not yet optimized GDEs. The formation of significant amounts of hydrocarbons from CO2 was observed only on the Cu- based material. The analysis of the potential dependence during product formation under different gases fed to the GDE allows initial conclusions about the underlying mechanisms of the CO2RR. Figure 1

Two iridium-based catalysts (namely IrSn and IrNi) are synthesised via a polyol route involving capping agents. The capping agents are removed according to a time-consuming multistep heat-treatment protocol described in the literature (N2 → N2/O2 → H2). In this work the effect of each of these steps on the structural composition and catalytic activity is investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and electrochemistry. It is shown that heating in nitrogen is not required, whereas air is the most effective for the removal of the capping agents. Besides FT-IR, the analysis of sp3 carbon (from XPS) turned out to give reasonable insights into capping agent removal. Induced by hydrogen treatment no further change of the surface occurs, while particles tend to grow and become more crystalline. While structural changes are similar for both catalysts, the impact of each of the steps on the catalysis is different: the activity per gram of iridium becomes even lower for IrSn (an electrochemical cleaning agent that was used as reference measurements, seems best suited) while the activity is doubled for the IrNi case. Our results illustrate that the selection of the cleaning procedure strongly depends on the investigated system and should be optimised individually.

Herein, Fe–N–C catalysts are prepared from surface functionalized carbon nanotubes (CNTs) in combination with iron acetate and phenanthroline. An improved performance and structural composition is obtained by surface functionalization of the CNTs with indazole or pyridine. Catalyst composition and morphology are characterized by transmission electron microscopy, N2 sorption, photoelectron spectroscopy, and 57Fe transmission Mössbauer spectroscopy. However, activity and selectivity toward oxygen reduction reaction are determined from rotating ring disc electrode (RRDE) experiments. The durability and stability are evaluated by accelerated stress tests (0.0–1.2 V) and differential electrochemical mass spectroscopy (DEMS), respectively. It is shown that surface functionalization with indazole enables the direct attachment of FeN4 centers to CNTs so that no impurity species are detected and a high activity is achieved, that can be attributed to an improved turnover frequency and higher mass‐based site density. Even more striking is the excellent durability and stability of the realized catalyst. While these trends are well pronounced in RRDE and DEMS, challenges in the preparation of membrane electrode assemblies make the trend not as obvious in fuel cells (FCs).

In this work a comprehensive study of the activity and stability of a non-precious metal catalyst of type Fe- N- C in acidic media is reported. The catalyst was prepared from polyaniline, dicyandiamide and iron acetate as precursors. Temperature-dependent rotating-disk electrode experiments were performed to determine the activation energy of the catalyst. Besides, load cycle durability tests with and without the addition of methanol show that there is no additional deactivation caused by methanol addition. In a Direct Methanol Fuel Cell (DMFCs) our catalyst performed similarly good in comparison to other Fe-N-C catalysts. Raman and Mossbauer spectroscopy provide valuable information on the structural composition and chemical changes induced by durability and stability testing of the catalyst. While the maximum power density during DMFC operation decreases by 85%, the qualitative distribution of iron sites might indicate the formation of iron and iron oxide clusters as decomposition product associated with the disintegration of FeN4 sites.

CO oxidation is an extensively studied reaction in heterogeneous catalysis due to its seeming simplicity and its great importance for emission control. However, the role of particle size and more specifically structure sensitivity in this reaction is still controversial. In the present study, colloidal "surfactant-free" Pt nanoparticles (NPs) in a size regime of 1-4 nm with narrow size distribution and control over particle size were synthesized and subsequently supported on Al2O3 to prepare model catalysts. CO oxidation was performed using Pt NPs catalysts with particles sizes of 1, 2, 3, and 4 nm at different reaction temperatures. It is shown that the reaction exhibits a particle size effect that depends strongly on the reaction conditions. At 170 degrees C, the reaction seems to proceed within the same kinetic regime for all particle sizes, but the surface normalized activity depends strongly on the particle size, with maximum activity for nanoparticles 2 nm in diameter. A temperature increase to 200 degrees C leads to a change of the kinetic regime that depends on the particle size. For Pt NPs 1 nm in diameter a reaction order of 1 for O-2 was observed, indicating that O-2 adsorbs molecularly and dissociates in a following step, which represents the generally accepted mechanism on Pt surfaces. The reaction order of -1 for CO demonstrates that the surface is saturated with CO under reaction conditions. With increasing particle size, the reaction orders of O-2 and CO change. For particles 2 nm in size, an increase in temperature also results in reaction orders of 1 for O-2 and -1 for CO; NPs of 3 and 4 nm, even at higher temperatures, show no clear kinetic behavior that can be explained by a single reaction mechanism. Instead, the Boudouard reaction between two adjacent adsorbed CO molecules was identified as an important additional reaction pathway that occurs preferentially on large particles and causes more complex kinetics. (C) 2019 Elsevier Inc. All rights reserved.

Electrochemical energy conversion in proton exchange fuel cells (PEFCs) is very promising device for world-wide energy demand, especially in the transportation sector [1]. Fe-N-C catalysts are promising materials as alternative for platinum-based catalyst because of the low cost and high activities especially for the ORR. However, a fundamental understanding of the nature of active site structures and the ORR reaction mechanism is missing. Especially also considering the question, to what extent differently prepared catalysts obtain similar trends or not. 57Fe Mössbauer Spectroscopy (MS) is a very good technique in distinguishing FeN4 centers with changing local coordination environment [2,3]. So far, the electronic and geometric structure have been investigated under in-situ / operando conditions by spectroscopic techniques, like XAS [4], XES [5]. However, only once Mössbauer spectroscopy was applied quasi in-situ by Bouwkamp-Wijnoltz et al. on frozen electrodes [6] and there are only a rare number of publications on carbon supported macrocycles [7,8]. In this work, we have designed a feasible electrochemical cell for in-situ Mössbauer spectroscopy. Two different types of Fe-N-C catalysts were studied under in-situ conditions and we will present the structural changes induced by the effect of applied potential. By comparing different catalysts, we are able to show to what extent Mössbauer sites remain the same, independent of the preparation route or are changing. The data will be correlated with selectivity measurements of the catalysts. Based on this, further fundamental insights on the nature of the active site structure in Fe-N-C catalysts can be gained which could guide the preparation for highly active non precious metal catalyst for commercialization. References: [1] Debe, M. K., Nature., 486 (2012), 43−51. [2] Kramm, U.I. et al., J. Am. Chem. Soc. 136 (2014), 978-985. [3] Kramm, U. I. et al., Phys. Chem.Chem.Phys. 14 (2012), 11673-11688. [4] Zitolo, A. et al., Nat Mater 14 (2015), 937-942. [5] Niwa, H. et al., Electrochemistry Communications 35 (2013), 57-60. [6] Bouwkamp-Wijnoltz et al., J. Phys. Chem. B, 106, (2002), 12993-13001. [7] Blomquist, J.et al., Electrochimica Acta, 1982, 27(10), 1445-1451. [8] Scherson. D.A. et al., J. Electroanal. Chem., 184 (1985), 419-426.

Cobalt hydroxide species are at the origin of OER activity of multiheteroatom doped carbon catalysts.