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Tuneable Catalyst Surfaces for Heterogeneous Catalysis – Electrochemical Switching of Selectivity and Activity

Periodic Reporting for period 2 - TUCAS (Tuneable Catalyst Surfaces for Heterogeneous Catalysis – Electrochemical Switching of Selectivity and Activity)

Reporting period: 2019-07-01 to 2020-12-31

As a result of recently growing efforts in the area of sustainable energy and resulting increased interest in energy storage and conversion, the fields of catalysis, electrochemistry, and materials research are now experiencing a peak. Numerous research groups focus their efforts on technology needed for the production of renewable energy or the conversion and storage of sustainable electricity. Involved areas are fuel cell technology, water electrolysis, battery research, and solar cells. For most of the processes needed in such technologies surfaces decorated with uniformly dispersed, catalytically highly active nanoparticles play a key role. Typically, these structures are prepared by standard techniques - with limitations in controlling catalyst surface structure (i.e. particle size distribution or dispersion) – followed by catalyst activation via oxidation and reduction cycles. Alternatively, a novel method was recently discovered achieving said surface structuring by growing the nanoparticles in-situ under catalytic reaction conditions, directly from the host material itself (by exsolution), which represents a new, easy, highly time- and cost-efficient way of catalyst preparation.
The materials we utilize to perform this task are perovskite-type oxides of nominal composition ABO3 with transition metal cations on the B-site that can exsolve the B-site transition metal atoms upon controlled reduction. In this exsolution process, the transition metal emerges from the oxide lattice and migrates to the surface where it forms catalytically active nanoparticles. Doping the B-site with reducible and catalytically highly active elements offers the opportunity of tailoring properties of exsolution catalysts.
In the current research project we tackle this research topic by combining surface science with heterogeneous catalysis and electrochemistry. With this unique approach, it will be possible to gain knowledge about the electronic and geometric structure of the perovskite surface and to correlate it with catalytic activity/selectivity.
With this project we aim to go one step further: Perovskite type oxides offer the opportunity to design the active catalysts as fuel cells with working and counter electrodes and electric contacts. This enables us to apply voltage to the sample and apply an electric potential to the catalyst surface. This potential can be utilized to control and fine tune the exsolution process and thus the nature of the formed nanoparticles, and also to enhance the catalytic reactivity of the surface. This allows control over the surface reactivity and catalyst structure in real time by an external signal (here the applied voltage) and offers the possibility to design catalysts exactly for their respective catalytic application.

To achieve our project goals and to investigate the novel perovskite catalysts, in-situ surface structure characterisation by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and by operando X-ray diffraction (XRD) are the methods of choice. These are extremely powerful spectroscopic techniques that give information on chemical composition, oxidation state, and surface/bulk structure. With NAP-XPS it will be possible to monitor the surface chemistry (especially exsolution of nano-particles, changes in redox state and electronic structure) directly during catalytic reactions. Catalytic performance (conversion, selectivity) will be monitored in real time by mass spectrometry (MS) and gas chromatography (GC). As our experiments are very specialized and have a really high demand on the spectroscopic equipment, we set up a specialized NAP-XPS system in our lab during the first project year. In addition to the changes in structure and catalysis due to the influence of the applied voltage, the related electrochemical properties of the perovskite samples will be analysed by electrochemical impedance spectroscopy (EIS) and current voltage curves (IV). Furthermore, we perform theory calculations (using mainly density functional theory – DFT) to support our experimental findings and to gain additional information on the newly designed perovskites. The obtained results will provide extensive understanding of the nature of electrochemical promotion of catalytic reactions (e.g. switching from normal to outstanding activity, or changing selectivity) and the underlying structural changes.

To test the capabilities of our novel perovskite catalyst, we use them in catalytic reactions that are highly relevant for chemical energy conversion (see reactions A - D). Chemical energy conversion is needed to be able to store excess renewable energy from e.g. wind or solar power. The reason is that one of the major drawbacks of renewable energy is its strong dependence on regional or seasonal aspects. For example, wind power plants deliver excess energy during storm seasons that is currently not utilized. With chemical energy conversion this excess energy can be stored and then released again when needed.
The catalytic reactions of interest are reversible (that means depending on the direction of the reaction different processes occur) – from the viewpoint of sustainable energy production, storage, and conversion the most impactful reactions are:

A) water splitting 2 H2O ⇌ 2 H2 + O2 hydrogen oxidation
B) water gas shift CO + H2O ⇌ CO2 + H2 reverse water gas shift
C) CO oxidation 2 CO + O2 ⇌ 2 CO2 CO2 electrolysis
D) methanol reforming CH3OH + H2O ⇌ 3 H2 + CO2 methanol synthesis

Due to the reversibility of the reactions (for energy storage and later release) our novel perovskites are tested for both reaction directions.
The first project year was dedicated to the set-up of the in-situ NAP-XPS (near ambient pressure X-ray photoelectron spectroscopy) system, as described in WP 1. This included initiating the call for bids, planning of the system and layout, constant interaction with the company building the main parts of the major equipment, installation at TU Wien, and commissioning. During this process, we decided to build parts of the major equipment on our own. We planned and assembled the gas supply unit and the laser heating system. This included programming of control software and an interlock system. We also implemented a micro-GC and a mass spectrometer for gas analysis. Furthermore, Huber Scientific built the sample stage on basis of a custom design. Concerning the timeframe for the in-situ NAP-XPS, there was a delay of about two 2 months in reaching operability which was mainly caused by SPECS (manufacturer of the in-situ system) – the X-ray sources were not working properly. Upon operation of the system at TU Wien a further problem was observed: energy fluctuations in the spectra which were caused by magnetic fields from the metro in Vienna. As a result, we had to implement active magnetic field compensation. In conclusion, we successfully finished the task of WP 1 (milestone reached).
In parallel to the setup of the in-situ NAP-XPS System, we started the synthesis and characterisation of novel perovskite materials in the first project year. The perovskites La0.9Ca0.1FeO3 La0.6Ca0.4FeO3 Nd0.9Ca0.1FeO3 Nd0.6Ca0.4FeO3 Nd0.6Ca0.4Fe0.9Ni0.1O3 and Nd0.6Ca0.4Fe0.9Co0.1O3 were synthesised. An in-depth characterisation of the structures and exsolution properties was conducted. For this, XRD measurements of the synthesised materials were conducted with additional in-situ measurements in the second project year to investigate stability in reducing reaction atmosphere and exsolution capabilities. Whereas the B-site undoped materials exhibited iron exsolution, the B-site doped materials showed either Co or Ni exsolution. Depending on the amount of Ca-doping of the A-site, the exsolution temperature could be lowered (more Ca, lower T).
In the second project year we started with the theoretical calculations of electronic properties and structure of the novel perovskites (WP 6). This task was performed a year ahead of time (PhD 3 for the theoretical calculations was planned to start in the third project year), since there was an extremely good candidate for the position available. Thus, we got the first theory results much earlier which could be used to support our results obtained from XRD, enabling exact proposals for the structure (with distortion) and insights into the conductivity of the perovskites. Both experimental results and theoretical calculations were compiled into a manuscript which was submitted to the journal “Acta Crystalographica Section B”.
Subsequently, we performed catalytic tests of the novel perovskites in the second project year (including benchmark tests with industrial catalysts), WP 3. With this, we could choose the most suitable catalyst material for in-situ NAP-XPS studies. These results were published in the journal Catalysts. A change in the sequence of the WP was made, since the topic of water gas shift (WGS) and reverse water gas shift (rWGS) reactions are very current and cutting-edge for chemical energy conversion (WP 3 was now conducted before WP 2; the sequence of WP 2, 3, 4, and 5 can be changed without affecting the project execution outlined in the DoA).
For the WGS reaction all materials were tested in terms of their catalytic performance and benchmarked against LSF and an industrial WGS catalyst. The novel cobalt doped perovskite (Nd0.6Ca0.4Fe0.9Co0.1O3) even outperformed the benchmark. For the B-site doped material exsolution, could be observed directly under reaction conditions without further pre-reduction.
Then, the more relevant rWGS reaction was studied on all novel perovskites. rWGS is particularly interesting for the utilization of CO2 as a sustainable carbon source (and for the removal of the greenhouse gas CO2 from the atmosphere). Again, the novel materials were benchmarked against LSF and an industrial catalyst. The highest performance was observed for the cobalt doped catalyst, which performs similarly to industrial catalysts. Nanoparticle exsolution occurs in-situ during the reaction. Our research results were summarized in a comprehensive paper that is currently finalized for submission in the Journal “Applied Catalysis B: Environmental”.
In the third year we started to investigate the suitability of the novel perovskites for water splitting and CO oxidation (WP 2 and 4). In parallel, we began to synthesise new perovskites with a lower reducibility including characterisation by XRD and in-situ NAP-XPS.
A high risk, high gain goal of the project is to design perovskites that can exsolve alloy particles in-situ. Tuning the alloy composition of nanoparticles (e.g. via applied polarisation) opens the possibility to modify the electronic structure of catalytically active centres such that they exhibit improved reactivity and coking resistance.
Our NAP-XPS measurements for Nd0.6Ca0.4Fe0.9Ni0.1O3 and Nd0.6Ca0.4Fe0.9Co0.1O3 have revealed that, indeed, we can control the exsolution and formation of metal nanoparticles with the applied voltage. For the Co doped perovskite at moderate reducing conditions (either reducing atmosphere or applied voltage,) Co is exsolved first. Only when for example the voltage is increased (stronger applied potential), Fe starts to exsolve as well and formation of metallic Fe on the surface is visible. Currently, we are investigating if Fe and Co form separate nanoparticles or rather a FeCo alloy with varying composition, depending on the strength of the applied potential. Upon clarification, we plan on publishing our findings in a high impact journal.
From an application point of view, the formation of alloy nanoparticles on the surface is particularly interesting for catalysis as e.g. FeNi alloys prevent surface poisoning by carbon for the dry reforming reaction.

An even more ambitious goal beyond the state of the art (and experiments that to the best of our knowledge have not been performed before) will be to dope the perovskite host lattice with two (or more) catalytically active elements (with different reducibility) and to exsolve them consecutively as nanoparticles depending on the degree of polarisation. This opens a completely new perspective of reversible switching of catalyst activity and selectivity. As example, reversible exsolution of CuZn or PdZn nanoparticles has enormous potential for technological application for renewable energy production and storage via methanol reforming or synthesis (3 H2 + CO2 ⇌ CH3OH + H2O), where catalysts often suffer activity loss over time. It is therefore expected that related results will have an extremely high impact in the scientific community and for application in industry (for later industrial applications, upscaling of catalyst design and production will be needed).
Schematic depiction of chemical energy conversion (gases + energy are converted to fuel)
Stylized depiction of B-site elements emerging from bulk, forming nanoparticles on surface