Skip to main content

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

Periodic Reporting for period 3 - TUCAS (Tuneable Catalyst Surfaces for Heterogeneous Catalysis – Electrochemical Switching of Selectivity and Activity)

Reporting period: 2021-01-01 to 2022-09-30

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. Involved areas are fuel cell technology, water electrolysis, battery research, and solar cells. A novel method for achieving controlled 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. They 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 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.
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. An in-depth characterisation of the structures and exsolution properties was conducted. 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). 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).
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). The highest performance was observed for the cobalt doped catalyst, which performs similarly to industrial catalysts. Our research results were published in the high impact 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.
In the fourth project year we started to work on the application of perovskites for methanol synthesis and methanol reforming. We could show that Cu doped perovskites are highly active for both reactions. Furthermore we started to work extensively on electrocatalytic CO2 reduction in the fifth project year.
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 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