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Scanning probes & catalytic systems

Latest updated: December 2, 2019

Overview:

Catalysis is incredibly important in industrialized processes. Developments of novel catalysts, and improvements to existing catalysts are therefore of great interest. Scanning probe electrochemistry is used by researchers to better understand catalytic systems. Through the use of scanning probe electrochemistry, it is possible to answer a number of questions, as outlined in the table below.

 

Question Technique Information Measured Example
Can the effect of sample treatment on catalytic activity be detected? SKP SKP measures the Contact Potential Difference between a sample and probe, which is related to the work function of the sample. Changes in work function reflect changes in sample catalytic activity before and after treatment. S. S. Zance, S. Ravichandran, Applied Physics A 125 (2019) 456
Can combinatorial libraries of catalysts be screened? dc-SECM In dc-SECM the probe can generate the species of interest for catalysis. The sample is then biased to interact with this species. The sample current is collected and correlated to the probe position. High current magnitude areas indicate sample compositions with high electrocatalytic activity. G. Lu, J. S. Cooper, P. J. McGinn, Electrochimica Acta 52 (2007) 5172–5181
SDC SDC confines the electrochemical measurement to the area under the droplet, to locally measure electrocatalytic activity. By performing scans across catalyst combinatorial libraries, the effects of the structure (i.e. thickness, chemical composition, etc.) on the current can be determined. D. Schäfer, et al., Analytical Chemistry 83 (2011) 1916-1923
SKP The contact potential difference between a probe and sample measured by SKP reflects the sample work function. With SKP a local map reflecting the of a combinatorial library can be measured. In a combinatorial library the work function of the sample is dependent on the catalyst composition, crystal structure, grain orientation, and surface electron density, which affect the catalytic activity. I. Pötzelbergera, et al., Applied Catalysis A: General 525 (2016) 110–118
Can the of the metal adsorbed H bond be compared at different metal catalysts? dc-SECM SECM can be used in surface interrogation mode (SI-SECM) which analyses adsorbed species through electrochemical titration. When the SECM probe is very near a sample surface the adsorbed species can be analysed based on changes to the mediator electrochemistry as measured by the probe. Different currents measured on the different metals can then be compared to determine relative coverage, with higher current reflecting higher coverage. When linear sweep voltammetry is performed the time taken to reach the high current adsorbed H state reflects the length of time required for H adsorption on a given metal. A. Papaderakis et al., Electrochemistry Communications 83 (2017) 77–80
Can the effect of different catalyst compositions on catalytic activity be investigated? dc-SECM Using substrate generation – tip collection SECM the sample can be biased to undergo its electrocatalytic reaction. The species resulting from the electrocatalytic reaction can then be measured by the SECM probe. The current measured at the probe is directly related to the concentration of the species generated by the sample. Higher currents, therefore reflect more electrocatalytically generated species and therefore higher electrocatalytic activity. This can be compared across different catalyst compositions. J. A. Díaz-Real, et al., Applied Catalysis B: Environmental 222 (2018) 18-25
What is the catalyst microstructure? dc-SECM When a catalytic system has homogeneous activity SECM in constant height mode can be used to measure the sample topography. In constant height mode the probe to sample distance will change due to changes in sample topography. Because the SECM signal is dependent on the probe to sample distance, with positive feedback from a homogeneously active sample, the result is a map with high current regions due to raised topography and low current regions from lower topography. Z. Wang, Y. Liu, V. M. Linkov, Journal of Power Sources 160 (2006) 326-333

 

Glossary:

  • Scanning ElectroChemical Microscopy (SECM): Measurement of local electrochemical activity of a sample with chemical selectivity. The local impedance of a sample can also be measured.
  • Localized Electrochemical Impedance Spectroscopy (LEIS): Measurement of local impedance differences and point-by-point Electrochemical Impedance Spectroscopy
  • Scanning Kelvin Probe (SKP): Local non-contact measurement of sample work function. Topography can also be measured.
  • Scanning Vibrating Electrode Technique (SVET): Measurement of the local current distribution of a sample in electrolyte. Also known as Vibrating Probe.
  • Scanning Droplet Cell (SDS): Local electrochemical and impedance measurements of a sample within a droplet.
  • Optical Surface Profiler (OSP): Non-contact measurement of the local sample topography.