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Electrocatalysis Discussion Group-1: 529627044
Key scientific problems addressed and main research content
1 Electrocatalytic active site structure, studying step sites and aggregated step sites
2 Characterizing the electrochemical performance of different sites, revealing the relationship between the structure and electrochemical performance of different sites
3 Step aggregation can affect the number of active sites, thus influencing the activity of electrocatalytic reactions
The atomic surface structure of electrocatalysts plays a crucial role in electrocatalytic activity. It has been found that in many electrocatalytic reactions, a pattern emerges: when the number of step sites is low, the ORR activity of Pt catalysts is linearly correlated with the number of step sites; however, when the density of step sites is very high, this linear relationship is disrupted.
In light of this, Marcel J. Rost and others from Leiden University reported a study on the catalytic activity of Pt(111) using in situ electrochemical scanning tunneling microscopy (EC-STM), structurally revealing that the reason for this anomalous change in catalytic reaction properties is due to densely clustered steps (multiple steps close together in space).
It was found that the widths of steps and terraces on Pt(554) matched the predicted results, but most step site structures on the Pt(553) crystal surface became more compressed than predicted, with the heights of steps and terraces being twice the standard value.
This study challenges the widely accepted notion in the field of electrochemistry that step surfaces consist of uniform single-atom heights. The aggregation of steps explained the anomalously high step density surface structure, providing a reasonable explanation for the reported catalytic activity and zero charge anomalies.
Scheme 1. Step aggregation on the surface of Pt catalysts and electrocatalysis
In situ electrochemical characterization
Figure 1. EC-STM testing of Pt(111) related crystal faces
Using 3D EC-STM imaging characterization technology, the Pt(554) and Pt(553) surfaces were characterized with different potentials (Us=0.1 V, Ut=0.15 V). The characterization found that the terrace widths of Pt(554) and Pt(553) samples were 23.1 Å and 23.9 Å respectively, which is quite strange, as theoretically the widths should be 22.4 Å and 10.4 Å.
Figure 2. Model of Pt(553) crystal face
Theoretical terrace widths can be calculated through modeling
Using STM, 50 random points were selected from Pt(554) and Pt(553) to calculate the heights of steps and widths of terraces. The testing results showed that the average width of the terrace on Pt(553) was 21.9±0.8 Å, consistent with the theoretical result (22.4 Å). However, the testing results for the terrace widths on Pt(553) were 11.3±0.5 Å, 20.6±0.5 Å, and 30±1 Å, which correspond to the standard, double, and triple terrace widths, indicating the existence of double and triple steps. Therefore, the data characterization calculations showed that the proportions of single, double, and triple steps on Pt(553) were 35±4%, 51±5%, and 14±3% respectively. This result indicates that the Pt(554) crystal face has single steps, but the Pt(553) crystal face exhibits a characteristic of clustered steps (the phenomenon of multiple steps being close together in space).
Aggregation leads to instability
Figure 3. Analysis of surface structure
The authors found that the occurrence of step aggregation on the surface can be explained through parameter calculations. Specifically, the relationship between fstep and Bstep can reduce the total free energy ftotal, where fstep refers to the free energy of steps and Bstep refers to the interaction constant. The repulsive interaction between steps leads to a change in ftotal. The research results can be expressed as a relationship between ftotal and the width of terraces, where when the width of the terrace is reduced to n=8, the interaction between steps has a more significant effect. The characterization results also indicate that when the width of the terrace is less than or equal to 8 atoms, the steps reduce free energy by forming aggregated steps. This explains why the Pt(554) crystal face has single steps, while the Pt(553) crystal face has double steps. According to EC-STM characterization, the width of the terrace between the (100) steps of Pt(553) is 4 atoms, where the steps mainly exhibit an aggregated form. Literature reports on surface XRD characterization have similarly found this phenomenon in studies of Pt(311) and Pt(331) crystal faces, although the authors attributed this phenomenon to surface reconstruction of the catalyst.
Figure 4. Thermodynamic properties of aggregated steps
Impact of step aggregation on electrocatalysis
Figure 5. Effects of aggregated steps on Pt electrocatalytic properties
In 0.1 M HClO4 electrolyte, the performance of Pt(111) and Pt catalysts containing (111) steps was tested.
Impact of electrochemical CV. First, the CV curve was tested, and it was found that in the region below 0.4 V there is a broad peak, and at 0.13 V there is a sharp peak, corresponding to the terrace and step sites respectively. The Pt(553) and Pt(221) crystal faces also exhibited an additional peak at 0.185 V, corresponding to hydrogen desorption from aggregated step sites. Similar peaks were also observed in Pt(110) containing (111) steps, indicating that this conclusion is reliable. Pt(775) did not exhibit similar peaks, but the potential corresponding to single steps shifted, indicating the presence of aggregated steps.
Impact on Epztc (zero charge potential). Epztc is closely related to the work function; the authors found that step aggregation leads to an increase in terrace width, thus increasing Epztc.
Additionally, it is known that acidic ORR reactions are most active at the concave sites at the step edges, where the bonding of the Ohad intermediate is weaker than that at the (111) terrace. Therefore, it is believed that as the number of step sites increases, the number of concave sites increases, leading to a linear increase in ORR catalytic activity. However, testing results showed that this is not the case; the ORR half-wave potential (a descriptor of catalytic activity) only exhibited an increasing trend with increasing step density when n<9.
Step aggregation on different crystal faces. Pt(775), Pt(221), and Pt(331) exhibited step aggregation phenomena, and led to a reduction of the number of step edge sites by 1/2 (steps appeared in pairs) and 1/3 (steps appeared in groups of three). Testing results showed that Pt(221) mainly formed paired steps, Pt(331) mainly formed three-step aggregates, and Pt(775) mainly formed pairs of two steps.
Step aggregation can affect other electrocatalytic reactions sensitive to step sites, such as CO oxidation reaction and NH3 oxidation reaction.
References and original article link
Valls Mascaró, F., Koper, M.T.M. & Rost, M.J. Step bunching instability and its effects in electrocatalysis on platinum surfaces.Nat Catal (2024).
DOI: 10.1038/s41929-024-01232-2
https://www.nature.com/articles/s41929-024-01232-2
Electrocatalysis Discussion Group-1: 529627044
Photocatalysis Group-2: 927909706
Homogeneous Catalysis and Enzyme Catalysis Group-2: 929342001
Nano-Catalysis Group-1: 256363607
Porous Materials Group-2: 813094255
Theoretical Calculation Group-2: 863569400
Synchrotron Radiation丨Spherical Aberration Electron Microscopy丨FIB-TEM
In Situ XPS, In Situ XRD, In Situ Raman, In Situ FTIR
