# Unraveling the Mixing Entropy-Activity Relationship in High Entropy Alloy Catalysts: The More, The Better?

## TL;DR

This paper explores how the complexity of high-entropy alloy catalysts affects their activity, finding that more elements aren't always better.

## Contribution

The study proposes a new hypothesis explaining the surface complexity-activity relationship in HEA catalysts.

## Key findings

- Adding elements initially boosts activity due to ligand interactions.
- Beyond a point, activity declines due to dilution of active sites.
- An optimal surface complexity exists for maximum catalytic activity.

## Abstract

The variety of publications reporting high-entropy alloy
(HEA)
catalysts with exceptional activities creates a survivor bias, implying
that the mixing entropy directly increases the activity. However,
many screening studies show a different picture. In a multielement
composition-activity space, often a low to medium entropic 2- or 3-element
composition emerges as the most active catalyst. In this work, we
investigate the relationship between the complexity of an alloy, which
can be expressed in mixing entropy, and its maximum possible activity
using theory and statistical modeling. Based on our analysis, we propose
a hypothesis for the surface complexity-activity relationship of HEA
catalysts. Namely, the intrinsic activity of an alloyed surface is
defined by two opposing forces: positive ligand interactions that
enhance the activity and the statistical dilution of active sites.
As a result, the relationship between the surface complexity-activity
shows qualitatively a volcano-like behavior. At first, adding elements
increases the activity due to favorable ligand interactions. Yet,
at some point, the catalytic benefit from increasing the complexity
of the surface gets outweighed by the dilution of the catalytic sites.
Correspondingly, this hypothesis states that there is an optimal ratio
between the surface complexity and catalytic activity.