Discovering a descriptor to design single-site alloy catalysts

Published in Chemistry

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A confusing question

The regulation of electronic properties and geometric features on extended solid surfaces is indispensable in the pursuit of advanced catalysts. However, researchers are always confused to interpret their effects on the microenvironment of catalytic surfaces, which directly relate to the catalytic performance.

Long attempts, many failures

We attempt to elucidate this question for Pt-based catalysts that are always used in propane dehydrogenation (PDH). As an industrially important reaction, the target product propylene is one of the most important chemical raw materials with a rich downstream industrial chain. In contrast to experimental trial-and-error methods, the development of predictive descriptors through density functional theory (DFT) calculations can accelerate the design process of catalysts.

On the basis of separately tracking electronic properties and geometric features, we made some progress. Preliminarily, we found that the electronic environment of active site can be qualified by electronegativity difference (Dc) between Pt and M (M = transition metals). Furthermore, the interatomic distance between the nearest neighbor of Pt (Dd) was recognized as a dominating geometric feature of active site. Nonetheless, we struggled with the point that how to comprehensively describe the microenvironment based on the two aspects consisting of electronic properties and geometric features. In the original manuscript, the rough combination of these two aspects to construct a single descriptor did not work well, as pointed out by the reviewers. For about one and a half years, we have strived to improve the effectiveness of descriptor, while I keep failing.

A watershed

Having thought for a long time, we realized that something is missing. There exists something that builds a bridge between the electronic properties and microenvironment, along with the geometric features and microenvironment. Moreover, it ought to reflect the impact of the microenvironment over catalytic performance.

By investigating the interaction between the active site and adsorbates, we found that “Pt-C repulsion” is the best choice. “Pt-C repulsion” relates to both electronic and geometric changes in the microenvironment, which quantifies the strength of surface-adsorbate interaction and directly influences the performance. Then, we introduced electron term and geometric term to describe “Pt-C repulsion”, respectively. As these two terms describe the same thing, they can be combined straightly. This is exactly a watershed in the research.

The descriptor called “degree-of-isolation”

Considering the repulsion is a kind of isolation effect, a descriptor, called “degree-of-isolation” (ϕ), is proposed. The volcano-shaped “isolation-selectivity” plot reveals a Sabatier-type principle for designing selective alloys, underscoring that moderate Pt-C repulsion contributes to optimal performance. It provides a direct approach and more intrinsic insight to evaluate selectivity, which is complex to be determined by two variables in competition, regarding the desorption and the further dehydrogenation of propylene. Additionally, the active center vividly changes from the left side to the right side of the volcano curve, stressing the great impact on selectivity via the alternation of adsorption configuration.

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