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Bowl Active Site Design: An Effective Breaking of Scaling Relations for N2-to-NH3 Conversion

A vital chemical process that provides nitrogen sources for modern industry and agriculture is the conversion of N2 to NH3. Massive efforts have been expended since the Haber-Bosch process was developed, but achieving N2-to-NH3 conversion under benign conditions remains a complex problem.

A bowl active site on the lanthanide intermetallic electride, composed of four surface La cations and one subsurface Si atom, plays the key role in efficient catalysis of N2–to–NH3 conversion by breaking the scaling relations through specific electrostatic interactions. This bowl active site thus presents a design concept of highly efficient heterogeneous catalyst for N2–to–NH3 conversion. Image Credit: Chinese Journal of Catalysis

The scaling relations present an apparent paradox between a catalyst’s capacities to activate N2 and release NH3, cause a universal problem.

This causes a volcano curve of the catalytic activity for the conversion of N2 to NH3, which limits the catalyst's performance by the best catalyst design.

According to the Sabatier principle for catalyst design, the adsorption of a pertinent intermediate on the best catalyst should be just right—neither too strong nor too weak. In other words, a compromise should be the ideal trigger.

To avoid this dilemma and create highly effective heterogeneous catalysts, it is tempting to discover and explain the catalytic mechanisms that are not constrained by scaling constraints.

An examination into the thermocatalytic pathways for N2-to-NH3 conversion on the intermetallic electride LaRuSi was recently conducted by a research team at Tsinghua University in China under the direction of Prof. Hai Xiao.

They discover that the secret to the effective catalysis of N2-to-NH3 conversion is a bowl active site made up of surface La cations and negatively charged subsurface Si atoms derived from the electride nature.

While destabilizing the adsorptions of NHx (x = 1, 2, 3) species that contain positively charged H atoms, which facilitate the desorption of the final NH3 product, the electrostatic and orbital interactions between this bowl active site and reaction intermediates significantly improve the N2 activation that results in negatively charged N2 for facile cleavage of NN bond.

The scaling laws for extremely efficient N2-to-NH3 conversion are broken by this specific bowl active site, which consists of f-block La cations and electride Si anion.

They unambiguously confirm the breakdown of scaling relations between the adsorptions of NHx species and those of N by comparison with other intermetallic electride catalysts that are isostructural to LaRuSi. The bowl active site’s adaptive electrostatic interactions are crucial in shattering the scaling equations for N2-to-NH3 conversion.

They also point out the potential existence of active bowl sites with a similar structure in other classes of extremely effective heterogeneous catalysts. To build extremely effective heterogeneous catalysts for the N2-to-NH3 conversion and other catalytic processes that are constrained by the scaling equations, they suggest this bowl active site with adjustable electrostatic interactions.

Chinese Journal of Catalysis published the findings.

Journal Reference:

Jiang, Y. F., et al. (2022) Breaking the scaling relations for efficient N2-to-NH3 conversion by a bowl active site design: Insight from LaRuSi and isostructural electrides. Chinese Journal of Catalysis. doi:10.1016/S1872-2067(22)64129-9.


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