Mechanistic and Electronic Insights into a Working NiAu Single-Atom Alloy Ethanol Dehydrogenation Catalyst

Abstract

Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multi-pronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge which increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogenous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step towards developing structure function relationships that inform the design of new catalysts.

Stick or Spill? Scaling Relationships for the Binding Energies of Adsorbates on Single-Atom Alloy Catalysts

“… 32 − 37 There are examples in the literature where charge analyses on DFT calculations were performed to explain experimental shifts observed in X-ray absorption and photoemission spectra. 38 , 39 However, the origin of these charges on the dopant atom and their impact on the stability of chemical intermediates adsorbed on the dopant site of SAAs (electrostatic repulsion/attraction) are yet to be understood.…”

“…On PtCu ( q d = −0.61 e ), the O–H cleavage is reported to be the first elementary step, 46 whereas on NiAu ( q d = +0.32 e ), the C–H cleavage occurs first. 39 …”

Research progress of asymmetrically coordinated single-atom catalysts for electrocatalytic reactions

“…9,10 At present, the common carriers to anchor single metal atoms include metal oxides, 11 transition metal sulfides (TMDs), 12 MXenes, 13 carbon materials, 14 and alloys. 15,16 Among them, the carbon support has a large specific surface area, good electrical conductivity, high porosity, outstanding chemical stability and corrosion resistance, and low cost, making it the most used substrate for SACs. 17–19 In 1964, Jasinski successfully prepared a cobalt phthalocyanine catalyst with Co–N 4 coordination, and found that it can catalyze the ORR under alkaline conditions.…”

Observation and Characterization of Dicarbonyls on a RhCu Single-Atom Alloy

“…Single-atom alloys (SAAs) are a type of single-atom catalyst consisting of a somewhat inert host metal doped with single atoms of a more reactive metal. − Unlike most catalysts, single-atom alloys were discovered by using fundamental microscopy and desorption measurements on single crystals under model-system ultra-high vacuum (UHV) conditions before making the transition to nanoparticle analogues. Since these initial studies, close synergy between model-system work under UHV and nanoparticle studies under industrially relevant conditions has enabled the design and understanding of these materials.…”