Zhang et al. (Nano Letters, 2017) showed that ultrathin Rh75W25 alloy nanosheets (~1.8 nm thick, fewer than eight atomic layers) combine one-dimensional quantum confinement, which shifts the surface dd-band center upward and strengthens CO2 adsorption, with a W-to-Rh alloy charge-transfer effect, which promotes CO2 activation and formate binding, giving high CO2-to-methanol activity and selectivity. The synthesis, however, pins the nanosheet thickness near 1.8 nm: CO released by decomposition of the W(CO)6 precursor caps the (111) facets and blocks growth along [111], so varying reaction time changes only the lateral size, never the thickness. The open problem is to make thickness a controllable, quantitative design variable in this system: develop a synthesis that systematically varies the number of atomic layers of Rh–W nanosheets while keeping the alloy composition homogeneous, and use such a thickness series to quantify how the dd-band center, W-to-Rh charge transfer, and CO2 hydrogenation activity/selectivity change with thickness — completing the confinement axis that the source paper, which mapped only the composition axis, could not address.

Background

Converting CO2 to methanol by heterogeneous hydrogenation is a target reaction for carbon utilization, and the activity of metal catalysts for it is often rationalized through electronic descriptors. Two such descriptors are central here. The dd-band center is the centroid of a transition metal's projected dd-electron density of states relative to the Fermi level; in the Hammer–Nørskov picture, a dd-band center closer to the Fermi level generally strengthens adsorption of CO2 and reaction intermediates because antibonding adsorbate–metal states become depopulated. Charge transfer between alloy components, commonly quantified by Bader analysis (a computational scheme that partitions the total electron density among atoms), determines whether the active metal carries negative surface charge, which facilitates electron donation into CO2's antibonding orbitals to form the activated, bent CO2δ− species.

Both descriptors can be engineered in ultrathin metal nanocrystals. Free-standing two-dimensional nanosheets of non-layered metals such as Rh (metals whose crystals do not consist of stacked layers and therefore cannot be exfoliated) must be grown colloidally, with surfactants or adsorbed species arresting growth along one crystallographic direction. When the remaining dimension shrinks to a few atomic layers, the electronic density of states is modified by quantum confinement — the same physics as a particle in a box — which shifts the dd-band center relative to that of bulk-like nanoparticles. Alloying with a more electropositive metal such as tungsten additionally transfers electron density to Rh.

Zhang et al. (Nano Lett. 2017, 17, 788–793) combined both effects. They synthesized hexagonal Rh75W25 nanosheets of ~1.8 nm thickness (fewer than eight atomic layers) with (111)-bounded faces, and measured CO2 hydrogenation at 150 °C and 32 bar (H2:CO2 = 3:1): a turnover frequency of 592 h−1 per Rh atom and 97% methanol selectivity, i.e., 5.9, 4.0, and 1.7 times the activity of Rh nanoparticles (4.0 nm), pure Rh nanosheets (~1.5 nm), and Rh73W27 nanoparticles (6 nm), respectively. Their density functional theory (DFT) calculations gave dd-band centers of −1.49 eV for Rh nanosheets versus −1.58 eV for Rh nanoparticles (a confinement-induced upshift of ~0.09 eV) and −1.56 eV for Rh75W25 nanosheets, with Bader charges indicating W-to-Rh electron transfer of about −0.2 e per surface Rh atom, corroborated by a −0.6 eV shift of the Rh 3d5/2 X-ray photoelectron spectroscopy (XPS) binding energy in the alloys. CO2 temperature-programmed desorption and in-situ diffuse-reflectance infrared Fourier-transform (DRIFT) spectroscopy connected these descriptors to CO2 adsorption strength, CO2δ− formation, and formate (HCOO*) intermediate binding. Varying the W(CO)6 amount produced a composition series (Rh90W10 to Rh64W36) with a volcano-type activity maximum — activity rising and then falling as W content increases — peaking at the intermediate Rh75W25 composition.

The thickness axis, however, was not variable. Monitoring the synthesis over 10–60 min showed the lateral size growing from ~5 to ~137 nm while the thickness stayed at ~1.8 nm. The authors attributed this to CO generated by decomposition of the tungsten hexacarbonyl precursor, W(CO)6: CO adsorbs on the (111) facets (bridge-bound CO bands at 1900–2100 cm−1 in infrared spectra) and acts as a capping agent that prevents growth along the [111] direction. The same agent that stabilizes the ultrathin sheet thereby pins its thickness. Consequently, quantum confinement in this system was demonstrated only at a single fixed thickness (~1.5–1.8 nm) compared against bulk-like nanoparticles, and the composition axis was mapped without the thickness axis.

Problem Statement

Can the thickness (number of atomic layers) of Rh–W alloy nanosheets be made a controllable synthesis variable, and how does thickness quantitatively govern their CO2 hydrogenation catalysis? Concretely: (1) develop a synthetic route that overcomes the CO-capping mechanism by which CO from the W(CO)6 precursor pins Rh–W nanosheet thickness at ~1.8 nm, so that a series of Rh–W nanosheets with systematically varied thickness — from the ultrathin, quantum-confined regime toward thicker, weakly confined sheets — can be prepared while keeping the alloy composition and its homogeneity fixed; and (2) using such a thickness series, quantify how the surface dd-band center, the W-to-Rh charge transfer, and consequently CO2 adsorption/activation and the activity and methanol selectivity of CO2 hydrogenation vary with the number of atomic layers — in particular, whether the confinement-induced dd-band upshift and activity benefit observed at the single fixed thickness persist, saturate, or reverse as thickness increases, complementing the known composition volcano (activity peaking at the intermediate Rh75W25 composition). The two parts jointly serve one objective: establishing thickness as a quantitative design knob for Rh–W nanosheet CO2-hydrogenation catalysis, in the same experimental-plus-computational framework (activity and selectivity measurements, X-ray photoelectron spectroscopy (XPS), CO2 temperature-programmed desorption (CO2-TPD), in-situ diffuse-reflectance infrared Fourier-transform (DRIFT) spectroscopy, and density functional theory (DFT) dd-band and Bader-charge analysis) used in the source work. A completed resolution must address both parts: a concrete synthesis strategy that decouples nanosheet thickness from the CO-capping that pins it, and a quantitative thickness dependence — obtained by DFT or another explicitly stated model, not an unquantified qualitative trend — for each of the five quantities the task tracks: the surface dd-band center, the W-to-Rh charge transfer, CO2 adsorption/activation, the CO2-hydrogenation activity, and the methanol selectivity. A proposed or executed synthesis route together with that quantitative thickness dependence satisfies this standard, as does an evidence-backed demonstration that thickness cannot be decoupled from composition and homogeneity, in which case all five quantities are to be quantified theoretically over the accessible thickness range; where a route is executed, the corresponding measurements and an explicit synthesis-to-measurement plan are expected.

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