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Methanol synthesis on ZnO(000). III. Free energy landscapes, reaction pathways, and mechanistic insights
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Image of FIG. 1.
FIG. 1.

Putative reaction network: Scheme of possible reaction channels and intermediates involved in the synthesis of methanol from CO on the defective ZnO() surface. The name and formal charge state of the intermediates was chosen following the nomenclature for stable gas phase species. Reduction steps (via or involving two electrons from the vacancy site), proton transfer reactions, and configuration changes according to the assigned formal charges are indicated by arrows with a dot, plain arrows and arrows with a tilde, respectively (see text for further explanations). The Roman numbers correspond to the number of H atoms bound to carbon. The same classification is used in Fig. 3. The reaction channels are termed formyl (cyan), formate (magenta), hydroxymethyl (green), methoxide (red), and hydroxymethoxide (blue) according to the main species involved. Structures corresponding to the species in the scheme as they appear in the explorative metadynamics simulation Mtd-I are depicted in the right panel. CO above an arbitrarily chosen initial F0/H2 oxygen vacancy is shown in the uppermost row. With the stage of the hydrogenation of CO the formal charge state of the vacancy successively changes from the initial F0/H2 via F0 to F++ after methanol has desorbed.

Image of FIG. 2.
FIG. 2.

Schematic (a) top and (b) side view of the slab model for the explorative metadynamics simulation of CO hydrogenation to methanol. The polar O–terminated ZnO(000) surface was made charge neutral by hydroxylation before an O vacancy with formal charge state F0/H2 was introduced (see text). The exposed Zn atoms in the O vacancy form a triangular cluster (green color). In this active site a H atom with hydride character is bound. In (b) the frozen atoms of the bottom layers of the slab are rendered pale for better view. d(C − Zn*) defines the distance between a plane in which the C atom of CO is allowed to move and the plane of the uppermost (second) layer of fixed Zn atoms.

Image of FIG. 3.
FIG. 3.

Free energy landscape obtained from an explorative multiple walker ab initio metadynamics sampling of the hydrogenation of CO at the partially hydroxylated ZnO(000) surface hosting an F0/H2 oxygen vacancy (see Fig. 2). Collective variables are the coordination numbers of carbon to hydroxylic atoms c[C-H] and to the hydridic inside the vacancy c[C − Hvac]. The C atom was constrained to move in a plane defined by d(C − Zn*) = 4.20 Å above the topmost fixed Zn layer as shown in Fig. 2(b). Relative free energies ΔF (in eV) are provided according to the color scale. The pathways (capital letters) interconnecting the free energy minima (Roman numbers), which are found in the metadynamics simulation, are sketched by black lines to guide the eye. The free energy minima (0) to (III)/(III) correspond to various molecular species as introduced in Fig. 1 for the four hydrogenation states (0, I, II, III) of the C site.

Image of FIG. 4.
FIG. 4.

Free energy landscape for the hydrogenation reaction of a tridentate formaldehyde (left inset) bound to an F0 center leading to a methoxide species (right inset). Only the forward reaction was sampled sufficiently so that only the forward free energy barrier can be estimated.

Image of FIG. 5.
FIG. 5.

Free energy landscapes for the hydrogenation of formaldehyde. Iso-surface representations of the FES are shown for (a) an F0 and (b) an F−− center. A free energy profile together with the structures of the intermediates at the FEM are sketched in (c) for F0 and in (d) for F−−. FEM are marked by numbers, MFEP by capital letters, and wiggly lines denote shallow minima on the FES.

Image of FIG. 6.
FIG. 6.

Free energy landscapes (a) of the decomposition of hydroxymethyl species on F++ and (b) of the methanol formation on F0 defect sites. The corresponding atomic configurations of the FEM of reactant and product state are given as insets.


Generic image for table
Table I.

Metadynamics simulation setups: collective variables (CVs) and height ω of the biasing functions (in units of k B T 500 = 0.043 eV). The reactant state of the systems is denoted using a shorthand notation explained in the text. In addition to the number of metadynamics (Mtd) sampling steps performed in each simulation, the underlying AIMD simulation (fictitious) “time” is reported.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Methanol synthesis on ZnO(0001¯). III. Free energy landscapes, reaction pathways, and mechanistic insights