Island Ripening via a Polymerization-Depolymerization Mechanism

In catalytic methanol oxidation on ultrathin vanadium oxide layers on Rh(111) (${\mathrm{\Theta }}_{\mathrm{V}}\approx 0.2$ monolayer equivalent) we observe a 2D ripening of the ${\mathrm{VO}}_x$ islands that is controlled by the catalytic reaction. Neighboring ${\mathrm{VO}}_x$ islands move under reaction conditions towards each other and coalesce. The motion and the coalescence of the islands are explained by a polymerization-depolymerization equilibrium that is sensitive to gradients in the adsorbate coverages.

Figure 1

Pattern formation and evolution of the reaction rates in methanol oxidation over a $\mathrm{Rh}(111){\mathrm{VO}}_{\mathrm{x}}$ catalyst during heating up. Formaldehyde (red), CO (blue), and ${\mathrm{CO}}_2$ (black) production rates together with PEEM images demonstrating different types of patterns during heating up. Inset: Structural model of $(\sqrt{7}\times \sqrt{7})R19.1°$ of ${\mathrm{VO}}_{\mathrm{x}}$ on Rh(111) associated with the stoichiometry ${\mathrm{VO}}_3$ and with ${\mathrm{\Theta }}_{\mathrm{V}}=0.43$ MLE. (From Ref. [11]. Reproduced with permission of the authors.) Experimental conditions: ${\mathrm{\Theta }}_{\mathrm{V}}=0.23$ MLE, $p({\mathrm{CH}}_3\mathrm{OH})=1\times {10}^{-4}\mathrm{mbar}$, $p({\mathrm{O}}_2)=1\times {10}^{-4}\mathrm{mbar}$, heating $\mathrm{rate}=0.2°\mathrm{C}/\mathrm{s}$.

Figure 2

Coalescence of ${\mathrm{VO}}_{\mathrm{x}}$ islands under reaction conditions. (a) PEEM images showing different stages in the coalescence of two nearby islands. Note that each island has a substructure with a bright core, an inner boundary, and a dark gray ring. Experimental conditions: ${\mathrm{\Theta }}_{\mathrm{V}}=0.23$ MLE, $T=750°\mathrm{C}$, $p({\mathrm{CH}}_3\mathrm{OH})=1\times {10}^{-4}\mathrm{mbar}$, $p({\mathrm{O}}_2)=1\times {10}^{-4}\mathrm{mbar}$. (b) $x(t)$ plot of the coalescence experiment displayed in (a) with $x$ representing the distance between the boundaries of the islands.

Figure 3

Concentration profile of a ${\mathrm{VO}}_{\mathrm{x}}$ island and chemical characterization with photoelectron spectroscopy. (a) Coverage profile of V and O under reaction conditions in the ${10}^{-6}\mathrm{mbar}$ range. The position $x=0$ refers to the center of the island. The inset displays a PEEM image of a ${\mathrm{VO}}_{\mathrm{x}}$ island (taken from a different experiment). The roman numbers I—IV refer to different positions inside and outside the island. Experimental conditions: ${\mathrm{\Theta }}_{\mathrm{V}}=0.28$ MLE, $T=630°\mathrm{C}$, $p({\mathrm{CH}}_3\mathrm{OH})=6.5\times {10}^{-7}\mathrm{mbar}$, $p({\mathrm{O}}_2)=6.5\times {10}^{-7}\mathrm{mbar}$. (b) $\mathrm{V}2p_{3/2}$ photoelectron spectra taken with microprobe XPS from various positions inside and outside (arrow) the island indicated in the inset of Fig. 3. The values of the thin film oxides for $V^{3+}$, ${\mathrm{V}}^{4+}$, and ${\mathrm{V}}^{5+}$ are shown as a guideline [18]. For details of the fitting, see the Supplemental Material [20].

Figure 4

Polymerization-depolymerization mechanism for coalescence of ${\mathrm{VO}}_{\mathrm{x}}$ islands. (a) Mechanistic scheme illustrating the movement of two neighboring ${\mathrm{VO}}_{\mathrm{x}}$ islands via a PD mechanism under reaction conditions. The intensity of the blue background represents the oxygen coverage. (b) Gradients of oxygen coverage around ${\mathrm{VO}}_{\mathrm{x}}$ islands as taken from PEEM images. Left: PEEM image of two neighboring ${\mathrm{VO}}_{\mathrm{x}}$ islands under reaction conditions in the ${10}^{-4}\mathrm{mbar}$ range. Right: Cross section of the PEEM brightness taken along the bright line shown in the PEEM image left. Experimental conditions: ${\mathrm{\Theta }}_{\mathrm{V}}=0.23$ MLE, $T=750°\mathrm{C}$, $p({\mathrm{CH}}_3\mathrm{OH})=8\times {10}^{-5}\mathrm{mbar}$, $p({\mathrm{O}}_2)=1\times {10}^{-4}\mathrm{mbar}$.