Insights into the Coadsorption and Reactivity of O and CO on Ru(0001) and Their Coverage Dependence

Abstract

Using density functional theory and an exchange–correlation functional that includes the van der Waals interaction, we study the coadsorption of CO on Ru(0001) saturated with 0.5 ML of oxygen. Different coexisting CO coverages are considered that are experimentally motivated, the room temperature coverage consisting of 0.5 ML-O + 0.25 ML-CO (low coverage), the saturation coverage achieved at low temperatures (0.5 ML-O + 0.375 ML-CO, intermediate coverage), and the equally mixed monolayer that is stable according to our calculations but not experimentally observed yet (0.5 ML-O + 0.5 ML-CO, high coverage). For each coverage, we study the competition between the desorption and oxidation of CO on the corresponding optimized structure by analyzing their reaction energies and minimum energy reaction paths. The desorption process is endothermic at all coverages, although the desorption energy decreases as the CO coverage increases. The process itself (and also the reverted adsorption) becomes more involved at the intermediate and high coverages because of the appearance of a physisorption well and concomitant energy barrier separating it from the chemisorbed state. Remarkably, the oxidation of CO, which is endothermic at low coverages, turns exothermic at the intermediate and high coverages. In all cases, the minimum reaction path for oxidation, which involves the chemisorbed and physisorbed CO2, is ruled by one of the large energy barriers that protect these molecular states. Altogether, the larger activation energies for oxidation as compared to those for desorption and the extreme complexity of the oxidation against the desorption paths explain that CO desorption dominates over the oxidation in experiments.

Why Ultrafast Photoinduced CO Desorption Dominates over Oxidation on Ru(0001)

“…The fact that CO desorption dominates over CO oxidation is particularly striking considering that previous DFT calculations of the minimum energy reaction paths showed that desorption requires around 0.38 eV of additional energy compared to oxidation at this coverage. 4 The natural question that arises is whether the laser-induced oxidation dynamics does or does not proceed through that minimum energy oxidation path, in which the less bound O fcc abandons its adsorption well, crosses the bridge site between two Ru atoms, and recombines with the nearby CO that tilts to form the chemisorbed bent CO 2 (hereafter denoted bCO 2 ). The selected snapshots depicted in Figure 2 for one of the trajectories confirm that this is the case.…”

“…6,19,[22][23][24][25][26] All (T e , T l )-AIMDEF simulations are performed with vasp 27,28 and the AIMDEF module 19,[29][30][31][32][33][34] using the same computational parameters and the same van der Waals exchange-correlation functional by Dion et al 35 that were used in our previous structural study. 4 Electronic friction coefficients are calculated with the local density friction approximation (LDFA). 36,37 The O K-edge XAS have been obtained with the transition potential and ∆-Kohn-Sham methods as implemented in GPAW.…”

“…As remarkable as it is, the precise dynamics of the oxidation process in this emblematic experiment remains unknown. In fact, already the reasons behind the very low probability for CO 2 formation are unclear because density functional theory (DFT) calculations of the minimum energy path (MEP) of these reactions show that CO 2 desorption is energetically favored against CO desorption. , Altogether, the general questions that naturally arise are how do the ultrafast CO oxidation and CO desorption proceed in the highly excited and nonequilibrated system, and ultimately, what mechanisms make the photoinduced CO desorption more probable than oxidation.…”

“…The experimental (2O+CO)/Ru(0001) honeycomb surface, in which CO adsorbs atop a Ru atom and the O atoms occupy the second nearest hcp and fcc sites forming a honeycomb arrangement around the CO, is modeled using a periodic slab with five Ru layers and the adlayer (see Figure S1 in SI). The employed (4 × 2) surface cell containing two equivalent adsorbates of each kind is the minimum cell that permits including out of phase movements of the adsorbates and a reliable description of the interadsorbates interactions, that are expected to be relevant at sufficiently large coverages. ,,− All ( T e , T l )-AIMDEF simulations are performed with vasp , and the AIMDEF module ,− using the same computational parameters and the same van der Waals exchange-correlation functional by Dion et al that were used in our previous structural study . Electronic friction coefficients are calculated with the local density friction approximation (LDFA). , …”

“…From the chemisorbed state the molecule evolves toward the physisorbed linear CO 2 ( t ≃1820 fs) and finally desorbs at t ≳ 2 ps. Altogether, the figure details the complexity of the oxidation process as compared to the simpler CO desorption dynamics, which does not involve intermediate states and barriers . By analyzing the details of the dynamics, we conclude that it is the reduced configurational space of the former, and particularly the access to the transition state, that explains why the energetically less favorable CO desorption process dominates.…”