Adsorption of alkanes on stoichiometric and oxygen-rich RuO₂(110)

Abstract

We investigated the molecular adsorption of methane, ethane, propane and n-butane on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory (DFT-D3) calculations. We find that each alkane adsorbs strongly on the coordinatively-unsaturated Ru (Rucus) atoms of s-RuO2(110), with desorption from this state producing a well-defined TPD peak at low alkane coverage. As the coverage increases, we find that alkanes first form a compressed layer on the Rucus atoms and subsequently adsorb on the bridging O atoms of the surface until the monolayer saturates. DFT-D3 calculations predict that methane preferentially adsorbs on top of a Rucus atom and that the C2 to C4 alkanes preferentially adopt bidentate configurations in which each molecule aligns parallel to the Rucus atom row and datively bonds to neighboring Rucus atoms. DFT-D3 predicts binding energies that agree quantitatively with our experimental estimates for alkane σ-complexes on RuO2(110). We find that oxygen atoms adsorbed on top of Rucus atoms (Oot atoms) stabilize the adsorbed alkane complexes that bind in a given configuration, while also blocking the sites needed for σ-complex formation. This site blocking causes the coverage of the most stable, bidentate alkane complexes to decrease sharply with increasing Oot coverage. Concurrently, we find that a new peak develops in the C2 to C4 alkane TPD spectra with increasing Oot coverage, and that the desorption yield in this TPD feature passes through a maximum at Oot coverages between ∼50% and 60%. We present evidence that the new TPD peak arises from C2 to C4 alkanes that adsorb in upright, monodentate configurations on stranded Rucus sites located within the Oot layer.

Hydrogen oxidation on oxygen-rich IrO2(110)

“…The broadening of the O 2 TPD feature towards lower temperatures suggests that neighbouring O ot atoms interact repulsively, and thereby lower the energy barrier for recombinative O ot desorption. Prior studies also report strong repulsive interactions between O ot atoms on RuO 2 (110) [26,27], and a significant broadening of the O 2 TPD feature with increasing O ot coverage [28]. The total desorption yield in the δ 1 + δ 2 feature is equal to 0.46 ML at saturation of the on-top oxygen layer at 85 K. Adsorption of O 2 at 300 K also produces a δ TPD feature that saturates at an O ot coverage of 0.44 ML (Figure 2(b)).…”

Adsorption and Oxidation of CH4 on Oxygen-Rich IrO2(110)

“…For example, prior work shows that O ot atoms strongly promote the complete oxidation of C 2 H 4 on O-rich RuO 2 (110), whereas ethylene reacts only modestly on the stoichiometric RuO 2 (110) surface. 19−21 While the light nalkanes (C 1 −C 3 ) react negligibly on RuO 2 (110) surfaces under ultrahigh vacuum (UHV) conditions, with the exception of nbutane, 6,7,22 Li et al find that O ot atoms stabilize alkane σcomplexes on RuO 2 (110) in addition to blocking the Ru cus sites needed for alkane adsorption. 6 We investigated the adsorption and oxidation of CH 4 on O ot precovered IrO 2 (110) in the present study and find that O ot atoms strongly enhance the complete oxidation of CH 4 over CO and recombinative CH 4 desorption during temperatureprogrammed reaction spectroscopy (TPRS).…”

“…19−21 While the light nalkanes (C 1 −C 3 ) react negligibly on RuO 2 (110) surfaces under ultrahigh vacuum (UHV) conditions, with the exception of nbutane, 6,7,22 Li et al find that O ot atoms stabilize alkane σcomplexes on RuO 2 (110) in addition to blocking the Ru cus sites needed for alkane adsorption. 6 We investigated the adsorption and oxidation of CH 4 on O ot precovered IrO 2 (110) in the present study and find that O ot atoms strongly enhance the complete oxidation of CH 4 over CO and recombinative CH 4 desorption during temperatureprogrammed reaction spectroscopy (TPRS). Density functional theory (DFT) calculations predict that key steps in the CH 4 oxidation pathway have significantly lower barriers on Orich compared with stoichiometric (s)-IrO 2 (110) because the steps involve abstraction of O ot atoms rather than the more strongly bound O br species.…”

Mixed-Anion Control of C–H Bond Activation of Methane on the IrO2Surface

“…On the basis of their DFT calculations, Jiang et al pointed out that the origin of IrO 2 ’s reactivity is due to the strong σ–d interaction between a C–H σ bond orbital of methane and a d orbital of a coordinatively unsaturated surface metal site. When coordinated to the coordinatively unsaturated metal atom, the C–H bond is weakened by the σ donation to the metal. , Generally, alkanes, including methane, are strongly bound by the σ–d interaction with the coordinatively unsaturated metal atom on the surface of the late transition metal oxides, forming what is called a σ complex in analogy with organometallic chemistry. − In the literature, one can often see many cases where the σ complex functions as a precursor for the C–H bond activation. , Such an interaction between the C–H bond and the metal atom is also known as an agostic interaction …”