In Situ Studies of Methanol Decomposition Over Cu(111) and Cu₂O/Cu(111): Effects of Reactant Pressure, Surface Morphology, and Hot Spots of Active Sites

Abstract

The dissociative adsorption of methanol was investigated on Cu(111) and ultrathin Cu 2 O films. We employed synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Scanning Tunneling Microscopy (STM) to study the dynamics of gas−solid interactions, and calculations based on Density Functional Theory (DFT) were used to examine the reaction path. C 1s XPS spectra revealed that methanol underwent dissociative adsorption on plain Cu(111) to form methoxy (CH 3 O), formaldehyde (H 2 CO), and formate (HCOO) at a pressure range of 0.5−10 mTorr, with these species remaining on the surface after evacuation. This was accompanied by the appearance of a low coverage (∼0.05 ML) of O ads in the O 1s which can be considered a highly active site for methanol activation. The high activity is apparent by a coverage of 0.8 ML of methoxy at room temperature. STM was unable to image these species at room temperature as they were highly mobile on metallic copper. In contrast, for CH 3 OH on Cu 2 O/Cu(111), STM showed clear hot spots for reaction and a complex array of adsorption structures. On the oxide substrate, there was decomposition of methanol to H 2 CO, CH 3 O, HCOO, and hydrocarbon species (CH x ) due to the subsequent interactions of methanol with lattice oxygen. Cu(111) remained entirely saturated with decomposition products under 10 mTorr of methanol (θ ≈ 0.97 ML), whereas the Cu 2 O overlayer was saturated at a much lower coverage (θ ≈ 0.30 ML). STM revealed rows and step edges of Cu 2 O decorated with decomposition products and metallic Cu islands ∼5 nm in size. The difference in activity between Cu(111) and Cu 2 O/ Cu(111) is attributed to the significant amount of O present on the oxide surface. Density Functional theory (DFT) calculations described the XPS measurements well, showing a likely methanol dissociation to *CH 3 O and therefore a surface reduction. More importantly, the DFT results revealed that it was the chemisorbed oxygen on Cu 2 O/Cu(111) which oxidized the dissociated *CH 3 O to *HCOO and eventually CO 2 , while the reaction only involving upper oxygen on the Cu 2 O hexagonal ring led to the formation of H 2 CO.

Understanding Methanol Synthesis on Inverse ZnO/CuOx/Cu Catalysts: Stability of CH3O Species and Dynamic Nature of the Surface

“…In a previous work, we have examined in detail the surface chemistry of methanol on a plain Cu 2 O/Cu­(111) substrate . Our work and previous studies indicate that the dissociation of the O–H bond in methanol is strongly dependent on the temperature of the surface. , Theoretical calculations gave an energy barrier of ∼0.8 eV for the cleavage of this O–H bond . The barrier was easily overcome at experimental temperatures of 300–400 K. However, the dissociation of the molecule at low temperatures (<200 K) was difficult, as reported in previous STM studies .…”

“…The barrier was easily overcome at experimental temperatures of 300–400 K. However, the dissociation of the molecule at low temperatures (<200 K) was difficult, as reported in previous STM studies . On the Cu 2 O/Cu­(111) substrate, at 300 K, methanol decomposed, yielding CH 3 O, H 2 CO, HCOO, and hydrocarbon species (CH x ) . Subsequent heating to 450 K led to desorption of most of the adsorbed CH 3 O, H 2 CO, and HCOO species.…”

“…In a previous work, we have examined in detail the surface chemistry of methanol on a plain Cu 2 O/Cu­(111) substrate . Our work and previous studies indicate that the dissociation of the O–H bond in methanol is strongly dependent on the temperature of the surface. , Theoretical calculations gave an energy barrier of ∼0.8 eV for the cleavage of this O–H bond .…”

“…The sample was held at the indicated pressure for ∼5 min, away from the beam, to allow for an equilibrium chemistry to establish. Coverages were calculated by taking the O 1s-to-Cu 3p ratio for each species and comparing it to a known coverage of O for Cu 2 O/Cu­(111). , …”

“…This type of basic information is necessary to optimize the performance of catalysts utilized to facilitate the CO 2 → CH 3 OH transformation. The chemical reactivity of methanol, as the simplest carbon-containing alcohol, has garnered considerable attention related to the transformation of C–H and C–OH bonds on surfaces of metals, − oxides, − and mixed oxides or more complex materials. − In this work, we study the dynamic interaction of methanol with ZnO/Cu 2 O/Cu­(111) catalysts employing ambient pressure X-ray photoelectron spectroscopy (AP-XPS), scanning tunneling microscopy (STM), and calculations based on density functional theory (DFT). Preferential adsorption is observed on the ZnO regions of the catalysts, with the adsorbate undergoing desorption as CH 3 OH or full decomposition (CH 3 O → CH 2 O → CHO → CO).…”