Copper (sub)oxide formation: a surface sensitive characterization of model catalysts

Abstract

Model studies on the catalytic methanol oxidation over single and polycrystalline copper have been performed. The catalytic activity was investigated by means of temperature-programmed techniques (thermal desorption and temperature-programmed reaction spectroscopy, TDS and TPRS, respectively). The TPRS experiments call for the existence of chemically inequivalent species of atomic oxygen accessibly for catalytic processes on the copper surface. The surface morphological changes after the combined action of oxygen and methanol were observed by using atomic force (AFM) and scanning electron miscroscopy (SEM) and indicate the participation of not only the surface but to a great extend also the bulk. Furthermore, ex situ X-ray absorption spectroscopy (XAS) at the O K-edge shows that a copper suboxide phase of is formed Cu (xw2.5) O at the surface/near-surface region up to a depth of about 100 Core-level (XPS) and valence band (UPS) A. photoemission suggests that the suboxide phase can be viewed as an oxygen-deÐcient copper(I) oxide phase exhibiting an increased density-of-states at the Fermi level pointing to an electrically conducting phase. The depth-selective recording of X-ray absorption spectra gives clear evidence of the formation of a protective copper(I) oxide Ðlm underneath the suboxide layer covering the bulk metal phase.

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

“…The oxygen is not desorbing but rather transforming some Cu into oxide species [56], similar to the oxide grains in Fig. 6b.…”

Subsurface oxide plays a critical role in CO 2 activation by Cu(111) surfaces to form chemisorbed CO 2 , the first step in reduction of CO 2

“…For the other experimental conditions and investigated surfaces (see Table 1 and Supporting Information for further details), the CO2 partial pressure (p(CO2)) was kept at 0.35 Torr whereas the total pressure (ptot) was kept constant at 0.7 Torr by codosing H2O. The APXPS measurements were performed while dosing CO2 on both metallic Cu(111) and Cux=1.5O surfaces, whereas CO2 and H2O were codosed on metallic Cu(111), Cux=1.5O, and Cux=2.5O suboxide surfaces (18). The sample surface was clean and no evident C-or O-based contaminations were observed after the cleaning procedure, as shown in Fig.…”

“…As with C 1s, we partition the O 1s spectral window into three regions. At low BEs we identify the states of O bonded as follows: (i) surface adsorbed O (Cu-O ads )on metallic Cu and on suboxidic Cux O structures (Cux O-O ads ) at 531.0 eV and 529.6 eV, respectively (15,(25)(26)(27); (ii) subsurface adsorbed O (O sub ) on metal Cu (Cu-O sub ) at 529.8 eV (27) (as we discuss in a later section, such a presence of suboxide plays an important role in stabilizing the l -CO2); and (iii) for Cux>1O the O 1s is centered at 530.3 eV (15,18,25). It is noteworthy that O ads groups on the Cu surface can serve as nucleation sites for hydroxylation when in the presence of H2O.…”

Adsorbate coverages and surface reactivity in methanol oxidation over Cu(110): An in situ photoelectron spectroscopy study

“…[7][8][9][10][11][12][13][14][15][16] Different in situ techniques were applied in order to identify the active surface phases on polycrystalline Cu at higher pressure (mbar range). [17][18][19][20][21][22][23][24][25][26] Cu(110) surfaces were investigated with temperatureprogrammed desorption (TPD), molecular beam techniques, low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). 7,8,[10][11][12]15,[27][28][29][30] These techniques were typically applied under non-stationary conditions using sequential dosing and temperature-programmed experiments.…”