Copper oxides, chemisorption

The studies of Garner and his co-workers in the years 1928-1939, which had established the existence of two types of carbon monoxide and hydrogen chemisorption on oxides and which identified irreversible chemisorption with incipient reduction, were followed in the immediate postwar period by an intensive study of the properties of copper oxide (12-15). The work was later extended to nickel oxide (16) and cobalt oxide (17,18). With each of these oxides it was established that carbon monoxide was capable of reacting not only with lattice oxygen, but also with adsorbed oxygen. The concept of irreversible chemisorption involving a carbonate ion and ulti-... 

In general, chemisorption will produce new spectral bands which are not characteristic of the adsorbate or the adsorbent. However, absence of such bands cannot be taken as evidence of an absence of chemisorption. A difficulty present in any attempt to make kinetic measurements is that extinction coefficients are often significantly altered as a result of adsorption. These changes, which cannot as yet be interpreted theoretically, make it difficult to correlate the observed absorbance with the coverage of adsorbed molecules. The change in extinction coefficient is dependent on both the adsorbate and the adsorbent. For example, an increase of e was observed with increasing coverage for ethylene adsorbed on copper oxide, whereas the reverse occurred with nickel oxide . ... 

Total smface areas were measured by nitrogen adsorption at -196 C, using an automated instrument (Omnisorp lOOCX, Coulter Electronics Limited). The cross sectional area of the nitrogen molecule was assumed to be 16.2 x 10 m. Pore type and volume data were also obtained by this method, using t-plot analysis. Metal areas were measured by selective chemisorption of hydrogen at 30 °C in the same instrument. Copper surface areas were measured in a flow system by nitrous oxide chemisorption at 60 C. 

Of crucial significance in deciding between various models have been estimates of the number of copper atoms required to transform the surface into a (2 x 3)N phase. This was the approach adopted by Takehiro et al 2 in their study of NO dissociation at Cu(110). They concluded that by determining the stoichiometry of the (2 x 3)N phase that there is good evidence for a pseudo-(100) model, where a Cu(ll0) row penetrates into the surface layer per three [ll0]Cu surface rows. It is the formation of the five-coordinated N atoms that drives the reconstruction. The authors are of the view that their observations are inconsistent with the added-row model. The structure of the (2 x 3)N phase produced by implantation of nitrogen atoms appears to be identical with that formed by the dissociative chemisorption of nitric oxide. 

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

The IR and Raman spectra of benzotriazole, benzotriazole anion and its Cu(I) complex have been measured. The characteristic peaks in the IR spectrum of the triazole moiety in benzotriazole anion occur at 1163 cm , 1134 cm , and 1115 cm . A broad band with a main peak at 1151 cm occurs in the spectrum of the Cu(I)-BTA complex <85JST(l00)57i>. The chemisorption of benzotriazole on clean copper and cuprous oxide surfaces is investigated by combining XPS, UV-PE and IR reflection absorption spectroscopy (IRAS). Coordination geometry including the triazole-... 

The present study was initiated to provide a direct comparison of IETS and IR spectra for an identical molecule adsorbed on an aluminum oxide covered, evaporated aluminum substrate. Further, it was of interest to see if a weakly acidic C-H bond, such as that present in 1,3-dialkanediones, would show dissociative chemisorption similar to the well-known chemisorptions of Bronsted acids containing acidic O-H bonds (see above). The molecules chosen for this study were acetic acid and 2,4-pentanedione. Both oxide covered copper and aluminum were used as substrates in order to see the effects of substrate oxide on the chemisorption spectra. 

Acetic acid chemisorption has been previously studied using IETS by Lewis, Mosesman and Weinberg for oxide covered aluminum surfaces. Using reflection IR Tompkins and Allara have reported spectra for adsorption on oxidized copper and Hebard, Arthur and Allara S for adsorption on oxidized indium. All these studies demonstrate that chemisorption from the gas phase involves proton dissociation since the observed spectra are those of acetate ion species. 

The formation of some organic hydroperoxides by oxidation with molecular oxygen is catalytically promoted by metals like silver or copper 171). A dissociative chemisorption of oxygen cannot be active in these processes they probably proceed via the chemisorption of O7 ions (or O2 molecules forming a covalent bond resonating with an ionic bond). 

Adsorption methods may be used to provide information about the total surface area of a catalyst, the surface area of the phase carrying the active sites, or possibly even the type and number of active sites. The interaction between the adsorbate and the adsorbent may be chemical (chemisorption) or physical (phys-isorption) in nature and ideally should be a surface-specific interaction. It is necessary to be aware, however, that in some cases the interaction between the adsorbate and the adsorbent can lead to a chemical reaction in which more than just the surface layer of the adsorbent is involved. For example, when using oxidizing compounds as adsorbates (O2 or N2O) with metals such as copper or nickel or sulfides, subsurface oxidation may occur. 

Paetow and Riekert (28, 29) in a careful study have compared the relative activities of Cu2 +-exchanged zeolite T and mordenite with various copper-containing compounds. On the basis of turnover numbers per CO chemisorption site the Cu2 +-exchanged zeolites are 2-4 orders of magnitude less active than CuO, CuMn204, and CuCr204. This was considered to be consistent with the involvement of lattice oxygen as an intermediate which is easier to remove in oxides than zeolites. 

BET area (Table V) and the copper area from oxygen chemisorption. Table VII summarizes the copper and zinc oxide areas so determined for the whole compositional range. The oxygen chemisorption method suffers from the uncertainty that some oxygen may be adsorbed on the copper solute and on defects in the zinc oxide surface that are formed only in the presence of copper. There is indirect evidence from a comparative study of carbon monoxide and oxygen chemisorption, however, that this is not the case and that oxygen titrates only the copper metal surface. 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

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