Surface-adsorbed carbon monoxide

In these processes, a carbon monoxide containing gas is fed to an adsorber bed containing copper, typically dispersed on a high surface area support such as alumina or carbon. The copper is present predominately as Cu", which selectively adsorbs carbon monoxide. The remainder of the gas stream passes through the adsorbent bed. The carbon monoxide is then removed from the adsorbent by lowering the pressure. Figure 6 shows a typical process for a CO-PSA process. Process conditions are typically adsorption pressures of 0.68—204 MPa (6.8—20.4 atm) and temperatures of 313—373 K. Regeneration occurs at reduced pressure or by vacuum. 

An alternative surface reaction which has been suggested is a reaction between an adsorbed oxygen atom with an adsorbed carbon monoxide molecule to form carbon dioxide which is immediately desorbed. The reaction rate is again given by the equation above. 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

The simplest theories of reactions on surfaces also predict surface rate laws in which the rate is proportional to the amount of each adsorbed reactant raised to the power of its stoichiometric coefficient, just like elementary gas-phase reactions. For example, the rate of reaction of adsorbed carbon monoxide and hydrogen atoms on a metal surface to produce a formyl species and an open site,... 

Figure 2.8 The surface reaction between adsorbed carbon monoxide and hydrogen to methane over rhodium catalysts occurs at lower temperatures in the presence of a vanadium oxide promoter, which is known to enhance the rate of CO dissociation (from Koerts el al. 113]). 

The first scheme was ruled out by showing that, at room temperature, a surface formed by very brief exposure of the oxygen-saturated surface to carbon monoxide is stable after removal of the carbon monoxide from the reaction chamber. In other words, no further surface carbide formed by lateral reactions of adsorbed carbon monoxide with surface oxygen atoms. The second scheme was ruled out by showing that exposure of the surface formed in the latter experiment to oxygen had no effect. Consequently the third scheme is believed to represent the mechanism of oxidation of carbon monoxide at the step and kink sites of platinum. 

Anode Investigations using cyclovoltammetry confirm an important effect of surface oxides (see Vols. 3, 4). A known example of the different anodic activity is the poisoning of platinum by adsorbed carbon monoxide species, for example, in the direct methanol fuel cell (DMFC),... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

According to UHV investigations (156), the cracking reaction of methanol at Pt surfaces yields strongly adsorbed carbon monoxide. There had been much dispute as to whether the species that blocks the electrocatalyst platinum in methanol-containing electrolytes is really carbon monoxide (157), but today the different schools seem to converge with the opinion that it is CO or CO, some species resembling adsorbed carbon monoxide very closely in its adsorptive, optical, and electrochemical properties. 

Reaction Mechanism. The rate of production of heat as a function of time gives indications of the velocity of the process taking place on the catalyst surface (Figure 4B). For instance, it has been shown (20) that, on NiO(200), the adsorption of oxygen and the formation of CO.r(adfo ions are fast processes compared with the adsorption of carbon monoxide or the reaction between CO and CCV ds)- From calorimetric results and a kinetic study of the reaction, it has been concluded (8) that the decomposition of COa uds) ions by adsorbed carbon monoxide to yield carbon dioxide is the slowest step of the reaction mechanism on NiO(200) (Mechanism I). 

A rate equation was derived on the assumption that in the first step formaldehyde is formed on the catalyst surface from adsorbed carbon monoxide and hydrogen. The subsequent conversion of formaldehyde to methanol was assumed to be the rate-determining step. The experimental data were best expressed by a rate equation of the Langmuir-Hinshelwood type ... 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

Van Ho and Harriott have postulated a different mechanism involving intermediate adsorbed carbon atoms.188 This is based on their observation that a larger fraction of the surface of the catalyst was covered by adsorbed carbon monoxide during reaction. The rate determining step was assumed to... 

Prompted in part by the results obtained in the previous section, the effects of adsorbed carbon monoxide and sulfur on the voltammetric behavior of single crystal Ni surfaces were examined in more detail. 

The hypothesis of formation of oxygenated compounds as intermediate products was rejected by Eidus on the basis of experiments on the conversion over cobalt of methyl and ethyl alcohols and formic acid which were found to form carbon monoxide and hydrogen in an intermediate step of the hydrocarbon synthesis (76). Methylene radicals are thought to be formed on nickel and cobalt catalysts (76) by hydrogenation of the unstable group CHOH formed by interaction of adsorbed carbon monoxide and hydrogen, while on iron catalysts methylene radicals are probably formed by hydrogenation of the carbide (78,81). Carbon dioxide was found to interact with the alkaline promoters on the surface of iron catalysts as little as 1 % potassium carbonate was found to occupy 30 to 40% of the active surface area. The alkali also promotes carbide formation and the synthesis reaction on iron (78). 

Adsorbed carbon monoxide is a matter of special interest in ultrahigh vacuum as well as in electrochemical systems. CO has been used as probe molecule in surface vibrational spectroscopy. For important reviews of CO adsorbed from the gas phase, see [21, 42, 45, 46]. The rather large dynamic dipole moment (9///0Q) of adsorbed CO is particulary suited for infrared spectroscopy at electrochemical interfaces, where submonolayer amounts of species must be observed in the presence of IR-active solution compontents. 

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