Carbon monoxide surface impurities

Figure 2. Auger electron spectrum of the surface of two Ru electrodes after deactivation by reduction of carbon monoxide and methanol at higher temperatures (75 and 90 °C respectively in 0.2 M Na2SC>4 at pH 4 and -0.545 V vs SCE ). The presence of K on the surface must result from the adsorption of K+ ions present as an impurity in the electrolyte.

If it is assumed that the mobile oxygen differs from the extralattice oxygen by the absence of an additional electron supplied by the solid, it is quite likely that modifications of the electronic levels of nickel oxide by impurities will not affect substantially the low-temperature rate of carbon monoxide oxidation. Indeed, the rate depends on surface diffusion with subsequent reaction of the adsorbed partners if our scheme is correct. On the contrary such modifications might affect the rate of the high-terapera-ture process insofar as it depends on the availability and heat of adsorption of the extralattice oxygen. As will be seen later, this prediction is correct. 

The method outlined above in the case of zinc oxide will now be applied to the carbon monoxide oxidation on nickel oxide catalysts modified in both ways. If it is assumed, as before, that semiconductivity trends in the bulk and in the surface layer are qualitatively the same, a correlation between semiconduetivity and catalysis will be established if cationic impurities of valences lower and higher than 2 are found to affect the catalytic rate in opposite directions. 

Such a proof of the carbon monoxide oxidation was first given by one of us (28). It is very important to remark that the catalysts containing impurities were prepared by firing together in air at 600°C. for 3 hr. a mechanical mixture of the required components in adequate proportions. As pointed out by Fensham (54), this is much too low a temperature to ensure homogeneous solid solutions. Consequently, when a catalyst is described as NiO + 0.1% Li2O, there is no assurance that this nominal composition is realized at all either in the surface layer or in the bulk of the sample. As will be shown, this reservation is quite essential. 

It has long been established that Pt is the most efficient singlemetal electrode for the catalysis of both reactions (1) and (2). In the case of ddiydrogen activation, no metal electrocatalyst performs better than platinum. However, aside from the fact that platinum is a precious metal, a major drawback is that commercial (fossil-based) hydrogen contains residual amounts of impurities (e g., carbon monoxide) that only serve to poison the catalyst surface." To address this particular problem, present research has focused on the employment of metal additives (e.g., Ru) or of molecular catalysts that mimic the impressive activity of biological materials (e g., hydrogenase enzymes) " the use of molecular catalysts appears to be the more attractive option since such com-... 

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

A method to elude those defects, induced reconstructions, or anion adsorption is to transfer the electrodes under well-controlled conditions including atmosphere. Thus, undesirable effects from oxygen adsorption or impurities as a source of voltammogram modifications can be avoided. These requirements are fulfilled in the iodine-carbon monoxide substitution method which was proposed for the preparation of clean and well-ordered Pt( 111) [66] and applied to Pt(100) clean surface preparation [67]. An interesting alternative to this method would be to find experimental conditions that maintain a carbon monoxide adlayer for surface protection during the transfer, assuming that this adsorption is innocuous for the surface structure itself. If this efficient protection makes no detectable surface-order modifications for Pt(100) electrodes as deduced from the cyclic voltammetric contour, we can conclude that this protection method is convenient for studying the influence of anion adsorption on the surface structure in transfer experiments. 

The open hearth proeess (Fig. 21.37) uses a dishlike container that holds 100 to 200 tons of molten iron. An external heat source is required to keep the iron molten, and a concave roof over the container reflects heat back toward the iron surface. A blast of air or oxygen is passed over the surface of the iron to react with impurities. Silicon and manganese are oxidized first and enter the slag, followed by oxidation of carbon to carbon monoxide, which causes agitation and foaming of the molten bath. The exothermic oxidation of carbon raises the temperature of the bath, causing the limestone flux to calcine ... 

Now warm the tube C gently with a flame until it is just too hot to touch—between 60 and 100 —and increase the flow of carbon monoxide. Nickel carbonyl vapor is carried over and decomposes in C, depositing a mirror of nickel on the surfaces. The principle of the Mond process for purifying nickel is to pass carbon monoxide over the metal and then to decompose the nickel carbonyl on the surface of heated nickel balls. Cobalt, the main impurity to be removed, does not react with carbon monoxide under these conditions, and in any case its carbonyl is not volatile. Nickel completely free from cobalt is thus obtained. 

Another Important concept introduced by Taylor was that of heterogeneity of surface-active centers.(25-26) This stemmed from observation of R. N. Pease that minute amounts of carbon monoxide, much smaller than the amount necessary to cover the surface, were sufficient to poison the surface of a copper catalyst. Taylor proposed that there were active centers on the surface while others argued that nickel impurities segregated preferentially on the surface and acted as catalyst. The variation of the heats of adsorption with surface coverage as determined by R. Beebe was used as evidence supporting the concept of active centers. In spite of the contradictory interpretation of the same experimental data, the concept of active centers has been a fruitful one. It inspired Imaginative research in the field of metal and oxide catalysis and has its present day expression in sophisticated surface physics studies. Subsequent work by coworkers of Turkevich at Princeton refined the nature of active centers in monodisperse metal particles and crystalline oxide catalysts. 

Other atmospheric factors such as gaseous impurities in the air (e.g. sulfur dioxide, ammonia, carbon monoxide, ozone, carbon dioxide, halogen compounds, or formaldehyde) and solid impurities in the atmosphere (airborne dust, sand, and soot) result in an acceleration of aging processes in polymers. Whereas the effects of the gaseous impurities on polymers are mainly chemical in nature, the solid particles mostly cause abrasive damage to the plastic surface. 

Impurities that adsorb mito the electrocatalysts surface inhibit the charge-transfer processes, resulting in performance loss. Common fuel impurities include carbon monoxide, ammonia, hydrogen sulfide, hydrogen cyanide, hydrocarbons, formaldehyde, and formic acid [93]. On the cathode side, ambient air may contain impurities such as sulfur dioxide, nitrogen oxides, and particulate matter (including salts) that can affect fuel cell performance [93]. 

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