Carbon monoxide linear adsorption

Chemisorption state of carbon monoxide. The adsorption of carbon monoxide on the surface of solid catalyst has been studied extensively. As early as in 1950s, Esschens et al. have concluded that there are two states of the CO chemisorption by infrared spectrum One is linear structure, which is a single metal atom adsorptive site, that is, the carbon atom is adsorbed by a metal atom, with the wave number being at about 2,050 cm the other state is the bridge-like structure, which is double-metal atoms adsorptive site, that is, carbon atom is adsorbed by two adjacent metal atoms, with the wave number being at about 1,905 cm. ... 

The adsorption of carbon monoxide on supported ruthenium has been extensively studied by IR spectroscopy (ref. 4). General agreement exists on the presence of three IR bands. The LF band at 1990-2030 cm" is assigned to the vibration of carbon monoxide linearly bonded on ruthenium crystallites. The bands at 2080 and 2140 cm correspond to the vibrations of a multicarbonyl. In a recent investigation (ref. 5) this species was shown to be a tricarbonyl associated with Ru cations bonded directly to the support. 

The Pd-Sn/C catalysts (1 to 7.5% Pd containing 0 to 1% Sn) were heated under vacuum at 150°C and then exposed to hydrogen. These preactivated samples were then titrated with carbon monoxide, a veiy specific ligand for Pd, up to 800 Torr at 30°C. A general linear trend of carbon monoxide concentration with % Pd in Figure 15.3 indicates that the carbon monoxide adsorption is directly correlated to Pd concentration, as expected. The trend is independent of Sn content. This linear Pd-CO trend indicates that the particle size distribution is similar for the different catalysts. However, Figure 15.3 also indicates no relationship between % H2S irreversibly adsorbed and % Pd. 

Data for the adsorption of carbon monoxide on charcoal at 273 K analyzed with three equations in linearized form, are... 

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... 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

According to the data of analysis of many adsorption systems, the first term in Equation 9 corresponding to the second order appears only v hen considering adsorption of relatively small molecules. They include molecules of linear shape, such as the diatomic gases, carbon dioxide, carbon monoxide, etc. Experimentally realizable orders, n, are integers from 3 to 6 in the general case. With larger polyatomic molecules, no adsorption space remains in the zeolite voids for final adsorption under the effect of dispersion forces. Then Equation 9 retains only the second term, and Uon is expressed by Equation 12. 

The polymerization of ethylene from the gas phase by CrO, /SiOa has been studied [280, 281]. A linear relationship was observed between the rate of polymerization of ethylene and the capacity of the catalyst for adsorption of carbon monoxide — this latter increasing with time of pretreatment at 300°C. 

Evidence for ensemble effects in VIIIC/IB alloys has been obtained by examination of carbon monoxide adsorption by infrared spectroscopy. This technique has been applied to the systems Pd-Ag, Ni-Cu, and Pd-Au. It is generally accepted that carbon monoxide may chemisorb in bridged or linear forms, the former providing an absorption band in the region 1900-1950 cm and the other in the region 2000-2050 cm . There may be a distinguishable contribution to the former from CO bonded... 

Figure 8 shows the adsorption isotherms of carbon monoxide at 25° for two temperatures of activation. If one takes the linear portions of the isotherms at pressures above 60 torr as the Henry s law portion of physical adsorption and extrapolates to zero pressure, one obtains values of chemisorption nearly identical with those measured at —78° as described above. 

We attempted to measure an adsorption isotherm for the adsorption of ethylene at 25° on a chromia activated at 337°. At 2, 8, 18, 28, 36, and 48 torr, the weight rapidly reached a steady state at 60 torr, the weight very slowly increased at 90 torr it increased rather rapidly. In 12 minutes at 90 torr, 0.5 rnolecules/100 of extra ethylene adsorbed. Adsorption from about 25 to 50 torr is linear with pressure. If one extrapolates the linear region to zero pressure as we did with carbon monoxide, one computes a chemisorption of 0.6 molecules/100 A. Most of the ethylene adsorbed at 90 torr is not removable by helium flushing at either 25 or 100°. 

Second, the stoichiometry of adsorption must be known in order to calculate surface concentrations. This is extremely difhcult to establish. Hydrogen, for example, adsorbs as M-H species over crystalline planes. This is conhrmed with parallel hydrogen chemisorption and BET measurements on nonsupported nickel. However, the stoichiometry of the bond M Hn appears to increase for low coordination sites (see Chapter 3) so that overall values for very small crystallites may be greater than one. Carbon monoxide is even more troublesome. Several modes coexist linear Ni-CO, bridged, Ni CO, and subcarbonyl, Ni(CO), so that some assumptions are inherent in its use. ... 

Therefore, a linear Langmuir plot is obtained by plotting 1 /o against 1 /P. Such a plot is shown for the adsorption of oxygen, carbon monoxide, and carbon dioxide on silica in Figure 3.18. 

If the poisoning molecule (e.g., carbon monoxide) must make many collisions with the catalyst surface before adsorption can occur, then poisoning molecules will have the opportunity to diffuse deep into the catalyst pellet before being cleaned up by the pore walls. Such poisons will be evenly distributed along the wall of a typical pore. If a fraction of the pore surface a is poisoned, the simplest assumption is that the intrinsic activity k of the pore wall decreases to fc(l — a), that is, linearly with the fraction poisoned. The activity of the whole pore, or whole pellet, is not directly proportional to A (l — a) for a fast reaction, but for first order reactions is proportional to h tanh h, as given by eq. (35). For a poisoned catalyst we find h by substitutii i (l — a) for k in eq. (33) and find ... 

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