CO heats of adsorption

Another gas easy to measure is CO. This does not mean that better agreement has already been achieved here. One group of authors (67) did not find any systematic variation in the CO heat of adsorption with alloy composition of evaporated metal films other authors (65) found a moderate... 

Electrode Potential -V vs.SHE Faradaic efficiency/ % CO heat of adsorption kcal mol" ... 

Fig. 6 CO heats of adsorption as a function of Au cluster size on a TiO2(110) support. View this art in color at www.dekker.com.)...

Results obtained by temperature-programmed desorption suggested that in the case of bimetallic catalysts there was a reduction in the number of strong CO-adsorption sites. This finding allows conclusion that the alloying effect of these systems leads to the lowering of the CO heat of adsorption. This finding was confirmed by direct measurement of differential heat of CO chemisorption in the microcalorimetry experiment (see Fig. 4.20). 

For example, by examining the initial and differential heats of adsorption measured on Pt/Al203 powders calcined at different temperatures, Uner and Uner [50] concluded that CO adsorption processes is not structure-sensitive. CO heats of adsorption values obtained by the authors are plotted against carbon monoxide coverage in Fig. 12.4. The heat of adsorption data for all catalysts fell on the same curve. 

The heat of adsorption is an important experimental quantity. The heat evolution with each of successive admissions of adsorbate vapor may be measured directly by means of a calorimeter described by Beebe and co-workers [31]. Alternatively, the heat of immersion in liquid adsorbate of adsorbent having various amounts preadsorbed on it may be determined. The difference between any two values is related to the integral heat of adsorption (see Section X-3A) between the two degrees of coverage. See Refs. 32 and 33 for experimental papers in this area. 

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

The Langmuir equation is based on the assumption that the heat of adsorption does not vary with the coverage 6 it is interesting that in the systems just quoted, the heat of adsorption varies with the amount adsorbed in the case of CO, but is virtually constant in the case of Ar. 

From TPD data obtained at low CO coverages, it is possible to estimate the initial heats of adsorption 45,46 AH 0, of CO on alkali modified surfaces. Figure 2.15 a shows the dependence of AH°0 on alkali coverage, for CO adsorption on alkali modified Ru(1010) It is clear that up to moderate alkali coverages the... 

The activity of all catalysts were evaluated for the CO hydrogenation reaction. The histogram shown in Fig. 8 reveals that the bimetallic Co-Mo nitride system has appreciable hydrogenation activity with exception of samples 2 and 4. This apparent anomaly was probably due to the relatively high heat of adsorption for these two catalysts, which offered strong CO chemisorption but with imfavourable product release. 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

Figure 6.36. Calculated variation in the heats of adsorption of molecular CO and NO compared with the heats of adsorption of the dissociation products. Open symbols follow from the Newns— Anderson model, closed symbols from density functional theory. [Adapted from B. Hammer and J.K. N0rskov, Adv. Catal. 45 (2000) 71.]...

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

Why does CO dissociate readily on iron and not at all on platinum even though the heats of adsorption of CO on these metals are similar ... 

It can be expected that the electronic structure changes would be reflected by the heats of adsorption of suitable chosen molecules. Indeed, Shek et al (17) report that one maximum in the thermal desorption profile of CO shifts to lower temperatures when the Cu content of alloys increases. If the variations in the entropy changes upon adsorption can be neglected (probably - they can) this would indicate a lower heat of adsorption of CO on alloys than on Pt from abt. 33 Kcal/mol on pure Pt,to 26 Kcal/mol for an alloy with abt. 20% Cu. 

However, before we accept this conclusion as the definitive one, a word of caution is necessary. Due to the CO-CO interactions the heats of adsorption depend on the coverage 0. Shek et al (17) compared Pt and alloys at a constant dosage and this could mean that the coverage of Pt was (at the same dosage) lower on Pt than on alloys, and consequently - the heat of adsorption higher. Shek et al report the above mentioned data for the (111) faces of Pt and alloys. They studied also the (110) faces, but there the effect of alloying is masked by the reconstruction of the surface upon CO-adsorption (18). 

When CO is adsorbed on, say Pt, interaction of dipoles of individual molecules is repulsive, and it decreases the heat of adsorption and increases the v(Pt,CO) frequency. This effect on frequency is a resonance effect when a l CO layer is diluted by l co or C °0, etc., the interaction is much weaker (26,27). mole-... 

Although the spectral developments shown In Figure 2 have been obtained by cooling the Pd/S102 surface to 80K, we have shown that species L may also be formed at 300K at CO pressures of several hundred Torr. This behavior Is consistent with species L having a fairly low heat of adsorption. 

So, in the latter case the apparent activation energy is increased by the heat of adsorption of CO, amounting to about 40-60 kJ/mol as calculated from the IR experiments. Hence, for both the Co and the Cu samples E is slightly larger than 2 (table 2) while for iron ai is considerably lower. All these values are compatible with values reported in the literature for Fe-zeolites [6,7,10,11] or dilute solid solutions of Co in MgO [31]. The kinetic and IR results with NO indicate that, like CO, it can remove the oxygen from the... 

The heat of adsorption of 2-nitropropane is very high, so carbon-containing respirators should not be used in high vapour concentrations. Also, if Hopcalite catalyst (co-precipitated copper(II) oxide and manganese (IV) oxide) is present in the respirator cartridge, ignition may occur. 

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