Microcalorimetry of CO adsorption

Karmazyn AD, Fiorin V, JenMns SJ, King DA (2003) First-principles theory and microcalorimetry of CO adsorption on the 211 surfaces of Pt and Ni. Surf Sci 538 171... 

The surface acidity and basicity of iron oxide (a-Fe203) catalysts, pure and surface-doped with variable amoimts of sulfate groups, have been characterized by microcalorimetry of CO adsorption at room temperature. An appreciable decrease of Q with surface coverage was detected [108]. The surface basicity of iron oxide, tested by adsorption of CO2 and monitored by FTIR, which revealed the formation of carbonate-like surface species, is gradually decreased by sulfates, but not suppressed. The vacuum reducibility of iron oxide, which can be spectroscopically evidenced both by a colour change (UV-VIS spectra) and by the formation upon CO adsorption of surface carbonyl-like species with a... 

Guerrero-Ruiz et al. [139] have studied the catalytic behaviour of ruthenium for the conversion of -hexane when supported on non-reducible carriers such as activated carbon and high-surface-area graphite. The samples were also characterized by microcalorimetry of CO adsorption. The higher initial heat of CO adsorption observed for ruthenium/graphite (135 kJ mol ) compared to ruthenium/activated carbon (115 kJ mol ) indicates an enhanced electron density of the ruthenium particles caused by electron transfer from the graphite. The catalytic results show that ruthenium particles with an increased electron density have a higher activity for -hexane conversion. 

Abstract To date, microcalorimetry of CO adsorption onto supported metal catalysts was mainly used to study the effects induced by the nature and the particle size of supported metallic clusters, the conditions of pretreatment and the support materials on the surface properties of the supported metallic particles. The present chapter focuses on the employ of adsorption microcalorimetry for studying the interaction of carbon monoxide with platinum-based catalyst aimed to be used in proton exchange membrane fuel cells (PEMFCs) applications. 

However, although the determination of the heat of adsorption of carbon monoxide on platinum have been reported in the literature (Table 12.2), the possible use of adsorption microcalorimetry as an available technique for giving valuable information in PEM fuel cells smdies has not been reported. In what follows, recent results from our work in the field of microcalorimetry of CO adsorption on Pt/C catalysts are presented [65-67]. The catalysts used in these studies were different commercial carbon-supported platinum, with high Pt loading, aimed to be used in PEMFCs applications. Particular emphasis on the sample history (preparation method, the support material, the metal loading) and the pre-treatment of the catalysts on the CO poisoning effect on supported platinum is paid. 

A. Maroto-Valiente, I. Rodriguez-Ramos, A. Guerrero-Ruiz, Determination of the surface states of metallic clusters supported on alumina using microcalorimetry of CO adsorption. Thermochim. Acta 379, 195-199 (2001)... 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

The adsorption of CO on a series of platinum based catalysts was also carried out by microcalorimetry technique, supplying information about the number, the strength distribution and the heat associated to the carbon monoxide adsorption on available Pt surface sites. Table 12.2 lists some literature reports on the average heats of CO adsorption over different platinum supported catalysts. In this table, the experimental data on powdered catalysts were collected from the vicinity of room temperature up to 130 C. Prior to CO adsorption, the samples were reduced at 200 or 500 C under hydrogen flow. 

In these literature reports [43-58], the data obtained by adsorption microcalorimetry are considered together with those obtained from complementary techniques (i.e. infrared spectroscopy, temperature programmed desorption, X-ray photoelectron spectroscopy) in order to elucidate the influence of loading and dispersion of Pt, type of support material and the reduction temperature, on energetics and mechanism of CO adsorption on supported Pt catalysts for a better understanding of their catalytic performances. 

The results of CO adsorption microcalorimetry described in this overview were collected with a differential and isothermal microcalorimeter (Tian-Calvet Microcalorimeter) linked to a static volumetric system. The equipment permits the introduction of successive small doses of CO onto the catalyst. Both the calorimetric and the volumetric data were stored and analyzed by microcomputer processing. The obtained data are presented as differential heats versus the amount of CO adsorbed... 

The effect of adsorption temperature on metals or supported metals on the mobility of adsorbed probe molecules has not received as much attention as on metal oxides. Gelin and co-workers (96) used adsorption microcalorimetry at 296 and 423 K and IR spectroscopy to study the adsorption of CO on Ir supported on NaY zeolite reduced from 383 to 923 K and on Ir supported on silica. At 296 K it was observed that for intermediate coverages (6 > 0.3) the kinetics of adsorption changed, with the thermograms displaying long tails... 

Fubini et al. (123) studied the adsorption of CO on reduced and oxidized Cu/ZnO catalysts. The differential heat for the reduced catalyst decreased rapidly with coverage from 120 to 40 kj mol and remained constant at that value. The site energy distribution on the oxidized sample showed a maximum concentration at about 90 kJ moC and no sites with diflerential heats lower than 65 kJ mol. The authors proposed that adsorption microcalorimetry is a good probe for the detection of different adsorption sites, namely, Cu° and Cu. ... 

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

We again point out that there are a number of experimental values of Eads for CO atop buUc Pt(l 11) suii ce. Early studies measured the Eads to be 1.43 eV [42], 1.40 [43] and 1.55 eV [45]. King et al. [46], by single crystal adsorption microcalorimetry, have recently measured a value of 1.90 0.07 eV for the low-coverage adsorption of CO on Pt(l 10) surface. Although this value refers to a different metal bulk surfece, adsorption energies of many small molecules on Pt do not vary much with facial structure. The calculated Eads (= 1-92 eV) of CO on Pt o cluster is closer to the most recent experimental value [46]. 

P-15 - A combination of high resolution manometry, gravimetry and microcalorimetry to study the co-adsorption of Ar/Nj mixtures on 5A and 13X zeolites... 

A series of binary (Cu/ZnO and C11/AI2O3) and ternary (Cu/ZnO/A Oa) copper catalysts employed in methanol synthesis has been investigated by d Alnoncourt et al. [142]. The results obtained using CO adsorption microcalorimetry showed that Cu/ZnO/AlaOa had the lowest initial heat of adsorption of 68 kJ mof and was the most active catalyst for methanol synthesis. Cu/ZnO showed a heat of adsorption of 71 kJ mol" and a lower activity, and CU/AI2O3 had the highest initial heat of adsorption of 81 kJ mof and the lowest activity. The decrease of about 10 kJ mol in the heat of adsorption of CO induced by the presence of ZnO has been attributed to strong metal-support interactions. 

Ashton and co-workers have studied MPBD adsorption onto magnesium hydroxide using Fourier Transform Infrared (FTIR) spectroscopy, flow microcalorimetry and classical adsorption techniques [43]. They confirmed the formation of a carboxylate salt at the filler surface and found the chemisorbed layer to be only about 15 A thick. This implies loopy adsorption and multiple point attachment. 

A calorimetric and IR study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [192]. The active sites for both N2O and CO are Cu(I) ions, which are present as a result of the pre-treatment in vacuum at 673 K. The measured amounts of chemisorbed species in the investigated systems and the values of differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mol ) and carbon monoxide (between 140 and 40 kJ mor ) demonstrate the dependence of the adsorption properties on the copper content. The samples were additionally characterized by ammonia adsorption microcalorimetry at 423 K [192]. 

A co-incorporation of aliuninum and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [233] in boron-substituted ZSM-5 and ZSM-11 zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of about 65kJmol for H-B-ZSM-11 and showed that B-substituted pentasils have only very weak acidity [234]. Calcination at 1073 K increased the heat of NH3 adsorption to about 170kjmol by creation of strong Lewis acid sites. The lack of strong BrOnsted acid sites in H-BZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

E. Garrone, G. Ghiotti, E. Giamello, B. Fubini, Entropy of adsorption by microcalorimetry. Part 1—Quasi-ideal chemisorption of CO onto various oxidic systems. 1. Chem. Soc., Faraday Trans. 1 Phys. Chem. Condensed Phases 77(11), 2613-2620 (1981)... 

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). 

The CO adsorption microcalorimetry was also used to explain the promoting effect of Pt in bimetallic Ni-Pt catalysts supported on alumina nano-flbre (Alnf) tested for the liquid phase reforming of sorbitol to produce hydrogen [51]. The differential heat of adsorption for Ni-Pt/Alnf reduced to around 111 kJ/mol, which was 12 and... 

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