Probing Surface Basic Properties

The number of acidic probe molecules that are able to cover wide range of strength is rather small. Moreover the difficulty in evaluating surface basic properties stems from the fact that these molecules may interact simultaneously with cations (such as Na+). The ideal probe molecule should be specific to basic sites, it should distinguish interaction with oxide ion and hydroxyls and it should not give rise to chemical reactions. 

Sulphur dioxide (pK = 1.89) is another molecule commonly used to investigate basicity of solid catalysts. SO2 adsorption on the surface of metal oxides is complex. Several types of species can be formed according to hydroxylation state and the acidity or basicity of surface [58]. Interaction of SO2 with basic 0 leads to formation 

The number of CO2 adsorption sites is always much lower than that found for SO2 suggesting that adsorption of CO2 is more specific than that of SO2. The latter, having a very strong acidic character, therefore probes almost all the basic sites regardless of their strength. 

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The Br0nsted basicity of a surface is related to its deprotonation ability, which can be probed by investigating the dissociative adsorption of protic molecules (Bailly et al., 2005a Chizallet et al., 2006). The 0Lc2 0Lc H transformation thus induced can be followed by PL, which is one of the few techniques able to simultaneously characterize oxide ions and their protonated forms. The same kind of equilibrium is also involved when a hydroxylated surface is undergoing thermal pretreatment (Section 2.1). PL is thus an interesting tool to evaluate the surface basic properties of alkaline earth oxides. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

Beside its use for the direct characterization of solids, PL has been applied in combination with probe molecules, luminescent or not, to investigate the acidic and basic properties of oxide surfaces (Section 3). In relation to catalysis, the investigation of the formation of hydroxyl groups from hydrogen-containing reactants (such as hydrocarbons or alcohols) or reaction products (notably water) is an important step forward. Various types of hydroxyl group can be formed upon adsorption of such molecules. In the case of water adsorption on MgO, this can be illustrated schematically by the following reaction ... 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Earlier Paukshtis and Davydov [7, 9] proposed this probe-molecule as a test for basic properties of oxide catalysts, but it has been noted that this molecule may be converted upon adsorption on strong surface sites (probably, Lewis acid sites) to other species or polymeric moieties. 

For these reasons, the effect of support on Ni based catalysts is better shown when comparing the MEA selectivity at low acetonitrile conversions (Table 2). The improvement of primary amine selectivity upon Mg addition could arise from a modification of the acido-basic properties of the support surface. To check any differences in these properties, the acid sites were probed by TPD of NH3 and adsorption of MEA followed by calorimetry. 

A selection of typical probes used to assess surface characteristics, along with their relative acidic and basic properties (indicated by AN and DN parameters respectively) and boiling points, is presented in Table 1 compiled from pnblished data. ... 

In this case a homologous series of alcohols and amines were employed as the acidic and the basic probes, respectively. The interaction parameter, O, reflects the acidic/ basic properties of the sohd surface and is defined as follows for acidic surfaces, the specific retention volume,... 

Basic properties were also measured by TPD of probe molecules such as CO2, CO and H2. Although CO2 appears to be the proper probe molecule because of its acidic nature, TPD profile of CO2 varies depending on the adsorption condition of COi Only a broad desorption peak appeared if too much CO2 was adsorbed. The alkaline earth oxide surfaces react with CO2 to form different surface structures depending on the adsorption time and temperature. TPD profiles of adsorbed CO2 on MgO, CaO, SrO, and BaO measured under controlled adsorption conditions are shown in Fig.2.9. >... 

Ammonia is among the smallest strongly basic molecules and its diffusion is little affected by the porous structure, if at all. Because of this, it is the most commonly used probe molecule for testing surface acid properties. It is adsorbed as an ammonium ion, and the corresponding heat of adsorption depends both on the proton mobility and on the affinity of ammonia for the proton. 

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