Surface acid-base properties study

Gas-phase methylation of catechol by methanol was studied on y -AI2O3 modified by the basic elements K, Li, Mg and Ca. Addition of 7.5 at.% Mg to y-AljOa was optimal and increased the 3-methyl catechol selectivity from 0.26 to 0.65. X-ray diffraction experiments showed the diffusion of Li and Mg cations into the y -AI2O3 bulk. This induces a change in the surface species (XPS data) and the surface acid-base properties (TPD experiments). Ca and K addition to y-alumina was ineffective due to formation of basic oxide layers on the sur ce. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

Tu, M., Shen, J. Y. and Chen, Y. (1997). Microcalorimetric studies of surface acid/base properties of magnesium-iron catalysts prepared from hydrotalcite-type precursors. J. Solid State Chem. 128, 73. 

Nagy, N. M., and J. Konya. 2007. Study of pH-dependent charges of soils by surface acid-base properties. J. Coll. Interface Sci. 305 94—100. 

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

The surface acid-base properties of polycrystalline MgO surfaces have been assessed by means of thermogravimetry and DSC of desorption of pyridine and CO2 in the room temperature to 400 °C temperature range [44]. The endotherms and corresponding AH of desorption were discussed in relation with results determined previously using differential adsorption calorimetry and taking into account the structure, surface area and defects of the studied surfaces. 

The measurement of the acidity of solid acid surfaces has been the focus of a vast number of studies. The most commonly used techniques are Hammett titrations, chemisorption of bases and TPD. Extensive discussions of diese methods and their shortcomings are available in the literature [4], The use of adsorption calorimetry makes it possible to determine quantitatively the surface acidity and the acid-strength distribution of solid acids. Surface acid-base properties of catalytic solids can also be studied by base desorption using TG [71]. 

The adsorption of CO2 has been studied in order to compare the surface acid-base properties of yttria-stabilized tetragonal Zr02, either plain or sulfated to various extents [104]. The site populations and their energy distributions were studied by microcalorimetry, which evidenced the modification of the basicity of zirconia induced by the sulfation process. Upon sulfation die initial heats of CO2 adsorption decreased fi om about 120 kJ mol to less than 85 kJ mol" , depending on the amount of loaded sulfate. 

In the fiumework of realistic surface complexation models (realistic in the sense of realistically accounting for expected features), site densities are known for well-defined sorbent samples. For natural sorbent samples and powders, this is not the case. Realistic acid-base constants for such sorbent samples are difficult to obtain. Spline analysis of proton consumption functions or numerically fitting a model to such sorbents are presently the preferred approaches. However, they involve the respective assumptions, and reahstic models await improved experimental (most probably spectroscopic) methods to define the amount and nature of surface sites. Otherwise, in generic models, acid-base constants are model-dependent parameters, which have some use when apphed self-consistently, but should not be interpreted as realistic parameters. This is a very controversial statement, which is in conflict with many modeling studies in which generic models of the surface acid-base properties are combined with spectroscopic information on sorbing ions. 

Activity and selectivity of nanosized nickel ferrites have been studied for hydrogen and syngas prodnction via the CO2 reforming of methane (DRM). The catalysts were prepared by two different methods (i) co-precipitation (CP) route using nitrates salts as precursors and (ii) hydrothermal (HT) method using chlorides as starting salts. The materials were characterized by several techniques HT-XRD, TGA-DTA, XRD, BET, LRS, TPR, SEM. Surface acid-base measurements were performed by 2-propanol decomposition (IPA) and catalysts were tested in DRM reaction. A relationship is established between the method of preparation, the solid structure, the surface acid-base properties and the catalytic activity of iron-nickel solids in DRM reaction. The surface acid-base properties seem to play an important role in DRM reaction. 

Liquid-phase hydrogenation of 1,4 butynediol to cis-1,4-butenediol and 1,4-butanediol has been carried out on nickel catalysts supported on thirteen different supports. Some commercial nickel catalysts were used as references. Furthermore, metal loading and Ni-Cu alloying have also been studied. The results obtained indicates that catalytic activity, selectivity and metal surface area of catalysts are closely correlated to some textural and/or acid-base properties of the corresponding support. Similarly, the influence of Cu as a second metal in catalyst behaviour is also related to the nature of the support. 

Relatively few studies have focused on influence of the acid/base properties of the support on the chemisorption of reactants on supported metal clusters. A NMR study by Tong et al.23 showed that the stretching frequency of CO chemisorbed on zeolite supported Pt particles correlates with the surface local density of states (LDOS) of the Pt. The LDOS also showed a correlation with the faujasite framework acidity, but an explanation of this correlation is lacking. Several infrared studies on similar supported Pt catalysts show that the mode of CO... 

In order to try to clarify the different types of mechanism involving either redox cycles and/or acid-base properties, a study of the surface chemistry of single, doped and mixed oxides is of much interest. The calorimetric technique, by allowing heat transfer measurements, can provide very informative data on the thermodynamics of solid-gas interactions and for the study of the surface and reactivity of these metal oxides. 

The oxidative dehydrogenation (ODH) of lower alkanes is an attractive process for the formation of alkenes. The ODH of propane to produce propene has been particularly studied, given its high demand for the production of polypropene, acrylonitrile and propene oxide. There is a combined influence of the redox and acid-base properties of the surface of the oxides used for propane ODH. Intermediate reducibility, weak Lewis acid centers and oxygen mobility represent the essential requirements for selective ODH, as they are consistent with the trends in ODH rates observed in VO, MoO and WO based catalysts. 

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,... 

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