Catalyst physicochemical methods

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

The TA adsorbed on the nickel catalyst (DNi) prepared from nickel formate had been studied by chemical and physicochemical methods by Yasumori (64), and by electrochemical methods by Fish and Ollis (70), respectively. The number of nickel atoms occupied by TA on the surface of the catalyst was estimated to be 30% by both authors. 

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

PtRu catalysts with controlled atomic ratios were prepared by adjusting the nominal concentrations of platinum and ruthenium salts in the solution, whereas different mean particle sizes could be obtained by adjusting some electric parameters of the deposition process, e.g., ton (during which the current pulse is applied) and toff (when no current is applied to the electrode), as determined by different physicochemical methods (XRD, EDX, and TEM) [40], Characterization by XRD led to determine the crystallite size, the atomic composition and the alloy character of the PtRu catalysts. The atomic composition was confirmed using EDX, and TEM pictures led to evaluate the particle size and to show that PtRu particles formed small aggregates of several tens of nanometers (Figure 9.10). 

Available vast information in the scientific literature on synthesis and mechanism of heterogeneous metal-complex catalysts (HMC) action points to combination of advantages in both homogeneous and heterogeneous catalysts. The catalytic activity of HMC has been investigated in a series of processes, including oligomerization of lower olefins [1]. However, in connection with limitations of physicochemical methods of research of HMC, the data on formation of catalytic active centers on the surface of the support and on mechanism of their action are not numerous and are contradictory. 

The aim of this chapter is to show that the choice of a catalyst formulation leading to increase the activity and the selectivity of a given electrochemical reaction involved in a fuel cell can only be achieved when the mechanism of the electrocatalytic reaction is sufficiently understood. The elucidation of the mechanism caimot be obtained by using only electrochemical techniques (e.g. cyclic voltammetry, chronopotentiometry, chrono-amperometiy, coulo-metry, etc.), and usually needs a combination of such techniques with spectroscopic and analytical techniques. A detailed study of the reaction mechanism has thus to be carried out with spectroscopic and analytical techniques under electrochemical control. In short, the combination of electrochemical methods with other physicochemical methods cannot be disputed to determine some key reaction steps. For this purpose, it is then necessary to be able to identify the nature of adsorbed intermediates, the stractuie of adsorbed layers, the natirre of the reaction products and byproducts, etc., and to determine the amormt of these species, as a fimction of the electrode potential and experimental conditions. 

The original material (subsequently called URPAC) is accessible in various morphologies with a large number of Ziegler-Natta type and other transition metal catalysts. It has been studied in detail with spectroscopic and other physicochemical methods. Quantum theoretical model calculations have provided insights into the energetics of conjugated double bond systems. 

MTT zeolite crystals with siliceous layer on their external surface were synthesized using a two step hydrothermal procedure in which cores prepared from an aluminosilicate gel were transferred into a siliceous gel for farther crystal growth. The thickness of the siliceous shell was varied by varying the proportion of the two gels. The samples were characterized using physicochemical methods (XRD, XPS, Al MAS NMR, SEM, Na adsorption). The zeolites were converted into bifunctional catalysts and evaluated in hydroisomerization of decane. 

In addition to these purely experimental approaches, strategies have also been put forward where possible candidate catalysts have been selected based on theoretical considerations, for example, based on thermodynamic guidehnes [43] or even on DFT calculations [17,104). Nevertheless, progress in fuel cell catalysis requires the understanding of structure-activity relationships, which is usually attempted by a detailed characterization of the prepared materials with state-of-the-art physicochemical methods, including transmission electron microscopy (TEM), X-ray... 

One of the most attractive features of unsupported metal nanoparticles is the possibility to apply physicochemical methods to investigate them and to catalyze chemical reactions by exactly the same material. Thus, xmsupvported metal nanopartides are frequently used as a model system for conventional heterogeneous catalysts as many physicochemical methods cannot be applied directly to conventional catalysts due to their heterogeneous nature, interfering support effects, and low transparency for electromagnetic radiation. In other words, nanopartides can serve as models for ideal surfaces and are good candidates to fill the so-called pressure material gap. [24,25]... 

Recent advances in the preparation of ceria-based gold catalysts for hydrogen production by the WGS and PROX reactions are reviewed in this chapter. Considerable emphasis is placed on the catalyst characterization by a number of physicochemical methods X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), temperature programmed reduction (TPR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. The relation between the structure, properties, and catalytic activity, as well as the nature of the active sites is also discussed. 

A feasibility study on the application of H-NMR petroleum product characterization to predict physicochemical properties of feeds and catalyst-feed interactions has been performed. The technique satisfactorily estimates many feed properties as well as catalyst-feed interactions to forecast products yield. There are, however, limitations that have to be understood when using the H-NMR method. The technique, in general, is not capable either to estimate the level of certain contaminants such as nitrogen, sulfur, nickel, and vanadium when evaluating feed properties or the effect of these contaminants on products yields while testing catalyst-feed interactions. 

Titanium containing pure-silica ZSM-5 (TS-1) materials are synthesized using different methods. The activity of the titanium containing catalysts for the oxidation of alkanes, alkenes and phenol at temperatures below 100 °C using aqueous H2O2 as oxidant is reported. The relationships between the physicochemical and catalytic properties of these titanium silicates are discussed. The effects of added duminum and sodium on the catalytic activity of TS-1 are described. The addition of sodium during the synthesis of TS-1 is detrimental to the catalytic activity while sodium incorporation into preformed TS-1 is not. The framework substitution of aluminum for silicon appears to decrease the amount of framework titanium. 

Several preparation methods have been reported for the synthesis of TS-1. In this work, we have investigated the physicochemical properties of TS-1 samples synthesized by different preparation metiiods and tested these materials as catalysts for the oxidation of n-octane, 1-hexene and phenol using aqueous hydrogen peroxide (30 wt%) as oxidant at temperatures below 100 C. For comparison, Ti02 (anatase) and the octahedral titanium-containing silicate molecular sieve (ETS-10) (5) have been studied. The effect of the presence of aluminum and/or sodium on the catalytic activity of TS-1 is also discussed. 

Summarizing, there are still many scientific challenges and major opportunities for the catalysis community in the field of cobalt-based Fischer-Tropsch synthesis to design improved or totally new catalyst systems. However, such improvements require a profound knowledge of the promoted catalyst material. In this respect, detailed physicochemical insights in the cobalt-support, cobalt-promoter and support-support interfacial chemistry are of paramount importance. Advanced synthesis methods and characterization tools giving structural and electronic information of both the cobalt and the support element under reaction conditions should be developed to achieve this goal. 

Therefore, heterogeneous catalysts present a greater potential for the application of HT and Combinatorial methods, because they involve diverse compositional phases that are usually formed by interfacial reactions during their synthesis, which in turn produce a variety of structural and textural properties, often too vast to prepare and test by traditional methods. In this respect the HT and Combinatorial methods extend the capabilities of the R D cycle, which comprises the synthesis, the characterization of physicochemical properties and the evaluation of catalytic properties. The primary screening HT method gives the possibility of performing a rapid test of hundreds or thousands of compounds using infrared detection methods [27-29]. Alternatively, a detection method called REMPI (Resonance Enhanced Multi Photon Ionization) has been used, which consists of the in situ ionization of reaction products by UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam [30, 31]. 

More details on the contribution of the preparation methods and chemical composition to the physicochemical properties, activity and selectivity of Ni,H-Z M-5 containing catalysts (similar to those used in the dewaxing process) can be found in Ref (15-17]. The present study concentrates on the contribution of acidity and metal content on the properties of cokes deposited on the investigated catalysts. 

Perfecting an industrial catalyst is thus the culmination of a long and complicated process that requires a knowledge as broad as possible of the methods relative to the preparation of catalysts, to the study of catalytic and mechanical properties, and to the determination of the physicochemical characteristics... 

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