Characterization hydrogen chemisorption

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

The effects of tin/palladium ratio, temperatnre, pressnre, and recycling were studied and correlated with catalyst characterization. The catalysts were characterized by chemisorption titrations, in situ X-Ray Diffraction (XRD), and Electron Spectroscopy for Chemical Analysis (ESCA). Chemisorption studies with hydrogen sulfide show lack of adsorption at higher Sn/Pd ratios. Carbon monoxide chemisorption indicates an increase in adsorption with increasing palladium concentration. One form of palladium is transformed to a new phase at 140°C by measurement of in situ variable temperature XRD. ESCA studies of the catalysts show that the presence of tin concentration increases the surface palladium concentration. ESCA data also indicates that recycled catalysts show no palladium sulfide formation at the surface but palladium cyanide is present. 

Highly mesoporous carbon supported Pd catalysts were prepared using sodium formate and hydrogen for the reduction of the catalyst precursors. These catalysts were tested in the enantioselective hydrogenation of isophorone and of 2-benzylidene-l-benzosuberone. The support and the catalysts were characterized by different methods such as nitrogen adsorption, hydrogen chemisorption, SEM, XPS and TPD. 

Hydrogen chemisorption has frequently been used for catalyst characterization. Considerable detailed information is available for hydrogen... 

In any of the schemes presented, hydrogen dissociative adsorption on Pt is possible after Sn addition, as was checked by hydrogen chemisorption. From these results, it is possible to think of a scheme to represent the main reaction pathway during the hydrogenation of a,(3-unsaturated aldehydes. Depending on the catalyst used, such a scheme is shown in Figure 6.12, which summarizes the results from the characterizations and catalytic tests performed in this work [47]. 

The prepared catalysts and the chemical compositions measured by atomic absorption, are listed in Table 1. Complementary characterization experiments such as hydrogen chemisorption in a pulse apparatus and temperature-programmed reduction (TPR) were performed using experimental systems and methods described in detail elsewhere [10]. 

Hydrogen chemisorption is not yet a routine characterization method for supported metal clusters because stoichiometries of chemisorption on the clusters are not well known and are different from those of chemisorption on metal crystallites [13]. Chemisorption of CO is also of limited value because CO typically reacts with supported clusters, leading to changes in cluster structure. Research is needed to clarify these matters. 

J. de Graaf, A.J. van Dillen, K.P. de Jong, and D.C. Koningsberger, Preparation of Highly Dispersed Pt Particles in Zeolite Y with a Narrow Particle Size Distribution Characterization by Hydrogen Chemisorption, TEM, EXAFS Spectroscopy, and Particle Modeling. J. Catal., 2001, 203, 307-321. 

When plotted versus crystallite size, independently of the support used, a similar crystallite size effect appears to exist for all three adsorbates— heats of adsorption increase somewhat as crystallite size decreases from 6 nm to below 2 nm. All Pd crystallite sizes were based on hydrogen chemisorption and the assumption of 1. 2 x 10 9 Pdam2-1 (f)). At this time we believe there is a possibility that the values reported for the most poorly dispersed samples—particle sizes of 10 nm and higher—may actually represent a bimodal crystallite distribution because of their preparation by sintering in H2. Consequently, the observed AH(aq) values would be higher than expected for large supported Pd crystallites. Characterization by TEM is now underway to better characterize these catalysts. Further study is necessary to determine the reasons for this apparent crystallite size effect on heats of adsorption, especially since the trend is opposite to that found in supported Pt catalysts (16.1J). 

Treatment and Characterization of Catalysts. A dynamic pulse adsorption apparatus (16) was used to carry out in situ pretreatments, reductions, degassing and hydrogen chemisorption measurements. 

The carbon fiber support and the catalysts before and after reduction were characterized with various techniques, viz. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), temperature programmed reduction (TPR), Nz-physisorption, inductively coupled plasma emission spectrometry (Vista AZ CCD simultaneous ICP-AES) and hydrogen chemisorption. 

Ruthenium-copper aggregates of the type described have been studied with chemical and physical probes. Chemical probes that have been very informative include hydrogen chemisorption and the hydrogenolysis of ethane to methane. Physical probes useful in these characterizations include X-ray diffraction and electron spectroscopy. 

As competent textbooks [5] and review papers [6] exist on the characterization of metal sites, only a brief overview will be given here. Hydrogen chemisorption is often used to obtain a rough estimate of the metal dispersion. It will, however, be shown below that this method is unreliable when small clusters of a transition metal coexist with strong Bronsted... 

Pt concentration in the various samples, determined by XR-fluorescence method, ranged from 0.5 to 2.5%, depending on the adsorption capacity of the carbon fibers. The samples were characterized by hydrogen chemisorption and XRD. 

Characterization of Alloy Type Sn-Pt/Si02 Catalysts. The alloy type Sn-Pt/Si02 catalysts were characterized by chemisorption and Mossbauer spectroscopy. Results of CO and hydrogen chemisorption are summarized in TaWe 2. From data presented in Table 2 the following conclusions can be drawn ... 

The catalysts were characterized by hydrogen chemisorption, transmission electron microscopy (TEM), temperature programmed reduction (TPR) and X Ray diffraction (XRD)... 

All the samples were also characterized by hydrogen chemisorption, performed following a particular procedure to determine the metal dispersion (dispersion index, ID%) [9, 10]. 

Table 1. Characterization of samples. He amount of hydrogen chemisorptions (ji mole/gcataX Dhc metal dispersion (%).

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