Hydrogen chemisorption, characterisation

Table 4 Chemisorption characterisation of fresh Pt and Pt-Bi catalysts supported on carbon and graphite. H2 chemisorption data calculated from the charge in the hydrogen under-potential deposition (H-UPD) region of the cyclic voltammetry profiles. Values given as m2/g (total catalyst).

The preparation of a successful supported bimetallic catalyst is quite a difficult proposition. The main problem is to ensure that the two components reside in the same particle in the finished catalyst, and to know that it is so. The main physical techniques to characterise bimetallic particles are hydrogen chemisorption, XRD, TEM, EDX, XPS, XAFS,197Au Mossbauer (Section 3.3) and CO chemisorption coupled by IR spectroscopy (Section 5.3). The characterisation of bimetallic catalysts is not always thoroughly done, and there is the further complication of structural changes (particularly of the surface) during use. In situ or post-operative characterisation would reveal them, but it is rarely done. 

Section 4.3.2 will be devoted to the chemical characterisation studies. Because of the relationship existing between the support reduction degree and the occurrence of the deactivation phenomena mentioned above, we shall review first some of the major problems to be faced in relation to the redox characterisation of ceria and related oxide supports, sub-section 4.3.2.1. Then, we shall discuss the chemisorptive properties of these catalysts. In particular, section 4.3.2.2, will be devoted to the adsorption of H2 and CO, by far the two most commonly used probe molecules. Special attention will be paid to the relationship existing between chemisorptive behaviour and reduction temperature. We shall also report on some recent hydrogen chemisorption studies, in accordance with which, the sensitivity to the deactivation phenomena may vary from one noble metal to the other (97,117,235), being also influenced by the presence of chlorine in the support (163). 

At 300K, and in the absence of hydrogen, the characterisation of butene chemisorbed phase is made impossible since it undergoes transformations such as dehydrogenation on Pd(lll) and probably formation of butylidyne on Pt(lll). By extrapolation of the data to lower temperature, i.e under associative chemisorption conditions, we can however speculate that butene, with a single C=C bond, is n adsorbed on Pd(lll) and di-CT adsorbed on Pt(lll). These results are corroborated by theoretical calculations [32] whose mains results are reported in Table 2. 

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

The textural and the structural characterisation performed on the Pd/AbOs catalyst prepared by sol-gel method, shows a BET surface area of 270 m /g, a mesoporous texture and a uniform porous distribution with an average pore diameter of 3.3 nm. The metallic dispersion obtained by hydrogen chemisorption is 45 %. This later result is confirmed by the palladium particles diameter varying between 1 and 10 nm with an average of 3 nm obtained form the MET analysis. The palladium content determined by inductively coupled plasma is closely to 1.9 %. No significant BET surface area decrease nor a metallic dispersion loss were observed when the catalyst is aged under catalytic conditions up to the steady state. Since the thermal stability of the catalyst is needed to minimize the modification of the palladium particles structure, the later result justifies the choice of the sol-gel synthesis method and the calcinations temperature (700°C) selection. 

The ethylidyne species, CH3C, is formed by the dissociative chemisorption of ethene on metals. Although present on a metal during catalytic hydrogenation of ethene, ethylidyne is not an intermediate it is a spectator [78]. Chemisorbed ethylidyne has been characterised by vibrational spectroscopy and low energy electron diffraction by comparison with the model compound... 

Onto the supports HTl, HT2, HT3, HT4 and HT5 acidified solution containing PdCb was impregnated so as to get lwt% of Pd on the support. Different palladium precursors were used to load Iwt% of palladium on HTl support. Characterisation and catalytic activity The above supports and supported metal catalysts were characterised by XRD, sur ce area and CO chemisorption. The catalytic activity was studied for phenol hydrogenation at 4S3 K in a vertical down flow reactor. The reaction mixture containing phenol and cyclohexane (1 4 wtAvt) was added fi om the top of the reactor at a controlled rate with the help of a motorised syringe. H/Phenol (mol/mol) was maintained at 4. The experimental setup and conditions are given elsewhere (12). 

The catalysts were characterised by BET surface area and CO-chemisorption (carried out at 25°C), An all-glass high vacuum unit capable of producing a vaccum of lO" torr was used for this. X-ray diffraction patterns were recorded on a Phillips PW-101 difractometer using CuKa radiation. Prior to CO chemisorption experiment the catalysts were reduced in hydrogen flow at 400°C for 4h, Irreversible CO uptake was obtained by the double isotherm method. Pd- dispersion was calculated considering a 1 1 stoichiometry between CO and Pd... 

Me]x-MCM-41 containing nanosized particles of platinum, palladium, rhodium, ruthenium and iridium were directly synthesised from surfactant stabilised spherical metal nanoparticles in the synthesis gel, and characterised with XRD, ICP-AES, TG/DSC, TEM, nitrogen physisorption and carbonmonoxide chemisorption, and Si MAS NMR. During the synthesis some agglomeration of the particles took place, but the metal particles were present inside the pore system of MCM-41. The matericils were active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbomene. 

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