Chemisorption palladium supported catalysts

Second, we shall present our approach as to how to probe the palladium surfaces in more complex Pd/support catalysts, especially when the so-called metal-support interactions are expected. We shall develop our idea of how to use such chemical probes as a catalytic reaction (alkane catalytic conversion) or chemisorption in order to see important changes in the catalytic behavior. When possible, an adequate reference to available data from more sophisticated physical techniques is made. 

Mahata, N.. and Vishwanathan, V., Dependency of phenol hydrogenation activity on hydrogen chemisorption over supported palladium catalysts, Adsorpt. Sci. Technol.. 15(3), 165-172 (1997). 

Aben PC. Palladium areas in supported catalysts determination of palladium surface areas in supported catalysts by means of hydrogen chemisorption. J Catal. 1968 10 224. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

This argument is confirmed by the study of CO pulse chemisorption by Biffis at al., mentioned above. In this piece of investigation, the authors prepared a 2% (w/w) palladium catalyst supported by Lewatit UCP 118, a macroreticular resin (nominal cld = 18 %) from Bayer. Its TEM characterization showed a remarkably heterogeneous distribution of the metal nanoclusters, which are apparently located close to the surface of the polymer nodules [62] (Figure 9). 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

L. F. Liotta, G. A. Martin, and G. Deganello, The influence of alkali metal ions in the chemisorption of CO and C02 on supported palladium catalysts, a Fourier transform infrared spectroscopic study, J. Catal. 164, 322-333 (1996). 

Narayanan, S. and Krishna, K. (1998). Hydrotalcite-supported palladium catalysts Part I preparation, characterization of hydrotalcites and palladium on uncalcined hydrotalcites for CO chemisorption and phenol hydrogenation. Appl. Catal. A 174, 221. 

Supported palladium catalysts for syngas conversion were characterized by CO chemisorption, ammonia TPD and SEM. Compositional characterization using XRD, MAS NMR and argon adsorption were done at VPI. 

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. 

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 performance in this reaction (Eq. (2.5)) of heterogeneous Pd colloid catalysts on CaCOs modified by a number of surfactants was compared with conventional Pd/C and Lindlar catalysts. The selectivity was found to depend on the support and various promoters. The highest activity and the best selectivity (98.1%) towards the desired cis-3-hexen-l-ol was found when employing a lead-acetate-promoted palladium colloid on CaCOs modified by the zwitterionic surfactant SB-12 (N,N-dimethyl-dodecylammoniopropanesulfonate). According to chemisorption results. 

Oxidized carbon supports. As follows from HREM and CO chemisorption study of palladium catalysts supported on modified Sibunit carbons, the dimension of the Pd particles depends only slightly, if at all, on the content of the surface oxides, the blocking effect of the modified supports being retained the same as the one of the initial carbon. This may be explained in view of the fact that, under the reduction conditions, palladium can catalytically decompose the surface oxides [15] so that the chemical state of some part of the carbon surface around the Pd particles for a modified support and for the initial one becomes similar, which gives rise to similar conditions of sintering as the former Pd(0) clusters on these supports. 

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