Hydrogen on Supported Metals

Electrochemical methods have also been applied to characterising adsorbed hydrogen on supported metals. 

Kovtunov KV, Beck IE, Bukhtiyarov VI, Koptyug IV (2008) Observation of parahydrogen-induced polarization in heterogeneous hydrogenation on supported metal catalysts. Angew Chem 120 1514-1517... 

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

It is well established that when ethylene is admitted to a freshly prepared evaporated metal film, self-hydrogenation resulting in the rapid production of ethane is observed [50—52]. A similar phenomenon is observed when ethylene is adsorbed on supported metal catalysts [49,53] (see Fig. 5). These observations have been interpreted as indicating that ethylene is first chemisorbed dissociatively, viz. 

Kitajima, N., Fukuzumi, S., and Ono, Y., Formation of superoxide ion during the decomposition of hydrogen peroxide on supported metal oxides, /. Phys. Chem., 82, 1505, 1978. 

The key to the success of the oxidation examples cited above is the ability of the catalysts used to exert proper kinetic control on the possible side reactions. Without it, thermodynamically favorable but undesired products such as CO2 and H2O are made instead. Controlling oxidation kinetics to stop at the desired oxygenated products is quite difficult, and has yet to be solved for many other systems. For instance, although many attempts have been made to develop a commercial process for the oxidation of propylene to propylene oxide, both the activity and the selectivity of the systems proposed to date, mostly based on silver catalysts, are still too low to be of industrial interest " propylene oxide is presently manufactured by processes based on chlorohydrin or hydrogen peroxide instead. In spite of these difficulties, though, recent advances in selective liquid phase oxidation of fine chemicals on supported metal catalysts have shown some promise, offering high yields (close to 100%) under mild reaction conditions." ... 

Fig. 11. NMR spectra of several supported metal catalysts under 30 Torr of H2 gas. The peak around 0 ppm is due to hydroxyl groups on the surface of the carrier oxide the other peak is hydrogen on the metal surface. The shift of the latter can have cither sign (but H/Cu is the only known case of positive shift for chemisorbed H). [Reproduced with permission from Pruski (5). Copyright 1996 Wiley.]...

In oxide-supported catalysts, the NMR signal from the hydrogen on the metal is distinct from that due to hydroxyl groups on the oxide surface, although often the wings of the two signals have a strong overlap (Fig. 11)... 

When in situ dosing onto two different samples of Pt/silica (45, 48) and onto Cu/MgO was used (49), no evidence for spillover was found from NMR. Only one detailed study based on fully relaxed spectra led to observation of a non zero spillover (4T). In a Ru/Si02 catalyst, the silanol protons w ere exchanged for deuterons, the sam.ple was evacuated at 623 K, and a reference NMR spectrum w as taken at room, temperature. The sample was then exposed to 20 Torr of H2, an NMR spectrum was taken, and the difference with respect to the reference was calculated (line in Fig. 14). This represents the sum of reversible and irreversible hydrogen on the metal (resonating at -65 ppm) and spilled over on the support (at about 3 ppm). Then the sample was pumped out at room temperature for 10 min, and again a difference spectrum with the reference state was obtained (dashed line in Fig. 14) this represents irreversible hydrogen both on the support and on the metal. Similar in situ NMR techniques were used... 

Nuclear magnetic resonance (NMR) spectroscopy continues to gain importance in catalyst characterization, and van der KJink contributes the first chapter on metal NMR to the Advances the focus is on supported platinum catalysts, including those with adsorbed carbon monoxide or hydrogen. The method provides evidence of metal dispersions and distinguishes between hydrogen on the metal and hydrogen on the support. 

Hayek K, Fuchs M, Klotzer B, Reichl W, Rupprechter G (2000) Studies of metal-support interactions with real and inverted model systems Reactions of CO and small hydrocarbons with hydrogen on noble metals in contact with oxides. Topics Catal 13 55... 

Selective chemisorptions of hydrogen and carbon monoxide have also been used to determine the surface area of other Group VIII metals, especially by Yates et al. (6). Development of methods applicable to other metals is only a matter of ingenuity and from now on every investigation of catalysis on supported metals must include a determination of the surface area of the metal. 

Selective catalytic hydrogenation of 2,4-dinitrotoluene to nitroarylhydroxylamines on supported metal catalysts... 

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