Hydrogen form temperature-programmed desorption

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

The temperature programmed desorption (t.p.d) of n-hexane from the sodium and hydrogen forms of ZSM-5. ZSM-11 and THETA-1 have been studied. The t.p.d profiles have been analysed by a newly developed method. From these analyses peak temperatures, peak widths, maximum rates of desorption and activation energies of desorption as a function of coverage have been obtained. The saturation capacities of these high silica zeolites for n-hexane have also been determined. The effect of change of cation on all of these quantities is demonstrated. 

A second assumption is that CO and dihydrogen absorb competitively in molecular form on the remaining surface sites. Because Sexton and Somorjai s temperature-programmed desorption studies (12) show that molecularly chemisorbed CO is desorbed rapidly from the rhodium surface only above 250°C at 10" Torr, virtual saturation of these associ-atively adsorbing sites by carbon monoxide can be assumed to occur at 300°C and 50 atm. The fact that molecular hydrogen does not compete... 

The same reaction regenerates the Pt +(NH3)4 complex after reactions (6.32) and (6.35). In the presence of oxygen, hydrogen will be oxidized to form H2O, which drives this reaction thermodynamically. In the overall reaction scheme, N2 and N2O formation compete in the absence of water through reactions (6.32) and (6.36). In the presence of water, N2 formation is promoted because of reactions (6.33) and (6.35). There is ample evidence that the reduction of the zeolitic protons by reduced metal atoms such as Pt is actually an easy reaction stepl . In the temperature-programmed desorption spectra (Fig. 6.28), N2O formation occurs predominantly in the peak in which excess oxygen is consumed. The other two peaks follow N2 formation. In the absence of water, the low-temperature peak that corresponds to the reaction sequence initiated by reaction step (6.33) is suppressed. 

In temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR), and TPRx, the sample is exposed to a reactant gas while it is being heated. For example, examination of the desorption spectroscopy of previously adsorbed surface carbon species in a flow of hydrogen is useful for understanding the amounts and types of deactivating coke that may have formed during reaction. Different surface carbon... 

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