Surface during ammonia synthesis

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions The changes in the apparent reaction order dependence in ammonia partial pressure suggest that to elucidate the effects of potassium on both iron single crystals and the industrial catalyst, it is necessary to understand the readsorption of gas-phase ammonia on the catalyst surface during ammonia synthesis The fact that the rate of ammonia synthesis is negative order in ammonia synthesis. Once adsorbed, the ammonia has a certain residence time (t) on the catalyst which is determined by its adsorption energy on iron [t cx tq exp (AH /RT)]... 

Another important factor in the catalytic properties of Fe and other metallic catalysts is the structure of these catalysts. It is a well-known fact that during ammonia synthesis, when it is exposed on the surface of a Fe catalyst s (111) plane, the catalyst is more active [14],... 

The direct comparison of the catalytic activity and selectivity of surfaces with different orientations provides information abont the influence of the atomic structure. This has been well described (for example, see [15]). It is well established that catalytic reactions may depend on the atomic structure of the surface (i.e., they are structure sensitive). A classic example of a catalytic reaction that is sensitive to the atomic sUucture of the catalyst s surface is ammonia synthesis on iron surfaces [15], The (111) and (211) surfaces of iron exhibit a significantly higher reaction rate than the (100), (210), and (110) faces. This structural effect has been ascribed to C7 sites (i.e., Fe atoms with a coordination number of 7, or number of nearest neighbors), which exist only on the (111) and (211) surfaces. Now, what if the structure of the catalytic surface during the reaction differs substantially from the initially pure, well-defined crystalline metal surface For example, depending on the gas pressure (i.e., the chemical potential) new structures may become stable (see Sect. 8.2.2). Or what if only a small percentage of uncontrolled or varying defects and steps completely dominate the activity In the remainder of this chapter, these questions will be addressed. 

Changes in the surface concentrations of the promoter elements are also dependent on the mode of reduction. Subsequent to wet reduction, potassium and aluminum are destributed at the surface, while their concentrations are lowered when dry hydrogen is used. Redispersion of aluminum occurred in samples of both catalysts that had been reduced in dry hydrogen during ammonia synthesis. The redispersion was complete within ca 30 hours of exposure to synthesis gas mixture and did not occur when the samples were kept at 790 K in pure hydrogen. 

It turned out " that of all the surface species which may be present during ammonia synthesis at 700 K, only atomic nitrogen (Nad) is sufficiently stable to survive evacuation. Determination of its concentration in vacuo after high-pressure operation will therefore yield information about this quantity under reaction conditions. In a series of experiments with Fe(lll) performed at 580 K and Pn2 =150 torr with varying hydrogen pressure it was found that the steady-state Nad concentration decreases continuously with increasing This result demonstrates directly that indeed hydrogenation of Nad is an essential reaction step, and not that of N2.ad as was sometimes speculated in the earlier literature. ... 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

The state of iron ammonia catalysts is dealt with in the following chapters, and x-ray, magnetic, and electric data will be discussed together with adsorption measurements. Information about the catalysts combined with kinetic experiments has led to a fairly good qualitative understanding of ammonia synthesis on iron catalysts, but owing to the extremely complicated nature of the catalyst surface during reaction, a quantitative treatment based on data of catalyst and reactants will not be attained in the near future. 

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