Iron catalysts single crystal studies

New results of styrene formation over iron oxide single-crystal model catalysts were reported.326 In ultra-high-vacuum experiments with Fe304(lll) and a-Fe203(0001) films combined with batch reaction studies only Fe203 showed catalytic activity. The activity increased with increasing surface defect... 

Prom recent single crystal studies, DFT calculations, and studies of supported catalysts, it was found that ammonia synthesis reaction over ruthenium is an even more structure-sensitive reaction than over iron-based catalysts. In order to... 

A target of the work described below is the preparation of technical catalyst surfaces under in situ conditions. The state of nitrogen chemisorbed onto these surfaces will subsequently be characterized by X-ray photoelectron spectroscopy for comparison with the iron single-crystal studies. 

In the case of Haber-type catalysts the potassium is present in the unreduced catalyst, predominantly in a compound form associated with both the iron oxide and alumina phases, while in supported platinum group metal catalysts the alkali may be added in salt form (nitrate, carbonate, hydroxide, etc.) or as the metal. In single-crystal studies the alkali may be fired at the crystal surface from molecular sieve ion sources or evaporated onto the surface as the metals. 

B. Fastrup, Microkinetic analysis of ammonia synthesis based on siarface reaction studies of iron catalysts as compared to single-crystal studies, J. Catal. 168 (1997) 235. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

In this exercise we shall estimate the influence of transport limitations when testing an ammonia catalyst such as that described in Exercise 5.1 by estimating the effectiveness factor e. We are aware that the radius of the catalyst particles is essential so the fused and reduced catalyst is crushed into small particles. A fraction with a narrow distribution of = 0.2 mm is used for the experiment. We shall assume that the particles are ideally spherical. The effective diffusion constant is not easily accessible but we assume that it is approximately a factor of 100 lower than the free diffusion, which is in the proximity of 0.4 cm s . A test is then made with a stoichiometric mixture of N2/H2 at 4 bar under the assumption that the process is far from equilibrium and first order in nitrogen. The reaction is planned to run at 600 K, and from fundamental studies on a single crystal the TOP is roughly 0.05 per iron atom in the surface. From Exercise 5.1 we utilize that 1 g of reduced catalyst has a volume of 0.2 cm g , that the pore volume constitutes 0.1 cm g and that the total surface area, which we will assume is the pore area, is 29 m g , and that of this is the 18 m g- is the pure iron Fe(lOO) surface. Note that there is some dispute as to which are the active sites on iron (a dispute that we disregard here). 

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

The catalytic properties of a surface are determined by its composition and structure on the atomic scale. Hence, it is not sufficient to know that a surface consists of a metal and a promoter - perhaps iron and potassium - but it is essential to know the exact structure of the iron surface, including any defects and steps, as well as the exact location of the promoter atoms. Thus, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom, and under reaction conditions. The well-defined surfaces of single crystals offer the best perspectives for atom-by-atom characterization, although occasionally atomic scale information can also be obtained from real catalysts under in-situ conditions (for some examples, see Chapter 9). The many aspects that we need to study in order to properly understand supported catalysts on a fundamental level are shown schematically in Figure 1.4 [13]. 

Matsui has found for an I.G. catalyst that the aluminum oxide at its surface is present in the form of ferrous aluminate which covers parts of the metal surface, consisting of a-iron. The same author prepared a small single crystal of iron coated by an oxide film and studied its structure by the transmission method of electron diffraction. The pattern shows, Fig. 14, that the (111) plane of magnetite is produced parallel to the (111) plane and also to the (110) plane of iron, suggesting that with the... 

In previous papers by the authors [9,10], the temperature-programmed desorption of N2 from an iron-based catalyst has been studied experimentally. The microkinetic analysis of these results is based on the kinetic simulation of ammonia synthesis by Stoltze and Nprskov [22-24] using the approach by Dumesic and Trevino [2]. On Fe single crystal surfaces it was possible to detect a di-molecular precursor labelled a-N2 — forN2 dissociation... 

Figure 6 Correlation between both N2 adsorption and NH3 production rates and the structure of the catalytic surface, as determined by studies with various single crystals of iron. These results explain the strong dependence of the performance of commercial ammonia synthesis catalysts on their method of preparation ...

Investigations into these topics are presented in this volume. Iron, nickel, copper, cobalt, and rhodium are among the metals studied as Fischer-Tropsch catalysts results are reported over several alloys as well as single-crystal and doped metals. Ruthenium zeolites and even meteo-ritic iron have been used to catalyze carbon monoxide hydrogenation, and these findings are also included. One chapter discusses the prediction of product distribution using a computer to simulate Fischer-Tropsch chain growth. 

It is a pity that open surface of a-Fe single crystal is possibly obtained only under rigorously controlled laboratory conditions. In industrial production, we are ignorant of how the (111) surface of iron exposed fully on the siu face of a-Fe crystallites during the manufacture and reduction of catalysts and also how the stability of the (111) surface is retained in the reactor. According to the studies of chemisorptions of CO and CO2, more than 50% of a-Fe surface is covered by K2O or KOH. Studies by Ertl et indicated that when there are K2O or KOH present on the a-Fe... 

The synthesis of ammonia from its elements ranks as one of the most important discoveries in the history of the science of catalysis, not only because of its industrial application in which synthetic fertilizers have contributed enormously to the survival of mankind, but also from the viewpoint of fundamental science. Even today, some eighty years after the first demonstration of ammonia synthesis, many original scientific papers on the mechanism of the catalytic synthesis of ammonia are still published. Every time a new method, technique, or concept has appeared in the field of heterogeneous catalysis, it has been applied to this reaction. Specific examples of these applications over the years include the concepts of gas equilibrium, activated adsorption, structure sensitivitystoichiometric number and kinetic studies, " nonuniform surfaces, the measurements of surface area, surface composition and promoter distributions, and the use of isotopic and spectroscopic techniques. In particular, various surface science techniques have been applied successfully to this reaction system over well-defined single crystal surfaces in recent years. In this way the effect of promoters on the iron catalyst has been elucidated. Accordingly, the history of ammonia synthesis parallels not only that of industrial catalysis, but also the development of the science of catalysis. 

It should be stressed that not only is there a pressure gap between the UHV studies and the high-pressure measurements, but also a material gap. The UHV studies use carefully cleaned iron metal in the form of single crystals as the catalyst. On the other hand, elemental polycrystalline iron without surface purification does not catalyze ammonia synthesis at high pressure (1 bar), as has been described in Section 2.2. This may be rationalized in terms of the extreme sensitivity of the... 

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