Long-Range Diffusion of K Promoter on an Ammonia Synthesis Catalyst Surface—Ionization of Excited Potassium Species in the Sample Edge Fields

The potassium ions K⁺which diffuse out to the surface from the K promoted ammonia synthesis iron catalyst material do not desorb from there but diffuse rapidly along the surface until they reach the edges of the sample. This is shown by angular distributions of the ion emission at the normal operating temperatures of 900–1100 K. The ionic emission at low field strengths, of the order of 2–50 V cm⁻¹, has a minimum or even a zero signal in the direction of the surface normal. Instead of desorbing from the surface, the ions interact strongly with the surface and give electronically excited states K^*on the surface, which diffuse rapidly along the surface over a distance of several millimeters to the edges of the sample. A detailed model is proposed for this process, based on recent kinetic results. At the edges, ions are formed in the stronger electric field just outside the surface giving lobes along the surface. With the highest field strengths used, these lobes are transformed into strongly peaked distributions at 45–70° from the normal, with a strong minimum in the normal direction. From the open surface, only clusters K_nand neutral atoms K can be emitted. Trajectory calculations show that ions, which are emitted from the edges of the sample with higher then thermal energy, appear in the experimentally observed angular range. Their excess energy may be derived from the work function difference between the catalyst sample and its Ta holder.     

Fused iron surface composition as measured by low energy ion scattering

Low energy He⁺-scattering was used to simply demonstrate that the surface of a typical fused iron ammonia synthesis catalyst is largely covered by promoter oxides of calcium and/or potassium. Complementary photoemission results indicate that atomic nitrogen is deposited in the near surface region during catalyst activation in NH₃/H₂-mixtures.     

Promotion in ammonia synthesis: A pressure dependent phenomenon

The origin of the pressure dependence of potassium promotion of iron catalysts has been investigated by measuring the turnover numbers for ammonia synthesis on polycrystalline iron, potassium-doped polycrystalline iron and two triply promoted (K₂O, Al₂O₃, C_aO) industrial ammonia synthesis catalysts at 1, 2 and 31 bar in the temperature range 550–800 K. At 1 and 2 bar the turnover numbers of polycrystalline iron and potassium doped polycrystalline iron are greater than those of the industrial catalysts, suggesting that the promoters are merely decorating the surface of the iron, blocking off part of it for reaction. It is only at 31 bar and at temperatures above 670 K, that the promoting effect of the additions to the industrial catalyst becomes apparent, the turnover numbers of these materials being roughly 20 times greater than that of polycrystalline iron. Potassium doped polycrystalline iron, however, has the same turnover number as those of the industrial catalysts. It is concluded that at temperatures above 670 K in a hydrogen/ nitrogen (3 : 1) mixture and at 31 bar the morphology of the surface of the industrial catalysts is changed with the iron, probably as iron nitride, coating the potassium aluminate, the identity of the turnover numbers of the catalysts and of potassium doped polycrystalline iron showing that potassium plays a key role in this promotion. The surface restructuring of the industrial catalysts does not produce dominantly the (111) surface, the observed turnover numbers being 10³ times lower than that of the (111) surface. The activation energy for ammonia synthesis in the temperature range 670–770 K at 31 bar on these materials is much higher (32–37 kcal mol^–1) than that of iron (111) (19.2 kcal mol^–1) since it incorporates the activation energy for surface reconstruction of iron. The molecular nitrogen desorption spectra from the industrial catalysts exhibit three states: (i) N₂, bonded end on, (ii) N₂, -bonded and (iii) -bonded N₂, vicinal to some form of potassium.     

Mechanism of potassium loss by desorption from an iron oxide catalyst for the styrene process

Several types of experiments have been done with molecular beam and mass spectrometric methods to characterize the desorption processes of potassium from a commercial styrene (potassium promoted iron oxide) catalyst. The loss of potassium as desorption of K is found to be mainly thermal, with an activation energy close to 1.0 eV (97 kJ/mole), which probably is associated with release of K from the initially bound positions in the bulk. The directly measured rate of loss as K appears too small to account for the experimentally observed total rate of loss. A new loss mechanism as electronically excited but not easily field ionizable potassium atoms is detected with an activation barrier of 1.7 eV (164 kJ/mole). Excited states may be important for the total loss both through direct emission of excited K atoms, and since excited states may give rise to cluster formation at the surface and more rapid loss as clusters K_n.     

High Pressure Desorption of K+ from Iron Ammonia Catalyst – Migration of the Promoter Towards Fe Active Planes

The thermal desorption of potassium ions from industrial iron catalysts was studied in situ in the wide pressure range of 10^?8–10 bar of Ar, N₂ and synthesis gas mixture of N₂:3H₂. While high activation energy of 284 ± 1 kJ/mol, for K⁺ was determined for the catalyst precursor, in the reaction conditions it drops down to 231 ± 5 kJ/mol, corresponding well to that found for iron single crystals in UHV studies. The results are rationalized in terms of potassium migration from oxide storage phases towards the iron facets developed during the catalyst activation.     

Application of Ru exchanged zeolite-Y in ammonia synthesis

Na-Y zeolite was cation exchanged with Ru(NH₃)₆Cl₃ yielding at 25% exchange level a light-purple solid which was active in ammonia synthesis at atmospheric pressure. Pulse conversion experiments show that the catalyst stores nitrogen as it was observed with the conventional iron catalyst. At 810 K the conversion reached about 20% of the maximum conversion of the iron catalyst. The catalyst deactivated reversibly within 30 h due to agglomeration. The active species in the catalyst is most likely a cluster-like Ru metal particle prevented from sintering under the reducing conditions of catalysis by the zeolite framework.     

The mechanism of the poisoning of ammonia synthesis catalysts by oxygenates O2, CO and H2O: an in situ method for active surface determination

The effect of adding an oxygenated poison (O₂, CO or H₂O) to a hydrogen/nitrogen stream producing ammonia over a triply promoted (K₂O, CaO, Al₂O₃) commercial catalyst is not unsurprisingly rapidly to poison the catalyst. However, immediately the oxygenated poison reacts with the catalyst and before total poisoning has occurred, which in these experiments took 10 min, there was an explosive release of ammonia producing concentrations in the gas phase in excess of the equilibrium value. This is thought to be due to a convulsive reorganisation of the surface of the catalyst in forming regions of an oxide overlayer, resulting in the expulsion of the standing surface nitrogen atom coverage as ammonia. However, in contradistinction to the observation of complete poisoning of the triply promoted catalyst shortly after switching the water (2.9%) into the hydrogen/nitrogen stream, when polycrystalline iron was used as the catalyst, after the initial pulse of ammonia was observed, the small quantity of water (2.9%) in the hydrogen/nitrogen stream resulted in an increased rate ( ×3) of ammonia synthesis which declined only slightly over the twenty minute duration of the experiment. The difference in behaviour between the triply promoted catalyst and the polycrystalline iron is thought to be due to the relative ease of reduction of the latter, so that submonolayer quantities of oxide can be stabilised on the surface of the polycrystalline iron. The promoting effect of this oxide overlayer is either structural or electronic; no distinction can be made from these experiments. The technique of injecting either O₂ or CO into a hydrogen/nitrogen stream which is producing ammonia over promoted catalysts in quantities insufficient to cause complete poisoning and measuring the oxygen coverage of the catalyst to a measured decrease in the ammonia synthesis rate, appears to be a ready, in situ method for the determination of the active catalyst area.     

The nature of the active phase of the Fe/K-catalyst for dehydrogenation of ethylbenzene

The industrial catalyst for high temperature dehydrogenation of ethylbenzene based on iron and potassium oxides undergoes, under reaction conditions, essentially a transformation into magnetite, Fe₃O₄, and a mixture of ternary oxides containing trivalent iron, viz. K₂Fe₂₂O₃₄ and KFeO₂. The latter compound constitutes the outside of the catalyst particles and is indeed the catalytically active phase.     

On the rate enhancement of ammonia synthesis over iron single crystals by coadsorption of aluminum oxide with potassium

The behavior of doubly promoted iron catalysts utilized for ammonia synthesis is modelled by the coadsorption of aluminum oxide and potassium on iron single crystal surfaces that were employed in high pressure reaction rate studies. The promoter effect of aluminum oxide is due to its interaction with iron oxide during the preparation stage of the industrial catalyst. After reduction, aluminum oxide stabilizes the most active Fe(111) and Fe(211) crystal surfaces. Potassium does not appear to be involved in the structural promotion but its presence on the active iron surfaces increases the rate of dinitrogen dissociation mostly by lowering the concentration of adsorbed ammonia thus, making more catalytic sites available for dinitrogen dissociation. Co-adsorbed potassium and alumina form a potassium aluminate compound that a) inhibits the aluminum oxide induced restructuring of iron and b) covers up the active iron sites for ammonia synthesis.     

Combining molybdenum carbide with ceria overlayers to boost Mo/CeO2 catalyzed ammonia synthesis

The design of an efficient non-noble metal catalyst is of burgeoning interest for ammonia synthesis. Herein, we report a Mo₂C/CeO₂ catalyst that is superior in ammonia synthesis activity. In this catalyst, molybdenum carbide coexisted with the ceria overlayers which is from the ceria support as the strong metal–support interaction. There is a high proportion of low-valent Mo species, as well as high concentration of Ce³⁺ and surface oxygen species. The presence of Mo₂C and CeO₂ overlayers not only leads to enhancement of hydrogen and nitrogen adsorption, but also facilitates the desorption and exchange of adsorbed species with the gaseous reagents. Compared with the Mo/CeO₂ catalyst prepared without carbonization, the Mo₂C/CeO₂ catalyst is more than sevenfold higher in ammonia synthesis rate. This work not only presents an explicit example of designing Mo-based catalyst that is highly efficient for ammonia synthesis by tuning the adsorption and desorption properties of the reactant gases, but opens a perspective for other elements in ammonia synthesis.     

Effect of potassium on the high pressure kinetics of ammonia synthesis over fused iron catalyst

Measurements were performed of reaction rate in the process of ammonia synthesis (T=370–470°C) on doubly promoted (DP) (Al₂O₃, CaO) and triply promoted (TP) (K₂O, Al₂O₃, CaO) iron catalysts. The latter were obtained by impregnation of the reduced and subsequently passivated DP precursors with alcoholic solution of KOH. The studies were carried out under high total pressure (10 MPa) in a wide range of ammonia partial pressure in the gas phase: from 0.25 to about 7 bar. The results are shown to be authoritative for the so-called kinetic regime. The effect of the presence of K⁺ cations in the catalyst was the stronger, as the temperature of the reaction was the lower and, in particular, the ammonia pressure in the gas phase the higher. The obtained results are in good accordance with the results of Somorjai's studies on activity of iron single crystal surfaces both clean and covered with (K+O) adlayer.     

Potassium-Promoted Carbon-Based Iron Catalyst for Ammonia Synthesis. Effect of Fe Dispersion

Potassium-promoted iron catalysts supported on thermally modified, partly graphitized carbon were studied in the ammonia synthesis reaction. Iron nitrate was used as a precursor of the active phase and KOH or KNO₃ were used as promoters. The kinetic studies of NH₃ synthesis were carried out in a differential reactor under 63 bar and 90 bar pressure. Hydrogen chemisorption, X-ray diffraction and transmission electron microscopy experiments were performed to determine the dispersion of iron in the specimens. All the K⁺–Fe/C catalysts proved to be sensitive to ammonia, the NH₃ partial pressure dependencies of their reaction rates being close to that of the commercial magnetite catalyst (KM I, H. Topsoe). The catalytic properties of the promoted Fe particles on carbon were shown to be dependent upon the iron dispersion, i.e. smaller particles exhibited higher turnover frequency in NH₃ synthesis. It is suggested that either small Fe crystallites expose more highly active sites, e.g. C-7 (B-5) or the promotion of small crystallites by the alkali is more efficient.     

Potassium promotion of the ammonia synthesis reaction: A combined UHV-high pressure investigation

The ammonia synthesis rate has been measured as a function of the potassium coverage of a polycrystalline iron foil. The results clearly demonstrate a promoting effect of the potassium in agreement with the work of Ertl et al. on the dissociative dinitrogen adsorption at iron single crystal surfaces. However, the results also imply a marked difference between the state of the surface under ammonia synthesis conditions and during the dinitrogen adsorption experiments in ultra-high vacuum. It is suggested that in the former case the potassium atoms are surrounded by several strongly adsorbed atoms effectively blocking the corresponding sites for dinitrogen dissociation.     

Kinetics of the dehydrogenation of ethylbenzene to styrene over unpromoted and K-promoted model iron oxide catalysts

The dehydrogenation of ethylbenzene to styrene over unpromoted and potassium-promoted model iron oxide catalysts has been studied using ultrahigh vacuum techniques in conjunction with elevated pressure reaction kinetics. Model iron oxide catalysts were prepared by oxidizing a polycrystalline Fe sample that was subsequently dosed with metallic potassium. At 875 K the unpromoted catalyst exhibited a turnover frequency of 5×10^–4 molecules/ site s and an activation energy of 39 kcal/mol, both in excellent agreement with the results found for an analogous iron oxide powder catalyst. Potassium promotion increased the turnover frequency to 1.0×10^–3 molecules/site s and lowered the activation energy to 36 kcal/mol for the dehydrogenation reaction. Similarities between the activation energies on the unpromoted and promoted catalysts indicate that the active site is the same on both catalysts. Creation of the active site was dependent upon the formation of an Fe³⁺ metastable species, consistent with the formation of a KFeO₂ phase, upon the addition of potassium.     

Study on the properties of iron–cobalt alumina supported catalyst for ammonia

The addition of Co to Fe/Al₂O₃ increases the catalytic activity in NH₃ synthesis. The maximum effect is observed for 20% by weight of Co in the metallic phase. Bivalent cobalt atoms replace bivalent iron atoms (a similar ionic radius) in the crystal lattice. This process changes the reducibility of the samples. The Fe-Co compound and its formation results in the fairly high temperature of reduction (873 K) which is needed to prepare the most active catalyst. Changing the reactor atmosphere from reducing to inert causes the disappearance of free iron (escape of Fe to the crystal lattice of support with formation of a new compound with a spinel character). This is the effect of the iron-hydrogen interaction. The formation of an intermetallic iron-cobalt compound is crucial to the catalyst activity. This might be due to the surface restructuring by exposing the most active iron surface, Fe(111). The potassium addition in the form of KOH causes an increase in the catalytic activity. The increase is not as high as for a ‘super basic’ Fe-Co magnesium hydroxide carbonate supported catalyst studied earlier. A part of the potassium hydroxide is used to neutralize the acid sites on the surface of alumina.     

The effect of cobalt on the activation of fused iron catalyst for ammonia synthesis

BET, scanning electron microscopy, X-ray diffraction, Mössbauer spectroscopy, X-ray fluorescence spectroscopy and McBain thermobalance were used to investigate the effect of cobalt on the reduction behavior and activity of used iron catalyst for ammonia synthesis. Activity tests were carried out under 10 MPa in the 350–450°C temperature rang. Studies were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approx. 5.5% wt Co. Cobalt changed the reduction behavior of the catalyst. The rate of the surface change during reduction was higher for the case of the ‘cobalt catalyst’; however the rate of mass change was higher for a typical iron catalyst. The process of reduction was probably followed by the formation of an Fe₃Co compound and by the surface faceting, with the exposure of an Fe(111) plane.     

Carbon Formation Thresholds and Catalyst Deactivation During CH4 Decomposition on Supported Co and Ni Catalysts

Carbon deposition during catalytic CH₄ decomposition (CH₂?C+2H₂), occurs at a given reaction temperature when K _M < K _M ^*, where K _M ^* is the carbon formation threshold defined as the value of K_M = (P _H2 ² /PCH₄) at which the net rate of carbon deposition is zero (Snoeck et al., J. Catal. 169 (1997) 240). Carbon deposition can produce encapsulating carbon that results in catalyst deactivation, or filamentous carbon that ensures stable catalyst activity for extended periods of time. In the present study, the rate of catalyst deactivation during CH₄ decomposition at 773 K on supported Co and Ni catalysts decreased as K _M increased. A filamentous carbon formation threshold K _M ^f is therefore defined as the value of K _M at which the rate of catalyst deactivation equals zero as a consequence of filamentous carbon formation. Results presented herein demonstrate that stable activity and filamentous carbon formation during CH₄ decomposition on supported Ni and Co catalysts can be guaranteed by choosing K _M such that the inequality K _M ^f < K _M < K _M ^* is satisfied, whereas if K _M < K _M ^f < K _M ^*, encapsulating carbon accompanied by catalyst deactivation occurs.     

Evolution of Ti–Sn-rutile-supported V2O5–WO3 catalyst during its use in nitric oxide reduction by ammonia

This paper concerns the relation between surface structure of crystalline vanadia-like active species on vanadia–tungsta catalyst and their activity in the selective reduction of NO by ammonia to nitrogen. The investigations were performed for Ti–Sn-rutile-supported isopropoxy-derived catalyst. The SCR activity and surface species structure were determined for the freshly prepared catalyst, for the catalyst previously used in NO reduction by ammonia (320 ppm NO, 335 ppm NH₃ and 2.35 vol% O₂) at 573 K as well as for the catalyst previously annealed at 573 K in helium stream containing 2.35 vol% O₂. The crystalline islands, exposing main V₂O₅ surface, with some tungsten atoms substituted for V-ones, were found, with XPS and FT Raman spectroscopy, to be present at the surface of the freshly prepared catalyst. A profound evolution of the active species during the catalyst use at 573 K was observed. Dissociative water adsorption on V⁵⁺OW⁶⁺ sites is discussed as mainly responsible for the catalyst activity at 473 K and that on both V⁵⁺OW⁶⁺ and V⁴⁺OW⁶⁺ sites as determining the activity at 523 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

Potassium Promotion of Cobalt Spinel Catalyst for N2O Decomposition—Accounted by Work Function Measurements and DFT Modelling

The beneficial effect (decrease of the half conversion temperature by 100 °C) of potassium doping, in the range of 0–5 atoms/nm², on N₂O decomposition over Co₃O₄ was analyzed by work function measurements and DFT calculations. The optimal potassium surface loading was found to be 1.8 atoms/nm². The effect was explained in terms of electronic promotion gauged by lowering of the catalyst work function by 0.48 eV (for K₂CO₃ precursor) and 0.44 eV (for KOH). The promotional effect is discussed in relation to the theoretical and experimental surface dipoles determined from Hirshfeld atomic charges and geometry of the postulated potassium adspecies and from the Topping model, respectively.     

Perturbed Angular Correlation Characterization of Indium Species on In/H-ZSM5 in the Presence of Water and Catalytic Deactivation Studies During the SCR of NO x with Methane

An In(4 wt%)-impregnated H-ZSM5 catalyst was characterized under wet and dry conditions by time-differential perturbed angular correlation (PAC) and by temperature-programmed reduction (TPR). Different indium species were quantified, correlating their structure with the catalyst deactivation due to the presence of water during the NO selective catalytic reduction (SCR) with methane. The fresh sample contains 60% of indium oxide and 40% of (InO)⁺ species at exchange sites, the latter being the active species for the reaction under study. Under wet atmosphere, hyperfine interactions determined by PAC indicate the formation of two types of In hydroxide species and the decrease of both (InO)⁺ and In₂O₃. TPR and PAC characterizations also show that deactivation is due to the decrease of (InO)⁺ at exchange sites to form, after water is removed, different non-active In oxide species.