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.     

On the mechanism of poisoning and promotion of ammonia synthesis

The effect of potassium in combination with alumina in promoting the rate of production of ammonia over polycrystalline iron has been shown to occur only at high temperatures (> 670 K) and high pressures (> 30 bar). The effect is the result of the iron, probably in the form of a nitride, migrating over the potassium aluminate promoter. The addition of H₂O (2.9%) to the H₂/N₂ stream causes the complete cessation of ammonia production before which, however, there is a virtually instantaneous pulse of NH₃ to concentration three times greater than equilibrium. This is thought to result from a convulsive rearrangement of the nitride to an oxide overlayer, with the concomitant expulsion of the nitrogen as ammonia. The effect is reversible upon removing the H₂O from the H₂/N₂ stream.     

Property influence and poisoning mechanism of HgCl2 on V2O5-WO3/TiO2 SCR-DeNOx catalysts

In this paper, HgCl₂ was loaded by impregnation method to study its property influence and poisoning mechanism on V₂O₅-WO₃/TiO₂ SCR-DeNO_x catalyst. For HgCl₂-containing catalysts, deactivation was observed in simulated gas stream, and its catalytic activity declined with the increase of HgCl₂ loadings. Brønsted acid sites (VOH) and VO bond were affected evidently by HgCl₂, new NH₃ adsorption site ClVOH was generated after HgCl₂ addition. Interactions between HgCl₂ and V₂O₅ resulted in the loss of SCR active sites, and –HgCl existed as the stable form on the bridge site. Finally, the probable schematic diagram of HgCl₂ poisoning mechanism was also proposed.     

The effect of microwave energy on three-way automotive catalysts poisoned by SO2

Recent studies suggest that microwave energy can be employed in catalysis and that the results differ from conventional heating. The influence of microwave energy on automotive exhaust catalysis in the presence and absence of a catalyst poison (SO₂) was studied. The conclusion is that microwave energy can induce carbon monoxide (CO) lightoff (the temperature where 50% of final conversion is achieved) more efficiently than conventional heating and can reverse the poisoning by SO₂ for a commercial three-way catalyst.     

Enantioselective hydrogenation of ethyl pyruvate on tin promoted Pt/Al2O3 catalysts

The enantioselective hydrogenation of ethyl pyruvate has been studied on a Pt/Al₂O₃-dihydrocinchonidine catalyst promoted with different amount of tin. The surface reaction between hydrogen adsorbed on Pt and tin tetraalkyls is used for the tin introduction. This reaction leads to the formation of surface organometallic complexes (I), with SnR_(4-x) moieties anchored to the platinum surface. The enantioselectivity of the Pt/Al₂O₃-dihydrocinchonidine catalyst is found to change only slightly upon promotion with tin, while the rate of ethyl pyruvate hydrogenation depends strongly on the amount and the form of tin introduced. The hydrogenation activity is suppressed completely at relatively low tin coverage (Sn/Pt_s < 0.06). The highest hydrogenation rate is measured over catalysts containing complex (I) (Sn/Pt_s = 0.025) on the platinum surface. On Sn-Pt alloy type active sites, which are formed after decomposition of (I) in hydrogen, the rate of hydrogation is considerably lower than on the unpromoted reference Pt/Al₂O₃ catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences.     

Photo-enhanced catalytic decomposition of isopropanol on V2O5

The effect of UV-visible photo irradiation has been investigated on the decomposition of isopropanol over V₂O₅. A 2.5-fold photo-enhancement in the rate of the dehydration (-H₂O) reaction was observed under UV-visible photo irradiation compared to thermal heating of the catalyst. The increase in propene yield under photo irradiation is believed to be initiated from photo excitation (photo-reduction) of the V₂O₅ complex to [-V⁴⁺-O^–] * thus, increasing surface concentration of V⁴⁺ species. The V⁴⁺ active surface species is assumed to serve as the center for adsorption of isopropanol. The observed increase in water formation along with the formation of propene suggests that the rate determining step for the dehydration reaction is desorption of water. Selectivity toward the dehydrogenation (-H₂) reaction has also been measured to be 1% under UV-visible photo irradiation as compared to 15% under thermal heating of the catalyst.     

The effect of cobalt on the reactants adsorption and activity of fused iron catalyst for ammonia synthesis

Modern adsorption study facilities as well as scanning electron microscopy, X-ray diffraction, Mössbauer spectroscopy and X-ray fluorescence spectroscopy were used to investigate the effect of cobalt on the adsorption of nitrogen, hydrogen and ammonia on the surface of cobalt modified iron fused catalysts. Adsorption studies were carried out at the temperature range specific for ammonia synthesis (385C). Activity tests were carried out under 10 MPa in the 350–450 C temperature range. Investigations were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Introduction of cobalt changed considerably the sample behaviour during activation and ammonia synthesis. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approximately 5.5 wt% Co. Cobalt changed the adsorption behaviour of the catalyst. Chemisorption of nitrogen is much higher for cobalt catalysts. Growth of nitrogen chemisorption and decrease of ammonia adsorption resulted in the growth of catalytic activity of cobalt catalysts in ammonia synthesis.     

Poisoning and Regeneration of an Ni/SiO2 Catalyst

The poisoning effect of thiophene during the hydrogenation of styrene on an Ni/SiO₂ catalyst, and the regenerating role of both pure hydrogen and 2-butyne in the presence of hydrogen on the poisoned catalyst, were studied. The treatments induced the elimination of sulfur and promoted an important recovery of the catalytic activity and selectivity. XPS analyses show that the sulfur species adsorbed during the catalyst poisoning is thiophene. Part of the sulfur remained irreversibly adsorbed after the regeneration treatments carried out at 473 K; a modification on the adsorbed sulfur electronic state was detected, which can be ascribed to thiophene hydrogenolysis induced by the regenerating temperature. © 1997 SCI.     

抑制碳负载钌基氨合成催化剂甲烷化的研究

The effects of promoters K, Ba, Sm on the resistance to carbon-methanation and catalytic activity of ruthenium supported on active carbon (Ru/AC) for ammonia synthesis have been studied by means of TG-DTG (thermalgravity-differential thermalgravity), temperature-programmed desorption, and activity test. Promoters Ba,K, and Sm increased the activity of Ru/AC catalysts for ammonia synthesis significantly. Much higher activity can be reached for Ru/AC catalyst with bi- or tri-promoters. Indeed, the triply promoted catalyst showed the highest activity, coupled to a surprisingly high resistance to methanation. The ability of resistance of promoter to methanation of Ru/AC catalyst is dependent on the adsorption intensity of hydrogen. The strong adsorption of hydrogen would enhance methanation and impact the adsorption of nitrogen, which results in the decrease of catalytic activity.     

Selective hydrodehalogenation of an olefinic compound on doubly poisoned palladium—carbon catalyst; the mechanism of metal ion poisoning

Hydrodehalogenation of 7,7-dichlorobicyclo (3,2,0)hept-2-en-6-one was studied on a commercial palladium—carbon catalyst in liquid phase. The simultaneous saturation of the CC function could be suppressed by pyridine and metal ion (Cu²⁺, Pb²⁺, As³⁺) poisoning. The two types of catalyst poison had a synergetic effect. The mechanism of selective poisoning was found to be metal deposition on palladium partly covering the active sites. Alternative explanations of metal ion poisoning are reviewed.     

Surface behaviour of carbon supported ruthenium catalysts

Chemisorption of H₂, CO, CO₂ and N₂ on carbon-supported ruthenium catalysts, with and without alkali promoters, was studied to ascertain the nature of binding of these gases to the catalyst surface. It was observed that the chemisorption of hydrogen is significantly enhanced when a carbon-supported ruthenium catalyst is promoted either with K₂O or K₂O + BaO. This enhancement may be due to the penetration of some of the hydrogen atoms to lower crystal planes of the catalyst, or some of the hydrogen atoms were migrated to ruthenium atoms lying immediately below the surface promoter molecules. In contrast, nitrogen chemisorption is reduced on the promoted catalysts but shows higher activity for ammonia synthesis. Presorption experiments indicated an inhibiting effect of nitrogen on hydrogen chemisorption. This effect, which was approximately one to one, suggests that the chemisorbed nitrogen occupied the same surface sites. Data on adsorption of N₂, H₂ and CO and their mutual inhibiting effects led to the conclusion that the chemisorption of nitrogen and hydrogen on the transition metal surface was predominantly of atomic nature while that of carbon monoxide was molecular. Carbon dioxide, however, was sorted molecularly by the alkali-containing molecules. There appeared to be significant influence of the surface acidity-basicity functions of the support material on the chemisorption of nitrogen on ruthenium catalysts.     

Studies on Chlorided Pt/Al2O3 Catalysts: Preparation, Characterization and n-Butane Isomerization Activity

A series of chlorided Pt/Al₂O₃ (both and ) catalysts were prepared and characterized for various physicochemical properties. The chloride content of the catalysts was found to increase with chloride treatment time up to a certain level and then decrease owing to prolonged exposure at high temperature. The surface area and pore volume of the catalysts were decreased by chloride treatment. The activity of the prepared catalysts were tested in n-butane isomerization. The platinum content of the catalysts was found to have no effect on catalytic activity up to 0.2 wt% whereas the chloride content of the catalyst strongly influenced the activity and a >20-fold increase in activity was observed on chloriding Pt/Al₂O₃ catalysts. The catalyst activity was found to be directly related to its acidity.     

Hydrogenolysis of 1,4-dimethylcyclohexane on silica supported iridium catalyst: influence of time on stream on activity and selectivity

Opening cyclic or polycyclic alkanes by carbon–carbon bond cleavage can be a good route for the valorization of certain alkanes. A key point of any proposed catalytic system would be the catalyst poisoning. In this paper, we determine experimentally the influence of the time on stream on the distribution of products when 1,4-dimethylcyclohexane is introduced together with hydrogen over a Ir/SiO₂ catalyst. A simplistic model of selective catalyst poisoning is proposed to explain the influence of the time on stream on the activity and product selectivity.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Preparation and Characterization of Copper Catalysts Supported on Mesoporous Al2O3 Nanofibers for N2O Reduction to N2

The gas-phase hydrogenation of benzene to cyclohexane over Ce_{1 - x} Pt _x O_{2 -} (x = 0.01, 0.02) catalyst was investigated in the temperature range 80-200 °C. A 42% conversion of benzene to cyclohexane with 100% specificity was observed at 100 °C over Ce_0.98Pt_0.02O_{2 -} with a catalyst residence time of 1.22 × 10⁴ g s/mol of benzene. The activity of the catalyst was compared with those of Pt metal, combustion-synthesized Pt/-Al₂O₃ and Pt/-Al₂O₃. The turnover frequency value of Ce_0.98Pt_0.02O_{2 -} is 0.292, which is an order of magnitude higher than those of the other Pt catalysts investigated. The kinetics of reaction and the deactivation behavior of the catalyst were studied and a regeneration methodology was suggested. The deactivation kinetics and structural evidence from XRD, XPS, TGA and H₂ uptake studies suggest that the oxidized Pt in Ce_0.98Pt_0.02O_{2 -} is responsible for the high catalytic activity towards benzene hydrogenation.     

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.     

Phenol alkylation with methanol: effect of sodium content and ammonia selective poisoning of an HY zeolite

Sodium exchange and ammonia selective poisoning of the acid sites of an HY zeolite (Si/Al=20) were carried out and their effects on the catalytic properties for the alkylation of phenol with methanol (200C, 1 atm and N₂/reactants molar ratio of 4) were evaluated. Results show that the reaction is highly sensitive to the number and strength of the acid sites of the catalyst. A decrease in the number of acid sites by sodium exchange of the protons or by ammonia selective poisoning produces important changes in the selectivity of the reaction. In fact, a high increase in the anisole/cresol ratio is observed as the percentage of exchanged sodium in the zeolite increases, while the ammonia selective poisoning shows that at low desorption temperatures (250C) only anisole is formed while at higher desorption temperatures both anisole and cresols were observed. These results show that anisole formation requires sites with lower acid strength compared to those necessary for cresol formation.     

Ammonia Decomposition on Ir(100): From Ultrahigh Vacuum to Elevated Pressures

Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al₂O₃ 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9±0.1) with respect to ammonia and negative order (–0.7 ±0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed Ir(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N₂ is the rate-determining step with a desorption activation energy of 63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NH _x + H NH _x+1, x=0–2) in the presence of excess H atoms on the surface.     

Chemisorption of H2 on supported Pt clusters probed by129Xe NMR

The surface of Pt clusters with average size between 1 and 8 nm supported on SiO₂, -Al₂O₃ or Y-zeolite was probed by¹²⁹Xe NMR as the Pt surface coverage with hydrogen, , was increased. A distinct change in the structure of the hydrogen overlayer at 0.3 was inferred from the NMR spectra. This change is believed to take place when the chemisorbed hydrogen fills all the next nearest neighbor metal sites and the nearest neighbors start to be occupied. These new observations clarify previously reported determinations of the average number of Pt atoms in supported clusters by means of Xe NMR and other techniques. It also appears that interfacial metal-support interactions may be probed by Xe NMR.