Rh Gauze Research Articles

Monolithic FeO x /Al2O3 catalysts for ammonia oxidation and nitrous oxide decomposition

(1.2–8.3)%FeOх/Al2O3 monolith catalysts have been prepared by impregnating alumina with aqueous solutions of iron(III) nitrate and oxalate and have been tested in NH3 oxidation and in the selective decomposition of N2O in mixtures resulting from ammonia oxidation over a Pt–Rh gauze pack under conditions of nitric acid synthesis (800–900°C). In the case of the support calcined at 1200°C, the catalyst is dominated by bulk Fe2O3 particles localized on the Al2O3 surface. The activity of these samples in both reactions decreases with a decreasing active component content, thus limiting the potential of Fe2(C2O4)3 · 5H2O, an environmentally friendlier but poorly soluble compound, as a substitute for Fe(NO3)3 · 9H2O. Decreasing the support calcination temperature to 1000°C or below leads to the formation of a highly defective Fe–Al–O solid solution in the (1.2–2.7)%FeOх/Al2O3 catalysts. The surface layers of the solid solution are enriched with iron ions or stabilize ultrafine FeOх particles. The catalytic activity of these samples in both reactions is close to the activities measured for ~8%FeOх/Al2O3 samples prepared using iron nitrate.

Formation of N2 and N2O in industrial combustion of ammonia over platinum

The catalytic combustion of ammonia over a 95%Pt/5%Rh gauze has been investigated in an oxygen/steam environment using a flow reactor. Direct measurements of byproduct N2 and N2O selectivities have been made over a wide range of process conditions: temperature 450–815°C; pressure 1–3bar; mass flux 0.12 to 1.12kgm−2s−1; ammonia mole fraction 7–13%; and inlet oxygen-ammonia feed ratio 1.4 to 3. The results show that, while both N2 and N2O selectivities decrease with increasing temperature, the influence of other process variables is important. The formation of N2O is significantly reduced at higher values of the oxygen-ammonia feed ratio so attempts to increase burner throughput by reducing this ratio may lead to increased N2O formation in the burner.A model for product selectivity based on mass-transfer-limited combustion and detailed microkinetic description of the surface N2- and N2O-forming steps is developed. Explicit expressions incorporating all the process and gauze parameters in the system are obtained for the individual products. The model is found to provide excellent descriptions of the entire data set, both for oxygen and temperature dependence. The model also describes available literature data even for data obtained under very different conditions from ours (gauze dimensions, combustion in air, temperatures up to 1000°C and mass fluxes up to 50 times those studied in this work).Fundamental relations between the activation energies for the reactions of surface nitrogen atoms that lead to NO and byproduct formation are extracted from the data. These values provide a target for fundamental studies.The approach developed here is applicable to a wide range of catalysed mass-transfer-limited reactions such as combustion and partial oxidation.

Oxidation of ammonia to NOx in a two bed reactor (Pt gauzes + oxide monolytic layer): Experimental studies and mathematical modelling

The behavior of one- and two-bed systems consisting of Pt–Rh gauze pack (the first bed) and monolith honeycomb catalyst (the second bed) in the conditions corresponding to those in the industrial medium pressure plant for NH3 oxidation to NOx was experimentally studied. At complete NH3 conversion in the gauze pack installation of the monolith results in the increase of the temperature in the gauzes and NOx yield. One dimensional mathematical model with heat and mass exchange between gas and solid phases and considering the effect of monolith as a heat shield was developed. The effect of monolith was supposed to consist preferentially in increasing the coefficients of heat and mass exchange between gas and solid phases in the gauze pack and determined by “hydrodynamic factor” γ depending on monolith geometry. It results in the shift of temperature front towards the frontal gauze and the reasonable increase of NOx yield.

Mechanistic aspects of the Andrussow process over Pt–Rh gauzes. Pathways of formation and consumption of HCN

Reaction pathways governing HCN selectivity in the oxidative coupling of methane and ammonia were investigated over three commercial Pt–Rh gauzes (fresh, activated, and spent) in the temporal analysis of products reactor with submillisecond-time resolution using isotopic traces. These gauzes differed in the extent of reaction-induced restructuring as well as impurity content. The ability of the gauzes to form HCN in the dual interactions of CH 4 with NH 3 or NO was compared considering the morphological differences of the gauzes. It was found that the progressive structural changes increased the gauze activity for methane conversion and facilitate the stabilization of methane fragments on the catalyst surface, but did not influence significantly the surface residence time of N-containing species. The well-known increase in HCN selectivity within first hours on-stream in the Andrussow process was suggested to be likely due to restructuring-induced stabilization of surface methane fragments. The decrease in the HCN selectivity after a stable phase of operation is mainly related to consecutive oxidation of HCN over iron oxide accumulated on the catalyst surface under industrial conditions of the Andrussow process.

Decomposition of N2O over Hexaaluminate Catalysts

Catalytic N2O decomposition has been studied over metal-substituted hexaaluminates with the general formula ABAl11O19, where A = La, Ba, and B = Mn, Fe, Ni. The materials were prepared by coprecipitation via the carbonate route followed by calcination at 1473 K for 10 h. Inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), transmission electron microscopy (TEM), and N2 adsorption techniques were used to characterize the catalysts. The activity in direct N20 decomposition was evaluated by means of temperature-programmed reaction and steady-state experiments. Fe- and Mn-containing hexaaluminates present the highest activities. The Ni-containing catalysts are significantly less active, comparable to the hexaaluminates without metal substitution. The catalytic activity was practically not influenced by the A cation (La or Ba) in the structure. The Fe- and Mn-substituted hexaaluminates exhibit high activity and stability for N2O decomposition in mixtures simulating the outlet of the Pt-Rh gauzes in ammonia oxidation reactors, containing N2O, NO, O2, and H2O. These materials are promising for high-temperature abatement of this powerful greenhouse gas in the chemical industry, particularly in nitric acid and caprolactam production.

Transient studies on the effect of oxygen on the high-temperature NO reduction by NH 3 over Pt–Rh gauze

The effect of oxygen on the activity and selectivity of the NO reduction by NH 3 over Pt–Rh (95–5 wt.%) alloy gauze at 1023–1073 K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH 3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O 2-pretreated gauze accelerate the reaction of NH 3 with NO with respect to the as-received gauze. N 2 was the main reaction product and traces of N 2O were comparatively formed (N 2O/N 2 ∼ 10 −3). Pulsing of an equimolar O 2– 15NH 3–NO mixture over the Pt–Rh gauze mainly produces 15NO, while the formation of N 2 is largely suppressed. The selectivity to 15NO and N 2O in the ternary O 2– 15NH 3–NO system diminished upon decreasing the O 2/( 15NH 3 + NO) ratio, in favor of N 2. This ratio was qualitatively varied by changing the time delay between O 2 and 15NH 3–NO in sequential pulse experiments. Our results indicate that the NH 3 oxidation by O 2 to NO is much faster than the NO reduction by NH 3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N 2 and N 2O in ammonia burners within nitric acid manufacture.

Selectivity-directing factors of ammonia oxidation over PGM gauzes in the Temporal Analysis of Products reactor: Primary interactions of NH 3 and O 2

The Temporal Analysis of Products (TAP) reactor has been applied to study selectivity-directing factors of the high-temperature NH 3 oxidation to NO, N 2O, and N 2 over commercial knitted Pt and woven Pt–Rh alloy gauzes at 973–1173 K. The unique features of the TAP technique enable investigation of the mechanism of this highly exothermic process under isothermal conditions over catalysts of industrial relevance, and the applicaton of much higher peak pressures as compared to surface science techniques in ultrahigh vacuum. This article focuses on the investigation of primary interactions of NH 3 and O 2. The overall reaction mechanism was found to be very similar over both Pt and Pt–Rh gauzes. NH 3 activation is favored over O 2-pretreated gauzes, while the as-received gauzes are virtually inactive for NH 3 decomposition to N 2. The selectivity to NO primarily depends on the concentration of adsorbed oxygen species. A high ratio of adsorbed O/NH x species favors NO formation, confirming that undesirable secondary reaction paths are minimized at a high O coverage. Besides, our results suggest that the nature of oxygen species influences the distribution of NO and N 2 in the product. It is put forward that weakly bounded oxygen species lead to a high NO selectivity, while strongly bounded oxidize NH 3 into N 2. The interaction of NH 3 and NO also contributes to N 2 formation, while direct NO decomposition is practically suppressed over the oxidized gauzes. Application of isotopically labeled 15NH 3 and high peak pressures were essential for detecting N 2O formation during ammonia oxidation. Analysis of secondary interactions of NH 3 and NO in the authors' next project is required to further unravel the origins of reaction by-products like N 2O and N 2.

Evidences of the origin of N2O in the high-temperature NH3 oxidation over Pt-Rh gauze.

Transient isotopic experiments reveal that the mechanism of N2O formation in the high-temperature NH3 oxidation over Pt-Rh gauze involves the reaction of adsorbed ammonia intermediate species (NHx) and NO.

Transient Studies of Direct N2O Decomposition over Pt–Rh Gauze Catalyst. Mechanistic and Kinetic Aspects of Oxygen Formation

For elucidating the mechanistic aspects of oxygen formation during N2O decomposition over commercial woven Pt–Rh gauze, transient experiments were carried out in the temporal analysis of products (TAP) reactor by pulsing N216O over 18O-pretreated gauze catalyst at temperatures typical of industrial ammonia burners (1073–1273 K). The transient responses of N2O and the products of its decomposition (O2 and N2) were fitted to two different mechanistic models. From the isotopic studies and the fitting of transient experiments, two separate routes of oxygen formation during catalytic N2O decomposition have been identified. Oxygen is produced via both (i) interaction of N2O with adsorbed oxygen species formed from N2O and (ii) recombination of adsorbed oxygen species on the catalyst surface. The relative contribution of these reaction pathways depends on the reaction temperature.

New insight into the role of gas phase reactions in the partial oxidation of butane

The partial oxidation of n-butane at high alkane to oxygen ratio was studied in the presence of a Pt–Rh gauze and in an empty tubular reactor under identical conditions. Temperature-programmed reaction (TPR) experiments with the metal gauze showed dramatic changes in the product distribution in the range 25–500 °C. Total oxidation of butane at low temperature is followed by selective conversion to olefins and oxygenates around 400 °C; a further increase in the oven temperature enhances selectively olefins formation. In situ infrared and visible imaging of the reacting gauze revealed remarkable ignition/extinction phenomena of the surface reactions. Fast ignition on the metal catalyst was observed around 200 °C; upon further raising the oven temperature, suppression of these reactions occurred at a temperature level corresponding to the transition to high selectivity conditions. These results indicate a shift from heterogeneous to homogeneous reaction mechanism, the latter being responsible for high selectivity to partial oxidation products (84% olefins+oxygenates). With the empty reactor, conversion and selectivity were similar to those obtained in the presence of the catalyst. The study shows that the gauze plays no significant role in butane partial oxidation, as reactions take place in the void upstream of the catalyst, presumably via an alkylperoxy intermediate.

Short contact time oxidative dehydrogenation of propane

Short contact time oxidative dehydrogenation of propane has been investigated over a Pt/10%Rh gauze catalyst and a catalyst consisting of VMgO supported on a monolithic substrate. The main focus has been on obtaining information about the roles of homogeneous and heterogeneous reactions by comparing the behavior of the two catalysts with blank runs with no active catalyst in the reactor. The reaction was studied at temperatures ranging from 500 to 950°C and with propane/oxygen ratios between 1.1 and 2.3 at atmospheric pressure. It was observed that ethene, methane and synthesis gas (with a hydrogen/carbon monoxide ratio close to 1.0) were the main products at high conversions of propane. Propene becomes increasingly more important at lower conversions, especially in the presence of an active VMgO catalyst. Very low selectivities to ethene and high selectivities to carbon oxides were obtained with the VMgO catalyst at low conversions of propane (<20%). At high temperatures and high conversions no significant effect of the presence of neither the noble metal catalyst nor the VMgO catalyst was found. Small amounts of ethyne, propyne and benzene were always observed in the product gas. The conversion of oxygen and the selectivity to carbon oxides were higher over the Pt/10%Rh gauze and the VMgO catalyst than in the empty reactor, particularly at the lower conversions of propane. One important role of both catalysts seems to be to ignite and to facilitate the internal production of heat for the gas phase formation of alkenes. The effect of varying the flow rate on the performance of the VMgO catalysts indicates that the heterogeneous reactions are subject to transport limitations as would be expected at these conditions.

Hydrogen addition to the Andrussow process for HCN synthesis

The effect of hydrogen addition on HCN production by the ammoxidation of methane over Pt–Rh gauze catalyst has been examined. Upon adding small amounts of H 2, the HCN selectivity increased from ∼74 to 82% on NH 3 basis at 35% N 2 dilution and (CH 4+NH 3)/O 2=2, while selectivities on CH 4 basis were nearly unchanged. Using air instead of oxygen, the HCN selectivity increased from 76 to 86%. However, the increase in HCN selectivity with H 2 addition was accompanied by a decrease in the NH 3 conversion. H 2 addition with different (CH 4+NH 3)/O 2 and CH 4/NH 3 ratios and with preheating of feed gases has also been investigated. The H 2 produced was observed to be greater than that fed for all conditions, so that with recycle the process would require no additional source of H 2. Experimental results obtained here can be qualitatively explained on the basis of a surface reaction model for Andrussow reactor described previously.

Conditions for HCN synthesis and catalyst activation over Pt–Rh gauzes

We have examined different processing conditions for the synthesis of HCN by the ammoxidation of methane over woven Pt–Rh gauzes in an autothermal bench scale reactor. Compared to the conventional air processes, the HCN yield can be improved 10–15% by preheating the reactant gases, and HCN throughput can be increased 140% by removing N 2 from the feed stream. We were able to attain operation in a high pressure bench scale reactor, and HCN yields were maintained above 0.60 up to 3.5 atm at 300% of the throughput achievable at 1 atm. We also investigated activation of the Pt–Rh gauze catalyst which occurs through facet and pit formation on the metal surface. A high temperature treatment reduced activation times from 30 to 3 h. Pits on the catalyst surface resulted from increased temperatures and NH 3 in the reactant gas, but HCN processing conditions were necessary for the catalyst to achieve best performance.

Oxygenate Formation from n-Butane Oxidation at Short Contact Times: Different Gauze Sizes and Multiple Steady States

The production of olefins and oxygenates by partial oxidation of n-butane over a single layer of Pt–10% Rh gauze has been examined at atmospheric pressure in the fuel rich regime and in the space velocity range of 3.3×106to 1.6×107h−1(contact times of ∼1000 to 200 μs) using a single gauze reactor. We show that there is an optimum regime of space velocities to maximize the oxygenate selectivity and that at least 75% selectivity to olefins and oxygenates can be obtained. Nitrogen diluent in the feed lowers the production of oxygenates, and coarser mesh gauze gives higher oxygenate yield than finer mesh. In addition, at a specific range of fuel/oxygen feed ratios, multiple steady states are observed on the more open gauze. Analysis of the product distribution of these steady states indicates that initially the exothermic complete combustion of the butane on the gauze surface to CO2and H2O provides most of the necessary heat for this process. Oxygenates and olefins are then produced by homogeneous reaction in the quench zone downstream of the gauze. A detailed model of homogeneous chemistry confirms that oxygenates are produced by homogeneous reactions.

Partial oxidation of methane to synthesis gas over a Pt/10% Rh gauze

The partial oxidation of methane to synthesis gas has been studied over a Pt/10% Rh gauze catalyst. The experiments were carried out at atmospheric pressure using a single gauze in a quartz reactor heated in an electric furnace. The furnace temperature was varied in the range 200--1050°C and the space time in the range 0.00015--0.0005 s. The feed consisted of a mixture of CH4 : O2 : ≈2 : 1 : 10 and oxygen was only partly consumed. The products were carbon oxides, water, hydrogen and traces of C2-hydrocarbons with compositions far from equilibrium. Compared to a Pt metal gauze a higher methane conversion and more favourable selectivities of synthesis gas, and in particular of H2, were obtained at similar furnace temperatures. The Pt/10% Rh gauze showed a better stability with time on stream compared to the Pt gauze. No loss of metal was observed from the Pt/10% Rh gauze after 8 h on stream. The oxidation reactions were studied separately from steam reforming and the water-gas shift reaction at temperatures up to 1050°C at a space time of 0.00021 s. At 960°C the results indicate that CO and H2 are formed beside H2O as primary products.

Catalyst for HCN synthesis by Andrussov's method, modified by aluminium oxide

The properties of a Pt-10% Rh gauze catalyst, modified by coating its surface with alumina film containing dispersed platinum metals, have been studied. The conversion, process yield and morphological changes of the surface of the modified catalyst are compared with those of a standard gauze applied in the HCN synthesis. For the first time it has been demonstrated, basing on HRTEM observations, that formation of a Pt-Al intermetallic compound on the surface of the gauze catalyst occurs under the conditions of HCN synthesis by Andrussov's method.

Surface Areas of Pt–Rh Catalyst Gauzes Used for Ammonia Oxidation

The changes in structure and surface composition of new and used Pt–Rh catalyst gauzes have been studied by gas adsorption methods. The total surface area was measured by physical adsorption of Kr (Kr-BET). The surface area of used samples varied between 0.01 and 0.06 m2/g representing an increase in surface area of 2 to 20 times compared with the surface area of new catalysts. A gauze with Rh2O3whiskers exhibited the largest increase in surface area. Except for this abnormal sample, the top gauze in the catalyst pack exhibited the largest surface area. The active surface area of the Pt–Rh gauze catalysts was measured by H2-titration/adsorption. It was assumed that Pt represents the active part of the catalyst. When exposing a spent gauze to H2at 298 K, only platinum oxide on the catalyst surface was titrated. During operation the catalyst deactivates partly by the formation of Rh2O3, which was found to be inert against H2at 298 K. However, reduction of used catalyst gauze in H2at 623 K and reoxidation at 623 K made all the Rh2O3reactive toward H2at 298 K, indicating the presence of two forms of Rh2O3. It was observed that only the first gauze in a gauze pack from a NH3-oxidation reactor comprised a noteworthy active area. All the other gauzes investigated were more or less blocked by Rh2O3.

HCN Synthesis by Ammoxidation of Methane and Ethane on Platinum Monoliths

We have examined HCN synthesis by the ammoxidation of CH4, C2H6, and CH4/C2H6 mixtures in air or O2 at atmospheric pressure on α-Al2O3 foam monoliths coated with 1−10 wt % Pt and on Pt−10% Rh gauzes. The HCN reaction system was found to be highly sensitive to the catalyst microstructure because we observed activation and differences in performance on catalysts with different support materials. For CH4 ammoxidation on Pt monoliths we observe HCN yields >63% based on CH4 and NH3 fed which is comparable to yields obtained on commercially used Pt−10% Rh gauzes (∼65%). With C2H6 rather than CH4, we achieve HCN yields of ∼42% (carbon and nitrogen basis). We also examined CH4/C2H6 mixtures (at fixed C/N and (C + N)/O2) for HCN synthesis. HCN yields were found to decrease by 5−7% with increasing C2H6 in the feed hydrocarbon mixture (up to 14%). Results for experiments with Pt−10%Rh gauzes used commercially are shown in all cases to allow comparisons with Pt monoliths. C2H6 ammoxidation on Pt monoliths and gauzes ...

Effect of pressure on three catalytic partial oxidation reactions at millisecond contact times

The effects of pressure on reactant conversion and product selectivities in three catalytic oxidation systems have been examined at pressures between 1 and > 5 atm. Reaction was sustained autothermally near adiabatic operating conditions at temperatures of ∼1000°C with residence times over the noble metal catalysts between 10−4 and 10−2 s. The three systems investigated were (1) HCN synthesis over Pt-10% Rh gauze catalysts, (2) methane oxidation to synthesis gas (CO and H2) over rhodium-coated monoliths, and (3) ethane conversion to ethylene over platinum-coated monoliths. We find that selectivities in all three reactions do not change dramatically with approximately a five-fold increase in pressure. This strongly suggests that free radical homogeneous chain reactions are not significant in these processes and that they can be operated reliably above atmospheric pressure. For the synthesis of HCN over Pt-10% Rh gauzes, the selectivity to HCN can be maintained above 0.75 at pressures up to 5.5 atm. Selectivities to synthesis gas (CO and H2) from a methane-air mixture over a Rh-coated foam monolith at pressures up to 5.5 atm were maintained above 0.90. Over a Pt-coated foam monolith, the selectivity to ethylene from ethane-air and ethane-O2 mixtures was independent of pressure up to 6.5 atm and conversion rose slightly although it was necessary to maintain constant velocity and residence time over the catalyst to avoid carbon formation.

Studies of rubidium aluminosilicates as thermionic emitters of Rb + ions

Filament sources of Rb + ions were created using 90% Pt-10% Rh gauze coated with synthetic rubidium aluminosilicate compounds of various compositions. The performance characteristics of these compounds as thermionic emission sources of Rb + ions based on total emission current and purity of ion emission as functions of time were studied and compared. In addition, these characteristics were investigated of β-eucryptite (Rb 2O·Al 2O 3·2SiO 2) and spodumene (Rb 2O·Al 2O 3·4SiO 2) samples at four different filament surface temperatures. It was concluded that the β-eucryptite composition of rubidium aluminosilicate gives the most satisfactory results as a Rb + ion emitter at a temperature of 1100°C.