Low-temperature H2-SCR of NO on a novel Pt/MgO-CeO2 catalyst

The selective catalytic reduction of NO by H₂ under strongly oxidizing conditions (H₂-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La₂O₃, MgO, Y₂O₃, CaO, CeO₂, TiO₂, SiO₂ and MgO-CeO₂). The Pt/MgO and Pt/CeO₂ solids showed the best catalytic behavior with respect to N₂ yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO₂ support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N₂ production (mols N₂/g_cat s). High NO conversions (70–95%) and N₂ selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h⁻¹ with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H₂, 5 vol% O₂ and He as balance gas. Addition of 5 vol% H₂O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO₂ during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO₂ in the feed stream following a given support pretreatment. N₂ selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO₂ catalyst in the 100–200 °C range in the presence of water and SO₂ in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO₂ catalyst towards a new NO_x control technology in the low-temperature range of 100–200 °C using H₂ as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO_x species with H₂ experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO_x intermediate species, NO_x spillover).     

The role of lanthana loading on the catalytic properties of Pd/Al2O3-La2O3 in the NO reduction with H2

The role of La₂O₃ loading in Pd/Al₂O₃-La₂O₃ prepared by sol–gel on the catalytic properties in the NO reduction with H₂ was studied. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO.

The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La₂O₃ inclusion. The selectivity depends on the NO/H₂ molar ratio (GHSV = 72,000 h⁻¹) and the extent of interaction between Pd and La₂O₃. At NO/H₂ = 0.5, the catalysts show high N₂ selectivity (60–75%) at temperatures lower than 250 °C. For NO/H₂ = 1, the N₂ selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H₂O vapor. The high N₂ selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdO_x phase interacting with La₂O₃. The formation of this phase is favored by the spreading of PdO promoted by La₂O₃. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al₂O₃ the O₂ uptake is consistent with the oxidation of PdO to PdO₂, and when La₂O₃ is present the O₂ uptake exceeds that amount (1.5 times). La₂O₃ in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N₂.     

TAP study of NOx storage and reduction on Pt/Al2O3 and Pt/Ba/Al2O3

A systematic mechanistic study of NO storage and reduction over Pt/Al₂O₃ and Pt/BaO/Al₂O₃ is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H₂ pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N₂ on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al₂O₃ is an order of magnitude higher than on Pt/Al₂O₃ showing participation of Ba in the storage even in the absence of gas phase O₂. Either oxygen spillover or transient NO oxidation to NO₂ is postulated as the first steps for NO storage on Pt/BaO/Al₂O₃. The storage on Pt/Ba/Al₂O₃ commences as soon as Pt–O species are formed. Post-storage H₂ reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H₂ pump-probe experiments. NO conversion to N₂ by decomposition is sustained on clean Pt using excess H₂ pump-probe feeds. With excess NO pump-probe feeds NO is converted to N₂ and N₂O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N₂ production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H₂ feed ratio.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Lean reduction of NO by C3H6 over Ag/alumina derived from Al2O3, AlOOH and Al(OH)3

The effectiveness of Ag/Al₂O₃ catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al₂O₃, and Al(OH)₃ was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, O₂, NO + O₂-temperature programmed desorption (TPD), H₂-temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and activity test, with a focus on the correlation between their redox properties and catalytic behavior towards C₃H₆-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C₃H₆ conversion was achieved over Ag/Al₂O₃ (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al₂O₃ (I) is deemed to be crucial for NO selective reduction into N₂. By contrast, a high C₃H₆ conversion simultaneously with a moderate N₂ yield was observed over Ag/Al₂O₃ (II) prepared from a γ-Al₂O₃ source. The larger particles of Ag_mO (m > 2) crystallites were believed to facilitate the propene oxidation therefore leading to a scarcity of reductant for SCR of NO. An amorphous Ag/Al₂O₃ (III) was obtained via employing a Al(OH)₃ source and 500 °C calcination exhibiting a poor SCR performance similar to that for Ag-free Al₂O₃ (I). A subsequent calcination of Ag/Al₂O₃ (III) at 800 °C led to the generation of Ag/Al₂O₃ (IV) catalyst yielding a significant enhancement in both N₂ yield and C₃H₆ conversion, which was attributed to the appearance of γ-phase structure and an increase in surface area. Further thermo treatment at 950 °C for the preparation of Ag/Al₂O₃ (V) accelerated the sintering of Ag clusters resulting in a severe unselective combustion, which competes with SCR of NO reaction. In view of the transient studies, the redox properties of the prepared catalysts were investigated showing an oxidation capability of Ag/Al₂O₃ (II and V) > Ag/Al₂O₃ (IV) > Ag/Al₂O₃ (I) > Ag/Al₂O₃ (III) and Al₂O₃ (I). The formation of nitrate species is an important step for the deNO_x process, which can be promoted by increasing O₂ feed concentration as evidenced by NO + O₂-TPD study for Ag/Al₂O₃ (I), achieving a better catalytic performance.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers

The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO_x/Al₂O₃ and MO_x/TiO₂ supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H₂O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts, it is shown that the presence of MO_x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al₂O₃ or Pt/TiO₂, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO_x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H₂O, 20% H₂ and 6% CO₂, showed that certain composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.     

Study of n-hexane isomerization on mixed Al2O3–ZrO2/SO4 catalysts

Mixed oxides of alumina and zirconia having a relative composition of 50, 80 and 100% Zr₂O were synthesized by means of sol–gel methods. The catalysts were sulfated with H₂SO₄ 1N, and were loaded with 0.3% Pt metal using the incipient wetness technique. The characterization of the physicochemical properties was carried out using XRD, N₂-adsorption at 78 K, and SEM. The catalytic properties of the Al₂O₃–ZrO₂ series were studied by means of dehydration of 2-propanol at 180°C and isomerization of n-hexane at 250°C, 1 atm. The sulfated solids presented a high surface acidity and a limited crystallinity, together with high activity for alcohol dehydration (i.e. 2-propanol). On the other hand, the Al₂O₃–ZrO₂ solid solutions (i.e. those having a 20–80% composition) turned out to be the most active ones for the isomerization of n-hexane.     

Investigation of CoNiMo/Al2O3 catalysts: Relationship between H2S adsorption and HDS activity

Sulfidation of trimetallic CoNiMo/Al₂O₃ catalysts was studied by thermogravimetry at 400 °C under flow and pressure conditions. Results were compared with those obtained on prepared and industrial CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts. The amount of sorbed H₂S on the sulfided solids was measured at 300 °C in the H₂S pressure range 0–3.5 MPa at constant H₂ pressure (3.8 MPa). The adsorption isotherms were simulated using a model featuring dissociated adsorption of H₂S on supported metal sulfides and bare alumina. The amount of sulfur-vacancy sites could thus be determined under conditions close to industrial practice. A relationship with activity results for thiophene HDS and benzene hydrogenation was sought for.     

Comparative study of structural properties and NOx storage-reduction behavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3

Differences in the NO_x storage-reduction (NSR) behavior of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ have been identified and traced to their different chemical and structural properties. The results show that Pt/Ba/CeO₂ exhibits inferior NO_x storage and, particularly, reduction (regeneration) activity compared to the Al₂O₃ supported catalyst. The incomplete reduction of the stored NO_x-species in Pt/Ba/CeO₂ seems to be caused by a faster and more profound reoxidation of Pt particles during the lean period as evidenced by in situ X-ray absorption spectroscopy. Interestingly, the reduction activity could be significantly improved by a pre-reduction step at mild conditions. Exposure of the Pt/Ba/CeO₂ catalyst to reducing H₂ atmosphere in the temperature range 300–500 °C lead to a moderate increase of Pt particle size which beneficially influenced the regeneration activity. In contrast, pre-reduction at temperatures above 500 °C was unfavorable and resulted in a severe decrease of the regeneration activity, probably due to migration of the partially reduced CeO₂ onto the surface of Pt particles.     

Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO–H2–O2 reactions

The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H₂ in the presence of excess O₂ (NO–H₂–O₂ reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H₂, 10% O₂, and He balance yielded N₂ with less than 10% selectivity, which could not be increased by changing Pt loading or H₂ concentration in the gas feed. Co-impregnation of NaHCO₃ and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO_x conversion (>90%) and the N₂ selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N₂ selectivity was attained by the post-impregnation of NaHCO₃ onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO₂⁻-type species, which would play a role of an intermediate to yield N₂. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H₂–O₂ reaction at low temperatures.     

Impact of redox conditions on thermal deactivation of NOx traps for diesel

Performance of NO_x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO_x trap, Pt–Ba/Al₂O₃. These were done at 950 °C for 3 h, in air and in 1% H₂/N₂, respectively. Lean aging had a severe impact on NO_x trap performance, including HC and CO oxidation, and NH₃ and N₂O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO_x trap and two model NO_x traps, Pt–Ba/Al₂O₃ and Pt–Ba–Ce/Al₂O₃. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO_x, HC, CO, NH₃ and N₂O activities for the ceria-containing NO_x traps, but had no impact on Pt–Ba/Al₂O₃. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO_x traps for light-duty diesel vehicles.     

Preparation of supported Pt-M catalysts (M = Mo and W) from anion-exchanged hydrotalcites and their catalytic activity for low temperature NO-H2-O2 reaction

The NO-H₂-O₂ reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO₂)₄²⁻, MoO₄²⁻, and/or WO₄²⁻ and subsequent heating at 600 °C in H₂. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H₂ in the absence of O₂. The Pt-HT catalyst showed the almost complete NO conversion at 70 °C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O₂, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30 °C, compared to ≥60 °C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO₃ and promote the catalytic conversion of NO to N₂O at low temperatures.     

Quantification of the in situ DRIFT spectra of Pt/K/γ-A12O3 NOx adsorber catalysts

A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO_x adsorber catalyst, Pt/K/Al₂O₃, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al₂O₃ phase with associated hydroxyl groups at the surface, and an alkalized-Al₂O₃ phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al₂O₃ and 65% for the alkalized-Al₂O₃. NO₂ and CO₂ chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al₂O₃ adsorbs 0.087 μmol CO₂/m²and 2.0 μmol NO₂/m², and Pt/K/Al₂O₃ adsorbs 2.0 μmol CO₂/m²and 6.4 μmol NO₂/m². This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al₂O₃ during both lean and rich periods of the NO_x adsorber catalyst cycle.     

Hydrodesulfurization of model diesel using Pt/Al2O3 catalysts prepared by supercritical deposition

Pt/Al₂O₃ catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO₂) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane (n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H₂]₀/[DBT]₀ by either increasing [H₂]₀ or decreasing [DBT]₀, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium.     

Optimal promotion by rubidium of the CO + NO reaction over Pt/γ-Al2O3 catalysts

The reduction of NO by CO over Rb-promoted Pt/γ-Al₂O₃ catalysts has been investigated over a wide range of temperature (ca. 200–500°C), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/γ-Al₂O₃ catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N₂-selectivity. Rate increases by factors as high as 110 and 45 for the production of N₂ and CO₂, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N₂-selectivity (e.g. from 24 to 82% at 350°C and [CO]=0.5%, [NO]=1%). Under stoichiometric conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N₂ at T350°C and at an effective reactant contact time of only 0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N₂-selectivity (approximately <40%); even at 480°C the conversion was only 60%. The observed promotional effects are ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products which lead to the observed activity enhancement and dependence of N₂-selectivity on promoter loading. The effects of K-promotion mirror those of Rb-promotion, but are significantly less pronounced. Rb is the best alkali promoter.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Syngas production from butane using a flame-made Rh/Ce0.5Zr0.5O2 catalyst

The capability of flame-made Rh/Ce_0.5Zr_0.5O₂ nanoparticles catalyzing the production of H₂- and CO-rich syngas from butane was investigated for different Rh loadings (0–2.0 wt% Rh) and two different ceramic fibers (Al₂O₃/SiO₂ and SiO₂) as plugging material in a packed bed reactor for a temperature range from 225 to 750 °C. The main goal of this study was the efficient processing of butane at temperatures between 500 and 600 °C for a micro-intermediate-temperature SOFC system. Our results showed that Rh/Ce_0.5Zr_0.5O₂ nanoparticles offer a very promising material for butane-to-syngas conversion with complete butane conversion and a hydrogen yield of 77% at 600 °C. The catalytic performance of packed beds strongly depended on the use of either Al₂O₃/SiO₂ or SiO₂ fiber plugs. This astonishing effect could be attributed to the interplay of homogeneous and heterogeneous chemical reactions during the high-temperatures within the reactor.     

Non-steady activity during methane combustion over Pd/Al2O3 and the influences of Pt and CeO2 additives

Methane combustion over Pd/Al₂O₃ catalysts with and without added Pt and CeO₂ in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al₂O₃, which was greatly enhanced over Pd–Pt/Al₂O₃. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al₂O₃ and Pd/Al₂O₃. Such non-steady behaviour was almost eliminated over Pd/Al₂O₃–CeO₂. Under a very narrow range of conditions and over a Pd/Al₂O₃ packed bed, oscillation of methane combustion was observed.