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.     

Potential rare-earth modified CeO2 catalysts for soot oxidation: Part III. Effect of dopant loading and calcination temperature on catalytic activity with O2 and NO + O2

CeO₂ and CeReO_x_y catalysts are prepared by the calcination at different temperatures (y = 500–1000 °C) and having a different composition (Re = La³⁺ or Pr^3+/4+_, 0–90 wt.%). The catalysts are characterised by XRD, H₂-TPR, Raman, and BET surface area. The soot oxidation is studied with O₂ and NO + O₂ in the tight and loose contact conditions, respectively. CeO₂ sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La³⁺ or Pr^3+/4+ hinders the grain growth of CeO₂ and, thereby, improving the surface catalytic properties. Using O₂ as an oxidant, an improved soot oxidation is observed over CeLaO_x_y and CePrO_x_y in the whole dopant weight loading and calcination temperature range studied, compared with CeO₂. Using NO + O₂, the soot conversion decreased over CeLaO_x_y catalysts calcined below 800 °C compared with the soot oxidation over CeO₂_y. CePrO_x_y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O₂. The improvement in the soot oxidation activity over the various catalysts with O₂ can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO_x_y with NO + O₂ is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO_x_y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO₂ oxidation activity, that determines soot oxidation activity, is improved over all CePrO_x catalysts.     

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.     

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.     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

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.     

A new concept in high performance ceria–zirconia oxygen storage capacity material with Al2O3 as a diffusion barrier

CeO₂–ZrO₂ solid solution ((Ce,Zr)O₂) is an indispensable oxygen storage capacity (OSC) material for emission control in gasoline-fuelled automobiles. The high performance OSC material developed in this study is composed of Al₂O₃ as “a diffusion barrier” and (Ce,Zr)O₂ particles in intervening layers on a nanometer scale, and is abbreviated as “ACZ”. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of ACZ after durability testing in air at 1000 °C was 20 m²/g, which is higher than that of conventional CZ (2 m²/g) composed of (Ce,Zr)O₂ without Al₂O₃. After heat treatment at 1000 °C in air, the particle size of (Ce,Zr)O₂ in ACZ was about 10 nm and that without Al₂O₃ was one-half of the size in pure CZ. The OSC was roughly characterized by the total capacity (OSC-c1) and the oxygen release rate (OSC-r). In a fresh catalyst, ACZ and CZ had almost the same OSC-c1; however, the OSC-r of ACZ was twice as fast as CZ. After durability testing, the OSC-r of both ACZ and CZ were reduced significantly, but the OSC-r of ACZ was about five times as fast as CZ. While the OSC-c1 was hardly influenced by the (Ce,Zr)O₂ crystallite size and Pt particle size on the supports, the OSC-r was influenced by both of these parameters. The improvement of the OSC-r in the fresh catalyst and inhibition of the decrease in the OSC-r after durability testing were achieved by suppression of particle growth of (Ce,Zr)O₂ in ACZ by introducing Al₂O₃ as a diffusion barrier with resultant inhibition of sintering of Pt particles.     

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.     

Selective catalytic reduction of NO by C3H8 over CoOx/Al2O3: An investigation of structure–activity relationships

A series of CoO_x/Al₂O₃ catalysts was prepared, characterized, and applied for the selective catalytic reduction (SCR) of NO by C₃H₈. The results of XRD, UV–vis, IR, Far-IR and ESR characterizations of the catalysts suggest that the predominant oxidation state of cobalt species is +2 for the catalysts with low cobalt loading (≤2 mol%) and for the catalysts with 4 mol% cobalt loading prepared by sol–gel and co-precipitation. Co₃O₄ crystallites or agglomerates are the predominant species in the catalysts with high cobalt loading prepared by incipient wetness impregnation and solid dispersion. An optimized CoO_x/Al₂O₃ catalyst shows high activity in SCR of NO by C₃H₈ (100% conversion of NO at 723 K, GHSV: 10,000 h⁻¹). The activity of the selective catalytic reduction of NO by C₃H₈ increases with the increase of cobalt–alumina interactions in the catalysts. The influences of cobalt loading and catalyst preparation method on the catalytic performance suggest that tiny CoAl₂O₄ crystallites highly dispersed on alumina are responsible for the efficient catalytic reduction of NO, whereas Co₃O₄ crystallites catalyze the combustion of C₃H₈ only.     

Kinetic modelling of sulfur deactivation of Pt/BaO/Al2O3 and BaO/Al2O3 NOx storage catalysts

In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO_x storage performance of BaO/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts. The model is based on a previous model for NO_x storage under sulfur-free conditions. In the present model the storage of NO_x is allowed on two storage sites, one for complete NO_x uptake and one for a slower NO_x sorption. The adsorption of SO_x is allowed on both of these NO_x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO_x storage under sulfur-free conditions; (ii) SO₂ storage on NO_x storage sites; (iii) SO₂ oxidation; (iv) SO₃ storage on bulk sites; (v) SO₂ interaction with platinum in the presence of H₂; (vi) oxidation of accumulated sulfur compounds on platinum by NO₂. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO_x storage before exposure to sulfur and after pre-treatments either with SO₂ + O₂ or SO₂ + H₂. The simulations show that the model is able to describe the main features observed experimentally.     

Effect of Rh loading on the performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al₂O₃ catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH₄ and reaction products (CO, H₂, and CO₂) were observed. Comparative studies using H₂-TPR, O₂-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al₂O₃ with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO_x), that intimately interacting with the Al₂O₃ surface (RhⁱO_x) and the Rh(AlO₂)_y species formed by diffusion of rhodium oxides in to sublayers of Al₂O₃ [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H₂-TPR and O₂-TPD experiments. Among them, the first two species can be easily reduced by H₂ at temperature below 350 °C, while the last one can only be reduced by H₂ at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO_x and RhⁱO_x species can be reduced by CH₄ in the reaction mixture. Compared to the Rh/Al₂O₃ with high Rh loadings, the catalysts with low Rh loadings contain more RhⁱO_x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO_x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al₂O₃ with low Rh loadings can be related to the behaviour of Rh(AlO₂)_y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction.     

助剂Ba对Pd/Al_2O_3和Pt/Al_2O_3催化剂的C1-C3烷烃催化燃烧性能的影响

通过浸渍法制备了Al_2O_3负载的Pd和Pt催化剂,考察催化剂的甲烷、乙烷和丙烷催化燃烧活性,以及助剂Ba对催化性能的影响。对于Pd/Al_2O_3催化剂,加入Ba使活性物种PdO颗粒变大和还原温度升高,形成更稳定的PdO活性物种,是Pd-Ba/Al_2O_3催化剂活性提升的主要原因。对于Pt/Al_2O_3催化剂,加入Ba助剂使活性物种Pt0含量降低,PtO_x与Al_2O_3载体相互作用增强,使PtO_x物种更难被还原为Pt~0,导致Pt-Ba/Al_2O_3催化剂活性降低。Pd和Pt催化剂催化烷烃氧化反应活性规律一致:丙烷乙烷甲烷。Pd/Al_2O_3催化剂有利于C—H键活化,Pt/Al_2O_3催化剂有利于C—C键活化。Pt/Al_2O_3催化剂对C1-C3烷烃氧化活性的差别明显大于Pd/Al_2O_3催化剂。Pt/Al_2O_3催化剂对碳比例高的烷烃活性更高。     

Water vapor sensitivity of nanosized La(Co, Mn, Fe)1−x(Cu, Pd)xO3 perovskites during NO reduction by C3H6 in the presence of oxygen

A series of La(Co, Mn, Fe)_1−x(Cu, Pd)_xO₃ perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O₂, NO + O₂, C₃H₆, in the absence or presence of 5% H₂O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C₃H₆, 1% O₂, 0 or 10% H₂O, and 50,000 h⁻¹ space velocity. The objective was to investigate the influence of H₂O addition on catalytic behavior. A good performance (100% NO conversion, 77% N₂ yield, and 90% C₃H₆ conversion) was achieved at 600 °C over LaFe_0.8Cu_0.2O₃ under a dry feed stream. With the exposure of LaFe_0.8Cu_0.2O₃ to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N₂ yield, and 86% C₃H₆ conversion. A competitive adsorption between H₂O vapor with O₂ and NO molecules at anion vacancies over LaFe_0.8Cu_0.2O₃ was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C₃H₆ + NO + O₂ reaction. This H₂O deactivation was also verified to be strictly reversible by removing steam from the feed.     

Catalytic activity of Pd loaded on WO3/Al2O3 for NO–CH4–O2 in the presence of water vapor

Pd loaded on various kinds of monolayer supports was applied for selective reduction of NO by methane in the presence of O₂ and water vapor. Pd/WO₃/Al₂O₃ exhibited the highest conversion of NO to N₂ among Pd loaded monolayer supports. The catalyst was relatively tolerant and reversible upon the exposure of water vapor. This is due to the enhanced amount of Brønsted acid sites under the moisture as evidenced by the IR measurement of adsorbed pyridine. The Brønsted acid sites generated on WO₃/Al₂O₃ support were required to give the dispersed Pd species, similar to on the zeolite.     

Preferential oxidation of CO over CuO/CeO2 and Pt-Co/Al2O3 catalysts in micro-channel reactors

Micro-channel plates with dimension of 1 mm × 0.3 mm × 48 mm were prepared by chemical etching of stainless steel plates followed by wash coating of CeO₂ and Al₂O₃ on the channels. After coating the support on the plate, Pt, Co, and Cu were added to the plate by incipient wetness method. Reaction experiments of a single reactor showed that the micro-channel reactor coated with CuO/CeO₂ catalyst was highly selective for CO oxidation while the one coated with Pt-Co/Al₂O₃ catalyst was highly active for CO oxidation. The 7-layered reactors coated with two different catalysts were prepared by laser welding and the performances of each reactor were tested in large scale of PROX conditions. The multi-layered reactor coated with Pt-Co/Al₂O₃ catalyst was highly active for PROX and the outlet concentration of CO gradually increased with the O₂/CO ratio due to the oxidation of H₂ which maintained the reactor temperature. The multi-layered reactor coated with CuO/CeO₂ showed lower catalytic activity than that coated with Pt catalyst, but its selectivity was not changed with the increase of O₂/CO ratios due to the high selectivity. In order to combine advantages (high activity and high selectivity) of the two individual catalysts (Pt-Co/Al₂O₃, CuO/CeO₂), a serial reactor was prepared by connecting the two multi-layered micro-channel reactors with different catalysts. The prepared serial reactor exhibited excellent performance for PROX.     

Catalytic performance of Ag–Al2O3 catalyst for the selective catalytic reduction of NO by higher hydrocarbons

Silver–aluminum mixed oxide catalyst (Ag–Al₂O₃) prepared by the sol–gel method was studied for the selective reduction of NO by various alkanes in the presence of water vapor. As the carbon number of alkanes increases, the de-NO_x activity and water tolerance were markedly increased. In the case of n-octane as a reductant, the presence of water vapor markedly promoted NO reduction. The results of reaction studies and in situ IR experiment showed that the possible reasons for the promoting effect by water vapor are the inhibition of the n-octane oxidation by O₂ and the suppression of the poisoning effect caused by carboxylate and carbonate species. Among various alumina-supported transition metal catalysts, Ag–Al₂O₃ showed the highest activity for SCR by n-octane. Ag–Al₂O₃ showed higher NO conversion to N₂ and selectivity than alumina-supported Pt and Cu-ZSM-5 catalysts for the selective reduction of NO by n-octane and i-octane.     

The effect of potassium on the Ir/C3H6 + NO + O2 catalytic system

The C₃H₆ + NO + O₂ reaction has been studied in a wide range of temperatures (ca. 250–400 °C) and oxygen concentrations (0–5% O₂) over potassium-modified Ir surfaces. The in situ electrochemical controlled concept of catalysts promotion was used by interfacing a polycrystalline Ir thin film with a potassium β″-Al₂O₃ solid electrolyte disc, a K⁺ conductor. At low oxygen concentrations (i.e., at reducing conditions), the effect of potassium on the Ir activity and selectivity is negligible. However, at higher oxygen concentrations (oxidizing conditions), strong K-induced poisoning on both propene and NO turnover consumption rates, as high as 85% and 65%, respectively, were recorded. Significant reduction on the system selectivity towards N₂ was also recorded under these conditions (from 100% over K-free Ir surface to 70% on K-modified Ir surfaces). The performance of Ir under alkali promotion is dramatically different to that reported in the literature for Pt or Pd under similar conditions, where strong promotional effects have been found. This very different behaviour may be understood in terms of the electronic influence of co-adsorbed potassium on the adsorption strengths of the neighbor reactants on the Ir surface.     

Regeneration of S-poisoned Pd/Al2O3 catalysts for the combustion of methane

Regeneration of S-poisoned Pd/Al₂O₃ catalysts for the abatement of methane emissions from natural gas vehicles was addressed in this work.

Investigations were devoted to determine the temperature threshold allowing for catalyst reactivation under different CH₄ containing atmospheres. Under lean combustion conditions in the presence of excess O₂, partial regeneration took place only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Short CH₄-reducing, O₂-free pulses led to partial catalyst reactivation already at 550 °C and to practically complete regeneration at 600 °C. Also in this case reactivation was associated with SO₂ release due to the decomposition of stable support sulphates likely promoted by CH₄ activation onto the reduced metallic Pd surface. Rich combustion pulses with CH₄/O₂ = 2 were equally effective to CH₄-reducing pulses in catalyst regeneration.

These results suggest that a regeneration strategy based on periodical natural gas pulses fed to the catalyst by a by-pass line might be efficient in limiting the effects of S-poisoning of palladium catalysts for the abatement of CH₄ emissions from natural gas engine.