Simultaneous Removal of NO x and Soot Over Pt–Ba/Al2O3 and Pt–K/Al2O3 DPNR Catalysts

The effect of NO _x storage on the soot combustion activity when alkaline- and alkaline/earth-containing model DPNR catalysts are used is investigated in this work. The influence of different experimental conditions (NO concentration, temperature, and particulate loading) is addressed and discussed in relation to the NO _x storage efficiency and soot oxidation activity as well.     

Flame-Made Pt/K/Al2O3 for NO x Storage–Reduction (NSR) Catalysts

High surface area Pt/K/Al₂O₃ catalysts were prepared with a 2-nozzle flame spray method resulting in Pt clusters on γ-Al₂O₃ and amorphous K storage material as evidenced by Raman spectroscopy. The powders had a high NO _x storage capacity and were regenerated fast in a model exhaust gas environment. From 300 to 400 °C no excess NO _x was detected in the off gas during transition from fuel lean to fuel rich conditions, resulting in a highly effective NO _x removal performance. Above 500 °C, the NSR activity was lost and not recovered at lower temperatures as K-compounds were partially crystallized on the catalyst.     

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

Promotional Effects of H2O Treatment on NO x Storage Over Fresh and Thermally Aged Pt–BaO/Al2O3 Lean NO x Trap Catalysts

A simple liquid water treatment applied to fresh and thermally aged Pt(2 wt%)–BaO(20 wt%)/Al₂O₃ lean NO _x trap catalysts at room temperature induces morphological and structural changes in the barium species as followed by XRD and TEM analysis. During the water treatment, liquid water sufficient to fill the catalyst pore volume is brought into contact with the samples. It was found that irrespective of the original barium chemical state (highly dispersed BaO or crystalline BaAl₂O₄), exposing the sample to this liquid water treatment promotes the formation of BaCO₃ crystallites (about 15–25 nm of its size) without changing the Pt particle size. Such transformations of the barium species are found to significantly promote NO _x uptake from 250 to 450 °C. The increase in the NO _x uptake for the water-treated samples can be attributed to an enhanced Pt–Ba interaction through the redistribution of barium species. These results provide useful information for the regeneration of aged lean NO _x trap catalysts since water is plentiful in the exhaust of diesel or lean-burn engines.     

FTIR and Transient Reactivity Experiments of the Reduction by H2, CO and HCs of NO x Stored Over Pt–Ba/Al2O3 LNTs

The reduction of NO _x stored over a Pt–Ba/Al₂O₃ Lean NO _x Trap is analysed when H₂, CO or heptane are used as reductants. In all cases, the reduction proceeds via a Pt-catalyzed process involving the formation of intermediate species like ammonia and isocyanates in the case of H₂ and CO, respectively. No specific intermediates have been observed when heptane is used as reductant. It is claimed that the role of the reductant is to keep Pt in a reduced state; this favours nitrate decomposition and reduction over the Pt sites. The effect of water on the reaction is also investigated.     

Effect of V2O5 Loading on Propane Combustion over Pt/V2O5–Al2O3 Catalysts

Propane combustion was studied on Pt(0.4%)/V₂O₅–Al₂O₃ catalysts containing up to 20% V₂O₅. The density, strength and nature of surface acid sites were determined by TPD of NH₃ and FTIR spectra of adsorbed pyridine. The sample acidity increased with the vanadium content, essentially because the addition of vanadium oxide generated Brønsted acid sites. The Pt dispersion as determined by H₂ chemisorption increased with increasing V₂O₅ loading. The sample activity for propane combustion was evaluated through both conversion versus temperature (light-off curves) and kinetically-controlled conversion versus time catalytic tests. The propane combustion turnover rate on Pt/V₂O₅–Al₂O₃ increased with the amount of vanadium, probably because the intrinsic Pt oxidation activity increases with the sample acidity.     

Coke Formation on Pt?CSn/Al2O3 Catalyst in Propane Dehydrogenation: Coke Characterization and Kinetic Study

The influences of gas compositions on the rates of coke formation over a Pt?CSn/Al₂O₃ catalyst are studied. The coke formed on the catalyst is characterized by thermal gravimetric analysis, IR spectroscopy, Raman spectroscopy and elemental analysis. Two kinds of coke are identified from the TPO profiles and assigned to the coke on the metal and the coke on the support, respectively. The coke formed on the metal is softer (containing more hydrogen) than that formed on the support. The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen pressures but increasing with the propane pressure, while the rate of coke formation on the support is increasing with the propane and propylene pressures and decreasing with the hydrogen pressure. Based on the kinetic analysis, a mechanism for the coke formation on the Pt?CSn/Al₂O₃ catalyst is proposed, and the dimerization of adsorbed C₃H₆ is identified to be the kinetic relevant step for coke formation on the metal.     

Spectrokinetic Analysis of the NOx Storage Over a Pt–Ba/Al2O3 Lean NOx Trap Catalyst

In this work the kinetics of the (reactive) lean accumulation phase of the NO_x storage-reduction process is described through a detailed kinetic model, involving both the gas-phase molecules and the adsorbed species. Kinetic data have been collected following a novel approach based on simultaneous operando spectroscopic measurements and on-line pulse reactor effluent analysis. To our knowledge, this is the first time the temporal evolutions of the concentration of both the surface and the gas species are used jointly to describe the kinetic of a transient catalytic process.     

NOx Adsorption Study over Pt–Ba/Alumina Catalysts: FT-IR and Reactivity Study

The NO _x storage process over Ba/Al₂O₃ and Pt–Ba/Al₂O₃ NSR catalysts has been analyzed in this study by performing experiments at 350 °C with NO₂ and NO/O₂ mixtures using different complementary techniques (Transient Response Method, in situ FT–IR and DRIFT spectroscopies). The collected data suggest that over the Pt–Ba/Al₂O₃ catalyst the NO _x storage process from NO/O₂ mixtures occurs forming at first nitrite species, which progressively evolve to nitrates. In addition, a parallel nitrate formation via disproportionation of NO₂ (formed upon NO oxidation) cannot be excluded.     

Effect of Acid–Base Properties on the Catalytic Activity of Pt/Al2O3 Based Catalysts for Diesel NO Oxidation

The activity of Pt catalysts supported on Al₂O₃ modified with various acid–base additives has been investigated for oxidation of NO to NO₂. Although Pt dispersion was changed by the additives, there was no clear effect of Pt dispersion on the catalytic activity. The measurement of solid acid–base properties of the modified Pt/Al₂O₃ indicated that the NO oxidation activity increased by the increase of surface density of strong acid sites and decreased by the increase of basic sites. It was suggested that platinum on the acidic supports keeps its highly active metallic state for NO oxidation, while the formation of nitrate/nitrite on the basic supports inhibits the reaction on the Pt surface.     

Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO_x by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al₂O₃ NO_x storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO_x due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO_x to main product N₂ (selectivity >70 %) and byproduct N₂O. The addition of H₂O at these temperatures has a significant promoting effect on NO_x conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH₃ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO_x by H₂ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N₂O and CO₂ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al₂O₃ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al₂O₃ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O₂ upon switching to the subsequent lean phase, as evidenced by the evolution of N₂, N₂O and CO₂. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH₃, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH₃ with stored NO_x. The data reveal more efficient utilization of CO for reducing NO_x in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO_x by CO under dry and wet conditions.     

NO–H2–CO–O2 Reactions Over Pt Catalysts Supported on Ln-incorporated FSM-16 (Ln = La, Ce and Pr)

Catalytic NO–H₂–CO–O₂ reaction was studied over Pt-supported Ln-incorporated FSM-16 (Ln = La, Ce and Pr). Pr-FSM-16 exhibited the highest activity for NO _x reduction at ≤200 °C. Pr has an effect of increasing the basicity to promote the oxidative adsorption of NO, which is a key for efficient de-NO _x .     

Influence of Indium Content on the Properties of Pt–Re/Al2O3 Naphtha Reforming Catalysts

The influence of indium on the properties of Pt–Re/Al₂O₃ catalysts used in naphtha reforming is studied. The addition of indium to the Pt–Re/Al₂O₃ catalyst produces a big decrease of acidity. It also produces an inhibition of the metal function, i.e., dehydrogenation and hydrogenolysis activity. The reaction of n-C₅ isomerization shows that indium addition decreases the total activity of the Pt–Re catalyst but increases the selectivity to the i-C₅ isomers. The selectivity to low cost light gases (C₁–C₃) is particularly decreased. The reaction of n-C₇ reforming showed that addition of indium increases the stability of the catalyst and the selectivity to aromatics, and decreases the production of light gases.     

Influence of Mn and Fe Addition on the NO x Storage–Reduction Properties and SO2 Poisoning of a Pt/Ba/Al2O3 Model Catalyst

This work deals with the effect of Mn or Fe addition on the NO _x storage–reduction properties of a Pt/Ba/Al₂O₃ model catalyst. NO _x storage capacity, SO₂ poisoning and regeneration and NO _x removal efficiency under rich/lean cycling conditions are studied. Fe addition to Pt/Ba/Al₂O₃ leads only to a small increase of NO _x storage capacity, and more interestingly, to a better sulfur removal due to the inhibition of bulk barium sulfate formation. Unfortunately, the NO _x storage property cannot be fully recovered. Moreover, Fe addition results in a decrease in the NO _x removal efficiency. Mn addition also improves the NO _x storage capacity, but no significant influence on the sulfur elimination is observed. Mn-doped catalyst does not improve the NO _x removal efficiency, but NH₃ selectivity is found to drastically decrease at 400 °C, from 20 to 3%. In addition, the NO _x conversion can be improved at higher H₂ concentration in the rich pulse, always keeping NH₃ selectivity at low level.     

Transesterification of Soybean Oil Over Me/Al2O3 (Me = Na, Ba, Ca, and K) Catalysts and Monolith K/Al2O3-Cordierite

Four different Me/Al₂O₃ (Me = Na, Ba, Ca, and K) powder catalysts prepared by incipient-wetness impregnation, and a K/Al₂O₃-cordierite monolithic catalyst produced by the dipcoating technique were used for biodiesel production. The samples were characterized and studied in the transesterification of soybean oil with methanol at 120 °C and 500 rpm, with a alcohol/oil molar ratio = 32, and a catalyst load = 1 wt% for the powder catalyst and 0.5 wt% for the monolith. The Ca/Al₂O₃, Na/Al₂O₃ and K/Al₂O₃ powder catalysts reported a FAME (fatty acid methyl esters) formation of 94.7, 97.1, and 98.9% respectively after 6 h of reaction. On the other hand, Ba/Al₂O₃ showed little activity (7.6%). The leaching of the alkali and alkaline earth metal species during reaction was important, what indicates that the activity could be explained in terms of a homogeneous–heterogeneous catalyst effect. When the monolithic sample and the powder catalyst were compared (under identical reaction conditions), the production of FAME for the latter was 89.5–59.1% for the monolithic catalyst. After two consecutive runs, the monolithic catalyst presented a partial deactivation of 8% in the FAME yield. The present work shows that the use of monolithic catalysts in the transesterification of vegetable oils is a viable alternative.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

Transient Surface Processes of Storage and Conversion of NOx Species on Pt–Me/Al2O3 Catalysts (Me = Ba,Ce, Cu)

The transient reactivity and surface phenomena of storage and conversion of NO _x species on Pt(1%)–Me/Al₂O₃ catalysts, where Me = Ba, Ce and Cu, were studied by the RWF (rectangular wavefront) method. The Me component has a relevant influence on the processes of surface storage and transformation. The reduction of NO _x by propene in the presence of oxygen is promoted by adding Cu to a Pt/Al₂O₃ catalyst, while cerium promotes transient conversion of NO in the absence of propene, but inhibits the reduction of NO _x in the presence of propene. Copper is suggested to be a promising element to add together with Ba for new NO _x storage-reduction catalysts due to its capacity to act both as a storage element and as promoter for NO _x reduction.