Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NO x Storage–Reduction Performance

The effect of Pt location in Pt/Ba/CeO₂ catalysts for NO _x storage–reduction (NSR) was analyzed. The Pt location on BaCO₃ or CeO₂ support was controlled by changing the angle (φ) between the two flame sprays producing these two components. As-prepared flame-made catalysts contain PtO _x which must be reduced during the fuel rich phase to become active for NO _x storage and reduction of NO _x . For Pt on BaCO₃ this process was significantly faster than for Pt on CeO₂. The increased reduction ability of Pt on Ba is reflected in the light off temperatures: for Pt on CeO₂ temperatures around 330 °C were needed to combust 20% of C₃H₆ in air while for Pt on BaCO₃ only 250 °C were required for the same conversion. The ability to control the location of Pt or other noble metals is, therefore, essential to optimize the catalysts for a given Pt/Ba/CeO₂ weight ratio. The best performance was observed when most of the Pt constituent was located near Ba-containing sites.     

Influence of Pt–Ba Proximity on NO x Storage–Reduction Mechanisms: A Space- and Time-Resolved In Situ Infrared Spectroscopic Study

The influence of Pt–Ba proximity on the performance and mechanism of NO _x storage–reduction (NSR) was investigated by a comparative study of Pt–Ba/CeO₂, Pt/CeO₂ and mechanically mixed Pt/CeO₂–Ba/CeO₂. NO _x storage capacity, regeneration activity and selectivity to nitrogen and ammonia during periodic lean (NO + O₂)–rich (H₂) cycles were evaluated and the chemical gradients along the axial direction of the catalyst beds were monitored by space- and time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The presence of Ba and its proximity to Pt greatly influenced the NSR process. In particular, the proximity was crucial to achieve better utilization of bulk Ba components as well as enhancing selectivity to N₂. The space-resolved approach is shown to be a powerful tool to understand the impact of the proximity of Pt and Ba constituents on the final NSR performance.     

Platinum–tin bimetallic nanoparticles for methanol tolerant oxygen-reduction activity

Carbon-supported Pt–Sn/C bimetallic nanoparticle electrocatalysts were prepared by the simple reduction of the metal precursors using ethylene glycol. The catalysts heat-treated under argon atmosphere to improve alloying of platinum with tin. As-prepared Pt–Sn bimetallic nanoparticles exhibit a single-phase fcc structure of Pt and heat-treatment leading to fcc Pt₇₅Sn₂₅ phase and hexagonal alloy structure of the Pt₅₀Sn₅₀ phase. Transmission electron microscopy image of the as-prepared Pt–Sn/C catalyst reveals a mean particle diameter of ca. 5.8 nm with a relatively narrow size distribution and the particle size increased to ca. 20 nm when heat-treated at 500 °C due to agglomeration. The electrocatalytic activity of oxygen reduction assessed using rotating ring disk electrode technique (hydrodynamic voltammetry) indicated the order of electrocatalytic activity to be: Pt–Sn/C (as-prepared) > Pt–Sn/C (250 °C) > Pt–Sn/C (500 °C) > Pt–Sn/C (600 °C) > Pt–Sn/C (800 °C). Kinetic analysis reveals that the oxygen reduction reaction on Pt–Sn/C catalysts follows a four-electron process leading to water. Moreover, the Pt–Sn/C catalyst exhibited much higher methanol tolerance during the oxygen reduction reaction than the Pt/C catalyst, assessing that the present Pt–Sn/C bimetallic catalyst may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell.     

Development and Application of Sintering Dynamics Simulation for Automotive Catalyst

Sintering behavior of Pt/γ-Al₂O₃, Pt/ZrO₂ and Pt/CeO₂ catalyst was studied using an originally developed 3D sintering simulator. Experimental results were well reproduced. While Pt on the γ-Al₂O₃ sintered significantly, Pt on CeO₂ presented the highest stability against sintering. On the other hand, grain growth of supports was significant in the order; ZrO₂ > CeO₂ > γ-Al₂O₃.     

Significant Improvement in Activity and Stability of Pt/TiO2 Catalyst for Water Gas Shift Reaction Via Controlling the Amount of Na Addition

Na promoted Pt/TiO₂ catalysts have been studied under high severity, near equilibrium, conditions for use as a single stage WGS catalyst. Addition of 3 wt% Na to a 1 wt% Pt/TiO₂ catalyst has been found to improve water gas shift activity significantly compared to Pt/TiO₂, Pt/CeO₂, and Pt–Re/TiO₂ catalysts. This catalyst is stable when the reaction temperature is higher than 250 °C. Deactivation occurred when the reaction temperature was lower than 250 °C, however, returning the temperature to higher than 250 °C fully recovered activity. TEM observations revealed that addition of Na inhibited Pt particle sintering. These results suggest that Na promoted Pt/TiO₂ is a promising single stage water gas shift catalyst for small scale hydrogen production.     

Catalytic Removal of Toluene over Co3O4–CeO2 Mixed Oxide Catalysts: Comparison with Pt/Al2O3

The catalytic oxidation of toluene, chosen as VOC probe molecule, was investigated over Co₃O₄, CeO₂ and over Co₃O₄–CeO₂ mixed oxides and compared with the catalytic behavior of a conventional Pt(1 wt%)/Al₂O₃ catalyst. Complete toluene oxidation to carbon dioxide and water was achieved over all the investigated systems at temperatures below 500 °C. The most efficient catalyst, Co₃O₄(30 wt%)–CeO₂(70 wt%), showed full toluene conversion at 275 °C, comparing favorably with Pt/Al₂O₃ (100% toluene conversion at 225 °C).     

Selective Production of H2 from Ethanol at Low Temperatures over Rh/ZrO2–CeO2 Catalysts

A series of Rh catalysts on various supports (Al₂O₃, MgAl₂O₄, ZrO₂, and ZrO₂–CeO₂) have been applied to H₂ production from the ethanol steam reforming reaction. In terms of ethanol conversion at low temperatures (below 450 °C) with 1wt% Rh catalysts, the activity decreases in the order: Rh/ZrO₂–CeO₂ > Rh/Al₂O₃ > Rh/MgAl₂O₄ > Rh/ZrO₂. Support plays a very important role on product selectivity at low temperatures (below 450 °C). Acidic or basic supports favor ethanol dehydration, while ethanol dehydrogenation is favored over neutral supports at low temperatures. The Rh/ZrO₂–CeO₂ catalyst exhibits the highest CO₂ selectivity up to 550 °C, which is due to the highest water gas shift (WGS) activity at low temperatures. Among the catalysts evaluated in this study, the 2wt% Rh/ZrO₂–CeO₂ catalyst exhibited the highest H₂ yield at 450 °C, which is possibly due to the high oxygen storage capacity of ZrO₂–CeO₂ resulting in efficient transfer of mobile oxygen species from the H₂O molecule to the reaction intermediate.     

Effect of the Incorporation Order of Pt- and Ba-Precursors on the Structure and Catalytic Performance of NSR Catalysts

The structural properties and the performance of two NO _X storage-reduction (NSR) catalysts prepared as a function of the deposition order of Pt- and Ba-precursors was investigated. The catalyst obtained by incorporating first Ba turned out to be more active mainly due to the larger regeneration extent achieved at temperatures below 350 °C. This behaviour was related to the larger external Pt/Ba interface exhibited by this catalyst.     

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.     

Two Reaction Pathways in the Selective Catalytic Reduction of NO x by C6H14 Over Ag–Al2O3 with H2 Co-Feeding

The NO _x reduction with n-C₆H₁₄ was studied over a 3% Ag/Al₂O₃ catalyst in the presence of hydrogen. The catalyst performance was evaluated by varying the H₂ concentration from 0 to 1600 ppm and by comparing the results with blank runs in which an empty reactor with no catalyst was used. Two distinct reaction pathways were revealed: one at low-temperature (T_react < 370 °C) and another one at high-temperature (T_react > 370–390 °C). Co-feeding of H₂ promotes the reaction within the 150–360 °C interval. The high-temperature pathway (T_react > 370–390 °C) seems to be almost independent of hydrogen co-feeding. The homogeneous gas-phase NO _x oxidation initiated by NO presumably plays an important role in this high-temperature pathway.     

Thermal ageing phenomena and strategies towards reactivation of NO x - storage catalysts

The thermal ageing and reactivation of Ba/CeO₂ and Ba/Al₂O₃ based NO _x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl₂O₄ and BaCeO₃, which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition of BaCeO₃ in an atmosphere containing H₂O/NO₂ leads again to NO _x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO₂ based model catalysts. This can be technically exploited, particularly for the Ba/CeO₂ catalysts, as reactivation studies on thermally aged Ba/CeO₂ and Ba/Al₂O₃ based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantly.     

Effects of alkaline earths and rare earths added to sol-gel alumina on the thermostability and the oxidation activity of the mixed alumina-supported Pt catalysts

Various alumina-based supports were prepared by adding metal elements such as alkaline and rare earths to sol-gel alumina synthesized from aluminium isopropoxide using 2-methylpentane-2,4-diol as a solvent and the influence of the additives on the thermostability of the supports was investigated. It was found that Ba and Sr are the most effective in improving the thermostability of the alumina, La is moderately effective while Zr, Mg and Ca are ineffective. The influence of the additives on the catalytic activities of Pt/CeO _x /MO _y -Al₂O₃ (M=Ba, Sr, La) after the calculation at 1273 K was also investigated. Pt/CeO _x /BaO-Al₂O₃ and Pt/ CeO _x /La₂O₃-Al₂O₃ showed higher catalytic activity than Pt/CeO _x /Al₂O₃, while Pt/CeO _x / SrO-Al₂O₃ showed lower catalytic activity. The activity order was Ba > La = none > Sr in terms of CO and C₂H₄ oxidation.     

CO2 effect on activity and stability of NOx storage reduction catalysts

A catalyst for NO_x storage/reduction was prepared to improve the activity of Ba–Pt/γ-Al₂O₃ by replacing Ba with a mixture of Ba and Mg. The catalyst was prepared by impregnating Pt and then co-impregnating Ba and Mg (Mg:Ba molar ratio = 1) on commercial γ-Al₂O₃. The tests have been carried out in the presence of CO₂ at temperatures between 200 and 400 °C in order to understand the role of both the feed and various alkaline-earth metals. The storage capacity of the two catalysts was different like the mechanism in the reduction process.     

Hydrogen Production with a Microchannel Reactor by Tri-Reforming; Reaction System Comparison and Catalyst Development

In this work, Pt based mono and bimetallic catalysts were tested under conditions of tri-reforming (TR). All the catalysts contained 25% of CeO₂ and a metal loading of 2.5 or 5.0% (wt%). The bimetallic catalysts contained 2.5% Pt and 2.5% of Me, where Me?=?Ni, Co, Mo, Pd, Fe, Re, Y, Cu or Zn. For all the experiments, a synthetic biogas which consisted of 60% CH₄ and 40% CO₂ (vol.) was mixed with water, S/C?=?1.0, and oxygen, O₂/CH₄?=?0.25, and fed to a fixed bed reactor (FBR) system or a microreactor. The 2.5Pt catalyst was used in order to compare the performance of each reaction system. The tests were performed at reaction temperatures between 700 and 800?°C, and at volume hourly space velocities (VHSV) between 100 L_N/(h g_cat) and 200 L_N/(h g_cat) for the FBR system and between 1000 L_N/(h g_cat) and 2000 L_N/(h g_cat) for the microreactor, at atmospheric pressure. Then, all catalysts were deposited into microchannel reactors and tested at a constant VHSV of 2000 L_N/(h g_cat) and reaction temperatures between 700 and 800?°C. Catalysts under investigation were characterized applying the following techniques: inductively coupled plasma optical emission spectroscopy (ICP-OES), N₂ Physisorption, Temperature Programmed Reduction (TPR), CO chemisorption, Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The microreactor was identified as the most efficient and promising reaction system, and the 2.5(Pt–Pd) catalyst as the bimetallic formulation with the highest activity. Therefore its activity and stability was compared with the reference 5.0Pt catalyst at 700?°C and VHSV of 2000 L_N/(h g_cat) for more than 100 h. Although slightly lower activity was measured operating with the 2.5(Pt–Pd) catalyst, a significant reduction of the Pt content compared to the reference 5.0Pt catalyst was achieved through the incorporation of Pd.     

Preparation of Highly Active Alumina-Pillared Clay Montmorillonite-Supported Platinum Catalyst for Hydrodesulfurization

Effect of Pt precursor and pretreatment on hydrodesulfurization (HDS) activity of Pt/Al-PILM catalyst was examined to prepare highly active Pt-supported HDS catalyst. The order of HDS activities of Pt/alumina-pillared clay montmorillonite (Al-PILM) catalysts prepared by various Pt precursors was Pt(C₅H₇O₂)₂ > H₂PtCl₆ · 6H₂O > [Pt(NH₃)₄](NO₃)₂ > [Pt(NH₃)₄]Cl₂ · H₂O > H₂Pt(OH)₆. This order was in accordance with that of Pt dispersion. Thus, high Pt dispersion is essential factor to prepare highly active Pt/Al-PILM catalyst for HDS reaction. On the other hand, the effect of pretreatment on the HDS activities of Pt/Al-PILM catalysts prepared by various Pt precursors was also evaluated. The UC-TPS Pt/Al-PILM catalyst showed the highest HDS activity among various pretreated Pt/Al-PILM catalysts, in which uncalcined catalyst was sulfided by temperature programmed sulfidation (TPS). We assumed that high HDS activity of UC-TPS Pt/Al-PILM catalyst is caused by partly sulfided Pt particle with high dispersion. It is concluded that the highly active Pt/Al-PILM catalyst for the HDS reaction could be prepared by using Pt(C₅H₇O₂)₂ as a precursor and UC-TPS treatment.     

Integrated DeNO x –DeSoot Catalytic Systems with Improved Low-Temperature Performance

Performance of the combined catalysts [CeO₂–ZrO₂ + FeBETA] and [Mn/CeO₂–ZrO₂ + FeBETA], prepared by mechanical mixing of the component powders, was studied in soot oxidation and NH₃-SCR. [CeO₂–ZrO₂ + FeBETA] catalyst (with component volumetric ratio of 3/1) demonstrated efficient soot oxidation at 400–450 °C and DeNO _x performance similar to that of FeBETA catalyst, despite the fact that the amount of zeolite was reduced by four times indicating a strong synergistic effect between the components. Low-temperature DeNO _x performance of the [CeO₂–ZrO₂ + FeBETA] system can be additionally enhanced by doping the CeO₂–ZrO₂ with Mn. It was found that NO _x conversion over [Mn/CeO₂–ZrO₂ + FeBETA] at 150–250 °C was steadily improved by increasing the Mn loading, and [5.4 % Mn/CeO₂–ZrO₂ + FeBETA] attained NO _x conversion above 90 % at T_react ~190 °C. Comparative studies of soot oxidation and NH₃-SCR over combined catalysts and individual components indicated that the soot oxidation activity is assigned to the soot oxidation over ceria–zirconia component, while enhanced low-temperature SCR performance is a result of a “bi-functional” SCR mechanism comprising NO oxidation to NO₂ over CeO₂–ZrO₂ component followed by fast-SCR over FeBETA.     

NOx storage capacity,SO2 resistance and regeneration of Pt/(Ba)/CeZr model catalysts for NOx-trap system

NOx storage capacity, sulphur resistance and regeneration of 1wt%Pt/Ce_0.7Zr_0.3O₂ (Pt/CeZr) and 1wt%Pt/10wt%BaO/Ce_0.7Zr_0.3O₂ (Pt/Ba/CeZr) catalysts were studied and compared to a 1wt%Pt/10wt%BaO/Al₂O₃ (Pt/Ba/Al) model catalyst submitted to the same treatments. Pt/Ba/CeZr presents the best NOx storage capacity at 400 °C in accordance with basicity measurements by CO₂ TPD and Pt/CeZr shows the better performance at 200 °C mainly due to a low sensitivity to CO₂ at this temperature. For all samples, sulphating induces a detrimental effect on NOx storage capacity but regeneration at 550 °C under rich conditions generally leads to the total recovery of catalytic performance. However, the nearly complete sulphur elimination is only observed on Pt/CeZr. Moreover, an oxidizing treatment at 800 °C leads to partial sulphates elimination on the Pt/CeZr catalyst whereas a stabilization of sulphates on Ba containing species is observed.     

Synthesis and Dispersion of Dendrimer-Encapsulated Pt Nanoparticles on γ-Al2O3 for the Reduction of NO x by Methane

Dendrimer encapsulated Pt nanoparticles were prepared by using hydroxyl terminated generation four (G4OH) PAMAM dendrimers (DEN) as the templating agents. The encapsulated Pt nanoparticles were dispersed on γ-Al₂O₃ at room temperature by impregnation. Pt/Al₂O₃ (DEN) catalysts were then subjected to thermal treatments in oxidizing and reducing atmospheres at different temperatures. These catalysts were characterized by Transmission Electron microscopy (TEM) and In situ Fourier-Transform Infrared (FTIR) spectroscopy. The TEM analysis of the as synthesized catalysts revealed that the Pt nanoparticles were found to be 2–4 nm in size. It is observed that the Pt particle size in 0.5% Pt/Al₂O₃ (DEN) catalyst increased upon thermal decomposition of the dendrimer. The in situ FTIR results suggested that the presence of oxygen and the Pt nanoparticles in the Pt-dendrimer nanocomposite accelerate the dendrimer decomposition at low temperatures. All the catalysts were tested for the reduction of NO _x with CH₄ in the temperature range of 250–500 °C. NO _x reduction efficiency of Pt/Al₂O₃ (DEN) catalysts were compared with the Pt/Al₂O₃ (CON; conventional) catalyst. The conversion of NO _x was started from the low temperatures over Pt/Al₂O₃ (DEN) catalysts. The high selectivity of NO _x to N₂ of 74% was obtained over 0.5% Pt/Al₂O₃ (DEN) catalyst at low temperatures around 350 °C.     

Effect of Ceria on the Sulfation and Desulfation Characteristics of a Model Lean NO x Trap Catalyst

The effect of ceria addition on the sulfation and desulfation characteristics of a model Ba-based lean NO _x trap (LNT) catalyst was studied. According to DRIFTS and NO _x storage capacity measurements, ceria is able to store sulfur during catalyst exposure to SO₂, thereby helping to limit sulfation of the main (Ba) NO _x storage phase and maintain NO _x storage capacity. Temperature programmed desulfation experiments revealed that desulfation of a model ceria-containing catalyst occurred in two stages, corresponding to sulfur elimination from the ceria phase at ~450 °C, followed by sulfur loss from the Ba phase at ~650 °C. Significantly, the ceria-containing catalyst displayed relatively lower sulfur evolution from the Ba phase than its non-ceria analog, confirming that the presence of ceria lessened the degree of sulfur accumulation on the Ba phase.     

In-situ XPS Study on the Reducibility of Pd-Promoted Cu/CeO2 Catalysts for the Oxygen-assisted Water-gas-shift Reaction

Cu/CeO₂, Pd/CeO₂, and CuPd/CeO₂ catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO₂ catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO₂ components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO₂ greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce³⁺ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO₂ catalysts, possibly by helping to maintain Cu in a reduced state during reaction.