NOx storage properties of Pt/Ba/Al model catalysts prepared by different methods: Beneficial effects of a N2 pre-treatment before hydrothermal aging

The influence of a pre-treatment at 700 °C, either under a O₂/N₂ mixture or only under N₂, and followed by a hydrothermal aging at 700 °C under wet air, was studied for Pt/Ba/Al NSR model catalysts prepared by different methods: (i) successive impregnation of Ba and Pt, (ii) co-addition of Pt and Ba and (iii) barium precipitation followed by Pt impregnation. The catalysts were evaluated by NOx storage capacity measurements and were characterized by N₂ adsorption, XRD, CO₂-TPD, H₂ chemisorption and H₂-TPR. The pre-treatment under N₂ largely improves the NOx storage performance in the whole studied temperature range (200–400 °C), with or without H₂O and CO₂ in the inlet gas. The better NOx storage properties of the catalysts treated under N₂ before aging are due to: (i) a higher NO oxidation activity (mainly linked to a higher platinum dispersion), (ii) a higher number of NOx storage sites resulting from a higher barium dispersion, and consequently to (iii) a higher Pt-Ba proximity.     

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.     

Comparison of NO adsorbing ability and process on Pt/Mg/Al oxide catalysts prepared by different methods

Pt/Mg/Al metal oxide catalysts were prepared by impregnation and co-precipitation methods, respectively. These samples were characterized by BET, XRD and NO-TPD; their NO _X storage property and adsorbing intermediate species were investigated with NSC and FTIR. The results showed that the prepared methods exert significant influence on the physical structure properties and the adsorption abilities of NO. (Pt)/Mg/Al samples prepared by impregnation (IM) have larger specific areas and higher NO _X storage capacity than (Pt)/Mg/Al catalysts prepared by co-precipitation (CP). The intermediate species of NO adsorbing process indicated that NO was firstly adsorbed as bridged nitrites both on Pt/Mg/Al (IM) and on Pt/Mg/Al (CP), then on Pt/Mg/Al (IM) the nitrites transferred into monodentate and bidentate nitrate species while on Pt/Mg/Al (CP) the nitrites only transferred into monodentate nitrate species.     

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.     

Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts

This study provides insight into the effect of Pt dispersion on the overall rate and product distribution during NOx storage and reduction. The storage and reduction performance of Pt/BaO/A₂O₃ monoliths with varied Pt dispersion (3%, 8%, and 50%) and fixed Pt (2.48 wt.%) and BaO (13.0 wt.%) loadings is reported. At low temperature (<200 °C), the differences in storage and reduction activity were the largest between the three catalysts. The amount of NOx stored increased with increased dispersion, as did the amount of stored NOx that was reduced. These trends are attributed to larger Pt surface area and Pt–BaO interfacial perimeter, the latter of which enhances the spillover of surface species between the precious metal and storage components. At high temperature (370 °C), the stored NOx was almost completely regenerated for the three catalysts. However, the regeneration of the 3% dispersion catalyst was much slower, suggesting a rate limitation involving the reverse spillover of stored NOx to Pt and/or of adsorbed hydrogen from Pt to BaO. The results indicate that the catalyst dispersion and operating conditions may be tuned to achieve the desired ammonia selectivity. For the aerobic regeneration feed, the most (net) NH₃ was generated by the 50% dispersion catalyst at the lowest temperature (125 °C), by the 3% dispersion catalyst at the highest temperature (340 °C), and by the 8% dispersion catalyst at the intermediate temperatures (170–290 °C). Similar trends were observed for the net production of NH₃ with an anaerobic regeneration feed. A phenomenological picture is proposed that describes the effects of Pt dispersion consistent with the established spatio-temporal behavior of the lean NOx trap.     

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.     

Sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al₂O₃ and Pt/SrO/Al₂O₃ NO _x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO _x storage sites are regenerated when H₂ is used in combination with CO₂ compared to H₂ only.     

Model for NOx storage/reduction in the presence of CO2 on a Pt–Ba/γ-Al2O3 catalyst

We have constructed a global reaction kinetic model to better understand and describe the NO_x storage/reduction process in the presence of CO₂. Experiments were performed in a packed-bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings. The model is based on a multiple storage sites mechanism and considers that fast NO_x storage occurs at surface barium sites, as determined by the reaction kinetics. Slow NO_x storage occurs at the semi-bulk and bulk barium sites, where diffusion plays a major role. It is assumed that surface, bulk, and semi-bulk sites differ not only in physical appearance, but also in chemical reactivity. The distribution of these sites is obtained from 9-h lean-phase and 15-h rich-phase cycling experiments and thermogravimetric analysis of fresh catalyst. The model adequately describes the NO and NO₂ breakthrough profiles during 9 h of lean exposure, as well as the subsequent release and reduction of the stored NO_x. Furthermore, the model is also capable of simulating transient reactor experiments with 240-s lean-cycle and 60-s rich-cycle timings.     

不同金属离子催化剂制备氢化天然橡胶的热氧老化性能对比

采用不同金属离子催化剂，通过乳液氢化方法制备了氢化天然橡胶（HNR），研究了生胶的等温氧化诱导时间（OIT）和硫化胶热氧老化前后的拉伸性能变化，考察了金属离子催化剂对HNR热氧老化性能的影响。结果表明，过氧化氢/水合肼/铜离子催化体系中的铜离子对材料的热氧老化性能影响较大，采用铜离子制备的HNR热氧老化后拉伸性能变化率高于天然胶乳，OIT缩短至2.5 min。将铜离子替换为锌离子后制备的HNR，100 ℃热氧老化72 h后拉伸强度变化率减小至23.91%，扯断伸长率变化率为32.12%，150 ℃ OIT延长至32.4 min，具有更好的热氧老化性能。     

硅担载的铜基催化剂制备方法对乙醇一步法合成乙酸乙酯催化性能的影响

采用离子交换法(IE)和浸渍法(IM)制备了Cu/SiO_2催化剂,考察了两种方法合成的催化剂对乙醇脱氢合成乙酸乙酯反应的催化性能的影响,结果表明,在催化反应中IE-Cu/SiO_2催化剂的催化性能和IM-Cu/SiO_2催化剂对比更佳。同时,催化剂进行了XRD、TPR、TPD、FTIR表征,结果表明,离子交换法制备的催化剂,铜物种高分散在载体表面上,而且,铜与载体具有较强相互作用,这部分较难还原的Cu~(2+)以及Cu+可以作为L酸中心,为催化剂表面提供一定的酸量。     

Catalytic wet air oxidation of phenol aqueous solutions by 1% Ru/CeO2–Al2O3 catalysts prepared by different methods

The wet air oxidation of phenol aqueous solutions (5 g/L) has been studied in a trickle bed reactor (140 °C and 7 atm of oxygen pressure). The experiments were performed over several 1%Ru/5%CeO₂–Al₂O₃ samples, prepared by different methods: impregnation, co-impregnation, co-precipitation and surfactant. Phenol conversion and chemical oxygen demand were evaluated. Results indicate that performance of the catalysts is influenced by the preparation method. Phenol conversions diminished with reaction time; deactivation was attributed to the deposition of carbonaceous solids on the surface and to the transformation of the support into boehmite phase. The selectivity toward CO₂ production was, in all cases, nearly complete.     

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.     

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.     

Characterisation by TPR,XRD and NO x storage capacity measurements of the ageing by thermal treatment and SO2 poisoning of a Pt/Ba/Al NO x -trap model catalyst

Topics in Catalysis - The deactivation of a Pt/Ba/Al2O3 NO x -trap model catalyst submitted to SO2 treatment and/or thermal ageing at 800&nbsp;°C was studied by H2 temperature programmed...     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

FT-IR study of NO X storage mechanism over Pt/BaO/Al2O3 catalysts. Effect of the Pt–BaO interaction

NO _x storage mechanism over a model NSR catalyst has been analysed by means of in-situ FTIR. The results indicated that a two-step mechanism involving nitrite formation, without requirement of NO evolution to NO₂, followed by oxidation to nitrate species, being both steps assisted by O₂, would describe the overall process at 350 °C. This mechanism could be also extended to a wider temperature range. The interaction between Pt and Ba sites was crucial in this mechanism, since spillover process of oxidising agents appeared to play a key role. NO₂ direct interaction with BaO surface may also occur, but this process was only dominant on Ba sites away from Pt interaction.     

CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods

A composition of Cu/ZnO/Al₂O₃ catalysts prepared by the impregnation method was optimized for water gas shift reaction (WGSR) coupled with CO oxidation in the reformed gas. The optimum composition of the impregnated catalyst for high WGSR activity was 5 wt.% Cu/5 wt.% ZnO/Al₂O₃. The optimum loading amounts of Cu and ZnO in the impregnated catalyst were smaller than those in the coprecipitated catalyst. Its catalytic activity above 200 °C was comparable to that of the conventional coprecipitated Cu/ZnO/Al₂O₃ catalyst. However, the activity of the impregnated Cu/ZnO/Al₂O₃ catalysts was significantly lowered at 150 °C, whereas no deactivation was observed for the coprecipitated catalyst at the same temperature. It was found that deactivation occurred over impregnated catalysts with H₂O and/or O₂ in the reaction gas; it prevented CO adsorption on the surface.     

The effects on the field emission properties of silicon nanowires by different pre-treatment techniques of Ni catalysts layers

Silicon nanowires (SiNWs) were synthesized directly from silicon substrates via a catalytic reaction by thermal CVD system. Ni catalyst layers were deposited on silicon substrates by RF sputtering and electroless-plating pre-treatment techniques, respectively. It was found that the average diameters, lengths, and growth densities of SiNWs by the RF sputtering pre-treatment technique was larger than that by the electroless-plating pre-treatment technique. Furthermore, a better and more stable emission property of the SiNWs was also observed by the RF sputtering pre-treatment technique. Therefore, it was then concluded that the growth mechanism and the emission properties were both strongly influenced by the Ni catalyst layers of different pre-treatment techniques.     

Soot oxidation on a catalytic NOx trap: Beneficial effect of the Ba–K interaction on the sulfated Ba,K/CeO2 catalyst

The Ba,K/CeO₂ catalyst is active both for NO_x trapping and soot combustion. In this work we report a Ba–K interaction that prevents K sulfation when NO_x is present, thus preserving the activity of K towards soot combustion during the working period of the trap. This effect is originated in the K₂SO₄(s) + Ba(NO₃)₂(s) → 2KNO₃(s) + BaSO₄(s) reaction, which is thermodynamically favored. In the absence of NO_x, the soot combustion reaction is strongly depressed by SO₂ whereas when NO_x is present both the sulfated and the non-sulfated catalysts present similar TPO patterns.     

Structure,properties and roles of the different constituents in Pt/WOx/ZrO2 catalysts

Six samples of platinum-promoted tungstated zirconia catalysts (Pt_0.5/WO_x/ZrO₂) with W loadings between 6.5 and 12.5 wt.% are investigated using X-ray diffraction (XRD) and magnetic susceptibility measurements (4 to 350 K). Studies are also carried out on some of these samples after annealing them in air at temperatures up to 1000 °C. In the as-prepared samples, the absence of any lines due to WO₃, irrespective of W loading, suggests the highly dispersed state of WO₃. However, the dispersed WO₃ appears to be necessary to stabilize the tetragonal phase of zirconia (t-ZrO₂). Ex-situ XRD studies show that on heating the samples to 1000 °C, the fraction of m-ZrO₂ (monoclinic) increases, with the simultaneous appearance of crystalline m-WO₃. This leads us to infer that the dispersed WO_x species are associated with t-ZrO₂ only. By comparing the magnitude and the temperature variations of the magnetic susceptibility χ of the samples with those of Pt, α-PtO₂, Pt₃O₄, WO₃ and ZrO₂, we infer that Pt in the as-prepared catalysts is primarily in the oxidized form, α-PtO₂ and/or Pt₃O₄, relative magnitudes of the two oxides being dependent on the preparation procedures, thermal treatments and aging. The oxides are converted to Pt in reducing atmosphere.