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1.
Activated carbon-supported Pt-Cu, Pt-Ag, Pt-Co, Pt-Fe, and Pt catalyze the formation of oligomerization products from CF2Cl2 and H2 mixture (1:1 ratio) at 523 K. All catalysts deactivate with time on stream. The Pt-Co/C catalyst exhibits the highest selectivity toward C2–C3 hydrocarbons (50%), whereas Pt-Cu/C is the most selective toward tetrafluoroethylene (20%). The other catalysts (Pt, Pt-Ag, Pt-Fe) exhibit negligible oligomerization activity, CH4 and partially halogenated C1 molecules are the main products. The performance of each catalyst is understood in terms of the difference in the stability of bimetallic particles toward segregation under dechlorination conditions.  相似文献   

2.
The interactions of H2 and H2S molecules with Pt–Pd bimetallic catalysts were investigated at the molecular level using a DFT (density functional theory) approach to better understand the structures and properties of active sites, and the relations between structural changes and sulfur resistance. It was found that when alloying the Pt catalyst with a small amount of Pd at a particular surface atomic ratio range, both H2 and H2S showed different adsorption properties compared to those on monometallic Pt or Pd catalyst. The adsorptions of both H2 and H2S were enhanced, but the adsorption energy of H2 increased more than that of H2S, indicating that the adsorption of H2S became less favorable compared with H2 on the bimetallic Pt–Pd catalyst surface. The desorption energy of hydrogen from monometallic Pt or Pd, as well as bimetallic Pt–Pd supported on zeolite, were calculated by temperature-programmed desorption (TPD), the values were compared against the DFT results to explain experimentally and theoretically why the bimetallic Pt–Pd catalyst has better sulfur resistance than monometallic Pt catalyst.  相似文献   

3.
Novel NOx storage-reduction (NOxSR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NOx storage than Pt–Ba/Al2O3 Toyota-type NOxSR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO2. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al2O3 Toyota-type NOxSR catalysts.  相似文献   

4.
The mechanism of the liquid phase methanol reforming reaction over silica supported Pt–Ru catalyst was investigated by kinetic studies, employing a pyrex glass reactor with reflux condensers connected to a closed gas circulation system under ambient pressure. The rate of H2 formation over Pt–Ru/SiO2 catalysts was more than 20 times faster than that over Pt/SiO2 catalysts with high selectivity for CO2 (72.3%), indicating a marked addition effect of Ru. In the case of HCHO–H2O reaction over Pt–Ru/SiO2, the H2 formation rate was five times larger than that in the CH3OH–H2O reaction but selectivity to CO2 was only 4%. On the contrary, in the HCOOCH3–H2O and HCOOH–H2O reactions, both high activity and selectivity were observed over Pt–Ru/SiO2. These results clearly indicate that the CO2 formation does not proceed via HCHO decomposition and following water gas shift reaction. We propose the following pathway for liquid phase methanol reforming reaction over Pt–Ru/SiO2; a partly dehydrogenated methanol (CH2OH*) is the initial reaction intermediate, from which H2 and CO2 are formed through HCOOCH3 and HCOOH as the successive reaction intermediates.  相似文献   

5.
We have investigated the catalytic behavior of Pt encapsulated TiO2 nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO2 nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH3)4](HCO3)2 complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO2 catalysts, and the selectivity for methanol in CO–H2 reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO2 (Ti3+ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti3+ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H2 reaction, similar promotion effect of Ti3+ species may be operating for selective methanol formation by supplying active OH(a).  相似文献   

6.
Performance of NOx 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 NOx 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 NOx 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 NOx trap and two model NOx 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 NOx, HC, CO, NH3 and N2O activities for the ceria-containing NOx 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 NOx traps for light-duty diesel vehicles.  相似文献   

7.
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.  相似文献   

8.
Pt–xMo/γ-Al2O3 catalysts of different molybdenum loading (2–20 wt.%) and with 1 wt.% of platinum were prepared by successive wet impregnation after intermediate calcination. The structure, morphology and surface were characterized by various methods. The DRS results indicate the presence of octahedral Mo6+ and tetrahedral Mo6+ phases. It also evidences the presence of polymeric MoOx species, responsible for the formation of a well dispersed surface sublayer and bulk MoO3 crystalline phase. XPS results after reduction and passivation of the 1Pt and 1Pt2Mo revealed the presence of residual chlorine, in the form of surface species such as [Pt(OH)xCly]s and [PtOxCly]sfavoring the formation of well dispersed platinum particles. The TPD and FTIR results are consistent with the existence of new active sites of Pt in the presence of molybdenum loading. For low Mo content there is a H2 spillover effect. These results confirm the decoration model of Pt encapsulation by partially reduced Mo species as well as H2 storage and backspillover due to the generation of a bronze compound.  相似文献   

9.
The oxidation of CH4 over Pt–NiO/δ-Al2O3 has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH4 conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH4 and O2 compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol−1) than either Pt (86.45 kJ mol−1) and NiO (103.73 kJ mol−1). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH4 partial pressure but was inhibited by O2. At low partial pressures (<30 kPa), H2O has a detrimental effect on CH4 conversion, whilst above 30 kPa, the rate increased dramatically with water content.  相似文献   

10.
The reaction intermediates formed during hydrodechlorination of CF2Cl2 catalyzed by Pd supported on AlF3 have been investigated using steady state and transient kinetics experiments. The formation of the coupling product C2H6 and its dependence on H2 partial pressure have been used to investigate the pathways by which possible surface carbene species react. Reactions of surface species formed during the CF2Cl2 hydrodechlorination with scavenging agent C2H4 yielded addition products typical of metal-carbenes. Information from these experiments suggests that for carbene and fluorocarbene species formed on the surface of a Pd/AlF3 catalyst the rates of hydrogenation vs. coupling reactions are different.  相似文献   

11.
PtSn/TiO2 catalysts containing 2 wt% Pt and a Pt:Sn atomic ratio of 2:1 and 1:1 were prepared by coimpregnation or successive impregnation method with aqueous solutions of SnCl2·2H2O and H2PtCl6·6H2O of a commercial TiO2 (P25, from Degussa). Both catalyst series, independent of the preparation method, were reduced at 473 and 773 K. XPS results show that tin was in an oxidized state after reduction at 473 K, and that a fraction was in the metallic state after reduction at 773 K. By use of in situ FTIR spectroscopy of adsorbed CO, the presence of bimetallic Pt–Sn phases was assessed after reduction at 773 K. Microcalorimetric analysis of CO adsorption enthalpy indicates that reduction at 773 K causes the appearance of a more heterogeneous distribution of active sites, as well as a loss in the amount of sites. The catalytic activity for the gas phase hydrogenation of crotonaldehyde was greatly improved when the catalysts were prepared by coimpregnation, at both reduction temperatures. The selectivity toward crotyl alcohol was higher after reduction at 773 K and independent of the preparation method, although it increased with the amount of tin, suggesting a promoting effect of tin on this reaction.  相似文献   

12.
Local structure around Pd and Pt in the bimetallic Pd–Pt catalysts supported on ultra stable Y (USY) zeolite (SiO2/Al2O3=680) was investigated by an extended X-ray absorption fine structure (EXAFS) method during oxidation, reduction, and sulfidation. The Pt L III-edge EXAFS spectra showed that a new bond that was significantly different from Pt–Pt to Pt–Pd metallic bonds was formed in the bimetallic Pd–Pt (4:1) reduced catalysts supported on USY zeolite. This new bond may reflect the ionic properties of Pt through the Pt–Pd interaction. Furthermore this new bond survived sulfidation indicating that the bond has a cationic property and sulfur-tolerance property. The Pt–Pd ionic interaction in these catalysts allows some of the Pd metal to survive as metallic phase. The existence of this metallic phase under sulfidation condition may result in high activity of Pd–Pt (4:1) catalyst supported on USY zeolite in the aromatics hydrogenation.  相似文献   

13.
A series of 1 wt.%Pt/xBa/Support (Support = Al2O3, SiO2, Al2O3-5.5 wt.%SiO2 and Ce0.7Zr0.3O2, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NOx 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 NOx 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 NOx uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NOx 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 NOx 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 NOx 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.  相似文献   

14.
The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h−1) in air was studied in this work. Experiments were carried out at both dry and wet (20,000 ppm of H2O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO2, HCl, Cl2, C2Cl4, CCl4 and CHCl3), and stability at reaction conditions.

As general trend, the activity of the catalysts decreases in the order Ru  Pd > Rh > Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C2Cl4 the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl4 and CHCl3 being the main organic by-products.

Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts.

Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes.  相似文献   


15.
A series of Pt and Pt,Cu supported catalysts were prepared by wet impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds. These novel NOx storage-reduction (NOxSR) catalysts show improved performances in NOx storage than Pt,Ba/alumina NOxSR catalysts at reaction temperatures lower than 200 °C. These catalysts show also improved resistance to deactivation by SO2. The effect is attributed to the formation of well dispersed Mg(Al)O particles which show good NOx storage properties. The promoted low temperature activity is explained by the lower basicity of the Mg(Al)O mixed oxide in comparison to BaO, which induces on one hand a lower inhibition on Pt activity (NO to NO2 oxidation and/or hydrocarbon oxidation) due to electronic effect, and on the other hand a lower thermal stability of the stored NOx. The presence of Cu slightly inhibits activity at low temperature, although improves activity and resistance to deactivation at 300 °C. On these catalysts FT-IR characterization evidences the formation of a Pt–Cu alloy after reduction.  相似文献   

16.
17.
《Catalysis Today》2002,73(3-4):343-353
Low temperature oxidation of CO over alloy type Sn–Pt/SiO2 catalysts with different Sn/Pt ratios has been investigated at different CO partial pressure using thermal programmed oxidation (TPO) technique and time on stream (TOS) experiments. The introduction of tin into platinum strongly increased the activity of the catalyst. The activity had a maximum, which depended on both the Sn/Pt (at./at.) ratio and the CO partial pressure. TOS experiments revealed the aging of the Sn–Pt/SiO2 catalysts. FTIR and Mössbauer spectroscopy has been used to follow compositional and structural changes of Sn–Pt/SiO2 catalysts during the catalytic run. The results show that the in situ formed, highly mobile “Snn+–Pt” ensemble sites are responsible for high activity, while formation of relatively stable SnOx type surface species are involved in the catalyst deactivation.  相似文献   

18.
The Pd–Pt/Al2O3 bimetallic catalysts showed high activities toward the wet oxidation of the reactive dyes in the presence of 1% H2 together with excess oxygen. Palladium was believed to act as a co-catalyst to spillover the adsorbed H2 onto the surface of the oxidized Pt surface, and thereby the reducibility of the Pt increased greatly. The organic dye molecule adsorbed on the reduced Pt surface more easily than the oxidized Pt surface under the competition with excess oxygen, which is an essential step for the catalytic wet oxidation (CWO). The Pd–Pt/Al2O3 catalysts also produced H2O2 from H2/O2 mixture, and the hydroxyl radical was formed through the subsequent decomposition of H2O2. Additional oxidation of the reactive dyes was obtained with hydroxyl radical. The high activities of the Pd–Pt/Al2O3 catalysts were believed to be due to the combined effects of the faster redox cycle resulting from the increased reducibility of Pt surface and the additional oxidation of the reactive dyes with hydroxyl radical.  相似文献   

19.
Bimetallic palladium-based supported catalysts were tested in the liquid phase hydrogenation of nitrates. They were characterised by XPS, CO chemisorption, TPD–TPR and DRIFT. The effect of the preparation method, the support, the precursors, the relative amount of active metals and their role in the formation of intermediates and products are tentatively discussed. The catalytic activity and the formation of intermediate nitrite depend on the Pd–Cu ratio. Catalysts presenting a Pd/Cu atomic ratio >1 display the highest activity and the lowest intermediate nitrite than those presenting a Pd/Cu atomic ratio <1. Sol–gel method gives catalysts with a high activity and a low nitrite formation. The Pd–Cu-based catalyst supported on zirconia is more active and selective in N2 compared to the corresponding Pd–Sn catalyst. An enrichment of the surface by Pd is responsible for a low intermediate nitrite formation and high selectivity in N2. The reduction of NO is activated on Pd–Cu catalysts, contrary to Pd–Sn catalysts. Sn promotes the formation of ammonia.  相似文献   

20.
The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO2 catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO2 catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO2 and H2O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.  相似文献   

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