Decomposition of CCl2F2 over metal sulfate catalysts

Activity for hydrolysis of CCl₂F₂ (CFC12) on various metal sulfate was investigated. Zr(SO₄)₂ was found to be the most active while FeSO₄, Cr₂(SO₄)₃, Al₂(SO₄)₃, La₂(SO₄)₃ and Ce₂(SO₄)₃ had intermediate activity. MnSO₄, CoSO₄, and MgSO₄ showed low activity and SrSO₄, CaSO₄, and BaSO₄ had even less activity. The major carbon containing product was CO₂ and small amount of CClF₃ and CO were formed over several sulfates. The crystal structure of the sulfates was stable during decomposition of CCl₂F₂, and the conversion reached a steady state after initial decrease at 275 °C over Zr(SO₄)₂ catalyst. The concentration of surface hydroxyl was larger than that over AlPO₄-based catalysts and a reaction mechanism similar to that over AlPO₄-based catalysts was proposed.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

Effect of the aluminium fluoride phase for the Cl/F exchange reactions in CCl2F2 (CFC-12) and CHClF2 (HCFC-22)

Two different aluminium trifluorides (-AlF₃ and γ-AlF₃) of high-area have been prepared, characterised by XRPD, N₂ physisorption, IR, X-ray fluorescence, TPD and SEM techniques, and tested for the Cl/F exchange reaction of CCl₂F₂ (CFC-12) and CHClF₂ (HCFC-22) in the gas phase.

Catalyst -AlF₃ is more active than catalyst γ-AlF₃ for both reactions. This is due to both its higher amount of Lewis acid sites, as deduced from ammonia-TPD and pyridine FT-IR studies, and its higher TOF values, which favours the Cl/F exchange in CFC and HCFC compounds. The main products obtained are those which result from the exchange of one chlorine by one fluorine, CClF₃ and CHF₃ for the exchange reaction of CCl₂F₂ and CHClF₂, respectively.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

Non-steady activity during methane combustion over Pd/Al2O3 and the influences of Pt and CeO2 additives

Methane combustion over Pd/Al₂O₃ catalysts with and without added Pt and CeO₂ in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al₂O₃, which was greatly enhanced over Pd–Pt/Al₂O₃. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al₂O₃ and Pd/Al₂O₃. Such non-steady behaviour was almost eliminated over Pd/Al₂O₃–CeO₂. Under a very narrow range of conditions and over a Pd/Al₂O₃ packed bed, oscillation of methane combustion was observed.     

Comparison of catalytic activity and selectivity of Pd/(OSC + Al2O3) and (Pd + OSC)/Al2O3 catalysts

The effect of the Pd addition method into the fresh Pd/(OSC + Al₂O₃) and (Pd + OSC)/Al₂O₃ catalysts (OSC material = Ce_xZr_1−xO₂ mixed oxides) was investigated in this study. The CO + NO and CO + NO + O₂ model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 °C lower for the fresh Pd/(OSC + Al₂O₃) catalyst than for the fresh (Pd + OSC)/Al₂O₃ catalyst during the CO + NO + O₂ reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than Pd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger Pd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al₂O₃) catalyst was higher than that of the (Pd + OSC)/Al₂O₃ catalyst.     

Enhanced activity of metal oxide-doped Ga2O3–Al2O3 for NO reduction by propene

Effect of additives, In₂O₃, SnO₂, CoO, CuO and Ag, on the catalytic performance of Ga₂O₃–Al₂O₃ prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H₂O, CoO, CuO and Ag showed good additive effect. When H₂O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga₂O₃–Al₂O₃ was depressed considerably, while an intensifying effect of H₂O was observed for In₂O₃- and SnO₂-doped Ga₂O₃–Al₂O₃. Of several metal oxide additives, In₂O₃-doped Ga₂O₃–Al₂O₃ showed the highest activity for NO reduction by propene in the presence of H₂O. Kinetic studies on NO reduction over In₂O₃–Ga₂O₃–Al₂O₃ revealed that the rate-determining step in the absence of H₂O is the reaction of NO₂ formed on Ga₂O₃–Al₂O₃ with C₃H₆-derived species, whereas that in the presence of H₂O is the formation of C₃H₆-derived species. We presumed the reason for the promotional effect of H₂O as follows: the rate for the formation of C₃H₆-derived species in the presence of H₂O is sufficiently fast compared with that for the reaction of NO₂ with C₃H₆-derived species in the absence of H₂O. Although the retarding effect of SO₂ on the activity was observed for all of the catalysts, SnO₂–Ga₂O₃–Al₂O₃ showed still relatively high activity in the lower temperature region.     

ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming

Surface-phase ZrO₂ on SiO₂ (SZrOs) and surface-phase La₂O₃ on Al₂O₃ (SLaOs) were prepared with various loadings of ZrO₂ and La₂O₃, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH₄/CO₂ reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al₂O₃ and Pt/SiO₂ catalysts. CO₂ or CH₄ pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.

The zirconia can be homogeneously dispersed on SiO₂ to form a stable surface-phase oxide. The lanthana cannot be spread well on Al₂O₃, but it forms a stable amorphous oxide with Al₂O₃. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO₂ and Pt/Al₂O₃ by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO₂ and Pt/Al₂O₃ catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO₂ adsorbed on the basic sites of SZrOs and SLaOs.     

Transient studies of carbon dioxide reforming of methane over Pt/ZrO2 and Pt/Al2O3

The mechanism of the CO₂ reforming of methane reaction over the Pt/ZrO₂ catalyst was investigated using a temporal analysis of products (TAP) reactor system. For comparative purposes, the reaction pathway using a Pt/Al₂O₃ catalyst was also examined. A reaction sequence is suggested for both catalysts. Over both catalysts, methane decomposition takes place over platinum. The main difference between the two catalysts concerns the carbon dioxide dissociation. Over Pt/Al₂O₃ this step is assisted by hydrogen. Over Pt/ZrO₂ this step takes place over the zirconia support and involves surface vacancies. Moreover, large pools of formate and carbonate species are present on the zirconia. Transient studies conducted to determine the origin of carbon species accumulated during CO₂ reforming revealed that more than 99% of the carbon was derived from the methane molecule over both catalysts. Over the Pt/ZrO₂ catalyst, only a single very reactive carbon species was detected, while over the Pt/Al₂O₃ a second less active species was also formed.     

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。     

Decomposition of chlorofluorocarbons in the presence of water over zeolite catalyst

The decomposition of chlorofluorocarbons (CFCs) in the presence of water was examined over a variety of solid acid catalysts. More than 40% of the conversion of CFC was observed on HY zeolite, H-mordenite, H-ZSM-5, γ-Al₂O₃, and SiO₂TiO₂ catalysts, and the selectivity to CO and CO₂ was nearly 100% except on γ-Al₂O₃. Although the H-mordenite had the highest activity among the tested catalysts, it was gradually deactivated during the reaction due to the elimination of Al atoms from the zeolite framework. A good relationship was found between the reactivity on H-mordenite and the bond energy of CCl in compounds of CCl₄, CCl₃F, CCl₂F₂, and CClF₃, suggesting that the rate controlling step was the cleavage of the CCl bond.     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.     

Catalytic hydrodehalogenation of halon 1211 (CBrClF2) over γ-alumina-supported Ni, Pd and Pt catalysts

The hydrodehalogenation of halon 1211 over Ni, Pd and Pt supported on γ-alumina was studied. The effect of reaction temperature and hydrogen/halon 1211 input ratio was examined. Steady state catalytic activities of Pd and Pt are very similar, and much higher than that of Ni. Hydrodehalogenation over Pd favours the formation of CH₂F₂, while Pt and Ni produce primarily CH₄ and CH₃F, respectively. Product profiles suggest that the hydrodehalogenation of halon 1211 over Pd and Pt follows a reaction mechanism which is similar to hydrodehalogenation of CFC-12. Over Ni catalysts, the primary hydrodehalogenation reactions are those which first remove one halogen (Cl or Br) and then remove two halogens (one F and either Br or Cl). The higher mobility of hydrogen atom on the surface of Pt is consistent with the observation that only trace amounts of C₂₊ hydrocarbons were detected when Pt was used compared with Pd and Ni. Increasing selectivity to CH₂F₂ with time on stream over Pd and Pt is mainly ascribed to the transformation of support alumina to partially fluorinated alumina.     

Transient kinetics investigations of reaction intermediates involved in CF2Cl2 hydrodechlorination

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.     

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.     

Aromatization of hydrocarbons over Pt- and Pt-Re-containing zeolite-Al2O3 catalysts

The typical reaction of ethane in a hydrogen flow over Pt-Re/ZSM-Al₂O₃ catalysts in hydrogenolysis of the C₂H₆ molecule. Hydrogenolysis activity is thereby enhanced by a high dispersion of the metal phase. However, in a helium atmosphere, the formation of aromatics can be observed which reaches a maximum for ZSM contents between 50 and 70 wt % and surpasses the hydrogenolysis activity of the pure Pt-Re/ZSM catalyst.

The utilization of this ability to aromize during the conversion of higher hydrocarbons is hindered by the high hydrocracking activity of the zeolites. However, by adding small amounts of both a ZSM and a Y type zeolite to Al₂O₃, the aromatization activity and selectivity in the conversion of n-hexane and n-heptane in the presence of about 300 ppm water vapour can be improved.     

CO2 hydrogenation over Pd-modified methanol synthesis catalysts

The effect of palladium incorporation on the performance of Cu–ZnO(Al₂O₃) during the hydrogenation of carbon dioxide has been assessed. Temperature-programmed reduction profiles and X-ray photoelectron spectra of copper revealed that Pd enhances copper oxide reduction. Carbon dioxide conversion and methanol yield were found to increase on Pd-loaded catalysts. The importance of the palladium incorporated to the base Cu–ZnO(Al₂O₃) catalyst in determining the catalytic activity is discussed in terms of the relative ease with which hydrogen is dissociated on the Pd particles and then spilt over the Cu–ZnO phase of the base catalyst.     

Comparison of supports for the direct synthesis of hydrogen peroxide from H2 and O2 using Au–Pd catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using a range of supported Au–Pd alloy catalysts is compared for different supports using conditions previously identified as being optimal for hydrogen peroxide synthesis, i.e. low temperature (2 °C) using a water–methanol solvent mixture and short reaction time. Five supports are compared and contrasted, namely Al₂O₃, -Fe₂O₃, TiO₂, SiO₂ and carbon. For all catalysts the addition of Pd to the Au only catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Of the materials evaluated, the carbon-supported Au–Pd alloy catalysts give the highest reactivity. The results show that the support can have an important influence on the synthesis of hydrogen peroxide from the direct reaction. The effect of the methanol–water solvent is studied in detail for the 2.5 wt% Au–2.5 wt% Pd/TiO₂ catalyst and the ratio of methanol to water is found to have a major effect on the rate of hydrogen peroxide synthesis. The optimum mixture for this solvent system is 80 vol.% methanol with 20 vol.% water. However, the use of water alone is still effective albeit at a decreased rate. The effect of catalyst mass was therefore also investigated for the water and water–methanol solvents and the observed effect on the hydrogen peroxide productivity using water as a solvent is not considered to be due to mass transfer limitations. These results are of importance with respect to the industrial application of these Au–Pd catalysts.     

Tetralin hydrogenation catalyzed by Mo2C/Al2O3 and WC/Al2O3 in the presence of H2S

Supported molybdenum and tungsten carbides were synthesized by temperature-programmed reactions. These materials were characterized by XRD, EDS analysis, HRTEM and CO chemisorption. Hydrogenation of tetralin was carried out at a total pressure of 4 MPa (3.06 MPa of H₂), at 573 K, without or with sulfur (200 ppm of sulfur as DMDS). The resulting activities were compared with those of MoS₂/Al₂O₃ and Pt (1% (w/w) metal loading) supported on Al₂O₃ or SiO₂. In the absence of sulfur, WC/Al₂O₃ showed an initial activity similar to that of Pt/SiO₂, higher than that of MoS₂/Al₂O₃ but lower than that of Pt/Al₂O₃. In the presence of H₂S, WC/Al₂O₃ showed a steady-state activity similar to that of Pt/Al₂O₃ (which suffered a marked deactivation). Post-reaction characterization did not show any sulfur poisoning of the supported carbides. Therefore the supported carbides are sulfur-tolerant and promising catalysts for the hydrogenation of aromatics in diesel fuels in the presence of small amounts of S-containing compounds.     

Pd/Ce0.6Zr0.4O2/Al2O3 as advanced materials for three-way catalysts: Part 1. Catalyst characterisation, thermal stability and catalytic activity in the reduction of NO by CO

Pd-loaded Ce_0.6Zr_0.4O₂ solid solutions supported on Al₂O₃ are investigated as catalysts for the reduction of NO by CO. The attention is focused on the role of the Ce_0.6Zr_0.4O₂ and of the Pd dispersion on the catalytic activity. The system shows a very high activity below 500 K, which is almost independent on the Pd dispersion. The high activity is attributed to a promoting effect of the Ce_0.6Zr_0.4O₂ on the NO conversion. Investigation of the influence of high temperature treatments disclosed a thermal stabilisation of both Ce_0.6Zr_0.4O₂ and Al₂O₃ in the Ce_0.6Zr_0.4O₂/Al₂O₃ system.