Pt催化丙烷脱氢过程中结焦反应的粒径效应与Sn的作用

用乙二醇还原法制备了Pt颗粒平均粒径分别为2.0、4.6、12.1 nm的Pt/Al₂O₃催化剂,同时用浸渍法制备了PtSn/Al₂O₃双金属催化剂,并考察了各催化剂在丙烷脱氢过程中的结焦行为。分别用H₂化学吸附、透射电镜、热重分析、元素分析、红外光谱、拉曼光谱等手段对催化剂进行了表征。表征结果显示,催化剂金属上的结焦速率与Pt金属颗粒粒径密切相关。具有较小Pt颗粒的催化剂金属上的结焦速率明显大于具有较大Pt颗粒的催化剂。具有较小Pt颗粒的催化剂上生成的焦含有较少的氢,其石墨化程度也较高。本研究中PtSn/Al₂O₃催化剂金属上的结焦速率高于Pt/Al₂O₃催化剂,并且在双金属上生成的焦具有更高的石墨化程度。结合Pt/Al₂O₃催化剂上的结焦机理,对高性能丙烷脱氢催化剂提出了新的概念设计。     

Correlation of the deactivation of CoMo/Al2O3 in hydrodesulfurization with surface carbon species

The deactivation of CoMo/Al₂O₃ in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was investigated under laboratory conditions that allowed the accelerated deposition of coke on the catalyst. The coke deposition was enhanced at low H₂ pressures and when naphthalene was added to the reaction solution. Characterization of deactivated catalysts by elemental analysis (EA) and temperature-programmed oxidation (TPO) identified two types of carbonaceous species deposited on the catalysts, the reactive and the refractory species. The refractory deposit, or hard coke, was a major contributor to the deactivation and, therefore, the amounts of hard coke present on the catalyst determined the overall activity. A correlation was established in this study between the activity and the amounts of deposited hard coke based on the results of accelerated deactivation treatment. A similar relation was also observed between the two parameters when the catalyst was used in an industrial process for long periods. The above findings suggest that the reaction periods of two different scales, i.e., in laboratory and industrial processes, can be correlated with each other based on the amounts of hard coke when coking is the major mechanism of catalyst deactivation.     

Differences in coke burning-off from Pt–Sn/Al2O3 catalyst with oxygen or ozone

Pt–Sn/γ-Al₂O₃ catalysts with different Sn loadings were prepared by incipient wetness coimpregnation of γ-Al₂O₃ with H₂PtCl₆ and SnCl₂. The Pt–Sn interaction was tested by temperature-programmed reduction and the catalytic activity was measured by cyclohexane dehydrogenation. The catalysts were coked by cyclopentane at 500 °C and totally or partially decoked with O₂ at 450 °C or O₃ at 125 °C. Coke deposits were studied by TPO and the catalytic activity of coked catalysts, partially or totally regenerated, by cyclohexane dehydrogenation.The TPO with O₃ shows that coke combustion with O₃ starts at a low temperature and has a maximum at 150 °C, that is a compensation between the increase of the burning rate and the rate of O₃ decomposition when increasing the temperature. Meanwhile O₂ burns coke with a maximum at 500 °C. When performing partial decoking with O₃ (125 °C) the remaining coke is more oxygenated and easier to burn than the coke that remains after decoking with O₂ (450 °C).After burning with O₃ the dehydrogenation activity of the fresh catalyst is recovered, while after burning with O₂ the activity is higher than that of the fresh catalyst. The burning with O₃ practically does not change the original Pt–Sn interaction while the burning with O₂ produces a decrease in the interaction, producing free Pt sites with higher dehydrogenation capacity.The differences in coke combustion with O₃ and O₂ are due to the different form of generation of activated oxygen, the species that oxidizes the coke. O₃ is activated by the γ-Al₂O₃ support at low temperatures firstly eliminating coke from the support while O₂ is activated by Pt at temperatures higher than 450 °C and the coke removal starts on the metal. Then, the recovery of the Pt catalytic activity as a function of coke elimination is faster with O₂ than with O₃.     

Effect of temperature on the reaction of H2S with a coke

The effect of temperature on reaction of H₂S with carbon structures of a coke were studied in a fixed-bed quartz tube reactor coupled with two parallel detectors, flame photometric detector (FPD) and mass spectrum (MS). The uptake of H₂S with the coke matrix was studied through a sulfur uptake/temperature programmed desorption process (SU/TPD) and a temperature programmed oxidation process (TPO). The results show that the sulfur imbibed by a demineralized coke at elevated temperatures is very stable, which can only be decomposed and released to gas phase under combustion conditions. The chemical imbibition of sulfur takes place at an elevated temperature. At relatively lower temperatures, H₂S was adsorbed physically by the sample and then transformed to stable sulfur species. At higher temperatures, the chemical reactions between H₂S and DM-Coke led to the formation of more stable sulfur-containing forms and consequently increased H₂S uptake ability. This is essence of the temperature effect on the uptake of H₂S by a de-mineralized coke. The irregular behavior with the temperature was caused by the different interactions.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

The chemistry of coke deposits formed on a PtSn catalyst during dehydrogenation of n-alkanes to mono-olefins

This work presents a detailed study of coke deposition on a PtSn catalyst under industrial conditions. The spent catalyst was submitted to Soxhlet extraction and the “soluble coke” was characterized by means of IR, UV and GC-MS. The catalyst before and after coke extraction was characterized by means of IR, carbon content and temperature-programmed oxidation (TPO). Coke formation on the catalyst involves several processes: (i) successive dehydrogenation/cyclization of alkyl chains (polyene route); (ii) n-alkane oligomerization; (iii) Diels-Alder-type reactions. The structure of insoluble coke is quite different from that of soluble coke: the elimination of the latter one causes disappearance of the low-temperature oxidation zone (TPO). Soluble coke contains dominant proportion of aliphatic groups whereas polyaromatic structures predominate in insoluble coke.     

Regeneration of cracking catalyst influence of the homogeneous CO postcombustion reaction

Accompanying the fast burning of coke on a cracking catalyst is the oxidation of the coke burning product, carbon monoxide. This carbon monoxide combustion involves a homogeneous and a catalytic reaction. The homogeneous reaction was isolated by injecting pulses of CO, CO₂ and O₂ into a laboratory scale reactor at controlled temperature and pressure. The coke burning and catalytic CO combustion reactions were studied by oxidizing coke deposited on a zeolitic catalyst in the same microcatalytic reactor. The role of homogeneous CO oxidation during coke combustion under these conditions was determined.     

Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studied on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂, and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst, For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane, and its behavior was explained by a change in the reaction mechanism.     

Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studio on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂ and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst. For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane and its behaviour was explained by a change in the reaction mechanism.     

On‐line addition of catalyst in high‐temperature reactors: high selectivity to olefins

By adding solutions containing catalyst salts to hot monolith catalysts during partial oxidation processes, the salt decomposes instantly and is deposited selectively near the front face. We have used this technique to deposit extremely small amounts of Pt on α-Al₂O₃ monoliths during ethane oxidation to olefins. We find that on this catalyst with H₂ addition, the selectivity to ethylene rises from ∼ to over 80% at an ethane conversion of ∼60% and at complete O₂ conversion. We also examine the addition of promoters including Sn, which gives improved performance compared to the Pt catalyst alone. This appears to be a general effect that could be useful in preparing catalysts with different loadings and distributions in high‐temperature processes. It can also be used for rapid and accurate diagnosis of catalysts and additives and to modify catalysts on‐line in situations where deactivation or catalyst loss occurs. This revised version was published online in July 2006 with corrections to the Cover Date.     

The nature of surface species on modified Pt-based catalysts for the SCR of NO by C<Subscript>3</Subscript>H<Subscript>6</Subscript> under lean-burn condition

The catalytic activity behavior for the selective catalytic reduction of NO by C₃H₆ under excess oxygen and the nature of surface species on the active sites of Pt/Al₂O₃ catalyst after adding a second metal (Fe, Sn, Co, Cr or W) were investigated. It has been found that an important role of second metals is on TONs of C₃H₆ and NO conversions and the nature of surface species produced on the catalyst surface at low temperature instead of the catalytic activity behavior towards the temperature programmed reaction. Although the introduction of each second metal distinctly disturbs the characteristic of surface species, the reaction mechanism is presumably similar. The observation of few surface species and the investigation about their reactivity indicate that few mechanisms are simultaneously proceeding at the same reaction condition.     

Dehydrogenation of Propane on Pt or PtSn Catalysts with Al2O3 or SBA-15 Support

Dehydrogenation of propane on Pt or PtSn catalyst over Al₂O₃ or SBA-15 support was investigated. The catalysts were characterized by CO-pulse chemisorption, thermogravimetry, temperature-programmed-reduction of H₂, and diffuse reflectance infrared Fourier transform spectroscopy of absorbed CO. The results show that the platinum species is in oxidation state in the catalyst on Al₂O₃ support, so the catalyst must be reduced in H₂ before dehydrogenation reaction. Addition of Sn improves the Pt dispersion, but the catalyst deactivates rapidly because of the coke formation. The interaction of Pt and Al₂O₃ is strong. On SBA-15 support, the platinum species is completely reduced to Pt⁰ in the calcination process, so the reduction is not needed. Addition of Sn improves the activity and selectivity of the catalyst. The interaction of Pt and SBA-15 is weak, so it is easy for Pt particles to sinter.     

Deactivation of VMgO x Catalysts by Coke in the Process of Isobutane Dehydrogenation with Carbon Dioxide

The process of isobutane dehydrogenation in the presence or absence of carbon dioxide was carried out over VMgO _x catalysts with different vanadium loading. The performed tests show that both the reaction atmosphere and physicochemical properties of the catalysts (related to vanadium content) have a great influence on the activity decrease and the carbonaceous deposit formation. Despite small ability of carbon dioxide to remove coke in the Boudouard reaction, the amounts of carbonaceous species deposited on the catalysts after the isobutane dehydrogenation under CO₂ atmosphere were even twice greater in comparison to those deposited in helium stream. Moreover, the rate of coke deposition during the dehydrogenation in the inert gas flow was only slightly dependent on the reaction time, in contrast to the process in carbon dioxide atmosphere. The results show that the coke formation on VMgO _x is enhanced predominantly by surface acidity of the catalysts, which grows with the vanadium content and the presence of CO₂ in the feed.     

Highly selective and stable multimetallic catalysts for propane dehydrogenation

Different mono (Pt), bi (Pt–Sn, Pt–Pb, Pt–Ga) and trimetallic (Pt–Sn–Ga) catalysts based on Pt and supported on different materials (Al₂O₃, Al₂O₃–K and ZnAl₂O₄) were tested under severe process conditions in the propane dehydrogenation reaction (both in continuous and in pulse reactors). Results show that the Pt–Sn–Ga/ZnAl₂O₄ catalyst has a better and more stable performance in propane dehydrogenation (high yield to propene and low coke deposition), than the other bi‐ and trimetallic systems and a commercial catalyst. Thus, the use of an adequate support (ZnAl₂O₄) in combination with the addition of Ga to the Pt–Sn bimetallic system enhances the catalytic performance. © 2000 Society of Chemical Industry     

The effects of Cl-induced alloying in Pt–Sn/Al2O3 catalysts on butane/H2 reactions

The effects of oxidation/reduction regeneration treatments, with and without 1,2-dichloropropane present as a chlorinating agent, on the structure of Pt(3%)–Sn(4.5%)/Al₂O₃ catalysts have been correlated with selectivities for butane/H₂ reactions. Particles of Pt⁰ fin Cl-free catalysts were partly covered by Sn⁰, but retained exposed ensembles of Pt atoms which were active for isomerisation, hydrogenolysis and dehydrogenation reactions, the latter becoming dominant at high reaction temperatures. Coking reduced Pt ensemble size and, hence, also favoured high selectivities for dehydrogenation as hydrogenolysis and isomerisation sites became poisoned. In contrast, the addition of 1,2-dichloropropane in an oxychlorination step before reduction promoted 1:1 Pt⁰–Sn⁰ alloy formation after reduction, the proportion of the total Pt in alloy being enhanced by increasing 1,2-dichloropropane concentration and oxychlorination temperature. The alloy surfaces were inactive for isomerisation and hydrogenolysis reactions, giving dehydrogenation as the sole catalytic reaction.     

FTIR study of CO adsorption on coked Pt–Sn/Al2O3 catalysts

Infrared spectra of adsorbed CO have been used as a probe to monitor changes in Pt site character induced by the coking of Pt/Al₂O₃ and Pt–Sn/Al₂O₃ catalysts by heat treatment in heptane/hydrogen. Four distinguishable types of Pt site for the linear adsorption of CO on Pt/Al₂O₃ were poisoned to different extents showing the heterogeneity of the exposed Pt atoms. The lowest coordination Pt atoms (ν(CO) < 2030 cm⁻¹) were unpoisoned whereas the highest coordination sites in large ensembles of Pt atoms (2080 cm⁻¹) were highly poisoned, as were sites of intermediate coordination (2030–2060 cm⁻¹). Sites in smaller two‐dimensional ensembles of Pt atoms (2060–2065 cm⁻¹) were partially poisoned, as were sites for the adsorption of CO in a bridging configuration. The addition of Sn blocked the lowest coordination sites and destroyed large ensembles of Pt by a geometric dilution effect. The poisoning of other sites by coke was impeded by Sn, this effect being magnified for Cl‐containing catalyst. Oxidation or oxychlorination of coked catalyst at 823 K followed by reduction completely removed coke from the catalyst surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Coke formation on an industrial reforming Pt–Sn/γ‐Al2O3 catalyst

The characterization of the coke deposited on an industrial Pt–Sn/γ‐Al₂O₃ catalyst, used in a continuous reforming process, was performed with AFM, XRD, FTIR, EPR, NMR, TG‐DTG and DTA techniques. Composition, structure and location of the coke on the catalyst were investigated. The coke was predominantly deposited on the catalyst surface and in the interstices between the catalyst particles. Its content increased along the reactor from top to bottom. Coke was deposited in the form of uniform films and clusters of three‐dimensional disks with diameters between 0.12 and 0.18 μm. It had a pseudo‐graphite structure produced by the dehydrogenation and polymerization of the aromatic precursor compounds. The coked catalyst showed a good combustion behavior; it was regenerated below 550°C. These results are important to elucidate the coke formation mechanism, to generate new continuous reforming catalysts, and to optimize the reactor operation parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Promotion effect of Sn in alumina-supported Pt catalysts for CO-PROX

In this work we have studied the effect of the addition of Sn to alumina-supported Pt catalysts towards the catalytic performance in CO-PROX reaction. Monometallic Pt and Sn catalysts supported on alumina, and bimetallic Pt–Sn supported on alumina (with Pt/Sn atomic ratios of 1.92, 0.53 and 0.28) was prepared by successive impregnation, with high dispersion of the metal. The addition of Sn to Pt does not substantially increase the activity in CO-PROX at low temperatures; however, the temperature interval where the CO conversion is maximum was significantly increased. The optimum Pt/Sn atomic ratio was found to be 0.53. In a wide operation window with respect to temperature, the catalyst with optimum Pt to Sn ratio shows a maximum CO conversion of 78% for λ = 2 with constant selectivity (about 40%) and with 31%CO yield. In the presence of either CO₂ or H₂O the performance of Sn promoted catalyst was seen to show improved activity.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Improvement of Pt/ZrO2 by CeO2 for high pressure CH4/CO2 reforming

CH₄/CO₂ reforming over Pt/ZrO₂, Pt/CeO₂ and Pt/ZrO₂ with CeO₂ was investigated at 2 MPa. Pt/ZrO₂, which shows stable activity under 0.1 MPa, and Pt/CeO₂ showed gradual deactivation with time at the high pressure. The deactivation was suppressed drastically on Pt/ZrO₂ with CeO₂ prepared by different impregnation order (co-impregnation of Pt and CeO₂ on ZrO₂, and consecutive impregnation of Pt and CeO₂ on ZrO₂). The amount of coke deposition was found insignificant and similar among all the catalysts (including Pt/ZrO₂ and Pt/CeO₂). Catalytic activity after the reaction for 24 h was in agreement with Pt particle size after the reaction for same period, indicating that the difference of the catalytic stability is mainly dependent on the extent of Pt aggregation through catalyst preparation, H₂ reduction, and the CH₄/CO₂ reforming. Pt aggregation and the amount of coke deposition were least pronounced on (Pt–Ce)/ZrO₂ prepared by impregnation of CeO₂ on Pt/ZrO₂ and the catalyst showed highest stability.