Catalytic oxidation of saturated hydrocarbons on multicomponent Au/Al2O3 catalysts: Effect of various promoters

The performance of unpromoted and MO_x-(M: alkali (earth), transition metal and cerium) promoted Au/Al₂O₃ catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH₄ (above 400 °C), compared with C₃H₈ (above 250 °C). The addition of various MO_x to Au/Al₂O₃ improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al₂O₃ are FeO_x and MnO_x. For C₃H₈ oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation.     

The nature of metal oxide on adsorptive and catalytic properties of Pd/MeOx/Al2O3 catalysts

The role of ceria, niobium and molybdenum oxides on the promotion of the NO reduction by CO was studied. A bifunctional mechanism was discussed as a function of both the nature of interaction between metal oxide and palladium and the redox properties of each metal oxide.

The NO dissociation was better on the Pd/MoO₃/Al₂O₃ catalyst than on the Pd/CeO₂/Al₂O₃ and Pd/Nb₂O₅/Al₂O₃ catalysts. The explanation for the very high N₂ production on Pd–Mo catalyst during the TPD analysis may be attributed to the NO+Me^δ+ stoichiometric reaction.

The promoting effect of a reducible oxide for the NO+CO reaction at low temperature can be ascribed mainly to its easiness for a redox interchange and its interaction with the noble metal particles. This would increase the surface redox ability and favor the dynamic equilibrium needed for high N₂ selectivity.     

Promotion of palladium-based catalysts on metal monolith for partial oxidation of methane to syngas

Four different modifications of alumina were prepared for use as the support for a Pd catalyst used for the partial oxidation of methane to syngas. The catalysts were washcoated on a metallic monolith in order to determine their activities at high gas flow rates. Compared with the Pd/Al₂O₃ catalyst, enhanced partial oxidation activities were observed with the Pd/CeO₂/Al₂O₃, Pd/CeO₂/BaO/Al₂O₃ and Pd/CeO₂/BaO/SrO/Al₂O₃ catalysts. The palladium particles were better dispersed in the presence of CeO₂ and SrO. Adding BaO, CeO₂ and BaO–CeO₂ to γ-Al₂O₃ prevented the transformation of the alumina phase during the 3-day aging process at 1000 °C, providing the support with some level of thermal stability. The addition of small amounts of SrO to the CeO₂/BaO/Al₂O₃ support enhanced the thermal stability of the Pd particles and minimized their sintering. The triply promoted Pd catalyst studied in this work was effective in carrying out partial oxidation at high temperatures, with BaO and CeO₂ promoting the thermal stability of the support, CeO₂ and SrO dispersing the Pd particles and SrO anchoring the Pd particles strongly to the support. The composition of the catalyst which gave both the highest partial oxidation activity and the best thermal stability was Pd(2)/CeO₂(23)/BaO(11)/SrO(0.8)/Al₂O₃.     

Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

In situ measurements under flow condition of the CO oxidation over supported gold nanoparticles

In situ FT-IR measurements for Au/TiO₂ and Au/Al₂O₃ have been carried out under the flow condition of CO oxidation at atmospheric pressure. It has been found that the Au particles remain neutral (Au⁰) in the presence of oxygen, while negatively charged particles (Au^δ−) is formed in the absence of oxygen, as a result of the charge transfer from the oxygen vacancies. Moisture did not significantly affect the adsorption states of CO over Au/TiO₂ and Au/Al₂O₃. Enhancement of the CO₂ production by moisture was observed over Au/Al₂O₃, which is accompanied by the decomposition of carbonate-like species by moisture.     

Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2O3 for the combustion of CO, ethyl acetate and ethanol

Combustion of CO, ethyl acetate and ethanol was studied over CuO_x/Al₂O₃, CuO_x–CeO₂/Al₂O₃, CuMn₂O₄/Al₂O₃ and Mn₂O₃/Al₂O₃ catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO_x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO_x–CeO₂/Al₂O₃ catalyst is more active than the CuMn₂O₄/Al₂O₃ catalyst for the oxidation of CO but the CuMn₂O₄/Al₂O₃ catalyst is more active for the combustion of ethyl acetate and ethanol.

Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuO_x–CeO₂/Al₂O₃ and CuMn₂O₄/Al₂O₃ catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed.

Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn₂O₄ and Mn₂O₃, respectively, on Al₂O₃.     

Influence of BaO on Pd/Al2O3-based catalysts in C2H4 and CO oxidation as well as in NO reduction

In this study, Pd/Al₂O₃ and Pd/BaO/Al₂O₃ metallic monoliths were used to investigate the effect of BaO in C₂H₄ and CO oxidation as well as in NO reduction. A FT-IR gas analyser was used to study the activity of the catalysts. Several activity experiments carried out with dissimilar feedstreams revealed that BaO enhances CO and C₂H₄ oxidation as well as NO reduction reactions in rich conditions. This effect is due to BaO, which causes a decrease in the ethene poisoning of palladium. In lean conditions BaO is present in the form of Ba(OH)₂ which reacts with oxidised NO releasing water. Therefore, NO was stored during the lean reaction.     

Supported base metal catalysts for the preferential oxidation of carbon monoxide in the presence of excess hydrogen (PROX)

Supported base metal catalysts were tested for the preferential oxidation of CO (CO PROX). The catalysts we investigated covered a wide range of transition metals (Co, Cr, Cu, Ni, Zn) supported on oxides with very different acidic, basic and redox properties (MgO, La₂O₃, SiO₂–Al₂O₃, CeO₂, Ce_0.63Zr_0.37O₂). The influence of the metal loading (Cu), the support properties (acidity, basicity, redox, surface area) and the reaction conditions (reaction temperature, feed composition) on the catalyst activity and selectivity was evaluated. The activity of ceria and ceria–zirconia supported copper catalysts was comparable to the performances of noble metal samples classically used for the PROX reaction. In addition, Cu–CeO₂ catalysts showed a practically constant and high selectivity towards CO oxidation in the temperature range of 50–150 °C. Due to the strong synergetic effect between copper and ceria, only a small amount of copper (0.3 wt.%) was necessary to get an active catalyst. The best catalytic performances were obtained for the samples containing 1–3 wt.% copper. The presence of small copper particles in close interaction with the ceria support was shown to be responsible for the enhanced activity. Except for the hydrogen oxidation, no parallel reactions (CO or CO₂ methanation reactions, coking, RWGS) could be detected over these catalysts. Classically, an increase of the oxygen excess led to an increased CO conversion with a simultaneous loss of selectivity towards CO₂. Finally, the presence of CO₂ in the feed negatively affected the catalytic activity. This effect was attributed to the adsorption of CO₂ on the copper sites, probably as CO.     

Pd–Ce interactions and adsorption properties of palladium: CO and NO TPD studies over Pd–Ce/Al2O3 catalysts

We have examined the adsorption of CO and NO on powder Pd/Al₂O₃, Pd–Ce/Al₂O₃ and CeO₂/Al₂O₃ catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al₂O₃ TPD profiles are identical to those observed for Pd/Al₂O₃, suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al₂O₃ and Pd–Ce/Al₂O₃. On oxidized catalysts, Pd/Al₂O₃ is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al₂O₃ and react to N₂O and N₂. In comparison with Pd/Al₂O₃, reduced Pd–Ce/Al₂O₃catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N₂ over N₂O.     

Total oxidation of propene and propane over gold–copper oxide on alumina catalysts: Comparison with Pt/Al2O3

Oxidation of propene and propane to CO₂ and H₂O has been studied over Au/Al₂O₃ and two different Au/CuO/Al₂O₃ (4 wt.% Au and 7.4 wt.% Au) catalysts and compared with the catalytic behaviour of Au/Co₃O₄/Al₂O₃ (4.1 wt.% Au) and Pt/Al₂O₃ (4.8 wt.% Pt) catalysts. The various characterization techniques employed (XRD, HRTEM, TPR and DR-UV–vis) revealed the presence of metallic gold, along with a highly dispersed CuO (6 wt.% CuO), or more crystalline CuO phase (12 wt.% CuO).

A higher CuO loading does not significantly influence the catalytic performance of the catalyst in propene oxidation, the gold loading appears to be more important. Moreover, it was found that 7.4Au/CuO/Al₂O₃ is almost as active as Pt/Al₂O₃, whereas Au/Co₃O₄/Al₂O₃ performs less than any of the CuO-containing gold-based catalysts.

The light-off temperature for C₃H₈ oxidation is significantly higher than for C₃H₆. For this reaction the particle size effect appears to prevail over the effect of gold loading. The most active catalysts are 4Au/CuO/Al₂O₃ (gold particles less than 3 nm) and 4Au/Co₃O₄/Al₂O₃ (gold particles less than 5 nm).     

The reduction phase in NOx storage catalysis: Effect of type of precious metal and reducing agent

The effect of different reducing agents (H₂, CO, C₃H₆ and C₃H₈) on the reduction of stored NO_x over PM/BaO/Al₂O₃ catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO₂-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H₂ and CO have a relatively high NO_x reduction efficiency compared to C₃H₆ and especially C₃H₈ that does not show any NO_x reduction ability except at 350 °C over Pd/BaO/Al₂O₃. The type of precious metals affects the NO_x storage-reduction properties, where the Pd/BaO/Al₂O₃ catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al₂O₃ catalyst shows a high reduction ability but a relatively low NO_x storage capacity.     

碱金属K、Li对Ru/γ-Al_2O_3催化剂上CO选择性氧化的助催化作用

以Ru为活性组分,碱金属氧化物K2O和Li2O为助剂,采用浸渍法制备Ru/γ-Al2O3、Ru1K10Oy/Al2O3和Ru1Li10Oy/Al2O3催化剂,对制备的催化剂进行TPR、XRD和SEM表征,采用固定床石英反应器考察催化剂的CO氧化性能。结果表明,碱金属助剂的引入对Ru/γ-Al2O3的催化性能有一定影响,K2O和Li2O的掺杂,显著降低Ru/γ-Al2O3催化剂上CO选择性氧化温度。Ru1K10Oy/Al2O3和Ru1Li10Oy/Al2O3催化剂低温具有较高的CO转化率和选择性,随着反应温度的升高,提高CO转化率的同时,存在更多的氢气消耗,导致CO选择性降低。     

Oxidative degradation properties of Co-based catalysts in the presence of ozone

Four series of cobalt-based catalysts, such as bare Co₃O₄ and CoO, CoO_x–CeO₂ mixed oxides, CoO_x supported over alumina and alumina–baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV–visible diffuse reflectance spectroscopy and TG/DT were performed.

The CoNiAl hydrotalcite exhibits, after 4 h of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al₂O₃–BaO and CoMgAl. The samples Co(1 wt%)/Al₂O₃–BaO and Co(1 and 3 wt%)/Al₂O₃ show a comparable medium activity, while the oxidation properties of bare oxides Co₃O₄, CoO and CoO_x–CeO₂ are really low. Leaching of cobalt ions in the water solution was detected during the reaction, the amount varied depending on the nature of catalysts. A massive release was observed for the CoMgAl and CoNiAl hydrotalcites, while cobalt catalysts over alumina and alumina–baria look much more stable. The recycle of CoO_x/Al₂O₃ and CoO_x/Al₂O₃–BaO was studied by performing three consecutive cycles in the phenol oxidation. Because of the potential interest of the cobalt-supported catalysts in the ozonation process, the oxidative degradation of naphtol blue black was also investigated.

On the basis of TPR and UV–visible results it appears that highly dispersed Co²⁺ ions especially present over Co(3 wt%)/Al₂O₃–BaO are the main active sites for phenol and naphtol blue black oxidative degradation by ozone.     

CO oxidation over promoted Pt catalysts

A study of CO oxidation by O₂ over Pt catalysts, promoted by MnO_x and CoO_x, is described. The activities of Pt/SiO₂, Pt/MnO_x/SiO₂ and Pt/CoO_x/SiO₂ are compared with commercial Pt/Al₂O₃, Pt/Rh/Al₂O₃ and Pt/CeO_x/Al₂O₃ catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO_x/SiO₂ > Pt/MnO_x/SiO₂, Pt/CeO_x/Al₂O₃ > Pt/Al₂O₃, Pt/Rh/Al₂O₃, Pt/SiO₂. Over Pt/CoO_x/SiO₂ CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO_x/SiO₂ in the CO/O₂ reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO_x/SiO₂, Pt/MnO_x/SiO₂ or Pt/CeO_x/Al₂O₃ catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO_x/SiO₂ catalyst in CO oxidation.     

Influence of metal oxides on the catalytic behavior of Au/Al2O3 for the selective reduction of NOx by hydrocarbons

The reduction of NO by propene in the presence of excess oxygen over mechanical mixtures of Au/Al₂O₃ with a bulk oxide has been investigated. The oxides studied were: Co₃O₄, Mn₂O₃, Cr₂O₃, CuO, Fe₂O₃, NiO, CeO₂, SnO₂, ZnO and V₂O₅. Under lean C₃H₆-SCR conditions, these oxides (with the exception of SnO₂) convert selectively NO to NO₂. When mechanically mixed with Au/Al₂O₃, the Mn₂O₃ and Co₃O₄ oxides and, to a much greater extent, CeO₂ act synergistically with this catalyst greatly enhancing its SCR performance. It was found that their synergistic action is not straightforwardly related to their activity for NO oxidation to NO₂. The exhibited catalytic synergy may be due to the operation of either remote control or a bifunctional mechanism. In the later case, the key intermediate must be a short-lived compound and not the NO₂ molecule in gas-phase.     

Processing of hot pressed Al2O3–Cr2O3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al₂O₃ particles were prepared from the decomposition of precursor Cr(CO)₆ by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr₂O₃, CrC_1−x, and C. A solid solution of Al₂O₃–Cr₂O₃ and an Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al₂O₃–Cr₂O₃/Cr-carbide (Cr₃C₂ and Cr₇C₃) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr₃C₂ and Al₂O₃ is non-coherent, while the interface between Cr₇C₃ and Al₂O₃ is semi-coherent.     

Ethylene hydroformylation and carbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts

Ethylene hydroformylation and carbon monoxide hydrogenation (leading to methanol and C₂-oxygenates) over Rh/SiO₂ catalysts share several important common mechanistic features, namely, CO insertion and metal–carbon (acyl or alkyl) bond hydrogenation. However, these processes are differentiated in that the CO hydrogenation also requires an initial CO dissociation before catalysis can proceed. In this study, the catalytic response to changes in particle size and to the addition of metal additives was studied to elucidate the differences in the two processes. In the hydroformylation process, both hydroformylation and hydrogenation of ethylene occurred concurrently. The desirable hydroformylation was enhanced over fine Rh particles with maximum activity observed at a particle diameter of 3.5 nm and hydrogenation was favored over large particles. CO hydrogenation was favored by larger particles. These results suggest that hydroformylation occurs at the edge and corner Rh sites, but that the key step in CO hydrogenation is different from that in hydroformylation and occurs on the surface. The addition of group II–VIII metal oxides, such as MoO₃, Sc₂O₃, TiO₂, V₂O₅, and Mn₂O₃, which are expected to enhance CO dissociation, leads to increased rates in CO hydrogenation, but only served to slow the hydroformylation process slightly without any effect on the selectivity. Similar comparisons using basic metals, such as the alkali and alkaline earths, which should enhance selectivity for insertion of CO over hydrogenation, increased the selectivity for the hydroformylation over hydrogenation as expected, although catalytic activity was reduced. Similarly, the selectivity toward organic oxygenates (a reflection of the degree of CO insertion) in CO hydrogenation was also increased.     

The size effect and high activity of nanosized platinum supported catalysts for low temperature oxidation of volatile organic compounds

Pt/Al₂O₃ catalysts with smaller size of Pt nanoparticles were prepared by ethylene glycol reduction method in two different way and their oxidation activities for three typical VOCs (volatile organic compounds) were evaluated. The catalyst prepared by first adsorption and then reduction procedure is denoted as L-Pt/Al₂O₃ while the catalyst prepared by first reduction and then loading procedure is defined as R-Pt/Al₂O₃. The results show that L-Pt/Al₂O₃ with the stronger interaction between Pt species and Al₂O₃ exhibit smaller size of Pt nanoparticles and favorable thermal stability compared with R-Pt/Al₂O₃. L-Pt/Al₂O₃ is favor of the formation of more adsorbed oxygen species and more Pt²⁺ species, resulting in high catalytic activity for benzene and ethyl acetate oxidation. However, R-Pt/Al₂O₃ catalysts with higher proportion of Pt⁰/Pt²⁺ and bigger size of Pt particles exhibits higher catalytic activity for n-hexane oxidation. Pt particles in R-Pt/Al₂O₃ were aggregated much more serious than that in L-Pt/Al₂O₃ at the same calcination temperature. The Pt particles supported on Al₂O₃ with~10 nm show the best catalytic activity for n-hexane oxidation.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Physico-chemical characteristics and CO oxidation studies over Pd/(Mn2O3 + SnO2) catalyst

The mixed oxide catalyst (Mn₂O₃ + SnO₂) prepared by the coprecipitation method has been impregnated with Pd metal and it's catalytic behaviour for CO oxidation reaction has been investigated. In the coprecipitated material, Mn₂O₃ and SnO₂ were found to crystallise at 875 K and 1175 K, respectively, which are significantly higher than the crystallisation temperatures of individual oxides prepared under similar conditions. Results of catalytic oxidation of CO, carried out using the pulse method for the mixed oxide system and the individual oxides, suggest significant synergistic effects between these two oxides. The impregnation of palladium metal facilitated CO oxidation and the catalyst Pd/(Mn₂O₃ + SnO₂) was found to be quite effective for CO oxidation even at room temperature. Further, the CO disproportionation has been observed on palladium sites in the temperature range 350 to 400 K for the individual oxide systems.