Patent Application
20090302275
The present invention relates to a new type of catalysts comprising refractory and ionic conductive oxide supporting noble metal(s).
A large number of companies are currently working on the production of synthesis gas (mixture of H2 and CO) starting from methane. Two main technologies have been deeply studied: Steam methane reforming (SMR) and catalytic partial oxidation of methane (CPO).
In the SMR technology, methane reacts with steam to produce a rich mixture of hydrogen and carbon monoxide. Two main reactions are involved:
CH4+H2OCO+3H2(ΔH=205.8 kJ/mol) (1)
CO+H2OCO2+H2(ΔH=−41.6 kJ/mol) (2)
This industrial process is thus based on the methane oxidation by water, and can lead to the production of H2 tonnage.
The reaction (1) is highly endothermic, and is promoted at high temperature, low pressure, high steam/carbon ratio (hereafter named: S/C ratio) (2 up to 4 times the stoechiometric composition) and low hourly volumetric flow rate (hereafter named: VVH) (1600-3000/h). The industrial reactors are working in a temperature range from 650° C. to 950° C., and under high pressure from 1.5 106 Pa to 4 106 (15 bars to 40 bars). Temperatures equal or greater than 850° C. are nevertheless required to achieve a significant conversion into CO and H2 (e.g., H2 yield around 70%). The reaction kinetics is improved by the use of catalysts.
Commercial catalysts are typically based on metals or metals doped with an alkali element (K), which are deposited on Magnesium or Calcium Aluminate supports, such as Ni/MgAl2O4 or Ni/CaAl4O7. Companies, such as Haldhor Topse A. S, Süd Chemie or Johnson Matthey, commercialize them.
European patent application publication EP 1 013 603 A1, discloses new based catalysts which comprise from 0.01 wt. % to 10 wt. % stabilizing elements as for example Zirconium (Zr), Yttrium (Y), Tungsten (W), Lanthanum (La) or Cerium (Ce).
International publication WO 031106332 discloses catalysts wherein the previously mentioned supports are considered as “inorganic oxide supports”. It is referred to materials composed of Alumina (A2O3), Zirconia (ZrO2), Titania (TiO2), rare earth metal oxides or materials, which are formed from mixtures of these compounds, such as Zirconia-Cerium oxide.
The active metal is deposited using incipient wetness impregnation, which means that the impregnation of the catalyst support is carried out with a volume of metal salt solution roughly equal to the pore volume of the support material.
New catalysts have been proposed since the beginning of year 2000. They are based on a noble metal or a mixture of noble metals deposited on an inorganic oxide support, but these catalytic materials are still obtained by the impregnation method:
A preparation of such catalysts includes first, the impregnation of the classical support (γ-Alumina or α-Alumina) by a salt (generally a nitrate precursor) containing Mg, Ce, La, . . . or a mixture of them. After drying and calcination, some stabilized compounds are formed, like La2O3— Al2O3, CeO2—Al2O3, MgO—Al2O3, . . . . These compounds can completely cover the initial support or can be present as clusters on the alumina surface. The next step then consists in the impregnation of the stabilized support by the active phase (noble metal(s) or nickel; generally as a nitrate precursors).
In the methane CPO technology, three main reactions are involved:
CH4+1.5O2CO+2H2O(ΔH=−44 kJ/mol) (3)
CO+H2OCO2+H2(ΔH=−41.6 kJ/mol) (2)
CH4C+2H2(ΔH=74.5 kJ/mol) (4)
This process is highly exothermic; temperature reaches above 900° C. The H2/CO molar ratio, which is reached is close to 1.5 up to 2, depending on the operating conditions. A lot of bibliographic references relate to this technology:
U.S. Pat. No. 5,720,901 discloses a CPO process of hydrocarbons using noble metal catalysts (Rh, Ru or Ir).
J. K. Hockmuth [CPO of methane . . . Applied Catalysis B, Environmental, 1 (1992), 89-100] reports the use of a combination of Pt & Pd supported on cordierite for methane CPO.
European patent application publication EP 1 570 904 A1, discloses a catalyst with a noble metal (Rh) supported on a Zirconia-Ceria material. The advantage of the use of Rh noble metal as the active phase for the synthesis gas production processes, is the increase of the methane conversion, a better H2 selectivity, a better Carbon gasification rate, while allowing to process at a lower S/C molar ratio in (SMR), a lower O/C ratio in (CPO) or a lower ratio CO2/C in dry reforming and a higher VVH, because of the higher reactivity.
One of the main advantages in using noble metal catalysts in SMR process, is related to soot formation, because Ni, which is a common metal catalyst is considered as being responsible of the formation of Carbon. As an example, G. Q. Lu et al. (G. Q. Lu-Shaobin Wang-University of Queensland (Australia)-Chemtech (1999)-37-43) noted that Ni/Al2O3 and Ni/CaO—Al2O3 were not suitable for CO2 reforming of methane because they lead to the formation of Carbon and that the addition of a promoter like CeO2 to the catalyst (5 wt. % of CeO2 in Al2O3) avoids this phenomenon, thanks to its redox properties, which promote the oxidation of the carbon species that are generated at the surface of the catalyst.
The Carbon formation depends on the operating conditions, such as the choice of oxidizing agent, the methane ratio, the pressure or the temperature and of the type of catalyst namely the nature and the size of the metallic particles, the nature of the support (acidity), the morphology of the support, and the chemical interactions which are developed between the support and the active phase.
Using a noble metal, the rate of elimination of carbon along the reaction is greater than the rate of formation. But the use of noble(s) metal(s) only, deposited on classical inorganic supports is not the best solution because these active elements (Rh, Pt. Pd or a mixture of them), must be attached together with inorganic supports, which are acceptable in terms of (i) stability under hydrothermal conditions, (ii) ionic conductivity to suppress the carbon formation like Ceria and/or Zirconia and which are able to oxidize the deposited carbon species. Ceria-containing supports have been recently studied (“On the catalytic aspects of steam-methane reforming”, a literature survey, P. van Beurden, 12.2004). Among them, CeO2—Al2O3, CeZrO2 and CeZrOx—Al2O3 supports were more specifically investigated.
It was found that in the catalytic system: Ni/Ce0.15Zr0.85O2, two kinds of active sites exist, one for the methane activation (on Ni) and one for steam and/or oxygen activation (on CeZrO2 support) (Dong et al., “Methane reforming over Ni—Ce—ZrO2 catalysts: effects of nickel content”, Appl. Cata. A 226, 63-72). Because of the addition of Ceria, the ability to store, release and transfer oxygen species (O, O2−, OH, . . . ) is acquired, and results in an enhanced ability to prevent from forming Carbon, which would normally appear on the metal or on the metal-support interface. Strong interactions between NiO and the CeZrO2 matrix were also observed.
The crystallographic structure seems to play an important role in the reactivity of the support and of the active phase. Other authors also confirm this approach (Roh et al., “carbon dioxide reforming of methane over Ni incorporated into Ce—ZrO2 catalysts, 2004, Appl. Cata. A 276, 231-239).
Other authors pointed out the interest to use ionic conductors like refractory ceramics as support. Specific effects of the addition of Ceria (CeO2) to Zirconia (ZrO2) were thus demonstrated in terms of stability and resistance to coke formation. The highest stability of Pt/Ce0.2 Zr0.8 O2, compared with the activity of Pt/ZrO2 in methane reforming is due to the higher density of the oxygen vacancies of the support, which favors the “carbon cleaning mechanism” of the metallic particles (Noronha et al., 2003, “catalytic performances of Pt/ZrO2 and Pt/Ce—ZrO2 catalysts on CO2 reforming of CH4 coupled with steam reforming or under high pressure; Cata. Letters 90, 13-21).
As described above, the resistance of a catalyst to coke formation is due (i) to the choice of metal active phase and support, (ii) to the properties of the support (ionic conductivities, . . . ) but also (iii) to the size and dispersion of the metallic nanoparticles. This last point is a direct consequence of above items (i) and (ii) and of the elaboration process (from precursors to the final object).
The present invention provides a new type of catalyst which improves the yield of the synthesis gas production by methane reforming, without damaging their stability and having a beneficial effect on the resistance to carbon formation. The subject matter of the present application relates to a new type of catalytic materials which are based on noble metal(s) mixed with refractory and ionic conductive oxides like Ceria (CeO2), Zirconia (ZrO2) or mixed (CexZryO2-δ) or CexZryNzO2-δ wherein N is a doping element like Yttrium (Y), Erbium (Er), Magnesium (Mg), Lanthanum (La), Praseodium (Pr), and which can be used in steam methane reforming (SMR) processes, Catalytic partial oxidation (CPO) of methane processes, Ethanol steam reforming (ESR) processes, as well as in synthesis gas production processes involving the working of a ceramic membrane reactor (CMR).
a provides a processing route for depositing Rh on CeO2, CeZrO and CeZrY nanopowder.
b provides the liquid precursor route to prepare RhUCe0.75Zr0.25O2-δ plus Rh2O3 grafted nanopowder after thermal treatment.
FIG. 4.1.a-nanostructure after nitrate precursor decomposition at 500° C. for 2 hours (just after Step 3 of
FIG. 4.1.b-nanostructure after RhxCeO2-d solid solution.
FIG. 4.1.c-nanostructure after metallic RH ex-situ growth under SMR operating conditions.
FIG. 4.2.a-nanostructure after nitrate precursor decomposition at 500° C. for 2 hours (just after Step 3 of
FIG. 4.2.b-nanostructure after RhxCeO2-d solid solution.
FIG. 4.2.c-nanostructure after metallic RH ex-situ growth under SMR operating conditions.
a provides TPR analysis of RhUCeXO2-δ solid solution plus free Rh2O3 catalyst for Rh content 5% and 15 wt. %.
b provides TPR analysis of RhUCeXO2-δ solid solution plus free Rh2O3 catalyst for Rh content ranging from 0.1 to 15 wt. %.
According to a first embodiment, the present invention relates to a catalytic composition comprising a catalytically active metal and a solid support, characterized in that a proportion of said catalytically active metal is dispersed on the outer surface of said support and another proportion is included into the core structure of said solid support, and said solid support is a refractory and preferably an ionic and/or mixed conductive oxide.
In the new catalytic material as described above, one part of the catalytically active metal is included inside the structure of the support below its surface; the maximum quantity of said included catalytically active metal, mainly depends on the crystallographic structure of the support, of the metal atom size and of the electronic interactions. The other part of the catalytically active metal, which is a proportion in excess, which cannot be included inside the sub-surface structure of the support, is grafted as metal particles or as metal oxide particles on the sub-surface of said support.
As an example of inclusion of metal in a support, reference can be made to the “solid solutions” or to other forms of intimate mixtures. For a solid solution, the maximum quantity of catalytically active metal is the solubility limit of the metal in the solid, which is generally between 5% molar to 10% molar; it means for example, that a solid solution of 1% molar Rhodium in Ceria contains 1 Rh atom per 100 CeO2 moles. In such intimate mixtures, the included catalytically active metal cannot be considered as being separated from the support, since both materials are intimately mixed before the use in operating conditions. As other intimate mixtures, which are not a solid solution, are the new crystalline structures incorporating both the metal atoms and the atoms constituting the oxide. According to the first embodiment of the invention, the catalytically active phase of the catalytic composition consists both, in the intimate mixtures of the catalytically active metal with the support and the excess of the catalytically active metal, which is grafted on the surface of the support.
According to a particular embodiment of the invention, the catalytic composition as defined above, is a saturated solid solution, or other forms of intimate mixtures, of said catalytically active metal in said solid support, together with a dispersion of nanoparticles of said catalytically active metal which are grafted on the outer surface of said solid solution.
By nanoparticles, it is understood that the average size of the particle is less or equal to 10−7 m.
According to another particular embodiment of the invention, in the catalytic composition as defined above, the catalytically active metal is selected from Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Rhenium (Re), Osmium (Os), Iridium (Ir) Platinum (Pt) or combinations thereof.
According to another particular embodiment of the invention, in the catalytic composition as defined above, the catalytically active metal is Rhodium or a combination of Rhodium with Platinum.
According to another particular embodiment of the invention, in the catalytic composition as defined above, the refractory and ionic conductive oxide is selected from Ceria (CeO2), Zirconia (ZrO2), mixed oxides of the formula (I):
Ce(1-x)ZrxO(2-δ) (I),
wherein 0<x<1 and δ ensures the electrical neutrality of the oxide,
or doped mixed oxides of the formula (II):
Ce(1-x-y)ZrxDyO2-δ (II),
wherein D is selected from Magnesium (Mg), Yttrium (Y), Strontium (Sr), Lanthanum (La), Praseodium (Pr), Samarium (Sm), Gadolinium (Gd), Erbium (Er) or Ytterbium (Yb); wherein 0<x<1, 0<y<0, 5 and δ ensures the electrical neutrality of the oxide.
As a more specific embodiment, which must not be regarded as a limitation of the present invention, in the catalytic composition as defined above, the catalytically active metal is Rhodium (Rh), and the refractory and ionic conductive oxide is selected from Ceria (CeO2) or from the mixed oxide of the formula (I′):
Ce(1-x′)Zrx′O(2-δ) (I′),
wherein 0<x′<0.5 and δ ensures the electrical neutrality of the oxide.
As another more specific embodiment, the catalytic composition as defined above, contains from 5.0 wt % to 15 wt. % of catalytically active metal per 100 wt % of refractory and ionic conductive oxide.
Another embodiment of the present invention is a process for the preparation of a catalytic composition as defined above, comprising the following successive steps: Step 1: Mixing of a precursor of the catalytically active metal with a powder of the refractory and ionic conductive oxide in a proportion to reach a final amount of said catalytically active metal greater than its solubility limit in said refractory and ionic conductive oxide, in a solvent containing a dispersing agent, to form a suspension;
Step 2: Drying the suspension obtained at Step 1, to form a powdered mixture;
Step 3: Heating of the powdered mixture obtained at Step 2, to decompose the precursor of the catalytically active metal and to obtain the final catalytic composition.
A particular embodiment of the above process also comprises a subsequent Step 4 of ageing the catalytic composition obtained at Step 3.
Another particular embodiment of the above process also comprises the preparation of the powder of the refractory and ionic conductive oxide, which is used at Step 1, said preparation comprising the subsequent following steps:
Step P1: Preparation of an aqueous solution of salt(s) precursor(s) of said refractory and ionic conductive oxide;
Step P2: Partial evaporation by heating and agitation of the water of the solution obtained at Step P1, to form a gel of the metal salt(s) precursor(s) of the refractory and ionic conductive oxide;
Step P3: Heat drying of the gel obtained at Step P2,
Step P4. Grinding of the dry gel obtained at Step P3, to form a powder of the gel of the metal salt(s) precursor(s) of the refractory and ionic conductive oxide.
Step P5: Heating of the powder obtained at Step P4 to decompose the precursors and to obtain the powder of the refractory and ionic conductive oxide.
Another embodiment of the present invention is a process for the preparation of a catalytic composition as defined above, comprising the following successive steps:
Step 1′: Preparation of a mixture of salt(s) precursor(s) of said refractory and ionic conductive oxide and of precursor(s) of the catalytically active metal, comprising the formation of a dispersion of said precursors, wherein the proportion of metal salt precursor to reach a final amount of said catalytically active metal is greater than its dissolving maximum amount in said refractory and ionic conductive oxide, in a liquid medium followed by the solvent removal;
Step 2′: Calcination of the mixture formed at Step 1′ under air or under oxygen, to decompose said precursors and to obtain a mixture of said refractory and ionic conductive oxide and of the catalytically active metal;
Step 3′: Attrition milling of the mixture obtained at Step 2′, to obtain the catalytic composition.
According to another particular embodiment of the above process, namely the Step 2′-Step 3′ sequence is repeated twice.
In the above defined processes and their particular embodiments, the elemental operations such as powders mixing, suspension making, suspension heat drying, powder grinding, powder heating, calcination, attrition milling, are implemented in an usual way for the man skill in the art of ceramics manufacturing.
Another embodiment of the present invention is the use a catalytic composition as defined above, as a catalyst in hydrocarbons Steam Reforming, hydrocarbons catalytic partial oxidation or hydrocarbons dry reforming and more particularly, the use of a catalytic composition, as defined above, wherein hydrocarbons is natural gas, methane, ethane, propane, butane or mixtures thereof.
A last embodiment of the present invention concerns synthesis gas production either by steam reforming, by catalytic partial oxidation or by dry reforming of natural gas (methane, ethane, propane, butane), naphta or mixtures thereof, involving as reaction catalyst, the catalytic composition as defined above.
The following considerations tend to propose some mechanisms of action of the catalytic composition, however they must not be regarded as limiting the scope of the invention.
(SMR, CPO . . . ), an “ex-situ” growth of the metal from the inner part of the support towards its surface, can occur leading to the appearing of isolated metallic clusters (typically below 1 to 100 nanometers in size, preferably between 1 to 50 nm). The presence of these clusters is demonstrated by Field emission scanning electron microscopy (hereafter named: FESEM), by Transmission Electron Microscopy (hereafter named: TEM) and chemisorption analyses. The appearing of the clusters, as well as their number and size, depend on the operating conditions (temperature, pressure, atmosphere . . . ), as well as on the quantity of metal initially present in the solid solution or other forms of intimate mixtures. This operation is reversible. After operation (SMR conditions for example) and an oxidative treatment (air/1000° C.) the catalytic material seems to be the same as after synthesis and initial thermal treatment (before reaction). During the same operation the free metal (and/or oxide) due to the initial excess of metallic phase is present as metallic particle and is grafted to the support sub-surface. These particles can be observed after thermal treatment, the size directly depend of:
In summary, the catalysts consist of nanosized noble metal(s) clusters extracted out of a solid solution or other forms of intimate mixtures with the support+nanoparticles of the excess of free noble metal(s) grafted on the surface structure of the support. The remaining solid solution (or other forms of mixtures) may also take part to the catalytic activity. The extracted noble metal clusters, extracted or due to the excess, (Rh, Pt, . . . ) act as active sites (which accelerate the establishment of a chemical equilibrium without themselves being consumed), whereas the support acts as an anchor for these clusters, thus preventing from coalescence effects in operating conditions. This support can be itself a nanosized powder with a high specific surface area to improve the effectiveness of the catalytic material, which is directly linked to the noble metal surface area (i.e. the number of active sites). The higher the active surface area of the catalyst is, the greater the number of molecules produced per time unit is. The reactions occurring in an heterogeneous phase, the nature of the noble metal (Pt, Rh, . . . ), the initial size, the spatial distribution and the presence of preferential crystalline planes are key parameters that mainly depend on the elaboration process. The catalyst stability results of the physico-chemical properties of the metals used (chemical reactivity, melting point . . . ) and on the interactions with the support oxide. This support oxide must have (i) refractory properties (to prevent from grain coarsening due to sintering effects under hydrothermal conditions and/or CPO conditions, (ii) ionic conductivity (to prevent from coke formation), and (iii) strong interactions with the noble metal(s) (a minimum solubility of the noble metal(s) in the crystallographic structure of the oxide support must exist).
The support oxide has hence a strong influence on the steam reforming reaction. It not only determines the distribution of the metal clusters exhibiting a catalytic activity, but also limits the coalescence phenomenon of active sites in operating conditions, depending on how strong the cluster anchorage is, as mentioned hereabove. In addition, the support must be thermally stable under operating conditions to keep its initial high specific surface area (resistance to sintering and as a consequence to the encapsulation of the active sites). Finally, the support also affects the reactivity and resistance to coke formation and may even participates in the catalytic reaction itself. The advantage of oxide presenting oxygen transport properties is to provide an extra source of oxygen that prevents the accumulation of carbon on the catalyst.
The catalyst final microstructure control (noble metal incorporated in the ceramic support+free noble metal nanoparticule grafted on the surface structure of the support) is a key point for the stability and the activity in operating conditions. Consequently, the catalyst elaboration process must be perfectly reliable with regards to the nanostructure it conducts to.
Soft chemical routes can be followed to adapt the elaboration process. These routes may result of different approaches:
In all cases, after the thermal treatment, the objectives are (i) to obtain the solid solution (or other forms of intimate mixtures) with presence after synthesis of free metallic element(s) or free oxide(s) on the surface of the support, (ii) to obtain a small support nanopowder (typically less than 200 nm in diameter and preferably around 20 nm but in all cases with a high resistance to sintering). After reaction in operating conditions, the nanostructure of the catalytic material is characterized by a mixture of (i) some metallic particles extracted out of the solid solution (or other forms of intimate mixtures) and well dispersed onto the support surface, (ii) a solid solution with a lower amount of metal which may evidence a catalytic activity, (iii) presence of “large” nanoparticles of free noble metal (big means more than 10 nm after thermal treatment, before operating conditions. by using a processing route based on liquid precursors only.
After thermal treatment, the elements are present in the general formulae MuCexZryNzO2-δ+free noble metal(s) oxide(s). The quantity of noble metal(s) introduced should be above the solubility limit of M in the support oxide (the intimate mixture of M with the support can be in another form than that of a solid solution).
After the reaction in operating conditions, the nanostructure of the catalytic material is characterized by a mixture of:
(i) Metallic clusters extracted out of the solid solution (or others forms of intimate mixtures) onto the MuCexZryO2-δ support surface;
(ii) A solid solution with a lower amount of metal, which may show evidence of a catalytic activity.
(iii) Metallic nanoparticules present and grafted on the surface structure of MuCexZryO2δ after the elaboration process. This presence is due to the excess of the noble metal(s) (quantity introduced above the solubility limit),
The nanosized claimed catalytic composition may have the following specific features:
This novel claimed catalytic composition can be used in a large range of operating conditions: from 1 up to 40 atm., from 650 up to 1200° C. with high space velocities (3000-360000/h). The SMR and CPO thermodynamic conditions for the specific case of RhuCexO2-δ+extracted Rh°+free Rh0 are very close or equal to the equilibrium.
The claimed catalytic composition may be deposited on various substrates such as ceramics (cordierite . . . ) or metal alloys (FeCrAIY . . . ). In addition, it can easily be shaped into balls, pellets, and monoliths . . . as a function of the targeted industrial applications.
In order to illustrate this approach, examples are described hereafter.
The powder resulting from Step 3 of
The different steps of an example of a processing route to deposit Rh on CeO2 are presented on
FIG. 4.1.a is a FESEM observation of the nanostructure after nitrate precursor decomposition at 500° C. for 2 hours (just after Step 3 of
FIG. 4.1.b is a FESEM observation of the nanostructure after RhxCeO2-d solid solution (or other forms of intimate mixtures) formation at 1000° C. for 48 hours (just after Step 4 of
FIG. 4.1.c is a FESEM observation of the nanostructure after metallic Rh “ex-situ growth” under SMR operating conditions (900° C., 20 bars, S/C=1, 5,
FIG. 4.2.a: is a FESEM observation of the nanostructure after nitrates precursors decomposition at 500° C. for 2 hours (just after Step 3 of
FIG. 4.2.b: is a FESEM observation of the nanostructure just after Step 4 of
FIG. 4.2.c: is a FESEM observation of the nanostructure after SMR conditions
b is a diagrammatic representation of a chemical route, which involves only liquid precursors as Rh Ce and Zr nitrates salts to elaborate RhuCe0.75Zr0.250O2-δ. This route provides a homogeneous and reliable distribution of all the elements before the formation of the RhuCeO2-δ solid solution+free noble oxide(s) by thermal treatment.
The catalytic activity of catalyst compositions of the type:
RhuCeO2-δ+grafted Rh2O3
with several amounts of Rh were tested under SMR and CPO conditions.
In the past, several authors have studied the solubility of Rh in YSZ and ZrO2 cristallographic structures (Ruckenstein et al., “Effect of support on partial oxidation of methane to synthesis gas over Rhodium catalyst”, 1999 Journal of Catalysis 187, 151-159; Y-C. Zhang, et al., “Stabilization of cubic ZrO2 with Rh(III) and/or La(III”). 1988 Journal of Solid State Chemistry 72, 131-136; E. Ruckenstein, H. Y. Wang, “Temperature-Programmed Reduction and XRD Studies of the Interactions in Supported Rhodium Catalysts and Their Effect on Partial Oxidation of Methane to Synthesis Gas”. 2000 Journal of Catalysis 190, 32-38). They concluded that the solubility limit of Rh in ZrO2 is 8% mol thus the solid solution has the following chemical composition: Rh0.08Zr0.92O1.96. Above 8% mol, the excess of Rhodium, which does not go into solid solution (or other forms of mixtures) in the support material, stands as “free” rhodium-based islands on the support surface.
In the present experiment, catalyst compositions with several values of Rh amount were tested (0.1% wt Rh, 1% wt Rh, 5% wt Rh, 15% wt Rh).
1. Characterization of the Samples
The B.E.T. surface areas of RhuCexO2-δ (with or without “free” excess Rh) were measured using a SORPTY™ 1750 instrument.
The metal dispersion, surface area and crystal size are determined using a CHEMISORB™ 2750 instrument according to the H2 chemisorption method.
The results are recorded in Table 1.
As expected (see Table 1), the crystal size increases together with the Rh loading, whereas the surface area and the dispersion state of metallic clusters is decreasing.
(i) to improve the homogeneity of the catalytic material, as well as
(ii) to have two types of actives sites: Rh extracted from RhuCeO2-δ combined with free Rh2O3 grafted on the sub-surface structure of the solid solution.
FESEM images (
The sample reducibility was determined by TPR analysis. The TPR patterns of
In our system, the surface sites are nanometric. At high temperature and under reducing atmosphere and depending on the saturation, a part of the nanometric entities initially located at the surface of the nanoparticles of Ceria leave this surface, towards the inner part of the support leading to the formation of a solid solution of which is comparable to an alloy. To be active, this system further requires a reduction step, which is carried out at high temperature(>800° C.), in order to conduct to the “ex-situ” growth of metal Rh sites at the Ceria surface upon the solid solution. The performances of this new material were evaluated for SMR &CPO processes.
With a large excess of Rh (more than 5 wt %), the presence of free Rhodium is confirmed by TPR-TPO. As observed for lower quantities (1 wt % and 5 wt %) some free Rh2O3 was detected by TPR-TPO, but not by FESEM and XRD. No free rhodium oxide was detected by TPR-TPO for 0.1 wt %.
2. Methane Catalytic Partial Oxydation (CPO) with the Inventive Catalytic Composition
The evaluation in the partial oxidation of CH4 (CPO) of the activity of samples with different Rh wt. % was carried out under various mixtures of CH4/O2/He (v/v/v representing volume flow ratios), either with or without a pre-reducing step (500° C. for 3 h with H2/N2 flow) of the catalytic composition. Tests were carried out on the non-reduced and on after-reduced 15 wt. % Rh/CeO2 catalytic composition (Tables 2 and 3).
The 15% Rh/CeO2, is active in all the reaction conditions. In particular at 500° C. and diluted mixture, high CH4 conversions were observed, both for the tests carried out with (pre-reduced) and without (not reduced) the reduction step at 500° C. Repeating this test as final test (after higher temperature and higher concentration of the feed) the catalytic activity is approximately the same than that of the initial test. This is probably due to the over-sizing of the amount of Rh, which cannot discriminate the part of the activity due to free Rh and to the solid-solution Ce3− Rh. The CO2 selectivity seems to be high and don't decrease with time on stream.
New tests were carried out on 5 wt. % Rh/CeO2 catalyst with and without the reduction step at 500° C. (Tables 4 and 5).
The pre-reduced 5 wt. % Rh/CeO2 sample shows evidence of higher performances than the non-reduced sample in all the reaction conditions. In addition, the CH4 conversion increases, whereas the CO2 selectivity decreases with time on stream due to the on-going reduction of samples in operating conditions (more & more reducing and hotter stream). This explains also why the pre-reduced catalyst is always more efficient than the non-reduced one at the beginning of the tests.
The benefits in term of performances of these systems are due:
The most important difference between the 5 and 15 wt % Rh, concerns the initial methane conversion. At 500° C. as observed in Tables 3 and 5 or in Tables 2 and 4 (cases of the non reduced samples) the catalytic activity of the 15 wt % Rh is higher than the 5 wt % Rh (54.3% versus 36%, 51% versus 43.1%). This means that at 500° C. with or without pre reduction treatment of the sample, the catalytic activity is probably due to the free Rhodium. During time on stream, the increase of oven temperature (750° C.) promotes the reduction of RhuCexO2-δ and increases the catalytic activity. After return to initial conditions (500° C., 2/1/20), the methane conversion and the CO selectivity are higher than before particularly for the 5 wt % Rh catalytic composition.
Tables 6 and 7 record the catalytic activity in CPO conditions of 0.1 wt % and 1 wt % Rh.
The same behavior appears on other refractory and ionic conductor support. Table 8 records the results obtained under CPO conditions on Rh(1 wt %)/Ce0.75Zr0.25O2 composition.
A comparison of the CPO catalytic activity between pre-reduced samples exhibiting different Rh loadings (from 0.1 to 15 wt % on CeO2=RhuCexO2-δ+Rh° extracted+Rh° free) is given in
3. Steam Methane Reforming Results (SMR) with the Inventive Catalytic Composition
The samples from 0.1 wt % to 15 wt. % Rh/CeO2 were tested under SMR reaction conditions for 40 hours spread over 5 days of experimental work.
The activity of our Rh/Ceria catalyst was then compared to a commercial catalyst Ni/MgAl2O4 (ref HT as R67-7H). The comparison presented on
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 06301133.2 | Nov 2006 | EP | regional |
The present application is a continuation-in-part of International PCT Application No. PCT/EP2007/061365, filed Oct. 23, 2007, which claims priority to European Patent Application No. 06301133.2, filed Nov. 8, 2006, each incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2007/061365 | Oct 2007 | US |
| Child | 12437748 | US |
