JP2005125148A - Methanol reforming catalyst and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a methanol reforming catalyst improved in the gas conversion ratio of methanol and reducing the amount of CO generated. <P>SOLUTION: This methanol reforming catalyst is a Pd/ZnO catalyst wherein Pd particles are supported on the surfaces of ZnO particles and a metal M is contained as a third component so that the atomic ratio (M/Zn) of the metal M and Zn is 0.02-0.05. Further, F or Cr is selected as the metal M. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a methanol reforming catalyst used for a reformer of a solid fuel cell and a method for producing the same.

  In recent years, hydrogen has attracted attention as a new energy source to replace fossil fuels as global environmental problems increase. There is a polymer electrolyte fuel cell (hereinafter referred to as PEFC (Polymer Electrolyte Fuel Cell)) that generates electricity by electrochemically reacting hydrogen and oxygen with hydrogen as a fuel.

  Hydrogen for PEFC is generally produced from city gas and natural gas mainly composed of methane, liquefied petroleum gas (LPG), alcohol such as methanol, naphtha, and the like. Among these fuel gases, methanol is an inexpensive liquid fuel that is easily synthesized from fossil fuels, and has the feature that hydrogen can be produced relatively easily using a catalyst. As a hydrogen source for As a method for producing hydrogen from methanol, a steam reforming method has become the mainstream.

  The steam reforming reaction (see the following formula (1)), which is one of the reforming reactions that generate hydrogen from methanol, is a relatively large endothermic reaction, so that the following formula ( A method (Oxygen Steam Reforming or Autothermal reaction) in which at least one partial oxidation reaction of methanol represented by 2) to (4) is used in combination.

CH ₃ OH + H ₂ O → 3H ₂ + CO ₂ Formula (1)
CH ₃ OH + 1 / 2O ₂ → CO ₂ + 2H ₂ Formula (2)
CH ₃ OH + 1 / 2O ₂ → CO + H ₂ + H ₂ O Formula (3)
CH ₃ OH + O ₂ → CO ₂ + H ₂ + H ₂ O Formula (4)

  As a conventional methanol reforming catalyst used for the oxidative steam reforming reaction, a Cu / ZnO-based methanol catalyst in which third and fourth components (Al, Zr) are added to Cu and Zn (see, for example, Patent Document 1) And a Pd / ZnO-based methanol catalyst in which a third component (Ce, Zr, etc.) is added to Pd and ZnO and the Pd loading is 0.1 to 10 wt% (for example, see Patent Document 2). .

JP 2001-347169 A JP 2001-232193 A

  By the way, in the honeycomb catalyst using the Cu / ZnO-based catalyst described in Patent Document 1, a rapid temperature rise occurs at the methanol introduction side end of the honeycomb catalyst as the reforming progresses. As a result, there was a problem that the activity of the catalyst was reduced in a short time of about 100 hours.

  On the other hand, the honeycomb catalyst using the Pd / ZnO-based catalyst described in Patent Document 2 has a lower degree of catalyst activity decrease with the progress of reforming than the honeycomb catalyst using the Cu / ZnO-based catalyst. However, compared with the Cu / ZnO-based catalyst, there is a problem that the amount of CO generated is relatively large when the gas conversion rate is the same. Since CO is a poisoning substance for the PEFC anode, it is preferable that the amount of CO contained in the reformed gas is as small as possible.

  An object of the present invention, which was created in view of the above circumstances, is to provide a methanol reforming catalyst having a good methanol gas conversion rate and a low CO generation amount, and a method for producing the same.

  In order to achieve the above object, the methanol reforming catalyst according to the present invention is a Pd / ZnO-based catalyst in which Pd particles are supported on the surface of ZnO particles, and includes a metal M as a third component, a metal M and Zn. And the atomic ratio (M / Zn) is in the range of 0.02 to 0.05.

  Here, the metal M is preferably Fe or Cr.

  According to the above, a predetermined amount is added to the Pd / ZnO-based catalyst as the third component, for example, Fe or Cr (metal M), and the atomic ratio (M / Zn) between the metal M and Zn is 0.02 to 0.00. By adding in the range of 05, the selectivity of CO can be lowered while maintaining a good gas conversion rate of methanol.

  According to the present invention, it is possible to obtain a Pd / ZnO-based methanol reforming catalyst having good methanol gas conversion and low CO generation.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a preferred embodiment of the invention will be described with reference to the accompanying drawings.

  The methanol reforming catalyst according to a preferred embodiment of the present invention is a catalyst used for oxidative steam reforming or normal steam reforming of methanol. Specifically, it is a Pd / ZnO-based catalyst, most of which is composed of ZnO particles having finer Pd particles supported on the surface of the ZnO particles, and a metal M as a third component. The atomic ratio (M / Zn) between the metal M and Zn is within a range of 0.02 to 0.05. Further, as the metal M, Fe or Cr is used, and Fe is particularly preferable.

  Here, in the composite particles (Pd / ZnO particles) in which the Pd particles are supported on the surfaces of the ZnO particles, the contact portion between the ZnO particles and the Pd particles is an alloy portion of PdZn. In addition, very fine metal M particles are adhered around each composite particle, thereby preventing adjacent composite particles from being bonded to each other.

  Further, the supporting ratio of the Pd component in the entire catalyst is not particularly limited. For example, the supporting ratio of the Pd component in a Pd / ZnO-based catalyst conventionally used as a methanol reforming catalyst (0.1 to 10 wt. %) Or higher.

  The reason for selecting Fe or Cr as the metal M and limiting the amount of addition thereof will be described below.

Changes in CO ₂ + CO conversion (μmol / g-cat / s) and CO selectivity (%) due to addition of various third components are shown in FIGS. 1 and 2, respectively. Here, in FIG. 1 and FIG. 2, the hatched bar graph on the left is a catalyst reduced at 500 ° C., and the white bar graph on the right is a catalyst reduced at 400 ° C. Further, the supporting ratio of the Pd component in the entire catalyst is 6.5 wt%, and the atomic ratio (M / Zn) of the metal M and Zn as the third component is 0.04. Furthermore, the reaction temperature (reforming temperature) for reforming methanol is 250 ° C., the molar ratio of water to methanol (water / methanol) in the raw material to be reformed is 1.5, and the molar ratio of oxygen to methanol (oxygen) / Methanol) is 0.1. The supply ratio of methanol (mixed solution of water and methanol) is W _CH3OH HSV (Wood Alcohol Hourly Space Velocity) = 50 (h ⁻¹ ). A WHSV of 50 (h ⁻¹ ) means that 50 g of methanol (or 78 g of a mixed solution of water and methanol) is supplied per 1 g of the catalyst.

As shown in FIG. 1, the change in the CO ₂ + CO conversion rate due to the addition of various third components (Cr, Mn, Fe, Co, Ni, Cu, Mg, Al, Zr, La, Ce, and Ru) Conversion rate of the Pd / ZnO catalyst without addition of three components (about 250 (μmol / g-cat / s) for 500 ° C. reduction, about 280 (μmol / g-cat / s) for 400 ° C. reduction) In comparison, the 500 ° C. reduction catalyst to which Cr, Fe, Zr, La, or Ce was added improved the conversion rate to about 255 to 270 (μmol / g-cat / s). All the catalysts reduced at 400 ° C. did not improve the conversion even when the third component was added.

  Further, as shown in FIG. 2, the change in CO selectivity due to the addition of various third components (Cr, Mn, Fe, Co, Ni, Cu, Mg, Al, Zr, La, Ce, and Ru) Compared with the CO selectivity of the Pd / ZnO catalyst with no component added (about 8% for 500 ° C reduction, about 9% for 400 ° C reduction), Cr, Fe, or Cu added The 500 ° C. reduction catalyst had a CO selectivity of about 5 to 6%. All of the 400 ° C. reduction catalysts did not decrease CO selectivity even when the third component was added.

According to the trade-off between the CO ₂ + CO conversion rate in FIG. 1 and the CO selectivity rate in FIG. 2, Cr, Fe, or Cu is selected as the third component, preferably Cr or Fe. Further, the preferred reduction temperature of the Pd / ZnO catalyst without addition of the third component is 400 to 500 ° C., but as shown in FIGS. 1 and 2, the reduction temperature is higher, that is, than 400 ° C. Further, the temperature of 500 ° C. is more preferable because the CO selectivity is lowered.

On the other hand, the change in CO ₂ + CO conversion (μmol / g-cat / s) and the change in CO selectivity (%) by changing the atomic ratio (M / Zn) of Fe or Cr are shown in FIG. As shown in FIG. Here, the supporting ratio of the Pd component in the entire catalyst is 6.5 wt%. Further, the reaction temperature (reforming temperature) for reforming methanol is 250 ° C., the molar ratio of water to methanol (water / methanol) in the raw material to be reformed is 1.5, and the molar ratio of oxygen to methanol (oxygen) / Methanol) is 0.1. Furthermore, the supply ratio of methanol (mixed solution of water and methanol) is W _CH3OH HSV = 47 (h ⁻¹ ).

  As shown in FIG. 3, when Fe is added as the third component, the conversion rate gradually decreases as the amount of Fe added increases. When the atomic ratio is about 0.02, the minimum value (about 227 (μmol / g-cat / s)). Thereafter, the conversion rate gradually increases, and when the atomic ratio is about 0.05, the conversion rate when the third component is not added (about 243 (μmol / g-cat / s)) is almost the same. In addition, when Cr is added as the third component, the conversion rate is drastically reduced by the addition of Cr and becomes the minimum value (about 207 (μmol / g-cat / s)) when the atomic ratio is about 0.01. . Thereafter, the conversion rate gradually started to increase. When the atomic ratio was about 0.04 to 0.08, the conversion rate was once saturated, and the conversion rate when the third component was not added (about 243 (μmol / g- cat / s)).

  In addition, as shown in FIG. 4, when Cr is added as the third component, the CO selectivity rapidly decreases due to the addition of Cr, and the minimum value (about 5.6%) when the atomic ratio is about 0.01. It becomes. Thereafter, the conversion rate gradually increases, and when the atomic ratio is about 0.05, the CO selectivity (about 6.4%) when the third component is not added is substantially equal. Further, when Fe is added as the third component, the CO selectivity gradually decreases as the amount of Fe added increases, and reaches a minimum value (about 4.7%) when the atomic ratio is about 0.04. Thereafter, the CO selectivity gradually increases, and when the atomic ratio is about 0.07, the CO selectivity when the third component is not added (about 6.4%) is almost the same.

Due to the trade-off between the CO ₂ + CO conversion ratio of FIG. 3 and the CO selectivity of FIG. 4, the third component Fe or Cr has an atomic ratio (M / Zn) of the third component metal M to Zn of 0. It is preferable to add in the range of 0.02 to 0.05. In particular, in the case of Fe, the atomic ratio (M / Zn) is preferably 0.03 to 0.05, and in the case of Cr, the atomic ratio (M / Zn) is preferably 0.02 to 0.04.

  Next, a method for producing a methanol reforming catalyst according to the present embodiment will be described.

In the method for producing a methanol reforming catalyst according to the present embodiment, first, a zinc metal salt, a palladium metal salt, and a third component metal salt are dissolved in distilled water to form a first solution. Here, examples of the zinc metal salt include zinc hexahydrate [Zn (NO ₃ ) ₂ .6H ₂ O], and examples of the palladium metal salt include palladium nitrate [Pd (NO ₃ ) ₂ ]. However, as the metal salt of the third component, iron nitrate nonahydrate [Fe (NO ₃ ) ₃ · 9H ₂ O] or chromium nitrate nonahydrate [Cr (NO ₃ ) ₃ · 9H ₂ O] Can be mentioned. Moreover, the amount of each added is adjusted so that the mixing ratio (zinc metal salt / palladium metal salt) of zinc metal salt and palladium metal salt may be, for example, 1600 to 10, preferably 35 to 15. By adjusting the mixing ratio, the supporting ratio of the Pd component in the entire catalyst described later is adjusted. Further, the mixing ratio of the third component metal salt is adjusted so that the atomic ratio (M / Zn) of the third component metal M to Zn is in the range of 0.02 to 0.05. The

  Meanwhile, sodium carbonate is dissolved in distilled water to form a second solution. Here, the amount of sodium carbonate added is adjusted so that the concentration of sodium carbonate is 0.5 to 5 mol / l, preferably 1 to 2 mol / l.

  Next, while slowly dropping the first solution and the second solution into distilled water, a precipitate is generated while maintaining the pH at a constant value of about 7 to 10 at room temperature (coprecipitation method). At this time, the first solution is dropped at a rate of 0.5 to 10 ml / min, preferably about 1 to 5 ml / min, and the dropping rate of the second solution so that the pH of the mixed solution becomes 10 ± 0.5. Is adjusted. The obtained precipitate is aged at 0.5 to 5 hr, preferably 1 to 3 hr, particularly preferably about 2 hr at a temperature of 50 to 80 ° C., and the coprecipitation reaction proceeds completely (or almost completely).

  Next, after discarding the supernatant in the mixed solution, distilled water is poured into the precipitate, which is the residue, and stirring, washing with water, and vacuum filtration are repeated as appropriate to take out the filtrate. At this time, washing and filtration are repeated until the pH of the waste liquid (filtrate) becomes 7 ± 0.5.

The filtrate is dried at 50 to 150 ° C., preferably about 100 ° C., for 1 to 15 hours, preferably 6 to 10 hours, particularly preferably about 8 hours. Thereafter, the filtrate is subjected to a baking treatment in an air atmosphere at a temperature of 300 to 600 ° C., preferably 400 to 500 ° C., particularly preferably 450 to 500 ° C. for 1 to 5 hours, preferably about 3 hours. As a result, a Pd / (ZnO + Fe ₃ O ₄ ) or Pd / (ZnO + Cr ₂ O ₃ ) -based methanol reforming catalyst (calcined product) having a Pd component loading ratio of 0.1 to 10 wt% is obtained.

  The obtained methanol reforming catalyst is appropriately subjected to press (pulverization) treatment and classification treatment. The powder body is used as it is as a pellet catalyst, or the powder body is supported on a metal (or ceramic) honeycomb carrier and used as a honeycomb catalyst. When using a catalyst as a product, it is preferable to perform a reduction treatment at 400 to 500 ° C. before use. Here, as the pellet catalyst, it can be used even in a powder state, but the powder catalyst may be compression-molded and formed into a pellet body (cylindrical body, sphere). This improves the handleability of the catalyst. Examples of the compression molding method include a tableting molding method, a rolling granulation method, and an extrusion molding method, and the tableting molding method is most preferable.

  Next, the operation of the present embodiment will be described.

  Since the methanol reforming catalyst according to the present embodiment is manufactured by the above-described manufacturing method (coprecipitation method), the obtained Pd particles and ZnO particles are Pd / ZnO-based catalysts manufactured by a conventional slurry method. It becomes finer than the Pd particles and ZnO particles. Here, the finer the Pd particles and ZnO particles in the Pd / ZnO-based catalyst, the more the portion of Pd and ZnO alloyed (PdZn) (the contact region (bonding region) between the Pd particles and the ZnO particles). Alloying between each particle easily proceeds. As a result, the part of alloying increases and the activity of the Pd / ZnO-based catalyst itself increases.

  Further, in the methanol reforming catalyst according to the present embodiment, the third component metal M (Fe or Cr) is added. However, if the metal M is added in an excessive amount, the gas conversion rate of methanol is remarkably reduced. . However, in the methanol reforming catalyst according to the present embodiment, the metal M is added while adjusting the atomic ratio (M / Zn) between the metal M and Zn to 0.02 to 0.05. Thus, it is possible to obtain a conversion rate substantially equal to that when no metal M is added (see FIG. 3).

  In addition, the methanol reforming catalyst according to the present embodiment adds the predetermined amount of the metal M (Fe or Cr) as the third component so that the reactions of the following formulas (5) and (7) The reaction of formula (6) is suppressed. As a result, the amount of CO generated (by-product) can be suppressed.

CH ₃ OH + H ₂ O → 3H ₂ + CO ₂ Formula (5)
CH ₃ OH → CO + 2H ₂ Formula (6)
CO + H ₂ O → CO ₂ + H ₂ Formula (7)

  Here, if the metal M (Fe or Cr) as the third component is added excessively, the CO selectivity is remarkably increased. However, in the methanol reforming catalyst according to the present embodiment, the metal M is added while adjusting the atomic ratio (M / Zn) between the metal M and Zn to 0.02 to 0.05. Thus, the CO selectivity can be made lower than when no metal M is added without substantially reducing the methanol reforming activity (see FIG. 4).

  As described above, the honeycomb catalyst using the methanol reforming catalyst according to the present embodiment is improved while maintaining the methanol reforming rate substantially the same as the honeycomb catalyst using the conventional Pd / ZnO-based catalyst. The CO concentration (CO selectivity) contained in the quality gas can be lowered (lower by about 2% at the maximum).

The reaction temperature (reforming temperature) for reforming methanol is preferably 250 to 350 ° C. In addition, the molar ratio of water to methanol (water / methanol) in the raw material to be reformed in the steam reforming reaction or the oxidative steam reforming reaction is preferably 1 to 3, particularly preferably around 1.5. Further, the molar ratio of oxygen to methanol (oxygen / methanol) in the raw material to be reformed in the oxidative steam reforming reaction is preferably 0 to 0.2, particularly preferably around 0.1. Methanol feed rate (water and a mixed solution of methanol) is, W _{CH3 OH} HSV is 30 to 200 (h ^-1), more preferably 30 to 80 (h ^-1), 50 before and after (h ^-1) Is particularly preferred.

In addition, a methanol reforming catalyst, particularly a methanol reforming catalyst for PEFC, must have the following characteristics.
(1) Gas conversion rate (reforming rate) of methanol is good.
(2) The amount of CO that is a poisoning substance for the PEFC anode is minimized. Since CO is a poisoning substance for the PEFC anode, it is generally necessary to reduce the CO concentration to a level of 50 ppm or less. A small amount of CO in the reformed gas is oxidized into CO ₂ that is harmless to the PEFC anode by a selective oxidation reaction with oxygen. At this time, H ₂ that is the main component of the reformed gas is also simultaneously with the oxidation of CO. Since partial combustion occurs, it is preferable that the amount of CO contained in the reformed gas is as small as possible.
(3) It has a catalyst life satisfying a durability life of tens of thousands of hours, for example, about 40,000 hours, which is a durability target of a general power generation PEFC.

  The methanol reforming catalyst according to the present embodiment sufficiently satisfies the above (1) to (3), and is suitable as a methanol reforming catalyst for PEFC.

  On the other hand, the methanol reforming catalyst (powder catalyst) according to the present embodiment is supported on the honeycomb carrier of the reactor, and a supply line for supplying methanol and water vapor into the reactor is connected to the reactor. A methanol reformer for steam reforming reaction can be obtained by connecting a hydrogen line for discharging hydrogen produced by reforming the steam. Here, in the case of a methanol reformer for so-called autothermal reforming reaction in which the steam reforming reaction of methanol is performed in an oxidizing atmosphere, a supply line for supplying air is provided in addition to a supply line for supplying methanol and steam. It is necessary to provide it. Further, when a pellet catalyst is used as the catalyst, it is only necessary to load the pellet catalyst into the reactor.

  In addition, methanol supply means (storage tank) and steam supply means (or methanol supply means (storage tank), steam supply means, and air supply means) are connected to the supply lines of these methanol reformers, By connecting the fuel cell to the hydrogen line of the methanol reformer, it is possible to obtain a polymer electrolyte fuel cell system that can generate power using hydrogen generated by reforming methanol. Here, the steam generated by generating electricity using hydrogen may be recovered and fed back to the steam supply means.

  Each methanol reformer using the methanol reforming catalyst according to the embodiment described above has high thermal efficiency, is compact, can be easily started and stopped, and has durability to withstand practical use. Become. Therefore, a polymer electrolyte fuel cell system using these methanol reformers is a system suitable for a fuel cell vehicle, a power generator for home / portable use, and a distributed power source.

  As mentioned above, it cannot be overemphasized that embodiment of this invention is not limited to embodiment mentioned above, and various things are assumed in addition.

Is a graph showing changes in conversion of CO ₂ + CO by the third component added various (μmol / g-cat / s ). It is a figure which shows the change of CO selectivity (%) by various 3rd component addition. Is a diagram showing changes in the atomic ratio of Fe or Cr (M / Zn) by varying the CO ₂ + CO conversion (μmol / g-cat / s ). It is a figure which shows the change of CO selectivity (%) by changing the atomic ratio (M / Zn) of Fe or Cr.

Claims (11)

  A Pd / ZnO-based catalyst in which Pd particles are supported on the surface of ZnO particles, the metal M as the third component, and the atomic ratio (M / Zn) of the metal M to Zn is 0.02 to 0.03. A methanol reforming catalyst characterized by comprising a range of 05.   The methanol reforming catalyst according to claim 1, wherein the metal M is Fe or Cr.   The methanol reforming catalyst according to claim 1 or 2, wherein a supporting ratio of the Pd component in the entire Pd / ZnO-based catalyst is 0.1 to 10 wt%.   A zinc metal salt, a palladium metal salt, and a metal salt of the third component are dissolved in distilled water to form a first solution, sodium carbonate is dissolved in distilled water to form a second solution, and the first solution is added to the distilled water. The solution and the second solution are added dropwise, and a precipitate is generated while maintaining the pH at about 7 to 10 at room temperature. The precipitate is filtered to take out the filtrate, and the filtrate is subjected to 400 to 500 in an air atmosphere. The manufacturing method of the methanol reforming catalyst characterized by performing a baking process of 1 degreeC for 5 degreeC.   The method for producing a methanol reforming catalyst according to claim 4, wherein the calcined product after the calcining treatment is subjected to a reduction treatment at 400 to 500 ° C.   A methanol reforming method comprising steam-reforming methanol with the methanol reforming catalyst according to claim 1 to generate hydrogen.   A methanol reforming method characterized in that methanol is steam reformed in an oxidizing atmosphere using the methanol reforming catalyst according to any one of claims 1 to 3 to generate hydrogen.   A reactor containing the methanol reforming catalyst according to any one of claims 1 to 3, a supply line for supplying methanol and water vapor into the reactor, and a hydrogen line for discharging hydrogen generated by reforming the methanol A methanol reformer characterized by comprising:   A reactor containing the methanol reforming catalyst according to any one of claims 1 to 3, a supply line for supplying methanol, water vapor, and air into the reactor, and hydrogen generated by reforming the methanol is discharged. A methanol reformer characterized by comprising a hydrogen line.   9. A methanol reformer according to claim 8, a methanol supply means and a steam supply means connected to each supply line of the reformer, and a fuel cell connected to a hydrogen line of the reformer. A polymer electrolyte fuel cell system. 10. A methanol reformer according to claim 9, a methanol supply means, a steam supply means, an air supply means connected to each supply line of the reformer, and a fuel connected to a hydrogen line of the reformer. A solid polymer fuel cell system comprising a battery.

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