Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-06-19
2003-04-22
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S329000, C502S330000, C502S331000, C502S332000, C502S334000, C502S339000
Reexamination Certificate
active
06551960
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the synthesis of mixed-metal catalysts, more particularly, high activity, long life, alcohol reforming catalysts, especially methanol, based upon nanosize Pt/Ru particles supported on an electroactive support, especially carbon.
BACKGROUND ART
Battery packs are currently the worldwide portable/emergency power source of choice for electrical devices. Researchers have long sought to develop small footprint fuel cells to replace rechargeable battery packs. Fuel cells offer efficient and direct conversion of the chemical energy stored in fuels to electricity in a very environmentally friendly (low polluting) fashion. In principle, fuel cells offer the potential to achieve higher power densities per unit volume, longer use times, and longer total equipment lifetimes than standard battery packs. Long term, this translates to lower cost, higher utility, and increased mobility.
For example, depending on device performance specifics, a battery pack for a laptop computer can provide ≈40-50 W-h of energy. If the laptop requires an average of 20 watts of power to run, then the battery pack can provide only 2-3 h of running time before requiring recharging. Although larger batteries can be used, one pays a price in weight and convenience (size). Furthermore, recharging requires access to a power grid. In contrast, a similar sized fuel cell based on methanol is anticipated to produce 50 W of power and last for 10-20 h before total methanol consumption. In this instance, replacing a used canister of methanol with another does not require access to a power grid (not rechargeable), provides instant continuity and saves weight if it replaces a second, backup battery. Finally, if lost or destroyed, a methanol canister will be easier to replace and much lower in cost than a high-tech, high-density battery pack.
The most efficient fuel cells use H
2
as the reductant, and oxygen or air as the oxidant. The more advanced H
2
based fuel cells can produce 0.8-1.0 A/cm
2
at ≈0.7 V (0.5 W/cm
2
) with performance lifetimes measured in hundreds of hours. Unfortunately, the cost and weight required to store large quantities of gaseous H
2
, even as metal hydrides, are major drawbacks. Hence, fuel cells that use liquid hydrocarbon fuels, especially methanol (MeOH), are the focus of commercialization efforts.
Two of the more promising direct methanol fuel cell systems are the polyphosphoric acid fuel cell and the proton exchange membrane fuel cell (PEMFC). PEM based fuel cells are more convenient to work than polyphosphoric acids because they employ a solid acid electrolyte, e.g. Nafion® membrane.
The drawback to using MeOH as a fuel is that energy output can be much lower than hydrogen, typically in the 300-500 mA range at 0.5 to 0.3 V. For short runs, 0.8 A/cm
2
at ≈0.5 V (0.25 W/cm
2
) have been achieved. In part, the lower performance is due to CO and/or methanol poisoning of the cathode due to crossover through the membrane (CO and MeOH compete with O
2
for active catalyst sites). In part, this difference is due to the need to catalytically reform MeOH at the anode coincident with reacting the product hydrogen with oxygen, some efficiency is lost in the process. The methanol reforming reaction (1) is shown below:
CH
3
OH+H
2
O→CO
2
+3H
2
(1)
For example, platinum metal by itself is an excellent catalyst for hydrogen fuel cells based on:
However, CO (a typical impurity in many H
2
sources) competes with H
2
for active catalytic sites on Pt metal particles and readily poisons the catalyst. Thus, CO coverage of active catalyst sites limits the rate at which reaction (2) proceeds.
MeOH reforming, as shown in reaction (1), can actually proceed via two stepwise processes that can involve the formation of CO and/or CO
2
:
The CO produced via reaction (4), is very effective in poisoning simple Pt catalysts. The actual problem lies in the fact that Pt metal alone is not an effective catalyst for the water-gas shift reaction, reaction (7), making CO difficult to remove from the surface.
For the direct methanol fuel cell to be successful, an effective catalyst that promotes reaction (7) as part of the overall methanol reforming reaction is needed. Ruthenium is one of several metals that aid in promoting reaction (7).
Thus, improving the efficiency and activity of the MeOH reforming catalyst is desirable. A higher efficiency catalyst means less of the precious metal catalyst is required, and higher activity will minimize CO crossover poisoning of the cathode. It will be appreciated that there is a need in the art for highly active and efficient methanol reforming catalysts.
DISCLOSURE OF THE INVENTION
The present invention is directed to high activity, supported, nanosized mixed-metal catalysts, especially Ru/Pt catalysts for methanol reformation, and to methods of fabricating such catalysts. These methanol reformation catalysts are useful in methanol fuel cells, particularly portable, small footprint fuel cells such as polymer electrolyte membrane fuel cells (PEMFCs) that use methanol as a primary fuel source.
In a currently preferred embodiment within the scope of the present invention, the soluble metals are dissolved in a polyhydroxylic alcohol (polyol). The ratio of M
1
:M
2
:M
3
:M
4
will typically vary from (0.001 to 1):(0.001 to 1):(0.001 to 1):(0.001 to 1). Presently preferred catalysts typically contain Ru and Pt, with or without additional metals. The ratio of Ru:Pt will typically vary from 0.001:1 to 1:0.001, and preferably from 0.1:1 to 1:0.1, and more preferably from 0.5:1 to 1:0.5. The polyols are preferably viscous alcohols to minimize diffusion and thereby prevent particle growth. Typical polyols used in accordance with the present invention include organic diols, triols, and tetraols. Ethylene glycol, glycerol, triethanolamine, and trihydroxymethylaminomethane are examples of currently preferred polyols. In the polyol process one has two choices, (1) make the colloid in the absence of support and then deposit it on the support or (2) make it in the presence of the support such that the support aids in minimizing particle growth. A typical example of each option is described below, realizing that variations of these examples can be made by persons having ordinary skill in the art.
Typical Colloid Preparation Procedure
An amount of metallic precursor (or precursors) is added to 100 mL of refluxing ethylene glycol. The reaction mixture is refluxed for 15 min. A first aliquot is taken out and quenched in water at ice-water bath temperatures. The quenched solution is centrifuged several times by decanting supernatant and washing with ethanol. A second aliquot is taken after 1 h and same workup process is applied. Samples are then vacuum dried overnight. These materials can then be redispersed in alcohol and deposited on a known amount of pretreated support material, such as carbon black.
Typical Supported Powder Preparation Procedure
An amount of metallic complex (M
1
such as Ru complex) and an equivalent amount (by weight or mole) of a second metallic complex (M
2
, such as Pt complex) are dissolved in 10 mL of ethylene glycol, respectively. The two solutions are mixed and then added to a dispersion consisting of a weighed amount of support material, such as activated carbon, in 80 mL of ethylene glycol. The resulting mixture is refluxed and samples are taken after 15 minutes and 1 hour. Samples are quenched as above. The quenched solutions are centrifuged several times by decanting supernatant and washed with ethanol. Finally samples are vacuum dried overnight. The resulting catalyst is ready to use as is. Sometimes, it is flammable if kept from air during the preparation procedure.
The resulting catalysts include nanometallic powders on a support, bimetallic powders on a support, polymetallic nanopowders on a support, high surface area powders on a high surface area support, and low porosity metal nanopowders on a support.
The polyol solution is heated to a temperature in the range from 20° C. to 300° C.
Laine Richard M.
Sellinger Alan
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Cam N.
Silverman Stanley S.
LandOfFree
Preparation of supported nano-sized catalyst particles via a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Preparation of supported nano-sized catalyst particles via a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Preparation of supported nano-sized catalyst particles via a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3043363