Patent Application
20140298714
1. Field of the Disclosure
The present disclosure relates generally to fuel additives, and more particularly to fuel borne catalysts of use in high sulfur fuels.
2. Background information
Diesel engines are highly regarded for their efficiency and reliability. However, they may produce a level of pollution higher than that desired, and may need to have after-treatment strategies, including one or more of either a catalyzed Diesel Particulate Filter (DPF) or Diesel Oxidation Catalyst (DOC)—to control Particulate Matter (PM), Hydrocarbon (HC), and Carbon Monoxide (CO) emissions. High sulfur concentrations in the fuel may make some after-treatment devices ineffective due to fouling by sulfate and vigorous chemical attack by sulfur oxides on the DPF/DOC and the catalysts, as most active PGM catalysts for these oxidation reactions typically also catalytically oxidize SO2 to SO3, which in conjunction with water vapor forms sulfuric acid. SO2 may also compete with oxidizable species for active sites on catalysts, and hence may affect NO, CO, HC, and Carbon oxidation. This in turn may result in an increase of the temperature required for Diesel Particulate Filter regeneration and an increase in Balance Point Temperature.
As a result, a number of countries have mandated the use of Ultra Low Sulfur Diesel (USLD). However, fuels containing sulfur levels of up to 3000 ppm are still in use in various parts of the world—in some cases in spite of regulations to the contrary. These levels of sulfur may cause one or more of the aforementioned problems for after-treatment devices of use in the reduction of Particulate Matter (PM) emissions, including particulate filters and diesel oxidation catalysts. Reasons for these problems may include the ability of catalysts to oxidize both NO to NO2, a desired reaction, and SO2 to SO3, an undesired reaction.
The present disclosure relates to Fuel Borne Catalyst (FBC) formulations of use with High Sulfur fuels, which may include platinum, palladium, iron, manganese, cerium, yttrium, lithium, sodium, calcium, strontium, vanadium, silver and combinations thereof.
Embodiments of the present disclosure are described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. Unless indicated as representing prior art, the figures represent aspects of the present disclosure.
As used herein, the following terms have the following definitions:
“Fuel Borne Catalyst (FBC)” refers to any material suitable for use as a catalyst able to be stored in fuel as one or more of a solute, colloid, or otherwise suspended material.
“Conversion” refers to the chemical alteration of at least one material into one or more other materials.
“Catalyst” refers to one or more materials that may be of use in the conversion of one or more other materials.
“High Sulfur Fuel” refers to fuel with a sulfur content of about 100 ppm or greater.
“Low Sulfur Fuel” refers to fuel with a sulfur content of about 50 ppm or fewer.
“Platinum Group Metals (PGMs)” refers to platinum, palladium, ruthenium, iridium, osmium, and rhodium.
Various example embodiments of the present disclosure are described more fully with reference to the accompanying drawings in which some example embodiments of the present disclosure are shown. In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. Detailed illustrative embodiments of the present disclosure are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present disclosure. This disclosure however, may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein.
Platinum and palladium compounds of use as Fuel Borne Catalysts (FBCs) include those described in U.S. Pat. No. 4,892,562; U.S. Pat. No. 5,034,020; and U.S. Pat. No. 6,003,303. These include soaps, B-diketonates and alkyl and arylalkyl metal complexes. One or more of said compounds may be suspended in fuel or may be fuel soluble and fuel stable at dose rates, including rates below 0.5 ppm metal and as discussed in the patents cited above.
Transition metals suitable for use in FBCs include iron, manganese, silver, and vanadium. These metals can be present in FBCs as any suitable compound, including long chain carboxylate salts with carbon chain lengths of 5 to 20 carbon atoms or, for iron, as ferrocene and ferrocene derivatives. Suitable forms of silver may include: fuel soluble carboxylates, such as a long chain alkyl soap with 5-20 carbon atoms; a substituted benzoate salt with 10 or more carbon atoms, including a benzene ring; an acetylacetonate; and any suitable derivatives.
Forms of Vanadium suitable for incorporation as a fuel stable complex include various forms and oxidation states, such as VO(acac)2 and derivatives, divalent V(cpd)2 and derivatives, and V(OR1)2(R2)2 (vanadium oxyalkyls and alkyls) where R contains at 4 or more carbon atoms and can be mixed oxyalkyl with alkyl or the alkyl or oxyalkyl may be all the same as in V(R)4. Vanadium soaps of +4 and +5 oxidation states may also be of use, including amongst others V(OOCR)n, where R is an alkyl group containing 5 or more carbon atoms and n is 4 or 5.
Other transition metals suitable for use in FBCs include one or more rare earth metals, such as cerium and yttrium, and may be employed in any suitable form, including those listed in patents cited in this disclosure. Suitable forms include one or more soaps, acetylacetonates, and the like.
Other suitable metals include Calcium, Strontium, Sodium, and Lithium. Calcium and strontium may be present in the FBC in any suitable form, including long chain soaps, M(OOCR)2, where R contains at least 5 carbon atoms. Sodium and lithium may also be supplied as long chain carboxylates with more than 9 carbon atoms.
These may combine with long chain polar co-solvents to increase stability of relatively more polar alkali metal compounds, such as compounds of the type HOR1OR2OR3, where R is alkyl. Suitable co-solvents of this type include diethylene glycol monobutyl ether. Other suitable co-solvents include ROH where R contains at least 6 carbons, octanol and other higher alcohols.
SO3 and H2SO4 Formation
In the absence of catalysts, about 1-2% of SO2 in the engine exhaust may be converted to S03. High SO3 concentrations are favored by low temperatures. However, at low temperatures, the un-catalyzed reaction rates are insignificant-explaining why engine out SO3 is normally only about 1% of total sulfur oxides. SO3 reacts exothermically with the moisture present in the exhaust to form sulfuric acid and other sulfates that may readily foul filter surfaces and may cause operational problems.
The exhaust gas of a typical diesel engine operating on a fuel containing 1000 ppm of Sulfur may contain 30 ppm of SO2 and about 0.3 to 0.6 ppm of S03. Some catalysts, including those using Pt on Al203, may increase the SO2 oxidation rate to 50% or more at temperatures as low as 250° C., which may result in over 15 ppm SO3 in the exhaust. SO2 may also compete with oxidizable species for active sites on catalysts, and hence may affect NO, CO, HC, and Carbon oxidation. This in turn may result in an increase of the temperature required for Diesel Particulate Filter regeneration and an increase in Balance Point Temperature by as many as 33° C., though these increases may stop once sulfur concentrations in fuel reach 30 ppm or higher.
The use of similar metals in an FBC formulation may not induce these effects when the size of the catalyst particle is below about 40 nm.
SO2 may also diminish FBC PGM catalytic activity at concentrations above 10 ppm, including concentrations between 10 ppm and 30 ppm. This may result in unreliable regeneration at low FBC concentrations in the fuel and at moderate temperatures. Li2O2—formed by combustion—may partially alleviate the problem of SO2 deactivation by wetting the Pt crystallite. However, lithium may not be a very effective metal oxidization catalyst, and and auxiliary heat pre-filter may be routinely required.
FBC Formulations
FBC formulations of use with high sulfur fuel include formulations containing one or more of the following and combinations thereof:
Additional materials of use in the fuel include:
FBCs including aforementioned materials may facilitate NO2 carbon oxidation mechanisms, while producing relatively small amounts of NO2 in the exhaust when compared to other catalyst systems. The use of certain metallic FBC components—including platinum, palladium and combinations thereof—may provide negligible SO3 formation while maintaining carbon oxidation catalysis by nearest neighbor atom NO2 formation as described below. At temperatures similar to those commonly found in DOCs and DPFs, these NO2 enhanced oxidation reactions may dominate.
Increasing the concentration of NO2 in may be beneficial, since NO2 may be a much more effective oxidizing agent than O2 for carbon-based PM soot, Soot loading is one of the limiting factors in DPF operation.
When used in FBCs with no PGM content, transition metals including Mn, Fe, and Cu, may foul DPFs at the concentrations necessary. The addition of Na and Li additives to these types of FBC may allow regeneration at temperatures low enough to permit regeneration at high doses of metal and may show some promise in improving trap regeneration. However, CO and HC reduction may not occur.
Methods suitable for enhancing DOC/FBC soot oxidation performance in high sulfur fuels include those functioning at the nanoscale level. These methods may not drive the bulk gas phase NO2 concentration to hundreds of ppm, but may instead enhance NO2 formation and reduce the ill effects of sulfur oxides at the Platinum-Transition Metal Oxide-Rare Earth Metal Oxide [Pt-TMO-REO] (or Platinum-Base Metal oxide [Pt-BMO]) crystallite surface.
SO2 may temporarily inhibit the aforementioned mechanism by weak adsorption of SO2 on the nanoparticulate Pt-BMO complex. This may increase filter regeneration temperatures, which may be suitable if operating temperatures are suitably high. Suitable alkali metals, including sodium or lithium, may be used as part of the FBC to continuously wet the Pt/BMO crystallite with a suitable peroxide film, including Na2O2/Li2O2, to reduce this problem.
According to K. Krishna, M. Makkee, (“Pt—Ce-Soot generated from Fuel Borne Catalysts: Soot Oxidation Mechanism”, Topics in Catalysis, 42-43, 229-236 (2000)), the evidence is that nitrate rather than free NO2 itself is the most effective carbon oxidizer at the lowest temperatures and that NO2 complex formation is key to forming the C—ONO2 (or the precursors such as Pt-BM(NO2)(O)) carbon-nitrate oxidation couple that directly leads to carbon oxidation (and to some extent NOx reduction) by this mechanism. The oxidation of soot may take place via the so-called surface oxygen complexes. For soot oxidation to proceed readily, defects in the soot surface may be necessary. NO2, and especially nitrates, are very active surface defect initiators, and may thereby assist in the formation of surface oxygen complexes and the acceleration of soot oxidation. SO2 may be adsorbed by the Pt and Pt-BMO crystallites in a weak surface association and inhibit the desired oxidation sequence.
Fuel Borne Catalysts may promote soot oxidation at low temperatures. Close contact between the (Pt)-BM oxide nanostructure and the carbon may allow the nitrate decomposition product to readily react with carbon in the soot, thereby creating high efficiency defects in the carbon soot structure.
In this example, a comparison of the DPF backpressure characteristics for two FBC formulations is made. In this example, the Reference FBC is a bi-metallic formulation containing only platinum and cerium that may very effectively cause filter regeneration (also as noted in the charts) used in low sulfur fuel but may be ineffective in a High Sulfur fuel containing 1000 ppm Sulfur. In this example, the “HSF FBC” contains platinum, cerium and iron with a similar amount of total metal, but the amount of metal corresponding to cerium in “Reference FBC” is instead split between iron and cerium in “HSF FBC”. This use of this composition causes regeneration to occur with 1000 ppm S fuel under the same engine duty cycles (400 C exhaust temperature max), where the “reference” formulation failed.
Analysis of PM from these cycles indicate neither Reference FBC nor HSF FBC significantly generate SO3 or SO4 in high S fuel, as sulfate is below 10% of the PM for both cases by analysis and comparable to baseline measurements and, in neither case, is the amount of FBC metal in the PM greater than the amount of lube oil metals that may be normally present. The difference in filter regeneration performance is not due to the differences in total metal present or to differences in sulfate in the PM, but due to a differing mechanism which may reduce the adverse effects on PM carbon oxidation catalysis caused by high concentrations of SO2 in the exhaust when platinum, cerium and iron are all present, as compared to the reference case where similar amounts of only platinum and cerium are present.
