Abstract:
Described is a method of slowly releasing a catalyst for, inter alia, the purpose of soot oxidation in a particulate filter. An example method includes incorporating an alkali metal oxide in a glass. Slow leaching of the alkali from the glass provides a means to gradually deliver the catalyst over extended periods. Additionally, the example method includes increasing the amount of alkaline metal ions that may be leached from the glass.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 60/898,851, entitled “Application of Glass Catalysts for Diesel Soot Oxidation,” filed on Feb. 1, 2007, and U.S. Provisional Patent Application No. 61/012,870, entitled “Catalyst Development for Diesel Particulate Filters,” filed on Dec. 11, 2007, which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to catalysts, and, more particularly, to catalysts with slow, passive release of alkali ions. 
     BACKGROUND 
     Combustion engines such as diesel engines typically produce unburned fuel residues or particulates i.e., soot, which is usually composed mainly of amorphous carbon. Exhaust from vehicles containing soot becomes airborne particulate matter and increases pollution, particularly in urban areas. Soot also is carcinogenic and, therefore, very hazardous to the lungs and general health when inhaled. 
     Diesel Particulate Filters (DPFs) have been developed to remove soot from the exhaust gas of diesel engines. After a period of time, however, enough soot may have collected on a DPF to cause an increase in pressure drop across the DPF, which results in compromised operation of the engine. Therefore, the DPF must be cleaned of accumulated soot. Some DPFs are designed for single use and are disposable, while other filters are designed to burn off the soot through carbon soot combustion or oxidation, which is known as filter regeneration. Filter regeneration may occur actively through a fuel burner that heats the soot to combustion levels or passively through the use of a catalyst. 
     Several types of filters have been used with various vehicles. For example, Flow-Through Oxidation Catalysts are used in filters to remove diesel particulate matter, CO and hydrocarbons, including the ones that form the soluble organic fraction of the total particulate mass. The oxidation catalyst used with Flow-Through Oxidation Catalysts is typically a platinum-rhodium-platinum catalyst deposited on a flow through monolith, where soot particles are not trapped. The oxidation catalysts convert CO and hydrocarbons at ˜200° C. but achieve less than 5% oxidation of the particulate matter. Platinum, however, is an excellent SO 2  oxidation catalyst. At temperatures above approximately 300-350° C., the catalyst oxidizes SO 2  to SO 3 , which quickly combines with water to form sulfuric acid and contributes significantly to the total particulate mass. To limit emissions above 300° C., a more specific catalyst should be used to minimize SO 2  oxidation. A tailored catalyst with comparable activity may be made by alloying less active oxidation catalysts, rhodium and palladium. In addition, base metals also may be used to tailor the activity of platinum such as, for example, where only soluble organic portions of the total particulate matter need to be lowered to meet any particular emission standards. 
     In addition, NO x -Aided CRTs are a type of filter typically used with trucks and buses. The NO x -Aided CRT includes a wall-flow monolith with an upstream flow-through diesel oxidation catalyst, called a preoxidizer, such as a platinum catalyst, and a cordierite wall-flow monolith downstream. The preoxidizer converts 90% of the CO and hydrocarbons to CO 2  and 20-50% of the NO to NO 2 ; the particles are trapped on a cordierite wall-flow monolith and subsequently oxidized by the NO 2 . NO x -Aided CRTs effectively oxidize all of the carbon components in diesel exhaust that include small particles and unregulated compounds. In addition, NO x -Aided CRTs reduce NO x  concentration by approximately 3-8%. Furthermore NO x -Aided CRTs have a reasonable temperature window of approximately 200-450° C. (200° C. is needed for CO and hydrocarbon oxidation, and 450° C. relates to the chemical equilibrium between NO and NO 2 , which is not favorable above 450° C.). Also, NO x -Aided CRTs have higher stability because of the continuous regeneration, which avoids extreme temperatures and enhances stability. However, these NO x -Aided CRT systems also have some limitations including requiring low-sulfur fuel, which makes wide-scale introduction unfeasible. 
     For light-duty vehicles such as, for example, passenger cars, an Integrated Catalytic Trap filter may be used that includes a silicon carbide wall-flow monolith, engine-controlled heating through fuel-injection-timing controls, cerium fuel additives and a preoxidizer (e.g., platinum catalyst). These systems include two catalyst technologies and make several catalyst mechanisms available including, for example, cerium-aided periodically induced self-supporting regeneration; cerium-catalyzed spontaneous local regeneration reactions at low temperatures; cerium-catalyzed continuous soot oxidation at high temperatures; cerium-catalyzed reduction of black smoke after some initial cerium deposition in the combustion chamber and exhaust system (cerium fuel additive reduces the raw particulate emissions by approximately 20%); platinum-catalyzed oxidation of volatile hydrocarbons and CO; platinum-catalyzed production of NO 2  at favorable temperatures; and platinum- and cerium-catalyzed synergetic oxidation of soot. However, this system is oftentimes complex and expensive and the trap should be cleaned periodically to remove cerium deposits. 
     Furthermore, alkali metals such as potassium improve the activity of catalysts for carbon soot combustion. Unfortunately, because of the high temperatures needed for carbon soot combustion and because traditional K 2 CO 3 , KOH, and KO 2 , potassium catalysts typically have low thermal stability, if the potassium is mobile and not tightly bound in the compound (i.e. if it is “free” potassium) potassium may be lost via evaporation or sublimation, etc. This results in a subsequent reduction in desired catalytic activity and, therefore, limits the usefulness of some alkali metal containing compounds as catalysts if there is no way to replenish the catalyst over time. This is true in a DPF environment and in other industries such as, for example, in coal gasification reactions wherein potassium-based catalysts are quickly consumed as a result of the combustion process. 
     U.S. Pat. No. 6,631,612 describes a device and method of filter regeneration used to avoid a reduction in desired catalytic activity. The device and method described in U.S. Pat. No. 6,631,612 adds seawater containing alkali metals to the filter to replenish the alkali metals used a catalyst in the filter regeneration. This device and method require a readily available supply of seawater, which is impractical for land-based vehicles. Furthermore, the device and method described in U.S. Pat. No. 6,631,612 require constant, repetitive steps, i.e., adding seawater, which can be onerous. This is an example of “active” catalyst replenishment, as opposed to “passive” replenishment as detailed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is flow chart describing an example process of forming an example oxide catalyst. 
         FIG. 2  shows a temperature versus carbon weight percentage comparison for a mixed-K 0.1 Fe 0.9 MnO 3  system and a K 0.1 Fe 0.9 MnO 3  system. 
         FIG. 3  shows thermogravimetric analysis curves comparing KO 2  and K 2 Mn 2 O δ  soot oxidation catalysts. 
         FIG. 4  is flow chart describing an example process of forming an example glass catalyst. 
         FIG. 5  is an enlarged view from a scanning electron microscope (SEM) of an example glass catalyst made from the example process of  FIG. 4 . 
         FIG. 6A  shows the same T ig  curve for K 2 Mn 2 O δ  along with the T 50  and T max  curves. 
         FIG. 6B  is a graph showing example thermogravimetric analysis curves for soot combustion using K 2 Mn 2 O δ  as a catalyst. 
         FIG. 6C  a graph showing the ignition temperature, (T ig ), the temperature at which half of the carbon has combusted (T 50 ) and the temperature at which maximum soot combustion rate is reached (T max ) for an example series of thermogravimetric analysis runs for an example potassium glass as a catalyst. 
         FIG. 6D  is a graph showing example thermogravimetric analysis curves for soot combustion using an example potassium glass as a catalyst. 
         FIG. 7  is a graph showing the ignition temperature (T ig ) and T 50  values for an example series of thermogravimetric analysis runs for glass catalysts comprising particles of varying sizes. 
         FIG. 8  is a partial cross-sectional and exploded view of an example diesel particulate filter. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are methods of slowly releasing catalysts to promote soot oxidation. The example catalysts described herein include an oxide catalyst and a glass catalyst. The example catalysts described herein can be used to facilitate removal of particulate carbon and residual hydrocarbonaceous material from engine exhaust filters, especially particulates emanating from diesel engines. In the example potassium-doped catalysts described herein, the potassium is made available through a controlled-release mechanism. In the case of the example crystalline oxide (K 2 Mn 2 O δ ), the delivery occurs through gradual decomposition of the starting compound. In the case of the glass described later, this occurs through ions leaching from a glass. Both mechanisms provide a means to slowly deliver the catalyst (K ions) for soot oxidation. 
     A first example catalyst for use in a soot filter is the oxide catalyst, which is described herein. Any number of methods such as, for example, solid state oxide processing, sol-gel processing, etc. are suitable to produce the example oxide compound in a powder form. One example method is shown in  FIG. 1 . 
     In particular, the example oxide catalyst may be synthesized by the example synthesis process  100  shown in  FIG. 1 . In the example process  100 , a chemical system is chosen (block  102 ), which may be, for example, mixed and complex metal oxides, including alkali-substituted perovskite and spinel type oxides. The system may include binary, ternary, or even more components. The component elements may focus on alkali metals and transition metals such as, for example, manganese, molybdenum, vanadium, lanthanum, copper, cerium, and iron. 
     Metal nitrates, carbonates, or oxides may be used as the source of metal cations (block  104 ) and dissolved in a solvent (block  106 ). The metals and the solvents are chosen based on their properties to form stable solution precursors with appropriate element ratios so the concentration of each component is not too low. The precursors may include Cu(NO 3 ) 2 .3H 2 O, Ce(NO 3 ) 3 .6H 2 O, Co(NO 3 ) 2 .6H 2 O, La(NO 3 ) 3 .6H 2 O, Fe(NO 3 ) 3 .9H 2 O, MoO 3 , Mn(NO 3 ) 2 , LiNO 3 , CsNO 3 , NaNO 3 , V 2 O 5  or any other suitable precursor. The solvents could be, for example, H 2 O (for most metal salts), or NH 3 .H 2 O (for MoO 3 ), or any other suitable solvent. Ethylene glycol (“EG”) also may be used as a complexing agent. 
     At least a portion of the example process  100  is known as the Polymerizable Complex Method (“PCM”), which is also known as the in-situ polymerization route. In PCM, the metal ions are dissolved in solution with a chelating agent (block  108 ), which itself may be formed by, for example, combining citric acid (“CA”) and a polyhydroxyl alcohol such as, for example, EG, with a solvent (block  110 ). In this example, the CA and EG may be dissolved in distilled water in a molar ratio of approximately 60 to 40. The metal ions are chelated by the CA and are evenly distributed throughout the solution. These organic precursor solutions may be transferred to an alumina crucible along with metal nitrate solutions in a molar ratio of approximately 38 metal to 62 organic, for example. Though other ratios maybe used as well. 
     After the metal cations form stable chelated solutions in the citric acid, they are dried (block  109 ). In one example, the chelated solutions are dried at, for example, 95° C. for a suitable time, such as, for example one hour (block  111 ) to evaporate the solvent and to form viscous resins. Heat is applied at, for example, approximately 105-180° C. for an hour or any suitable time (block  112 ) to the resins so the EG undergoes polyesterficiation and forms polymer precursors (block  114 ). During this time, the polymerization occurs between carboxylic acid groups and alcoholic groups. The result is a polymer resin with homogenously distributed metal ions. Heat may also be applied at, for example, approximately 300-400° C. for an hour or any suitable time (block  116 ) to slowly heat the combination to decompose the precursor, i.e., the resin, (block  118 ). Additional heat may be added, for example 600-800° C. for three hours or any suitable time (block  120 ) to calcinate and form the oxide powder (block  122 ). Because the PCM is based on a liquid mixing process, the metal ions are mixed on a molecular level facilitating a lower processing temperature and a faster synthesis time than a comparable solid state process. The oxide powders produced in this manner typically have higher surface areas than solid state processed powders. 
     There have been many reported catalysts that show activity for diesel soot combustion. These include noble metals, single, mixed, and complex metal oxides as well as low melting point compounds. Alkali doped compounds such as, for example, those based on potassium, lithium, cesium, or sodium, show better catalytic activity than those compounds without alkali doping. In particular, potassium is a very active alkali metal catalyst for carbon oxidation and many potassium-rich oxides ignite soot at about 300° C., which is among the lowest known oxidation temperatures of all the alkali metals. 
     The active catalytic species in the example potassium-doped catalysts described herein is termed “free” potassium, i.e., potassium that is mobile and not tightly bound in a lattice or matrix. This is illustrated by the example of K 0.1 Fe 0.9 MnO 3 . If the compound is synthesized by reacting at elevated temperatures (yielding K 0.1 Fe 0.9 MnO 3 ), the potassium is bound in a perovskite lattice while in a mixed-K 0.1 Fe 0.9 MnO 3  system (Fe 0.9 MnO 3  and K 2 CO 3  powders were mixed to have the ratio of K:Fe:Mn=0.1:0.9:1.0, termed “mixed-K 0.1 Fe 0.9 MnO 3 ”), potassium is in a more active/reactive state in the form of potassium carbonate. In the mixed-K 0.1 Fe 0.9 MnO 3  system, the ignition temperature is lowered to about 300° C., while the ignition temperature is only lowered to about 450° C. in the K 0.1 Fe 0.9 MnO 3  system, as shown in  FIG. 2 . Thus, systems with potassium in a “free” or mobile state more effectively lower the ignition temperature. The high activity of potassium-rich catalysts described herein is due to the existence of dissolvable, active potassium such as, for example, potassium in the form of carbonate, oxides, hydroxides, or other binary or ternary oxides that can be decomposed by polar solvents and release potassium cations such as, for example, KFeO 2 , which decomposes into Fe 2 O 3  and K +  in water or methanol. The activity of the potassium-rich catalysts is not a synergetic function of transition metal elements and potassium in the oxide lattice. 
     Furthermore, as mentioned above, the use of polar solvents may improve the soot/potassium contact and thereby lower the soot ignition temperature. Water vapor, in particular, is present in diesel exhaust and when condensed, likely will dissolve potassium cations and create intimate soot-potassium contact. The more intimate the contact, the greater the catalytic activity. Thus, such potassium-containing catalysts could be advantageous for use in diesel soot combustion. However, potassium has low thermal stability, which may be a factor considering the high temperatures associated with a diesel engine. 
       FIG. 3  shows thermogravimetric analysis (TGA) results comparing KO 2  and K 2 Mn 2 O δ  soot oxidation catalysts. TGA is a type of testing that is performed on samples to determine changes in weight in relation to change in temperature. Shown are the carbon soot ignition temperatures, (T ig ), as a function of repeated cycling in the TGA. Despite starting with equivalent amounts of potassium in the two cases, the KO 2  activity diminishes after the 13 th  cycle, while the K 2 Mn 2 O δ  does not start decreasing until after 19 combustion cycles. X-ray diffraction data during this cycling verifies that the K 2 Mn 2 O δ  (K:Mn ratio of 1:1) slowly transforms to K 2 Mn 4 O 8  (K:Mn ratio of 1:2), releasing “free” K in the process, providing the catalyst for soot combustion. 
     Comparing the TGA cycling lifetime results for K 2 Mn 2 O δ  and KO 2 , shows that K 2 Mn 2 O δ  has longer lasting activity than KO 2 . The longer activity life is due to the slower loss of potassium from K 2 Mn 2 O δ  than from KO 2 , due to the K being delivered more slowly as the phase transformation from K 2 Mn 2 O δ  to K 2 Mn 4 O 8  occurs. Therefore, the slow release potassium results in an increased catalyst activity lifetime. 
     The presence of free or mobile potassium can be reduced by sublimation during the soot combustion reaction, which results in the loss of active potassium on the catalyst surface over time and, therefore, decreases the activity stability of the catalyst. To counteract the reduction of potassium, a method for passive, slow release, which is also known as “timed released” or “controlled released” of catalysts or compounds to provide for replenishment of the active catalyst may be used. This timed release may be obtained by not only structure and composition modifications of potassium-bearing materials such as, for example, K 2 Mn 2 O x , (discussed above) but also by KAlSi 3 O 8 , K(Fe,Mg) 3 AlSi 3 O 10 (OH) 2 , intercalation compounds, and any other compounds that allow for slow release of active catalyst ions through processes including ion exchange, leaching, etc. 
     An alternative example catalyst utilizes a glass that can slowly release a catalyst. An alternative example method  400  of forming a glass catalyst for use in a soot filter is described herein and shown in  FIG. 4 . The alternative example method  400  includes combining a silicon oxide with an alkali bearing compound such as an oxide or carbonate, to form a mixture (block  402 ), which is then heated (block  404 ). A glass formed during the process is removed (block  406 ), cooled (block  408 ), and reduced to a powder (block  410 ) to form the catalyst used in the soot filter. 
       FIG. 5  is an enlarged view from a scanning electron microscope (SEM) of an example glass catalyst made from the example process of  FIG. 4 , that has been dispersed onto, and subsequently sintered to a cordierite substrate. A glass is an amorphous solid completely lacking in long range, periodic atomic structure, and exhibiting a region of glass transformation behavior. Any material, inorganic, organic, or metallic, formed by any technique, which exhibits glass transformation behavior is a glass. Thousands of chemical compositions may be made into a glass. Typical glass contains network-formers, modifiers and stabilizers. Network-formers form the random network of glasses such as, for example, SiO 2 , Al 2 O 3 , B 2 O 3 , P 2 O 5  and As 2 O 5 . In addition to the network-former, glasses may contain oxides that do not participate in forming the network structure. These oxides are the modifiers such as, for example, K 2 O and Na 2 O. In addition the stabilizers such as, for example, CaO (lime), make the glass strong and water resistant. Without stabilizers, water and humidity may attack and dissolve glass. 
     Elaborating on the example method  400  of  FIG. 1 , an example glass, referred to herein as “Glass-1” may be made by combining SiO 2 , K 2 CO 3  and CaCO 3  to form a combination or mixture (block  402 ). The mixture may be put into Al 2 O 3  crucibles and heated to, for example, approximately 1100° C. at 5° C./min. (block  404 ) and held there for approximately an hour or any suitable time. Because the glass does not wet Al 2 O 3 , the glass generally is easily removed from the crucibles (block  406 ). The obtained glass may then be ground into a powder and sifted, for example, with sieves (block  410 ). The glass may have a particle size of about 149 to about 177 microns (80-100 mesh), though other particle sizes may be used as well including down to about one micron or smaller. 
     The above-mentioned glass catalyst releases potassium from the bulk onto the surface at a controlled rate that is fast enough to supplement the lost surface potassium and, thus, decrease the degradation rate of the catalysts; while also being low enough to keep the catalysts&#39; activity lifetime at a reasonable level. The glass is particularly useful in DPFs because of its suitable potassium release rate, its stability at high temperatures (e.g., &gt;800° C.), and its adequate mechanical strength. 
     When Glass-1 is used as the catalyst in a wet methanol environment, the soot combustion temperature is about 20° C. higher than when K 2 Mn 2 O δ  is used, but still adequate when compared to other known non-alkali-doped catalysts. This temperature difference relative to K 2 Mn 2 O δ  is possibly due to a lower content of potassium on the glass surface (e.g., about 30%) than on K 2 Mn 2 O δ  (e.g., about 50%). Alternatively, it may be due to the slower leaching rate of potassium from Glass-1 than from K 2 Mn 2 O δ  in certain environments (e.g., methanol). However in alternative environments (e.g., dry, tight contact), Glass-1 may be more active than K 2 Mn 2 O δ . 
     For example,  FIGS. 6A and 6C  are graphs showing ignition temperature, (T ig ), the temperature at which half of the carbon has combusted (T 50 ) and the temperature at which maximum soot combustion rate is reached (T max ) for a series of TGA runs for K 2 Mn 2 O δ  and Glass-1, respectively.  FIGS. 6B and 6D  are graphs showing representative individual TGA curves for soot combustion using K 2 Mn 2 O δ  and Glass-1 as the catalysts, respectively. As shown in  FIG. 6A , when K 2 Mn 2 O δ  is used as the catalyst, T ig  increases from approximately 325° C. in the first run to approximately 425° C. in the twentieth run. In contrast, as shown in  FIG. 6C , when Glass-1 is used as the catalyst, the soot ignition temperature T ig  increased from approximately 311° C. in the first run to approximately 372° C. in run number 51. This is about a 61° C. increase after 51 runs. Because most of the carbon is combusted in the first step, T ig  and T max  change at approximately the same rate. T 50  of soot combustion varies somewhat in this example, with an average value of about 378° C. within the 51 runs. Therefore, the Glass-1 catalyst has a much longer activity lifetime than the potassium oxide catalyst K 2 Mn 2 O δ . This is because the glass structure facilitates the slow release of K +  ions to the surface, where the ions can continue promoting the soot oxidation reaction. 
     In this example, Glass-1 is approximately 35% K 2 O, 52% SiO 2 , and 13% CaO, but other compositions may be used in other examples. When SiO 2  is fixed at about 50%, the soot combustion temperature decreases with an increase in the percentage of K 2 O. When CaO is fixed at about 10%, the glass activity is maximized when K 2 O is between around 35-40%. When the percentage of K 2 O was fixed at about 35%, the soot combustion temperature increased with the percentage of CaO. Considering that real soot-catalyst contact is similar to loose contact, the optimum composition for SiO 2 -K 2 O—CaO glass is possibly around 50-55% SiO 2 , 30-35% K 2 O, and 10-15% CaO. Many other additions or substitutions to the glass can be made including, but not limited to TiO 2 , Cr 2 O 3 , Li 2 O, B 2 O, etc. In addition, the glass may include one or more of various other elements that complement the catalytic activity of sodium such as, for example, Ca, Sr, Ba, Mg, Mn, Cr, V, Ti, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Sb, La, Hf, Ta, W, Ir, Pt, Au, Bi, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or mixtures thereof. 
     Because of the exothermal oxidation of soot there is a local rise in temperature after ignition, the magnitude of which depends on the thermal properties of the filter material (thermal capacity and conductivity) as well as the exhaust temperature. In the worst case, temperatures of approximately 800° C. or higher can occur locally in the filter when the engine is stopped after ignition of the soot in a fully loaded trap. Such a high temperature may lead to sintering or even melting of a glass-based catalyst. Sintering and melting of glass powder will decrease the surface area of the glass and, thus, the catalytic efficiency of the glass. Sintering and melting also may block the micropores of the filter, which will lead to an increased back pressure. Therefore, the glass catalyst should be of optimum composition that has high activity and long activity lifetime, as well as a high softening/melting temperature. 
     In one example, when the percentage of SiO 2  is kept at around 50% and the percentages of CaO and K 2 O are varied, the softening temperature of glass dropped with an increase of K 2 O content. The same behavior is observed when the percentage of CaO is fixed and the percentages of SiO 2  and K 2 O are varied. However when the percentage of K 2 O is held constant at, for example, around 35%, a maximum softening temperature is observed at about 10% CaO and about 55% SiO 2 . K 2 CO 3 , for example, may be added as a flux or diffuser to decrease the melting point of the glass. Therefore, reducing the amount of K 2 O in glass raises the softening temperature and melting point. However, this also will decrease the amount of active phase in the glass catalyst and also its catalytic activity. When the percentage of K 2 O is fixed, the softening temperature will be determined by the relative amounts of other components in the glass. Thus, adding other elements to the class may increase the softening temperature of glass while maintaining activity. 
     Lubricants used in internal combustion engine oils can contain compounds that include phosphorous and/or zinc such as zinc dithiophosphate (ZDTP) and zinc dithiocarbonate (ZDTC). These compounds can pass from the engine and accumulate on the emission treatment catalyst, which can cause catalyst poisoning and deactivation. Sulfur in diesel fuel also has a major negative impact on catalyst performance. These poisons may accumulate on the surface of a washcoat, creating a physical barrier, or they may interact with the catalytic material in the washcoat, resulting in loss of catalytic activity, and/or become a barrier to particulate filters (such as foam, screens and wall-flow filters). However, the introduction of poisons such as for example, phosphorous, zinc or sulfur, in a DPF with a potassium-rich glass catalyst such as, for example, Glass-1, will not significantly influence the catalytic behavior of the potassium-rich glass. 
     Because degradation of catalytic activity of potassium-rich glass catalysts occurs due to the depletion of potassium from near the surface region of the catalyst, replenishment of the potassium will improve the catalytic activity and make the catalyst suitable for use in DPFs. The catalytic performance of the glass depends on the potassium ions being replenished at the surface through a leaching process. In addition, as described below, the potassium leaching rate also may be affected by the glass composition, composition of reaction gas, the soot/glass contact, and/or size of glass particles. 
     A high content of network modifiers (especially potassium) can be disadvantageous for the stability and durability of glasses. The presence of network modifiers will break A-O-A bonds (where A is the cation in the network-formers) and create non-bridging oxygen, which reduces connectivity of the glass network and, therefore, decreases the chemical resistance. With more modifiers in the glass, faster potassium leaching occurs, which yields a lower activity degradation rate. However, increasing the percentage of K 2 O favors a more rapid flux of potassium ions to the surface upon depletion and, therefore, a reduced activity degradation rate. Hence, it also is possible that the depleted region does not extend as far into the glass so the glass with a higher percentage of K 2 O has more potassium in the depleted region, and so degrades more slowly. 
     In alternative examples, various substitutions may be included in the composition of the glass. For example, phosphate may be introduced to produce phosphate glasses, which are often used as controlled released glasses in biological applications. Phosphate glasses typically dissolve more rapidly than silicate glasses due the asymmetry of the PO 4  tetrahedron unit. The dissolution of phosphate glasses in an aqueous medium is realized by the breakage of P—O—P bonds in the phosphate network within the hydrate layer. Substitution of SiO 2  by P 2 O 5  completely or partially may increase the potassium leaching rate. The addition of P 2 O 5  makes the glass easier to melt, and as the percentage of P 2 O 5  is decreased, the melting point increases. For example, when P-60-glass (i.e., approximately 60% P 2 O 5 , 30% K 2 O and 10% CaO) is used as the catalyst, the ignition temperature may be approximately 550° C., and when P-20-glass (i.e., approximately 20% P 2 O 5 , 30% K 2 O, 10% CaO and 40% SiO 2 ) is used, the ignition temperature may be about 535° C., where the ignition temperature for Glass-1 is about 500° C. The exotherm from soot combustion could lead to melting of phosphate glasses, leading to soot encapsulation by the glass. This will keep oxygen from reaching the soot particles, which might cause an incomplete combustion of the soot. 
     Cerium may also be substituted into the glass formulation. Cerium is often used as a fuel additive to remove soot from DPFs. When used in glass catalysts, cerium may increase the activity of the glass. In other examples, chromium (e.g., in the form of Cr 2 O 3 ) may be substituted for some of the SiO 2 . Chromium may also enhance the activity stability of glass catalysts. 
     In further examples, alumina may be used as a substitute. Glasses with a relatively low SiO 2 +Al 2 O 3  content have temporary increases in the leaching rate of potassium and other cations when initially exposed to water. Alumina may also raise the softening temperature. In particular, 5-7.5% Al 2 O 3  may provide a glass of approximately 50% SiO 2 , 40% K 2 O and 10% CaO with a high softening temperature and catalytic activity. As noted above, a decrease of the percentage of K 2 O will enhance the glass chemical durability, decrease the replenishment rate of potassium, and increase the degradation rate of the glasses activity. When K 2 O is substituted by Al 2 O 3  in a glass with the remaining composition remaining fixed, the glasses degrade at a similar rate. Thus, the substitution of K 2 O by Al 2 O 3  can counteract the acceleration of degradation due to the decreased percentage of K 2 O. 
     Furthermore, the activity is affected by the size of the particles of the catalyst. The smaller the glass particle size, the larger surface area the catalyst has, the faster the leaching of potassium may be and the more stable the catalytic activity the glass provides.  FIG. 7  is a graph that illustrates that in one example, the ignition temperature and the T 50  is generally lower when the particle size is smaller. 
     An example DPF  800  is shown in  FIG. 8 . The DPF  800  is designed to be positioned in the exhaust of a vehicle and collect solid and liquid particulate emissions such as, for example, soot. The DPF  800  is able be able to constantly withstand the high temperatures of diesel exhaust gas (e.g., up to approximately 975 K). The DPF  800  may also be subject to temperatures over 1250 K that are caused by exothermic reactions during oxidation of soot trapped in the DPF  800 . In addition, the pressure drop over the DPF  800  should be low to avoid decreasing engine performance. The example DPF  800  includes a wall flow monolith  802 , which is a high efficiency filter that may be made of cordierite or silicon carbide. Other example filters may include ceramic foam, candle filters, wire mesh, metal wool, and/or any type of suitable substrate. The example monolith  802  is a ceramic structure that include parallel channels  804 , of which half are closed at the upstream end in an alternate, checkerboard manner, and the other half are closed at the downstream end by, for example, a ceramic plug  806 . The channels  804  have porous walls  808  through which exhaust gases are forced. The walls  808  act as filters and trap particulates. Collected particulates in the DPF  800  accumulate over time and block the micropores of the walls  808  of the channels  804 , which increases the back pressure drop of the diesel engine and reduces the engine performance. To maintain maximum performance, the DPF  800  should be regularly regenerated or cleaned by, for example, periodically burning the accumulated particles, i.e., soot combustion, as detailed above. Because the spontaneous combustion temperature of carbon (soot) is about 600° C., and the temperature of diesel exhaust is in the range of 150-400° C., external energy would be needed to heat the diesel exhaust to the temperature necessary for soot combustion. However, catalytic regeneration of the DPF  800 , which may occur by adding a catalyst such as, for example the above-described Glass-1 catalyst or any other potassium or other alkali-doped catalysts, lowers the ignition temperature of the soot to a temperature within the range of the temperature of diesel exhaust. 
     The catalyst shown in  FIG. 5  is a Glass-1 catalyst that is coated via annealing onto a cordierite substrate, as typically used in a DPF. Alternately, the catalyst may be applied by injecting the catalyst into the channels  804  or upstream from the DPF  800  or coating the channels  804  with the catalyst (i.e., a washcoat). 
     In other possible implementations, metal wires comprising a filter can be coated by the glass. Additionally, glass catalysts can be spun into fibrous form and woven to form a filter, or glass beads or pellets can be used to fill a canister and provide filtration. In addition the glass may be incorporated by bonding glass particles to the substrate by a sintering process or, alternatively, by reacting precursor components in situ on a substrate, 
     There may be several applications for the DPF  800  including, for example, to replace or augment conventional filters such as, for example, the above-described Flow-Through Oxidation Catalysts, NO x -Aided Continuously Regenerated Trap, and/or Integrated Catalytic Trap. 
     Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.