Title of Invention: PALLADIUM DIESEL OXIDATION CATALYST Abstract: An oxidation catalyst composite for abatement of exhaust gas emissions from a lean burn engine is provided, the catalyst composite including a carrier substrate having a length, an inlet end and an outlet end, and an oxidation catalyst material coated on the carrier substrate. The oxidation catalyst material can include a first layer and a second layer. The first layer can include a first oxygen storage component that includes ceria and is impregnated with a palladium (Pd) component and a second component including one or more of magnesium (Mg), rhodium (Rh), and platinum (Pt). The second layer can include a refractory metal oxide component impregnated with platinum (Pt) and palladium (Pd), wherein the ratio of Pt to Pd is in the range of about 0:10 to about 10:0. Detailed Description: FIELD OF THE INVENTION The present invention relates to a diesel oxidation catalyst composition comprising a porous oxygen storage component impregnated with a palladium (Pd) component and at least one second component, methods for the preparation and use of such catalyst compositions, and catalyst articles and systems employing such catalyst compositions. BACKGROUND OF THE INVENTION Emissions of diesel engines include particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. The two major components of exhaust particulate matter are the soluble organic fraction (SOF) and the soot fraction. The SOF condenses on the soot in layers, and is generally derived from unburned diesel fuel and lubricating oils. The SOF can exist in diesel exhaust either as a vapor or as an aerosol (i.e., fine droplets of liquid condensate), depending on the temperature of the exhaust gas. Soot is predominately composed of particles of carbon. The HC content of exhaust can vary depending on engine type and operating parameters, but typically includes a variety of short-chain hydrocarbons such as methane, ethene, ethyne, propene, and the like. Platinum group metal (PGM) based catalysts are useful in treating the exhaust of diesel engines to convert hydrocarbon and carbon monoxide by catalyzing the oxidation of these pollutants to carbon dioxide and water. In addition, oxidation catalysts that contain platinum promote the oxidation of NO to NO2. For heavy-duty diesel systems, such catalysts are generally contained within regeneration diesel oxidation catalyst (DOC) systems, catalyst soot filter (CSF) systems, or combined DOC-CSF systems. These catalyst systems are placed in the exhaust flow path from diesel power systems to treat the resulting exhaust before it vents to the atmosphere. Typically, diesel oxidation catalysts are deposited on ceramic or metallic substrates. For additional reduction of NOx species, such systems also typically include at least one Selective Catalytic Reduction (SCR) catalyst downstream from the DOC catalyst. In light and medium-duty applications, the system may contain a lean NOx trap (LNT) which serves to store and reduce NOx, as well as remove carbon monoxide and unburned hydrocarbons from the exhaust stream. Catalysts are typically defined by their light-off temperature or the temperature at which 50% conversion is attained, also called T50. Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion to occur. This is particularly true for the downstream catalyst components, such as an SCR catalyst, which can take several minutes to reach a suitable operating temperature. There is a continuing need in the art to provide catalysts having low light-off temperatures. Such catalysts can sometimes have issues with sulfur poisoning of the PGM within the catalyst. As such, it is desirable to provide catalysts having light-off temperatures that are lower than traditional PGM catalysts, as well as being resistant to sulfur poisoning. SUMMARY OF THE INVENTION The invention provides a diesel oxidation catalyst (DOC) composition, the composition comprising at least one platinum group metal (PGM) material combined with a dopant useful in stabilizing the PGM material. In various embodiments, an oxidation catalyst composite for abatement of exhaust gas emissions from a lean burn engine is provided, the catalyst composite comprising a carrier substrate having a length, an inlet end and an outlet end and an oxidation catalyst material coated on the carrier substrate. The oxidation catalyst material can comprise a first layer and a second layer, wherein the first layer comprises a first oxygen storage component impregnated by a palladium (Pd) component and by a second component comprising one or more of magnesium (Mg), rhodium (Rh), and platinum (Pt), and wherein the second layer comprises a refractory metal oxide component impregnated by platinum (Pt), and palladium (Pd), wherein the ratio of Pt to Pd is in the range of about 0:10 to about 10:0 (e.g., 1:1, 2:1, 4:1, 1:2, 1:4, 1:10 etc.). In various embodiments, the carrier substrate can be selected from a flow-through monolith, a wall-flow monolith, a foam, or a mesh. In various embodiments of the catalyst composite, the first layer can be an under layer coated on the carrier substrate, and the second layer can be an upper layer coated on at least part of the first layer. In some embodiments, the second layer can be an under layer coated on the carrier substrate, and the first layer can be an upper layer coated on at least part of the second layer. The first oxygen storage component and the refractory metal oxide component can be mixed and in the form of a blended layer coated on the carrier substrate, for example. In certain embodiments, the first layer can be an under layer coated on the carrier substrate, and the second layer can be an upper layer coated only on a zone of the first layer. For example, the first layer can be coated on the entire length of the substrate, and the second layer can be only coated on the first layer at the outlet end of the substrate. The first layer can be coated on the entire length of the substrate, and the second layer can be only coated on the first layer at the inlet end of the substrate. In some embodiments, the first layer is an under layer coated on the carrier substrate, and the first layer is coated only on a zone of the carrier substrate In various embodiments of the DOC oxidation catalyst composite, the first oxygen storage component comprises ceria in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the oxygen storage component before being impregnated. In various embodiments, the palladium component impregnated in the first oxygen storage component can be present in an amount in the range of about 1 g/ft3 to 200 g/ft3. In certain preferred embodiments, the second component can comprise magnesium (Mg) present in an amount in the range of about 0 to about 5 weight percent, or about 0.1 to about 2 weight percent, or about 0.5 to about 1.0 weight percent, based on the total weight of the impregnated first oxygen storage component. In such embodiments, palladium and magnesium can be impregnated in the first oxygen storage component in a mole ratio of Pd to Mg in a range of about 1:0 to about 1:2. In certain preferred embodiments, the mole ratio of Pd to Mg is about 1:1. In some embodiments, the second component comprises rhodium present in an amount in the range of about 1 g/ft3 to about 200 g/ft3. In such embodiments, palladium and rhodium can be impregnated in the first oxygen storage component in a mole ratio of Pd to Rh in a range of about 0:10 to about 10:0 (e.g., 1:1, 2:1, 4:1, 1:2, 1:4, 1:10 etc.). In some embodiments, the second component comprises platinum present in an amount in the range of about 1 g/ft3 to about 200 g/ft3. In such embodiments, palladium and platinum can be impregnated in the first oxygen storage component in a mole ratio of Pd to Pt in a range of about 0:10 to about 10:0 (e.g., 1:1, 2:1, 4:1, 1:2, 1:4, 1:10 etc.). In certain embodiments, the first oxygen storage component can be further impregnated with a metal dopant selected from the group consisting of praseodymium (Pr), iron (Fe), yttrium (Y), lithium (Li), indium (In), tin (Sn), niobium (Nb), gallium (Ga), zirconium (Zr), iridium (Jr), silver (Ag), neodymium (Nd), tungsten (W), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co), and combinations thereof. In various embodiments of the DOC oxidation catalyst composite, the refractory metal oxide component is selected from the group consisting of alumina, silica, zirconia, titania, ceria, manganese, and combinations thereof. The refractory metal oxide component can comprise alumina in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the refractory metal oxide component before being impregnated, for example. In various embodiments, the refractory metal oxide component can further comprises barium oxide in an amount in the range of about 3 to about 100 g/ft3. In some embodiments, the refractory metal oxide component can comprise a hydrothermally stable zeolite selected from the group consisting of ZSM-5, beta, mordenite, Y-zeolite, a CHA framework type zeolite, ferrierite, and combinations thereof. In various embodiments, the palladium and the platinum impregnated in the refractory metal oxide component are present in an amount in the range of about 1 g/ft3 to 200 g/ft3. In some embodiments, one or more of the first oxygen storage component and the refractory metal oxide component can be impregnated by manganese in an amount in the range of about 0.1 to 25 wt. %, based on the total weight of the component impregnated by manganese. For example, in some embodiments, the first oxygen storage component can be impregnated by palladium, platinum, and manganese, wherein the manganese is present in an amount in the range of about 0.1 to about 10 weight percent, based on the total weight of the impregnated oxygen storage component. A catalyst article for abatement of exhaust gas emissions from an engine is also provided herein, comprising a substrate carrier having a plurality of channels adapted for gas flow, wherein a catalyst composition is positioned to contact an exhaust gas passing through each channel, and wherein the catalyst composition comprises a porous oxygen storage component impregnated with a palladium (Pd) component and by a second component comprising at least one of magnesium (Mg), rhodium (Rh), and platinum (Pt). In various embodiments, the oxygen storage component can be a rare earth metal oxide, optionally stabilized with a refractory metal oxide. In certain preferred embodiments, the oxygen storage component comprises ceria. Ceria can be present in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the oxygen storage component before being impregnated. The Pd component can be present in amount of about 1-10% by weight, based on the total weight of impregnated oxygen storage component. In various embodiments of the catalyst article, the second component comprises a Mg component, and the Pd and the Mg are present in molar ratio of about 1:0 to about 1:2. In some embodiments, the Mg component is present in amount of no more than about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. For example, the Mg component can be present in amount of about 0.1 to about 0.5% by weight, based on the total weight of impregnated oxygen storage component. In various embodiments of the catalyst article, the oxygen storage component is further impregnated with a rhodium (Rh) component. In some embodiments, the catalyst article can further include a second porous oxygen storage component impregnated by a rhodium (Rh) component. For example, the catalyst article can comprise a first layer and a second layer, wherein the first layer comprises the porous oxygen storage component impregnated with the palladium (Pd) component and the second component, wherein the second component comprises a magnesium (Mg) component, and wherein the second layer comprises a second porous oxygen storage component impregnated with a rhodium (Rh) component. In some embodiments, the catalyst article can further comprise a metal dopant impregnated within the oxygen storage component, the metal selected from the group consisting of praseodymium (Pr), iron (Fe), yttrium (Y), lithium (Li), indium (In), tin (Sn), niobium (Nb), gallium (Ga), zirconium (Zr), iridium (Ir), silver (Ag), neodymium (Nd), tungsten (W), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co), and combinations thereof. An emission treatment system for treatment of an exhaust gas stream is also provided herein, the emission treatment system comprising a diesel engine producing an exhaust gas stream, and an oxidation catalyst composite or catalyst article as described herein which is positioned in fluid communication with the exhaust gas stream and adapted for oxidation of carbon monoxide and hydrocarbon gases within the exhaust stream to form a treated exhaust gas stream. In some embodiments, the system can further include a catalyzed soot filter and an SCR catalyst situated downstream from the catalyst composite. The SCR catalyst can be present as a washcoat on the catalyzed soot filter. In some embodiments, the SCR catalyst is a flow through monolith situated downstream from the catalyst composite, and the catalyzed soot filter is situated downstream from the SCR catalyst. In certain embodiments, the SCR catalyst comprises a molecular sieve having a double six ring (d6r) unit. The SCR catalyst can be selected from a CHA, AEI, or AFX framework type zeolite, for example. In some embodiments, the SCR catalyst can be promoted with a metal selected from Cu, Fe, Co, Ni, La, Ce, Mn, V, Ag, or combinations thereof. A method of treating an exhaust stream from a diesel engine is also provided herein, the method comprising passing the exhaust stream through the oxidation catalyst composite or catalyst article described herein such that NO is oxidized within the catalyst. The method can further include passing the exhaust stream through an SCR catalyst situated downstream from the oxidation catalyst composite of claim 1. In some embodiments, the SCR catalyst can be disposed on a wall flow filter monolith. The invention includes, without limitation, the following embodiments. EMBODIMENT 1 An oxidation catalyst composite for abatement of exhaust gas emissions from a lean burn engine, the catalyst composite comprising: a carrier substrate having a length, an inlet end and an outlet end; and an oxidation catalyst material coated on the carrier substrate; wherein the oxidation catalyst material comprises a first layer and a second layer; wherein the first layer comprises a first oxygen storage component impregnated by a palladium (Pd) component and by a second component comprising one or more of magnesium (Mg), rhodium (Rh), and platinum (Pt); and wherein the second layer comprises a refractory metal oxide component impregnated by platinum (Pt), and palladium (Pd), wherein the ratio of Pt to Pd is in the range of about 0:10 to about 10:0. EMBODIMENT 2 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first layer is an under layer coated on the carrier substrate, and the second layer is an upper layer coated on at least part of the first layer. EMBODIMENT 3 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the second layer is an under layer coated on the carrier substrate, and the first layer is an upper layer coated on at least part of the second layer. EMBODIMENT 4 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first oxygen storage component and the refractory metal oxide component are mixed and in the form of a blended layer coated on the carrier substrate. EMBODIMENT 5 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first layer is an under layer coated on the carrier substrate, and the second layer is an upper layer coated only on a zone of the first layer. EMBODIMENT 6 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first layer is coated on the entire length of the substrate, and wherein the second layer is only coated on the first layer at the outlet end of the substrate. EMBODIMENT 7 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first layer is coated on the entire length of the substrate, and wherein the second layer is only coated on the first layer at the inlet end of the substrate. EMBODIMENT 8 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first layer is an under layer coated on the carrier substrate, and the first layer is coated only on a zone of the carrier substrate. EMBODIMENT 9 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first oxygen storage component comprises ceria in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the oxygen storage component before being impregnated. EMBODIMENT 10 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the refractory metal oxide component is selected from the group consisting of alumina, silica, zirconia, titania, ceria, manganese, and combinations thereof. EMBODIMENT 11 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the refractory metal oxide component comprises alumina. EMBODIMENT 12 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the palladium component impregnated in the first oxygen storage component is present in an amount in the range of about 1 g/ft3 to 200 g/ft3. EMBODIMENT 13 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein one or more of the first oxygen storage component and the refractory metal oxide component is impregnated by manganese in an amount in the range of about 0.1 to 25 wt. %, based on the total weight of the component impregnated by manganese. EMBODIMENT 14 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the second component comprises magnesium (Mg), and wherein magnesium is present in an amount in the range of about 0.1 to about 5 weight percent, based on the total weight of the impregnated first oxygen storage component. EMBODIMENT 15 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein palladium and magnesium are impregnated in the first oxygen storage component in a mole ratio of Pd to Mg in a range of about 1:0 to about 1:2. EMBODIMENT 16 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the second component comprises rhodium, and wherein the rhodium is present in an amount in the range of about 1 g/ft3 to about 200 g/ft3. EMBODIMENT 17 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein palladium and rhodium are impregnated in the first oxygen storage component in a mole ratio of Pd to Rh in a range of about 0:10 to about 10:0. EMBODIMENT 18 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the second component comprises platinum, and wherein the platinum is present in an amount in the range of about 1 g/ft3 to about 200 g/ft3. EMBODIMENT 19 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein palladium and platinum are impregnated in the first oxygen storage component in a mole ratio of Pd to Pt in a range of about 0:10 to about 10:0. EMBODIMENT 20 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the first oxygen storage component is further impregnated with a metal selected from the group consisting of praseodymium (Pr), iron (Fe), yttrium (Y), lithium (Li), indium (In), tin (Sn), niobium (Nb), gallium (Ga), zirconium (Zr), iridium (Jr), silver (Ag), neodymium (Nd), tungsten (W), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co), and combinations thereof. EMBODIMENT 21 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the second component comprises platinum, and where the first oxygen storage component is further impregnated with a manganese component in the amount of about 0.1 to about 10 weight percent, based on the total weight of the impregnated first oxygen storage component. EMBODIMENT 22 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the refractory metal oxide component further comprises barium oxide in an amount in the range of about 3 to about 100 g/ft3. EMBODIMENT 23 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the refractory metal oxide component comprises a hydrothermally stable zeolite selected from the group consisting of ZSM-5, beta, mordenite, Y-zeolite, a CHA framework type zeolite, ferrierite, and combinations thereof. EMBODIMENT 24 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the palladium and the platinum impregnated in the refractory metal oxide component are present in an amount in the range of about 1 g/ft3 to 200 g/ft3. EMBODIMENT 25 An oxidation catalyst composite of any preceding or subsequent embodiment, wherein the carrier substrate is selected from a flow-through monolith, a wall-flow monolith, a foam, or a mesh. EMBODIMENT 26 A catalyst article for abatement of exhaust gas emissions from an engine, comprising: a substrate carrier having a plurality of channels adapted for gas flow, wherein a catalyst composition is positioned to contact an exhaust gas passing through each channel, wherein the catalyst composition comprises a porous oxygen storage component impregnated with a palladium (Pd) component and by a second component comprising at least one of magnesium (Mg), rhodium (Rh), and platinum (Pt). EMBODIMENT 27 A catalyst article of any preceding or subsequent embodiment, wherein the oxygen storage component is a rare earth metal oxide, optionally stabilized with a refractory metal oxide. EMBODIMENT 28 A catalyst article of any preceding or subsequent embodiment, wherein the oxygen storage component comprises ceria. EMBODIMENT 29 A catalyst article of any preceding or subsequent embodiment, wherein ceria is present in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the oxygen storage component before being impregnated. EMBODIMENT 30 A catalyst article of any preceding or subsequent embodiment, wherein the second component comprises a Mg component, and wherein the Pd and the Mg are present in molar ratio of about 1:0 to about 1:2. EMBODIMENT 31 A catalyst article of any preceding or subsequent embodiment, wherein the Mg component is present in amount of no more than about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. EMBODIMENT 32 A catalyst article of any preceding or subsequent embodiment, wherein the Mg component is present in amount of about 0.1 to about 0.5% by weight, based on the total weight of impregnated oxygen storage component. EMBODIMENT 33 A catalyst article of any preceding or subsequent embodiment, wherein the oxygen storage component is further impregnated with a rhodium (Rh) component. EMBODIMENT 34 A catalyst article of any preceding or subsequent embodiment, further comprising a second porous oxygen storage component impregnated with a rhodium (Rh) component. EMBODIMENT 35 A catalyst article of any preceding or subsequent embodiment, wherein the catalyst composition comprises a first layer and a second layer, wherein the first layer comprises the porous oxygen storage component impregnated with the palladium (Pd) component and the second component, wherein the second component comprises a magnesium (Mg) component, and wherein the second layer comprises a second porous oxygen storage component impregnated with a rhodium (Rh) component. EMBODIMENT 36 A catalyst article of any preceding or subsequent embodiment, wherein Pd component is present in amount of about 1-10% by weight, based on the total weight of impregnated oxygen storage component. EMBODIMENT 37 A catalyst article of any preceding or subsequent embodiment, further comprising a metal impregnated within the oxygen storage component, the metal selected from the group consisting of praseodymium (Pr), iron (Fe), yttrium (Y), lithium (Li), indium (In), tin (Sn), niobium (Nb), gallium (Ga), zirconium (Zr), iridium (Ir), silver (Ag), neodymium (Nd), tungsten (W), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co), and combinations thereof. EMBODIMENT 38 An emission treatment system for treatment of an exhaust gas stream, the emission treatment system comprising: a diesel engine producing an exhaust gas stream; and an oxidation catalyst composite of any preceding or subsequent embodiment positioned in fluid communication with the exhaust gas stream and adapted for oxidation of carbon monoxide and hydrocarbon gases within the exhaust stream to form a treated exhaust gas stream. EMBODIMENT 39 An emission treatment system of any preceding or subsequent embodiment, further comprising a catalyzed soot filter and an SCR catalyst situated downstream from the catalyst composite. EMBODIMENT 40 An emission treatment system of any preceding or subsequent embodiment, wherein the SCR catalyst is present as a washcoat on the catalyzed soot filter. EMBODIMENT 41 An emission treatment system of any preceding or subsequent embodiment, wherein the SCR catalyst is a flow through monolith situated downstream from the catalyst composite, and wherein the catalyzed soot filter is situated downstream from the SCR catalyst. EMBODIMENT 42 An emission treatment system of any preceding or subsequent embodiment, wherein the SCR catalyst comprises a molecular sieve having a double six ring (d6r) unit. EMBODIMENT 43 An emission treatment system of any preceding or subsequent embodiment, wherein the SCR catalyst is selected from a CHA, AEI, or AFX framework type zeolite. EMBODIMENT 44 An emission treatment system of any preceding or subsequent embodiment, wherein the SCR catalyst is promoted with a metal selected from Cu, Fe, Co, Ni, La, Ce, Mn, V, Ag, or combinations thereof. EMBODIMENT 45 A method of treating an exhaust stream from a diesel engine, comprising passing the exhaust stream through an oxidation catalyst composite of any preceding or subsequent embodiment such that NO is oxidized within the catalyst. EMBODIMENT 46 The method of any preceding or subsequent embodiment, further comprising passing the exhaust stream through an SCR catalyst situated downstream from the oxidation catalyst composite of claim 1. EMBODIMENT 47 The method of any preceding or subsequent embodiment, wherein the SCR catalyst is disposed on a wall flow filter monolith. These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The invention includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed invention, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise. BRIEF DESCRIPTION OF THE DRAWINGS In order to provide an understanding of embodiments of the invention, reference is made to the appended drawings, which are not necessarily drawn to scale, and in which reference numerals refer to components of exemplary embodiments of the invention. The drawings are exemplary only, and should not be construed as limiting the invention. FIG. 1A is a perspective view of a honeycomb-type substrate which may comprise a diesel oxidation catalyst (DOC) washcoat composition in accordance with the present invention; FIG. 1B is a partial cross-sectional view enlarged relative to FIG. 1A and taken along a plane parallel to the end faces of the carrier of FIG. 1A, which shows an enlarged view of a plurality of the gas flow passages shown in FIG. 1A; FIG. 2 shows a schematic depiction of an embodiment of an emission treatment system in which a DOC of the present invention is utilized; FIG. 3 is a graph comparing CO light-off temperatures for diesel oxidation catalysts comprising alumina impregnated with palladium and diesel oxidation catalysts comprising ceria impregnated with palladium; FIG. 4 is a graph comparing CO light-off temperatures for diesel oxidation catalysts comprising alumina impregnated with palladium and diesel oxidation catalysts comprising ceria impregnated with palladium; FIG. 5 is a graph illustrating CO light-off temperatures for diesel oxidation catalysts comprising ceria impregnated with 2 weight percent palladium (Pd) and 0.5 weight percent of different dopants; FIG. 6 is a graph illustrating HC light-off temperatures for diesel oxidation catalysts comprising ceria impregnated with 2 weight percent palladium (Pd) and 0.5 weight percent of different dopants; FIG. 7 is a graph comparing the CO light-off temperatures of Pd/ceria catalyst powders and Pd/Mg/ceria catalyst powders; FIG. 8 is a graph comparing the HC light-off temperatures of Pd/ceria catalyst powders and Pd/Mg/ceria catalyst powders; FIG. 9 is a graph showing CO light-off temperatures for different loading levels of Mg in ceria; FIG. 10 is a graph illustrating CO light-off temperatures for fresh samples of several catalysts of the present invention; FIG. 11 is a graph illustrating CO light-off temperatures for aged samples of several catalysts of the present invention; FIG. 12 is a graph illustrating CO light-off temperatures for fresh and S-aged catalyst samples comprising Rh; FIG. 13 is a graph illustrating CO light-off temperatures for fresh and S-aged catalyst samples comprising Pt; FIG. 14 is a graph comparing CO light-off temperatures for hydrothermally aged Pd/ceria, Rh/ceria, and Pt/ceria catalyst samples; and FIG. 15 is a graph illustrating CO light-off temperatures for fresh, S-aged and regenerated Pd/Pt/ceria catalyst samples and Pd/Pt/(Mn+ceria) catalyst samples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention now will be described more fully hereinafter. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents. It is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “a reducing agent” means one reducing agent or more than one reducing agent. Any ranges cited herein are inclusive. The term “about” used throughout this specification are used to describe and account for small fluctuations. For example, the term “about” can refer to less than or equal to ±5%, such as less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.2%, less than or equal to ±0.1% or less than or equal to ±0.05%. All numeric values herein are modified by the term “about,” whether or not explicitly indicated. A value modified by the term “about” of course includes the specific value. For instance, “about 5.0” must include 5.0. The term “substantially” similarly is used to describe and account for small fluctuations. Accordingly, where a composition is referred to as “substantially free” of a given component, unless otherwise specified, this can mean, e.g., that the composition comprises less than about 5%, such as less than about 2%, less than about 1%, less than about 0.5%, less than about 0.1%, or less than about 0.05% of that component by weight of the composition. All measurements herein are performed at ambient conditions, 25° C. and 1 atm of pressure, unless otherwise indicated. Where ranges in the claims of this application do not find explicit support in the specification, it is intended that such claims provide their own disclosure as support for claims or teachings in a later amendment of this application. Numerical ranges of ingredients that are bounded by zero on the lower end (for example, 0-5 weight % Mg component) are intended to provide support for the concept “up to [the upper limit],” for example “up to 0.5 weight percent % Mg component,” vice versa, as well as a positive recitation that the ingredient in question is present in an amount that does not exceed the upper limit. An example of the latter is “comprises Mg, provided the amount does not exceed 0.5 weight percent %.” A recitation such as “8-25 weight percent % (PGM+second component+optional metal component)” means that any or all of PGM, second component, and/or optional metal component may be present in an amount of 8-25 weight percent % of the composition. The present invention generally provides a diesel oxidation catalyst (DOC) composition suitable for at least partial oxidation of NO to NO2, which may optionally further comprise a CSF component to treat the soluble organic fraction of particulate matter present in diesel exhaust. The DOC compositions disclosed herein comprise a porous refractory oxide support impregnated by a PGM component as well as a second component comprising one or more of magnesium (Mg), rhodium (Rh), and platinum (Pt), wherein such compositions provide enhanced NO oxidation as compared with comparable compositions comprising PGM complexes impregnated on a porous refractory oxide support. As used herein, “impregnated” or “impregnation” refers to permeation of catalytic material into the porous structure of the support material or attached to a surface of the support material. The DOC compositions can be prepared and coated onto a catalyst substrate using a washcoat technique as set forth more fully below. Catalyst Composition The DOC composition can include an oxygen storage component (OSC) impregnated with at least one platinum group metal (PGM) material combined with a dopant or a second PGM material useful in stabilizing the first PGM material. As used herein, “platinum group metal” or “PGM” refers to platinum group metals or oxides thereof, including platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Jr), and mixtures thereof. In certain embodiments, the platinum group metal comprises palladium. The concentrations of PGM component (e.g., Pd) can vary, but will typically be from about 0.1 wt. % to about 10 wt. % relative to the weight of the impregnated porous oxygen storage component (e.g., about 1 wt. % to about 6 wt. % relative to the OSC) in a given composition. In some embodiments, the concentration of the PGM component can be about 2 wt. % to about 4 wt. %, based on the total weight of the impregnated oxygen storage component. In various embodiments, the oxygen storage component is a rare earth oxide, optionally stabilized with a refractory metal oxide. “Refractory metal oxides” or “porous refractory oxides” refers to porous metal-containing oxide materials exhibiting chemical and physical stability at high temperatures, such as the temperatures associated with diesel engine exhaust. Exemplary refractory oxides include alumina, silica, zirconia, titania, ceria, and physical mixtures or chemical combinations thereof, including atomically-doped combinations and including high surface area or activated compounds such as activated alumina. Exemplary combinations of metal oxides include alumina-zirconia, alumina-ceria-zirconia, lanthana-alumina, lanthana-zirconia-alumina, baria-alumina, baria lanthana-alumina, baria lanthana-neodymia alumina, and alumina-ceria. Exemplary aluminas include large pore boehmite, gamma-alumina, and delta/theta alumina. Useful commercial aluminas used as starting materials in exemplary processes include activated aluminas, such as high bulk density gamma-alumina, low or medium bulk density large pore gamma-alumina, and low bulk density large pore boehmite and gamma-alumina, available from BASF Catalysts LLC (Port Allen, La., USA). High surface area refractory oxide supports, such as alumina support materials, also referred to as “gamma alumina” or “activated alumina,” typically exhibit a BET surface area in excess of 60 m2/g, often up to about 200 m2/g or higher. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. “BET surface area” has its usual meaning of referring to the Brunauer, Emmett, Teller method for determining surface area by N2 adsorption. Desirably, the active alumina has a specific surface area of 60 to 350 m2/g, and typically 90 to 250 m2/g. In certain preferred embodiments, the oxygen storage component comprises ceria. The ceria can be present in an amount of about 0.1 to about 100 weight percent, or about 70 to about 100 weight percent, based on the total weight of the oxygen storage component before it is impregnated. The ceria support can have a BET surface area of about 140 to 160 M2/g and a pore volume between 0.3 to 0.5 cc/g, for example. In various embodiments, the DOC catalyst composition can comprise a dopant useful in lowering the light-off temperature of the catalyst composition and/or stabilizing the PGM component. It was surprisingly discovered that certain metals can be useful in lowering the CO and HC light-off temperatures of a catalyst composition, as compared to catalyst compositions that do not include the metal dopant component. For example, the dopant can be a metal selected from the group consisting of magnesium (Mg), praseodymium (Pr), iron (Fe), yttrium (Y), lithium (Li), indium (In), tin (Sn), niobium (Nb), gallium (Ga), zirconium (Zr), iridium (Ir), silver (Ag), neodymium (Nd), tungsten (W), copper (Cu), zinc (Zn), rhodium (Rh), molybdenum (Mo), cobalt (Co), and combinations thereof. The metal component can be present in amount of about 0.1 to about 10% by weight, or about 0.1 to about 5% by weight, or about 0.1 to about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. In certain embodiments, the metal dopant component can be present in an amount of no more than about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. In some embodiments, the catalyst composition comprises a porous oxygen storage component comprising ceria impregnated with a palladium (Pd) component and a magnesium (Mg) component. The Mg component can allow the Pd catalyst composition to achieve lower light-off temperatures for fresh, aged, and regenerated catalytic articles. The Pd component and the Mg component can be present in a molar ratio of about 1:0 to about 1:2. In certain embodiments, the Pd component and the Mg component can be present in a molar ratio of about 1:1. The Pd component can be present in an amount of about 1-10% by weight, based on the total weight of impregnated oxygen storage component. For example, the Pd component can be present in an amount of about 2-4% by weight, based on the total weight of impregnated oxygen storage component. The Mg component can be present in an amount of about 0.1 to about 5% by weight, based on the total weight of impregnated oxygen storage component. In certain embodiments, the Mg component can be present in an amount of no more than about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. In certain preferred embodiments, the DOC catalyst composition comprises a porous oxygen storage component comprising ceria impregnated with a palladium (Pd) component and a second component comprising at least one of platinum (Pt), magnesium (Mg), rhodium (Rh), and combinations thereof. Without being limited by theory, the Mg, Pt, and/or Rh component can allow the Pd catalyst composition to be more resistant to sulfur poisoning and to achieve lower light-off temperatures for fresh, aged, and regenerated catalytic articles. The Pd component and the second component (i.e., the Pt, Mg, and/or Rh component) can be present in a molar ratio in the range of about 1:0 to about 1:2, or about 0:10 to about 10:0. In certain embodiments, the Pd component and the second component can be present in a molar ratio of about 1:1. The Pd component can be present in an amount of about 1-10% by weight, based on the total weight of impregnated oxygen storage component. For example, the Pd component can be present in an amount of about 2-4% by weight, based on the total weight of impregnated oxygen storage component. The second component can be present in an amount of about 0.1 to about 5% by weight, based on the total weight of impregnated oxygen storage component. In certain embodiments, the second component can be present in an amount of no more than about 0.5% by weight, based on the total weight of the impregnated oxygen storage component. In various embodiments of the DOC catalyst compositions described herein, the oxygen storage component can be impregnated with a Pd component and an Mg and/or Pt component, and the catalyst composition can further comprise a Rh component. In such embodiments, the Rh can be impregnated in the same oxygen storage component as the Mg and/or Pt, or the catalyst composition can further include a second porous oxygen storage component impregnated with a rhodium (Rh) component. For example, the catalyst composition can include a first layer and a second layer, wherein the first layer comprises the porous oxygen storage component impregnated with a palladium (Pd) component and a magnesium (Mg) component, and wherein the second layer comprises a second porous oxygen storage component impregnated with the rhodium (Rh) component. In various embodiments of the present invention, the catalyst composition can further comprise an additional refractory metal oxide and a molecular sieve, wherein the refractory metal oxide component is impregnated with a platinum (Pt) component and a palladium (Pd) component, and wherein the Pt and the Pd components are present in a molar ratio in the range of about 1:10 to about 10:1. In various embodiments, the refractory metal oxide can comprise alumina. In certain embodiments, the molecular sieve can comprise a zeolite. As used herein, the term “molecular sieves” refers to zeolites and other zeolitic framework materials (e.g. isomorphously substituted materials), which may, in particulate form, support catalytic metals. Molecular sieves are materials based on an extensive three-dimensional network of oxygen ions containing generally tetrahedral type sites and having a substantially uniform pore distribution, with the average pore size being no larger than 20 Å. The pore sizes are defined by the ring size. As used herein, the term “zeolite” refers to a specific example of a molecular sieve, further including silicon and aluminum atoms. According to one or more embodiments, it will be appreciated that by defining the molecular sieves by their structure type, it is intended to include the structure type and any and all isotypic framework materials such as silico-alumino-phosphate (SAPO), alumino-phosphate (ALPO) and metal-alumino-phosphate (MeAPO) materials having the same structure type, as well as borosilicates, gallosilicates, mesoporous silica materials such as SBA-15 or MCM-41, and the like. In certain embodiments, the molecular sieve may comprise a zeolite or zeotype selected from the group consisting of a chabazite, ferrierite, clinoptilolite, silico-alumino-phosphate (SAPO), beta-zeolite, Y-zeolite, mordenite, faujasite, ZSM-5, mesoporous materials, and combinations thereof. The zeolite may be ion-exchanged with a metal, such as a metal selected from the group consisting of La, Ba, Sr, Mg, Pt, Pd, Ag, Cu, V, Ni, Co, Fe, Zn, Mn, Ce, and combinations thereof. In various embodiments of the present invention, the catalyst composition can further comprise a manganese component impregnated on the oxygen storage component comprising ceria and/or on the additional refractory metal oxide component. The manganese component can be present in an amount of about 0.1 to about 25 weight percent, or about 1 to about 10 weight percent, based on the total weight of the catalyst composition or based on the total weight of the component impregnated by manganese for multi-layer catalyst compositions. For example, in some embodiments, the first oxygen storage component can be impregnated by palladium, platinum, and manganese, wherein the manganese is present in an amount in the range of about 0.1 to about 10 weight percent, based on the total weight of the impregnated oxygen storage component. Without being limited by theory, manganese can be useful in improving light-off performance by lowering the light-off temperature of the catalyst, as well as enhancing the catalyst resistance to sulfur poisoning. Method of Making Catalyst Composition Preparation of a porous support with a PGM component, a second component, or dopant typically comprises impregnating the porous support (e.g., a refractory oxide support material in particulate form such as particulate ceria) with a PGM or metal dopant solution. Multiple metal components (e.g., platinum and palladium) can be impregnated at the same time or separately, and can be impregnated on the same support particles or separate support particles using an incipient wetness technique. The support particles are typically dry enough to absorb substantially all of the solution to form a moist solid. Aqueous solutions of water soluble compounds or complexes of the metal component are typically utilized, such as palladium or platinum nitrate, tetraammine palladium or platinum nitrate, tetraammine palladium or platinum acetate, copper (II) nitrate, manganese (II) nitrate, and ceric ammonium nitrate. Following treatment of the support particles with the metal solutions, the particles are dried, such as by heat treating the particles at elevated temperature (e.g., 100-150° C.) for a period of time (e.g., 1-3 hours), and then calcining to convert the metal components to a more catalytically active form. An exemplary calcination process involves heat treatment in air at a temperature of about 400-550° C. for 1-3 hours. The above process can be repeated as needed to reach the desired level of impregnation. The resulting material can be stored as a dry powder or in slurry form. Catalyst Composite As noted above, a catalyst article comprising the DOC composition of the invention exhibits enhanced NO conversion activity. Accordingly, one or more embodiments of the invention provide a catalyst article comprising a calcined DOC composition as disclosed herein (where the refractory metal oxide, the PGM component, the second component, and any optional metal components are mixed or are layered), characterized by higher sulfur resistance and/or lower CO and HC light-off temperatures for fresh, aged, and/or regenerated catalyst compositions than exhibited by a comparable catalyst article that does not include the second component or the optional metal components. In certain preferred embodiments, an oxidation catalyst composite for abatement of exhaust gas emissions from a lean burn engine is provided, the catalyst composite comprising a carrier substrate having a length, an inlet end and an outlet end, and a washcoat of an oxidation catalyst material on the carrier. The carrier substrate can be selected from a flow-through monolith, a wall-flow monolith, a foam, or a mesh, for example. The oxidation catalyst material can comprise a first layer and a second layer. In various embodiments, the first layer of the oxidation catalyst material can comprise a porous oxygen storage component comprising ceria impregnated with a palladium (Pd) component and a second component comprising a material selected from the group consisting of a platinum (Pt), magnesium (Mg), rhodium (Rh), and combinations thereof. The porous oxygen storage component can comprise ceria in an amount in the range of about 0.1 to about 100 weight percent, based on the total weight of the oxygen storage component before it is impregnated with additional components. Without being limited by theory, the Mg, Pt, and/or Rh component can allow the Pd catalyst composition to be more resistant to sulfur poisoning and/or to achieve lower light-off temperatures for fresh, aged, and regenerated catalytic articles. The Pd component and the second component can be present in a molar ratio in the range of about 1:0 to about 1:2, or about 0:10 to about 10:0 (e.g., 1:1, 2:1, 4:1, 1:2, 1:4, 1:10 etc.). In certain embodiments, the Pd component and the second component can be present in a molar ratio of about 1:1. The Pd component can be present in an amount of about 1-10% by weight, based on the total weight of impregnated oxygen storage component. For example, the Pd component can be present in an amount of about 1-4% by weight, based on the total weight of impregnated oxygen storage component. In various embodiments, the second component can comprise magnesium. The second component comprising magnesium can be present in an amount of about 0.1 to about 5% by weight, based on the total weight of impregnated oxygen storage component. In certain embodiments, the second component comprising magnesium can be present in an amount of no more than about 0.5% by weight, or no more than about 1% by weight, based on the total weight of the impregnated oxygen storage component. The molar ratio of the palladium to the magnesium can be in the range of about 1:0 to about 1:2. In certain embodiments, the molar ratio of the palladium to the magnesium can
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