Title of Invention: Use of a supplemental promoter in conjunction with a carbon-supported, noble-metal containing catalyst in liquid phase oxidation reactions Abstract: This invention relates to the use of a supplemental promoter in conjunction with a noble-metal-containing catalyst comprising a carbon support in catalyzing liquid phase oxidation reactions, a process for making of an improved catalyst comprising such a supplemental promoter, and an improved catalyst comprising such a supplemental promoter. In a particularly preferred embodiment, a supplemental promoter (most preferably bismuth or tellurium) is used in conjunction with a noble-metal-containing catalyst comprising a carbon support in a liquid phase oxidation process wherein N-(phosphonomethyl) iminodiacetic acid (i.e., xe2x80x9cPMIDAxe2x80x9d) or a salt thereof is oxidized to form N-(phosphonomethyl) glycine (i.e., xe2x80x9cglyphosatexe2x80x9d) or a salt thereof. The benefits of such a process include increased oxidation of the formaldehyde and formic acid by-products, and, consequently, decreased final concentrations of those by-products as well as other undesirable by-products, most notably N-methyl-N-(phosphonomethyl) glycine (i.e., xe2x80x9cNMGxe2x80x9d). Brief Summary: This invention generally relates to liquid phase oxidation processes using a carbon-supported, noble-metal-containing catalyst (particularly a deeply reduced catalyst) in conjunction with a supplemental promoter (e.g., bismuth or tellurium). In an especially preferred embodiment, this invention relates to such a process wherein N-(phosphonomethyl)iminodiacetic acid (xe2x80x9cPMIDAxe2x80x9d) or a salt thereof is oxidized to form N-(phosphonomethyl)glycine (also known in the agricultural chemical industry as xe2x80x9cglyphosatexe2x80x9d) or a salt thereof. This invention also generally relates to enhancing the activity, selectivity, and/or stability of a carbon-supported, noble-metal-containing catalyst (particularly a deeply reduced catalyst) using a supplemental promoter. N-(phosphonomethyl)glycine is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in an aqueous formulation. Glyphosate is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz (U.S. Pat. No. 3,950,402) discloses that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of PMIDA with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support: Other by-products also typically form, such as formic acid (HCO2H), which is formed by the oxidation of the formaldehyde by-product; and aminomethylphosphonic acid (xe2x80x9cAMPAxe2x80x9d), which is formed by the oxidation of N-(phosphonomethyl)glycine. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., xe2x80x9cleachingxe2x80x9d) result because, under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form, and both PMIDA and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal. In U.S. Pat. No. 3,969,398, Hershman discloses that activated carbon alone, without the presence of a noble metal, may be used to effect the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine. In U.S. Pat. No. 4,624,937, Chou further discloses that the activity of the carbon catalyst disclosed by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formic acid and formaldehyde by-product when used to effect the oxidative cleavage of N-phosphonomethyliminodiacetic acid. These byproducts are particularly undesirable because they react with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as xe2x80x9cNMGxe2x80x9d) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107. Optimally, therefore, it has been suggested that the formic acid and formaldehyde be simultaneously oxidized to carbon dioxide and water as the PMIDA is oxidized to N-(phosphonomethyl)glycine in a single reactor, thus giving the following net reaction: As the above references suggest, such a process requires the presence of both carbon (which primarily effects the oxidation of PMIDA to form N-(phosphonomethyl)glycine and formaldehyde) and a noble metal (which primarily effects the oxidation of formaldehyde and formic acid to form carbon dioxide and water). Previous attempts to develop a stable catalyst for such an oxidation process, however, have not been entirely satisfactory. Like Franz, Ramon et al. (U.S. Pat. No. 5,179,228) disclose using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), however, Ramon et al. disclose flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%. Still, the amount of noble metal loss incurred with this method is unacceptable. In addition, re-depositing the noble metal can lead to loss of noble metal surface area which, in turn, decreases the activity of the catalyst. Using a different approach, Felthouse (U.S. Pat. No. 4,582,650) discloses using two catalysts: (i) an activated carbon to effect the oxidation of PMIDA into N-(phosphonomethyl)glycine, and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde into carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of PMIDA to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle. In PCT/US99/03402, Ebner et al. disclose a reaction process for making N-(phosphonomethyl)glycine compounds from PMIDA compounds using a deeply reduced, carbon-supported, noble metal catalyst which exhibits improved resistance to noble metal leaching and increased destruction of undesirable byproducts (e.g., formaldehyde). Still, this reaction process typically does not eliminate all the formaldehyde and formic acid byproduct, and, consequently, also does not eliminate all the N-methyl-N-(phosphonomethyl)glycine byproduct. Thus, a need continues to exist for an improved reaction process for oxidizing PMIDA to N-(phosphonomethyl)glycine using a catalyst which exhibits resistance to noble metal leaching and increased oxidation of formic acid and formaldehyde into carbon dioxide and water (i.e., increased formic acid and formaldehyde activity). This invention provides, in part, for an improved process for oxidizing PMIDA, salts of PMIDA, and esters of PMIDA to form N-(phosphonomethyl)glycine, salts of N-(phosphonomethyl)glycine, and esters of N-(phosphonomethyl)glycine, particularly such a process which uses a catalyst (or catalyst system) that (a) exhibits resistance to noble metal leaching, and (b) exhibits increased oxidation of formic acid and/or formaldehyde, and consequent decreased formation of NMG; an improved process for oxidizing a substrate in general wherein the activity, selectivity, and/or stability of a carbon-supported, noble-metal-containing catalyst used to catalyze the oxidation is enhanced by merely mixing the catalyst with a supplemental promoter (rather than using a catalyst which already contains the promoter, and, consequently, is more costly to manufacture); an improved process for making an oxidation catalyst system (particularly an oxidation catalyst system for oxidizing PMIDA compounds) having enhanced activity, selectivity, and/or stability; and an oxidation catalyst system (particularly an oxidation catalyst system for oxidizing PMIDA compounds) having enhanced activity, selectivity, and/or stability. Briefly, therefore, the present invention is directed to a process for oxidizing formic acid or formaldehyde in the presence of a catalyst and a supplemental promoter. Here, the catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the catalyst is at least about 1:15,000. The present invention is also directed to a process for oxidizing a substrate in general using a catalyst comprising a carbon support and a noble metal. In this embodiment, the process comprises contacting the substrate with oxygen in the presence of the catalyst and a supplemental promoter. Here, the mass ratio of the supplemental promoter to the catalyst is at least about 1:15,000. And, before the catalyst is used in the oxidation of the substrate, the catalyst: A. comprises a non-graphitic carbon support having a noble metal at a surface of the non-graphitic carbon support; and is identifiable as yielding no greater than about 0.7 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst in a helium atmosphere is heated from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or B. comprises a non-graphitic carbon support having a noble metal and a catalyst-surface promoter at a surface of the non-graphitic carbon support; and is identifiable as yielding no greater than about 0.7 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst, after being heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or C. comprises a non-graphitic carbon support having a noble metal, carbon, and oxygen at a surface of the non-graphitic carbon support, the ratio of carbon atoms to oxygen atoms at the surface being at least about 30:1, as measured by x-ray photoelectron spectroscopy; or D. comprises a non-graphitic carbon support having a noble metal, a catalyst-surface promoter, carbon, and oxygen at a surface of the non-graphitic carbon support; and is identifiable as having a ratio of carbon atoms to oxygen atoms at the surface which is at least about 30:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere; or E. comprises a non-graphitic carbon support having (i) a noble metal at a surface of the non-graphitic carbon support; and (ii) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising oxygen and carbon, the ratio of carbon atoms to oxygen atoms in the surface layer being at least about 30:1, as measured by x-ray photoelectron spectroscopy; or F. comprises a non-graphitic carbon support having: (a) a noble metal and a catalyst-surface promoter at a surface of the non-graphitic carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen; and is identifiable as having a ratio of carbon atoms to oxygen atoms in the surface layer of at least about 30:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere; G. is formed by a process comprising depositing a noble metal at a surface of a non-graphitic carbon support, and then heating the surface at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or H. is formed by a process comprising depositing a noble metal at a surface of a carbon support, and then exposing the surface to a reducing environment, wherein, before the noble metal deposition, the carbon support has carbon atoms and oxygen atoms at the surface of the carbon support in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or I. is formed by a process comprising depositing a noble metal at a surface of a non-graphitic carbon support, and then heating the surface at a temperature greater than about 500xc2x0 C. The present invention is also directed to a process for making an oxidation catalyst system. In one embodiment directed to a process for making an oxidation catalyst system, the process comprises mixing a noble-metal-containing catalyst with a supplemental promoter in the presence of formic acid or formaldehyde. Here, the noble-metal-containing catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000. In another embodiment directed to a process for making an oxidation catalyst system, the catalyst system is prepared using a carbon support having carbon atoms and oxygen atoms at a surface of the non-graphitic carbon support. In this process, a noble metal is deposited at the surface of the carbon support to form a noble-metal-containing catalyst. Oxygen-containing functional groups are subsequently removed from the surface of the noble-metal-containing catalyst to form a noble-metal-containing catalyst comprising a deoxygenated surface. This removal of oxygen-containing functional groups comprises: (i) heating the surface of the noble-metal-containing catalyst at a temperature of greater than about 500xc2x0 C.; or (ii) heating the surface of the noble-metal-containing catalyst at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or (iii) exposing the surface of the noble-metal-containing catalyst to a reducing environment, wherein, before the noble metal deposition, the ratio of carbon atoms to oxygen atoms at the surface of the non-graphitic carbon support is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or (iv) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that the ratio of carbon atoms to oxygen atoms at the deoxygenated surface of the noble-metal-containing catalyst comprising the deoxygenated surface is at least about 30:1, as measured by x-ray photoelectron spectroscopy; or (v) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that no greater than about 0.7 mmole of carbon monoxide per gram of the noble-metal-containing catalyst comprising the deoxygenated surface desorb from the deoxygenated surface when a dry sample of the noble-metal-containing catalyst comprising the deoxygenated surface is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes. After removing oxygen-containing functional groups from the surface of the noble-metal-containing catalyst, the noble-metal-containing catalyst is mixed with a supplemental promoter. Here, the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000. This invention is also directed to an oxidation catalyst system. In one embodiment directed to an oxidation catalyst system, the oxidation catalyst system is prepared by a process comprising mixing a noble-metal-containing catalyst with a supplemental promoter in the presence of formic acid or formaldehyde. Here, the noble-metal-containing catalyst comprises a noble metal and a carbon support; and the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000. In another embodiment directed to an oxidation catalyst system, the oxidation catalyst system is prepared using a carbon support. When preparing this catalyst system, a noble metal is deposited onto a surface of the carbon support to form a noble-metal-containing catalyst. Oxygen-containing functional groups are subsequently removed from the surface of the noble-metal-containing catalyst to form a noble-metal-containing catalyst comprising a deoxygenated surface. This removal of oxygen-containing functional groups comprises: (i) heating the surface of the noble-metal-containing catalyst at a temperature of greater than about 500xc2x0 C.; or (ii) heating the surface of the noble-metal-containing catalyst at a temperature of at least about 400xc2x0 C., wherein, before the noble metal deposition, the non-graphitic carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or (iii) exposing the surface of the noble-metal-containing catalyst to a reducing environment, wherein, before the noble metal deposition, the non-graphitic carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or (iv) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that the ratio of carbon atoms to oxygen atoms at the deoxygenated surface of the noble-metal-containing catalyst comprising the deoxygenated surface is at least about 30:1, as measured by x-ray photoelectron spectroscopy; or (v) exposing the surface of the noble-metal-containing catalyst to a reducing environment so that no greater than about 0.7 mmole of carbon monoxide per gram of the noble-metal-containing catalyst comprising the deoxygenated carbon support surface desorb from the deoxygenated surface when a dry sample of the noble-metal-containing catalyst comprising the deoxygenated surface is heated in a helium atmosphere from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes. After oxygen-containing functional groups have been removed from the surface of the noble-metal-containing catalyst, the noble-metal-containing catalyst is mixed with a supplemental promoter. Here, the mass ratio of the supplemental promoter to the noble-metal-containing catalyst is at least about 1:15,000. This invention also is directed to a general process for making N-(phosphonomethyl)glycine, a salt of N-(phosphonomethyl)glycine, or an ester of N-(phosphonomethyl)glycine. This process comprises oxidizing N-(phosphonomethyl)iminodiacetic acid, a salt of N-(phosphonomethyl)iminodiacetic acid, or an ester of N-(phosphonomethyl)iminodiacetic acid in the presence of an oxidation catalyst. Before the oxidation, this oxidation catalyst: A. comprises a carbon support having a noble metal at a surface of the carbon support; and is identifiable as yielding no greater than about 1.2 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst in a helium atmosphere is heated from about 20xc2x0 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes; or B. comprises a carbon support having a noble metal, carbon, and oxygen at a surface of the carbon support, the ratio of carbon atoms to oxygen atoms at the surface being at least about 20:1, as measured by x-ray photoelectron spectroscopy; or C. comprises a carbon support comprising: (a) a noble metal at a surface of the carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen, the ratio of carbon atoms to oxygen atoms in the surface layer being at least about 20:1, as measured by x-ray photoelectron spectroscopy; or D. is formed by a process comprising depositing a noble metal at a surface of a carbon support, and then heating the surface at a temperature of at least about 400xc2x0 C.; or E. is formed by a process comprising: depositing a noble metal at a surface of a carbon support, and then exposing the surface to a reducing environment, wherein, before the noble metal deposition, the carbon support has carbon atoms and oxygen atoms at the surface in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1, as measured by x-ray photoelectron spectroscopy; or F. comprises a carbon support having a noble metal, a promoter, carbon, and oxygen at a surface of the carbon support; or G. comprises a carbon support having: (a) a noble metal and a promoter at a surface of the carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen, the catalyst being identifiable as having a ratio of carbon atoms to oxygen atoms in the surface layer which is at least about 20:1, as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere. Other features of this invention will be in part apparent and in part pointed out hereinafter. Detailed Description: A. The Oxidation Catalyst The catalyst used in the present invention may be used to catalyze liquid phase (i.e., in an aqueous solution or an organic solvent) oxidation reactions, especially in acidic oxidative environments and in the presence of solvents, reactants, intermediates, or products which solubilize noble metals. The catalyst exhibits resistance to noble metal leaching from the catalyst surface under these conditions. The noble metal component of the catalyst serves various functions. For example, depositing a noble metal onto the surface of a catalyst consisting of a carbon support alone tends to reduce the rate of deactivation of the catalyst. To illustrate, when N-(phosphonomethyl)glycine is prepared by the liquid phase oxidative cleavage of PMIDA with oxygen in the presence of a catalyst consisting of an activated carbon support without a noble metal, the activated carbon is found to deactivate as much as 10% per cycle or more. Without being bound by any particular theory, it is believed that the deactivation of the activated carbon arises because the surface of the carbon support oxidizes under the reaction conditions. See Chou, U.S. Pat. No. 4,624,937. See also, Chou, U.S. Pat. No. 4,696,772, which provides a separate discussion related to deactivation of activated carbon by oxidation of the surface of the carbon. In the presence of the noble metal, however, the rate of deactivation of the activated carbon is diminished. It is believed that the noble metal can react with the oxidant at a faster rate than the activated carbon surface, and, thus, preferentially removes the oxidant from solution before extensive oxidation of the carbon surface can occur. Further, unlike many oxide species which form at activated carbon surfaces and require high temperature treatments to be reduced, oxide species which form at the surface of a noble metal typically are easily reduced by the reducing agents present in or added to the reaction mixture (e.g., the amine fragment cleaved, formaldehyde, formic acid, H2, etc.), thus restoring the noble metal surface to a reduced state. In this manner, the catalyst of this invention advantageously exhibits significantly longer life as long as the noble metal is not lost by leaching, or sintered (i.e., in the form of undesirably thick layers or clumps) by processes such as dissolution and re-deposition or noble metal agglomeration. Also, depending on the particular oxidation reaction, a noble metal may be more effective than carbon at effecting the oxidation. For example, in the context of the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine, although a carbon catalyst can be used in the oxidation of PMIDA to N-(phosphonomethyl)glycine, it is the noble metal component that primarily effects the oxidation of the undesirable formaldehyde and formic acid by-products into the more preferred by-products, carbon dioxide and water. Oxygen-containing functional groups (e.g., carboxylic acids, ethers, alcohols, aldehydes, lactones, ketones, esters, amine oxides, and amides) at the surface of the carbon support tend to increase noble metal leaching and potentially increase noble metal sintering during liquid phase oxidation reactions, and, thus, reduce the ability of the catalyst to oxidize oxidizable substrates, particularly formaldehyde and formic acid during the PMIDA oxidation reaction. As used herein, an oxygen-containing functional group is xe2x80x9cat the surface of the carbon supportxe2x80x9d if it is bound to an atom of the carbon support and is able to chemically or physically interact with compositions within the reaction mixture or with the metal atoms deposited on the carbon support. Many of the oxygen-containing functional groups that reduce noble metal resistance to leaching and sintering and reduce the activity of the catalyst desorb from the carbon support as carbon monoxide when the catalyst is heated at a high temperature (e.g., 900xc2x0 C.) in an inert atmosphere (e.g., helium or argon). Thus, measuring the amount of CO desorption from a fresh catalyst (i.e., a catalyst that has not previously been used in a liquid phase oxidation reaction) under high temperatures is one method that may be used to analyze the surface of the catalyst to predict noble metal retention and maintenance of catalyst activity. One way to measure CO desorption is by using thermogravimetric analysis with in-line mass spectroscopy (xe2x80x9cTGA-MSxe2x80x9d). Preferably, no greater than about 1.2 mmole of carbon monoxide per gram of catalyst desorb from the catalyst when a dry, fresh sample of the catalyst in a helium atmosphere is subjected to a temperature which is increased from about 20xc2x0 to about 900xc2x0 C. at about 10xc2x0 C. per minute, and then held constant at about 900xc2x0 C. for about 30 minutes. More preferably, no greater than about 0.7 mmole of carbon monoxide per gram of fresh catalyst desorb under those conditions, even more preferably no greater than about 0.5 mmole of carbon monoxide per gram of fresh catalyst desorb, and most preferably no greater than about 0.3 mmole of carbon monoxide per gram of fresh catalyst desorb. A catalyst is considered xe2x80x9cdryxe2x80x9d when the catalyst has a moisture content of less than 1% by weight. Typically, a catalyst may be dried by placing it into a N2 purged vacuum of about 25 inches of Hg at a temperature of about 120xc2x0 C. for about 16 hours. Measuring the number of oxygen atoms at the surface of a fresh catalyst support is another method which may be used to analyze the catalyst to predict noble metal retention and maintenance of catalytic activity. Using, for example, x-ray photoelectron spectroscopy, a surface layer of the support which is about 50 xc3x85 in thickness is analyzed. Presently available equipment used for x-ray photoelectron spectroscopy typically is accurate to within xc2x120%. Typically, a ratio of carbon atoms to oxygen atoms at the surface (as measured by presently available equipment for x-ray photoelectron spectroscopy) of at least about 20:1 (carbon atoms:oxygen atoms) is suitable. Preferably, however, the ratio is at least about 30:1, more preferably at least about 40:1, even more preferably at least about 50:1, and most preferably at least about 60:1. In addition, the ratio of oxygen atoms to metal atoms at the surface (again, as measured by presently available equipment for x-ray photoelectron spectroscopy) preferably is less than about 8:1 (oxygen atoms:metal atoms). More preferably, the ratio is less than 7:1, even more preferably less than about 6:1, and most preferably less than about 5:1. In general, the carbon supports used in the present invention are well known in the art. Activated, non-graphitized carbon supports are preferred. These supports are characterized by high adsorptive capacity for gases, vapors, and colloidal solids and relatively high specific surface areas. The support suitably may be a carbon, char, or charcoal produced by means known in the art, for example, by destructive distillation of wood, peat, lignite, coal, nut shells, bones, vegetable, or other natural or synthetic carbonaceous matter, but preferably is xe2x80x9cactivatedxe2x80x9d to develop adsorptive power. Activation usually is achieved by heating to high temperatures (from about 800xc2x0 to about 900xc2x0 C.) with steam or with carbon dioxide which brings about a porous particle structure and increased specific surface area. In some cases, hygroscopic substances, such as zinc chloride and/or phosphoric acid or sodium sulfate, are added before the destructive distillation or activation, to increase adsorptive capacity. Preferably, the carbon content of the carbon support ranges from about 10% for bone charcoal to about 98% for some wood chars and nearly 100% for activated carbons derived from organic polymers. The non-carbonaceous matter in commercially available activated carbon materials normally will vary depending on such factors as precursor origin, processing, and activation method. Many commercially available carbon supports contain small amounts of metals. Carbon supports having the fewest oxygen-containing functional groups at their surfaces are most preferred. The form of the carbon support is not critical. In one embodiment of this invention, the support is a monolithic support. Suitable monolithic supports may have a wide variety of shapes. Such a support may be, for example, in the form of a screen or honeycomb. Such a support may also, for example, be in the form of a reactor impeller. In a particularly preferred embodiment, the support is in the form of particulates. Because particulate supports are especially preferred, most of the following discussion focuses on embodiments which use a particulate support. It should be recognized, however, that this invention is not limited to the use of particulate supports. Suitable particulate supports may have a wide variety of shapes. For example, such supports may be in the form of granules. Even more preferably, the support is in the form of a powder. These particulate supports may be used in a reactor system as free particles, or, alternatively, may be bound to a structure in the reactor system, such as a screen or an impeller. Typically, a support which is in particulate form comprises a broad size distribution of particles. For powders, preferably at least about 95% of the particles are from about 2 to about 300 xcexcm in their largest dimension, more preferably at least about 98% of the particles are from about 2 to about 200 xcexcm in their largest dimension, and most preferably about 99% of the particles are from about 2 to about 150 xcexcm in their largest dimension with about 95% of the particles being from about 3 to about 100 xcexcm in their largest dimension. Particles being greater than about 200 xcexcm in their largest dimension tend to fracture into super-fine particles (i.e., less than 2 xcexcm in their largest dimension), which are difficult to recover. The specific surface area of the carbon support, measured by the BET (Brunauer-Emmett-Teller) method using N2, is preferably from about 10 to about 3,000 m2/g (surface area of carbon support per gram of carbon support), more preferably from about 500 to about 2,100 m2/g, and still more preferably from about 750 to about 2,100 m2/g. In some embodiments, the most preferred specific surface area is from about 750 to about 1,750 m2/g. The pore volume of the support may vary widely. Using the measurement method described in Example 1, the pore volume preferably is from about 0.1 to about 2.5 ml/g (pore volume per gram of catalyst), more preferably from about 0.2 to about 2.0 ml/g, and most preferably from about 0.4 to about 1.7 ml/g. Catalysts comprising supports with pore volumes greater than about 2.5 ml/g tend to fracture easily. On the other hand, catalysts comprising supports having pore volumes less than 0.1 ml/g tend to have small surface areas and therefore low activity. Carbon supports for use in the present invention are commercially available from a number of sources. The following is a listing of some of the activated carbons which may be used with this invention: Darco G-60 Spec and Darco X (ICI-America, Wilmington, Del.); Norit SG Extra, Norit EN4, Norit EXW, Norit A, Norit Ultra-C, Norit ACX, and Norit 4xc3x9714 mesh (Amer. Norit Co., Inc., Jacksonville, Fla.); Gl-9615, VG-8408, VG-8590, NB-9377, XZ, NW, and JV (Barnebey-Cheney, Columbus, Ohio); BL Pulv., PWA Pulv., Calgon C 450, and PCB Fines (Pittsburgh Activated Carbon, Div. of Calgon Corporation, Pittsburgh, Pa.); P-100 (No. Amer. Carbon, Inc., Columbus, Ohio); Nuchar CN, Nuchar C-1000 N, Nuchar C-190 A, Nuchar C-115 A, and Nuchar SA-30 (Westvaco Corp., Carbon Department, Covington, Va.); Code 1551 (Baker and Adamson, Division of Allied Amer. Norit Co., Inc., Jacksonville, Fla.); Grade 235, Grade 337, Grade 517, and Grade 256 (Witco Chemical Corp., Activated Carbon Div., New York, N.Y.); and Columbia SXAC (Union Carbide New York, N.Y.). The catalyst of this invention preferably has one or more noble metal(s) at its surface. Preferably, the noble metal(s) is selected from the group consisting of platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), iridium (Ir), silver (Ag), osmium (Os), and gold (Au). In general, platinum and palladium are more preferred, and platinum is most preferred. Because platinum is presently the most preferred noble metal, the following discussion will be directed primarily to embodiments using platinum. It should be understood, however, that the same discussion is generally applicable to the other noble metals and combinations thereof. It also should be understood that the term xe2x80x9cnoble metalxe2x80x9d as used herein means the noble metal in its elemental state as well as the noble metal in any of its various oxidation states. The concentration of the noble metal deposited at the surface of the carbon support may vary within wide limits. Preferably, it is in the range of from about 0.5 to about 20 wt. % ([mass of noble metal÷total mass of catalyst]xc3x97100%), more preferably from about 2.5 to about 10 wt. %, and most preferably from about 3 to about 7.5 wt. %. If concentrations less than 0.5 wt. % are used during the PMIDA oxidation reaction, there tends to be less formaldehyde oxidized, and therefore a greater amount of NMG produced, thereby reducing the N-(phosphonomethyl)glycine yield. On the other hand, at concentrations greater than about 20 wt. %, layers and clumps of noble metal tend to form. Thus, there are fewer surface noble metal atoms per total amount of noble metal used. This tends to reduce the activity of the catalyst and is an uneconomical use of the costly noble metal. The dispersion of the noble metal at the surface of the carbon support preferably is such that the concentration of surface noble metal atoms is from about 10 to about 400 xcexcmole/g (xcexcmole of surface noble metal atoms per gram of catalyst), more preferably, from about 10 to about 150 xcexcmole/g, and most preferably from about 15 to about 100 xcexcmole/g. This may be determined, for example, by measuring chemisorption of H2 or CO using a Micromeritics ASAP 2010C (Micromeritics, Norcross, Ga.) or an Altamira AMI100 (Zeton Altamira, Pittsburgh, Pa.). Preferably, the noble metal is at the surface of the carbon support in the form of metal particles. At least about 90% (number density) of the noble metal particles at the surface of the carbon support are preferably from about 0.5 to about 35 nm in their largest dimension, more preferably from about 1 to about 20 nm in their largest dimension, and most preferably from about 1.5 to about 10 nm in their largest dimension. In a particularly preferred embodiment, at least about 80% of the noble metal particles at the surface of the carbon support are from about 1 to about 15 nm in their largest dimension, more preferably from about 1.5 to about 10 nm in their largest dimension, and most preferably from about 1.5 to about 7 nm in their largest dimension. If the noble metal particles are too small, there tends to be an increased amount of leaching when the catalyst is used in an environment that tends to solubilize noble metals, as is the case when oxidizing PMIDA to form N-(phosphonomethyl)glycine. On the other hand, as the particle size increases, there tends to be fewer noble metal surface atoms per total amount of noble metal used. As discussed above, this tends to reduce the activity of the catalyst and is also an uneconomical use of the costly noble metal. In addition to the noble metal, at least one promoter may be at the surface of the carbon support. As defined herein, a xe2x80x9cpromoterxe2x80x9d is a metal that tends to increase catalyst selectivity, activity, and/or stability. A promoter additionally may reduce noble metal leaching. Although the promoter usually is deposited onto the surface of the carbon support in a promoter deposition step, the carbon support itself may also (or alternatively) naturally contain a promoter. A promoter which is deposited or exists naturally on the catalyst surface before the carbon support surface is finally reduced (see Section (B) (4) below) is referred to herein as a xe2x80x9ccatalyst-surface promoter.xe2x80x9d The catalyst-surface promoter may, for example, be an additional noble metal(s) at the surface of the carbon support. For example, depending on the application, ruthenium and palladium may act as catalyst-surface promoters on a catalyst comprising platinum deposited at a carbon support surface. The catalyst-surface promoter(s) alternatively may be, for example, a metal selected from the group consisting of tin (Sn), cadmium (Cd), magnesium (Mg), manganese (Mn), nickel (Ni), aluminum (Al), cobalt (Co), bismuth (Bi), lead (Pb), titanium (Ti), antimony (Sb), selenium (Se), iron (Fe), rhenium (Re), zinc (Zn), cerium (Ce), zirconium (Zr), tellurium (Te), and germanium (Ge). Preferably, the catalyst-surface promoter is selected from the group consisting of bismuth, iron, tin, titanium and tellurium. In a particularly preferred embodiment, the catalyst-surface promoter is tin. In another particularly preferred embodiment, the catalyst-surface promoter is iron. In an additional preferred embodiment, the catalyst-surface promoter is titanium. In a further particularly preferred embodiment, the catalyst comprises both iron and tin at its surface. Use of iron, tin, or both generally (1) reduces noble metal leaching for a catalyst used over several cycles, and (2) tends to increase and/or maintain the activity of the catalyst when the catalyst is used to effect the oxidation of PMIDA. Catalysts comprising iron generally are most preferred because they tend to have the greatest activity and stability with respect to formaldehyde and formic acid oxidation. In a preferred embodiment, the catalyst-surface promoter is more easily oxidized than the noble metal (in instances where the catalyst-surface promoter is a noble metal as well, the catalyst-surface promoter noble metal preferably is more easily oxidized than the non-promoter noble metal). A promoter is xe2x80x9cmore easily oxidizedxe2x80x9d if it has a lower first ionization potential than the noble metal. First ionization potentials for the elements are widely known in the art and may be found, for example, in the CRC Handbook of Chemistry and Physics (CRC Press, Inc., Boca Raton, Fla.). The amount of catalyst-surface promoter at the surface of the carbon support (whether associated with the carbon surface itself, metal, or a combination thereof) may vary within wide limits depending on, for example, the noble metal(s) and catalyst-surface promoter(s) used. Typically, the weight percentage of the catalyst-surface promoter is at least about 0.05% ([mass of catalyst-surface promoter÷total mass of the catalyst]xc3x97100%). The weight percent of the catalyst-surface promoter preferably is from about 0.05 to about 10%, more preferably from about 0.1 to about 10%, still more preferably from about 0.1 to about 2%, and most preferably from about 0.2 to about 1.5%. When the catalyst-surface promoter is tin, the weight percent most preferably is from about 0.5 to about 1.5%. Catalyst-surface promoter weight percentages less than 0.05% generally do not promote the activity of the catalyst over an extended period of time. On the other hand, weight percents greater than about 10% tend to decrease the activity of the catalyst. The molar ratio of noble metal to catalyst-surface promoter (and, in instances where the catalyst-surface promoter is a noble metal as well, the molar ratio of the non-promoter noble metal to the catalyst-surface promoter noble metal) may also vary widely, depending on, for example, the noble metal(s) and catalyst-surface promoter(s) used. Preferably, the ratio is from about 1000:1 to about 0.01:1; more preferably from about 150:1 to about 0.05:1; still more preferably from about 50:1 to about 0.05:1; and most preferably from about 10:1 to about 0.05:1. For example, a catalyst comprising platinum and iron preferably has a molar ratio of platinum to iron of about 3:1. In a particularly preferred embodiment of this invention, the noble metal (e.g., Pt) is alloyed with at least one catalyst-surface promoter (e.g., Sn, Fe, or both) to form alloyed metal particles (and, in instances where the catalyst-surface promoter is a noble metal as well, the non-promoter noble metal preferably is alloyed with the catalyst-surface promoter noble metal). A catalyst comprising a noble metal alloyed with at least one catalyst-surface promoter tends to have all the advantages discussed above with respect to catalysts comprising a catalyst-surface promoter in general. Catalysts comprising a noble metal alloyed with at least one catalyst-surface promoter also tend to exhibit greater resistance to catalyst-surface promoter leaching and further stability from cycle to cycle with respect to formaldehyde and formic acid oxidation. See, e.g., Example 17. The term xe2x80x9calloyxe2x80x9d encompasses any metal particle comprising a noble metal and at least one catalyst-surface promoter, irrespective of the precise manner in which the noble metal and catalyst-surface promoter atoms are disposed within the particle (although it is generally preferable to have a portion of the noble metal atoms at the surface of the alloyed metal particle). The alloy may be, for example, any of the following: 1. An intermetallic compound. An intermetallic compound is a compound comprising a noble metal and a promoter (e.g., Pt3Sn). 2. A substitutional alloy. A substitutional alloy has a single, continuous phase, irrespective of the concentrations of the noble metal and promoter atoms. Typically, a substitutional alloy contains noble metal and promoter atoms which are similar in size (e.g., platinum and silver; or platinum and palladium). Substitutional alloys are also referred to as xe2x80x9cmonophasic alloys.xe2x80x9d 3. A multiphasic alloy. A multiphasic alloy is an alloy that contains at least two discrete phases. Such an alloy may contain, for example Pt3Sn in one phase, and tin dissolved in platinum in a separate phase. 4. A segregated alloy. A segregated alloy is a metal particle wherein the particle stoichiometry varies with distance from the surface of the metal particle. 5. An interstitial alloy. An interstitial alloy is a metal particle wherein the noble metal and promoter atoms are combined with non-metal atoms, such as boron, carbon, silicon, nitrogen, phosphorus, etc. Preferably, at least about 80% (number density) of the alloyed metal particles are from about 0.5 to about 35 nm in their largest dimension, more preferably from about 1 to about 20 nm in their largest dimension, still more preferably from about 1 to about 15 nm in their largest dimension, and most preferably from about 1.5 to about 7 nm in their largest dimension. The alloyed metal particles need not have a uniform composition; the compositions may vary from particle to particle, or even within the particles themselves. In addition, the catalyst may further comprise particles consisting of the noble metal alone or the catalyst-surface promoter alone. Nevertheless, it is preferred that the composition of metal particles be substantially uniform from particle to particle and within each particle, and that the number of noble metal atoms in intimate contact with catalyst-surface promoter atoms be maximized. It is also preferred, although not essential, that the majority of noble metal atoms be alloyed with a catalyst-surface promoter, and more preferred that substantially all of the noble metal atoms be alloyed with a catalyst-surface promoter. It is further preferred, although not essential, that the alloyed metal particles be uniformly distributed at the surface of the carbon support. Regardless of whether the catalyst-surface promoter is alloyed to the noble metal, it is presently believed that the catalyst-surface promoter tends to become oxidized if the catalyst is exposed to an oxidant over a period of time. For example, an elemental tin catalyst-surface promoter tends to oxidize to form Sn(II)O, and Sn(II)O tends to oxidize to form Sn(IV)O2. This oxidation may occur, for example, if the catalyst is exposed to air for more than about 1 hour. Although such catalyst-surface promoter oxidation has not been observed to have a significant detrimental effect on noble metal leaching, noble metal sintering, catalyst activity, or catalyst stability, it does make analyzing the concentration of detrimental oxygen-containing functional groups at the surface of the carbon support more difficult. For example, as discussed above, the concentration of detrimental oxygen-containing functional groups (i.e., oxygen-containing functional groups that reduce noble metal resistance to leaching and sintering, and reduce the activity of the catalyst) may be determined by measuring (using, for example, TGA-MS) the amount of CO that desorbs from the catalyst under high temperatures in an inert atmosphere. However, it is presently believed that when an oxidized catalyst-surface promoter is present at the surface, the oxygen atoms from the oxidized catalyst-surface promoter tend to react with carbon atoms of the support at high temperatures in an inert atmosphere to produce CO, thereby creating the illusion of more detrimental oxygen-containing functional groups at the surface of the support than actually exist. Such oxygen atoms of an oxidized catalyst-surface promoter also can interfere with obtaining a reliable prediction of noble metal leaching, noble metal sintering, and catalyst activity from the simple measurement (via, for example, x-ray photoelectron spectroscopy) of oxygen atoms at the catalyst surface. Thus, when the catalyst comprises at least one catalyst-surface promoter which has been exposed to an oxidant and thereby has been oxidized (e.g., when the catalyst has been exposed to air for more than about 1 hour), it is preferred that the catalyst-surface promoter first be substantially reduced (thereby removing the oxygen atoms of the oxidized catalyst-surface promoter from the surface of the catalyst) before attempting to measure the amount of detrimental oxygen-containing functional groups at the surface of the carbon support. This reduction preferably is achieved by heating the catalyst to a temperature of about 500xc2x0 C. for about 1 hour in an atmosphere consisting essentially of H2. The measurement of detrimental oxygen-containing functional groups at the surface preferably is performed (a) after this reduction, and (b) before the surface is exposed to an oxidant following the reduction. Most preferably, the measurement is taken immediately after the reduction. The preferred concentration of metal particles at the surface of the carbon support depends, for example, on the size of the metal particles, the specific surface area of the carbon support, and the concentration of noble metal on the catalyst. It is presently believed that, in general, the preferred concentration of metal particles is roughly from about 3 to about 1,500 particles/xcexcm2 (i.e., number of metal particles per xcexcm2 of surface of carbon support), particularly where: (a) at least about 80% (number density) of the metal particles are from about 1.5 to about 7 nm in their largest dimension, (b) the carbon support has a specific surface area of from about 750 to about 2100 m2/g (i.e., m2 of surface of carbon support per gram of carbon support), and (c) the concentration of noble metal at the carbon support surface is from about 1 to about 10 wt. % ([mass of noble metal÷total mass of catalyst]xc3x97100%). In more preferred embodiments, narrower ranges of metal particle concentrations and noble metal concentrations are desired. In one such embodiment, the concentration of metal particles is from about 15 to about 800 particles/xcexcm27 and the concentration of noble metal at the carbon support surface is from about 2 to about 10 wt. %. In an even more preferred embodiment, the concentration of metal particles is from about 15 to about 600 particles/xcexcm2, and the concentration of noble metal at the carbon support surface is from about 2 to about 7.5 wt. %. In the most preferred embodiment, the concentration of the metal particles is from about 15 to about 400 particles/xcexcm2, and the concentration of noble metal at the carbon support surface is about 5 wt. %. The concentration of metal particles at the surface of the carbon support may be measured using methods known in the art. B. Process for the Preparation of the Oxidation Catalyst 1. Deoxygenation of the Carbon Support The surface of the carbon support preferably is deoxygenated before the noble metal is deposited onto it. Preferably, the surface is deoxygenated using a high-temperature deoxygenation treatment. Such a treatment may be a single-step or a multi-step scheme which, in either case, results in an overall chemical reduction of oxygen-containing functional groups at the surface of the carbon support. In a two-step high-temperature deoxygenation treatment, the carbon support preferably is first treated with a gaseous or liquid phase oxidizing agent to convert oxygen-containing functionalities in relatively lower oxidation states (e.g., ketones, aldehydes, and alcohols) into functionalities in relatively higher oxidation states (e.g., carboxylic acids), which are easier to cleave from the surface of the catalyst at high temperatures. Representative liquid phase oxidizing agents include nitric acid, H2O2, chromic acid, and hypochlorite, with concentrated nitric acid comprising from about 10 to about 80 grams of HNO3 per 100 grams of aqueous solution being preferred. Representative gaseous oxidants include molecular oxygen, ozone, nitrogen dioxide, and nitric acid vapors. Nitric acid vapors are the preferred oxidizing agent. With a liquid oxidant, temperatures in the range of from about 60xc2x0 to about 90xc2x0
{'bibliographic_information': "{'document_kind': 'B2', 'document_number': '06586621', 'document_date': '20030701', 'publishing_country_or_organization': 'US', 'title_of_invention': 'Use of a supplemental promoter in conjunction with a carbon-supported, noble-metal containing catalyst in liquid phase oxidation reactions'}", 'source_file': './grant/2003/pg030701.xml', 'source_data': 'USPTO-Grant'}