Metal utilization in supported, metal-containing catalysts
Generally, the present invention relates to improvements in metal utilization in supported, metal-containing catalysts. For example, the present invention relates to methods for directing and/or controlling metal deposition onto surfaces of porous substrates. The present invention also relates to methods for preparing catalysts in which a first metal is deposited onto a support (e.g., a porous carbon support) to provide one or more regions of a first metal at the surface of the support, and a second metal is deposited at the surface of the one or more regions of the first metal. Generally, the electropositivity of the first metal (e.g., copper or iron) is greater than the electropositivity of the second metal (e.g., a noble metal such as platinum) and the second metal is deposited at the surface of the one or more regions of the first metal by displacement of the first metal. The present invention further relates to treated substrates, catalyst precursor structures and catalysts prepared by these methods. The invention further relates to use of catalysts prepared as detailed herein in catalytic oxidation reactions, such as oxidation of a substrate selected from the group consisting of N-(phosphonomethyl) iminodiacetic acid or a salt thereof, formaldehyde, and/or formic acid.
1. A process for oxidizing a substrate selected from the group consisting of N-(phosphonomethyl)iminodiacetic acid or a salt thereof, formaldehyde, and formic acid, the process comprising:
contacting the substrate with an oxidizing agent in the presence of an oxidation catalyst comprising a particulate carbon support, a first metal, and a second metal, the support having at its surface particles comprising the first metal and the second metal, wherein:
the first metal constitutes from about 1% to about 10% by weight of the catalyst;
the first metal is selected from the group consisting of copper, tin, nickel, cobalt, and combinations thereof;
the second metal constitutes from about 1% to about 6% by weight of the catalyst;
the second metal is selected from the group consisting of platinum, palladium, ruthenium, rhodium, iridium, silver, osmium, gold, and combinations thereof;
the carbon support has a specific surface area of at least about 500 m 2 /g; and
wherein first metal and second metal-containing particles at the surface of the carbon support comprise a core comprising the first metal and a shell at least partially surrounding the core and comprising second metal, the shell having a thickness of no more than five second metal atoms.
2. The process as set forth in claim 1 wherein the shell has a thickness of no more than 3 second metal atoms.
3. The process as set forth in claim 1 wherein the shell has a thickness of no more than 2 second metal atoms.
4. The process as set forth in claim 1 wherein the first metal constitutes from about 2% to about 5% by weight of the catalyst.
5. The process as set forth in claim 1 wherein the second metal constitutes from about 1% to about 5% by weight of the catalyst.
6. The process as set forth in claim 1 wherein the first metal is copper.
7. The process as set forth in claim 1 wherein the first metal is iron.
8. The process as set forth in claim 1 wherein the first metal is cobalt.
9. The process as set forth in claim 1 wherein the second metal is platinum.
10. The process as set forth in claim 1 wherein at least about 10% of the second metal is present within the shell of the metal particles.
11. The process as set forth in claim 1 wherein the atom percent of second metal at the surface of first metal and second metal-containing particles at the surface of the carbon support is at least about 2%.
12. The process as set forth in claim 1 wherein the atomic ratio of second metal to first metal of the at least one particle is less than 1:1.
13. The process as set forth in claim 1 wherein the at least one particle has a largest dimension of at least about 6 nm.
14. The process as set forth in claim 1 wherein the at least one particle constitutes at least about 1% of the metal particles at the surface of the carbon support.
15. The process as set forth in claim 1 wherein the carbon support has a specific surface area from about 500 m 2 /g to about 1900 m 2 /g.