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44045-AC5
Electronic Structure Studies of Transition Metal Phosphides
The catalysts employed in traditional commercial hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes are sulfides of Mo or W promoted by Co or Ni (often designated as CoMoS or NiMoS catalysts) The need to develop ever more active HDS/HDN catalysts has led to research focused on the search for new catalytic materials. A number of studies have shown that MoP, WP, CoP, Co2P, Ni2P, CoMoP, and NiMoP are all active HDN and HDS catalysts, and several of the phosphides have been reported to exhibit higher HDS and/or HDN activity than the commercial CoMoS or NiMoS catalysts.
Although reports have appeared discussing certain aspects of the electronic structure of some of the phosphides there has been no comprehensive study of the electronic structure and bonding in this whole group of catalytically active phosphides. We have carried out such a comprehensive study. MoP, WP, CoP, Co2P, Ni2P, CoMoP, and NiMoP exhibit several different, but in some cases related, crystal structures. The results of our Fenske-Hall band structure calculations have enable us to understand the nature of the metal-metal, metal-phosphorous, and phosphorus-phosphorous bonding in these materials, and to compare the electronic structure of the phosphides which have similar or related crystal structures.
MoP, which exhibits a hexagonal WC-type structure, has strong covalent Mo-P bonds and very weak Mo-Mo bonds within the hexagonal layers. Although isoelectronic with MoP, WP has a different structure. Both WP and CoP have a distorted MnP structure (itself a distorted NiAs structure), and both WP and CoP are usually described as having the metal in a distorted octahedral geometry with six metal-phosphorous bonds. Our results indicate that in fact the metal in WP should be described as square pyramidal, since the metal only bonds to five P atoms. In addition, unlike MoP, there are strong W-W bonds evident in WP. The metal in CoP is truly six-coordinate, but in this case weak P-P bonds can be observed from the density of states and Crystal Orbital Overlap (COOP) curves. The same metal-metal interactions observed in WP occur in CoP, but the larger number of valence electrons in CoP means that the Co-Co bonding and antibonding bands are both occupied, resulting in no net Co-Co bond.
The structures of Co2P, CoMoP, Ni2P, and NiMoP are similar, though not identical. In Co2P and Ni2P the metal atoms occupy an equal number of tetrahedral and square pyramidal sites. In CoMoP and NiMoP, the square pyramidal metals are replaced by Mo. Our results show that while there are strong metal-P bonds in both Co2P and Ni2P, there is no net metal-metal bonding in either material, since both the metal-metal bonding and antibonding bands are occupied. Several marked changes in electronic structure can be observed in CoMoP and NiMoP, where all of the square pyramidal Ni or Co atoms are replaced by Mo. Since the Mo's are larger, have fewer electrons, and have higher energy atomic orbitals than Co or Ni, there is considerable metal-metal bonding observed in the ternary phosphides. There are strong Mo-Mo bonds, weaker Mo-Ni bonds, but still no Ni-Ni bonds. The nature of the states around the Fermi level are also altered in the ternary phosphides; In NiMoP, the majority of these states are Mo in character, while in CoMoP about half are Mo in character.
Comparisons of the electronic structures of Co2P, CoMoP, Ni2P, and NiMoP are particularly important, because, unlike the MoS2 based catalysts where addition of Co or Ni "promotes" the activity of the catalyst, the mixed Co/Mo and Ni/Mo phosphides show a marked decrease in activity compared to Co2P or Ni2P. This suggests that either the square pyramidal Ni atoms provide active sites in Co2P and Ni2P or that introduction of Mo alters the electronic structure of the tetrahedral Ni in CoMoP NiMoP . Since the catalytic activity is determined by the surface electronic structure, we have also carried out calculations on several surfaces of Ni2P, and NiMoP. Our results suggest that the pyramidal Ni is the active site
