Periodic Trends Transition Metal Sulfide Catalysis: Intuition and Theory

Abstract

The use of Transition Metal Sulfide (TMS) catalysts came about as the need to hydrogenate coal to liquid fuels became urgent in the 1920's. Sabatier won the Nobel Prize in 1912 for describing the phenomenon of hydrogenation by transition metals. However, when the transition metals were used to hydrogenate coal, the TMS emerged as the stable catalytic states due to high sulfur content in the coal liquids. From 1920–1930 I.G. Farbenindustrie A.G. tested over 6000 catalysts. It was from these origins that modern Co (Ni)/Mo/Al2 O3 arose. Since that time scientists have developed an understanding of the fundamental properties that lead toboth the activity of the simple binary sulfides and the mechanism by which two metals (Co+Mo ) acted together to enhance activity (promotion). Initial efforts focused on supported commercial catalysts with limited success. In the early 1980's the periodic trends of TMS catalysts on unsupported catalysts were discovered and these results formed the foundation for further basic understanding of the key properties that led to catalytic activity. These results have been extended over the years to include supported catalysts and many petroleum based substrates. Early results related catalytic activity to the heats of formation of the sulfides and Pauling % d-character of the transition metals. These correlations both pointed to the importance of the 4d an 5d electrons and also to the well established catalytic principle that the metal sulfur bonds needed to be “just right”, not too strong nor too weak thus allowing facile turnover of the adsorbing and desorbing molecules. Averaging of the heat of formation of Co9 S8 and MoS2 suggested that promotion operated through sulfur vacancies formed by atoms that shared both a Co atom and a Mo atom on the surface. Theoretical studies also began in this period that further supported the idea that d-electrons in the frontier orbitals of the catalysts were key in determining catalysis at the surface. The triumph of this approach was that it unified the promoted TMS systems with the binary TMS and provided a common rational for the activity of both. Constant progress since then has been achieved through the application of density functional theory (DFT) narrowing the gap between instinct and a formal description of catalyst structure/function. It is crucial to remember that for real understanding to develop we must study the catalytically stabilized materials and not materials that are changing under catalytic conditions. In the case of the TMS this means that we must study materials like MoSCx and RuSCx . It has been demonstrated that “surface carbides” are the catalytically stabilized state under hydrotreating conditions. The original relation between the d-electrons and later DFT calculations all point to the importance of these electrons in the catalytic reaction. However, more work is needed to define the relation between these electrons and the stabilized carbide surfaces before detailed “active site” structures can be developed with confidence. In addition the presence of Co metal in active hydroprocessing catalysts stabilized for four years in a commercial reactor, calls in to question current theories of the structure of promoted catalysts.