Accounts Novel Molecular Approaches to the Structure-Activity Relationships and Unique Characterizations of Co-Mo Sulfide Hydrodesulfurization Catalysts for the Production of Ultraclean Fuels Yasuaki Okamoto Graduate School of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori, Ako-gun, Hyogo 678-1297 Received July 22, 2013; E-mail: yokamoto@sci.u-hyogo.ac.jp Hydrodesulfurization (HDS) catalysts have been extensively used worldwide in refineries to protect the environ- ment. The present Account reviews our recent studies to establish the structure-activity relationships of HDS catalysts on a molecular level. We have developed two molecular approaches to this issue; a molecular cluster approach using intrazeolite Mo sulfide and CoMo binary sulfide clusters possessing well-defined structures and a CVD-CoMo sulfide designed catalyst approach in which Co is exclusively accommodated in the CoMoS structure, the active sites of HDS catalysts. It is revealed that the Co sites ofintrazeolite thiocubane-type [Co 2 Mo 2 S 6 ]clusters play a pivotal role in HDS reaction and that the Mo-Mo atomicdistance of Mo 2 S 4 dinuclear clusters determines the HDS activity. Designed CVD- CoMo catalysts supported on refractory oxides have been successfully prepared by CVD using [Co(CO) 3 NO] as a precursor of Co. Combined with magnetic properties and XAFS, it is concluded that the CoMoS structure isdinuclear Co sulfide clusters located on the edge of MoS 2 nanoparticles. It is shown that the intrinsic activity of the CoMoS structure substantially depends on the MoS 2 -edge where it is located. The effects of support and additives are discussed on the basis of the intrinsic activity. Both MoS 2 -support interactions and reaction or sulfidation conditions elucidate the local structure and intrinsic activity of the CoMoS structure. It is demonstrated that the CVD technique using [Co(CO) 3 NO] provides unique characterization methods of HDS catalysts. 1. Introduction The present age is the era of petroleum. While world energy consumption has been increasing from the 1950s at an alarming rate, petroleum has always been a major source of energy. At present, 34% of the energy consumption in the world is supplied by petroleum (2010). 1 With Japan 40% ofits energy consumption depends on petroleum and about 45% of the imported petroleum is used as transportation fuels such as gasoline, dieselfuel, and jet fuel (2010). The usage of such a large amount of petroleum, however, poses crucialissues, that is, 1) petroleum contains sulfur, nitrogen, oxygen, and metals in considerable amounts as naturalingredients of crude oil, 2) petroleum is one of the major origins of CO 2 emission as a fossil resource as well as coal and natural gas, accelerating global warming (400 ppm of CO 2 in 2013), and 3) petroleum reserves are inevitably very limited, prompting us to develop substitutive renewable energies. From the catalysis point of view, we focus in the present Account on the first issue, in particular on the removal of sulfur and nitrogen from fuels. The sulfur and nitrogen contents depend on the quality of crude oil and the fraction of distillates. Typically, the sulfur contents of naphtha (bp 310-450 K), diesel oil (500-630 K), and atmospheric residue (AR, >610 K) range from 0.01 to 0.05, 0.5-1.5, and 1.5-5 wt % S, respectively. Their corresponding nitrogen contents are 0.001, 0.01-0.05, and 0.2-0.5 wt %, respectively. The metals in crude oil remain in AR (5-200 ppm of Ni and 20-1000 ppm of V) after atmosphericdistil- lation. After combustion, such as in power plants or internal combustion engines, these sulfur and nitrogen organic com- pounds are transformed into SO x and NO x , severe air pollu- tants, which are not only hazardous to human health but also cause serious acid rain and photochemical smog. With diesel engines, the amount of particulate matter or soot increases with increasing sulfur content in the fuel. In addition, a small amount of SO x severely retards the performance of exhaust emission control systems. Furthermore, the sulfur and nitrogen com- pounds are notorious inhibitors of the catalysts used in refinery and petrochemical processes following distillation units, such as reforming, hydroisomerization, and catalytic cracking. The amounts of sulfur and nitrogen in fuels, thus, must be strictly controlled. The specifications for fuel oils, in particular for diesel oil, have recently become progressively stricter worldwide, pre- senting great challenges to refiners. 2-6 In Japan, the diesel oil sulfur content was regulated to 500 ppm in 1997 and 50 ppm in © 2013 The Chemical Society of Japan Published on the web September 21, 2013; doi:10.1246/bcsj.20130204 Bull. Chem. Soc. Jpn. Vol. 87, No. 1, 20-58 (2014) 20