13
higher HDS activity is attributed to the weaker metal-sulfur bond strength. In
Co-promoted MoS 2 -based HDS catalysts, the decrease in Mo-S bond strength and
improved HDS activity is attributed to the electron donation from promoter Co to
Mo atom. The promotion effect of Co in HDS catalysts by weakening the metalsulfur bond strength is well understood and confirmed using both theoretical estimations and experiments. Morphology of the sulfide phase can be tuned by
increasing the metal content and addition of organic additives [21]. Increased metal
loading results in a progressive increase in MoS 2 slab length with little effect on
stacking, while introduction of organic additives such as ethylene glycol and citric
acid resulted in the reduction of MoS 2 slab length and enhancement in the stacking
due to variation in metal–support interaction.
Topsoe et al. [22] used advanced catalyst characterization tools such as scanning
tunneling microscopy (STM) to identify a special type of Mo electronic edge sites
termed as “brim sites” in hydrotreating catalysts, which are altogether different
from Type I and Type II active sites. These brim sites tend to bind the sulfurcontaining hydrocarbon molecules due to their metallic character and react with
hydrogen available at neighboring edge sites. Brim sites are not co-coordinately
unsaturated sites and usually exhibit higher hydrogenation activity compared to the
conventional type of active sites. It was proposed that brim sites promote hydrogenation reactions and both the edges are responsible for direct sulfur removal. The
hydrogenation mechanism involving brim sites is used to explain several catalytic
phenomena such as inhibitions, steric hindrance of alkyl substituents, and poisoning
effects in a better way compared to the vacancy model.
Tuxen et al. [23] investigated the effect of carbon incorporation as carbide into
MoS 2 nanocluster during sulfidation with organic sulfur compounds such as
dimethyl disulfide (DMDS) or dimethyl sulfide (DMS) using STM, XPS, and density functional theory (DFT) calculations. Their studies showed the strong influence
of the choice of sulfiding agent on the morphology and dispersion of the sulfided
phase and ultimately on the catalytic activity of the freshly sulfided phase. The presence of carbon in the form of carbides was not observed at the edge sites of MoS 2
under HDS conditions due to unfavorable thermodynamics. There was no evidence
for the existence of surface or bulk carbide phases in HDS catalysts, and substitution of sulfur in MoS 2 nanoclusters incorporated with carbon was not energetically
favorable and stable.
Chen et al. [21] studied the morphology and selectivity of the sulfide phase of
CoMo catalyst in the HDS of 4,6-DMDBT and hydrodearomatization (HDA) of
1-methylnaphthalene (1-MN) by increasing the metal content and incorporation of
organic additives such as ethylene glycol and citric acid. The change in morphology
of the sulfide phase resulted in modifications in the DDS, HYD, and HDA activities
of the catalyst along with hydrogen consumption. Based on XPS and HR TEM
analysis of the corner and edge sites of sulfide slabs of CoMoS sites, it was revealed
that the corner and edge sites of the sulfide phase are responsible for the DDS and
HYD routes for the HDS of 4,6-DMDBT, respectively. Figure 3 shows the relationship between the ratio of corner and edge sites of the CoMoS phase and rate constants of DDS and HYD reactions on various catalysts, and the linear relationship
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…
higher HDS activity is attributed to the weaker metal-sulfur bond strength. In
Co-promoted MoS 2 -based HDS catalysts, the decrease in Mo-S bond strength and
improved HDS activity is attributed to the electron donation from promoter Co to
Mo atom. The promotion effect of Co in HDS catalysts by weakening the metalsulfur bond strength is well understood and confirmed using both theoretical estimations and experiments. Morphology of the sulfide phase can be tuned by
increasing the metal content and addition of organic additives [21]. Increased metal
loading results in a progressive increase in MoS 2 slab length with little effect on
stacking, while introduction of organic additives such as ethylene glycol and citric
acid resulted in the reduction of MoS 2 slab length and enhancement in the stacking
due to variation in metal–support interaction.
Topsoe et al. [22] used advanced catalyst characterization tools such as scanning
tunneling microscopy (STM) to identify a special type of Mo electronic edge sites
termed as “brim sites” in hydrotreating catalysts, which are altogether different
from Type I and Type II active sites. These brim sites tend to bind the sulfurcontaining hydrocarbon molecules due to their metallic character and react with
hydrogen available at neighboring edge sites. Brim sites are not co-coordinately
unsaturated sites and usually exhibit higher hydrogenation activity compared to the
conventional type of active sites. It was proposed that brim sites promote hydrogenation reactions and both the edges are responsible for direct sulfur removal. The
hydrogenation mechanism involving brim sites is used to explain several catalytic
phenomena such as inhibitions, steric hindrance of alkyl substituents, and poisoning
effects in a better way compared to the vacancy model.
Tuxen et al. [23] investigated the effect of carbon incorporation as carbide into
MoS 2 nanocluster during sulfidation with organic sulfur compounds such as
dimethyl disulfide (DMDS) or dimethyl sulfide (DMS) using STM, XPS, and density functional theory (DFT) calculations. Their studies showed the strong influence
of the choice of sulfiding agent on the morphology and dispersion of the sulfided
phase and ultimately on the catalytic activity of the freshly sulfided phase. The presence of carbon in the form of carbides was not observed at the edge sites of MoS 2
under HDS conditions due to unfavorable thermodynamics. There was no evidence
for the existence of surface or bulk carbide phases in HDS catalysts, and substitution of sulfur in MoS 2 nanoclusters incorporated with carbon was not energetically
favorable and stable.
Chen et al. [21] studied the morphology and selectivity of the sulfide phase of
CoMo catalyst in the HDS of 4,6-DMDBT and hydrodearomatization (HDA) of
1-methylnaphthalene (1-MN) by increasing the metal content and incorporation of
organic additives such as ethylene glycol and citric acid. The change in morphology
of the sulfide phase resulted in modifications in the DDS, HYD, and HDA activities
of the catalyst along with hydrogen consumption. Based on XPS and HR TEM
analysis of the corner and edge sites of sulfide slabs of CoMoS sites, it was revealed
that the corner and edge sites of the sulfide phase are responsible for the DDS and
HYD routes for the HDS of 4,6-DMDBT, respectively. Figure 3 shows the relationship between the ratio of corner and edge sites of the CoMoS phase and rate constants of DDS and HYD reactions on various catalysts, and the linear relationship
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…