Mo Hydrodesulfurization Catalysts Research Articles

Rationalizing the catalytic performance of γ-alumina-supported Co(Ni)–Mo(W) HDS catalysts prepared by urea-matrix combustion synthesis

A comparative study of the influence of Co (or Ni) promoter loadings and the effect of different sulfurizing agents and sulfurizing temperatures on the structure, morphology and catalytic performance of Mo- or W-based hydrodesulfurization (HDS) catalysts was carried out. Catalyst performance using a tubular fixed-bed reactor and the HDS of thiophene as a model reaction was evaluated. The oxidic and sulfurized states of the HDS catalysts were characterized by laser Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and high resolution transmission electron microscopy (HRTEM). It has been found that the urea-matrix combustion (UMxC) synthesis is a simple tool for preparing supported catalysts in a short period of thermal treatment. Several consecutive stages such as urea melting, metal precursor dissolution and chemical reactions take place before and upon combustion process. The C4H4S/H2-activated Co- (or Ni-) promoted MoS2 (or WS2) catalysts present a strong synergistic effect (SE) when the Co (or Ni)/Mo (or W) molar ratio is near to 0.5, whereas the C4/C 4 = molar ratios display a weak antagonistic effect. Alumina-supported Ni–W catalyst showed an optimal SE 2.5 times higher than those for Co (or Ni)-promoted Mo HDS catalysts. The kinetic parameters for thiophene-HDS reaction were also determined, suggesting that the C–S bond cleavage reaction for alumina-supported Co(Ni)–Mo HDS catalysts and H2 activation reaction for Ni-promoted WS2catalysts play an important role in the rate-limiting step.

Structure of the Active Sites of Co−Mo Hydrodesulfurization Catalysts as Studied by Magnetic Susceptibility Measurement and NO Adsorption

The elucidation of a molecular structure of the active sites (i.e., the Co-Mo-S phase) of Co-Mo hydrodesulfurization catalysts has received extensive attention. In the present study, we unambiguously determined, for the first time, the NO adsorption behavior and magnetic property of the Co-Mo-S phase by preparing unique Co-Mo/Al(2)O(3) catalysts (CVD-Co/MoS(2)/Al(2)O(3)), in which all the Co atoms are present as the Co-Mo-S phase. The catalysts were characterized by NO adsorption (pulse technique and FTIR), Co K-edge XANES, and the magnetic susceptibility and effective magnetic moment of Co. Nitric oxide molecules were adsorbed on 33% of the Co atoms in CVD-Co/MoS(2)/Al(2)O(3) after sulfidation and on only half of the Co atoms even after an H(2)-treatment of the sulfided catalyst at 573-673 K. The Co atoms in CVD-Co/MoS(2)/Al(2)O(3) exclusively exhibited an antiferromagnetic property, indicating that even-numbered Co atoms are interacting with each other in the Co-Mo-S phase. A Co-Mo/Al(2)O(3) catalyst, prepared by a conventional impregnation technique, was composed of the antiferromagnetic Co sulfide species as observed in CVD-Co/MoS(2)/Al(2)O(3) in addition to Co(9)S(8). On the basis of the NO adsorption behavior and magnetic property, it is empirically proposed that the structure of the Co-Mo-S phase is represented as a Co sulfide dinuclear cluster located on the edge of MoS(2) particles. The magnetic property of Co/Al(2)O(3) sulfide catalysts depended on the preparation method.

A model catalyst approach to the effects of the support on Co–Mo hydrodesulfurization catalysts

Co–Mo model sulfide catalysts, in which CoMoS phases are selectively formed, were prepared by means of a CVD technique using Co(CO) 3NO as a precursor of Co. It is shown by means of XPS, FTIR and NO adsorption that CoMoS phases form selectively when the Mo content exceeds monolayer loading. A single exposure of MoS 2/Al 2O 3 to a vapor of Co(CO) 3NO at room temperature fills the edge sites of the MoS 2 particles. It is suggested that the maximum potential HDS activity of MoS 2/Al 2O 3 and Co–Mo/Al 2O 3 catalysts can be predicted by means of Co(CO) 3NO as a “probe” molecule. An attempt was made to determine the fate of Co(CO) 3NO adsorbed on MoS 2/Al 2O 3. The effects of the support on Co–Mo sulfide catalysts in HDS and HYD were investigated by use of CVD-Co/MoS 2/support catalysts. XPS and NO adsorption showed that model catalysts can also be prepared for SiO 2-, TiO 2- and ZrO 2-supported catalysts by means of the CVD technique. The thiophene HDS activity of CVD-Co/MoS 2/Al 2O 3, CVD-Co/MoS 2/TiO 2 and CVD-Co/MoS 2/Al 2O 3 is proportional to the amount of Co species interacting with the edge sites of MoS 2 particles or CoMoS phases. It is concluded that the support does not influence the HDS reactivity of CoMoS phases supported on TiO 2, ZrO 2 and Al 2O 3. In contrast, CoMoS phases on SiO 2 show catalytic features characteristic of CoMoS Type II. With the hydrogenation of butadiene, on the other hand, the Co species on MoS 2/TiO 2, ZrO 2 and SiO 2 have the same activity, while the Co species on MoS 2/Al 2O 3 have a higher activity.

Fraction of the CoMoS phases accessible to NO in Co-Mo hydrodesulfurization catalysts.

It is established by using Co-Mo model sulfide catalysts, XAFS and FTIR that Co atoms constituting CoMoS phases are not oxidized by NO adsorption and that only 55% of the CoMoS phases is susceptible to NO adsorption even at the maximum coordinative unsaturation attainable under usual HDS reaction conditions (623-673 K).

An X-ray photoelectron spectroscopy study of zirconia-supported Mo and Ni–Mo hydrodesulfurization catalysts

Monometallic, Mo, and bimetallic, Ni–Mo, catalysts supported on zirconia have been prepared and characterized by X-ray photoelectron spectroscopy (XPS). These catalysts were also tested in the hydrodesulfurization of a vacuum distillate in a batch autoclave at 330°C and 60 atm. The investigation by XPS of the oxidic catalysts with different Mo loadings showed that most likely a growth of 3D MoO 3 crystallites takes place. The investigation of the sulfided catalysts showed that they contained molybdenum in a formal 4 + oxidation state. The quantitative analysis suggested that the sulfidation was within the experimental uncertainty complete and no structural changes occurred in the Mo phase. The comparison between monometallic and bimetallic catalysts indicated a strong enhancement of the molybdate species dispersion in the latter case, which leads to the conclusion that the main role of the Ni promoter is the improved spreading of the active phase. The strongest dispersion enhancement occurs for low MoO 3 loadings (7% MoO 3) and this is consistent with measurements of the catalytic activity whereby the 7% MoO 3 catalyst exhibited the largest enhancement of the catalytic activity for the hydrodesulfurization reaction.

Structure and catalytic properties of Al 2O 3 supported and unsupported CoMo HDS catalysts : the problem of synergy

Coimpregnated and consecutively impregnated (Mo first) catalysts were prepared on γ-Al 2O 3 support with various mole ratios r = Co/(Co+Mo). Samples were calcined at 723K for 4h then subjected to three types of pretreatment: none, prereduction in H 2 for 2h and presulfidation with H 2/H 2S mixture for 1h, both at 673K. They were then contacted with hydrogenthiophene (H 2/T) mixture and the activity in thiophene hydrodesulfurization (HDS) and hydrogenation selectivity (HYD) determined. The surface Co and S concentrations were monitored by XPS. Any of the treatment brought about a partial sulfidation only; upon sulfidation, a surface Co-enrichment occurred. No synergistic effects were observed with catalysts not prereduced; synergy appeared with presulfided and, coimpregnated and prereduced catalysts in HDS activity. Hydrogenation selectivity decreased with increasing Co-content. A comparison with unsupported catalysts led us to the conclusion that HDS synergy is a temporary phenomenon. It can be attributed to a certain state of Co which promotes Mo reduction. As Co is segregated to the surface, synergy disappears. This may be the main reason of deactivation of unsupported catalysts; the much less significant deactivation of supported samples may be connected with surface carbonization.

Structure and Function of the Catalyst and the Promoter in Co—Mo Hydrodesulfurization Catalysts

Etude de la structure d'un catalyseur utilise pour une hydrodesulfuration et plus particulierement etude de la localisation et de la structure des ions molybdene et cobalt a la surface de l'alumine. Discussion sur le role d'un promoteur modifiant un catalyseur, notamment determination du nombre de sites actifs

EPR studies on the influence of NO adsorption on sulfided CoMo hydrodesulfurization catalysts

EPR studies on the influence of NO adsorption on sulfided CoMo hydrodesulfurization catalysts

EPR studies on unsupported and alumina-supported sulfided CoMo hydrodesulfurization catalysts

EPR has been used to investigate the paramagnetic species present in unsupported and aluminasupported sulfided CoMo ( Co Mo = 0–2.3 ) catalysts for which the nature of the Co and Mo phases was previously characterized by Mössbauer emission spectroscopy (MES) and EXAFS. Three distinct EPR signals have been observed: MO, MS, and CMS. Signal MO ( g av = 1.93) corresponds to oxo-Mo 5+ ions interacting with alumina, whereas MS ( g ∥ = 2.06, g ⊥ = 1.98) is assigned either to bulk defect species in a MoS 2 phase or to thio-Mo 5+ species located at the edges of the layered MoS 2 crystallites. Addition of Co decreases the intensity of signals MO and MS and generates a new EPR resonance, CMS, with g av = 2.06 and ΔH = 100–200 G and best identified by its low-field peak at g = 2.08-2.10. Its intensity rapidly increases with Co content and reaches a maximum at Co Mo ∼- 1 . Furthermore, the intensity of signal CMS is found to parallel both the hydrodesulfurization activity and the amount of Co observed by MES to be present as CoMoS. Recent work has shown that the Co atoms in CoMoS are located at the edges of MoS 2. The presence of the CMS signal does therefore provide new evidence for the presence of special Mo atoms neighboring the Co edge atoms. A two-center model is presented to explain the observed properties of the CMS Signal.

Activity of Ni−Mo hydrodesulfurization catalysts supported on mixed alumina-zirconia

The hydrodesulfurization (HDS) of thiophene by nickel-molybdenum catalysts supported on alumina-zirconia has been investigated. The effect of varying the ratios of oxides in the support is to produce a minimum in the activity for HDS, and the coimpregnated catalysts are shown to be more active than the analogous co-precipitated catalysts.

In situ Mössbauer emission spectroscopy studies of unsupported and supported sulfided CoMo hydrodesulfurization catalysts: Evidence for and nature of a CoMoS phase

Information regarding the structure and type of phases present in sulfided alumina-supported, as well as unsupported, CoMo catalysts is obtained from in situ Mössbauer emission spectroscopy (MES) studies. The results give the first direct evidence of the presence of a CoMoS phase in alumina-supported and unsupported catalysts with similar Co Mo ratios. Information regarding the nature of the CoMoS phase is obtained from a detailed study of the Mössbauer parameters, their temperature dependence and their sensitivity to changes in the gaseous environment. Previously proposed structural models cannot explain all the observed features of the CoMoS phase. It is proposed that in alumina-supported catalysts the CoMoS phase is present as single SMoS slabs (i.e., one layer of the MoS 2 structure) with cobalt most likely present at molybdenum sites. For unsupported catalysts the CoMoS phase consists of several slabs with bulk MoS 2-like structure. The present observations suggest that the previously observed similarities between supported and unsupported catalysts are associated with the presence of the CoMoS phase in both catalyst systems. A possible reaction mechanism for hydrodesulfurization involving cobalt in the CoMoS phase is proposed. Information regarding other phases in the catalysts is obtained by MES studies of Co Al 2 O 3 , CoMo 2S 4, Co 9S 8, CoS 2, Co 3S 4, and CoS 1 + x samples. It is observed that for a sulfided CoMo Al 2 O 3 catalyst with a composition typical of those used industrially, part of the cobalt is located in the alumina. For unsupported catalysts the effect of changing the cobalt concentration and preparation method is investigated and it is observed that under certain conditions also the thermodynamically stable cobalt sulfide, Co 9S 8, may be formed.