Role of Citric Acid in Preparing Highly Active CoMo/Al₂O₃ Catalyst: From Aqueous Impregnation Solution to Active Site Formation

Abstract

In this work, the role of citric acid (CA) in preparing CoMo/Al 2 O 3 catalyst was investigated from aqueous impregnation solution to catalytic active site formation by laser Raman, UV−vis, in situ IR and mass spectrometry characterizations. COS and CO were employed as probe molecules to illustrate the formation of active site during and after catalyst sulfidation, respectively. Citric acid mainly coordinates with Mo, resulting in the presence of Mo−CA complex, which consequently enlarge the temperature window of MoS 2 formation. Parallels between in situ IR and activity testing suggest that citric acid addition increases the concentration of CoMoS active site without significantly changing its intrinsic activity. It is concluded that the essential role of CA in preparing CoMo/Al 2 O 3 catalyst is mainly ascribed to a tuning effect on the 2D morphology of MoS 2 slabs, resulting in MoS 2 exposes more S-edge to accommodate a cobalt atom to form more CoMoS sites.

Formation and stability of CoMoS nanoclusters by the addition of citric acid: A study by high resolution STEM-HAADF microscopy

“…The CoMoS edge sites of these catalysts were in-situ characterized by lowtemperature CO adsorption followed by IR spectroscopy [12,19,21,22]. First, the dried catalytic precursor in oxidic form was grounded and pressed into a self-supported wafer (10 mg for a disc of 2.01 cm 2 ) precisely weighted and positioned into a quartz IR cell with CaF2 windows.…”

“…Low-temperature (liquid nitrogen temperature) CO adsorption followed by in situ IR spectroscopy (IR/CO) allows distinguishing non-promoted (MoS2) sites and promoted (CoMoS) sites [19,23,24] on the supported CoMo catalyst. Figure 1 reports the IR/CO spectra obtained on the CoMo(CA/M = x)/Al2O3 catalysts.…”

“…The strong band at around 2072 cm −1 is attributed to the CO adsorption on Copromoted MoS2 edges (the so-called CoMoS sites), while the small shoulder at 2115 cm −1 is attributed to CO adsorption on unpromoted MoS2 sites [13,19]. In addition in CoMo(CA/M = 2)/Al2O3 sample, a second band in the promoted massive is observed at 2060 cm -1 , assigned by Dominguez et al [25] to partially promoted S-edge.…”

“…The addition of chelating agents have a special and efficient role in increasing the dispersion degree of Co(Ni)-Mo-S active phases [16][17][18] by interacting with the precursor ions and the support surface. However, the effect of the addition of citric acid in the preparation of promoted catalysts is always somewhat controversial since both increase [19] and decrease [20] in dispersion have been reported. Moreover, while in the non-promoted catalysts the variation of morphology with the addition of citric acid was indirectly observed by IR/CO method, that is, the S-edge/M-edge ratio was closer to one after CA addition, for the promoted systems this is still inconclusive.…”

Effect of sulfidation pressure on the structure and activity of Ni(CyDTA)W/γ-Al2O3 hydrodesulfurization catalysts

“…In this regard, Scheffer et al [26] suggested that the higher activity of CoMoS phase is attributed to the weaker active phase-support interaction. In the same way Chen et al showed that addition of chelating agent as citric acid limits the metal support interaction and leads to the formation of more active M and CoMo catalysts [17,27]. Consequently, the catalyst NiW with chelating agent could forms NiWS species more active than NiW.…”