The deposition of Mo on γ-alumina by the equilibrium adsorption method starting from ammonium heptamolybdate has been studied. Spectroscopic results converge to indicate that a previously unrecognized species, i.e., the Anderson-type heteropolymolybdate [Al(OH)6Mo6O18]3-, plays a major role in this type of synthesis as it is quantitatively formed in the solution within a few hours, by reaction of the heptamolybdate with dissolved aluminic species. This results in a considerable increase of alumina solubility in conditions generally thought to be nonaggressive. Furthermore, this species is also present in the solid catalyst after deposition, although it is harder to observe than in the liquid phase. A parallel is drawn with a well-known idea from the field of geochemistry, i.e., ligand-promoted oxide dissolution. The relevance of this phenomenon in catalyst preparation is evaluated in realistic conditions corresponding to published studies and/or industrial procedures. It is concluded that strong metal−support interaction in the deposition stage by surface dissolution followed by reaction in the liquid phase is most likely to be an important phenomenon, not only for cationic metal precursors as previously known but also for anionic precursors such as molybdates.

The role of the oxide support on the structure of the MoS2 active phase (size, morphology, orientation, sulfidation ratio, etc.) remains an open question in hydrotreating catalysis and biomass processing with important industrial implications for the design of improved catalytic formulations. The present work builds on an aqueous-phase surface-science approach using four well-defined α-alumina single crystal surfaces (C (0001), A (112̅0), M (101̅0), and R (11̅02) planes) as surrogates for γ-alumina (the industrial support) in order to discriminate the specific role of individual support facets. The reactivity of the various surface orientations toward molybdenum adsorption is controlled by the speciation of surface hydroxyls that determines the surface charge at the oxide/water interface. The C (0001) plane is inert, and the R (11̅02) plane has a limited Mo adsorption capacity while the A (112̅0) and M (101̅0) surfaces are highly reactive. Sulfidation of model catalysts reveals the highest sulfidation degree for the A (112̅0) and M (101̅0) planes suggesting weak metal/support interactions. Conversely, a low sulfidation rate and shorter MoS2 slabs are found for the R (11̅02) plane implying stronger Mo-O-Al bonds. These limiting cases are reminiscent of type I/type II MoS2 nanostructures. Structural analogies between α- and γ- alumina surfaces allow us to bridge the material gap with real Al2O3-supported catalysts. Hence, it can be proposed that Mo distribution and sulfidation rate are heterogeneous and surface-dependent on industrial γ-Al2O3-supported high-surface-area catalysts. These results demonstrate that a proper control of the γ-alumina morphology is a strategic lever for a molecular-scale design of hydrotreating catalysts.

The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the supportdependent genesis of MoS 2 on four planar, single crystal -Al 2 O 3 surfaces with different crystal orientations (C (0001), R , M and A). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, the Mo is introduced by impregnation from an aqueous solution, in order to mimic the standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. AFM, XAS, TEM and XPS show that the -Al 2 O 3 orientation has a clear impact on the strength of metal-support interactions at the oxide state with consequences on the sulfidation, size, stacking and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0001) plane suggesting weak metal-support interactions leading to high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R plane implies stronger metal-support interactions leading to a low sulfidation degree and shorter MoS 2 slabs. Both A and M facets, of similar crystallographic structure, display an intermediate behaviors in terms of sulfidation rate and MoS 2 size in line with intermediary metal-support interactions. Polarization-dependent Grazing-Incidence-EXAFS experiments as well as HR HAADF-STEM analysis allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. A predominant basal bonding is suggested on the C (0001) plane in agreement with the existence of weak metal-support interactions. Conversely, a random orientation (edge and basal-bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed based on the comparison of the surface structure of the various model -Al 2 O 3 orientations used in this work and the predominantly exposed -Al 2 O 3 surfaces ((110), (100) and (111)).

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A new zirconium-containing polyoxotungstate [{W 5 O 18 Zr(µ-OH)} 2 ] 6-2 was prepared by alkalinization of a solution of the monomeric form [W 5 O 18 Zr(H 2 O)] 2-1. The crystal structure of ( n Bu 4 N) 6 [{W 5 O 18 Zr(µ-OH)} 2 ]‚ 2H 2 O (monoclinic; space group C2/c; a ) 26.086, b ) 17.018, c ) 32.891 Å; β ) 112.23°) reveals that two Lindqvist-type units {ZrW 5 O 18 } 2-(shortly ZrW 5 ) are linked through two hydroxo bridges (Zr-OH-Zr) with Zr in sevenfold coordination. Both compounds 1 and 2 were characterized by IR and Raman spectroscopy. EXAFS analysis of 1 at the W L III -edge agrees with the hypothesis of a monosubstituted Lindqvist structure. The 183 W NMR spectrum of 2 in acetonitrile is consistent with the solid-state structure assuming a dynamic process implying likely the µ-hydroxo bridge. A brief discussion is given on the relevance of molecular zirconium-containing polyoxotungstates to heterogeneous catalysis since these compounds can be envisioned as molecular analogues of zirconia-supported tungsten catalysts.

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Despite the great commercial relevance of zinc-promoted copper catalysts for methanol synthesis, the nature of the Cu–ZnO x synergy and the nature of the active Zn-based promoter species under industrially relevant conditions are still a topic of vivid debate. Detailed characterization of the chemical speciation of any promoter under high-pressure working conditions is challenging but specifically hampered by the large fraction of Zn spectator species bound to the oxidic catalyst support. We present the use of weakly interacting graphitic carbon supports as a tool to study the active speciation of the Zn promoter phase that is in close contact with the Cu nanoparticles using time-resolved X-ray absorption spectroscopy under working conditions. Without an oxidic support, much fewer Zn species need to be added for maximum catalyst activity. A 5–15 min exposure to 1 bar H 2 at 543 K only slightly reduces the Zn(II), but exposure for several hours to 20 bar H 2 /CO and/or H 2 /CO/CO 2 leads to an average Zn oxidation number of +(0.5–0.6), only slightly increasing to +0.8 in a 20 bar H 2 /CO 2 feed. This means that most of the added Zn is in a zerovalent oxidation state during methanol synthesis conditions. The Zn average coordination number is 8, showing that this phase is not at the surface but surrounded by other metal atoms (whether Zn or Cu), and indicating that the Zn diffuses into the Cu nanoparticles under reaction conditions. The time scale of this process corresponds to that of the generally observed activation period for these catalysts. These results reveal the speciation of the relevant Zn promoter species under methanol synthesis conditions and, more generally, present the use of weakly interacting graphitic supports as an important strategy to avoid excessive spectator species, thereby allowing us to study the nature of relevant promoter species.

NO x adsorption-desorption experiments were carried out on a series of W-free and W-containing ZrO 2 samples. To the best of our knowledge, this study establishes for the first time NO x -TPD (temperature-programmed desorption of NO x ) as a reliable method to estimate the accessible ZrO 2 surface of complex oxides. The NO x uptake by ZrO 2 was found to be 5.8 ( 0.2 µmol/m 2 . On the tungstated zirconia series of samples, the NO x uptake decreases linearly with W density up to about 5 W/nm 2 , which is in excellent agreement with literature data for which the pseudo monolayer coverage was obtained by spectroscopic methods. The accessible ZrO 2 surface area of ZrO 2 /SBA-15 materials, which cannot be ascertained through common spectroscopic methods, was estimated by WO 3 thermal spreading and the NO x -TPD method. Both values compare very well but NO x -TPD, unlike WO 3 thermal spreading, does not alter the integrity of the investigated sample as the sample is pretreated at lower temperatures (500 instead of 700°C).