Insights into the WO_xCoverage-Dependent Location and Oxidation State of Noble Metals Supported on Tungstated Oxides: The Case of Rh/WO_x–Ce_0.62Zr_0.38O₂

Abstract

Noble metals-promoted tungstated oxides have been shown to be profitable in a wide variety of catalytic reactions of environmental interest but to be detrimental in the hydrogenation of aromatics. The origin of the deleterious effect of tungstates on the hydrogenation performance of noble metals is still being debated. To provide further insights into this, the location and the oxidation state of Rh were investigated as a function of the W surface density (0–10 W/nm2) of Rh/WO x –Ce0.62Zr0.38O2 (Rh/W–CZ) catalysts after high-temperature reduction. For that purpose, a thorough characterization of the oxide phases was performed through N2-sorption, X-ray diffraction, Raman spectroscopy, NO x temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS), whereas the metallic phases were characterized by low-temperature H2 chemisorption, XPS, N2 Fourier transform infrared spectroscopy and benzene hydrogenation. It was found that Rh deposited on both tungstates and CZ, and did not sinter with increasing W surface densities. The observed linear decrease in the Rh hydrogenation performance of the WO x -promoted Ce0.62Zr0.38O2 below pseudo monolayer coverage of CZ (4.8 W/nm2 CZ) was assigned to a strong metal support interaction effect between the Rh particles and the nonreducible underlying WO x phase, resulting in the formation of electron-deficient Rh species (Rh δ+ ).

Successive Strong Electrostatic Adsorptions of [RhCl6]3–on Tungstated-Ceria as an Original Approach to Preserve Rh Clusters From Sintering Under High-Temperature Reduction

“…The intensity of these peaks decreased as the W surface density increased and the HT desorption peak was found to vanish for W surface densities higher than or equal to 4.9 W/nm 2 CeO2. As was found on ZrO2 43,48 and Ce0.62Zr0.38O2, 35 the NOx uptake is found to decrease linearly as the W surface density increases up to about 4.9 W/nm 2 CeO2 before stabilizing at higher W surface densities (Figure S3). From this observation it can be concluded that pseudo monolayer coverage of CeO2 by the tungstates is achieved at a W surface density of 4.9 W/nm 2 CeO2 in agreement with the values found earlier by us on Ce0.62Zr0.38O2 35…”

“…As illustrated earlier on Ce0.62Zr0.38O2, 35 the NOx-TPD profile of CeO2 also displayed a low-temperature (LT) and a high-temperature (HT) NOx desorption peaks at about 90 and 350 °C (not shown). The intensity of these peaks decreased as the W surface density increased and the HT desorption peak was found to vanish for W surface densities higher than or equal to 4.9 W/nm 2 CeO2.…”

“…As has already been reported in refs. 35,43,48 , it should be noted that NOx chemisorption does not occur on WOx species. N2-FTIR spectra of adsorbed N2 on supported Rh samples were collected in transmission mode on a Bruker Vertex 70 FTIR spectrometer equipped with a liquid N2-cooled MCT detector and a data acquisition station.…”

Assessing carbon or tungstates coverage of ZrO2 nanoparticles supported on MWCNT via NOx-TPD

“…After each NO x -TPD experiment, the NO x analyzer was calibrated with a NO/He standard mixture containing 933 ppm of NO. As reported earlier [27,52,53], it should be noted that NO x chemisorption does not occur on WO x species. The reactor outflow was also analyzed periodically using a µ-GC (Agilent Technologies, CP4900) equipped with a 5A…”

Insights into the accessibility of Zr in Zr/SBA-15 mesoporous silica supports with increasing Zr loadings

“…Considering all the above, we found it of particular interest to study the influence of Zr loading on its accessibility when supported on SBA-15 using the NO x -TPD technique, which has been shown to provide useful characterization of various supported oxides [21][22][23][24][25][26], combined with gas physisorption, which is an important tool for the advanced analysis of complex pore structures. For that purpose, a series of Zr-SBA-15 samples was synthesized via NH 3 /water vapor-induced internal hydrolysis (VIH) [27] and was thoroughly characterized through N 2 -sorption, TEM-EDS, SEM, XRD, Raman spectroscopy and NO x -TPD.…”