Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂:  Activation by Rhodium and Effect of Chlorine

Abstract

Exchange of D2 with the OH groups of several oxides was carried out on the samples on which we investigated the 18O2/16OH exchange (part I, J. Phys. Chem. 1996, 100, 9429). Temperature-programmed isotopic exchange was first carried out on the bare oxides. In every case, the reaction follows a simple heteroexchange mechanism, with HD as a primary product. The maximal rates of exchange are obtained at the following temperature: CeO2 (prereduced), 100 °C > MgO, 120 °C > ZrO2, 145 °C > CeO2 (preoxidized), 160 °C > γ-Al2O3, 190 °C > Cl−Al2O3, 200 °C ≫ SiO2, 540 °C. The first two samples exchange their hydrogen at a significant rate at room temperature. Secondary peaks and shoulders show the multiplicity of the OH groups on most oxides. Compared to oxygen, hydrogen exchange occurs at much lower temperatures, the shift of the temperature ranges of O and H exchange being between 250 and 430 °C. Isothermal isotopic exchange was carried out between 25 and 100 °C over rhodium catalysts supported on these oxides. The presence of rhodium accelerates the hydrogen exchange, at least by 2 orders of magnitude. As with oxygen exchange, three main steps must be considered: (i) adsorption−desorption on the metal, (ii) transfer of D atoms onto the support (spillover), and (iii) hydrogen diffusion. Contrary to what was observed with O2 exchange, step i is never a rate-determining step (rds) for H2 exchange, the rate of H2 + D2 equilibration being much higher than the rate of exchange. The results are in accordance with a model where at T < 75 °C the rds of exchange would be the hydrogen transfer from the metal to the support (spillover step), while at T ≥ 75 °C, the rds would be the surface diffusion. This model is valid for all the supports except silica, for which most of the hydroxyl groups exchange at high temperatures. The rate of exchange depends linearly on the density of the hydroxyl groups and tends toward zero for a fully dehydroxylated support. Coefficients of surface diffusion give the following order for the hydrogen mobility at 75 °C (base 100 for γ-Al2O3): CeO2, 770 > MgO, 230 > γ-Al2O3, 100 > ZrO2, 23 ≫ SiO2, nd. There is no correlation between the hydrogen mobility and the surface acidity of the oxides, the highest hydrogen mobility being found on basic oxides with a very high oxygen mobility. A model of isotropic heterogeneous diffusion is proposed to explain certain discrepancies observed between the present isotopic exchange method and direct IR spectroscopic methods previously published.

EFFECT OF FLUORINE IN THE SYNERGISM Co-Mo VIA HYDROGEN SPILLOVER ON THE HYDRODESUL-PHURIZATION OF REFRACTORY MOLECULES

“…This behavior suggests that the amount of available OH groups through their replacement, have a significant influence on the neighboring O-H bonds since they lose proximity. This scheme is in agreement with previous studies [34][35][36][37] that indicates that the way in which fluorine ions interacts with the alumina, is replacing the hydroxyl groups present on their surface, preferably OH groups with lower net charge, i.e. most basic OH groups 38 .…”

“…Without the incorporation of fluorine the alumina surface has an amount of OH groups available for Hso migration, as well as some type Al-O-Al bonds (A). By incorporating fluorine scheme proposes an increase in the number of OH sites available for Hso migration, caused either by the breaking of bonds Al-O-Al, to incorporate fluoride ions 34 . Taking into account that fluorine replaced OH groups, causing weakening of neighbor O-H bond, and favoring transport of Hso through the surface (B).…”

Activation of dihydrogen on supported and unsupported silver catalysts

“…If one envisions rapid activation of D 2 on a metal particle and migration of an activated deuterium species across the surface to un-exchanged OH groups, diffusion indeed may become rate-limiting. This interpretation is in line with a switch to a diffusion limited D-H exchange reaction of various oxides in supported rhodium catalysts at temperatures above 353 K [68]. The reaction front should progress radially from the silver center and the rate law of two-dimensional diffusion, with the rate being proportional to -1/(ln (1 -Θ)), should be applicable [69].…”

Structuring Pt/CeO2/Al2O3 WGS catalyst: Introduction of buffer layer

“…These authors estimate the coverage of H-containing species for γ-Al2O3-supported Pt and Rh and state that labile hydroxyl groups and H species coming from dissociated water are involved in the reaction. However, just a small fraction of these species under WGS reaction conditions are energetically able to diffuse towards the metal particle for producing H2 gas [50,51].…”

“…If an out-gas reformate feed stream is submitted to the WGS catalysts the high partial pressure of H2 will results in the formation of partially reduced ceria surfaces favoring then water dissociation and stabilization on the support and on the buffer layer [50,53,58]. Moreover, the increased number of oxygen vacancies leads to an increase in the electronic densities of the metal particles [25,[59][60][61][62].…”