Cyclic Sulphidation/Reduction of Transition Metal Model Compounds: A Quasi in situ XPS Study

Abstract

INTR OD UCTlONAs the allowable level of sulphur in refinery products is reduced, the H2S concentrations to which catalysts are exposed during the final stages of hydrotreating can be expected to decrease. Although the precise H2S/H2 ratio will be dependent on the reactor configuration, this may lead to loss of sulphur from the catalyst surface with consequent effects on catalyst structure and activity. This problem Is likely to be more acute for catalysts based on precious metals with their potential use for deep desulphurization, slnce thermodynamic calculations suggest, for example, that Pt and Re sulphides are reducible in H2 atmospheres containing less than 1 % H2S [l ,2,3] whereas base metals show greater stability of their sulphldes towards reduction by H2.Studies of the sulphidation of hydrotreating catalysts show that their activlty depends on several factors. Coupled or stepwise sulphidatlon/reduction seems to yield the most active catalysts (41, but other factors also affect the final performance. For example, oxygen incorporation in the sulphided catalyst can give sites of higher activii [5], and reduction after sulphidation can give either higher acti-The present study forms part of a programme of research to probe the surface compositions of the suiphided state of various transition metal model compounds after cyclic treatments simulating low H2S concentration hydrotreating conditlons and resulphldation. For this task a quasi in situ XPS system has been built to allow measurement of surface sulphurlmetal ratios of transition metals after various cyclic treatments of the same sample without air exposure. This work concentrates on the precious metals Pt and Re, as well as Mo and Fe. These materials were investigated as thin films on Si wafers, as SiO2 supported catalysts in the case of Pt and Re, and as bulk materials in some cases. EXPERIMENTALFor this study a gas-cell-type reactor was constructed in the preparation vessel of a Vacuum Generators ESCA 111 XPS Instrument. Cyclic treatments and surface analysis were performed on the same sample without air contactlng the sample between treatment and analysis. A sketch of the reactor set-up is given in Figure 1. The sample is held on the end of a movable probe and the reactor hangs from a bellows system. For sample treatment, the reactor is lowered down onto a support, the sample is moved into the reactor and sealed with a metal gasket. Further details of the reactor construction and performance can be found in another publication [7]. XPS spectra were taken using Mg or A1 radiation and binding energles were referenced to the Cls binding energy of 264.6 eV or cross-referenced to Si2p312 at 103.2 eV in some cases. For the Mo/Si wafer and the MoS2 single crystal, the S2p was set at 162.0 eV, corresponding to the average of a range of values taken from literature [8,9]. This is discussed further below. Constant spectrometer pass energies of 50 or 20 eV were applied.