CO/FTIR Spectroscopic Characterization of Pd/ZnO/Al2O3 Catalysts for Methanol Steam Reforming

Abstract

An as-synthesized 8.8wt% Pd/ZnO/Al 2 O 3 catalyst was either pretreated under O 2 at 773 K followed by H 2 at 293 K or under H 2 at 773 K to obtain, respectively, a supported metallic Pd°catalyst (Pd°/ZnO/Al 2 O 3 ) or a supported PdZn alloy catalyst (PdZn/ZnO/Al 2 O 3 ). Both catalysts were studied by CO adsorption using FTIR spectroscopy. For the supported PdZn alloy catalyst (PdZn/ ZnO/Al 2 O 3 ), exposure to a mixture of methanol and steam, simulating methanol steam reforming reaction conditions, does not change the catalyst surface composition. This implies that the active sites are PdZn alloy like structures. The exposure of the catalyst to an oxidizing environment (O 2 at 623 K) results in the break up of PdZn alloy, forming a readily reducible PdO with its metallic form being known as much less active and selective for methanol steam reforming. However, for the metallic Pd°/ZnO/ Al 2 O 3 catalyst, FTIR results indicate that metallic Pd°can transform to PdZn alloy under methanol steam reforming conditions. These results suggest that PdZn alloy, even after an accidental exposure to oxygen, can self repair to form the active PdZn alloy phase under methanol steam reforming conditions. Catalytic behavior of the PdZn/ZnO/ Al 2 O 3 catalyst also correlates well with the surface composition characterizations by FTIR/CO spectroscopy.

Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

“…After 4 hours of exposure to steam reforming conditions, both these catalysts were analyzed via FTIR of adsorbed CO. Figure 8 shows that the surfaces of the two catalysts give nearly identical IR spectra after reaction. Note that the major peak for both catalysts comes from atop CO at ~2070 cm -1 (and a very small peak corresponding to the bridgebonded CO) confirming that the catalyst surface contains predominantly Pd-Zn in both cases [23,24]. The fact that the as-prepared catalyst shows a similar surface composition to the one reduced at 400 ºC is intriguing since the asprepared catalyst contains no Pd-Zn alloy before reaction.…”

“…There is also a measurable shift in the linear-bound CO frequency from 2065 cm -1 for Pd to 2076 cm -1 for Pd-Zn due to ligand effects. The assignment of the CO band on Pd-Zn is based on recent literature assignments derived from single crystal surfaces as well as high surface area catalysts [23,24]. The ligand effects were reported by Tsai et al [15] due to the differences in the density of states between Pd and Pd-Zn as calculated from density functional theory.…”

Single-Step Syngas-to-Distillates (S2D) Synthesis via Methanol and Dimethyl Ether Intermediates: Final Report

“…Note that the band at 1800 to 2000 cm -1 is almost undetectable for the 2.5Pd/ZnO/Al 2 O 3 -0.25 catalyst, suggesting that the amount of Pd° is low compared to the amount of bimetallic PdZn particles. Note that the presence of a band due to linearly CO species adsorbed on Pd° for the catalysts with Pd >2.5% can be ruled out since the Pd°/ZnO/Al 2 O 3 catalyst band is due to linearly adsorbed CO between 2100-2000 cm -1 is accompanied by a more intense band between 2000-1800 cm -1 [14]. one more band between 1800 and 2000 cm -1 , which is attributed to Pd°.…”

“…For all the catalysts, the IR spectra present one main band between 2069 and 2077 cm -1 attributed to the vibration of CO linearly adsorbed on the PdZn alloy particles [13,14]. The shift observed between the spectra for the different catalysts for the band at 2069 to 2077 cm -1 is not understood yet.…”

“…The shift observed between the spectra for the different catalysts for the band at 2069 to 2077 cm -1 is not understood yet. In addition, a closer look at the spectrum obtained for the 2.5Pd/ZnO/Al 2 O 3 -0.25 catalyst shows one broad band between 1800 and 2000 cm -1 due to multi-bonded CO species [13] and characteristic of Pd° particles [14]. Note that the band at 1800 to 2000 cm -1 is almost undetectable for the 2.5Pd/ZnO/Al 2 O 3 -0.25 catalyst, suggesting that the amount of Pd° is low compared to the amount of bimetallic PdZn particles.…”