Design on improved noble catalysts for the water -gas shift reaction

Luis Bollmann, Purdue University

Abstract

The water-gas shift (WGS) reaction (CO + H2O → CO 2 + H2) plays an important role in the areas of hydrogen generation and CO purification. My goal has been to synthesize a catalyst with a turnover rate (TOR) as high as the commercial Cu catalyst and the robustness of noble metals. Herein, the enhanced rate per mole of noble metal of Pd-Zn and Pd-Fe catalysts for the forward WGS is reported. The effect of Zn addition on the Pd TOR for the WGS reaction was studied on 2 wt% Pd on alumina with Zn content from 0 to 19% Zn by weight. The average Pd-Zn coordination number, as measured by EXAFS, increased monotonically from 0 to 4.5 as the Zn loading on alumina increased. At a loading of 2% Pd and 19% Zn on alumina the TOR increased by a factor of 12, as compared to the pure metallic Pd supported on alumina, to 3.6x10-2 mol H2 sec-1 (mol exposed Pd)-1 at 280°C, 6.8% CO, 8.5% CO2, 21.9% H2O, 37.4% H2 at 1 atm. The binding of adsorbed CO changed from 80% bridging on pure Pd to 90% linear on the 1:1 PdZn intermetallic compound as determined by diffuse reflectance infrared spectroscopy (DRIFTS) performed under WGS conditions. The fraction of CO binding linearly to Pd correlated linearly with the WGS rate per Pd on the surface. The correlation of the reaction rate with the types of sites on catalysts with six different Pd-Zn compositions shows that the data is consistent with an active site composed of surface Pd atoms with surface coordination of 2 Pd and 4 Zn atoms, which is the coordination expected for the (111) plane of the 1:1 PdZn intermetallic compound. The structure of the catalyst is destroyed upon oxidation at 280°C, however subsequent exposure to the WGS conditions is enough to regenerate the reduced alloy. On SiO2, TiO2, ZrO2 or CeO2 supports, PdZn catalysts had a higher TOR than on the pure Pd counterparts. However, the relative enhancement in TOR compared to pure Pd on the same supports was less pronounced than on Al 2O3. For the 2 wt% Pd catalysts with Fe wt% loading between 2 and 14%, the rate per gram of catalyst correlates with the amount of reducible Fe near Pd as estimated from O2-H2 titration at 100°C and CO chemisorption at 35°C. From X-ray photoelectron spectroscopy data, it is calculated that less than 15% of Fe is in the Fe0 state after reduction in H2 at 300°C. EXAFS data shows that on average only a third of the Pd atom neighbors are Fe. Since the particles are Pd-rich, the in-situ IR shows mainly CO adsorbed in the bridging conformation as in pure Pd. For all the Pd/Fe samples, the measured apparent reaction orders and activation energies are within 10 and 5% of each other, respectively. When the amount of Pd was decreased keeping the Fe loading constant, the rates per gram of catalyst decreased proportionally. These observations suggest that the activity of the catalysts is driven by reducible Fe species that are in close proximity to Pd centers.

Degree

Ph.D.

Advisors

Hillhouse, Purdue University.

Subject Area

Chemical engineering

COinS