Fundamental heterogeneous catalysis for environmental and energy-related applications

Jorge H Pazmino, Purdue University

Abstract

Catalyst technology has been an essential element of the industrial development of the 20th century and it had tremendous impact in improving the quality of life. Heterogeneous catalysis has been particularly important for processes involving petroleum refining, chemical production and environmental clean-up. As part of our contribution to the field, this work focuses on understanding specific phenomena linked to NO x abatement processes, hydrogenation of light hydrocarbons and H2 generation via the water gas shift (WGS) reaction. In all cases, kinetic measurements were conducted and complemented with a combination of techniques such as, x-ray absorption (XAS), infrared (IR), x-ray photoelectron spectroscopy (XPS) and electron microscopy. The NO oxidation reaction is the first step in the cyclic operation of Lean NOx Traps (LNT), a process designed to reduce the amount of NO and NO2 emitted from lean-burn combustion engines. LNT catalysts are multi-component, consisting of a noble metal (Pt, Pd, Rh), an alkaline or alkaline earth element, and a high surface area support. It is known that NO2 can be trapped more effectively than NO. Therefore increasing the rate of NO oxidation is highly desirable. Under realistic conditions, however, the trap is exposed to impurities from lubricants and gasoline that can detrimentally affect the catalyst performance. Among these contaminants, sulfur is well known to adsorb strongly on Pt. Part of the work described here focuses on evaluating the effect of pre-adsorbed sulfur on the rate of NO oxidation and the stability of Pt supported on mesoporous silica (SBA-15). It was found that pre-sulfiding the catalysts did not affect the rate or kinetics of reaction. However, the evidence suggests sulfur adsorption is dynamic depending on the environment. This outcome served as a platform to study the nature of catalyst deactivation with time. Irrespective of the presence of sulfur, it is suggested that platinum oxidation is the main source of deactivation. The effect of sulfur was also studied on a different catalytic system operating under reducing conditions. The goal was to compare the thio-resistance and regenerability of two Pt catalysts supported on Al2O3 and TiO2. Kinetic measurements for the hydrogenation of ethylene in addition to chemisorption and x-ray absorption (XAS) methods were used to conclude that both supports were similarly affected by sulfur. Regeneration treatments up to 450 ºC under H2 were found ineffective in restoring the rates of the S-free catalysts. The interaction between sulfur poisoning and the occurrence of strong metal support interactions (SMSI) on Pt/TiO2 is discussed. The final contribution concentrates on developing Pt-based catalysts with WGS turnover rates (TOR) comparable or higher than the commercial Cu formulations and the additional robustness of a noble metal component. First, the addition of alkali (Na, Li, K) was found to promote the TOR (250 ºC) by as much as 107 times that of Pt/Al2O3. X-ray absorption measurements confirmed no correlation between the rate promotion and the fraction of oxidized Pt. The change in kinetics suggest that alkali assist in water dissociation steps. The promotion followed the order Na > Li ∼ K. Based on IR and 13C isotopic exchange experiments it was proposed that alkali facilitate a parallel path likely involving formate intermediates. The rate enhancement by addition of Fe oxides to Pt catalysts was also investigated. The use of non-porous Al2O3 support with layers of Fe prepared by atomic layer deposition was intended to increase the surface sensitivity of XAS measurements. The data showed that the rate increases with the relative amount of reducible iron, quantified by chemisorption, XAS or XPS. The formation of PtFe bimetallic entities under WGS conditions was confirmed.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Inorganic chemistry|Chemical engineering

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