Studies On Co Selective Oxidation In H2 Over Silver Catalysts

Dr. Zhenping Qu(1999-2003)
Directed by Prof. Xinhe Bao
Abstract
University of Stuttgart
Germany
    Polymer electrolyte membrane fuel cells (PEMFC), which have better energy efficiency than the conventional combustion engines and a zero-emission of air pollutants, have received much attention as a potential power source of electric vehicles. Restricted by the distribution and storage of hydrogen, the H2 feed gas of PEMFC is usually produced by steam reforming, partial oxidation or combination of the above techniques from methanol, natural gas. Since the PEMFC is operated at relatively low temperatures (80°C), its Pt-anode catalyst is extremely sensitive to CO contaminant (1%) in reformed gases, which will poison the catalyst and decrease the performance.

    Therefore, it is essential to remove the trace CO in H2 feedgas. Among the currently available methods for removing CO from H2-rich feedgas, the selective catalytic oxidation of CO with molecular oxygen is undoubtedly the most straightforward, simplest, and the most economic one. Catalysts so far proposed for this process are mainly noble metals, such as Pt, Rh, Au. However, decreasing the reaction temperatures and seeking for more economic catalysts for CO selective oxidation are the research focuses of the near future. It is well known that silver catalysts are unique for partial oxidation reactions and is not so precious as other noble metals used currently.
    Based on this idea, silver catalyst is a good candidate for CO selective oxidation in H2 feedgas. The effects of supports, silver loading, and reaction temperatures et al. on the activities of silver catalysts, as well as the correlation between the surface structure of silver and reaction activity, are investigated. The surface restructuring, redispersion model and the reaction mechanism for CO oxidation over silver surface are proposed. The following research works have been conducted:

  1. SiO2 is a good support of silver catalysts for CO selective oxidation. An appropriate content and distribution of silanol favors the silver particles dispersion. For zeolite support, the catalytic activity increases with the increasing of Si/Al ratio of the zeolites. The strong interaction between Ag and Al decreases the catalysts activity. Mesoporous silica materials as SBA-15, MCM-41 zeolites are also found to be better supports of silver catalysts for CO selective oxidation at low temperatures.
  2. Ag/SiO2 catalysts have a good performance for CO selective oxidation at low temperatures, and are quite stable. With the increasing of silver loading, the reaction rate increases and reaction temperature decreases. TPD and CO-TPSR results show that the amount of sub-surface oxygen increases with the silver loading. Thus more active [O] species will be supplied, and react with CO at much lower temperatures (-10°C) over silver catalyst with higher loading.
  3. The surface oxygen species formed by oxygen treatment at low temperatures (100-300°C) following H2 treatment at 500°C block the adsorption of gas oxygen species, and further deactivate the silver catalysts, indicating that it is very essential to obtain a clean silver surface for CO selective oxidation. However, pretreatment with O2 at higher temperatures (>350°C) distinctly improves the catalytic performance at low temperatures. The stability of silver particles decreases at higher temperatures than 350°C, and more sub-surface oxygen species are formed. Reduction with H2 at low temperatures following the oxygen treatment at high temperatures increases the catalytic activity due to the further dispersion of silver particles; however, hydrogen reduction at higher temperatures decreases the activity and selectivity. Oxygen treatment at high temperatures will reactivate the silver catalysts. Interestingly, the changes in activity are mostly reversible.
  4. It is firstly observed that the reaction of CO and oxygen over silver catalysts treated by H2 at low temperatures (100-300°C) following oxygen treatment at 500°C may occur at -75°C by in-situ FTIR technique. Adsorbed CO gives a lower frequency after H2 reduction due to an increase in electron density on surface Ag atoms. The amount of adsorbed CO on the silver particles strongly depends on the CO partial pressure, and the CO coverage is significant ly below saturation under reaction temperatures. The non-competitive adsorption of CO and oxygen induces the better selectivity for CO oxidation over silver catalysts at low temperatures. The enhanced desorption of CO at elevated temperatures decreases the selectivity.
  5. The surface restructuring, redispersion model is proposed to account for the effect of pretreatment cycles of oxidation-reduction on the silver catalysts. Undisturbed (111) planes are formed after hydrogen treatment at high temperatures; oxygen treatment at high temperatures induces the refacetting and surface restructuring, forming more sub-surface oxygen species. CO is easily adsorbed on the restructured silver surface. A subsequent reduction at lower temperatures (RT-300°C) splits silver into smaller nano-particles, however continued reduction at higher temperatures agglomerates theses small particles. Reduction will also remove parts of sub-surface oxygen species, and the effect of pretreatment enhances with the reduction temperature. Oxygen treatment at high temperatures may restore the restructuring surface. Appropriate oxygen-hydrogen treatment is essential for the redispersion of silver particles.
  6. The mechanism for CO oxidation over silver surface is investigated, and a possible reaction model is proposed. CO promptly reacts with surface oxygen species firstly, and will accelerate the adsorption equilibrium of oxygen species over silver surface. CO also reacts with sub-surface oxygen species resulted from the oxygen treatment at high temperatures.
        The key postulate is that strongly adsorbed sub-surface oxygen species acts to facilitate the migration of surface oxygen into subsurface oxygen sites, with no restructuring of the silver surface being required. The oxidation-reduction cycle over silver surface is formed. Reduction at low temperatures following the oxygen treatment results in the formation of small, rough and round silver particles, enhancing the adsorption of oxygen and CO. While reduction at elevated temperatures aggregates the silver particle, moreover it removes so much sub-surface oxygen that migration into sub-surface during subsequent oxygen adsorption is inhibited, decreasing the activity of silver catalysts.
The activity of the CO hydrogenation on the Ag/SiO2 catalyst