XPS and Moessbauer investigation of potassium promoted iron catalysts for nitrile synthesis
Abstract
Potassium promoted Fe/C catalysts prereduced at 673 K have an improved nitrile selectivity with respect to hydrocabons in the reaction from CO, H$\sb2$ and NH$\sb3.$ A 3%K/12%Fe/C has an initial $\rm CH\sb3CN/CH\sb4$ ratio of $\sim$22, compared to a value of $\sim$3 for an 11.8%Fe/C catalyst. In spite of potassium mobility, shown by XPS, some promotion effect is retained by the catalysts after 24 hours on stream. The 3%K/12%Fe/C catalyst had a $\rm CH\sb3CN/CH\sb4$ ratio of 6, compared to $\sim$2 for the unpromoted catalyst. The nitrile and hydrocarbon distributions have different chain propagation probability, suggesting the nitrile/hydrocarbon ratio is not determined by simple competition of CN and H for a common hydrocarbon intermediate. Mossbauer experiments show initial activity loss of Fe/C prereduced at 673 K is caused by sintering. When reaction times exceed an hour, reversible and irreversible carbon overlayers also contribute to deactivation. At 473 K prereduction of Fe/C results in a slow growth to a stable activity. Mossbauer experiments indicate that the rate-limiting step in catalyst development after a 473 K prereduction is the conversion of Fe$\sp{2+}$ into iron carbides. XPS shows the dissociative adsorption of NO on reduced 11.8%Fe/C. A 4%Fe/SiO$\sb2$ catalyst adsorbs NO molecularly. The effect of nitriding on iron dispersion of the carbon-supported and silica-supported catalysts was characterized with XPS. A nitrided Fe/C sinters. The Fe/SiO$\sb2$ catalysts also sinters, with a dramatic change in the reducibility of the catalyst.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering|Analytical chemistry|Chemistry
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