CHARACTERIZATION OF CHROMIA ALUMINA CATALYSTS BY X-RAY PHOTOELECTRON-SPECTROSCOPY, PROTON-INDUCED X-RAY-EMISSION AND THERMOGRAVIMETRIC ANALYSIS

Chromia/alumina catalysts with different metal loading were characteri zed using X-ray photoelectron spectroscopy (XPS), proton induced X-ray emission (PIXE) and thermogravimetric (TG) techniques to elucidate th e surface structure of these catalysts. XPS studies on calcined sample s show a sharp increase of the Cr/Al ratio at calcination temperatures up to 500 degrees C while the ratio remains relatively unchanged at h igher calcination temperature. The surface state of chromium shows pre dominantly Cr6+. At calcination temperatures higher than 500 degrees C , calcination-induced reduction is observed of the Cr6+ to Cr3+, where the fraction of chromia in Cr3+ oxidation state increases with increa sing temperature. A progressive increase of the intensity of the peak due to Cr 2p of the Cr3+ oxidation state is also observed with increas ing amount of metal loading. The calcination-induced reduction of the alumina-supported chromia was found to be less than the corresponding reduction of bulk CrO3. Also, the size of the spin-orbit splitting of the Cr 2p level of chromia catalysts which had undergone calcination-i nduced reduction was found to be smaller than would be expected for bu lk Cr3+. The XPS spectra of chromium on the Cr/Al catalysts were found to be time dependent. Photoreduction of Cr6+ on Cr/Al samples was fou nd for irradiation times longer than 4.0 min. It was found from PIXE a nalysis that at higher calcination temperature, the Cr/Al atomic ratio approaches the values obtained by XPS. For all samples, the chromium particles were found to be homogeneously distributed on the alumina su pport for calcination temperatures up to 800 degrees C. Thermogravimet ric results on uncalcined bulk CrO3 agree well with the XPS observatio n as to the fact that the main phase transformation of Cr6+ compounds occurs at about 500 degrees C, resulting in reduction to Cr3+.