Reduction of chromia/alumina catalyst monitored by DRIFTS-mass spectrometry and TPR-Raman spectroscopy

Abstract

The reduction of alumina-supported chromia with 13 wt.% chromium was investigated by X-ray photoelectron and X-ray absorption spectroscopies, in situ temperature-programmed Raman spectroscopy and in situ temperature-programmed diffuse reflectance infrared Fourier transform spectroscopy combined with mass spectrometry. For comparison, unsupported chromia was also studied. The oxidised chromia/ alumina contained Cr 3+ and Cr 6+ , whereas after reduction by carbon monoxide or hydrogen mainly Cr 3+ was detected, although the formation of Cr 2+ in carbon monoxide reduction was not ruled out. The polymeric chromates on chromia/alumina were more reducible than the monomeric ones due to the weaker interaction of the polychromates with the support. During reduction with hydrogen at high temperatures, hydroxyl species formed on chromia and chromia/alumina. In addition, surface restructuring to more crystalline chromia species took place on chromia/alumina. Measurements on the reduction by carbon monoxide suggested that the reduction could occur through formation of bicarbonate species. With increasing temperature, formate speciesprobably formed by reaction of Cr 3+ -CO species with surface hydroxyls-were observed, which reacted further to carbonates on chromia, and inorganic carboxylates and other carbonaceous species on chromia/alumina. Experimental Sample preparation and characterisationTwo samples were used in the study: bulk chromia (Cr 2 O 3 , Aldrich, 98+) and chromia supported on alumina. The

Surface chemistry and catalytic performance of chromia/alumina catalysts derived from different potassium impregnation sequences

“…After reaction, the amount of isolated tetrahedral Cr VI species decreased dramatically, which may be due to the coke Therefore, it seems that coke formation is more favored on oligomeric chromium species than on polymeric ones. In addition, it can be seen that a structural rearrangement of polymeric chromium species on Cr/K/Al 2 O 3 to oligomeric species Page 17 of 40 A c c e p t e d M a n u s c r i p t may take place during reaction process [19,20,32]. ***FIGURE 10***…”

Mechanistic Investigation of Heterogeneous Catalysis by Transient Infrared Methods

“…Infrared (IR) spectroscopy has been one of the most important means of studying adsorbates on supported catalysts [2,[24][25][26][27]. The development of in situ IR cells has permitted investigation of IR-observable adsorbates during reaction [28][29][30][31][32][33][34][35][36][37].…”

Effect of support modification on the characterization and catalytic activity of Mo/Al2O3 catalysts

“…These results revealed the formation of polymolybdate species other than the isolated Td-Mo. Airaksimen et al [23] reported that the appearance of a band at 900 cm −1 is assigned to the vibration of Mo-O-Cr in molybdenum dichromate and that at 2000 cm −1 , it is assigned to chromate vibration. The band at 850 cm −1 is assigned to Cr-O-Cr vibration.…”