Characterization and CH₄ Oxidation Activity of Cr/Al₂O₃ Catalysts

Abstract

X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cr/Al2O3 catalysts (designated “Cry”). The information obtained from surface and bulk characterization has been correlated with the CH4 oxidation activity of the Cry catalysts. For catalysts with Cr/Al atomic ratio = 0.013, XPS results indicated that most of the Cr was present as a highly dispersed Cr6+ surface phase. Catalysts with intermediate Cr loadings (0.027 ≤ Cr/Al atomic ratio ≤ 0.080) showed no XRD peak characteristic of chromium oxide; however, XPS data indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. For catalysts with high Cr loadings (Cr/Al atomic ratio ≥ 0.107), large Cr2O3 crystallites were detected by XRD. The specific activity for CH4 oxidation increased with increasing Cr content up to Cr/Al = 0.107. This has been attributed to an increase in the amount of Cr(III)−Cr(VI) cluster present in the catalysts. A decrease in CH4 oxidation activity observed for Cr-rich catalysts (Cr/Al = 0.13) has been ascribed to the formation of poorly dispersed Cr2O3.

Ammonium hydroxide addition and its influence on the catalytic activities of Pt-based catalysts for methane oxidation

“…On this broad peak, a low peak is located at binding energies 576.7 eV, this binding energies indicates the presence of Cr 2p3/2. Our results agree well with numbers of researchers who have determined the position of Cr 2p on supported alumina [18,19]. The XPS spectra on curve-fitting area of Cr for 6.0Cr/2.0Pt/Al2O3 (pH 10) and 6.0Cr/2.0Pt/Al2O3 (pH 11) catalyst recorded higher intensities and clear peaks, the catalyst's peaks are shifted to the left compared with 6.0Cr/2.0Pt/Al2O3 (pH 4) catalyst.…”

Understanding the dispersion of Ag on high surface area TiO2 supports using XPS intensity ratios

“…XPS has been well established as a technique for studying dispersion of metal on supported systems [12][13][14][15][16][17][32][33][34][35][36][37][38][39]. Since there is no shadowing of the substrate, the Ti intensity from the TiO 2 support is not attenuated and can be assumed constant.…”

Dual Nanoparticle/Substrate Control of Catalytic Dehydrogenation

“…[3,4,6,7] It has been established that the catalytic activity of chromia/alumina catalysts first increases with chromia loading, but then starts to decrease. [3,5] Degradation at higher coverages has been correlated with the appearance of crystalline Cr 2 O 3 particles. Thus far, characterization of the low-loading regime has been pursued with a variety of techniques, all of which probe relatively large surface areas, yielding an average signal.…”

“…It has been found that oxidative catalytic dehydrogenation, that is, a process assisted by both a solid catalyst and a flow of oxygen gas, is most practical because of higher system longevity. [3][4][5] Chromia/alumina catalytic systems have been widely adopted by industry. They have also been the subject of scientific studies seeking understanding that may help optimization of the process.…”

“…Thus far, characterization of the low-loading regime has been pursued with a variety of techniques, all of which probe relatively large surface areas, yielding an average signal. [3,5,8,9] It has been inferred that "surface chromate species", as opposed to fragments of stoichiometric oxides, are responsible for catalysis. Efforts to determine the oxidation state of Cr in catalytically active species have yielded conflicting results, but a consensus has emerged that Cr 6+ is present.…”