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Acid sites development on Cr3+/SiO2 catalysts obtained by the sol–gel method and hydrothermal treatment: Effect of calcination temperature

“…The absence of diffraction lines on 5SG samples, corresponding to crystaline species of Cr 6+ , demonstrates the high dispersion of the Cr 6+ oxides. It leads to the formation of surface chromates as reported by other autors [2,4,13,14].…”

“…These bands are characteristic of oxidized species of Cr 6+ . They are assigned to symmetric and asymmetric stretching modes of O Cr O bonds present in chromates, dichromates and CrO 3 oxide [13,14]. The intensity of these bands depends on chromium loading and on the temperature of the hydrothermal treatment.…”

“…In catalysts with highly dispersed chromium ions prepared both by ion exchange of mono and binuclear Cr 3+ complexes, the alcohol dehydrogenation reaction rate was dependent on the amount of Cr 3+ centers anchored to the support surface [13][14][15]. These Cr 3+ centers developed Lewis acidity, as determined by both pyridine (Py) adsorption and by the 2-propanol dehydration reaction.…”

Spatially-resolved investigation of the water inhibition of methane oxidation over palladium

“…[76][77][78][79] Internal and external heat and mass transfer 80 are phenomena that can have a marked effect on the observed rates of reaction, particularly affecting species diffusion rates in the bulk and within the pore network of the wash coat. [81][82][83][84][85][86][87][88][89][90][91][92][93] Internal pore diffusion limitations are generally accounted for via the inclusion of an effectiveness factor in both the energy and species transport equations, assuming two discrete phases i.e. solid and gas.…”

“…solid and gas. The majority of research groups have estimated the effectiveness factor via the Thiele Modulus approach [81][82][83][84][85] however the Weisz-Prater criterion [89][90][91] and internal mass transfer coefficient 92,93 approaches have also been utilised in numerous works. External diffusion is generally allowed for via lumped parameter models, distributed models and empirically derived correlations.…”