Effects of SiO <b><sub>2</sub></b> Support on the Formation of Active Sites in HfO <b><sub>2</sub></b> /SiO <b><sub>2</sub></b> Catalysts for the Conversion of Ethanol and Acetaldehyde to 1,3-butadiene.

The estimated activation energies are almost equal for the investigated catalysts, indicating that the quality of active sites in HfO 2 /SiO 2 catalysts do not depend on the type of SiO 2 support. However, the estimated frequency factors exhibit an increasing trend (Entry 20 < Entry 9 < Entry 4 < Entry 17; the numbers 4, 9, 17, and 20 correspond to the entry numbers in Table ), indicating that the number of active sites depends on the type of SiO 2 support. 3.2.

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