Catalytic Consequences of Particle Size and Chloride Promotion in the Ring-Opening of Cyclopentane on Pt/Al₂O₃

Abstract

Ring-opening of cyclopentane on alumina-supported Pt particles was studied as a function of Pt particle size in the presence of different Cl contents. With catalysts prepared from a Cl-free precursor, measured turnover rates increased monotonically with increasing Pt particle size (1–15 nm). On catalysts derived from a Cl-containing precursor, the turnover rates fell into two separate trends with the change of Pt particle size, depending on the extent of Cl removal by increasing thermal treatment temperature. In both cases, catalytic activity increased with increasing particle size in the examined ranges of dispersions (D = 0.7–1.0 and 0–0.6) and for both series of catalysts, the apparent activation energies were higher on large Pt particles than on small ones, with only small differences in the reaction orders for H2 and cyclopentane on particles of widely varying average sizes. Therefore, the effect of particle size on the turnover rates stems mainly from intrinsic rate constants, rather than from coverage effects. The relative adsorption coefficients of toluene and benzene indicated lower electron densities at the surface Pt atoms in the catalysts prepared from the Cl-containing precursor than in those from the Cl-free precursor. This subtle electron deficiency, which seems not to stem from the local Cl enrichment near Pt, affects both the concentration of chemisorbed hydrogen under reaction conditions and the barrier for C–C bond cleavage. The Cl postintroduced to the catalyst, in contrast, does not induce a similarly positive effect.

Morphology-dependent phase transformation of γ-Al2O3

“…It has been well documented that impurities such as alkali metals (e.g., Na + ) can substantially modify the thermal stabilities of γ-Al 2 O 3 and also influence its phase transformations [11,13,14,[26][27][28][29].…”

“…Thus, the prepared crystallites have different relative ratios of specific facets allowing systematic studies aimed at finding correlations between phase transformations of γ-Al 2 O 3 and crystallite morphologies that otherwise would be very difficult with commercial γ-Al 2 O 3. These information will be very useful for potential development of thermally stable γ-Al 2 O 3 without any surface modifier such as La, Ba, etc., that can affect the catalytic properties [11,[13][14][15][16][26][27][28][29].…”

Effect of structural defects in alumina supports on the formation and catalytic properties of the active component of reforming catalysts

“…Data on the ionic state of platinum in alumina-platinum catalysts were first reported in [10] and then verified by others [5,11]. Modern ideas on the structure of platinum catalysts dictate an essential role for ionic platinum species in catalyzing the transformation of hydrocarbons [12]. Such ionic platinum species show an order of magnitude higher atomic activity towards the aromatization of n-alkanes when compared with disperse Pt 0 [13].…”

Hydroconversion of 1-methylnaphthalene over Pt/AlSBA-15–Al2O3 composite catalysts

“…ThePt δ + species have been suggested play a positive role in the hydrogenation of toluene. Shi et al [19] have observed a higher activity of the Pt/Al 2 O 3 catalyst obtained by impregnation with Cl-containing precursor in the ring opening of cyclopentane as compared to the one obtained by impregnation with Cl-free precursor (Pt(NH 3 ) 4 (NO 3 ) 2 ). In contrast, the activity enhancement was less marked when a similar concentration of Cl was introduced to the catalyst by means of postimpregnation.…”

“…Activation of the Pt catalysts is based on a high-temperature reduction step with H 2 . It was shownthat even after the heat treatment, not only platinum species, but also residualions originatingfrom theactive metalprecursor can still be found on the catalystsurface [18,19]. Those residues can influence the catalyst performance.…”