The Origin of Sulfur Tolerance in Supported Platinum Catalysts: The Relationship between Structural and Catalytic Properties in Acidic and Alkaline Pt/LTL

Abstract

The reactivity, structure, and sulfur tolerance is compared for platinum supported on acidic and alkaline LTL zeolite. In the absence of sulfur, EXAFS spectroscopy indicates that small metallic platinum particles of approximately 6 to 14 atoms/cluster are present. The TOF for neopentane hydrogenolysis and isomerization is ca 100 times higher on the acidic LTL due to the metalsupport interaction. Saturation of the platinum by H 2 S, results in the formation of surface Pt-S bonds with a bond length of 2.33Å. Comparison of the EXAFS results of the sulfur poisoned catalysts, indicates that the S to Pt ratio is lower for platinum on the acidic zeolite. Catalytically, the initial activity (per gram) of both catalysts is greatly reduced after sulfur poisoning due to the loss of exposed platinum, however, the initial neopentane TOFs are nearly unchanged. Because of its higher TOF, the catalytic activity (per gram) of sulfur poisoned, acidic Pt/LTL is comparable to that of nonsulfur poisoned alkaline Pt/LTL. In both sulfur poisoned catalysts the neopentane isomerization selectivity increases compared to the sulfur free catalyst. Although the initial TOFs of the nonsulfur poisoned and sulfur poisoned catalysts are the same, there is a rapid loss in activity due to coke formation in the sulfur poisoned, alkaline LTL, while rate of deactivation by coke in the sulfided, acidic LTL is much lower. The increased sulfur tolerance of acidic supported noble metal catalysts appears to result primarily from the higher intrinsic TOF. In addition, because of its resistance to coke deactivation, the acidic supported sulfur tolerant catalyst is able to maintain stable catalytic activity.

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

“…As observed previously, increasing the number of protons by replacing Na + in the support results in an increase in the Pt TOF for neo-pentane conversion [3,[9][10][11]13,20,39,[40][41][42]. The TOF of Pt in H-NaY is 8.5 times higher than that in NaY.…”

Pt–Co alloy nanoparticles synthesized on sulfur-modified carbon nanotubes as electrocatalysts for methanol electrooxidation reaction

“…We believe that during the temperature ramp, first PtCl 2À 6 ions are reduced to form metallic nuclei. The rate of nucleation is higher for the modified CNTs, because sulfur species on the CNT surfaces act as anchoring sites for the metal species, and specially, they have high tendency for noble metals [33][34][35], which arise from soft acid-soft base interactions. This behavior was evidenced by the smaller particle size obtained for Pt 3 Co/CNT than Pt 3 Co/CNTum.…”

n-Octane aromatization on Pt-containing non-acidic large pore zeolite catalysts

“…Many studies have been conducted to understand the uniqueness of this catalyst and its ability to aromatize n-hexane [7][8][9][10][11][12][13][14] as well as to improve its tolerance to sulfur, which is a weakness of this unique material [15][16][17][18][19][20][21][22]. A well-recognized fact is that the KL zeolite structure is able to stabilize extremely small Pt clusters (<1 nm) in a completely non-acidic environment, two conditions that are needed in the selective aromatization pathway [23].…”