Synthesis of Highly Dispersed Palladium Nanoparticles Supported on Silica for Catalytic Combustion of Methane

Abstract

Palladium nanoparticles supported on silica catalysts (Pd/SiO 2 ) were prepared by wet impregnation (WI), dry impregnation (DI), strong electrostatic adsorption (SEA), and charge-enhanced dry impregnation (CEDI) methods. The Pd/SiO 2 samples with highly dispersed and tight size-distributed palladium nanoparticles are obtained via SEA and CEDI methods based on strong electrostatic interactions between the dissolved metal precursor ([Pd(NH 3 ) 4 ] 2+ ) and positively charged SiO 2 support in an alkali-impregnating solution (initial pH = 12). The Pd/SiO 2 -SEA samples prepared by the SEA method usually showed higher palladium dispersions (>50%) than those prepared by CEDI (Pd dispersion = 32−45%). The surface loading (support surface area per liter of preparation solution), pH regulator (NaOH or NH 4 OH), Pd loading, and reduction temperature were shown to be key factors affecting the dispersion of palladium in the Pd/SiO 2 -SEA samples, as well as the leaching/dissolution of SiO 2 and palladium in the alkali solution. The Pd/SiO 2 -SEA samples prepared with proper SLs of 30,000−100,000 m 2 L −1 using NH 4 OH as the pH regulator exhibited not only very high Pd dispersions (64−97%) but also negligible losses of SiO 2 and Pd in the impregnating solution. The Pd/SiO 2 -SEA samples also exhibited better catalytic performance in methane combustion based on both the T 10 and T 50 temperatures and the intrinsic activities (mass-specific activity and/or turnover frequency (TOFs)). The TOFs generally decreased from 130 h −1 to 6.2 h −1 as Pd dispersion increased from 32% to 97% for the Pd/SiO 2 -SEA(NH 4 OH) catalysts. Moreover, the reaction activity of Pd/SiO 2 -SEA catalysts was significantly improved by increasing the fraction of Pd 0 in the range of 70−85%, indicating that this size-sensitive catalysis would be related to the redox properties of the supported Pd nanoparticles.

Atmospheric- and Low-Level Methane Abatement via an Earth-Abundant Catalyst

“…, 24 h at 350–400 °C , ); that is, successful activations generally exceed the time and temperature ranges explored here. Using relatively aggressive activations, successful conversions of 1000–10,000 ppmv methane (0.1–1% CH 4 , with 10% or less oxygen) at temperatures well below the ignition point (600 °C) have been recorded ,,,, with a few demonstrating conversion temperatures as low as 300 °C. , While the reaction temperatures we observed are moderately lower than previous demonstrations, the dramatically different activation procedures and simplified catalyst synthesis offer unique benefits. As a point of comparison, conversion efficiencies to CO 2 are not often reported and cannot be directly compared to our results; nevertheless, methanol production rates in previous studies have been quite low (order 0.1–0.3 mol methanol per mol Cu or less; methanol production was not systematically quantified in our study, but early spot checks in the two-step process revealed around 1–2 orders of magnitude lower methanol in the post-reaction, water-extracted catalyst, where the input methane was around 10 6 -fold lower than in previous studies).…”

“…Limited but quantifiable conversion was observed at temperatures as low as 100 °C (Figure a), reduced from previous experiments carried out between 150 and 200 °C. Catalytic conversion of methane using PGMs, − cobalt, or nickel–cobalt mixtures , has typically relied on higher activation temperatures (500–1000 °C) or long durations ( e.g. , 24 h at 350–400 °C , ); that is, successful activations generally exceed the time and temperature ranges explored here.…”

“…Catalytic conversion of methane using PGMs, 33−38 cobalt, 30 or nickel−cobalt mixtures 31,32 has typically relied on higher activation temper-atures (500−1000 °C) or long durations (e.g., 24 h at 350−400 °C33,35 ); that is, successful activations generally exceed the time and temperature ranges explored here. Using relatively aggressive activations, successful conversions of 1000−10,000 ppmv methane (0.1−1% CH 4 , with 10% or less oxygen) at temperatures well below the ignition point (600 °C50 ) have been recorded 31,33,34,36,38 with a few demonstrating conversion temperatures as low as 300 °C. 32,35 While the reaction temperatures we observed are moderately lower than previous demonstrations, the dramatically different activation procedures and simplified catalyst synthesis offer unique benefits.…”

Comparative Study of the Catalytic Oxidation of Hydrocarbons on Platinum and Palladium Wires and Nanoparticles

“…Most reported studies of catalytic oxidation employ supported Pt and Pd, − where the support is mainly a more accessible metal oxide such as aluminum oxide, − cerium oxide, , and silicon dioxide , or metal such as nickel, barium, zeolite, and zirconium. , While such configurations are relevant for practical applications, they come with the complexity of the role played by the support and uncertainty of mechanisms explaining the interchanging reactions between noble metals, support, and gaseous reactants. Only a handful of papers are reported for pure unsupported noble metals reacting with a limited range of hydrocarbon fuels .…”

“…20−22 Further details may be found in the following comprehensive reviews on this topic. 23−27 Most reported studies of catalytic oxidation employ supported Pt and Pd, 28−33 where the support is mainly a more accessible metal oxide such as aluminum oxide, 34−38 cerium oxide, 39,40 and silicon dioxide 41,42 or metal such as nickel, 43 barium, 44 zeolite, 45 and zirconium. 46,47 While such configurations are relevant for practical applications, they come with the complexity of the role played by the support and uncertainty of mechanisms explaining the interchanging reactions between noble metals, support, and gaseous reactants.…”