A novel Pd supported on TS-1 combustion catalyst was synthesized and tested in methane combustion under very lean and under highly humid conditions (<1%). A notable increase in hydrothermal stability was observed over 1900 h time-on-stream experiments, where an almost constant, steady state activity obtaining 90% methane conversion was achieved below 500 °C. Surface oxygen mobility and coverage plays a major role in the activity and stability of the lean methane combustion in the presence of large excess of water vapour. We identified water adsorption and in turn the hydrophobicity of the catalyst support as the major factor influencing the long term stability of combustion 7% palladium on carbon. While Pd/Al2O3 catalyst shows a higher turn-over frequency than that of Pd/TS-1 catalyst, the situation reversed after ca. 1900 h on stream. Two linear regions, with different activation energies in the Arrhenius plot for the equilibrium Pd/TS-1 catalyst, were observed. The conclusions were supported by catalyst characterization using H2-chemisorption, TPD, XPS analyses as well as N2-adsorption–desorption, XRD, SEM, TEM. The hydrophobicity and competitive adsorption of water with oxygen is suggested to influence oxygen surface coverage and in turn the apparent activation energy for the oxidation reaction.
The selective hydrogenation of a range of substrates is a key technology in both the bulk and fine chemicals industries [1]. In both contexts, selectivity to the desired product is usually a key attribute: loss of reagent to the formation of undesired products is economically undesirable and can lead to challenges in separation downstream. This means that there is a pressing need for more selective catalysts and processes for a range of selective hydrogenation reactions. One way to meet this need is the design and realization of catalytic materials with improved properties. The majority of commercial 5% palladium on carbon are made using a small number of synthesis methods (impregnation, precipitation, solid-state methods, etc.). There is good reason for this: they are reliable, economic, and can be performed at the necessary scale for commercial use. However, they are not always able to produce materials that are truly optimized.
In this work, we use gas-phase cluster deposition as a method to deposit size-controlled palladium series catalyst onto two typical commercial powder support materials. We employ the selective partial hydrogenation of 1-pentyne (Scheme 1) as a model reaction for the selective hydrogenation of alkynes relevant to both the bulk [17] and fine [18,19] chemicals industries. We have previously reported the good performance of a palladium catalyst prepared by gas-phase cluster deposition onto a flat graphite tape as a catalyst for the selective hydrogenation of 1-pentyne [20], and we have also observed changes in the atomic structure of size-selected palladium nanoparticles during this reaction [21]. Most recently, we have reported the performance of PdM bimetallic cluster catalysts in alkyne hydrogenation [14]. In this paper, we describe the performance of catalysts prepared by gas-phase nanoparticle synthesis in selective alkyne hydrogenation and offer some perspective on the nature of the reactive sites.
