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Towards understanding the improved stability of palladium supported on TS-1 for catalytic combustion
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.
Making an optimized catalyst requires control over the synthesis of the active site, as well as attachment of the active site to the catalyst support (which is typically needed for mechanical properties as well as to disperse the active sites). For the former, the use of nanoparticles synthesized in solution is an attractive proposition. They can be produced ex situ from the catalyst support by controlling the key properties such as particle size [2], shape [3], and the nature of the exposed surfaces [4] and can contain more than one metal with controlled location (such as a core–shell structure) [5]. Attaching these particles to supports is a complex process. Although in some cases the presence of stabilizers has been shown to be beneficial [6], often the stabilizers need to be removed for optimal performance. Ligand removal often changes the nature of the nanoparticle, for example through a loss of size control [7], rendering them poorly performing. Ligand removal has been addressed in a few selected cases, for example in a catalyst made with polymer-stabilized nanoparticles [8], but significant progress is still needed to find a general method that would allow manufacturing at scale to take place.
Synthesis of nanoparticles by aggregation of metal atoms or ions in the gas phase is a promising technology [9] that addresses many of these issues. In a typical configuration, atoms are generated from a metal source and these are condensed to form clusters. Typically, some of the particles formed are charged, which allows them to be manipulated using applied voltages, mass-selected if desired, and finally guided onto the support. The technique can offer particle-size control from less than 2 nm to over 10 nm [10] and also some control over the interaction between the nanoparticle and the support: the accelerating voltage can be used to control the impact of the particle into the support [11–13]. We [14] and others [15] have, in this way, made bimetallic clusters from a number of metals. Yang et al. [16] have demonstrated the selective deposition of silver clusters onto the top face of silicon pillars. A combination of these different features should allow the design of catalysts with a high degree of control.
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.
Figure 1. Representative bright-field aberration-corrected STEM images of the catalysts prepared by gas-phase cluster deposition: (A)–(B) Pd/α-Al2O3; (C)–(D) Pd/TiO2. Examples of palladium particles are indicated by red arrows, alpha alumina particles with yellow arrows, and titania particles with blue arrows.
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