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.

Palladium was deposited on two conventional support powders (alpha alumina and titania) to make representative catalysts for the vapor-phase selective hydrogenation of 1-pentyne to 1-pentene. Catalysts were prepared by four methods: gas-phase cluster beam deposition, incipient wetness impregnation, deposition-precipitation, and ion-exchange methods. Details of the methods used are presented in the Supporting Information. Table 1 compares the properties of the catalysts. The palladium content of the materials is low at 0.1wt%. This was driven by the experimental configuration for gas-phase cluster deposition. However, the efficient use of scarce precious metal resources is a key consideration, and synthetic methods for making good catalysts at these low loadings are valuable. Figure 1 shows representative TEM images of the catalysts synthesized by gas-phase cluster deposition, whereas images of the other catalysts are presented in Fig. S2. Table 1 lists the particle-size ranges for the catalysts. It was difficult to determine precise distributions of the nanoparticles due to clustering in some systems and low loading in the others.

In gas-phase cluster deposition on both supports, nanoparticles are observed only close to the support surface, where they often form agglomerates. In the case of titania, the support is present as a loose agglomerate of 20–30 nm particles, and the palladium particles are deposited on the surface of these agglomerates. The alpha alumina is present as much larger particles (20–40 µm), and here the heterogeneous catalyst of palladium are deposited on the alumina particle surface with little transport of the nanoparticles into the interior of the alumina. Although deposition on the external surface is a general feature of gas-phase cluster deposition processes, neither the alpha alumina nor the titania used in this work is significantly porous, so the materials are all expected to be surface enriched in palladium. Clearly, this would not be the case for a more porous support, such as a typical gamma alumina.

Catalyst testing

The catalysts’ performance in the selective hydrogenation of 1-pentyne (Scheme 1) were tested in a quartz microreactor using the as-prepared powders. 1-pentyne vapor and a hydrogen–helium mixture were flowed through a catalyst bed while the temperature was increased from ambient to 250°C. Full details of the catalytic testing methodology are presented in the Supporting Information. Figure 2 shows the performance of the eight catalysts when tested at equivalent palladium content and bed depth. None of the catalysts showed a significant amount of activity at low temperature (<50°C). As the temperature increases above this temperature, the 1-pentyne conversion increases. The most active of the catalysts studied were Pd/α-Al2O3 prepared by impregnation and by deposition-precipitation. The gas-phase cluster deposition materials were the least active, but also the most selective, with combined selectivity to 2-pentenes and pentane of less than 10% across the temperature range studied.

Figure 2. Catalyst testing in 1-pentyne hydrogenation. The 1-pentyne conversion is shown in blue, with selectivity to 1-pentene (red), 2-pentenes (green, solid line), and pentane (green, solid line) also shown. The catalysts are (a) Pd/TiO2 GCD; (b) Pd/α-Al2O3 GCD; (c) Pd/TiO2 impregnation; (d) Pd/α-Al2O3 impregnation; (e) Pd/TiO2 deposition-precipitation; (f) Pd/α-Al2O3 deposition-precipitation; (g) Pd/TiO2 ion exchange; and (h) Pd/α-Al2O3 ion exchange.

Given the difference in activity between the GCD and reference catalysts, it was of interest to compare their performance at close to iso-conversion. This was achieved by varying the catalyst mass at constant flow rates of hydrogen and 1-pentyne. Details of the procedure are presented in the Supporting Information. Table 2 shows the selectivity of each catalyst when the temperature was at a point where 80% conversion was achieved (T80). Under these conditions, the selectivity of the catalysts is much closer, although the GDC catalysts are still among the best for each support studied. The most selective catalysts are Pd/α-Al2O3 prepared by impregnation, deposition-precipitation, and gas-phase cluster deposition with over 90% selectivity to 1-pentene. The Pd/TiO2 catalysts are generally less selective. Intriguingly, the two catalysts prepared by the ion-exchange method have very similar performances.It is clearly of interest to understand the origin of the performance of the eight catalysts studied. The materials present a range of metal–support interaction types, and these can be used to understand how the nature of the active site affects catalytic performance. For the materials prepared by gas-phase cluster deposition, there is no contact between Pd2+ ions and the support, whereas for ion-exchange materials, the interaction is governed by the adsorption of Pd2+ ions onto reactive sites on the support, such as Al-O− or Ti-O−, by the replacement of two H+ ions with one Pd2+ ion. The isoelectric points of alpha alumina and titania are reported to be pH 9.3 [22] and pH 5.4 [23], respectively. The metal precursor used in this study, homogeneous catalyst of palldium nitrate, is acidic, which makes the impregnating solution acidic. However, even at lower pH, some negatively charged surface sites will exist [24]; clearly, the number and distribution will be affected by the nature of the palladium precursor solution and the support material. At ion-exchange sites, palladium will be transformed during subsequent thermal treatments (in this work, drying at 100°C and hydrogen reduction at 250°C). In the final catalyst, they will behave differently from the main nanoparticulate palladium phase and invariably lead to some loss of selectivity under reaction conditions. If these sites were highly active, as might be anticipated for a very well-dispersed phase, they could influence selectivity disproportionately.