Laurent Delannoy

Supported gold catalysts for selective hydrogenation of 1,3-butadiene in the presence of an excess of alkenes

Supported gold catalysts were investigated in the selective gas phase hydrogenation of 1,3-butadiene in an excess of propene (0.3% butadiene, 30% propene and 20% hydrogen), in order to simulate the process required for the purification of industrial alkenes streams to prevent poisoning of the polymerisation catalysts used for polyalkene production. Gold catalysts containing small gold particles (between 2 to 5 nm in average) are less active than commercial palladium catalysts, but they are much more selective. Under our experimental conditions, 100% of butadiene can be converted at ≈170°C into 100% butenes with 1-butene as the main product, and with only very small amount of alkanes formed (≈100 ppm). The absence or presence of propene does not drastically modify the rate of hydrogenation of butadiene.Parameters directly related to the nature of the gold catalysts were also investigated. For a given preparation method (deposition-precipitation with urea (DPU)), gold particle size and gold loading, the nature of the oxide support (alumina, titania, zirconia, ceria) does not influence the gold reactivity. The variations of gold particle size and gold loading do not induce changes in the TOF (expressed per surface gold atoms). The method of preparation has an influence when it leaves chlorine in the samples (impregnation in excess of solution and anionic adsorption). In such a case, the gold catalysts are less active.

Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

Gold supported on ceria or ceria–alumina mixed oxides are very active catalysts for total oxidation of a variety of molecules. The key step of the oxygen activation on such catalysts is still a matter of debate. Gold–ceria (Au/CeO2) and gold–ceria–alumina (Au/CeO2/Al2O3) catalysts were prepared by deposition–precipitation of gold precursor with urea as in former works where their efficiency to catalyze the oxidation of propene and propan-2-ol was demonstrated. To understand the phenomenon of oxygen activation over this class of catalysts, efficient techniques generally used to characterize the interaction between oxygen and cerium-based oxides were applied; the oxygen storage capacity (OSC) measurement, the 18O2/16O2 isotopic exchange study (OIE), as well as characterizations by in situ Raman and electron paramagnetic resonance (EPR) spectroscopies. Each of the techniques allowed showing the impact of the gold nanoparticles on the activation of dioxygen, on the kinetic governing the gas-phase/solid oxygen atom exchange, and on the nature and the location of the adsorbed oxygen species. Gold nanoparticles were shown to increase drastically the OSC values and the rate of oxygen exchange. OIE study demonstrated the absence of pure equilibration reaction (16O2(g) + 18O2(g) ↔ 2 16O18O(g)), indicating that gold did not promote the dissociation of dioxygen. Peroxo adspecies were observed by Raman spectroscopy only in the presence of gold. On the contrary, EPR spectroscopy indicated that the concentration of superoxo adspecies was lower for oxide-supported gold samples than for bare oxides. The combination of techniques allowed reinforcing the hypothesis that the gold nanoparticules promote the activation of dioxygen by generating extremely mobile diatomic-oxygenated species at the gold/ceria interfacial perimeter. This specific gold–ceria interaction, which leads to the increase in oxygen mobility, is probably also responsible for the higher catalytic performance of Au/CeO2 and Au/CeO2/Al2O3 in oxidation reaction compared to bare supports.

Silica-Supported Au-Ag Catalysts for the Selective Hydrogenation of Butadiene

Gold and silver are miscible over the entire composition range, and form an attractive combination for fundamental studies on bimetallic catalysts. Au–Ag catalysts have shown synergistic effects for different oxidation and liquid‐phase hydrogenation reactions, but have rarely been studied for gas‐phase hydrogenation. In this study 3 nm particles of Au, Ag and Au–Ag supported on silica (SBA‐15) were investigated as catalysts for selective hydrogenation of butadiene in an excess of propene. The Au catalyst was over an order of magnitude more active than the Ag catalyst at 120 °C. The initial activity of the Au–Ag catalysts scaled linearly with the Au‐content, suggesting a direct correlation between the surface and overall compositions of the nanoparticles and the absence of synergistic effects. All Au‐containing catalysts were highly selective to butenes (>99.9 %). The Au catalysts were stable, whereas the Au–Ag catalysts lost about half of their activity during 20 h run time at 200 °C, but the initial activity was restored by a consecutive oxidation‐reduction treatment. Near ambient pressure x‐ray photoelectron spectroscopy showed that exposure to H2 at elevated temperatures led to a gradual enrichment of the surface of the Au–Ag nanoparticles by Ag. These observations highlight the importance of considering progressive atomic rearrangements in bimetallic nanocatalysts under reaction conditions.

Superior Stability of Au/SiO2 Compared to Au/TiO2 Catalysts for the Selective Hydrogenation of Butadiene

Supported gold nanoparticles are highly selective catalysts for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about their stability during gas-phase catalysis and the influence of the support thereon. We report on the activity, selectivity, and stability of 2–4 nm Au nanoparticulate catalysts, supported on either TiO2 or SiO2, for the hydrogenation of 0.3% butadiene in the presence of 30% propene. Direct comparison of the stability of the Au catalysts was possible as they were prepared via the same method but on different supports. At full conversion of butadiene, only 0.1% of the propene was converted for both supported catalysts, demonstrating their high selectivity. The TiO2-supported catalysts showed a steady loss of activity, which was recovered by heating in air. We demonstrated that the deactivation was not caused by significant metal particle growth or strong metal–support interaction, but rather, it is related to the deposition of carbonaceous species under reaction conditions. In contrast, all the SiO2-supported catalysts were highly stable, with very limited formation of carbonaceous deposits. It shows that SiO2-supported catalysts, despite their 2–3 times lower initial activities, clearly outperform TiO2-supported catalysts within a day of run time.

Selective gas phase hydrogenation of nitroarenes over Mo2C-supported Au–Pd

We report the first synthesis of Mo2C-supported Au and Au–Pd catalysts (nominal Au/Pd = 10 and 30) obtained from colloidal nanoparticles stabilised by polyvinyl alcohol (PVA). Equivalent Au/Al2O3 and Au–Pd/Al2O3 were prepared and served as benchmarks. Residual PVA was removed by thermal treatment in N2, which was monitored by thermogravimetric analysis. The catalysts were characterised in terms of temperature-programmed reduction (TPR), BET surface area, H2 chemisorption, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. The reduced catalysts exhibited an equivalent metal particle size range (1–8 nm) and mean size (4–5 nm). The carbide samples showed greater H2 chemisorption capacity than the Al2O3 systems where inclusion of Pd enhanced H2 uptake. XPS measurements suggest electron transfer from Al2O3 to Au while the Au binding energy for the carbide samples is close to that of the metallic Au reference. The catalysts were tested in the gas phase hydrogenation of nitrobenzene, p-chloronitrobenzene and p-nitrobenzonitrile and delivered 100% selectivity to the target amine in each case. Inclusion of Pd served to increase selective hydrogenation rates where Au–Pd/Mo2C outperformed Au–Pd/Al2O3, a response that is attributed to increased surface hydrogen.

Evidence for an H2 promoting effect in the selective catalytic reduction of NO(x) by propene on Au/Al2O3.

This work provides the first experimental evidence of an H2 effect in C3H6-SCR over an Au/Al2O3 catalyst. This effect could only be observed when the number of Au catalytic sites was decreased. The N2 turnover rate estimated for the first time for the Au catalytic sites for H2-C3H6-SCR was found to be similar to that estimated for Ag ones supported on Al2O3.

Unusual behaviour of Au/ZnO catalysts in selective hydrogenation of butadiene due to the formation of a AuZn nanoalloy

The reaction of selective hydrogenation of butadiene performed in an excess of propene, which has been extensively studied over gold supported on different oxides and exhibited high selectivity to butenes, has been applied to the case of gold (1 wt%) supported on zinc oxide (8 m2 g−1), a strongly basic oxide, in order to try to thwart deactivation. Our results revealed, for the first time, a catalytic behaviour drastically different from what was previously observed over gold catalysts supported on alumina or titania, i.e. a much poorer activity when the catalyst was activated under usual conditions under hydrogen but a “normal” activity when it was activated in air. An in-depth characterisation study based on different hypotheses to explain the unusual result revealed that at variance with the other catalysts, Au1Zn1 alloy particles formed during activation under hydrogen at 300 °C and were responsible for the loss of activity.

In Situ XAS Studies on the Structure of the Active Site of Supported Gold Catalysts

Gold clusters supported on Al2O3 and TiO2 have been exposed to different mixtures of CO and O2. Their structure has been probed in situ using X-ray absorption spectroscopy (XAS) at the Au L3-edge. In all materials, the dominant phase during catalysis is Au0. Both samples show variations of the electronic structure of the gold clusters with changing reaction conditions as evidenced by changes in the X-ray absorption near-edge (XANES) region. These variations are caused by interaction between the gold clusters and the carbon monoxide present in the gas phase. The gold atoms remain zerovalent throughout all experiments confirming the importance of Au0 for catalytic activity.

Corrigendum: Silica-Supported Au–Ag Catalysts for the Selective Hydrogenation of Butadiene

In Table 2 of this Full Paper, the units for the turnover frequencies (TOF) are given as 10-13 s-1. The correct units for the TOF are 10-3 s-1. The authors and editorial office apologize for the oversight.