Deactivation of a single-site gold-on-carbon acetylene hydrochlorination catalyst: An X-ray absorption and inelastic neutron scattering study
journal contributionposted on 28.09.2018 by Grazia Malta, Simon Kondrat, Simon J. Freakley, Catherine J. Davies, Simon R. Dawson, X. Liu, Li Lu, K. Dymkowski, F. Fernandez-Alonso, S. Mukhopadhyay, Emma K. Gibson, Peter P. Wells, S.F. Parker, Christopher J. Kiely, Graham J. Hutchings
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Single-site Au species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. The evolution of these single-site species has been monitored by Au L3 X-ray Absorption Spectroscopy (XAS). Alternating between a standard reaction mixture of HCl/C2H2 and the single reactants, has provided insights into the reaction mechanism and catalyst deactivation processes. We demonstrate that oxidative addition of HCl across an Au(I) chloride species requires concerted addition with C2H2, in accordance with both the XAS measurements of Au oxidation state and the reaction kinetics being 1st order with respect to each reactant. The addition of excess C2H2 changes the Au speciation and results in the formation of oligomeric acetylene species which were detected by inelastic neutron scattering. Catalyst deactivation at extended reaction times can be correlated with the formation of metallic Au particles. The presence of this Au(0) species generated during the sequential gas experiments or after prolonged reaction times, results in the analysis of the normalised near edge white line intensity of the Au L3 X-ray absorption spectrum alone becoming an unsuitable guide for identifying the active Au species, affecting the strong correlation between normalized white line height and VCM productivity usually observed in the active catalyst. Thus, a combination of scanning transmission electron microscopy and detailed modelling of whole XAS spectrum was required to distinguish active Au(I) and Au(III) species from the spectator Au(0) component.
The authors thank Cardiff University for support as part of the MAXNET Energy Consortium. The UK Catalysis Hub is thanked for resources and support provided through our membership of the UK Catalysis Hub Consortium and funded by the Engineering and Physical Sciences Research Council (EPSRC) (grants EP/K014706/1, EP/K014668/1, EP/K014854/1EP/K014714/1, and EP/M013219/1). We used the B18 beamline at the Diamond Light Source (allocation numbers SP10306, SP11398, and SP15214) with the help of D. Gianolio and G. Cibin. We gratefully acknowledge the Science and Technology Facilities Council (STFC) for access to neutron beamtime at ISIS and also for the provision of sample preparation, Merlin facilities. C.J.K. acknowledges funding from the National Science Foundation Major Research Instrumentation program (GR no. MRI/DMR-1040229). We thank Johnson Matthey for their contribution to and funding of this work.