Abstract
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.
Highlights
Vinyl chloride monomer (VCM) is a large-scale chemical intermediate used almost exclusively as a precursor to polyvinyl chloride (PVC)
Active Au/C catalysts prepared by the impregnation of Au from a solution of aqua regia have previously been shown to consist of atomically dispersed cationic Au and be highly active for acetylene hydrochlorination.[9]
Given that the single-site cationic Au species, shown by X-ray absorption near-edge structure (XANES) analysis[9] to be present in catalyst, are undetectable by X-ray Diffraction (XRD), no diffraction peaks from Au were observed at room temperature
Summary
Vinyl chloride monomer (VCM) is a large-scale chemical intermediate used almost exclusively as a precursor to polyvinyl chloride (PVC). A similar correlation between the normalized white line height and VCM production for the reaction period after HCl treatment (step iii in Figure 2) is evident, except that the gradient of the trend line changed to 0.057 (±0.007). Linear combination fitting (LCF) of these three spectra and those of the catalyst under HCl/Ar and C2H2/Ar using Au(III), Au(I) and Au metal standards was performed to gain greater understanding of the changes in the XANES spectra and explain the loss in correlation between white line and VCM productivity in step v (Table 1). While the Au L3 edge XANES of this species was clearly different to Au (I) or Au(III) chloride, EXAFS shows that Au−Cl interactions are still present in the C2H2/Ar-treated sample The magnitude of this scattering in the Fourier transform data (Figure S3) is significantly lower under C2H2/Ar than the catalyst in the presence of both reactants (C2H2 + HCl/Ar). This suggests that ∼2/3 of atomic Au transformed into large Au particles during the reaction, leading to a significantly decreased Au dispersion
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