In situ study of Au/C catalysts for the hydrochlorination of acetylene

Very few commercial processes employ Au catalysts for the production of fine or commodity chemicals. The recent validation of Au/C catalysts to produce vinyl chloride monomer (VCM) via the acetylene hydrochlorination reaction, as a replacement of the traditionally used highly volatile and toxic mercuric chloride catalyst, represents a notable exception. However, the active form of the catalyst and the reaction mechanism are still not fully understood. The work presented in this thesis aims to provide detailed information about the nature of the catalyst’s active species and the possible reaction pathway, combining commonly used characterisation techniques with more challenging in situ experiments. The first part of this thesis (Chapter 3) aims to understand the influence of the choice of solvent and metal precursor during the catalyst preparation in the activity of the final material in order to identify the active state of Au during catalysis and to propose a reaction mechanism (Chapter 3). This investigation was carried out via a in situ X-ray absorption fine structure (XAFS) spectroscopy experiment. This study led to the conclusion that under reaction conditions highly active catalysts comprise single-site cationic Au species whose activity correlates with the ratio of Au(I)/Au(III) present, providing a new insight to the structure-function relationship of this reaction, while the mechanism has been hypothesised to proceed through the oxidative addition of HCl to Au chloride, followed by the insertion of acetylene and reductive elimination of VCM. The deactivation of gold on carbon catalysts during acetylene hydrochlorination has been attributed to two possible deactivation mechanisms: the formation of oligomers on the catalyst surface blocking the active site and the reduction of active cationic gold to inactive metallic Au. These two deactivation pathways have been shown to be influenced by both reaction temperature used and the detrimental effect of acetylene rich feeds. The second part of this thesis (Chapter 4) investigates the role of each reactant on the catalyst composition to further elucidate both the reaction and deactivation mechanism via an in situ gas switching experiment while recording the XAS spectra at the Au L3-edge. This study led to the hypothesis that the oxidative addition of HCl across the Au(I) chloride species requires the concerted addition with C2H2, in partial modification of the reaction mechanism proposed in chapter 3. An inelastic neutron scattering (INS) study of the catalyst exposed to C2H2 showed the formation of oligomeric acetylene species on the catalyst surface, which, upon re-introduction of both reactants, led to significant catalyst deactivation associated with the formation of metallic Au nanoparticles. The formation of Au(0) has been directly correlated with a decrease in VCM productivity when under reaction conditions also using an higher Au loading catalys. The recently validated Au/C catalyst by Johnson Matthey, prepared by using a sulphur containing Au complex, under industrial conditions, is more active and stable than the traditional Au/C catalyst made using hard donor ligands such as Cl. Clearly the choice of the ligand plays a major role in the final activity and stability of those catalysts. Chapter 5 reports in situ ligand K-edge XAS characterisation of gold on carbon catalyst for the hydrochlorination of acetylene to understand chlorine and sulphur speciation in the catalysts under operating conditions. In both catalytic systems, Cl is bounded directly to the gold and is directly involved in the reaction mechanism, re-affirming that AuClx speciation are active site for the acetylene hydrochlorination reaction

Acetylene hydrochlorination using Au / Carbon: a journey towards single site catalysis

The replacement of mercuric chloride in the production of vinyl chloride monomer, a precursor to PVC, would greatly reduce the environmental impact of this large scale industrial process. The validation of single Au cations supported on carbon as the best catalyst for this reaction at an industrial scale has resulted from nearly 35 years of research. In this feature article we review the development of this catalyst system and address the limitations of a range of characterisation techniques used previously which may induce damage to the fresh catalyst. Following our latest findings using X-ray absorption spectroscopy, we show that under operating conditions the catalyst is atomically dispersed and can be classed as a single site catalyst, we give a perspective on future directions in single atom catalysis

Gas-Phase Deposition of Gold Nanoclusters to Produce Heterogeneous Glycerol Oxidation Catalysts

Gold nanoparticles prepared by colloidal methods are effective catalysts for selective glycerol oxidation under basic conditions. Large-scale synthesis of catalysts by wet chemical methods leads to large amounts of waste and can result in polymer or salt residues remaining on the catalyst. In contrast, gas-phase cluster deposition (cluster beam deposition) offers a solvent-free method to synthesize controlled nanoparticles/clusters. We show that the deposition of bare gas-phase gold clusters onto carbon powder leads to a catalyst comparable to that prepared by colloidal methods. This shows the feasibility of the synthesis method to produce oxidation catalysts with reduced waste

Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

Palladium nanoparticles have been studied extensively as catalysts for the direct synthesis of hydrogen peroxide, where selectivity remains a key challenge. Alloying Pd with other metals and using acid and halide promoters are commonly employed to increase H2O2 selectivity; however, the sites that can selectively produce H2O2 have not been identified and the role of these additives remains unclear. Here, we report the synthesis of atomically dispersed PdClx/C as a model catalyst for H2O2 production without the presence of extended Pd surfaces. We show that these isolated cationic Pd sites can form H2O2 with significantly higher selectivity than metallic Pd nanoparticles in both the reaction of H2 and O2 and the electrochemical oxygen reduction reaction. These results demonstrate that catalysts containing high populations of isolated Pd sites are selective catalysts for this two-electron reduction reaction and that the performance of materials in the direct synthesis reaction and electrocatalytic oxygen reduction reaction has many similarities.</p

Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

Palladium nanoparticles have been studied extensively as catalysts for the direct synthesis of hydrogen peroxide, where selectivity remains a key challenge. Alloying Pd with other metals and the use of acid and halide promoters are commonly used to increase H2O2 selectivity, however; the sites that can selectively produce H2O2 have not been identified and the role of these additives remains unclear. Here, we report the synthesis of atomically dispersed Pd/C as a model catalyst for H2O2 production without the presence of extended Pd surfaces. We show that these isolated cationic Pd sites can form H2O2 with significantly higher selectivity than metallic Pd nanoparticles in both the reaction of H2 and O2 and the electrochemical oxygen reduction reaction (ORR). This demonstrates that catalysts containing high populations of isolated Pd sites are selective catalysts for this two-electron reduction reaction and that the performance of materials in the direct synthesis reaction and ORR have many similarities

Identification of single-site gold catalysis in acetylene hydrochlorination

There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species. View Full Tex

Can gold be an effective catalyst for the Deacon reaction?

The Deacon reaction is an important industrial process for the oxidation of hydrogen chloride, thereby enabling chlorine to be recycled. As gold is an efficient catalyst for reactions involving hydrogen chloride and oxygen, we have studied the use of gold as a potential catalyst for the Deacon reaction. Unfortunately, gold displays only limited activity; however, this is markedly increased if hydrogen is cofed as a reactant

Facile synthesis of precious-metal single-site catalysts using organic solvents

Single-site catalysts can demonstrate high activity and selectivity in many catalytic reactions. The synthesis of these materials by impregnation from strongly oxidizing aqueous solutions or pH-controlled deposition often leads to low metal loadings or a range of metal species. Here, we demonstrate that simple impregnation of the metal precursors onto activated carbon from a low-boiling-point, low-polarity solvent, such as acetone, results in catalysts with an atomic dispersion of cationic metal species. We show the generality of this method by producing single-site Au, Pd, Ru and Pt catalysts supported on carbon in a facile manner. Single-site Au/C catalysts have previously been validated commercially to produce vinyl chloride, and here we show that this facile synthesis method can produce effective catalysts for acetylene hydrochlorination in the absence of the highly oxidizing acidic solvents previously used