Single-Atom Gold Catalyst Supported On Modified Activated Carbon For Acetylene Hydrochlorination:A Theoretical Investigation

The innovation and preparation of efficient catalysts is critical to exploit of catalytic reaction processes.Supported catalysts have better catalytic performance than traditional catalyst,such as high activity,high selectivity or both,and can be applied to industrial production on a large scale.The catalytic performance of supported metal catalysts is closely related to the size of their active component on the carriers.While decreasing the size of the supported metal particles can increase the utilization efficiency of the metal atom of the catalyst.The theoretical limit of the high dispersion of the particle size of the supported metal catalyst is uniformly in the form of single atom on the supporter,that is,the so-called single atom catalyst(SAC).The first example of SAC,namely Pti/FeOx,was reported by Zhang's group in 2011 that exhibited high catalytic activity and stability in low-temperature selective oxidation of CO.Vinyl chloride monomers(VCM)is a primary chemical feedstock to synthesize poly(vinyl chloride)(PVC),a widely used chemical material.HgCl2 is generally used to catalyze the acetylene hydrochlorination to produce VCM in industrial application.However,mercury is toxic,which can easily lead to environmental pollution,and it has begun to limit its use internationally,so it is necessary to develop non-mercury VCM catalysts.Hutchings et al.reported that gold chloride is more active than other metal chlorides in hydrochlorination.It was sooner discovered that AuCl3 supported on activated carbon(AC)is more efficient,but with lower stability.Recently,Wang et al.found that AuCl3 supported on the phosphine-oxidation-pretreated AC exhibit even higher activity and stability in catalyzing acetylene hydrochlorination,but the underlying mechanism is unclear.We therefore conducted a systematic theoretical investigation to explore the possible structures and stability of active catalytic center(s)on such AC-based VCM catalysts as AuCl.supported on the carboxylic group-containing AC and on the phosphine-oxidation-pretreated AC,and the mechanism of VCM process over these sites.A summary of the aforementioned work is given as follows:(1)AuCb-anchored by carboxylic group on AC and the related mechanism of hydrochlorination of Acetylene.A variety of oxygen-containing functional groups,especially carboxylic group,are available on AC surface that can be used to anchor single metal atoms.A recent theoretical work by Wan et al.proposed that a tri-coordinated Au(?)center in the form of-CO2-AuCl2 be the active site in catalyzing hydrochlorination of alkynes.We performed cluster-model DFT calculations to re-invatigate this catalytic system in detail,but using different structural models for the surface sites,-CO(OH)-AuCl3 and the chelate model-CO2>AuCl2.The following remarks can be drawn:(1)The structure model proposed by Wan et al.that contains a 3-coordinated Au(?)center is far less stable than the currently proposed chelate model-CO2>AuCl2,suggesting that the latter is more reasonable than the former as the active site for the VCM reaction.(2)Among the two four-coordinated Au(?)models currently concerned,the energy barrier for the rate-limiting step of the catalytic cycle over the chelate model is lower than that over the-CO(OH)-AuCl3 model.(3)The carboxylic group coordinated to Au(?)center helps to prevent the active center from being reduced to Au(??),thereby increasing the stability of the catalyst.(2)Mechanism of acetylene hydrochlorination catalyzed by AuClx supported on the AC pretreated by phosphine oxidation.A cluster model was used to study the stability and activity of AuClx supported phosphine oxide modified AC surface.At first,our cluster-model DFT calculations confirmed that the phosphine-oxidation pretreated AC can strongly anchor AuClx species to afford such species as-PO(OH)2-AuCl3 and the chelate-PO2(OH)>AuCl2 that can be active sites to catalyze acetylene hydrochlorination.Further investigate the mechanism of acetylene hydrochlorination catalyzed by these species.The computations revealed that:(1)The catalytic cycle process includes-PO(OH)-AuCl3 react with C2H2 to release HCl?formation of the chelate species-PO2(OH)>AuCl2?activated adsorption of acetylene?addition of hydrogen chloride?protodeauration? desorption of vinyl chloride.(2)The catalytic Au(?)species anchored by Phosphonic acid group can hardly be reduced to Au(?),neither can occur the Au(?)/Au(?)cycling in the hydrochlorination process recently proposed by Hutchings at al.