CO₂ Hydrogenation To Methanol On The Indium Oxide Supported Ag And Pt Catalysts

With the development of hydrogen production technology via renewable energy,methanol synthesis from CO2 hydrogenation has attracted extensive attentions recently.The development on catalyst for CO2 hydrogenation to methanol with high selectivity and high activity has become the hotspot in catalysis research.The metal-supported indium oxide catalysts exhibit the excellent catalytic activity to methanol resulting from the oxygen vacancy for CO2 activation and the metal site for H2 activation,which can enhance the activity of CO2 hydrogenation to methanol over indium oxide as well as can improve the catalyst stability.Additionally,the metal-supported indium oxide catalyst is an excellent system to study the relationship between electronic structure and catalyst performance.The purpose of this thesis is to confirm the presence of strong interaction between indium oxide and silver or platinum experimentally,which makes the supported silver and platinum catalysts have high activity for CO2 hydrogenation to methanol.Since the supported silver and platinum catalysts are generally inactive for CO2 hydrogenation to methanol on other supports,the interaction mechanism of indium oxide with silver or platinum and the reaction pathway of CO2 hydrogenation to methanol with related catalysts are also studied by theoretical calculation in this paper.The results of density functional theory calculations indicate that the interfacial site of defective Ag4/1n2O3 model facilitates the activation and dissociation of CO2 with an activation barrier of 0.41 eV.The electron transfer between metal and support leads to the formation of a positively charged Ag4 cluster,which is favorable for H2 activation.The synergy between the supported Ag4 cluster and oxygen vacancy facilitates methanol production via the CO hydrogenation route.The Ag/In2O3 catalyst,prepared by the deposition-precipitation method,shows the enhanced activity of CO2 hydrogenation to methanol.CO2 conversion reaches 13.6%and methanol selectivity reaches 58.2%with the methanol space-time yield of 0.453 gmethanol gcat-1 h-1 at 300℃and 5 MPa.The results of characterizations indicate that the introduction of Ag not only creates a new active site for CO2 activation and dissociation but also suppresses the over-reduction of indium oxide.Therefore,the catalyst stability under the reaction conditions is improved.The reaction mechanism of methanol synthesis from CO2 and H2 over the defective Pt4/In2O3 model is investigated via density functional theory calculations.The presence of oxygen vacancy can enhance the interaction between the supported Pt4 cluster and In2O3,resulting in the improved stability of the supported Pt4 cluster.The interfacial site facilitates the activation and dissociation of CO2 with an activation barrier of 0.18 eV.CO hydrogenation route is the optimal-energy route for methanol synthesis due to the synergy between supported Pt4 cluster and oxygen vacancy.The Pt/In2O3 catalyst,prepared by the deposition-precipitation method,shows the superior activity of CO2 hydrogenation to methanol.CO2 conversion and methanol selectivity reach 17.3%and 54%,respectively,with the methanol space-time yield of 0.542 gmethanol gcat-1 h-1 at 300℃ and 5 MPa.The results of characterization indicate that the Pt nanoparticles are well-dispersed on In2O3 support with an average size of 1.47 nm.The Pt nanoparticles can enhance the H2 activation significantly,which can provide the active H atoms and promote the formation of oxygen vacancy of In2O3.Moreover,the strong interaction of Pt-In2O3 improves the stability of oxygen vacancy and inhibits the over-reduction of indium oxide,leading to the enhanced stability of catalyst under the reaction conditions.