Study On The Relationship Between The D Charge At Pd Sites And Performance Of Pd-based Catalysts

The chemical industry produces a variety of important chemicals,materials and daily necessities,and has made great contributions to human civilization and social progress.However,many traditional chemical production processes are accompanied by serious waste of resources and cause pollution to the environment.How to protect the environment and achieve sustainable development is a major challenge.Green chemistry is committed to eliminating pollution from the source,including the use of green catalysts,green solvents,and the use of synthetic routes with high utilization of raw materials to save resources and energy,achieve sustainable development,and be environmentally friendly.In a word,atomic economic reaction is an important way to realize green chemistry,and the key is the development of high-performance green catalysts.Pd catalysts are widely used in fine chemical synthesis,medicine and other fields.However,the traditional carbon-supported noble metal catalysts use activated carbon as the carrier,and the development of the supported metal catalysts is mostly based on the trial-and-error method,showing the major issues of homogenization,low value,heavy environmental load,low energy efficiency and resource bottleneck.The challenge for the design and optimization of high-performance Pd catalysts is the determination of catalytic activity descriptors.The d-band center descriptors of transition metals based on theoretical calculations encounter difficulties in guiding the controllable preparation of high-performance carbon-supported metal catalysts.Therefore,we have been seeking a simple and experimentally measurable descriptor that quantitatively correlates the surface electronic structure to the catalytic activity for transition metal alloy nanocatalysts.The research work of this paper focuses on ordered mesoporous carbon-confined Pd-based catalysts with clear electronic structure,and explores the quantitative relationship between the electronic structure of the catalyst and the reaction activity.It is proposed and confirmed that the experimental measurement of d charge can be used as the catalytic activity descriptor of the Pd catalyst.The d charge is used as the activity descriptor of the experimental measurement,which provides a research foundation for the rational design and optimization of high-performance catalysts.The main results are as follows:(1)A strategy of inducing surface atomic rearrangement by adsorption is proposed,which breaks through the thermodynamically stable Au shell Pd core structure,and obtains a mesoporous carbon-supported Au Pd alloy nanocatalyst with uniform size.Through high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),XANES and other technologies,the Au Pd alloy structure was confirmed at the atomic level,excluding the eutectic or core-shell structure.By adjusting the content of Au,the Pd-Au coordination number can be precisely adjusted.XANES results show that when the Au content is greater than 50 at%,the Pd in the alloy catalyst exists in the form of single atoms or diatoms.Using XPS and XANES techniques,the d-and non-d-charged charge on the active site of Pd was measured,and the d charge of the Pd site was obtained.As the Pd content increases,the d charge of Pd active sites presents a volcanic curve,and the d charge of the Au Pd alloy catalyst with a Pd content of 33-50 at%reaches the maximum.(2)The activity and selectivity of the Au Pd alloy catalyst in the alkali-free selective oxidation of benzyl alcohol as a strong adsorbent were investigated.With the increase of Pd content,the TOF_Pd value of Au Pd catalyst exhibits obvious volcanic characteristics.The TOF_Pd value of Au₅₀Pd₅₀ can reach 370 h^-1,which is 9 times the TOF value of single Pd nanoparticles.The Au₅₀Pd₅₀ and Au₆₇Pd₃₃ catalysts still show nearly 100%benzaldehyde selectivity at high conversion rates(90%).We attempt to relate the d charge gain to the activation entropy(?S^0*)and turn over frequency(TOF_Pd).The change indeed follows a linear relationship with the d charge at the Pd sites.We have identified the d charge at the Pd sites as a descriptor to predict entropy changes and activities in the oxidation of alcohols.The decisive influence of the d orbitals not participating in the metal bond in the hybrid orbital on the stabilization energy and catalytic activity of the adsorption complex is clarified.(3)Further expand the scope of application of the d charge descriptor.In the strong adsorbate quinoline hydrogenation system,the effect of metal surface segregation,aggregation and loss on the reaction activity is first eliminated.With the change of Pd content in Au Pd alloy catalyst,TOF_Pd and?S^0*present typical volcanic curves.Among them,Au₅₀Pd₅₀ and Au₆₇Pd₃₃ catalysts have the highest TOF_Pd and the largest negative?S^0*,while maintaining nearly 100%1,2,3,4-tetrahydroquinoline selectivity.We also attempt to relate the d charge gain to the activation entropy(?S^0*)and Turnover frequency(TOF_Pd).The change indeed follows a linear relationship with the d charge at the Pd sites.The experiment confirmed that the d charge is an appropriate descriptor for the activity of the catalyst.(4)Based on the d charge descriptor,the order mesoporous carbon-supported Pd interstitial solid solution catalyst was prepared by using the carbon atom reverse doping strategy.Through HAADF-STEM,XANES and other technologies and density functional theory(DFT)calculations,it is confirmed that the interstitial C atoms are located in the Pd octahedral gap.The interstitial carbon atoms significantly changed the electronic structure of Pd,and further modified with N atoms,the d charge gained to0.29 e.The selective hydrogenation reaction of quinoline was used to investigate the catalytic activity and selectivity of the interstitial solid solution catalyst.The TOF of C,N-Pd is as high as 185 h^-1,which is 3 times that of commercial Pd/C catalysis,and it is almost 100%converted into 1,2,3,4-tetrahydroquinoline.At the same time,the interstitial solid solution catalyst exhibits excellent anti-poisoning and anti-loss performance,and even after 6 cycles,it still maintains an activity equivalent to that of a fresh catalyst.The d charge is further correlated with the TOF_Pd,?S^0*,and the change indeed follows a linear relationship with the d charge at the Pd sites.Using the d charge descriptor,a high-performance mesoporous carbon-supported Pd catalyst was created.