Construction Of Pd Nanocubes-Loaded N-Doped Carbon Microspheres For Selective Alkyne Semi-Hydrogenation

Catalytic hydrogenation reaction is the core pillar of chemical industry,which is also one of the most popular research directions in basic scientific research.However,metal catalysts are generally too active to allow tuning of their selectivity,such as selective semi-hydrogenation of alkynes,selective hydrogenation of?,?-unsaturated aldehydes,and selective oxidation of alcohols.Poisoning,tuning of the steric effects and electronic effects,and specific molecular recognition have been used for solving the selectivity of nanocatalysts.However,the selectivity improvement is often accompanied by a significant sacrifice of catalyst activities.From the perspective of“green”chemistry,improving the utilization of active metal atoms and developing more efficient and highly selective heterogeneous catalytic engineering is of strategic importance for conducting sustainable chemistry.Herein,we developed a facile route to synthesize Pd nanocubes(NCs)via visible light-photoredox catalysis.Subsequently,these Pd NCs enclosed with uniform{100}facets were chosen as the model catalysts and loaded on N-doped carbon nanofibrous microspheres(NCMs)prepared from carbonated chitin microspheres.Based on the Mott-Schottky effect,NCMs with different electron structures as supporting media were used to regulate the charge density of Pd NCs,thereby adjusting the performance of selective alkyne hydrogenation.Meanwhile,the structure-performance relationship between the charge density of Pd NCs and its catalytic activity as well as selectivity in alkyne semi-hydrogenation was also discussed.A series of characterization methods were applied to study the morphology,structure and deformation charge density of Pd NCs@NCMs,they included scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray absorption spectroscopy(XAS),X-ray photoelectron spectroscopy(XPS),UV-photoelectron spectroscopy(UPS),H₂ temperature programmed desorption apparatus.Next,the Pd NCs hole concentration dependence of hydrogen adsorption energy and the influence of both hole concentration and H coverage of Pd NCs on the adsorption energy of phenylacetylene and styrene were examined via density functional theory calculations(DFT)to understand fundamental processes.Then,the size effects of Pd nanoparticles(NPs)on selective alkynes hydrogenation over Pd NCs@NCMs was studied.Finally,the applicability of Pd NCs@NCMS to the semi-hydrogenation of various terminal alkynes and internal alkynes was also investigated.The innovation of this thesis was listed as follows:(1)Developing a"green"visible light-photoredox catalysis for Pd NCs controllable synthesis in different sizes.(2)By using Pd NCs enclosed with uniform{100}facets as a model catalyst,the influence of size and morphology on the strong metal-support interactions(SMSIs)is avoided.(3)Owing to their uniform N heteroatom distributions,meso-/macropore hierarchical structures and large specific surface areas,three-dimensional N-doped carbon nanofibrous microspheres(NCMs)from carbonated chitin microspheres were fabricated to be used an ideal catalyst support.(4)Based on the Mott-Schottky effect at Pd/N-doped carbon interface,with the decrease of Pd charge densities,its catalytic selectivity in alkynes semi-hydrogenation was improved.(5)The construction of Mott-Schottky catalysts provides a new dimension to design efficient catalysts for selective hydrogenation reactions.The content and conclusions of this full text are described as below:It is noted that Pd catalysts generally require an elevated temperature or strong ultraviolet irradiation.We reported a novel route of synthesizing Pd NPs via facile visible?light photoredox catalysis for the first time,that Eosin Y(EY)was aroused to excited states(EY^*)and reductively quenched by several mild reductants,such as Et₃N,TEOA,and AA.It was followed by the generation of a strongly reduction state(EY^·-),and finally Pd²⁺/PdBr₄^2-reduced to zerovalent Pd metals.Consequently,Pd nanocubes were synthesized by using Br^-as the capping reagent in the presence of AA.And the sizes in the range from 7 to 374 nm could be modulated by changing the Br^-concentration or the composition of H₂O/CH₃CN solution.With the amount of KBr increasing,the effects of protection of the seeds by both PVP and Br^-from the size growth are larger than that decreasing the nucleation speed,eventually resulting in decreasing the size of Pd NCs.From a thermodynamic process perspective,with the increase of the ratio of CH₃CN in H₂O/CH₃CN solution,it would slow down the reduction of PdBr₄^2-to zerovalent Pd atoms and thus decrease the size of Pd NCs.Recently,strong metal-support interactions(SMSIs)have been reported having a significant effect on the electronic properties of metal species,which further influenced their catalytic behavior.NCMs having a p-type semi-conductive structure were used as supports to induce the electron transfer of Pd NCs to NCMs,which adjusted their performance of selective alkyne hydrogenation.The results of XPS,UPS,and XANES showed that the electron density of Pd NCs was transferred to the pyridine and pyrrole N of NCMs.According to the Mott-Schottky effect,loading of Pd NCs on N-doped carbon constructed a rectifying contact and decreased the electron density of Pd NCs.Thus,this Pd NCs@NCMs exhibited high reactivity and selectivity in alkyne semi-hydrogenation.For example,the hydrogenations of phenylacetylene to styrene and of 3-phenyl-2-propyn-1-ol to(Z)-cinnamyl alcohol with Pd NCs@NCMs were 12.9 and 18.3 times faster than that with Lindlar catalyst.Density functional theory(DFT)calculations results showed the hydrogen adsorption energy prominently diminished with increasing the hole concentration of Pd NCs,which leading to a decrease in the hydrogen coverage on Pd(100)facets.When the hydrogen coverage was less than 0.50,the adsorption energies of PhA maintained larger than those of St,indicating that the incoming PhA could replace the St adsorbed on Pd(100)facets,consequently hampering further hydrogenation.The roles of the Schottky effect and the size effect on the catalytic performance of alkynes semi-hydrogenation interplayed and competed with each other.To clarify further,5.7-nm Pd NPs@NCMs,8.8-nm Pd NCs@NCMs,16.7-nm Pd NCs@NCMs,and 23.5-nm Pd NCs@NCMs were prepared by using NCMs as the substrates in the test.The results showed that the selectivity of phenylacetylene semi-hydrogenation was increased but its reactivity was decreased with the larger Pd NPs size.There was on big difference in Schottky effect on the three sizes of Pd NCs@NCMs.The found variance in their catalytic activity and selectivity is caused by the different numbers of surface active sites of Pd NCs,which is called as geometric effects,rather than the electron effects.As the size of Pd NPs increased,the number of low coordination active sites declined,which improved the selectivity but decreased its reactivity.However,in the comparison with 23.5 nm Pd NCs@NCMs,the selectivity of phenylacetylene to styrene over 23.5 nm Pd NCs without any substrates was 47.9%with a full conversion in 5 hrs,and the over-hydrogenation of alkynes occurred without any modulation of the Mott-Schottky effect.Hence,the Mott-Schottky effect on the Pd/N-C interface has the dominant role of improving the selectivity of Pd NPs.To demonstrate the universality of the Mott-Schottky catalysts for selective hydrogenation reactions,16.7 nm Pd NCs@Cu₂O NCs was designed for phenylacetylene semi-hydrogenation,in which Cu₂O NCs was typical p-type semiconductors.The Cu₂O weaken the catalytic performance of Pd NCs but resulted in high selectivity.It played the same as that of p-type semiconductive N-doped carbon.Thus,this study provides an alternative strategy to design Mott-Schottky catalysts for selective hydrogenation reactions.Out of the above NCMs,N-doped carbon microspheres with different electronic structures were generated as the support of 16.7-nm Pd NCs,and used for regulating the charge density of Pd NCs.The results revealed that the conversion over Pd NCs@NCMs400 reached 100%in 3 hrs,but its selectivity of phenylacetylene to styrene was only 52.1%.The conversion over Pd NCs@NCMs750 reached 100%after 6 hrs with its selectivity of 87.1%.Pd NCs@NCMs900 had a conversion rate of100%and the selectivity reached 91.0%in 6 hrs,and its selectivity remained at 90%even after the reaction time was extended to 24 hrs.Namely,the catalytic activity of Pd NCs@NCMs decreases,but leading to the significant increase of its selectivity with the increase carbonation temperature.XPS and UPS results showed that as the carbonization temperature was higher,the work function of NCMs increased,and the difference in the work function between Pd NCs and NCMs rose up accordingly.As the result of more electrons was transferred from Pd NCs to NCMs.The finding is consistent with those calculated by DFT in the third chapter.This nanocomposites catalyst could achieve high catalytic selectivity of phenylacetylene to styrene.And,Pd NCs@NCM750 exhibited good applicability to the semi-hydrogenation of various terminal alkynes and internal alkynes.The work of dissertation constructed chitin-based N-doped carbon nanofibrous microspheres(NCMs)by using cooling alkali/urea aqueous system.NCMs with different electronic structures was obtained by controlling its carbonized temperature and also used as the support to adjust the charge density of Pd NCs.Therefore,the structure-performance relationship between the charge density of Pd NCs and their catalytic activity as well as the selectivity in alkyne semi-hydrogenation was clarified.The work opened up a new route to utilize N-doped carbon derived from chitin as a support for an improvement in the stability and homogeneous dispersion as well as selectivity of metal nanocatalysts.It was illustrated the catalytic mechanism of the Mott-Schottky catalyst for the selective alkynes hydrogenation.The new construction of Mott-Schottky catalysts provides another dimension to design a new generation of industrial catalysts for selective heterogeneous catalysis,biomass catalytic conversion and petrochemical refining process.Thus,the study exhibits obvious innovation,academic significance and application prospects,which is in accordance with national sustainable development strategy.