Preparation Of New Catalysts And Study On CO₂ Reduction

Carbon dioxide,as a greenhouse gas,has caused many environmental problems like global warming,sea level rising,and melting glaciers.The reduction of CO₂release and especially the reuse of CO₂have become a hot issue all over world.Many technologies of CO₂chemical conversion have been developed,including electrocatalytic CO₂reduction,simulated photosynthesis,and catalytic CO₂hydrogenation etc.Owing to advantages of environmental friendship,energy saving,and high efficiency etc.,both the thermocatalytic hydrogenation and photocatalytic CO₂reduction have caused more and more attentions.During the hydrogenation process,the target products could be achieved by adjusting the hydrogen-carbon ratio,reaction temperature and pressure.However,because CO₂is a very stable molecule,high temperature and pressure,together with the high hydrogen-carbon ratio for enhancing H₂adsorption and dissociation into active hydrogen atoms are necessary to achieve high-efficiency thermocatalytic CO₂hydrogenation and photocataltic CO₂reduction.In photocatalysis,it is also necessary to introduce noble metal like Pt,Pd etc.to promote the transfer of photoelectron and their separation from photo-induced holes,leading to the enhanced quantum efficiency of photocatalysis.Meanwhile,the noble metal could act as co-catalyst to promote the production of H-atoms from photocatalytic water splitting,which could further promote the photocatalytic CO₂hydrogenation.More importantly,the presence of noble metal co-catalyst could enhance the selectivity toward the target products in photocatalytic CO₂hydrogenation reactions.This work focuses on the preparation of new amorphous alloys and amorphous alloy-semiconductor nanocomposites.Their performances in thermo-and/or photo-catalytic CO₂hydrogenation/reduction reactions are studied under supercritical conditions.The research mainly covers the following aspects.（1）NiB and NiRu B ultrafine amorphous alloy catalysts were synthesized by chemical reduction under low temperature.Their performances in thermocatalytic CO₂hydrogenation were explored under supercritical conditions with low temperature and hydrogen-carbon ratio.The Ru-Ni alloying effect and the promoting effect of Ru on the CO₂hydrogenation activity were investigated and elucidated.It was found that the Ru-Ni alloy could adjust the catalyst surface electron density and the velocity of the H₂dissociation,leading to the enhanced activity and selectivity.（2）The NiRu B/Si O₂supported amorphous alloy catalyst was prepared by impregnation and the subsequent chemical reduction.Its activity and stability were examined in thermocatalytic CO₂hydrogenation under supercritical conditions.The Si O₂support could greatly promote the distribution of amorphous ally nanoparticles,together with the strong interaction with the amorphous ally nanoparticles,leading to the high catalytic activity and durability.（3）A NiB/In₂O₃amorphous alloy thermocatalyst-semiconductor photocatalyst was synthesized by impregnation and the subsequent chemical reduction.Their activity and stability were examined in photocatalytic CO₂hydrogenation reactions.The photo-excited electrons and holes generated from In₂O₃could enhance the desorption and migration rate of the hydrogen adsorbed on the catalyst surface,corresponding to the high catalytic activity.Meanwhile,they could also stabilize the NiB amorphous alloy,which could increase the catalyst lifetime.（4）A new Pt/In₂O₃/g-C₃N₄photocatalyst was synthesized by co-calcination and the subsequent photoreduction.Its performances in photocatalytic CO₂reduction were explored.The structure-function relationship between metal Pt nanoparticles and the In₂O₃/g-C₃N₄heterojunctions was investigated.The synergetic photocatalytic CO₂reduction reaction mechanism was also discussed.1.NiRu B amorphous catalyst for CO₂hydrogenation under supercritical conditionsGenerally,CO₂hydrogenation was usually conducted under high temperature and high hydrogen-carbon ratio,which may easily cause the catalyst sinter and deactivation.Meanwhile,the carbon deposition on the catalyst surface would further limit the catalyst performance.When the catalytic CO₂hydrogenation was performed under supercritical conditions,the catalyst sinter could be effectively avoided.Meanwhile,the carbon deposition on the catalyst surface could be washed clearly.Besides,owing to the coordination unsaturation and super high surface energy of NiB amorphous alloy,the catalytic CO₂hydrogenation can be realized at mild temperature and low hydrogen-carbon ratio.Furthermore,it is found that the addition of Ru results in the reduced catalyst particle size,corresponding to the increased number of exposed active sites.Meanwhile,the alloying Ru can increase surface electron density on Ni,leading to the enhanced H₂adsorption and dissociation.As a result,the as-prepared NiRu B amorphous alloy exhibited high activity and stability in thermocatalytic CO₂hydrogenation under supercritical conditions.2.NiRu B/Si O₂amorphous catalyst for CO₂hydrogenation under supercritical conditionsAlthough the amorphous alloys displayed remarkable advantages in catalytic hydrogenation reactions,they applications are still quite limited due to the easy oxidation and poor thermostability against crystallization,corresponding to the rapid deactivation.By loading amorphous alloy nanopaticles onto an inert support,we prepared a NiRu B/Si O₂.The Si O₂could highly distribute the active sites,leading to the enhanced catalytic activity in catalytic CO₂hydrogenation under supercritical conditions.Meanwhile,the supported catalyst could also decrease the use of metal and especially the noble metal,leading to the reduced cost.More importantly,the strong metal-support interaction could greatly protect the amorphous alloy from deactivation due to the oxidation and the crystallization,leading to long lifetime in catalytic CO₂hydrogenation.3.NiB/In₂O₃amorphous alloy-semiconductor for photocatalytic CO₂hydrogenation under supercritical conditionsAs a green and energy-saving catalytic method,photocatalysis will be widely used in future CO₂reduction.By designing nanocomposite with NiB amorphous alloy and In₂O₃semiconductor,we developed thermo-and photo-co-catalytic CO₂hydrogenation under supercritical conditions.During photocatalysis,the presence of NiB amorphous alloy could promote the light-harvesting by In₂O₃and the separation ability of photoelectrons from holes,leading to the enhanced photocatalyic efficiency.In other side,the photoelectrons resulted from photoexciting In₂O₃may enrich the electrons on the NiB surface,which could weaken the Ni-H bond and thus could promoted the desorption of adsorbed H-atoms.Meanwhile,it could also activate CO₂molecules for their easy reaction with the surface adsorbed H-atoms.As a result,such a catalyst exhibits high activity in CO₂hydrogenation under supercritical conditions.In addition,the rich Lewis acid active sites on the In₂O₃surface could promote the CO₂adsorption,which also favors the activity in CO₂conversion reactions.Furthermore,the photoelectrons can stabilize the NiB amorphous alloy against oxidation and crystallization deactivation,leading to the enhanced durability in thermocatalytic CO₂conversion reactions.4.Pt/In₂O₃/g-C₃N₄visible-photocatalytic CO₂reduction to HCOOHThe single semiconductor usually displays the inferior separation of photogenerated electrons and holes,corresponding to the low quantum efficiency in photocatalysis.Meanwhile,such a photocatalyst disfavors either the activation of CO₂molecules or the production of active H-atoms from photocatalytic water splitting.By constructing In₂O₃/g-C₃N₄heterojunction,the separation of photogenerated carriers could be greatly improved,leading to the enhanced efficiency in photocatalytic CO₂reduction reactions toward HCOOH product.Meanwhile,the existence of Pt nanoparticles also facilitates the electron transfer in the photocatalyst and also promoted the production of active H-atoms from photocatalytic water splitting,which further promotes the photocatalytic CO₂reduction.Furthermore,the unique composition and surface structure may favor the selectivity to the target product HCOOH during photocatalytic CO₂ reduction.