氧化銦基納米催化劑用于二氧化碳選擇性加氫的研究進展

劉瀚林，尹琳琳，陳西鳳，李國棟，3

（1.中國科學院納米系統(tǒng)與多級次制造重點實驗室,中國科學院納米科學卓越創(chuàng)新中心，國家納米科學中心,北京100190；2.鄭州大學化學學院,鄭州450001；3.中國科學院大學納米科學與技術學院,北京100049）

1 Introduction

Since the industrial revolution，carbon dioxide（CO2）as the main greenhouse gas has continued rising fromca. 0.028%（molar fraction）in the 1800s toca. 0.041% nowadays due to the excess utilization of carbon-rich fossil fuels［1，2］. This has gradually disrupted the balance of the carbon cycle and caused serious and irreversible environmental problems such as the global warming，ocean acidification，climate change and so on，posing great threats to the Earth’s ecosystems［3］. Although so，CO2is also regarded as a rich，cheap and renewable resource of carbon［4］. Developing effective strategies and advanced technologies to mitigate CO2emissions have attracted great attention in the past decades，containing CO2capture，storage and utilization.Among these modes，the chemical conversion of CO2into useful platform chemicals seems to be a more attractive and promising solution to fulfill the requirements of sustainable processes and renewable carbon sources［5］.

It is well known that the carbon atom in CO2is in its highest oxidation state，and the dissociation energy of C=O bond in CO2（803 kJ/mol）is much higher than the energy of C—C（336 kJ/mol），C—O（327 kJ/mol）or C—H（441 kJ/mol）bonds［6］. To date，the transformation of CO2into valuable products can be achieved through selecting high-energy co-reactants，such as unsaturated compounds，hydrogen，small-membered ring compounds，viathermal catalysis，electrocatalysis or photocatalysis［7—10］. Among them，the selective hydrogenation of CO2is considered as an outstanding way to attain valuable products for the relatively mature research system［11］.

Generally，catalytic CO2hydrogenation reaction can be simply divided into three possible modes according to the initially formed intermediate（Fig.1）：（1）CO2can be hydrogenated into formate intermediate（*HCOO），named formate pathway；（2）carboxyl intermediate（*COOH or *HOCO）is first generated and then hydrogenated into carbonyl intermediate（*CO），named RWGS（reverse water-gas shift）pathway；（3）C—O cleavage of CO2occurs directly to form *CO，but this pathway shows an excessively high energy barrier and is hardly favorable［12，13］. Notably，the hydrogenation of CO2into hydrocarbons and alcohols is exothermic，while RWGS reaction is endothermic. Therefore，rationally controlling the reaction condition is an important role in balancing the catalytic activity and selectivity of target products.

It is noted that the obtained products from CO2hydrogenation can be classified into two categories：C1 and C2+products. The C1 products，such as methane（CH4），carbon monoxide（CO），methanol（CH3OH）and formic acid（HCOOH），are mainly platform molecules，which can be used as renewable materials or further converted into value-added fuels and chemicals［14，15］. Compared with C1 compounds，C2+products ordinarily own higher economic value and energy density. Moreover，the C2+products are easily marketable fuels and chemicals［16］，such as light olefins，liquid fuels，higher alcohols，etc. However，owing to the high C—C coupling energy barrier and numerous competitive reactions，the highly selective synthesis of specific C2+products is more difficult and challenging［17］.

To effectively activate CO2and H2and regulate the product selectivity，it is of great importance to design and fabricate high performance nanocatalysts. Until now，many kinds of heterogeneous nanocatalysts have been developed for hydrogenation of CO2into diverse products［18—23］. Among them，In2O3based nanocatalysts have aroused great interest［24，25］，because of the unique sensitivity to oxidizing，reducing，acidic and basic reactants［26］. Both theoretical and experimental results have indicated that In2O3based nanocatalysts possess the promising catalytic performances for selective CO2hydrogenation into diverse products［27，28］.

To better guide the design on In2O3based nanocatalysts for CO2hydrogenation，it is necessary to summarize the recent progresses and challenges in this field. First，we discuss the design of In2O3samples with different crystal phases and modified In2O3with metal oxides or metal nanoparticles for hydrogenation of CO2into C1 products. Then，we discuss the selective hydrogenation of CO2into C2+hydrocarbons by In2O3combined with zeolites. Finally，we propose the emergent challenges and future developments of selective hydrogenation of CO2over In2O3based nanocatalysts.

Fig.1 Reaction modes of hydrogenation of CO2 into various products[12]

2 Selective Hydrogenation of CO2 into C1 Products

2.1 Single In2O3 Nanocatalysts

Single In2O3samples possess great potential for catalyzing CO2hydrogenation，due to the presence of abundant oxygen vacancies on the surfaces of In2O3that act as the active sites for not only the adsorption and activation of CO2，but also the adsorption and dissociation of H2［26］. Geet al.［29］discussed the adsorption and hydrogenation of CO2on the surface of In2O3through density function theory（DFT）calculation，proving that In2O3favored the formate pathway，and methanol was regarded as a favorable product for CO2hydrogenation reaction（Fig.2）. Liuet al.［30］prepared the In2O3nanocatalyst by simply calcining the commercial cubic In2O3powders in air. The obtained products were characteristic of 25—30 nm in diameter. When they were used as catalyst for CO2hydrogenation，the possible products including methanol and CO were detected.Furthermore，a methanol space-time yield（STY）of 0.78 mmol··h-1was achieved at 270 ℃under 4 MPa［n（H2）∶n（CO2）∶n（N2）=3∶1∶1］，while 3.69 mmol··h-1of methanol was obtained at 330 ℃. However，the selectivity of methanol reached 54.9%at 270 ℃but declined to 39.7%at 330 ℃. In comparison，pure Al2O3or Ga2O3showed no activity for hydrogenation of CO2into methanol because of the lower H2dissociation capability compared with In2O3. This is the first report for a single metal oxide showing catalytic activity for CO2hydrogenation. Pérez-Ramírezet al.［31］prepared single In2O3nanocatalysts by combining precipitation with calcination，which were characteristic of 8 nm in diameter，surface area of 123 m2/g and a total pore volume of 0.38 cm3/g. The catalyst exhibited a methanol STY of 6.88 mmolfor CO2hydrogenation at 300 ℃.Characterization results showed an obvious increase in particle size of In2O3from 8 nm to 18 nm and a decrease in specific surface area from 123 m2/g to 47 m2/g，indicating that sintering of In2O3occurred during the hydrogenation process.

Fig.2 Potential energy profiles of CO2 hydrogenation on the defective (110) surface of In2O3(A),a possible reaction route for CO2 hydrogenation into methanol(B)and two different types of oxygen vacancies on defective(110)surface of In2O3(C)[29]

Fig.3 Characterizations and catalytic tests of c-In2O3 and h-In2O3[33]

The crystal structure of In2O3is also an important factor that influences the catalytic performances on CO2hydrogenation reaction. For example，Guoet al.［32］prepared the cubic bixbyite-type In2O3（c-In2O3）and rhombohedral corundum-type In2O3（rh-In2O3）for catalyzing CO2hydrogenation. The respective average diameters of c-In2O3and rh-In2O3were 17.9 nm and 24.9 nm，respectively，and XPS spectra revealed that the oxygen vacancy concentration was 24.5% and 21.2%. When used as catalysts，c-In2O3showed a higher methanol STY up to 3.0 mmolat 340 ℃under 4 MPa with a H2/CO2molar ratio of 4，but lower methanol selectivity of 20% was observed with CO and CH4as byproducts. In contrast，as for rh-In2O3，the methanol STY was 1.8 mmol，while methanol selectivity reached up to 30%，since the only byproduct CO was observed under the same condition. Temperature-programmed reduction in hydrogen（H2-TPR）indicated a higher reducibility of c-In2O3than rh-In2O3，which favored generation of more oxygen vacancies to activate CO2. In addition，another type of In2O3with hexagonal structure（h-In2O3）was prepared for CO2hydrogenation along with c-In2O3［33］. Transmission electron microscopy（TEM）and high resolution transmission electron microscopy（HRTEM）images showed the spherical c-In2O3nanoparticles ofca. 8 nm in size，as well as h-In2O3nanorods of 36 nm in diameter and 200 nm in length（Fig.3）. Methanol and CO were produced over both c-In2O3and h-In2O3. It was noted that when the temperature rose from 240 ℃to 360 ℃，an increase in CO2conversion from 3.2%to 15.4%and a decrease in methanol selectivity from 97.9%to 73.4%for h-In2O3were achieved. As for c-In2O3，a higher CO2conversion from 5.2%to 27.3%was obtained in the same temperature range，but methanol selectivity decreased significantly from 95.9%to 17.8%. The methanol STY over h-In2O3at 360 ℃reached 10.9 mmol··h-1，twice higher than over c-In2O3. Moreover，a methanol STY value of 6.2 mmol··h-1was maintained at 300℃during 136 h on stream. DFT calculation proved that the oxygen defects on exposed（104）surface of h-In2O3disfavored CO formation，and a strong adsorption stability of *CH3O over h-In2O3（104）surface was revealed by diffuse reflectance infrared fourier transformations（DRIFT）spectra.

The catalytic stability of In2O3could be improved by introducing specific supports during CO2hydrogenation. Lu and co-workers［34］grew h-In2O3onto thin-felt Al2O3/Al-fiber by a facile mix-solvothermal method.H-In2O3（ca. 12 nm）was dispersed on microfibrous Al2O3/Al-fiber surface. A methanol STY of 6.25 mmol·and methanol selectivity of 67.6% were obtained at 325 ℃. Most importantly，the catalytic activity remained almost unchanged over 200 h on stream，and when the temperature rose to 350 ℃，a similar stability was observed. TEM images and X-ray diffraction（XRD）results showed the maintenance of h-In2O3crystal phase and crystallite size distribution，indicating that the microfibrous structure contributed to the stabilization of h-In2O3phase.

Besides direct thermo-catalytic CO2hydrogenation，In2O3is also regarded as a promising photocatalyst for CO2hydrogenation，because of the unique optical and electronic properties to generate frustrated Lewis pairs（FLPs）on the surface of In2O3［35］. Ozinet al.［36，37］studied the photocatalytic CO2hydrogenation over pristine In2O3and defective In2O3-x，In2O3（OH）yand In2O3-x（OH）y. CO2was hydrogenated with a decreased energy barrier in the presence of light compared with in the dark，especially on the defective surface of In2O3-x（OH）y，due to the FLPs excited by light-induced electronic transitions at the defective sites. The frustrated electronhole pairs are conductive for the adsorption and heterolytical dissociation of H2，and thus promote the hydrogenation of adsorbed CO2on the oxygen vacancies. In addition，a photocatalyst with heterostructure consisting of c-In2O3and rh-In2O3was prepared for CO2hydrogenation［38］. Both CO and methanol were formed and the STY was 0.092 and 1.12 mmol，respectively，during a continuous reaction of 50 h. TEM images indicated that a phase junction was established between rhombohedral and cubic In2O3-x（OH）ypolymorphs，which could facilitate the generation and separation of photogenerated charge carriers. Also，the defective states in In2O3-x（OH）yphase trapped the photogenerated electrons and holes and promoted the photocatalytic activation of H2and CO2.

In a word，single In2O3with different crystalline structures has exhibited the catalytic capability for CO2hydrogenation into C1 products. However，it is still highly desirable for developing effective strategies to tune the defective sites on In2O3for achieving high catalytic efficiency.

2.2 Hybrid Nanocatalysts of In2O3 and Other Metal Oxides

Though single In2O3could catalyze CO2hydrogenation，there are still more rooms for improving their catalytic performances. Recently，zirconia（ZrO2）was introduced to enhance the catalytic performance of In2O3for CO2hydrogenation［39］. For example，Pérez-Ramírezet al.［26］prepared monoclinic ZrO2supported c-In2O3as catalyst for CO2hydrogenation，in which the average crystal size of In2O3was 11 nm，and the mass-loading of indium was 9%（mass fraction）. It was noted that both c-In2O3and In2O3/ZrO2showed the methanol selectivity of nearly 100%at 300 ℃. Moreover，In2O3/ZrO2exhibited a methanol yield ofca. 10 mmol··h-1，far higher than ＜1 mmol··h-1over In2O3unsupported or on other carriers such as TiO2. The excellent performance of In2O3/ZrO2could be ascribed to the case that an electronic promotion of In2O3by ZrO2occurred，that is，reduced Zr centers abstract O atoms from In2O3to create more oxygen vacancies. Also，the catalytic activity of In2O3/ZrO2only decreased by 8%after 1000 h on stream. The crystallite size and surface area of In2O3phase in In2O3/ZrO2remained unchanged after reaction，suggesting that ZrO2also prevented the sintering of In2O3and thus avoided the deactivation.

The crystalline structure of the metal oxide supports can influence the catalytic activity of In2O3for CO2hydrogenation. For example，Pérez-Ramírezet al.［40］studied the catalytic properties of monoclinic（m-）and tetragonal（t-）ZrO2supported In2O3for CO2hydrogenation. The crystallite size of m-ZrO2and t-ZrO2were 9 and 11 nm respectively，and In2O3/t-ZrO2held a higher specific surface area of 162 m2/g than 100 m2/g of In2O3/m-ZrO2. However，In2O3/m-ZrO2exhibited a methanol STY of 8.7 mmol·g-1cat·h-1，8—10 times higher than In2O3/t-ZrO2catalyst. Kinetic analyses suggested that the oxygen vacancies on m-ZrO2surface contributed to the activation of CO2on defective In2O3/ZrO2surface，which led to a higher catalytic activity；in contrast，t-ZrO2had less oxygen vacancies and was negative in CO2adsorption.

Fig.4 Catalytic tests and possible reaction pathways of CO2 hydrogenation over In2O3/ZrO2 with different indium mass-loading[41]

In addition，the dispersion of In2O3phase on ZrO2affects the catalytic properties for CO2hydrogenation.Lattice mismatching leads to a lower dispersion of In2O3，and triggers the formation of more oxygen vacancies to better activate the reactants［40］. For example，Hanet al.［41］reported In2O3/ZrO2catalysts with a variety of indium mass-loading as the catalysts for CO2hydrogenation. Highly dispersed indium species were found in In0.1/ZrO2（In2O3/ZrO2with 0.1%indium mass-loading），while a clear aggregation of indium species with a size ofca. 20 nm was observed in In5/ZrO2. Catalytic results indicated that methanol and CO were detected over these catalysts at 280 ℃under 5 MPa［n（H2）∶n（CO2）=4∶1，with 25%N2］（Fig.4）. In5/ZrO2showed a methanol selectivity of approximate 70%，10% higher than bulk In2O3under the same condition. In contrast，CO was the main product over In0.1/ZrO2catalyst and a methanol selectivity of only 20% was achieved under the same conditions. It was proved by temperature-programmed desorption（TPD）andin-situDRIFT experiments that the adsorption strength of the key intermediate*HCOO on different In-Zr interfacial sites caused different product selectivities. Highly dispersed InOxon the surface of In0.1/ZrO2led to an excessively stable adsorption of *HCOO intermediate，which hampered its further hydrogenation into methanol. Conversely，moderate adsorption strength over In5/ZrO2favored an optimized selectivity towards methanol.

The catalytic performance of In2O3/ZrO2could be further enhanced by introducing other promoters. For example，Loboet al.42］modified In2O3/ZrO2catalyst with yttrium and lanthanum promoters. The BET surface area and pore volume of the un-promoted In2O3/ZrO2wasca. 87 m2/gand 0.22 cm3/g，while the introduction of Y or La did not change these properties. Both the unmodified and Y or La modified In2O3/ZrO2catalysts exhibited a similar methanol STY of around 14 mmol·at 300 ℃with CO as byproduct，but the Y and La modified catalysts showed the methanol selectivity of 69% and 66%，respectively，higher than 53% over unmodified In2O3/ZrO2. DFT and FTIR studies indicated that the incorporation of Y3+and La3+at In sites on the surface of In2O3/ZrO2affected the O—In bond energy and the surface reducibility，which suppressed the formation of*HOCO intermediateviaRWGS process，and thus enhanced the methanol selectivity.

In addition，metal oxides such as Nb2O5can enhance the photocatalytic efficiency of In2O3for CO2hydrogenation. For example，Ozinet al.［43］uniformly grew In2O3nanocrystals with an average size of 4 nm on Nb2O5nanorods as a heterostructured photocatalyst for CO2hydrogenation. The yield of CO reached 1.4 mmol··h-1under solar condition，44-fold higher than pristine In2O3，and moreover，the activity remained stable during 10 cycles. O1sXPS spectra revealed that the introduction of Nb2O5led to more defective interfaces and increased the concentration of oxygen vacancies on In2O3-x（OH）yphase，thus leading to the improved photocatalytic performance.

Altogether，modifying In2O3with oxide promoters such as ZrO2and Nb2O5is proved to boost the catalytic performances for CO2hydrogenation，due to the electronic and interfacial interactions between In2O3and the promoter. In addition，other kinds of metal oxides are required to combine with In2O3as catalysts for generating novel and intriguing interfacial interaction and achieving high catalytic performances.

2.3 In2O3-supported Metal Nanocatalysts

It should be pointed out that single In2O3or mixed oxides possess insufficient capability for adsorbing and splitting of H2，thus inhibiting the CO2hydrogenation. Transition metals often exhibit splendid ability to provide active hydrogen species in reduction reactions［44］. Therefore，In2O3catalysts modified with metallic active sites are developed to enhance the catalytic performances for CO2hydrogenation.

Palladium（Pd）catalysts are commonly used in hydrogenation reactions due to the excellent capability to adsorb dissociative hydrogen，so researchers attempt to modify In2O3with Pd. For example，Liuet al.［45］mixed In2O3powders with Pd/peptide composite and obtained a Pd/In2O3catalyst after post-treatment with an average Pd particle size of 3.6 nm. This catalyst showed the methanol STY of 27.8 mmol·with the CO2conversion ＞20%and methanol selectivity ＞70%with CO as byproduct at 300 ℃under 5 MPa with a H2/CO2ratio of 4. XPS and Raman spectra revealed that the ability of Pd nanoparticles to provide active hydrogen adatoms facilitated the formation of more oxygen vacancies on the surface of In2O3，and thus both the activation of CO2and the dissociation of H2were promoted. Pérez-Ramírez and co-workers［46］also emphasized the effect of the dispersion of Pd on the surface of In2O3on CO2hydrogenation. A Pd/In2O3catalyst characteristic of Pd sites atomically dispersed on In2O3by co-precipitation（CP）method was tested，along with a reference Pd/In2O3prepared by dry impregnation（DI）method. XRD results indicated that the average particle size of unmodified In2O3was 9 nm，but they were increased to 15 nm and 25 nm for both samples，respectively. During the catalytic tests，the Pd/In2O3（CP）catalyst maintained a methanol STY of 19.1 mmol··h-1over 94 h on stream at 280 ℃under 5 MPa with a H2/CO2ratio of 4，and the selectivity of methanol reached 97%compared with 89%for bulk In2O3，producing CO as byproduct. In contrast，the methanol selectivity over Pd/In2O3（DI）catalyst was only 78%，and half of the activity was lost after 74 h on stream. HRTEM revealed that Pd atoms aggregated into 2.8 nm nanoparticles in Pd/In2O3（DI）catalyst after reaction and the average size of In2O3increased up toca. 30 nm，while no significant change was observed in Pd/In2O3（CP）catalyst（Fig.5）. DFT calculation revealed that the Pd sites highly dispersed in Pd/In2O3（CP）catalyst favored the methanol formation with a lower energy barrier than RWGS，and the high diffusion energy barrier prevented Pd atoms from sintering.

Fig.5 Characterizations and catalytic tests of 0.75wt%Pd/In2O3 catalysts[46]

Other transition metals were also applied to combining with In2O3for catalyzing CO2hydrogenation. For example，Liu’s group［47］prepared In2O3catalysts modified with platinum（Pt）nanoparticles. Pt nanoparticles were well-dispersed and the average size was below 3 nm. Catalytic results showed a methanol formation rate of 16.9 mmol·over Pt/In2O3at 300 ℃，significantly higher than 10.5 mmol·over pure In2O3，and 54%methanol selectivity was obtained with CO as byproduct. In addition，a methanol selectivity of nearly 100%was obtained over Pt/In2O3at the milder temperature of 225 ℃，with a methanol STY of 4.7 mmol··h-1. The stability was also proved to be improved，that is，95%activity remained after 5 h on stream compared to 80% over unmodified In2O3，and then remained stable in several hundreds of hours. TEM and H2-TPR results proved that the interaction between Pt nanoparticles and In2O3improved the surface stability as well as the density of oxygen vacancy sites for CO2activation. Raman spectra showed that the strong interaction inhibited the over-reduction of In2O3to In0species，which led to deactivation. Notably，rhodium［48］and nickel［49］nanoparticles were attempted for the modification of In2O3by the same group，exhibiting similar In-metal interactions，and almost consistent catalytic selectivity and methanol yield under the same conditions.

Group 4 transition metals，especially copper and cobalt，were also attempted for the modification of In2O3because of the balance between catalytic activities and the cost. For example，Wuet al.［50］studied on Cu-In oxide catalysts prepared by co-precipitation，calcination and then reduction by H2. XRD results showed that CuO/In2O3was partially converted into a new Cu11In9phase after reduction at 350 ℃，and the crystallite size of Cu11In9wasca. 40 nm. The catalyst showed a methanol selectivity of 80.5% with CO as byproduct and a methanol STY up to 6.14 mmol··h-1at 280℃under 3 MPa［n（H2）∶n（CO2）∶n（N2）=3∶1∶1］. The catalytic activity was ascribed to the formation of Cu11In9-In2O3interface，contributing to the dissociation of H2and facilitating the hydrogenation of adsorbed CO2. Also，Penget al.［24］put forward the reaction pathway of CO2hydrogenation into methanol over the Cu-In2O3catalyst（Fig. 6）. Bavykinaet al.［51］prepared a novel Co-In oxide catalyst by hydrothermal method. A methanol STY of 26.9 mmol··h-1and a methanol selectivity of ＞80%were achieved with CO and CH4as byproducts at 300 ℃under 5 MPa with a H2/CO2ratio of 4. In contrast，metallic Co catalyst could catalyze CO2into methane with 100% selectivity，while the main product over unmodified In2O3was CO under the same conditions. Oxidized indium-cobalt layers around metallic cobalt nanoparticles

were proved to be the active phase. DFT simulations indicated that charge transfer between Co and In2O3-xphases could generate more oxygen vacancies on the surface of In2O3-x，thus leading to an improvement in the capability for CO2activation.

Fig.6 Schematic representation of CO2 hydrogenation to methanol over Cu/In2O3 catalyst[24]

In order to further enlarge the catalytic interfaces and improve the stability of In2O3，core-shell structured catalysts were developed and studied. For example，Wuet al.［52］designed a core-shell Cu-In oxides@SiO2catalyst for CO2hydrogenation using solvothermal method，and a reference Cu-In oxides/SiO2was prepared by an incipient wetness impregnation method. In both catalysts，the mass-loading of Cu and In was 8.0%and 6.6%，respectively，and the average particle sizes were 10.8 nm in Cu-In-O/SiO2and 8.1 nm in Cu-In-O@SiO2. A methanol selectivity of 78.1%was obtained over Cu-In-O@SiO2at 300 ℃，and a methanol STY of 6.55 mmol·was achieved，higher than 4.22 mmol·over Cu-In-O/SiO2. Moreover，the catalytic performance of Cu-In@SiO2was maintained over 100 h on stream，while 41.2% of the original activity was lost over Cu-In-O/SiO2. On the basis of XRD，TEM，XPS and TPR analyses，it revealed that higher metal dispersion denoted the formation of more Cu2In-In2O3interface in the core-shell catalyst，leading to a higher ability for CO2activation. Moreover，the core-shell structure prevented the small particles from sintering. In addition，Gasconet al.［53］synthesized In2O3@Co3O4derived from a metal-organic framework，ZIF-67（Co），followed by further reduction to obtain the partially reduced In@Co oxides. It showed a stable methanol production rate of 20.3 mmol·and a methanol selectivity of 87% at 300 ℃over 200 h on stream，with 11%CO and 2%methane as byproducts. TEM and XRD results revealed that a reorganization of Co3O4and In2O3under the reaction condition caused the formation of amorphous mixed Co-In oxide shell，which was proved to be responsible for the high methanol yield and selectivity in kinetic studies.

Briefly，due to the limited capability in activating H2by single In2O3，various kinds of metallic sites were introduced to enhance the CO2conversion while maintaining the high selectivity of methanol. Meanwhile，tuning the nanostructure of metal-In2O3catalysts should be emphasized，because of the regulation of the interaction between metallic sites and In2O3，which significantly causes an improvement in catalytic activity，selectivity and stability for CO2hydrogenation.

3 Selective Hydrogenation of CO2 into C2+Products

According to the relative studies，two reaction routes are considered feasible for CO2hydrogenation into C2+products［17］. One is a methanol-mediated route，in which methanol is formed，and then it is converted into key intermediates for C—C coupling. The other is a CO-mediated route，in which RWGS reaction is favorable first，and then follows the Fischer-Tropsch synthesis（FTS）process. As for both routes，hybrid nanocatalysts with bifunctional active sites are essential for each step. Since In2O3has been proved to hold a promising activity for CO2hydrogenation into methanol，it is suitable to act as an active component for the first step of methanol-mediate route，while a catalyst for converting methanol into C2+products is also in need. Notably，zeolites are widely applied for catalysis of C—C bond formation reactions，such as methanol to hydrocarbons（MTH）［54］，ethanol to C3+olefins［55］and syngas conversions［56］. During the MTH process，Br?nsted acid sites in zeolites contribute to the dissociation of methanol into intermediates containing CH3*or CH3O*species，which can couple and form CH3-CH2O*intermediates and then dehydroxylate into CH2=CH2，or insert into the formed olefins to promote the carbon chain growth into C3+products［57］. Liuet al.［58］discussed the mechanisms of the first C-C coupling step in MTH process，and Bhanet al.［59］summarized the plausible pathways of olefin methylation with methanol（Fig.7）. Gasconet al.［57］suggested that the unique pore structure and topology of zeolites led to a steric effect to regulate the product selectivity. Therefore，In2O3-zeolite hybrid catalysts are developed for selective hydrogenation of CO2into C2+hydrocarbons，mainly undergoing the methanol-mediate route. Besides，other catalysts such as zeolites and Zn-Al/Ga/Zr oxides undergo methanol-mediated pathway for producing C2+products. In addition，as for the RWGS-FTS pathway［17］，F(xiàn)e-based catalyst is another typical catalyst for CO2hydrogenation into C2+products.

Fig.7 Probable mechanisms of the first C—C bond formation step over zeolites[58](A) and probable mechanisms of olefin methylation with methanol over zeolites[59](B)

3.1 In2O3-zeolite Nanocatalysts

In the past few years，In2O3-zeolite catalysts are designed and studied for CO2hydrogenation. H-form Zeolite Socony Mobil-5（HZSM-5），an aluminosilicate zeolite with abundant solid acid sites，which was usually applied for catalytic methanol-to-gasoline reactions［60］，was attempted for the development of hybrid nanocatalysts for CO2hydrogenation. For example，Sunet al.［61］combined In2O3with HZSM-5 to construct the hybrid nanocatalysts，which consisted of In2O3with a small particle size ofca. 10 nm and specific surface area of 120 m2/g，and mesoporous structured HZSM-5 with a pore diameter of 4 nm and Si/Al ratio of 27. When used as catalyst for CO2hydrogenation，different products including C5+liquid fuels as well as methanol，CO，CH4and C2—C4hydrocarbons were produced. Moreover，HZSM-5-mixed Cu/ZnO/Al2O3catalyst was also prepared and tested as a comparison，showing CO selectivity of ＞95%and C5—C11selectivity of ＜65%. It should be pointed out that CO selectivity refers to the selectivity in all products，while C2+product selectivity refers to the distribution in all hydrocarbon products. In addition，only ＜5% C2+selectivity was achieved and no C5+product was obtained over pure In2O3under the same condition，suggestive of the key role of zeolite in C—C coupling. In addition，over In2O3/HZSM-5 with a mass ratio of 2∶1，the CO2conversion and C5—C11selectivity remained atca. 12% and 80% during 150 h on stream at 340 ℃under 3 MPa（H2/CO2/N2=73/24/3）with a C5+STY of around 3.0 mmol·，while CO selectivity was ＜45%in the total products.

Hydrogenation of CO2into light olefins（C2—C4）catalyzed by In2O3-zeolite catalysts also shows a promising prospect. For example，silicoaluminophosphate zeolites（SAPOs），which own weaker active sites and favors catalytic methanol-to-olefins reactions according to relative studies［62］，were combined with In2O3for CO2hydrogenation. Witton and co-workers［63］prepared the hybrid of In2O3with SAPO-34 for CO2hydrogenation，characteristic of spherical In2O3particles in a size range of 10—20 nm on cubic-like SAPO-34（Fig.8）. Light olefins，C2—C4alkanes，CH4，CO and methanol were detected as products. A CO2conversion of 34.6%，the light olefinsselectivity of about 70%，and a CO selectivity ofca. 60% in the total products was achieved over In2O3/SAPO-34 at 360℃under 2.5 MPa［n（H2）∶n（CO2）∶n（N2）=3∶1∶1］. Moreover，the CO2conversion was decreased to 30.8%after 200 h on stream，while the light olefin selectivity remained no change.

Fig.8 Characterizations and catalytic tests of In2O3/SAPO-34[63]

3.2 In2O3-ZrO2-zeolite Nanocatalysts

To further enhance the catalytic activity for CO2hydrogenation to C2+products，ZrO2was introduced into the In2O3/zeolite hybrid system. For example，Sunet al.［64］designed a bifunctional catalyst composed of indium-zirconium composite oxides and SAPO-34 zeolites for catalyzing CO2hydrogenation into lower olefins.In2O3-ZrO2with different In/Zr ratios and SAPO-34 with a Si/（Si+Al+P）molar ratio of 0.062 was prepared by co-precipitation and hydrothermal method，respectively，and the pore size of SAPO-34 wasca. 0.4 nm. The In2O3/ZrO2-SAPO-34 catalyst exhibited a selectivity to C2—C4olefins of 80%as more than 35%CO2conversion at 400 ℃under 3 MPa［n（H2）∶n（CO2）∶n（N2）=73∶24∶3］，and remained stable over 150 h on stream（Fig.9）.In addition，a series of In2O3/ZrO2-SAPO-34 catalysts with different In：Zr atomic ratios was investigated［65］.In2O3/ZrO2-SAPO-34 with In∶Zr atomic ratio of 4∶1 exhibited a CO2conversion of 26.2%and lower CO selectivity of 64.9%at 380 ℃，compared with 15.3%and 68.3%over In2O3-SAPO-34. Also，light olefin selectivity of 74.5%was observed. O1sXPS spectra indicated a concentration of oxygen vacancies up to 24.1%on the surface of In2O3/ZrO2，higher than 19.1% of pure In2O3and 18.6% of ZrO2. DFT calculations revealed that CO2chemisorption at the oxygen vacancy site on In1-xZrxOysurface was much stronger than that on pure In2O3，leading to a higher CO2conversion. Similar work on In2O3/ZrO2-SAPO-34 for CO2hydrogenation into light olefins was also reported by Liuet al.［66］and Tsubakiet al.［67］，showing the high light olefin selectivity of around 83%and 77.6%，respectively.

Fig.9 Catalytic tests for In-Zr-O/SAPO-34 catalysts and the reaction pathway[64,67]

Other types of zeolites combined with In2O3/ZrO2have been reported for C2+products as well. Guoet al.［68］studied on a bifunctional catalyst comprised of In2O3/ZrO2and SAPO-5 for CO2hydrogenation into C2—C4hydrocarbons. At a lower temperature of 300 ℃，a C2—C4selectivity of 83%was obtained at a CO2conversion of 6.7%，with ＜60%CO selectivity in the total products. As contrast，ZSM-5 and ZSM-11 admixed with In2O3/ZrO2were prepared and tested under the same condition，showing methane selectivity over 40%. Notably，a series of SAPO-5 with the ratio of Si/（Al+P）ranging from 0 to 0.3 was prepared and tested，indicating that SAPO-5 with Si/（Al+P）ratio of 0.3 exhibited the highest C2—C4selectivity. NH3-TPD characterization showed that the acid sites in SAPO-5 depended on the ratio of Si/（Al+P），and played a key role in C—C coupling of the obtained methanol.

Besides，the introduction of promoters into In2O3/ZrO2-zeolites is attempted for higher product selectivity.For example，Sunet al.［69］developed a bifunctional catalyst containing In2O3-ZnZrOxand SAPO-34 for CO2hydrogenation. As for In2O3/ZnZrOx-SAPO-34 with an oxide/zeolite mass ratio of 2∶1，the light olefin selectivity reached 85% among all hydrocarbons with very low CH4selectivity of only 1% at a CO2conversion of 17%at 380 ℃under 3 MPa［n（H2）∶n（CO2）∶n（N2）=73∶24∶3］，while a CO selectivity of 54%was obtained. Moreover，the selectivity to lightolefins was enhanced by about 5% after a treatment of SAPO-34 with nitric acid，due to the increase in the amount of Br?nsted acid sites and micro-/meso-/macropores. In addition，when the Zn/Zr molar ratio rose from 1.1 to 3.3，the CO2conversion was increased to 20.8%，which originated from the higher densities of oxygen vacancies responsible for CO2activation caused by the addition of Zn oxides.

In a word，C2+hydrocarbons could be attained from CO2hydrogenation through an In2O3-zeolite cascade system of CO2-to-methanol and methanol-to-hydrocarbon steps. The catalytic activity and selectivity of In2O3-zeolite nanocatalysts could be further enhanced by introducing ZrO2and other promoters. The studies on indium-containing bifunctional catalysts provide an effective and selective catalytic system for the hydrogenation of CO2into high-value C2+products，and the activity and selectivity of these catalysts still remain great potential for improvement.

4 Conclusions and Perspectives

Defective In2O3samples have already exhibited the catalytic capability for CO2hydrogenation into diverse C1 products. Moreover，combining In2O3with different components such as metal oxides，metal nanoparticles or zeolites could effectively tune the product selectivity from C1 to C2+products.

To date，many great progresses have been made in CO2hydrogenation over In2O3based nanocatalysts，but there are still many problems and challenges in this field. First，it is still challenging to develop effective strategies for fabricating In2O3samples with more effective defective sites，and the activation and conversion of both CO2and H2by the defective In2O3are still not clear. Second，deactivation on In2O3samples，which is caused by excessive reduction，often occurs under high reaction temperature and pressure. Third，the reaction process is relatively complex，and several products are often produced at the same time，thus leading to the difficulty in ensuring the selectivity of target products. Fourth，in-situcharacterization techniques are limited in observing the reaction process and getting the intermediate products［70］，thus leading to the difficulty in revealing the reaction nature of CO2hydrogenation.

To address above，developing novel In2O3based nanocatalysts is highly desired for achieving high selectivity of target products along with high activity and good stability. Tuning the oxygen vacancy sites on the surface of In2O3and the interaction between In2O3and other active sites should be emphasized. Furthermore，in-situtechniques such as X-ray absorption fine structure（XAFS），F(xiàn)TIR and Raman spectra need to be developed for observing the intermediate behaviors of CO2hydrogenation. Meanwhile，DFT calculation should be applied for providing some theoretical basis on understanding reaction mechanism by combining withex- andin-situexperimental data.

This work is supported by the Strategic Priority Research Program of Chinese Academy of Sciences（No.XDB36000000），the National Natural Science Foundation of China（Nos.21722102，51672053），the Beijing Natural Science Foundation，China（No. 2182087），and the Youth Innovation Promotion Association of Chinese Academy of Sciences（No.2016036）.