Xiaohao Liu

Particle size effects in the selective hydrogenation of cinnamaldehyde over supported palladium catalysts

This work investigated the catalytic performance of palladium catalysts in the selective hydrogenation of α,β-unsaturated aldehydes, and especially the effect of Pd particle size on the hydrogenation of cinnamaldehyde (CAL). An unsupported nanosized Pd catalyst and a series of supported Pd catalysts using supports of activated carbon, SiO2, TiO2, γ-Al2O3, SiC, and graphene oxide were prepared and evaluated in the selective hydrogenation of CAL. Varied sizes of Pd particles could be obtained directly by using different Pd precursors and indirectly by introducing an inactive Ag metal over a γ-Al2O3 supported palladium catalyst. Over a reducible TiO2 support, the Pd particle size can also be controlled indirectly by the effect of strong metal-support interaction (SMSI). Combined with their TEM observations and catalytic tests, density functional theory (DFT) calculations have confirmed that smaller Pd particles favored CC-centered adsorption of CAL leading to a higher selectivity to hydrogenation of the CC bond, forming hydrocinnamaldehyde (HCAL), while on larger ones, the CC-centered adsorption would be partly substituted by CO-centered adsorption resulting in a lowered selectivity to HCAL but an increased selectivity to CO bond hydrogenation, forming hydrocinnamyl alcohol (HCOL). It is clarified that the size dependence of the catalytic selectivity originates from the strong dependence of CC/CO-centered adsorption on Pd particle size. Finally, tests using solvents with different Pd–solvent interactions and α,β-unsaturated aldehydes with remarkable steric effect variations were applied to further regulate the adsorption between palladium and substrate to alter the catalytic activity and selectivity.

Insights into the influence of support and potassium or sulfur promoter on iron-based Fischer–Tropsch synthesis: understanding the control of catalytic activity, selectivity to lower olefins, and catalyst deactivation

The fundamental understanding of the control of catalytic activity, selectivity and deactivation in iron-based FTS is of prime scientific and industrial importance. In this work, the inter-effects of various supports and promoters (K, Na and S) in FTS have been studied intensively to elucidate and draw the key points of a better reaction performance. Logical results from abundant experimental work indicate that the catalytic activity is mainly affected by the reducibility of iron oxides which is related to the particle size, interaction with the support, and promoter characteristics, and by the particle size-dependent carburization. In order to reach a desired selectivity to lower olefins (C2=–C4=) beyond the limitation of the Schulz–Flory distribution, the use of K or Na alone is insufficient due to its favorable chain growth to form higher hydrocarbons. It is well confirmed that S can shift the selectivity toward short-chain C2–C4 hydrocarbons without an increased selectivity to methane. Combining this distinctive function of S with the favoring of β-hydride abstraction termination of K and a suitable support to have a weak interaction with iron particles, a substantially higher selectivity to lower olefins at about 45–55% with 10–15% methane and ∼6% C2–C4 paraffins could be obtained at a milder reaction temperature and elevated reaction pressure (300 °C, 1.0 MPa). In this study, the catalyst deactivation is mainly ascribed to K-induced carbon deposition while the reversible transformation of χ-Fe5C2 into Fe3O4 is another reason to some extent.

Iron‐Based Fischer–Tropsch Synthesis for the Efficient Conversion of Carbon Dioxide into Isoparaffins

Iron‐based Fischer–Tropsch (FT) synthesis in combination with hydroisomerization in the presence of zeolites for the synthesis of isoparaffins from CO2/H2 was conducted in a fixed‐bed reactor. Relative to supported iron catalysts, the precipitated one efficiently converted intermediate CO into hydrocarbons by supplying a high density of FT active sites on the catalyst surface. Removing water by interstage cooling and promoting the CO conversion step in the FT synthesis were effective approaches in achieving a high CO2 conversion, because of an increase in the driving force to the reaction equilibrium. Particle mixing of 92.6 Fe7.4 K with either 0.5 Pd/β or HZSM‐5 zeolite effectively hydroisomerized the resulting FT hydrocarbons into gasoline‐range isoparaffins. Particularly, HZSM‐5 displayed a higher isoparaffin selectivity at approximately 70 %, which resulted from easier hydrocracking and hydroisomerization of the olefinic FT primary products.

Two-dimensional graphene-directed formation of cylindrical iron carbide nanocapsules for Fischer–Tropsch synthesis

Iron-based Fischer–Tropsch synthesis provides a promising alternative as a direct and non-petroleum route for producing olefins from syngas (CO + H2). In general, a promoter is required to regulate the activity and olefin selectivity, and the major challenges come from catalyst deactivation due to carbon deposition and sintering under drastic reaction conditions (340 °C, high pressure). In this study, we show a totally different strategy for solving these problems. We find that the originally large spherical Fe2O3 particles of around 120 nm assisted by graphene without an additional promoter can gradually evolve during the FTS reaction, forming much smaller cylindrical Hagg carbide nanocapsules of about 10 nm with a single (510) crystal facet, that display excellent activity nearly 40 times that of the unmodified one, olefin selectivity of 76% (84% if methane is not considered) and high stability. In contrast, the original Fe2O3 catalyst and carbon-containing polymer material (e.g. PAA, PVP)-modified Fe2O3 catalysts exhibit significantly lower selectivity for olefins and higher selectivity for methane and deactivate rapidly. Different from the above-mentioned peculiar evolution of iron particles, the iron particles over these spent catalysts aggregate into larger ones having multiple crystal facets, e.g. χ-Fe5C2 (510), and χ-Fe5C2 (−202), and at the same time serious carbon deposition is observed, which mainly accounts for catalyst deactivation. In the case of activated carbon and carbon nanofiber supported iron catalysts with initially small iron particles below 10 nm, irregular morphology and multiple crystal facets of iron carbides are also observed over the spent catalysts. DFT calculations demonstrate that the χ-Fe5C2 (510) crystal facet is more active and favorable for the formation of olefins than the χ-Fe5C2 (−202) crystal facet, which is well in accordance with our experimental results. The presented methodology provides a new strategy to control the morphology and crystal facet of iron carbide during the catalytic reaction for solving the toughest problems in heterogeneous catalysis.

Investigation of the highly tunable selectivity to linear α-olefins in Fischer–Tropsch synthesis over silica-supported Co and CoMn catalysts by carburization–reduction pretreatment

Herein, the evolution of metallic Co through RCR treatment and during the FTS reaction has been comparatively investigated considering the effects of a Mn promoter and varied conditions. The results show that the selectivity to α-olefins can be fine-tuned. In detail, Co3O4 (311) is always first reduced to metallic Co (111), and then, Co (111) is carburized with CO to form Co2C (111). However, second reduction over Co2C (111) results in Co (101) and Co (002) for 15Co/SiO2-RCR. Contrary to the case of unmodified 15Co/SiO2, the addition of Mn alters the evolution of Co2C (111) into Co (101) and Co (100) for 15Co3.7Mn/SiO2-RCR. During the FTS reaction over R-treated catalysts, Co (111) remains almost unchanged for 15Co/SiO2-R, but gradually evolves into Co3C (101) over 15Co3.7Mn/SiO2-R. For comparison, 15Co3.7Mn/SiO2-RCR exhibits stable Co (101) and Co (100) in the FTS reaction. The abovementioned evolutions reflect a marked effect on catalytic performance. Co (101) and Co (002) display a notable increase in selectivity to total olefins as compared to Co (111). Furthermore, a number of experiments reveal that Co3C (101) is favorable for olefins, especially for lower olefins, with a low selectivity to methane; however, Co2C (111) shows negligible activity with undesirable high selectivity to methane. Based on the comprehensive experimental data, the selectivity to total olefins might be mainly affected not only by the Co phase state and crystal facet but also by the unpaired d orbital electron number modified with Mn and the H2/CO ratio in the feed gas, directly determining the ratio of H*/C* on the catalyst surface. As an optimal result, an excellent selectivity of 74.2% to total α-olefins is obtained. In addition, α-olefin distributions can be efficiently tailored by simply changing the reaction temperature.

Supported Fe/MnOx catalyst with Ag doping for remarkably enhanced catalytic activity in Fischer–Tropsch synthesis

Although the Mn promoter has been widely demonstrated to be beneficial to promoting the adsorption and dissociation of CO and therefore increasing the selectivity to light olefins in Fe-based FTS, it also brings about a negative effect on the catalytic activity due to its strong interaction with Fe oxides. In this study, Fe oxide was directly dispersed on a synthesized mesoporous spherical-like MnOx support and further doped with Ag nanoparticles to investigate its impact on the FTS reaction. XRD and H2-TPR characterization confirmed that Mn and Fe tend to form an FeMn solid solution over a 10 wt% Fe/MnOx catalyst. As expected, a small amount of Ag doping on it could remarkably increase the catalytic activity by 1–5 times and even the selectivity to light olefins; this could be attributed to easier reduction of the Fe oxide, Mn oxide and FeMn solid solution due to the hydrogen (H) spillover generated by Ag as the reduced metallic Fe favors the formation of active Fe carbide and more O vacancies in MnOx facilitate CO adsorption and the rapid removal of dissociated O atoms on Fe carbide. Furthermore, the FTS activity related to the H spillover was intensively investigated over the MnOx-supported catalysts with different impregnation sequences of Ag and Fe, and over the hybrid 10Fe/MnOx and 1Ag/MnOx catalysts with particle and powder mixing in order to regulate the distance between Fe and Ag. It was found that the primary and secondary ways of H spillover are involved in the promoted reduction process, which was illustrated by XRD, H2-TPR, H2-TPD and CO-TPD characterization combined with catalytic results. The secondary spillover exhibited a much milder effect on the reduction promotion with a farther distance of Fe and Ag. Compared with the co-impregnated 1Ag10Fe/MnOx catalyst, the sequential impregnation of Ag and Fe onto the MnOx support, named the 10Fe/1Ag/MnOx catalyst, showed higher catalytic activity, which might be due to less formation of FeMn solid solution resulting from firstly introduced Ag on MnOx. Interestingly, both Ag-doped and Ag-free 10Fe/MnOx catalysts showed a rapid increase in CO conversion after several tens of hours of stable reaction and the CO conversion can reach 80–90% after a 100 hour test, and the possible reason for this was discussed based on HAADF-STEM observations of spent catalyst samples.

Investigation on converting 1-butene and ethylene into propene via metathesis reaction over W-based catalysts

Supported W catalysts were extensively investigated for the conversion of 1-butene and ethylene into propene by metathesis reaction. The performance of catalysts was compared by using unsupported WO3, pure SBA-15, supported W/SBA-15 with different W loadings, varied calcination temperatures, and by changing the pretreatment gas atmosphere. The above catalytic results could be employed to deduce the reaction mechanism combined with characterization techniques such as BET, XRD, UV-vis DRS, Raman, pyridine-IR, XPS, and H2-TPR. In this study, over the investigated W/SBA-15 catalysts, the results showed that the silanol group (Si–OH) in SBA-15 could act as a weak Bronsted acid site for 1-butene isomerization. However, the metathesis reaction was catalyzed by W-carbene species. The initially formed W-carbenes (WCH–CH3) as active sites were derived from the partially reduced isolated tetrahedral WOx species which contained WO or W–OH bonds in W5+ species as corresponding Lewis or Bronsted acid sites. Furthermore, the W/SBA-15 being pretreated by H2O led to a complete loss of the metathesis activity. This was mainly due to the sintering of isolated WOx species to form an inactive crystalline WO3 phase as demonstrated by XRD patterns. On the other hand, the reduction of WOx species remarkably suppressed by H2O pretreatment was also responsible for the metathesis deactivation. This study provides molecular level mechanisms for the several steps involved in the propene production, including 1-butene isomerization, W-carbene formation, and metathesis reaction.

Unravelling the New Roles of Na and Mn Promoter in CO2 Hydrogenation over Fe3O4-Based Catalysts for Enhanced Selectivity to Light α-Olefins

The direct production of light α‐olefins (C2=‐C4=) from CO2 is of great importance as this process can convert the greenhouse gas into the desired chemicals. In this study, the crucial roles of Na and Mn promoter in CO2 hydrogenation to produce light α‐olefins via the Fischer‐Tropsch synthesis (FTS) over Fe3O4‐based catalysts are investigated. The results indicate that both Na and Mn promoter can enhance the reducibility of Fe3O4. In situ XPS and DFT calculations show that Na facilitates the reduction by electron donation from Na to Fe as the oxygen vacancy formation energy is reduced by Na. In contrast, Mn promotes the reduction by the presence of oxygen vacancy in MnO as the oxygen in Fe oxide can spillover to the vacancy in MnO spontaneously. For un‐promoted Fe3O4 catalysts, CO2 hydrogenation dominantly produces light n‐paraffins. The addition of Na remarkably shifts the selectivity to light α‐olefins with a sharp decline in the selectivity to light n‐paraffins, which is attributed to the electron donation from Na to Fe resulting in the promoted CO dissociation and the favorable β‐H abstraction of surface short alkyl‐metal intermediates. The addition of Mn into Na‐containing Fe3O4 catalysts can obviously further enhance the selectivity to light α‐olefins as the spatial hindrance of Mn suppresses the chain growth to increase the amount of surface short alkyl‐metal intermediates.

CO2 formation mechanism in Fischer–Tropsch synthesis over iron-based catalysts: a combined experimental and theoretical study

Fischer–Tropsch synthesis (FTS) is one of the most attractive routes to convert syngas (CO + H2) into liquid fuels and high value-added chemicals. However, FTS over Fe-based catalysts generates and emits large amounts of CO2, which reduces the carbon atom economy and causes huge greenhouse gas emission. In this work, CO2 formation mechanisms in FTS over Fe-based catalysts were systematically investigated by combining experiments and DFT calculations, aiming to provide atomic-scale insights into the CO2 formation process. Our results indicate that the Boudouard mechanism, in which the surface O* species formed by CO* dissociation reacts with another CO* to form CO2, plays a predominant role in CO2 formation on the active χ-Fe5C2 phase, while the hydrogenation of surface O* species to form H2O is hindered. The existence of the Fe3O4 phase is favorable for the reverse water-gas shift (RWGS) reaction, leading to the decrease of CO2 selectivity and increase of the amount of generated H2O. The modification by the potassium promoter does not alter the predominant reaction pathway for CO2 formation over Fe-based FTS catalysts and the Boudouard mechanism still plays the dominant role. The potassium promoter can increase CO2 selectivity and decrease the amount of H2O mainly through the following two ways: (1) potassium largely increases the proportion of the χ-Fe5C2 phase and thus increases the amount of active sites for the Boudouard reaction; (2) potassium leads to the disappearance of the Fe3O4 phase and thus suppresses the RWGS reaction. The electronic structures were systematically analyzed to shed light on the nature of the potassium effect. On the one hand, the potassium promoter makes the d-band center of the χ-Fe5C2(510) surface atoms shift toward the Fermi level, facilitating the back-donation of electrons from the χ-Fe5C2(510) surface to the adsorbed CO* antibonding orbital; on the other hand, the direct interaction between K2O and adsorbed CO* weakens the C–O bond by decreasing its electron density, which also contributes to the promoted CO dissociation.

Hydrogenation of CO2 into hydrocarbons: enhanced catalytic activity over Fe-based Fischer–Tropsch catalysts

CO2-FTS is one of the most practical routes for converting CO2 into valuable chemicals and fuels to reduce CO2 emissions. However, efficient conversion remains a challenge because of its chemically inert property. Previously, we have reported a remarkably enhanced CO2 conversion efficiency by driving the RWGS reaction via the removal of large amounts of formed water in the reaction system. In this study, we propose to effectively enhance the CO2 conversion by promoting the conversion of the CO intermediate in the FTS stage. For this purpose, a K or/and Co (Ru) component is introduced onto the precipitated Fe catalysts. Results show that the addition of K obviously increases the CO2 conversion due to the promoted formation of iron carbide sites as the active FTS reaction phase. Moreover, the selectivity to C2+ hydrocarbons, especially lower olefins, can be substantially enhanced owing to the electron donation from K to Fe which promotes chain growth and suppresses the direct hydrogenation of Fe-(CH2)n intermediates. The addition of Co (Ru) that has no WGS activity could further remarkably enhance the CO2 conversion for Co (Ru) only promotes the FTS reaction rate of the CO intermediate without catalyzing the conversion of CO into CO2. Furthermore, the effects of the intimacy between Co and Fe sites from the nanoscale to the meter scale have also been investigated. The results display that the intimate contact between Fe and Co sites favors a higher selectivity to C2+ hydrocarbons, which is ascribed to the easy spillover of the CO intermediate from Fe3O4 to Co sites and thereby yielding a higher CO concentration over Co sites. In contrast, the increase in the distance between Fe and Co sites leads to a remarkably higher selectivity to CH4, which could be because the direct methanation of CO2 is enhanced and the chain growth possibility of the FTS reaction is reduced due to the lower CO concentration over Co sites.