Metal-Foam/-Fiber-Structured Pd- And Au-based Catalysts For Catalytic Combustion And Selective Catalytic Oxidation Of Methanol

Catalytic reaction is based on the reaction kinetics over the surface/interface of catalysts, which is usually limited by the process of hydrodynamics and transport within the catalyst bed at the reactor level, leading to the deterioration of catalyst performance. The development and use of structured catalysts and reactors （SCRs） is a promising avenue to process intensification, which overcomes the major drawbacks encountered in the traditional packed-bed reactor due to the improved hydrodynamics and low pressure drop in the combination with enhanced heat/mass transfer, thus being a hot topic in the heterogeneous catalysis. The overwhelming majority of studies have been invested in the honeycomb ceramic and micro-channel SCRs with regular two-dimensional channels. Such catalysts consist of thousands of opening parallel channels in millimetric diameter （to offer high void fractions and to allow low pressure drop at high flow rates through the catalyst beds） with catalytic washcoat in micrometric thickness on the channel walls （to improve mass transfer due to the short gas diffusion distance）. However, using monolithic ceramic honeycomb still remains challenging because of their poor heat transfer and the lack of radial mixing, which is ill for the endo-/exo-thermic and/or fast reactions.Metal fiber-and foam-based supports have attracted ever-increasing interest within the last decade. Besides the high voidage and internal-diffusion as typically in honeycomb catalyst, the unique three-dimensional （3D） network and open structure as well as high thermal conductivity and mechanical strength undoubtedly allow low pressure drop, high mass/heat transfer and especially high contacting efficiency resulted from the radial mixing. Moreover, the metallic supports have unique form factors that provide a great flexibility in geometric appearance when filling up structured reactors. These characteristics are particularly beneficial for very fast and heat/mass transfer controlled processes, which are pronounced in the energy/environmental catalysis. However, the main stumbling blocks in developing metal fiber- and foam-based SCRs are that the conventional washcoating technique for their catalytic functionalization suffers from nonuniformity and binder contamination as well as high cost and fabrication difficulty. More importantly, the large thermal expansion coefficient of metal will lead to the crack initiation and even the exfoliation of coatings.Therefore, our search for innovative preparation strategy to achieve one-step homogeneous functionalization of the structured supports in non-dip-coating manner will occupy a decisive position. This thesis is aiming at addressing the special requirements of high throughput and low pressure drop in the environmental protection, as well as the problems of the strong heat dissipation and mass transport limitation in the selective catalytic oxidation of alcohols. Accordingly, commercial metal fiber and foam with 3D continuously open-cell structure were employed as the monolithic substrate. Based on the "top-down" design philosophy, metal-supported noble-metal monolithic catalysts were triumphantly prepared by the novel catalytic functionalization techniques, such as galvanic deposition method, selective chemical etching, in-situ reaction activation, hydrothermal growth, solvent-assisted incipient wetness impregnation or coupling-agent-assisted one-step organization. These monolithic catalysts were qualified for several typical catalytic reactions and exhibited excellent performance, including the catalytic combustion of lean CH4 and VOCs, catalytic deoxygenation of coalbed methane （CBM）, and oxidative coupling of methanol （OCM） to methyl formate （MF）. The detailed content of this thesis consisted of four parts as following:（1） Microfibrous-structured Pd/AlOOH/Al-fiber catalysts for low concentration CH4 and VOCs abatement via catalytic combustion.Thin-sheet 3D sinter-locked network structure of Al-fiber was employed as monolithic substrate and Al source. Microfibrous-structured AlOOH/Al-fiber nanoarrays were prepared by a facile hydrothermal surface oxidation of Al-fiber without any chemical additives. The nanoarrays consisting of Al-fiber core and pseudo-boehmite （y-AlOOH） shell with various surface morphologies could be obtained through controlling hydrothermal time and temperature. Dehydration of AlOOH via calcination resulted in the phase transformation to γ-Al₂O₃ without altering its surface morphology. The strong adhesion between the AIOOH/Al₂O₃ shell and Al-substrate in the nanoarrays provided excellent monolithic-supports with high surface area and mesopore structure for subsequent dispersing active component.Palladium catalysts supported on the AlOOH/Al-fiber nanoarrays （denoted as Pd/AIOOH/Al-fiber） were prepared by solvent-assisted incipient wetness impregnation. Such catalysts with 0.3 wt% Pd-loading showed high oxidative activity of CH4 and VOCs at high throughput. At gas hourly space velocity （GHSV） of 72000 mL g-1 h-1, complete CH4 conversion could be obtained at below 400℃. During the optimization of preparation conditions, we found that the performance of Pd/AlOOH/Al-fiber depended on the surface morphology of AlOOH nanoarrays, phase composition of the support, calcination temperature, and Pd-loading of the catalysts.Despite the high initial activity of the Pd/AlOOH/Al-fiber for lvol% CHU catalytic combustion, rapid deactivation occurred during the stability test at 400℃. Based on the characterization results, the deactivation was attributed to the instability of alumina support which accelerated the sintering of Pd nanoparticles （NPs）. Therefore, the improvement of hydrothermal stability of Al₂O₃ nanoarrays and thermal stability of Pd NPs to enhance the stability of Pd-based catalysts is imperative to develop monolithic CH4 combustion catalyst with practical value.（2） Microfibrous-structured analogous core-shell Pd@SiO₂ catalysts embedded onto Al₂O₃/Al-fiber by one-step coupling-agent-assisted organization for low concentration CH4 and VOCs abatement via catalytic combustion.Microfibrous-structured Pd@SiO₂ catalysts supported on the Al₂O₃/Al-fiber nanoarrays （denoted as Pd@SiO₂/Al₂O₃/Al-fiber） were successfully fabricated from nano-to macro-scale in one step, aiming at improving the thermal-stability of CH4 combustion. The cost-effective coupling agent （APTES） acted as the bidirectional bridging:the first bridging occurred between-NH₂ of APTES and Pd2+ by the preferential chelation; and the second was the silanisation reaction between the ethoxy groups of APTES and surface OH groups on A1OOH nanoarrays to form Al-O-Si bonds. Such successive spontaneous reactions finally afforded the one-step precise organization of Pd2+, APTES and AlOOH. Followed by the pyrolysis process which reduced Pd2+ to Pd NPs and formed mesopore, the Pd@SiO₂/Al₂O₃/Al-fiber was finally obtained. Optimization experiments showed that the preparation conditions such as the amount of hydroxy on the support surface, ratio of Pd to APTES, trace water in the system, calcination temperature and Pd-loading, had an impact on the catalyst performance.Several microscopy techniques confirmed that finely-dispersed Pd NPs were effectively encapsulated into mesoporous SiO₂ matrix. Such catalyst with 0.3 wt% Pd-loading showed high catalytic combustion activity of CH4 with complete conversion at 400 ℃ and GHSV of 72000 mL g-1 h-1 with 1 vol% CH4 in air. CH4 conversion could be sustained for 700 h running with no signs of deactivation, while the surface morphology and Pd NPs were well-preserved after the test. Therefore, the encapsulation of Pd NPs into mesoporous SiO₂ matrix can effectively prevent the Pd sintering while the SiO₂-shell has an stabilizing-effect on the Al₂O₃ nanoarrays. In addition, the Pd@SiO₂/Al₂O₃/Al-fiber exhibited promising catalytic combustion activity for many kinds of VOCs.When the reaction was performed in the presence of 5 vol% H₂O, the activity of Pd@SiO₂/Al₂O₃/Al-fiber was obviously inhibited and deactivated rapidly. XPS analysis showed that the water vapor strongly adsorbed on the active site （i.e. PdOx）, leading to the formation of palladium hydroxide （i.e. Pd（OH）x） which was less active for CH4 catalytic combustion-Most notably, the SiO₂ shell remained robust even in the presence of such high H₂O concentration. It is thus particularly desireable to enhance the stability of our monolithic Pd-based core-shell catalysts in the presence of water, for example, through choosing an alternative oxide shell （e.g. ZxO₂） with high resistance of water poisoning.Based on controllable chemical reactions, one-step precise organization from nano-to macro-scale of different building blocks into monolithic structured architectures represents a promising strategy to develop novel catalytic systems. According to the design philosophy of the Pd@SiO₂/Al₂O₃/Al-fiber, new nanosystems of oxide-encapsulated-NPs such as Ag@SiO₂, Ni@TiO₂ and Pd@ZrO₂, supported on the monolithic-structured substrates could be designed for other harsh reactions, on the basis of well-defined coupling agents consisting of different functional group （e.g.-O-C2H5 and -NH₂） and center ion （e.g. Si, Al, Ti and Zr）.（3） High-performance PdNi alloy in-situ structured on monolithic metal foam by galvanic deposition method for CBM deoxygenation via catalytic combustion.A high-performance PdNi（alloy）/Ni-foam catalyst to be used for CBM deoxygenation via catalytic combustion of CH4 was developed with the aid of galvanic deposition of Pd NPs onto the monolithic Ni-foam followed by in-situ reaction-induced Pd-Ni alloying. Such catalyst provided a unique combination of high low-temperature activity/selectivity, oscillation-free, high permeability and enhanced heat transfer. As an example, the catalyst with a low Pd-loading of 1 wt% could deliver a complete O₂ conversion for a simulated feed of CH4/O₂/N2 （40/3/57, vol%） at 350 ℃ with a high GHSV of 12000 mL goat.-1 h-1, and particularly, this catalyst was stable for at least 500 h without deactivation and oscillation.In-situ reaction-induced Pd-Ni alloying was clearly revealed and by nature was responsible for the low-temperature activity promotion and oscillation suppression. Special Pd@NiO （Pd nanoparticle partially wrapped by tiny NiO fragments） ensembles were formed in galvanic deposition stage and could merely be transformed into PdNi nano-alloys in the real reaction stream at elevated temperature （e.g.,450 ℃ or higher）. Additionally, the PdNi nano-alloy particles did not hold still under reaction conditions, exhibiting obvious structure and composition changes as a function of reaction temperature.Density functional theory （DFT） calculations were performed to reveal the role of in-situ formed PdNi alloy and the Ni decoration at Pd for the CBM deoxygenation. We constructed several catalyst models, i.e. pure Pd（111） and corresponding surface alloy containing Ni. It was found that the adsorbed O atoms （O*） adsorbed more strongly at Pd（lll）. However, with pre-adsorbed O at different sites, the strong adsorption of O* at Pd（111） was unfavourable to CH4 activation, which would then significantly decrease the O₂ conversion rate until the excessive surface adsorbed O atoms were consumed, inducing an oscillatory behaviour over the fresh catalyst. O* at PdNi（111） was more active, leading to fast O₂ conversion. Thus the decrease of O* surface coverage was beneficial to CHU activation and avoiding the oscillatory behaviour caused by accumulated surface O*. We inferred that the capability of direct 02*/O* hydrogenation at PdNi（111） shifted the original cycles composed of stepwise 02-adsorption/activation and oxidation-of-CH4 at a Pd catalyst to the present transient-balancing cycles at our PdNi alloy catalyst thereby eliminating the oscillation. At the same time, the Ni decoration at Pd can modify the electronic structure of surface Pd and lead to a decrease in the O adsorption energy which can be taken as the activity descriptor for the CBM deoxygenation.The kinetic study of CBM deoxygenation indicated that the Ni decoration at Pd by Pd-Ni alloying lowered the apparent activation energy compared to the pristine Pd catalyst, confirming the enhanced catalytic activity of PdNi alloy. The reaction order for CH4 over these two catalysts was about 1, being consistent with the result under O₂-rich condition. The oxygen order was negative over the two catalysts, unlike the reported zero order in the case of O₂-rich combustion, revealing the origin of oscillatory behavior for CBM deoxygenation. Notably, the Ni decoration at Pd led to an increase of the reaction order of O₂ to -0.32 from-0.67 of the pristine Pd catalyst. This result demonstrated that O₂ concentration had a smaller influence on the reaction rate over PdNi alloy. Therefore PdNi alloy provided the ability to transiently balance the cycles of 02-adsorption/activation and oxidation-of-CH4 thereby eliminating the oscillatory phenomenoa（4） Thin-sheet microfibrous-structured nanoporous-gold（NPG）/Al-fiber catalysts prepared by galvanic deposition method for OCM to MF.The NPG/Al-fiber catalysts were obtainable by galvanically depositing Au-Ag alloy onto a 3D network structure using Al-fiber followed by selective chemical etching of Ag. Such catalyst was highly active, selective and stable for the OCM to MF while associated with enhanced heat transfer. At 100℃, the NPG-7/Al-fiber （Au loading:7 wt%） was capable of achieving-100% MF selectivity with-25% methanol conversion in a 10 vol% methanol feed at 5000 mL gcat.-1 h-1. The catalyst was stable for at least 300 h without NPG sintering.The effect of preparation conditions on the OCM was investigated. We found that the NPG/Al-fiber with different Au loading showed equivalent intrinsic activity; catalyst performance was influenced visibly by the alloying temperature of AuAg precursor but a little by the alloying time length; too high initial proportion of gold in the AuAg alloy will result in poor porosity evolution and a low density of active sites, thereby leading to a lower catalytic activity; Ag residue contents could be controlled by the Ag leaching time length while both selectivity and conversion for the NPG/Al-fiber were decreased along with the increase of Ag residue contents.The XPS and O₂-TPD indicated that the Ag residue content was unable to adjust the amounts of the surface Ag sites and formed surface O₂2-/O- species, but showed an ability to finely tune the chemical properties of such formed surface O₂2-/O- species. Lowering Ag residue content was essential for the OCM performance especially the selectivity to MF, by nature, due to reduction of the amount of unselective surface O species.