Hydrogen Production Based on Liquid Organic Hydrogen Carriers through Sulfur Doped Platinum Catalysts Supported on TiO₂

Abstract

In order to move from a carbon-based energy system to a more sustainable one, focus is put on liquid organic hydrogen carrier (LOHC) systems for CO 2 -free hydrogen storage and release. In this study sulfur as a dopant for the Pt/TiO 2 catalyst was identified to be a selective poison resulting in high performing catalysts in the dehydrogenation experiments with the LOHC system perhydro dibenzyltoluene/dibenzyltoluene (H18-DBT/H0-DBT). The Pt/TiO 2 and S−Pt/TiO 2 catalysts were compared to the current benchmarks Pt/γ-Al 2 O 3 and S−Pt/γ-Al 2 O 3 . S−Pt/TiO 2 was found to achieve a high degree of dehydrogenation of 98% which is competitive to the benchmark S−Pt/γ-Al 2 O 3 . In addition, analyses of the side products resulted in a higher selectivity toward dibenzyltoluene for S−Pt/TiO 2 than for the γ-Al 2 O 3 supported catalysts. Characterization with infrared spectroscopy (IR), transmission electron microscopy (TEM), and diffuse reflectance infrared Fourier transform spectroscopy with CO as adsorbing molecule (CO−DRIFTS) suggest the presence of strongly chemisorbed sulfur species as well as SMSI effects of the Pt/TiO 2 catalysts. Based on kinetic studies, a batch and plug flow reactor for the dehydrogenation reaction were simulated for both titania supported catalysts. According to the calculations, the sulfur doped catalyst displays higher conversions in both batch and plug flow operation as compared to the unmodified system Pt/TiO 2 .

Catalytic Dehydrogenation of Liquid Organic Hydrogen Carrier Model Compounds by CpM+ (M = Fe, Co, Ni) in the Gas Phase

“…Cyclic alkanes and N-heterocyclic species are of special interest due to their stability and high energy density. Cycloalkane LOHC systems such as cyclohexane/benzene are good models for mechanistic studies. , Others that are viable on the industrial scale include perhydro-dibenzyl toluene/dibenzyl toluene , and methylcyclohexane/toluene. , Although the cyclohexane/benzene system (eq ) is not very useful in the context of scaling up owing to the carcinogenic properties of benzene, it is often used as a model due to its relatively easy access to mechanistic studies and has been investigated in the context of reactant/product separation. , N-Heterocycle couples such as piperidine/pyridine (eq 2, and derivatives), octahydroindole/indole, and perhydrophenazine/phenazine (eq 3) are all viable substrates for stable hydrogen storage. N-Heterocycles have the added benefits of relatively better biodegradability, low volatility, and more efficient hydrogen liberation …”

“…Cycloalkane LOHC systems such as cyclohexane/benzene are good models for mechanistic studies. 7,8 Others that are viable on the industrial scale include perhydro-dibenzyl toluene/dibenzyl toluene 9,10 and methylcyclohexane/toluene. 11,12 Although the cyclohexane/benzene system (eq 1) is not very useful in the context of scaling up owing to the carcinogenic properties of benzene, it is often used as a model due to its relatively easy access to mechanistic studies and has been investigated in the context of reactant/product separation.…”

“…A key requirement of these LOHC systems is the need for dehydrogenation catalysts to liberate hydrogen on demand, and much work has been carried out to develop both homogeneous and heterogeneous catalysts. ,− While heterogeneous catalysts seem to hold most promise for large-scale dehydrogenation, the use of homogeneous catalysts offers advantages in studying reaction mechanisms and characterizing reactive intermediates. Multiple groups have investigated molecular catalysts involving the dehydrogenation of not only homocyclic compounds, but also N-heterocycles.…”

F– Serve as Surface Trapping Sites to Promote the Charge Separation and Transfer of TiO2

“…As a typical semiconductor, titanium dioxide (TiO 2 ) has received extensive attention due to its high stability, nontoxicity, facile and low-cost synthesis, and tunable electronic band structure features. Particularly, TiO 2 -based semiconductor materials have been extensively used in a wide range of applications, such as H 2 production, 1 − 3 CO 2 reduction, 4 − 7 nitrogen fixation, 8 − 10 wastewater treatment, 11 − 13 gas sensors, 14 , 15 and solar cells. 16 , 17 However, the ineffective separation and transport of charge carriers as a consequence of a shorter carrier lifetime largely restricts its application of photogenerated carriers for photoelectrochemical and chemical reactions.…”

“…As a typical semiconductor, titanium dioxide (TiO 2 ) has received extensive attention due to its high stability, nontoxicity, facile and low-cost synthesis, and tunable electronic band structure features. Particularly, TiO 2 -based semiconductor materials have been extensively used in a wide range of applications, such as H 2 production, − CO 2 reduction, − nitrogen fixation, − wastewater treatment, − gas sensors, , and solar cells. , However, the ineffective separation and transport of charge carriers as a consequence of a shorter carrier lifetime largely restricts its application of photogenerated carriers for photoelectrochemical and chemical reactions. − Thus, research studies were tremendously intensified to overcome these bottlenecks, and momentous advancements were attained over the past few decades. Generally, strategies proposed to boost the separation and transport efficiency of charge carriers were developed, such as morphology modulation, − crystal facet engineering, − element doping, − heterostructure construction, ,− co-catalyst loading, , plasma enhancement, , and combinations thereof .…”