Low-temperature photocatalytic dry reforming of methane over porous cylindrical, gyroidal, and asymmetric catalyst structures

doi:10.21203/rs.3.rs-3830664/v1

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Low-temperature photocatalytic dry reforming of methane over porous cylindrical, gyroidal, and asymmetric catalyst structures

https://doi.org/10.21203/rs.3.rs-3830664/v1

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Recent advances in the photocatalytic activation of dry reforming of methane (DRM: CO₂ + CH₄ → 2CO + 2H₂) at low temperature and ambient pressure have generated considerable interest as a promising route to convert greenhouse gases into valuable synthetic gas (syngas). While detailed studies have revealed the mechanisms involved in photocatalytic DRM at metal-semiconductor interfaces, less attention has been devoted to how high surface area semiconductor supports may enhance such conversions. Here we structure triblock terpolymer self-assembly directed sol-gel derived transition metal oxide (Ta₂O₅ or TiO₂) supports of Rh-decorated photocatalysts into various equilibrium and non-equilibrium derived porous morphologies and show how they modulate single-pass conversion, total production rate, and material efficiency. Supported by in-depth materials characterization and flow simulations rationalizing observed trends, results reveal record catalyst performance. Our work suggests that asymmetric pore structures simultaneously optimizing mass transport and surface area may be well-suited to maximize photocatalyst performance.

Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis

Physical sciences/Materials science/Soft materials/Polymers

Physical sciences/Materials science/Materials for energy and catalysis/Porous materials

The drastic increase in atmospheric greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) is a primary driver of climate change. Dry reforming of methane (DRM: CO₂ + CH₄ → 2CO + 2H₄) has been attracting attention as one of the most promising ways to convert CO₂ and CH₄ into a valuable synthesis gas (syngas) consisting of carbon monoxide (CO) and hydrogen (H₂) [1, 2]. This technology can potentially reduce environmental harms associated with emissions from the extraction and utilization of natural gas, shale gas, biogas, methane hydrates, and other natural resources, while providing a pathway to creating feedstocks for organic chemistry precursors [3–6]. DRM is a highly endothermic reaction (ΔH = + 247 kJ/mol) that traditionally requires temperatures of 800°C or higher to proceed efficiently. Due to these high reaction temperatures, DRM catalysts suffer from deactivation by thermal aggregation of co-catalysts and carbon deposition, limiting their practical use.

Recent advances have been made toward photocatalytic, rather than thermally activated, DRM. These systems use metals supported by oxide semiconductors such as Ga₂O₃, SrTiO₃, TiO₂, or CeO₂, or metal localized surface plasmon resonances in reactant activation [7–13]. Very recently, it has been reported that rhodium nanoparticle-loaded strontium titanate (Rh/SrTiO₃) and rhodium intertwined with cerium oxide (Rh#CeO₂) considerably exceeded the theoretical thermal catalyst thermodynamic limit by utilizing light rather than heat [14, 15]. Mechanistic investigations via isotope studies of the photocatalytic DRM system identified that lattice oxygen in the oxide mediates oxidation, while charge density in the supported metal mediates the reduction of CO₂, suggesting that the metal-semiconductor interface is key to the photocatalytic reaction. In particular, the Rh#CeO₂ photocatalyst with many exposed nanoscopic metal-semiconductor interfaces exhibited a methane conversion rate exceeding 60% due to the efficient charge separation of photoexcited electron-hole pairs and diffusion of lattice oxygen. The photocatalysts described above were used in the form of non-porous solids, however, thus limiting the accessible surface area per unit volume. Furthermore, despite relatively high reactant conversions, these studies were performed at very low flow rates of dilute reactants, achieving impractically low production rates of CO and H₂.

Here photocatalysts for DRM were developed from porous semiconducting oxides decorated with rhodium metal nanoparticles. To that end, block copolymer self-assembly (BCP SA) directed periodically ordered mesoporous TiO₂ and Ta₂O₅ supports with hexagonally-packed cylinders (Hex), co-continuous double gyroid (G^D), or alternating gyroid (G^A) morphologies and homogeneous 15–30 nm diameter pores were synthesized from evaporation induced SA (EISA): a (close to) equilibrium process. For comparison, TiO₂ film supports combining asymmetric, hierarchical pore structures across the film normal with well-defined mesoporosity throughout the material were derived from a non-equilibrium BCP SA approach referred to as SNIPS (self-assembly plus non-solvent induced phase separation). Rhodium metal nanoparticles were generated on supports by reduction of rhodium chloride hydrate to rhodium metal. Photocatalytic DRM conversions were studied under light irradiation without external heating, achieving up to 75% (Rh/Ta₂O₅-G^D), 78% (Rh/TiO₂-Film) and 82% (Rh/Ta₂O₅-G^A) CO₂ and CH₄ conversion at low flow rates. In concentrated gas at high flow rates, CO/H₂ production rates up to 28,500 mmol / (hr·g) at 9.7% CO₂/CH₄ conversion were achieved (Rh/TiO₂-Film). This mass-normalized production rate is more than 400 times higher than that of previous state-of-the-art ambient-temperature photocatalysts. All mesoporous catalysts outperformed commercial non-mesoporous Ta₂O₅ and TiO₂ powder based catalysts. Furthermore, only moderate performance degradation was observed over a 72-hour long test run for Ta₂O₅-G^A supports, yielding a turnover frequency of 38,300/hr. Especially at high flow rates, G^A and asymmetric film supports demonstrated enhanced single-pass conversions as well as production rates on the bases of mass and surface area over hexagonal and G^D supports. With respect to the latter two performance metrics, the asymmetric thin film catalyst substantially outperformed all other samples across all flow rates tested. Results demonstrate the importance of asymmetric hierarchical pore structures that optimize both mass transport and surface area, thereby allowing substantial improvements in DRM photocatalytic reactivity.

Synthesis and characterization of porous supports and metal decorated photocatalysts

Seven porous catalyst supports were synthesized for this study from two materials and with four morphologies: mesoporous TiO₂ and Ta₂O₅ supports with alternating gyroid (G^A), double-gyroid (G^D), and hexagonal-cylindrical (Hex) structures, as well as a porous TiO₂ thin film combining asymmetric, hierarchical pore structures across the film normal with well-defined mesoporosity throughout the material (Figure 1, Materials and Methods) [16]. Gyroidal and hexagonal supports were fabricated via BCP SA with one of two poly(isoprene-block-styrene-block-ethylene oxide) (PI-b-PS-b-PEO or ISO) terpolymers (ISO-1 and ISO-2, Supplementary Table 1), depending on the desired mesostructure: G^A from ISO-1; G^D and Hex from ISO-2. Structure formation for gyroidal and hexagonal structures occurred overnight via evaporation-induced self-assembly (EISA) at 40°C. The TiO₂ thin film was fabricated via SNIPS using a poly(isoprene-block-styrene-block-4-vinyl-pyridine) (PI-b-PS-b-P4VP or ISV) terpolymer (ISV-1, Supplementary Table 1). Hybrid polymer/oxide materials were subsequently heat treated at 130°C for 5 hours in vacuum, followed by calcination in air to remove the ISO or ISV terpolymer and crystallize the oxide phase. TiO₂ and Ta₂O₅ materials were calcined at 550°C and 700°C, respectively. After calcination, porous supports were characterized via a combination of small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and nitrogen sorption to establish sample structure and porosity (Materials and Methods).

The corresponding scattering patterns for the six periodically ordered materials are shown in Figure 2a. The asymmetric SNIPS derived TiO₂-Film sample lacking mesoscale periodic order was not included in the SAXS analysis. The low signal intensity for all support materials due to the strongly absorbing nature of these crystalline oxides, together with the relatively broad observed peaks, makes assignments to underlying lattices challenging. Tentative indexing of the observed peaks is shown in the figure. The four peaks assigned to samples TiO₂-G^A and Ta₂O₅-G^A are consistent with an alternating gyroid (G^A) lattice with cubic unit cell lattice parameters of 52.3 nm and 44.8 nm, respectively. For sample TiO₂-G^D, the first two indexed peaks of the pattern have been tentatively assigned to a double gyroid lattice with cubic lattice parameter of a = 135.8 nm. For sample Ta₂O₅-G^D, three of the observed peaks have been tentatively assigned to a double gyroid lattice with cubic lattice parameter a = 94.0 nm. SAXS results for samples TiO₂-Hex and Ta₂O₅-Hex both show a very broad first order peak and one additional higher order reflection tentatively assigned to hexagonal lattices. Associated (10) lattice dimensions, d₁₀, were 47.6 nm and 39.8 nm, respectively. Interestingly, although TiO₂ and Ta₂O₅ samples were synthesized from the same terpolymers, all TiO₂ structures showed patterns shifted to smaller values of q, i.e., larger mesostructure unit cell sizes than Ta₂O₅ counterparts. This shift is likely due to the higher density of Ta₂O₅, resulting in higher degrees of shrinkage during calcination.

To corroborate lattice interpretations from SAXS (Table 1), sample mesostructure was further investigated via SEM (Figure 2b-i). Supports were imaged after calcination (to increase contrast), but prior to rhodium deposition. SEM images for G^A and G^Dsamples clearly show continuous network morphologies, while those of TiO₂-Hex and Ta₂O₅-Hex both show cylinders, consistent with gyroidal and hexagonally-packed cylinder lattice assignments from SAXS, respectively. In the G^A morphology, the d₁₀₀ distance can be measured as the distance between the wall of a pore, and the center of the neighboring pore. From the visible [111] projection of the G^A samples in Figure 2b, f lattice parameters of d₁₀₀ = 51 ± 2 nm for TiO₂-G^A and d₁₀₀ = 45 ± 3 nm for Ta₂O₅-G^Awere obtained. These are similar to the respective d₁₀₀ values obtained from SAXS (52.3 nm and 44.8 nm). Along the G^D structure [211] projection, the width of each row of coils is approximately 80% of the d₁₀₀ length, which was determined as d₁₀₀ = 132 ±5 nm for TiO₂ and d₁₀₀ = 88 ± 6 nm for Ta₂O₅ from suitably orientated grains in Figure 2c, g [17]. This is again similar to the values for TiO₂(d₁₀₀ = 135.8 nm) and Ta₂O₅ (d₁₀₀ = 94.0 nm) derived from SAXS. SEM images of the respective gyroid structures in Figure 2b, c, f, g have visible features in agreement with simulated projections of these structures (see insets) corroborating the lattice assignments. SEM analysis of the (10) spacing for the hexagonal structures (Figure 2d, h), the distance between rows of pores, yields 48 ± 2 nm and 42 ± 3 nm for the TiO₂-Hex and Ta₂O₅-Hex structures, respectively, consistent with values of 47.6 nm and 39.8 nm from SAXS results. SEM images of the asymmetric TiO₂ film (Figure 2e, i) show a finger-like asymmetric cross-section with mesoporous walls and a mesoporous top surface. In the context of photocatalysis applications, these images collectively suggest that pore accessibility should increase in the following order: hexagonally packed cylinder < networked gyroidal < asymmetric structures. Finally, all seven supports were characterized by quantitative sorption/desorption analyses (Supplementary Discussion, Supplementary Figure 1). Results summarized in Table 1 show systematic decreases in surface area for both TiO₂- and Ta₂O₅-supports across morphologies (G^A > G^D > Hex) and higher values for TiO₂ as compared to Ta₂O₅.

Table 1: Summary of structural characterization results for oxide catalyst supports and rhodium particles. BJH pore statistics do not include pores larger than 300 nm, thus the TiO₂ film values marked with asterisks (BET surface area, average pore width, pore volume, and overall porosity) are likely slight underestimations.

The seven catalyst supports were decorated with Rh metal nanoparticles via hydrothermal infiltration of Rh and subsequent photoreduction under illumination during catalytic evaluation (Materials and Methods). Resulting metal photocatalysts were characterized by X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) (Supplementary Figure 2), with quantitative XPS results summarized in Supplementary Table 2. All materials measured were phase-pure and fully crystalline, indicated by the lack of broad amorphous background scattering, with metallic Rh detected on all samples. Ta₂O₅ supports could be matched to the orthorhombic tantalum oxide lattice structure. TiO₂ supports exclusively showed an anatase titania lattice structure. XPS results confirmed no chemical discrepancies between different structures of each material (Supplementary Figure 2b, d). Corroboratory UV-vis measurements (Supplementary Figure 3) confirmed all oxides have approximately 3 eV band gaps. Samples were not intentionally doped but may have lower than expected band gaps due to either unintentional impurities or the higher surface-to-bulk ratio resulting from the 10-30 nm diameter strut networks. Rhodium particles were fully reduced to the metallic phase, without remnant rhodium chloride precursor detected in XRD, although some remnant Cl was detected in XPS as a low-intensity peak around 200 eV. Oxide crystallite sizes and rhodium particle sizes were calculated via Scherrer analysis from XRD patterns (Supplementary Figure 2a, c). XRD-based structural information for oxide supports and rhodium particles is summarized in Table 1. Oxide crystallite sizes can be compared to oxide domain dimensions as reflected by SEM results (Figure 2b-i). From SEM, the TiO₂-G^A and Ta₂O₅-G^Astructures have ~12 nm thick struts, slightly smaller than the XRD derived crystallite sizes of 14 nm and 18 nm. TiO₂-G^D and Ta₂O₅-G^Doxide strut dimensions were ~25 nm, larger than the TiO₂-G^D 9 nm crystallite size and close to the ~27-28 nm Ta₂O₅ XRD crystallite size. SEM derived oxide domain dimensions for both hexagonal supports were also ~25 nm, larger than the XRD based crystallite sizes of ~9 nm and ~18 nm for TiO₂-Hex and Ta₂O₅-Hex, respectively. Oxide crystallite sizes for most samples were therefore either close to (for G^D structures) or smaller (for hexagonal structures) than the associated oxide domain dimensions. Slightly larger domain sizes in G^A structures without loss of mesostructure may suggest elongation of crystallites along the sample strut direction. All this is consistent with periodic mesoscale structure retention after high temperature thermal processing, as substantial crystalline overgrowth beyond the confinement of BCP SA directed nanoscale domains is typically associated with loss of mesoscale structure.

TEM micrographs were collected for the three most active catalyst supports (TiO₂-Film and G^Acatalysts, vide infra) to confirm the presence of metallic rhodium. Figure 3a-c show open pore networks for all structures, including the TiO₂-Film (Figure 3c) which is likely a fragment from the more-ordered top surface layer. The micrograph of the TiO₂-G^A structure (Figure 3b) suggests slightly improved periodic order relative to the Ta₂O₅-G^Amaterial (Figure 3a), consistent with the slightly improved peak definition observed in its SAXS pattern (Figure 2a). Figure 3d-f depicts high-magnification images of the same structures revealing lattice spacings (see insets) that can be indexed to Rh metal (200) and various oxide lattice planes. Results agree with earlier XRD and XPS datasets suggesting the presence of metallic Rh in catalyst samples.

Photocatalytic DRM

Photocatalysts were evaluated in a flow-through setup (Materials and Methods). Feed gas streams of either 1%/1%/98% or 10%/10%/80% CH₄/CO₂/Ar were delivered to the photocatalysts at flow rates of 0.2-2.1 mmol / hr for the 1% feed gas and 2.5-25 mmol / hr for the 10% feed gas. A large range of flow rates was used to enable a more comprehensive investigation of catalyst behavior relative to singe flow rate studies. The gyroidal and hexagonal catalyst materials were held in a quartz glass reactor (Supplementary Figure 4a), while the TiO₂ film was held in a top-loading brass reactor designed to accept samples in a film geometry (Supplementary Figure 4b and 5). Both reactors were illuminated by a 300W Xe lamp (Supplementary Figure 6) and oriented to make the reactant gas incident to the top illuminated surface. For a performance comparison of both reactors, please see Materials and Methods section. Products were measured with a gas chromatograph (Supplementary Figure 7). Volumetric flow rates ranged from 10-100 mL / min for each gas concentration. Conversion (%) is defined as the amount of products generated relative to the complete conversion of all reactants to products.

Figure 4a-c shows the photocatalytic DRM performance of all seven materials. At the lowest 0.25 mmol / hr feed rate, maximum conversions for the highest performers Ta₂O₅-G^A, TiO₂-Film, and Ta₂O₅-G^D were 81.6%, 77.8% and 75.1%, respectively. These conversion values all exceeded the 50-64% conversion by the most active DRM room-temperature photocatalysts to date, i.e., Rh/SrTiO₃ and Rh#CeO₂ nanocomposites [14,15]. The G^A and thin film catalysts exhibit a similar flow-rate dependence above 1 mmol / hr, with both types of catalyst supports showing conversions that outpace the performance of all other samples. Both the G^A and asymmetric film catalysts are templated by terpolymers with only ~7-12 vol% hydrophilic blocks (as compared to ~46% for all other structures, Supplementary Table 1), consistent with more accessible surface area of openly porous materials compared to the other catalysts. Since the asymmetric thin film catalyst has ~50x less mass (Supplementary Figure 5) for the same illuminated area than other materials tested, its mass-normalized performance shows a record photocatalytic mass activity (Figure 4b), substantially outperforming all other samples across all flow rates tested in this performance metric. The two catalysts with G^Amorphology outperform all other equilibrium derived and periodically ordered photocatalysts in this metric (especially above a flow rate of ~1 mmol / hr) due to their high-porosity-derived reduced density. Typical plateaus in production occur at 10 mmol / hr for Hex / G^D catalysts, 10-20 mmol/hr for G^A catalysts, and 20 mmol / hr for the asymmetric TiO₂ film (Figure 4b).

For both studied oxides, surface area was significantly higher for the gyroidal supports as compared to their hexagonal counterparts (Table 1). But using the performance metric of production per surface area, the asymmetric film also comes out on top by a large margin across all flow rates tested (Figure 4c). Due to the inability of the BJH method to characterize pores larger than 300 nm, the measured surface area of the asymmetric TiO₂ film is likely slightly underestimated. But since the surface area of the limited number of >300 nm macropores is minute compared to that of the many < 50 nm mesopores, this overestimation cannot account for the large margin by which the TiO₂ film outperforms the remaining structures on a surface-area-normalized basis. Within the family of equilibrium derived periodically ordered catalysts, when normalized to the internal (BET) surface area, the production rates of tantalum oxide-based catalysts are higher than those of their titania counterparts across all flow rates measured. In part, this performance difference results from TiO₂’s higher surface area, around a factor of 3 across all morphologies (Table 1). Other contributing effects (e.g., side reactions) are discussed in the Supplementary Information. Within the set of samples for each oxide, at slow feed rates (< 2 mmol / hr), surface-area-normalized performance of the gyroidal and hexagonally structured samples are comparable (Figure 4c). In contrast, at high flow rates (> 2 mmol / hr), the surface-area-normalized production rates of catalysts with G^A morphology increasingly outperform those of catalysts with either hexagonal or G^D morphology (by 2-3x at 20 mmol / hr).

A durability test was conducted over 72 hours with photocatalyst Ta₂O₅-G^A at its maximum mass-normalized reaction condition, i.e., at a high flow rate of 18.5 mmol / hr (Supplementary Figure 8). Photocatalyst performance only moderately declined to ~75% of its initial activity over the 72hr time period, reaching a turnover frequency, defined as mole of CO and H₂ (averaged) produced per mole of Rh over time, of 38,320 / hr or 10.6 / s. Additional comparisons to the porous catalysts were performed with commercial non-porous Ta₂O₅ and TiO₂ particle powder catalysts (Supplementary Figure 9, Supplementary Table 3). These reference results verify that the used reactors adequately reproduce the performance of reference catalysts relative to the literature. As expected, all porous catalysts outperform the non-porous catalysts in percentage of reactants converted, mass-normalized production, and surface-area-normalized production (Supplementary Figure 9a-c).

The product ratio between CO and H₂ as a function of flow rate is shown in Figure 4d. Notably, the CO/H₂ ratio substantially changed with feed gas concentration, with ratios below and above 1 for feed gas concentrations of 1% and 10% CH₄/CO₂, respectively. Overall, there seems to be qualitatively different behavior for tantalum oxide and titania based photocatalysts, with product ratios at high flow rates (>2 mmol / hr) converging towards 1 for tantalum oxide-based catalysts, while those of titania diverge towards a higher CO to H₂ ratio in the product stream. The asymmetric TiO₂ film catalyst is the exception to this rule as its behavior converges towards 1, similar to the tantalum oxide-based materials. For further in-depth analysis and discussion of this behavior, we refer to the supplementary information (Supplementary Discussion; Supplementary Table 4).

Figure 4e and Supplementary Table 3 provide an overall comparison of the (meso-) porous catalysts studied in this work to published works in photocatalytic and photo-thermal DRM in the form of a bar chart of production per illuminated area (spot size). This measure of performance is relevant for photocatalyst deployment since reactor design is primarily limited by the illuminated area of the catalyst, rather than its mass. Notably, Rh/TiO₂-G^Acan be directly compared to a non-structured powder Rh/TiO₂ catalyst [15], where Rh/TiO₂-G^A demonstrates both a higher maximum reactant conversion (50.3% vs 20.7%) and a 240x improvement in the maximum measured production per illuminated area. Mesostructure variations alone account for this substantial uplift in both single-pass conversion and maximum production for Rh/TiO₂. The highest activity catalyst studied, the asymmetric TiO₂ film, demonstrated a 719x improvement over the previously studied room-temperature Rh/TiO₂ catalyst in terms of production per illuminated area. This performance metric is not affected by the very low mass of the thin film, rather, the highly porous material and hierarchical pore network drove improvements in both conversion and production over all other TiO₂ structures studied. When normalized by mass, the asymmetric TiO₂ film achieved between 150-1500x improvements (depending on flow rate) compared to previously studied Rh/TiO₂.

Flow simulations were performed to estimate the tortuosity for each of the catalyst structures used (Figure 5, Materials and Methods). Tortuosity in this study is defined as the path length of a simulated gas flow line versus the length of the structure studied. Each of the simulations provided qualitative insights into gas transport through these porous photocatalysts. As expected, of all the equilibrium-based periodically ordered structures tested, the highly porous G^Astructure with approximately 85% porosity had the lowest simulated tortuosity of T = 1.2 (Figure 5c). The path of gas through this structure would be 1.2x longer than a direct path. Tortuosity is inversely related to diffusivity through catalyst structures, so it follows that the G^A structure with the lowest tortuosity has the most advantageous gas flow characteristics for catalysis, and thus the highest activity. The G^D structure had an estimated tortuosity of T = 2.0, while the hexagonal structure resulted in an estimate of T = 3.3 (Figure 5a, b). Conceptually, 1-dimensional pores should have about 3x higher tortuosity compared to a 3-D pore network (G^A), which agrees with these estimates.

Determining tortuosity in the porous asymmetric thin film structure required a separate approach derived from a previously described method (Materials and Methods) [18]. From associated simulations, tortuosity varied as a function of membrane thickness (Supplementary Figure 10). The top 8 µm mesoporous layer of the film, dominated by BCP SA, has a tortuosity slightly above T = 1.3. However, when averaged over the entire ~100 μm thick asymmetric film (Figure 2i, Supplementary Figure 10), including the essentially open macroporous substructure, tortuosity of the membrane decreases to below T = 1.1, the lowest tortuosity of all catalyst structures studied. This is consistent with the high conversion efficiency of this structure measured in the catalysis experiments (Figure 4).

Rh metal nanoparticles on semiconducting titanium and tantalum oxide supports with porous periodic alternating gyroid, double gyroid, hexagonally-packed cylinder, and asymmetric thin-film structures were studied to evaluate the impact of pore geometry, connectivity, and asymmetry on photocatalytic DRM at ambient pressure in the absence of external heating. Porous supports were prepared by BCP SA directed sol-gel synthesis from either ISO triblock terpolymers (periodic structures) or an ISV triblock terpolymer (asymmetric films) followed by thermal processing, and Rh metal decoration. A combination of SAXS, WAXS, SEM, TEM, and nitrogen sorption/ desorption characterization demonstrated that the oxide supports had either periodically ordered mesopores or asymmetric hierarchical pore structures exhibiting meso- to macro-porosity derived from equilibrium or non-equilibrium formation processes, respectively. In all cases, oxide crystallization occurred without mesostructure collapse, while Rh metal nanoparticle sizes were significantly smaller than the pore diameters of the oxide supports, preventing pore clogging. Photocatalytic DRM showed conversion rates of over 81% for Ta₂O₅-G^A and 77% for the asymmetric TiO₂-Film at low flow rates. At higher flow rates, activity increased, reaching a maximum production rate of over 28,560 mmol / (hr·g) for the TiO₂-Film, and 780 mmol / (hr·g) for the Ta₂O₅-G^A structure. To the best of our knowledge, these photocatalysts showed the highest reported activities to date for photocatalytic DRM without external heating. Furthermore, comparing periodic titanium- to tantalum-based oxide mesostructure supports, the latter showed better performance and more optimal DRM product ratios, especially at high flow rates. Comparing samples with different periodic mesopore structures derived from the same material (e.g., Ta₂O₅-G^A vs Ta₂O₅-Hex) demonstrated that details of the pore geometry and connectivity are critical to optimizing photocatalysis. Widely open alternating gyroids with three-dimensionally co-continuous network pore structures enhanced activity, whereas one-dimensional hexagonal pore structures met early performance plateaus. Photocatalytic results align with insights from flow simulations, suggesting that interconnected pores improve gas diffusion dynamics, thereby significantly enhancing performance of photocatalysts for DRM. Additionally, to the best of our knowledge, this first-time study of an asymmetric, hierarchically porous DRM photocatalyst film suggests that such membrane-type structures, combining fast mass transport through (finger-like) macropores and high surface area from mesopores distributed throughout the material, may be well-suited for not only photocatalytic DRM, but also other photocatalytic reactions. Such low-density thin membrane-type catalysts effectively achieve high reactant conversion while minimizing the required mass of expensive metal promoters. While this study focused on improvements in catalytic materials using simple reactor designs, further performance gains should be possible through improved process designs to recycle unreacted gas, advanced reactor designs to minimize unilluminated catalyst, and optimized low-power LED illuminations. We hope that our results of enhancing photocatalytic performance of a given class of catalysts (here: of semiconductor-metal photocatalysts) via fine control over pore geometry, connectivity, and asymmetry in mesoporous supports will stimulate further efforts to not only control atomic level structure but also meso- to macro-structural aspects to develop next generation high-performance reaction systems.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (U.W.) and [email protected] (S.S.)

Author Contributions

S.S. and U.W. conceived this project. W.M. and S.S. contributed equally to the work. S.S. and W.M. conducted materials synthesis, structural analysis, and catalytic testing. P.T. and F.Y. synthesized and analyzed the ISO terpolymers. F.Y and L.T. collected SEM images and performed subsequent image analysis. W.T. assisted with the nitrogen sorption analysis. A.S. performed flow simulations of gyroid and hexagonal structures. M.S.R. constructed the thin-film model and performed flow simulations of the thin-film. A.R. fabricated and donated the top-loading reactor and assisted catalytic testing. U.W. and J.S. continuously reviewed and discussed the materials synthesis/characterization and catalysis results with W.M and S.S. to continuously advance the project. W.M., S.S., and U.W. wrote the manuscript, with input from all co-authors.

ACKNOWLEDGMENT

S.S. thanks the Kavli Institute at Cornell (KIC) for nanoscale science as well as the Japan Society for the Promotion of the Science (JSPS) for fellowship funding. U.W. and J.S. acknowledge support from NSF (DMR-2307013 to U.W. and CBET-1805400 to J.S.). This work made further use of the Cornell Center for Materials Research Shared Facilities which are supported through the NSF MRSEC program (DMR-1719875).

We acknowledge Karl Termini and the Cornell CAS Professional Glass Shop for their contributions to designing and fabricating equipment used in the study.

S.S. and W.M. thank Sarah Hesse for fabrication of the ISV terpolymer used in this study.

This research used beamline 11-BM (CMS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors thank Peter Beaucage for the collection of this data.

This work is based on research conducted at the Center for High-Energy X-ray Sciences (CHEXS), which is supported by the National Science Foundation (BIO, ENG and MPS Directorates) under award DMR-1829070.

We thank Rajesh Bhaskaran and ANSYS Inc. for the ANSYS License Key.

Proton nuclear magnetic resonance (¹H NMR) spectroscopy was performed at the Cornell University NMR facility, which is supported in part by the NSF through an MRI award (CHE1531632).

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Low-temperature photocatalytic dry reforming of methane over porous cylindrical, gyroidal, and asymmetric catalyst structures

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