Preferential Oxidation Of CO-PROX Research Articles

Recent Advances in Regulating Ceramic Monolithic Catalyst Structure for Preferential Oxidation of CO in H2.

Preferential oxidation of CO (CO-PROX) has tremendous significance in purifying hydrogen for fuel cells to avoid catalyst poisoning by CO molecules. Traditional powder catalysts face numerous challenges, including high pressure drop, aggregation tendency, hotspot formation, poor mass and heat transfer efficiency, and inadequate thermal stability. Accordingly, ceramic monolithic catalysts, known as their excellent thermal stability, high surface area, and superior mass and heat transfer characteristics, are gaining increasing research attention. This review examines recent studies on ceramic monolithic catalysts in CO-PROX, placing emphasis on the regulation of active sites (e.g., precious metals like Pt and Au, and non-precious metals like CuO and CeO2), monolith structures, and coating strategies. In addition, the structure-catalytic performance relationships, as well as the potential and limitations of different ceramic monolithic catalysts in practical application, are discussed. Finally, the challenges of monolithic catalysts and future research prospects in CO-PROX reactions are highlighted.

In situ and Operando Characterisation in the Preferential Oxidation of Carbon Monoxide over Base Metal Oxide Catalysts: A Review

AbstractPolymer electrolyte membrane fuel cells (PEMFCs) are the core technology of the steadily growing hydrogen (H2) economy as they can convert chemical energy, in the form of H2, to electrical energy. If the H2 is derived from (green) hydrocarbons, via steam reforming and the water‐gas‐shift reaction, then it would contain small amounts of carbon monoxide (CO, 0.5–2 %), among other gases. CO poisons the platinum‐based anode catalyst in the PEMFC, and the current recommendation is to decrease its concentration to below 0.01 % (or 100 ppm) in the H2‐rich PEMFC feed. The preferential oxidation of CO (CO‐PrOx) is a promising strategy for decreasing the CO concentration, and base metal oxide catalysts have shown great potential in this regard. However, such catalysts tend to undergo physicochemical changes that cause undesirable catalytic activity and selectivity changes. This review discusses the different base metal oxide catalysts that have been evaluated in CO‐PrOx, while paying special attention to the various in situ and operando techniques that have been used to monitor the physicochemical changes of base metal oxides during operation. We conclude the review by highlighting the recent and possible future attempts of circumventing the undesired physicochemical changes of base metal oxides during CO‐PrOx.

Influence of preparation effects on 10 wt. %CuO/MnOx catalytic activity in the preferential oxidation of CO (CO-PROX)

Influence of preparation effects on 10 wt. %CuO/MnOx catalytic activity in the preferential oxidation of CO (CO-PROX)

Preventing Pt Catalysts in PEM Fuel Cells from CO Poisoning by Getting COPROX to Work

Proton-exchange membrane fuel cells still require CO-scrubbed H2 feedstreams to sustain long-duration performance. Sufficiently CO-free H2 costs an order of magnitude more than steam refining fossil fuels to generate H2—and until green processes produce the bulk of H2, preferential oxidation of CO from H2-rich feedstreams, also known as COPROX, is a necessity. We design architected catalytic platforms as ultraporous metal nanoparticle–modified oxide aerogels in which a covalently bonded oxide nanoparticulate network (NPs) supports comparably sized catalytic nanoparticles (NPs). The degree of physical and chemical interfacial intimacy stabilizes the more catalytically active low valent state of Cu and the ultraporous catalytic platform demonstrates high activity and on-stream durability for thermal catalysis of CO [1]. We then evaluate the selectivity and activity of this architected catalyst and a related aerogel, Cu/GdxCe1–xO2, for preferential oxidation of CO (COPROX) in H2 feedstreams. The catalysts convert >95% of CO at temperatures that prevent oxidation (consumption) of the H2 fuel (a purification reaction necessary to prevent poisoning of the anode catalysts in H2 fuel cells). The architected Cu/ceria catalysts achieve >95% selectivity vs. H2 and maintain this unprecedented activity for >16 h at 100°C in dry or humidified feedstreams [2,3]. The ability to purify H2 from steam-reformed fuels for use in proton-exchange membrane fuel cells increases power source flexibility as the world transitions to more sustainable, carbon-neutral routes to highly purified H2.[1] Stabilization of reduced copper on ceria aerogels for CO oxidation. C. L. Pitman, A. M. Pennington, T. H. Brintlinger, D. E. Barlow, L. F. Esparraguera, R. M. Stroud, J. J. Pietron, P. A. DeSario, D. R. Rolison, Nanoscale Adv. 2020, 2, 4547–4556 (10.1039/D0NA00594K).[2] Preferential oxidation of CO in H2-containing gas. T.G. Novak, D.R. Rolison, P.A. DeSario, US Patent Application, filed Jan 27, 2023 [Navy Case #210,945].[3] Cu/CeO2 and Cu/Gd-substituted CeO2 aerogels for active, selective, and stable COPROX catalysis. T.G. Novak, P.A. DeSario, T.H. Brintlinger, R.H. DeBlock, J.W. Long, and D.R. Rolison, ACS Sus. Chem. Eng. 2023, 11, 2853–2860 (10.1021/acssuschemeng.2c06158).

Bi-doped CuCeO2−x nanorod catalysts for photothermal CO-PROX in H2-rich streams: Enhanced oxygen vacancy and light absorption capacity

The wide band gap of ceria limited the light absorption and photothermal conversion of copper-ceria catalyst for photothermal preferential oxidation of CO (CO-PROX). In the present work, doping Bi element was adopted to regulate the energy band and catalytic performance of CuCeO2−x nanorod catalysts for photothermal CO-PROX. Bi/CuCeO2−x nanorods were synthesized by co-precipitation hydrothermal and deposition precipitation method. The Bi/CuCeO2−x catalyst with the molar ratio of Bi/Ce 2.5% exhibited the best catalytic performance and the CO conversion reached 90% under 2.5 suns with a surface temperature of 120 °C. The doped Bi species were incorporated into the CeO2 lattice and promoted the formation of oxygen vacancy sites and the strong Cu-Ce interaction, as revealed by XRD, XPS, Raman and H2-TPR results. In addition, doping Bi reduced the band gap of ceria nanorod from 2.77 to 2.47 eV and remarkably enhanced the UV–vis light response and photogenerated carrier separation of Bi/CuCeO2−x-2.5.

Identification of active sites for preferential oxidation of CO over Ru/TiO2 catalysts via tuning metal–support interaction

Preferential oxidation of CO (CO-PROX) is promising to purify H2 resources for proton-exchange membrane fuel cells with the challenges of identifying active sites and developing efficient catalysts. Herein, we report the high CO-PROX performance of Ru/TiO2 catalysts prepared by chemical-reduction method. The kinetic results reveal that the CO-PROX mechanism over the Ru/TiO2 catalysts follows the Langmuir-Hinshelwood model. We discover that Ruδ+ sites are more active for O2 activation and CO oxidation, while Ru0 sites prefer CO methanation in CO-PROX. The enhanced metal–support interaction (MSI) induces the formation of flat Ru nanoparticles, thereby increasing the number of interfacial Ruδ+ sites. Especially, compared with the conventional catalyst prepared by conventional wetness impregnation method, the as-synthesized RuTi-CR-R200 catalyst with strengthened MSI exhibits about 27 and 13 times higher reaction rate and intrinsic activity, respectively, for CO-PROX. To the best of our knowledge, our catalyst surpasses the catalytic performance of the reported Ru-based catalysts, and achieves 100% CO conversion in the wide temperature range of 90–150 °C, as well as high stability.

3D homogeneous porous copper-ceria catalyst for solar light driven photothermal CO-PROX in H2-rich gas: Enhanced light absorption and abundant oxygen vacancy

Photothermal preferential oxidation of CO (CO-PROX) on copper-ceria catalyst has emerged as an efficient and energy-saving approach for trace CO purification in hydrogen-rich gas. However, the limited light absorption and photo-to-thermal conversion capability of the copper-ceria catalyst hinder its catalytic performance for photothermal CO-PROX. In this study, a three-dimensional homogeneous porous copper-ceria catalyst (3DHP) was developed by using ordered SiO2 microspheres as the hard template. Compared to 3DIP catalyst with disordered silica colloid as the template, the uniform 3D pore structure, smaller particle size and abundant oxygen vacancy in 3DHP remarkably promote its photothermal catalytic performance for CO-PROX driven by the simulated solar light. The uniform 3D pore structure of 3DHP catalyst greatly strengthens the light absorption and photo-to-thermal conversion ability due to the slow photon effect of photonic crystals, which obtained higher surface temperature under the light irradiation. The higher specific surface area contributed to the high dispersion of copper species on 3DHP catalyst. Additionally, the more abundant oxygen vacancy facilitated the activation of O2 and the efficient electron-hole separation in 3DHP catalyst. Moreover, the visible light contributes to the largest portion of the total CO conversion due to the strong absorption ability of 3DHP in this range.

Cooperative effect of Pt and Cu on CeO2 for the CO-PROX reaction under CO2–H2O feed stream

Catalyst improvement for the preferential oxidation of CO (CO-PROX) is essential in developing efficient fuel cell technologies. Here, we investigate the promotion of the Cu/CeO2 system with Pt, prepared by impregnation and alcohol-reduction methods, in the CO-PROX reaction under ideal and realistic feed compositions. The high Pt dispersion in PtCu/CeO2 prepared by impregnation led to a CO conversion of 62% and CO2 selectivity of 83% at 50 °C under a feed stream composed of H2/CO/O2, while monometallic Cu/CeO2 and Pt/CeO2 showed negligible activity at these conditions. By adding CO2–H2O to the feed stream, PtCu/CeO2 catalysts prepared by both methods presented similar activity. The maximum CO conversion temperature was shifted to 100 °C. Under these conditions, Cu/CeO2 was inactive, and Pt/CeO2 showed identical conversion but lower CO2 selectivity. In-situ XANES revealed that fast oxidation of Cu species at low temperatures is responsible for Cu/CeO2 deactivation, while preferential adsorption of CO on Pt0 sites in PtCu/CeO2 avoided deactivation. The use of deactivation-resistant Pt sites as complimentary sites for CO activation associated with improved oxygen mobility over Cu–CeO2 surface proved to be an effective strategy for CO-PROX under H2O/CO2 feed stream at low temperatures.

Highly porous CuZnAl layered double hydroxides prepared by biochar-templated co-precipitation method as catalysts for the preferential oxidation of CO reaction

This study evaluates the effect of the porosity of Layered Double Hydroxide (LDH) containing copper (Cu), zinc (Zn), and aluminum (Al) synthesized by a co-precipitation method with biochar as a hard template on the preferential oxidation of CO (CO-PROX) reaction. Experimental data showed biochar as a hard template promoted structural changes in the LDH, such as 9 to 42 nm increase in pore size, decrease in the copper oxide crystallite size, and increase in metallic area, and enhanced its activity in the CO-PROX reaction. In situ DRIFTS analyses revealed CO was adsorbed on both Cu+ and Cu0 sites and Cu+ was the main species during the reaction for all catalysts.

Insights into the structure-performance relationship of CuOx-CeO2 catalysts for preferential oxidation of CO: Investigation on thermally induced copper migration process

A series of CuOx-CeO2 catalysts is designed by coating composite copper-ceria precursor with different thickness of CeO2, followed by thermal treatment at 500 and 700 °C respectively to investigate the outward migration phenomenon of Cu species induced by high temperature thermal treatment. For the CuOx-CeO2 catalysts calcined at 500 °C, the coating of CeO2 covers the active surface Cu entities and degrades the low temperature catalytic performance for preferential oxidation of CO (CO-PROX). However, thermal treatment at 700 °C induces the migration of large amounts of Cu entities from bulk to the surface of CeO2 nanoparticles and recovers the low temperature catalytic performance. In addition, 700 °C thermal treatment of CuOx-CeO2 catalyst without CeO2 coating causes excessive outward migration of Cu species and severe CuO agglomeration, which decreases the catalytic performance at low reaction temperature, but broadens the high temperature operation window on account of the formation of inverse CeO2-CuO structure. This work provides deeper insight into the regime of thermally induced Cu migration phenomenon, and illuminated the structure-performance relationship of copper-ceria based catalysts for CO-PROX.

Enhanced stability for preferential oxidation of CO in H2 under H2O and CO2 atmosphere through interaction between iridium and copper species

Preferential oxidation of CO (CO-PROX) is one of the most investigated methods for reducing residual CO in H2-rich stream to acceptable level in proton exchange membrane fuel cells. However, development of catalyst with high stability under simulated practical conditions is still challenging. Herein, a series of CuxCe1-xO2 (x = 0, 0.05, 0.09, 0.17) supported Ir catalysts were prepared and 1 wt%Ir/Cu0.09Ce0.91O2 exhibited full conversion of CO in a wide temperature window (80–180 °C), excellent stability and resistance to CO2 and H2O poison. Characterization results reveal that the superior performance was mainly associated with the interaction between Ir and Cu species, which resulted in that the adsorbed H2O on Ir sites was activated to react with adsorbed CO on Cu sites to form easily decomposable bicarbonates and formate species instead of main intermediate of carbonates for 1 wt% Ir/CeO2 and Cu0.09Ce0.91O2. This work provides a new sight for developing high-performance heterogeneous catalysts.

Reactant adsorption modulation by Fe and K in Pt catalyst for highly effective CO preferential oxidation in practical conditions

Developing efficient and stable catalysts for preferential oxidation of CO (CO-PROX) within a wide temperature window is significant for the industrial hydrogen resource purification process. Herein, dual promoters of Fe and K were deliberately introduced to the mordenite supported Pt catalyst for optimizing the adsorption behaviors of reactants in CO-PROX. The optimal 1Pt0.1 K/Fe-MOR catalyst can achieve 100% CO conversion and 60–70% CO2 selectivity (the ratio of CO/O2 is 1:1) under the realistic application temperature range (130–200 °C), and remains catalytically stable during 65 h testing at 140 °C. A combination of detailed characterizations and density functional theory (DFT) simulations show that Fe species can avoid the co-adsorption of O2 on the Pt sites by providing more oxygen vacancy to activate O2, while K species will further enhance the adsorption of CO and repulse the adsorption of H2 on Pt surface. Therefore, the optimized adsorption of CO, O2 and H2 results in a total CO conversion and high CO2 selectivity in high temperature range.

Simulated solar light-driven photothermal preferential oxidation of carbon monoxide in H2-rich streams over fast-synthesized CuCeO2–x nanorods

Aiming for purification the trace amount of CO in H2-rich streams with reduced energy consumption and low cost, solar-driven photothermal preferential oxidation of carbon monoxide on the non-precious metal oxide catalyst is proposed in this work. Cu doped CuCeO2−x nanorods catalysts were synthesized with a fast and simple coprecipitation method at the room temperature, which shows high CO oxidation activity in photothermal preferential oxidation of CO (CO-PROX) under UV-Vis-IR light irradiation. Rely on the various characterization methods such as UV-Vis-IR diffuse reflection spectrum (UV-Vis-IR DRS), photoluminescence spectrum (PL), transient photocurrent test, HRTEM, XRD, XPS, UV-Raman and H2-TPR, the optical and chemical properties of the CuCeO2−x nanorods catalysts were uncovered. The photothermal catalytic activity of CuCeO2−x nanorods doped with 10 wt% Cu reaches to 90% CO conversion under Xe lamp illumination (2.5 suns), and the solar driven photothermal CO-PROX reaction on CuCeO2−x nanorods were proposed to be proceeded by the light-to-thermal conversion and subsequently to drive a thermal catalytic process. The catalytic performance of CuCeO2−x nanorods in photothermal CO-PROX is closely related to the photo-to-thermal conversion efficiency and Cu-Ce synergetic interaction of CuCeO2−x nanorods catalyst. The introduction of CuOx greatly broaden the optical absorption range and promotes the light absorption capacity of ceria nanorods, which induces the catalyst with high photo-to-thermal conversion capability. Moreover, the optimal copper dopant benefits to enhance the Cu-Ce synergetic interaction and accelerate the oxidation reaction taking place at low temperature. CuCeO2−x nanorods catalyst shows promising competitive activity and ultra-low cost compared with the noble-based catalyst for the purification of hydrogen streams by the clean and eco-friendly sunlight sources.

Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

The preferential oxidation of CO (CO-PrOx) to CO2 is an effective catalytic process for purifying the H2 utilized in proton-exchange membrane fuel cells for power generation. Our current work reports on the synthesis, characterization and CO-PrOx performance evaluation of unsubstituted and magnesium-substituted iron- and cobalt-based oxide catalysts (i.e., Fe3O4, Co3O4, MgFe2O4 and MgCo2O4). More specifically, the ability of Mg to stabilize the MgFe2O4 and MgCo2O4 structures, as well as suppress CH4 formation during CO-PrOx was of great importance in this study. The cobalt-based oxide catalysts achieved higher CO2 yields than the iron-based oxide catalysts below 225 °C. The highest CO2 yield (100%) was achieved over Co3O4 between 150 and 175 °C, however, undesired CH4 formation was only observed over this catalyst due to the formation of bulk fcc and hcp Co0 between 200 and 250 °C. The presence of Mg in MgCo2O4 suppressed CH4 formation, with the catalyst only reducing to a CoO-type phase (possibly containing Mg). The iron-based oxide catalysts did not undergo bulk reduction and did not produce CH4 under reaction conditions. In conclusion, our study has demonstrated the beneficial effect of Mg in stabilizing the active iron- and cobalt-based oxide structures, and in suppressing CH4 formation during CO-PrOx.

Unravelling the role of Fe in trimetallic Fe-Cu-Pt/Al2O3 catalysts for CO-PROX reaction

This work proposes a trimetallic Fe-Cu/Pt/Al2O3 catalyst as an appealing system for preferential oxidation of CO (CO-PROX) reaction. The excellent conversion rates achieved by the Fe-Cu/Pt/Al2O3 catalysts under realistic reforming-surrogated feed streams along with the catalyst stability, reproducibility, and scalability showcase a very competitive system for CO-PROX reaction units. Furthermore, the systematic analysis conducted for Pt/Al2O3, Cu/Pt Al2O3, and Fe-Cu/Pt/Al2O3 catalysts enabled establishing meaningful relationships between catalytic behaviour and the catalyst surface to reactants interactions. Thus, the enhanced CO oxidation performances attained by the incorporation of Fe species into bimetallic Cu/Pt/Al2O3 catalysts were associated to superior surface electron densities and inhibited CO adsorption process over Pt surfaces. Remarkably, operando-DRIFTS spectroscopy evidenced significantly larger H-containing surface species developed over the trimetallic system. The enhanced abilities for developing thermally instable intermediates favoured by small amounts of Fe should indeed determine the enhanced catalysts behaviours displayed by the trimetallic Fe-Cu/Pt/Al2O3 catalyst.

The Impact of Ceria Loading on the CuOx−CeO2 Interaction and Performance of AuCu/CeO2−SiO2 Catalysts in CO‐PROX Reaction

AbstractThe use of CeO2 as a promoter for non‐reducible supports, such as SiO2, enhances the catalytic activity by facilitating the activation of species such as O2 and H2O. However, in the case of the bimetallic nanoparticles (NPs), the presence of promoters may also influence the migration of the metallic species, dramatically impacting the stabilization of catalytic sites and as consequence, the overall catalytic performance. In the case of AuCu alloy supported catalysts, the reduced form, i. e., AuCu alloy, or the oxidized form, i. e., Au−CuOx species, present different activities/selectivities. Moreover, several parameters, such as reactional conditions, AuCu particle size, support nature, and pretreatments dictate which species are stabilized on the catalyst surface. The preferential oxidation of CO (CO‐PROX) reaction, that takes place under the H2‐rich stream, favors the stabilization of the reduced form of the AuCu catalysts. However, the Cu distribution in the crystal lattice and the final composition of the AuCu NPs are dependent on the mobility of the Cu species on the support. In this work, we show that the loading of the CeO2 as a promotor in AuCu/SiO2 catalysts impacts the CeOx species and the Cu reincorporation into the Au lattice and consequently, the activity and stability of the catalysts under CO‐PROX reaction.

Support and gas environment effects on the preferential oxidation of carbon monoxide over Co3O4 catalysts studied in situ

We have studied the effect of different supports (CeO2, ZrO2, SiC, SiO2 and Al2O3) on the catalytic performance and phase stability of Co3O4 nanoparticles during the preferential oxidation of CO (CO-PrOx) under different H2-rich gas environments and temperatures. Our results show that Co3O4/ZrO2 has superior CO oxidation activity, but transforms to Co0 and consequently forms CH4 at relatively low temperatures. The least reduced and least methanation active catalyst (Co3O4/Al2O3) also exhibits the lowest CO oxidation activity. Co-feeding H2O and CO2 suppresses CO oxidation over Co3O4/ZrO2 and Co3O4/SiC, but also suppresses Co0 and CH4 formation. In conclusion, weak nanoparticle-support interactions (as in Co3O4/ZrO2) favour high CO oxidation activity possibly via the Mars-van Krevelen mechanism. However, stronger interactions (as in Co3O4/Al2O3) help minimise Co0 and CH4 formation. Therefore, this work reveals the bi-functional role required of supports used in CO-PrOx, i.e., to enhance catalytic performance and improve the phase stability of Co3O4.

Improving the Catalytic Performance of Cobalt for CO Preferential Oxidation by Stabilizing the Active Phase through Vanadium Promotion

Preferential oxidation of CO (COPrOx) is a catalytic reaction targeting the removal of trace amounts of CO from hydrogen-rich gas mixtures. Non-noble metal catalysts, such as Cu and Co, can be equally active to Pt for the reaction; however, their commercialization is limited by their poor stability. We have recently shown that CoO is the most active state of cobalt for COPrOx, but under certain reaction conditions, it is readily oxidized to Co3O4 and deactivates. Here, we report a simple method to stabilize the Co2+ state by vanadium addition. The V-promoted cobalt catalyst exhibits considerably higher activity and stability than pure cobalt. The nature of the catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co2+ state on the surface was verified by in situ NEXAFS at 1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate surface layer, containing tetrahedral V5+ and octahedral Co2+ cations, with an electronic and geometric structure that is deviating from the standard mixed bulk oxides. In addition, V addition helps to maintain the population of Co2+ species involved in the reaction, inhibiting carbonate species formation that are responsible for the deactivation. The promoting effect of V is discussed in terms of enhancement of CoO redox stability on the surface induced by electronic and structural modifications. These results demonstrate that V-promoted cobalt is a promising COPrOx catalyst and validate the application of in situ spectroscopy to provide the concept for designing better performing catalysts.

Environment-Dependent Catalytic Performance and Phase Stability of Co3O4in the Preferential Oxidation of Carbon Monoxide StudiedIn Situ

The preferential oxidation of CO (CO-PrOx) with co-fed O2 is an attractive route for removing trace amounts of CO in the H2-rich reformate gas prior to being used for power generation in proton-exc...

Recent Advances on the Rational Design of Non-Precious Metal Oxide Catalysts Exemplified by CuOx/CeO2 Binary System: Implications of Size, Shape and Electronic Effects on Intrinsic Reactivity and Metal-Support Interactions

Catalysis is an indispensable part of our society, massively involved in numerous energy and environmental applications. Although, noble metals (NMs)-based catalysts are routinely employed in catalysis, their limited resources and high cost hinder the widespread practical application. In this regard, the development of NMs-free metal oxides (MOs) with improved catalytic activity, selectivity and durability is currently one of the main research pillars in the area of heterogeneous catalysis. The present review, involving our recent efforts in the field, aims to provide the latest advances—mainly in the last 10 years—on the rational design of MOs, i.e., the general optimization framework followed to fine-tune non-precious metal oxide sites and their surrounding environment by means of appropriate synthetic and promotional/modification routes, exemplified by CuOx/CeO2 binary system. The fine-tuning of size, shape and electronic/chemical state (e.g., through advanced synthetic routes, special pretreatment protocols, alkali promotion, chemical/structural modification by reduced graphene oxide (rGO)) can exert a profound influence not only to the reactivity of metal sites in its own right, but also to metal-support interfacial activity, offering highly active and stable materials for real-life energy and environmental applications. The main implications of size-, shape- and electronic/chemical-adjustment on the catalytic performance of CuOx/CeO2 binary system during some of the most relevant applications in heterogeneous catalysis, such as CO oxidation, N2O decomposition, preferential oxidation of CO (CO-PROX), water gas shift reaction (WGSR), and CO2 hydrogenation to value-added products, are thoroughly discussed. It is clearly revealed that the rational design and tailoring of NMs-free metal oxides can lead to extremely active composites, with comparable or even superior reactivity than that of NMs-based catalysts. The obtained conclusions could provide rationales and design principles towards the development of cost-effective, highly active NMs-free MOs, paving also the way for the decrease of noble metals content in NMs-based catalysts.