Dissociative Adsorption Of CO Research Articles

The active pair of Coδ+/Co0 tailored by nickel and CaTiO3 with perovskite phase for higher alcohols synthesis from syngas

Higher alcohols synthesis (HAS) from syngas (CO + H2) is much attractive, but achieving a high selectivity of alcohols is still challenging due to the difficult balance between dissociative adsorption and associative adsorption of CO. In this work, a series of Coδ+/Co0 tailored by nickel and CaTiO3 catalysts derived from a series of perovskite pre-catalyst Ca0.6La0.4Ti0.6Co0.4-xNixO3 were prepared and applied to HAS. Highly dispersed Co-Ni bimetals on CaTiO3 were formed after reduction owing to the uniform distribution of nickel and cobalt ions in the perovskite phase. Through the reduction process, the bimetallic nanoparticles of Co-Ni with Co enriched in surface and Ni in core are produced. In addition, Coδ+ in the active pairs of Coδ+/Co0 were obtained via the interaction between Co and perovskite; and the chemical state of Co0 in the active pairs was tuned by the interaction between nickel and cobalt. The resulted catalyst showed excellent selectivity to higher alcohols with the total alcohols as high as 45.34 %. The higher alcohols reached 70.29 wt% at 270 °C, 3.0 MPa and a GHSV of 3900 mL g−1h−1 with H2/CO = 2/1.

Selective N‐Methylation of N‐Methylaniline with CO2 and H2 over Cu/In2O3 Catalyst†

Comprehensive SummaryN‐Methylation of amines with CO2 and H2 is a potential approach for CO2 utilization because N‐methylated amines can be used as solvents and organic intermediates. In2O3‐supported Cu (Cu/In2O3) acts as an effective heterogeneous catalyst for N‐methylation reaction of N‐methylaniline (MA) with CO2 and H2, showing higher N,N‐dimethylaniline (DMA) selectivity than other supported Cu catalysts. On one hand, the dispersion of Cu can be improved by the defective In2O3 support. On the other hand, In2O3 support is active in the dissociative adsorption of CO2 through C—O bond breaking. In addition, the H2 dissociation ability of In2O3 can also be enhanced by Cu. The combination of Cu and In2O3 is effective in the activation of CO2, the adsorption of intermediate N‐methylformanilide (MFA), the hydrogenation of MFA to DMA and the prohibition of C—N bond cleavage side reactions, thereby enhancing the reaction rate of MA conversion and the selectivity to DMA.

Effect of alkaline-earth metals (Mg, Ca, Sr, and Ba) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis of light olefins

For the high-temperature Fischer-Tropsch synthesis (HTFT) of light olefins (C2=-C4=), alkaline-earth metals (Mg, Ca, Sr, and Ba) modified FeMn catalysts were prepared using precipitation and impregnation methods. It was demonstrated that the electron-giving interaction of Ca, Sr, and Ba decreased the adsorption of H species while improving the dissociative adsorption of CO. This interaction resulted in the production of χ-Fe5C2, which increased CO conversion and C2=-C4= selectivity. Compared with Ca and Ba, the FeMnSr catalyst had the strongest CO dissociative adsorption capacity and the highest χ-Fe5C2 content with the best FTS performance. When Sr and Na were introduced simultaneously, the synergistic effect of Sr and Na inhibited the formation of θ-Fe3C and the aggregation of catalyst particles, and also increased the χ-Fe5C2 content. This synergistic effect resulted in the highest surface basicity of FeMnSrNa catalyst, the highest electron cloud density on the surface of Fe atoms, the weakest H species adsorption, the strongest CO dissociative adsorption, and the highest total amount of active iron carbide, which improved the FTS performance. The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kgCat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kgCat)) and FeMnNa catalyst (80.6 g/(h·kgCat)).

Insights into the Reactivity of Gd2−xSrxFe2O7 (x = 0 ÷ 0.4) in CO Radical Hydrogenation

The effect of strontium substitution in the structure of the complex oxide Gd2SrFe2O7 on the production of light olefins by CO hydrogenation was investigated. Perovskite-type oxides Gd2−xSr1+xFe2O7 (x = 0; 0.1; 0.2; 0.3; 0.4) were synthesized by sol–gel technology and characterized by XRD, Mössbauer spectroscopy, BET specific area, acidity testing, and SEM. The experimental data revealed a correlation between the state of iron atoms, acidity, and catalytic performance. It was found that with an increase in the content of Sr2+ in the perovskite phase, the basicity of the surface and the oxygen diffusion rate increased. This contributed to the CO dissociative adsorption, formation of active carbon, and its further interaction with atomic hydrogen.

Ruthenium-cobalt single atom alloy for CO photo-hydrogenation to liquid fuels at ambient pressures

Photothermal Fischer-Tropsch synthesis represents a promising strategy for converting carbon monoxide into value-added chemicals. High pressures (2-5 MPa) are typically required for efficient C-C coupling reactions and the production of C5+ liquid fuels. Herein, we report a ruthenium-cobalt single atom alloy (Ru1Co-SAA) catalyst derived from a layered-double-hydroxide nanosheet precursor. Under UV-Vis irradiation (1.80 W cm−2), Ru1Co-SAA heats to 200 °C and photo-hydrogenates CO to C5+ liquid fuels at ambient pressures (0.1-0.5 MPa). Single atom Ru sites dramatically enhance the dissociative adsorption of CO, whilst promoting C-C coupling reactions and suppressing over-hydrogenation of CHx* intermediates, resulting in a CO photo-hydrogenation turnover frequency of 0.114 s−1 with 75.8% C5+ selectivity. Owing to the local Ru-Co coordination, highly unsaturated intermediates are generated during C-C coupling reactions, thereby improving the probability of carbon chain growth into C5+ liquid fuels. The findings open new vistas towards C5+ liquid fuels under sunlight at mild pressures.

Catalytic Performance of Alumina-Supported Cobalt Carbide Catalysts for Low-Temperature Fischer–Tropsch Synthesis

The determination of the catalyst’s active phase helps improve the catalytic performance of the Fischer–Tropsch (FT) synthesis. Different phases of cobalt, including cobalt oxide, carbide, and metal, exist during the reaction. The content of each phase can affect the catalytic performance and product distribution. In this study, a series of cobalt carbide catalysts were synthesized by exposure of Co/Al2O3 catalyst to CH4 at different temperatures from 300 °C to 800 °C. The physicochemical properties of the carbide catalysts (CoCx/Al2O3) were evaluated by different characterization methods. The catalytic performances of the catalysts were investigated in an autoclave reactor to determine the role of cobalt carbides on the CO conversion and product distribution during the reaction. XRD and XPS analysis confirmed the presence of Co2C in the prepared catalysts. The higher carbidation temperature resulted in the decomposition of methane into hydrogen and carbon, and the presence of graphitic carbon was confirmed by XRD, XPS, SEM, and Raman analysis. The Co2C also decomposed to metallic cobalt and carbon, and the content of cobalt carbide decreased at higher carbidation temperatures. Higher content of Co2C resulted in a lower CO conversion and higher selectivity to light alkanes, mainly methane. The higher carbidation temperature resulted in the decomposition of Co2C to metallic cobalt with higher activity in the FT reaction. The CO conversion increased by increasing the carbidation temperature from 300 °C to 800 °C, due to the higher content of metallic cobalt. In the presence of pure hydrogen, the Co2C could be converted mainly into hexagonal, close-packed (hcp) Co with higher activity for dissociative adsorption of CO, which resulted in higher catalyst activity and selectivity to heavier hydrocarbons.

Effect of alkalis (Li, Na, and K) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis

Alkalis (Li, Na, and K) decorated FeMnZr catalysts prepared by coprecipitation and impregnation methods were used to investigate the effects of three alkali metal promoters and promoter loading (for Na and K) on the iron-based catalyst for high-temperature Fischer-Tropsch synthesis (HTFT). The results showed that alkalis donated electrons to iron species, which promoted CO dissociative adsorption while suppressing hydrogen adsorption, then boosted the selectivity to light olefins and the improvement was enhanced with the increase of the atomic number of alkali metals and the content of alkali metals. Moreover, compared to alkalis-free catalysts, alkalis-promoted catalysts facilitated the formation of χ-Fe5C2 and ɛ’-Fe2.2C. In alkalis decorated catalysts, the χ-Fe5C2 content decreased and the ɛ’-Fe2.2C content increased with increasing basicity of the catalyst surface. In addition, compared with χ-Fe5C2, ɛ’-Fe2.2C has a stronger CC coupling ability and promotes chain growth more. Compared to Li and Na promoters, K was easier to facilitate the formation of ɛ’-Fe2.2C due to its strongest basicity and CO dissociative adsorption ability. The lowest CH4 and highest C5+ selectivity were accompanied by the highest content of ɛ’-Fe2.2C on K-promoted catalysts, which suggested that ɛ’-Fe2.2C had outstanding CC coupling ability.

Numerical investigation of the reaction kinetics of dry reforming of methane over the yolk–shell and single-atom alloy catalysts

Dry reforming of methane (DRM) over Ni-based supported catalysts has been widely studied due to Ni's low cost and high activity. Despite their excellent performance, side reactions such as methane decomposition and Boudouard reaction can cause severe coke formation leading to catalyst deactivation. Our recent work reported that both yolk–shell morphology and Pt-Ni interaction in single-atom-alloy (SAA) structures could maintain the DRM activity. However, the effect of morphology and Pt-Ni interaction could not be elucidated. In this work, we studied the reaction kinetics of DRM by proposing a detailed elementary reaction mechanism with 12 surface species and 17 elementary steps over the Ptx-NiCe@SiO2 and Pt0.25-NiCe/SiO2WI catalysts. Due to the different DRM activity over the wet-impregnated and yolk–shell structures, we examined multiple reaction models to explain the effect of morphology and Pt-Ni interaction on the DRM activity. Our results show that the reverse Boudouard reaction and dissociative adsorption of CO2 and CH4 are the rate-limiting steps. The desorption of CO* and H* is also critical to products yield and selectivity. Compared with Pt0.25-NiCe/SiO2WI catalyst, the C* removal followed by the fast CO* desorption is more favored on Pt0.25-NiCe@SiO2 SAA, suggesting the reverse Boudouard reaction prevents the catalyst deactivation by coking. The high O* and low H* coverages are observed on Pt0.25-NiCe@SiO2 SAA due to the confined yolk–shell morphology and enhanced Pt-Ni interaction in SAA structures, respectively. Both these effects can lead to facile C* removal in the Pt0.25-NiCe@SiO2 SAA catalyst leading to a stable DRM activity.

Insight into the role of lanthanum-modified Cu[sbnd]Co based catalyst for higher alcohol synthesis from syngas

A series of lanthanum oxide modified CuCo based catalysts for the direct conversion of syngas to higher alcohols were synthesized via the coprecipitation method. The influence of lanthanum on the textural properties and catalytic performance was investigated. It was found that lanthanum-modified CuCo based catalysts possessed a novel nanosheet structure to form more oxygen vacancies. Compared with CuCo catalyst, the dissociative CO adsorption and associative CO adsorption capacities of LaxCuCo catalysts were improved markedly, and formate and methoxy intermediates were more easily formed on the surface of LaxCuCo catalysts, which is attributed to the proper synergy of the surface active sites (Cu0, Co0, and oxygen vacancies). In particular, the addition of lanthanum species reduced the thickness of the nanosheets and facilitated the high dispersion of LaxCuCo after reaction by acting as isolation islands to restrain the aggregation and growth of crystal grain. The evaluation results showed that optimized La0.3CuCo catalyst exhibited the highest CO conversion of 25.4%, superior total alcohols selectivity of 61.4%, and C2+ alcohols selectivity of 79.03% among the total alcohol products, and excellent stability.

Boosting the Fischer-Tropsch synthesis performances of cobalt-based catalysts via geometric and electronic engineering: Construction of hollow structures

It is highly desirable but challenging to accelerate the dissociative adsorption of CO and lower the energy barriers of C–C coupling in the Fischer-Tropsch synthesis (FTS), both of which are prerequisite for high productivities of long-chain hydrocarbons. Herein, we report the fabrication of N-doped carbon nanotubes assembled hollow polyhedrons with embedded hollow Co (H-Co) nanoparticles (NPs) (H-Co@NCNHP) for highly efficient FTS. H-Co@NCNHP affords an extremely high C5+ space time yield of 1.67 × 10−5 mol gcat−1 s−1, outperforming most of Co-based catalysts reported to date. Theoretical calculations demonstrate that, as compared with solid Co NPs with symmetric arrangement of d-orbital electrons, H-Co NPs induce the electron rearrangement of d orbitals (especially dz2) toward the Fermi level, promoting the d-2π* /p coupling between Co active sites and adsorbed CO/intermediates.

High-temperature Fischer–Tropsch synthesis over the Li-promoted FeMnMgOx catalysts

Fischer–Tropsch-to-Olefins (FTO) over iron-based catalysts is a promising process for the production of light olefins from a non-petroleum source. However, the catalytic mechanism is unclear because of the variation in the iron phase during the reduction and activation processes and the complex iron phase composition under the reaction conditions. A series of Li-promoted FeMnMgOx catalysts is synthesized using co-precipitation and impregnation methods, after which the effect of promoters on the evolution of the iron phase, chemisorption, and FTO performance are analyzed. The results demonstrate that Mg enhances dissociative CO adsorption and hydrogen adsorption, improving the FTO activity. Mg facilitates the dispersion of α-Fe2O3 and suppresses the carbonization, which weakens the chain propagation, encourages the formation of short-chain hydrocarbons, and increases the selectivity of light olefins. Li also enhances dissociative CO adsorption and increases CO conversion. Moreover, Li facilitates the formation of the ε'-Fe2.2C phase, which reveals a higher FTS activity and better C–C coupling ability than the χ-Fe5C2 phase. This results in a decrease in CO2 and increase in light olefins. At the optimal Li loading, the FeMnMg-0.75Li sample displays the highest space time yield of light olefins at 355 g/(kgcat·h).

Direct Conversion of Syngas to Light Olefins through Fischer-Tropsch Synthesis over Fe-Zr Catalysts Modified with Sodium.

Fe–Zr–Na catalysts synthesized by coprecipitation and impregnation methods were implemented to investigate the promoting effects of Na and Zr on the iron-based catalyst for high-temperature Fischer–Tropsch synthesis (HTFT). The catalysts were characterized by Ar adsorption–desorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, CO temperature-programmed desorption, H2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy (MES). The results indicated that Na changed the active sites on the catalyst surface for the CO and hydrogen adsorption, owing to the electron migration from Na to Fe atoms, which resulted in an enhanced CO dissociative adsorption and a decrease in hydrogen adsorption on the metallic Fe surface. The decreased H/C ratio on the catalyst surface accounted for the increased chain propagation and weakened hydrogenation of light olefins. Besides, Na could also facilitate the carbonization of catalysts and protect the iron carbide against oxidation, which provides more active sites for HTFT reaction and is beneficial to the C–C coupling. Zr could decrease the hematite crystallite size and stabilize the active phase to improve the HTFT activity. At an optimal Na loading of 1.0 wt %, the Fe–Zr–1.0Na catalyst exhibited the highest light olefin selectivity of 35.8% in the hydrocarbon distribution at a CO conversion of 95.2%.

Tuning direct CO hydrogenation reaction over Fe-Mn bimetallic catalysts toward light olefins: Effects of Mn promotion

Highly dispersed FeMn bimetallic nanoparticles prepared by a low-temperature co-precipitation method were employed for direct CO hydrogenation for light olefins (C2∼C4=). An outstanding selectivity (60.6 %) for light olefins at a CO conversion level of 7.6 % was obtained over FeMn (4:1) catalyst activated by syngas (5%CO/5%H2). Characterization indicated that a strong interaction between Fe and Mn resulted in significant effects on stabilization of Fe1-xMnxO phase during activation period. Responding to the morphology effect, Mn decreased the surface energy of nanocrystals and weakened the agglomeration of nanoparticles, thus resulted in a decrease in particle size of Mn-promoted catalysts. The combined effects of inhibited carburization and improved metal dispersion contributed to the volcano pattern in the catalytic activity of Mn-promoted catalysts. In addition, Mn may act as an effective electron donor to Fe, enhancing CO dissociative adsorption while inhibiting secondary hydrogenation of olefins, thus giving rise to high olefin selectivity in CO hydrogenation.

DFT study for Structural and Electronic Properties of N2O3 Adsorption onto C20 Fullerene

Structural and electronic properties of N2O3 adsorbed on C20 fullerene were investigated by Density Functional Theory. Before N2O3 was adsorbed on C20 fullerene surface, the structural properties of the isomers of the N2O3 were calculated. According to the total energy calculations, asym-N2O3 is the most stable isomer since it has lower energy than the other two isomers. Data obtained for the structural properties of N2O3 molecule are in agreement with the previous studies. After full structural optimization without any restrictions, the most stable structures were obtained. The adsorption energies with no dissociation of N2O3 were in the range of −2.88 to −3.55 eV for LDA and −2.02 to −2.45 eV for GGA. On the other hand, the dissociative adsorption yields a lower total energy with Eads values of −3.72 and −2.93 eV in LDA and GGA, respectively. According to these values, adsorption can be evaluated as chemisorption for all stable structures. The adsorption occurs with no reaction barriers except for one configuration. Although the co-adsorption of NO and NO2 molecules on the same fullerene is energetically less favorable compared to their adsorptions on separate fullerenes, the dissociative co-adsorption occurs with no energy barrier. The electronic structures are dominated by charge transfer from the fullerene to the adsorbate. The obtained HOMO-LUMO gap (GapHL) values are in the range of 1.02 and 1.35 eV for both LDA and GGA, respectively. The results presented here can be expected to guide future experimental and theoretical studies as new hybrid material.

Effect of copper on highly effective Fe-Mn based catalysts during production of light olefins via Fischer-Tropsch process with low CO2 emission

The effect of Cu on Fe-Mn based catalysts system in production of light olefins via Fischer-Tropsch process was investigated. Catalysts with different Cu loadings were synthesized and evaluated. The 3.0 wt% Cu promoted Fe-Mn catalyst demonstrated an activity as high as 3.58×10−6 molCO gcat−1 s−1. The corresponding conversion of CO and selectivity to light olefins are 96.9 % and 40.1 %, respectively. Characterization results indicated that the Cu promoter facilitated the reduction process and enhanced the H-assisted CO dissociative adsorption, by changing the interactions between metals and the SiO2, which lead to an improved activity. The overall weakened surface basicity caused by Cu decreased the chain growth probability and lead to a higher selectivity to light olefins by shifting the product distribution toward short-chain hydrocarbons. Furthermore, all prepared catalysts showed superior efficiency of carbon utilization with CO2 selectivity as low as 23.0 % due to the hindered water-gas shift (WGS) reaction.

New insights on the defect sites evolution during CO oxidation over doped ceria nanocatalysts probed by in situ Raman spectroscopy

Among the factors affecting ceria activity, the defectiveness plays a key role in the case of CO oxidation. In this study, its connection with the catalytic performance was investigated via in-situ Raman spectroscopy on nanostructured pure and Cu/Mn-doped ceria, monitoring the defect sites and surface species evolution during the reaction. The accumulation of polyene-like chains, formed through CO dissociative adsorption at the catalyst surface, was observed and their disappearance was related to the catalyst light-off temperature. Moreover, the doped samples exhibited a rise of the Raman bands associated to defects after the tests, consequence of the structural rearrangements occurring during CO oxidation. Indeed, in-situ Raman measurements during reduction (CO/N2) and oxidation cycles at 400 °C evidenced the formation of oxygen vacancy clusters in reducing atmosphere, which could reorganize not only in O2 but also upon a temperature decrease, forming isolated vacancies and then evolving in Frenkel and extrinsic oxidized dopant-containing sites when exposed to oxygen.

Molecular and dissociative adsorption of CO and SO on the surface of Ir(111)

This study investigates the molecular and dissociative adsorption of CO and SO molecules on the perfect and a defective Ir(111) surface. It is aimed at providing a broad spectrum of adsorption sites in terms of coordination of Ir atoms and investigating the role of surface defects on the adsorption of small molecules on the surface Ir(111). First-principles density functional theory (DFT) simulation with the generalized gradient approximation as it is implemented in Vienna ab initio simulation package has been employed for this study. Preferred adsorption sites, adsorption energies, and surface electronic structures of CO and SO molecules on the perfect and defective Ir(111) surfaces were calculated to obtain a systematic understanding on the nature of adsorption and dissociative interactions. The DFT calculation reveals the possible molecular adsorption of CO on both perfect and defective Ir(111) surface by the end-on manner (CO bond perpendicular to the surface); the later surface is found to be energetically more favorable. However, no dissociative adsorption was obtained. For SO molecule, on the other hand, both molecular and dissociative adsorption was observed. The defective surface is now less favorable in terms of adsorption energy, but yields stronger activation of SO. The nudged elastic band method investigation also reveals that the Pt single-atom catalysis significantly reduces (up to 80% reduction) the energy barrier of the dissociative adsorption of SO. The electronic structure calculation reveals that all the adsorptions investigated in this study involve hybridization of different electronic states.

Structural evolution of bimetallic Co-Cu catalysts in CO hydrogenation to higher alcohols at high pressure

Bimetallic Co-Cu catalysts are widely applied in higher alcohol synthesis (HAS), but the formation of the final active structure has not yet been fully clarified, especially for Co-rich catalysts. We investigated the structural evolution of a Co-Cu catalyst (Co:Cu = 2) from the hydrotalcite precursor containing additional Al3+ and Zn2+ to the final active state after 80 h under reaction conditions at 280 °C and 60 bar. The reconstruction of the bimetallic Co-Cu nanoparticles obtained by H2 reduction was induced by the feed gas consisting of an equimolar H2 and CO syngas mixture resulting in fast phase separation and sintering of metallic Cu0 and Co0 in the first 2 h time on stream (TOS) and a continuous carbidization of Co0 forming Co2C and its sintering until steady state was reached after 40 h TOS. An intergrowth of metallic Cu0 nanoparticles with Co2C nanoparticles was observed to occur under reaction conditions. The high selectivity to oxygenates amounting to 41% compared with 29% to hydrocarbons is ascribed to the multi-functional Co2C/Cu0 interface enabling dissociative CO adsorption, hydrogenation and CO insertion. The formation of hydrogenated carbon species (CxHy) originating from dissociative CO chemisorption is assumed to be favored by hydrogen spillover from Cu0 to Co2C. The adsorption sites for molecular CO provided by both Cu0 and Co2C facilitate its insertion into the CxHy intermediates thus leading to a higher selectivity to alcohols following the Anderson-Schulz-Flory distribution.

Ni-sepiolite and Ni-todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS

The hydrogenation of CO2 to methane was studied on Ni-sepiolite and Ni-todorokite catalysts. Catalytic testing and operando DRIFTS studies revealed the excellent performance of in particular todorokite-based catalysts. The catalytic activity is related to specific reaction pathways depending on the support material. Based on operando DRIFTS studies, different mechanisms are proposed for the methanation reaction. Over Ni-sepiolite catalysts, a dissociative adsorption of CO2 is observed. Linearly and bridged bonded CO on Ni0 was identified as intermediate. On Ni-todorokite this dissociative mechanism is accompanied by an associative one. Here, carbonate species were identified as additional intermediates, the formation of which is facilitated by the MnOx support. The ability of Ni-todorokite catalysts to activate CO2 via a dissociative as well as an associative mechanism explains their excellent catalytic performance. The influence of the applied preparation method on the catalytic performance was demonstrated for the Ni-sepiolite catalysts.

Magnetization Reversal and Domain Nucleation in Ultra-Thin Co/Re(0001) Capped by Graphitic C

By using X-ray magnetic circular dichroism-photoemission electron microscopy, we study the magnetic behavior of ultra-thin Co on Re(0001) covered by a single layer of graphitic carbon. The carbon overlayer is lithographically fabricated via electron-stimulated dissociative adsorption of CO and subsequent annealing to 650 K. Graphitization is found to induce perpendicular magnetic anisotropy in cobalt at a thickness in the range 4-5 atomic layers, where out-of-plane domains separated by chiral Neél type walls are observed. The graphitic overlayer efficiently protects Co from oxidation upon exposure to ambient conditions, leaving its magnetic structure completely unaffected. The magnetic response of the Co film to an external field is imaged in situ across the hysteresis loop, allowing a quantitative analysis of the field-dependent domain area and structure. Our observations pinpoint the influence of the step morphology on the local magnetic behavior. Moreover, below the saturation field, the domains with chiral walls evolve into nanometric-sized magnetic structures resembling skyrmionic bubbles for applied fields <;20 G.