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Monitoring the spontaneous reconstruction of the surface of metal oxides under electrocatalytic reaction conditions is critical to identifying the active sites and establishing structure-activity relationships. Here, we report on a self-terminated surface reconstruction of Ruddlesden-Popper lanthanum nickel oxide (La 2 NiO 4+δ ) that occurs spontaneously during reaction with alkaline electrolyte species. Using a combination of high-resolution scanning transmission electron microscopy (HR-STEM), surface-sensitive X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (sXAS), as well as electrochemical techniques, we identify the structure of the reconstructed surface layer as an amorphous (oxy)hydroxide phase that features abundant under-coordinated nickel sites. No further amorphization of the crystalline oxide lattice (beyond the ∼2 nm thick layer formed) was observed during oxygen evolution reaction (OER) cycling experiments. Notably, the formation of the reconstructed surface layer increases the material's oxygen evolution reaction (OER) activity by a factor of 45 when compared to that of the pristine crystalline surface. In contrast, a related perovskite phase, i.e. , LaNiO 3 , did not show noticeable surface reconstruction, and also no increase in its OER activity was observed. This work provides detailed insight into a surface reconstruction behavior dictated by the crystal structure of the parent oxide and highlights the importance of surface dynamics under reaction conditions.

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature., (© 2024. The Author(s).)

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO 2 to methanol. In this study, we decipher the catalytically active structure of Ni-Ga-based catalysts. To this end, model Ni-Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H 2 leads to the formation of nanoparticles based on a Ni-Ga face-centered cubic (fcc) alloy along with a small quantity of GaO x . Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaO x species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaO x and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Ni δ- ). Both the Ni:Ga ratio in the alloy and the quantity of GaO x are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni-Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 mol Ni -1 ). Furthermore, operando infrared spectroscopy experiments indicate that GaO x species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH 4 formation), while oxidized Ga species enhance the methanol formation rate., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

This work provides insight into the local structure of Na in MgO-based CO 2 sorbents that are promoted with NaNO 3 . To this end, we use X-ray absorption spectroscopy (XAS) at the Na K-edge to interrogate the local structure of Na during the CO 2 capture (MgO + CO 2 ↔ MgCO 3 ). The analysis of Na K-edge XAS data shows that the local environment of Na is altered upon MgO carbonation when compared to that of NaNO 3 in the as-prepared sorbent. We attribute the changes observed in the carbonated sorbent to an alteration in the local structure of Na at the NaNO 3 /MgCO 3 interfaces and/or in the vicinity of [Mg 2+ ···CO 3 2- ] ionic pairs that are trapped in the cooled NaNO 3 melt. The changes observed are reversible, i.e., the local environment of NaNO 3 was restored after a regeneration treatment to decompose MgCO 3 to MgO. The ex situ Na K-edge XAS experiments were complemented by ex situ magic-angle spinning 23 Na nuclear magnetic resonance (MAS 23 Na NMR), Mg K-edge XAS and X-ray powder diffraction (XRD). These additional experiments support our interpretation of the Na K-edge XAS data. Furthermore, we develop in situ Na (and Mg) K-edge XAS experiments during the carbonation of the sorbent (NaNO 3 is molten under the conditions of the in situ experiments). These in situ Na K-edge XANES spectra of molten NaNO 3 open new opportunities to investigate the atomic scale structure of CO 2 sorbents modified with Na-based molten salts by using XAS., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 °C. Insight into the atomic-scale structure of these shells was obtained by high-field 71 Ga solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and 29 Si dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere ( [4] Ga (Si) ) and, according to 15 N DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Brønsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-μ 2 -OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaO x ), i.e., an increased relative fraction of pentacoordinate sites ( [5] Ga (Ga) ), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO 2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO 2-500 , a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO 2-500 . In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO 2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomic-scale mixing of Al sites with silica, leading to Al-μ 2 -OH-Si sites. Our results suggest that [4] Ga (Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of [5] Ga (Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaO x -based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Brønsted acidity (nature, strength, and abundance)., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The development of effective CO 2 sorbents is vital to achieving net-zero CO 2 emission targets. MgO promoted with molten salts is an emerging class of CO 2 sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO 3 -promoted, MgO-based CO 2 sorbent. During the first few cycles of CO 2 capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO 3 growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na 2 Mg(CO 3 ) 2 crystallites that act effectively as seeds for MgCO 3 nucleation and growth. Na 2 Mg(CO 3 ) 2 forms due to the partial decomposition of NaNO 3 during regeneration at T ≥ 450°C followed by carbonation in CO 2 .

We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr 0.8 Ca 0.2 FeO 3- δ ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N 2 physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr K -edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite via X-ray photoelectron and in situ Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO 2 -free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO 2 , surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn 3 O 4 :M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn 3 O 4 :M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δ f H°(MO), and the Lewis acidity of the M 2+ substituent. Incorporation of elements M with low Δ f H°(MO) enhances the oxygen binding strength in Mn 3 O 4 :M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

MgO-based CO 2 sorbents promoted with molten alkali metal nitrates (e.g., NaNO 3 ) have emerged as promising materials for CO 2 capture and storage technologies due to their low cost and high theoretical CO 2 uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO 2 capture reaction) remains debated and poorly understood. Here, we utilize 18 O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO 3 -promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the 18 O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high 18 O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the 18 O label in the solid (MgCO 3 ), molten (NaNO 3 ), and gas (CO 2 ) phases during the CO 2 capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO 2 and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO 3 . On the other hand, no oxygen exchange was observed between NaNO 3 and CO 2 or NaNO 3 and MgO. Combining the results of the 18 O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg 2+ ···CO 3 2- ] ionic pairs in the molten NaNO 3 promoter. Upon reaching the solubility limit, MgCO 3 crystallizes at the MgO/NaNO 3 interface., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Catalysts with well-defined isolated Ni(ii) surface sites have been prepared on three silica-based supports. The outer shells of the support were comprised either of an amorphous aluminosilicate or amorphous alumina (AlO x ) layer - associated with a high and low density of strong Brønsted acid sites (BAS), respectively. When tested for ethene-to-propene conversion, Ni catalysts with a higher density of strong BAS demonstrate a higher initial activity and productivity to propene. On all three catalysts, the propene productivity correlates closely with the concentration of C 8 aromatics, suggesting that propene may form via a carbon-pool mechanism. While all three catalysts deactivate with time on stream, the deactivation of catalysts with Ni(ii) sites on AlO x , i.e. , containing surface Ni aluminate sites, is shown to be reversible by calcination (coke removal), in contrast to the deactivation of surface Ni silicate or aluminosilicate sites, which deactivate irreversibly by forming Ni nanoparticles., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The thermal carburization of MoO 3 nanobelts (nb) and SiO 2 -supported MoO 3 nanosheets under a 1 : 4 mixture of CH 4  : H 2 yields Mo 2 C-nb and Mo 2 C/SiO 2 . Following this process by in situ Mo K-edge X-ray absorption spectroscopy (XAS) reveals different carburization pathways for unsupported and supported MoO 3 . In particular, the carburization of α-MoO 3 -nb proceeds via MoO 2 , and that of MoO 3 /SiO 2 via the formation of highly dispersed MoO x species. Both Mo 2 C-nb and Mo 2 C/SiO 2 catalyze the dry reforming of methane (DRM, 800 °C, 8 bar) but their catalytic stability differs. Mo 2 C-nb shows a stable performance when using a CH 4 -rich feed (CH 4  : CO 2 = 4 : 2), however deactivation due to the formation of MoO 2 occurs for higher CO 2 concentrations (CH 4  : CO 2 = 4 : 3). In contrast, Mo 2 C/SiO 2 is notably more stable than Mo 2 C-nb under the CH 4  : CO 2 = 4 : 3 feed. The influence of the morphology of Mo 2 C and its dispersion on silica on the structural evolution of the catalysts under DRM is further studied by in situ Mo K-edge XAS. It is found that Mo 2 C/SiO 2 features a higher resistance to oxidation under DRM than the highly crystalline unsupported Mo 2 C-nb and this correlates with an improved catalytic stability. Lastly, the oxidation of Mo in both Mo 2 C-nb and Mo 2 C/SiO 2 under DRM conditions in the in situ XAS experiments leads to an increased activity of the competing reverse water gas shift reaction., Competing Interests: The authors declare that there is no conflict of interest regarding the publication of this article., (This journal is © The Royal Society of Chemistry.)

This work explores how H 2 pretreatment at 550 °C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline γ/β-Ga 2 O 3 and amorphous Ga 2 O 3 (GaO x ) supported on silica (γ-Ga 2 O 3 /SiO 2 and Ga/SiO 2 , respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO 2 -H 2 shows poor activity and propene selectivity, no coking and no deactivation with TOS, similar to Ga/SiO 2 . In contrast, the high initial activity and propene selectivity of γ-Ga 2 O 3 /SiO 2 -H 2 decline with TOS but to a lesser extent than in calcined γ-Ga 2 O 3 /SiO 2 . In addition, γ-Ga 2 O 3 /SiO 2 -H 2 cokes less than γ-Ga 2 O 3 /SiO 2 . Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline γ/β-Ga 2 O 3 phases in γ-Ga 2 O 3 /SiO 2 -H 2 and the emergence of additional tetrahedral Ga sites (Ga IV ). Such Ga IV sites are strong Lewis acid sites (LAS) according to studies using adsorbed pyridine and CO probe molecules, i.e. , the abundance of strong LAS is higher in γ-Ga 2 O 3 /SiO 2 -H 2 compared to γ-Ga 2 O 3 /SiO 2 but lower than in Ga/SiO 2 and Ga/SiO 2 -H 2 . Dissociation of H 2 on the Ga-O linkages in γ-Ga 2 O 3 /SiO 2 -H 2 yields high-frequency Ga-H bands that are observed in Ga/SiO 2 and Ga/SiO 2 -H 2 but not detected in γ-Ga 2 O 3 /SiO 2 . We attribute the increased amount of Ga IV sites in γ-Ga 2 O 3 /SiO 2 -H 2 mostly to an increased disorder in γ/β-Ga 2 O 3 . X-ray photoelectron spectroscopy detects the formation of Ga + and Ga 0 species in both Ga/SiO 2 -H 2 and γ-Ga 2 O 3 /SiO 2 -H 2 . Therefore, it is likely that a minor amount of Ga IV sites also forms through the interaction of Ga + (such as Ga 2 O) and/or Ga 0 with silanol groups of SiO 2 ., Competing Interests: The authors declare no conflicts of interest., (This journal is © The Royal Society of Chemistry.)

Chemical looping is an emerging technology to produce high purity hydrogen from fossil fuels or biomass with the simultaneous capture of the CO 2 produced at the distributed scale. This process requires the availability of stable Fe 2 O 3 -based oxygen carriers. Fe 2 O 3 -Al 2 O 3 based oxygen carriers exhibit a decay in the H 2 yield with cycle number, due to the formation of FeAl 2 O 4 that possesses a very low capacity for water splitting at typical operating conditions of conventional chemical looping schemes (700-1000 °C). In this study, the addition of sodium ( via a sodium salt) in the synthesis of Fe 2 O 3 -Al 2 O 3 oxygen carriers was assessed as a means to counteract the cyclic deactivation of the oxygen carrier. Detailed insight into the oxygen carrier's structure was gained by combined X-ray powder diffraction (XRD), X-ray absorption spectroscopy (XAS) at the Al, Na and Fe K-edges and scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) analyses. The addition of sodium prevented the formation of FeAl 2 O 4 and stabilized the oxygen carrier via the formation of a layered structure, Na-β-Al 2 O 3 phase. The material, i.e. Na-β-Al 2 O 3 stabilized Fe 2 O 3 , showed a stable H 2 yield of ca. 13.3 mmol g -1 over 15 cycles., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Gallia-alumina (Ga,Al) 2 O 3( x  :  y ) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as 27 Al and 71 Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning 15 N{ 27 Al} and 15 N{ 71 Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using 15 N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al) 2 O 3 oxides. Notably, we demonstrate that (Ga,Al) 2 O 3 catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al) 2 O 3 catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The development of stable Ni-based dry reforming of methane (DRM) catalysts is a key challenge owing to the high operating temperatures of the process and the propensity of Ni for promoting carbon deposition. In this work, Al 2 O 3 -coated Ni/SiO 2 catalysts have been developed by employing atomic layer deposition (ALD). The structure of the catalyst at each individual preparation step was characterized in detail through a combination of in situ XAS-XRD, ex situ 27 Al NMR and Raman spectroscopy. Specifically, in the calcination step, the ALD-grown Al 2 O 3 layer reacts with the SiO 2 support and Ni, forming aluminosilicate and NiAl 2 O 4 . The Al 2 O 3 -coated Ni/SiO 2 catalyst exhibits an improved stability for DRM when compared to the benchmark Ni/SiO 2 and Ni/Al 2 O 3 catalysts. In situ XAS-XRD during DRM together with ex situ Raman spectroscopy and TEM of the spent catalysts confirm that the ALD-grown Al 2 O 3 layer suppresses the sintering of Ni, in turn reducing also coke formation significantly. In addition, the formation of an amorphous aluminosilicate phase by the reaction of the ALD-grown Al 2 O 3 layer with the SiO 2 support inhibited catalysts deactivation via NiAl 2 O 4 formation, in contrast to the reference Ni/Al 2 O 3 system. The in-depth structural characterization of the catalysts provided an insight into the structural dynamics of the ALD-grown Al 2 O 3 layer, which reacts both with the support and the active metal, allowing to rationalize the high stability of the catalyst under the harsh DRM conditions., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

Early transitional metal carbides are promising catalysts for hydrogenation of CO 2 . Here, a two-dimensional (2D) multilayered 2D-Mo 2 C material is prepared from Mo 2 CT x of the MXene family. Surface termination groups T x (O, OH, and F) are reductively de-functionalized in Mo 2 CT x (500 °C, pure H 2 ) avoiding the formation of a 3D carbide structure. CO 2 hydrogenation studies show that the activity and product selectivity (CO, CH 4 , C 2 -C 5 alkanes, methanol, and dimethyl ether) of Mo 2 CT x and 2D-Mo 2 C are controlled by the surface coverage of T x groups that are tunable by the H 2 pretreatment conditions. 2D-Mo 2 C contains no T x groups and outperforms Mo 2 CT x , β-Mo 2 C, or the industrial Cu-ZnO-Al 2 O 3 catalyst in CO 2 hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO 2 hydrogenation., (© 2021. The Author(s).)