Method for Surface Characterization Using Solid-State Nuclear Magnetic Resonance Spectroscopy Demonstrated on Nanocrystalline ZnO:Al.

Nanoscale zinc-oxide doped with aluminum ZnO:Al is studied by different techniques targeting surface changes induced by the conditions at which ZnO:Al is used as support material in the catalysis of methanol. While it is well established that a variety of 1H and 27Al resonances can be found by solid-state NMR for this material, it was not clear yet which signals are related to species located close to the surface of the material and which to species located in the bulk. To this end, a method is suggested that makes use of a paramagnetically impregnated material to suppress NMR signals close to the particle surface in the blind sphere around the paramagnetic metal atoms. It is shown that it is important to use conditions that guarantee a stable reference system relative to which it can be established whether the coating procedure is conserving the original structure or not. This method, called paramagnetically assisted surface peak assignment, helped to assign the 1H and 27Al NMR peaks to the bulk and the surface layer defined by the blind sphere of the paramagnetic atoms. The assignment results are further corroborated by the results from heteronuclear 27Al{1H} dipolar dephasing experiments, which indicate that the hydrogen atoms are preferentially located in the surface layer and not in the particle core.

Role of Nanoscale Inhomogeneities in Co2FeO4 Catalysts during the Oxygen Evolution Reaction.

Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co2FeO4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm2. In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co2+-rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co2+-rich domains transform during OER to structurally different Co3+-rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.

Substitution of Copper by Magnesium in Malachite: Insights into the Synthesis and Structural Effects.

The phase width of the copper hydroxycarbonate malachite, Cu2CO3(OH)2, upon substitution with magnesium has been studied in detail. In extension of a previous study on amorphous precursors, the introduction of a hydrothermal aging step allowed the retrieval of crystalline hydroxycarbonate samples with up to 37 atom % Mg (metal content) that are suitable candidates as precursors to Cu/MgO catalysts for CO hydrogenation. Simultaneous refinements of X-ray powder diffraction and pair distribution function (PDF) data as well as complementary spectroscopic insight (X-ray absorption and infrared spectroscopy) revealed that samples with up to 18 atom % Mg are phase-pure magnesian malachites but the magnesium content can be increased beyond this threshold when mcguinnessite (CuMgCO3(OH)2) is accepted as a side phase. In a complementary study, a continuous increase of the magnesium fraction was found during aging and the corresponding structural evolution was studied by means of PDF. These findings add significant insight into the aging chemistry of crystalline Cu,Mg hydroxycarbonates. Furthermore, both phase-pure magnesian malachite and mcguinnessite-containing samples with up to 37 atom % Mg have been examined by thermogravimetry, X-ray powder diffraction, and N2 physisorption and were found to be promising candidates for use as precursors for the preparation of Cu/MgO catalysts.

Synergistic Effects of Co and Fe on the Oxygen Evolution Reaction Activity of LaCox Fe1-x O3.

In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCox Fe1-x O3 . Phase pure perovskites were synthesized up to x = 0 . 300 in 0.025/0.050 steps. HAADF STEM and EDX analysis points towards FeO2 -terminated (001)-facets in LaFeO3 , in accordance with the stability diagram obtained from density functional theory calculations with a Hubbard U term (DFT+U). Linear sweep voltammetry conducted in a rotating disk electrode setup shows a reduction of the OER overpotential and a nonmonotonic trend with x, with double layer capacitance measurements indicating an intrinsic nature of activity. This is supported by DFT+U results that show reduced overpotentials for both Fe and Co reaction sites with the latter reaching values of 0.32-0.40 V, ∼0.3 V lower than for Fe. This correlates with a stronger reduction of the binding energy difference of the *O and *OH intermediates towards an optimum value of 1.6 eV for x = 0 . 250 , the OH deprotonation being the potential limiting step in most cases. Significant variations of the magnetic moments of both surface and subsurface Co and Fe during OER demonstrate that the beneficial effect is a result of a concerted action involving many surrounding ions, which extends the concept of the active site.

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions.

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co3 O4 , MgCo2 O4 , Co2 FeO4 , Co2 AlO4 and CoFe2 O4 . The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H2 O2 decomposition. Studying the effect of dominant surface termination, isotropic Co3 O4 and CoFe2 O4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co3+ plays the major role for high activity. In H2 O2 decomposition, Co2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction.

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols.

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure-performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

The Effect of Water on the 2-Propanol Oxidation Activity of Co-Substituted LaFe1- Cox O3 Perovskites.

Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1-x Cox O3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x>0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state.

Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane.

Cryptomelane (α-(K)MnO2 ) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts' microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2 O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm-2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions.

Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis.

Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2 O4 , can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+ , Co2+ , and Fe3+ during co-precipitation, a mixture of LDH, (FeII CoII )2/3 FeIII1/3 (OH)2 (CO3 )1/6 ⋅m H2 O, and the target spinel CoFe2 O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+ :Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel ∥[001]LDH . Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material.

Different routes to methanol: inelastic neutron scattering spectroscopy of adsorbates on supported copper catalysts.

We have investigated methanol synthesis with model supported copper catalysts, Cu/ZnO and Cu/MgO, using CO/H2 and CO2/H2 as feedstocks. Under CO/H2 both catalysts show chemisorbed methoxy as a stable intermediate, the Cu/MgO catalyst also shows hydroxyls on the support. Under CO2/H2 the catalysts behave differently, in that formate is also seen on the catalyst. For the Cu/ZnO catalyst hydroxyls are present on the metal whereas for the Cu/MgO hydroxyls are found on the support. These results are consistent with a recently published model for methanol synthesis and highlight the key role of ZnO in the process.

Promoting the Synthesis of Methanol: Understanding the Requirements for an Industrial Catalyst for the Conversion of CO2.

The hydrogenation of CO2 to methanol is a potential process for the sustainable production of synthetic liquid fuels. The Cu/ZnO catalyst employed for this reaction has been studied extensively for many years, and recent progress now has the potential to turn it into a prototype for complex promotional interactions in heterogeneous catalysis.

Bridging the Time Gap: A Copper/Zinc Oxide/Aluminum Oxide Catalyst for Methanol Synthesis Studied under Industrially Relevant Conditions and Time Scales.

Long-term stability of catalysts is an important factor in the chemical industry. This factor is often underestimated in academic testing methods, which may lead to a time gap in the field of catalytic research. The deactivation behavior of an industrially relevant Cu/ZnO/Al2 O3 catalyst for the synthesis of methanol is reported over a period of 148 days time-on-stream (TOS). The process was investigated by a combination of quasi in situ and ex situ analysis techniques. The results show that ZnO is the most dynamic species in the catalyst, whereas only slight changes can be observed in the Cu nanoparticles. Thus, the deactivation of this catalyst is driven by the changes in the ZnO moieties. Our findings indicate that methanol synthesis is an interfacially mediated process between Cu and ZnO.

Formation of a ZnO overlayer in industrial Cu/ZnO/Al2 O3 catalysts induced by strong metal-support interactions.

In industrially relevant Cu/ZnO/Al2 O3 catalysts for methanol synthesis, the strong metal support interaction between Cu and ZnO is known to play a key role. Here we report a detailed chemical transmission electron microscopy study on the nanostructural consequences of the strong metal support interaction in an activated high-performance catalyst. For the first time, clear evidence for the formation of metastable "graphite-like" ZnO layers during reductive activation is provided. The description of this metastable layer might contribute to the understanding of synergistic effects between the components of the Cu/ZnO/Al2 O3 catalysts.

Cu-based catalyst resulting from a Cu,Zn,Al hydrotalcite-like compound: a microstructural, thermoanalytical, and in situ XAS study.

A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH)2(CO3)0.17]⋅mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl2 O4 still containing stabilizing carbonate with a strong interaction of Cu(2+) ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized Cu(I) intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2 O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings.

Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules.

Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2.

Unveiling the Structure-Property Relationship of MgO-Supported Ni Ammonia Decomposition Catalysts from Bulk to Atomic Structure by In Situ/Operando Studies.

Ammonia is currently being studied intensively as a hydrogen carrier in the context of the energy transition. The endothermic decomposition reaction requires the use of suitable catalysts. In this study, transition metal Ni on MgO as a support is investigated with respect to its catalytic properties. The synthesis method and the type of activation process contribute significantly to the catalytic properties. Both methods, coprecipitation (CP) and wet impregnation (WI), lead to the formation of Mg1-xNixO solid solutions as catalyst precursors. X-ray absorption studies reveal that CP leads to a more homogeneous distribution of Ni2+ cations in the solid solution, which is advantageous for a homogeneous distribution of active Ni catalysts on the MgO support. Activation in hydrogen at 900 °C reduces nickel, which migrates to the support surface and forms metal nanoparticles between 6 nm (CP) and 9 nm (WI), as shown by ex situ STEM. Due to the homogeneously distributed Ni2+ cations in the solid solution structure, CP samples are more difficult to activate and require harsher conditions to reduce the Ni. The combination of in situ X-ray diffraction (XRD) and operando total scattering experiments allows a structure-property investigation of the bulk down to the atomic level during the catalytic reaction. Activation in H2 at 900 °C for 2 h leads to the formation of large Ni particles (20-30 nm) for the samples synthesized by the WI method, whereas Ni stays significantly smaller for the CP samples (10-20 nm). Sintering has a negative influence on the catalytic conversion of the WI samples, which is significantly lower compared to the conversion observed for the CP samples. Interestingly, metallic Ni redisperses during cooling and becomes invisible for conventional XRD but can still be detected by total scattering methods. The conditions of activation in NH3 at 650 °C are not suitable to form enough reduced Ni nanoparticles from the solid solution and are, therefore, not a suitable activation procedure. The activity steadily increases in the samples activated at 650 °C in NH3 (Group 1) compared to the samples activated at 650 °C in H2 and then reaches the best activity in the samples activated at 900 °C in H2. Only the combination of complementary in situ and ex situ characterization methods provides enough information to identify important structure-property relationships among these promising ammonia decomposition catalysts.

Highly loaded bimetallic iron-cobalt catalysts for hydrogen release from ammonia.

Ammonia is a storage molecule for hydrogen, which can be released by catalytic decomposition. Inexpensive iron catalysts suffer from a low activity due to a too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. Here, we show that this limitation can be overcome by combining iron with cobalt resulting in a Fe-Co bimetallic catalyst. Theoretical calculations confirm a lower metal-nitrogen binding energy for the bimetallic catalyst resulting in higher activity. Operando spectroscopy reveals that the role of cobalt in the bimetallic catalyst is to suppress the bulk-nitridation of iron and to stabilize this active state. Such catalysts are obtained from Mg(Fe,Co)2O4 spinel pre-catalysts with variable Fe:Co ratios by facile co-precipitation, calcination and reduction. The resulting Fe-Co/MgO catalysts, characterized by an extraordinary high metal loading reaching 74 wt.%, combine the advantages of a ruthenium-like electronic structure with a bulk catalyst-like microstructure typical for base metal catalysts.

Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al.

Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also.

The effect of Al-doping on ZnO nanoparticles applied as catalyst support.

A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al(2)O(3) methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO.

Phase evolution, speciation and solubility limit of aluminium doping in zinc oxide catalyst supports synthesized via co-precipitated hydrozincite precursors.

The preparation of Al-doped ZnO via thermal decomposition of crystalline precursors, with a particular emphasis on kinetic effects on the solubility limits, was studied. The promoting effect of Al3+ on the catalyst system is discussed for methanol synthesis where ZnO:Al is employed as a support material for copper nanoparticles. The synthesis of the Al-doped zinc oxides in this study was inspired by the industrial synthesis of the methanol synthesis catalyst via a co-precipitated crystalline precursor, here: hydrozincite Zn5(OH)6(CO3)2. To determine the aluminium speciation and the solubility limit of the aluminium cation on zinc positions, a series of zinc oxides with varying aluminium contents was synthesized by calcination of the precursors. Short precipitate ageing time, low ageing temperature and aluminium contents below 3 mol% metal were advantageous to suppress crystalline side-phases in the precursor, which caused an aluminium segregation and non-uniform aluminium distribution in the solid. Even if zinc oxide was the only crystalline phase, TEM revealed such segregation in samples calcined at 320 °C. Only at very low aluminium contents, the dopant was found preferably on the zinc sites of the zinc oxide structure based on the signal dominating the 27Al NMR spectra. The solubility limit regarding this species was determined to be approximately xAl = 0.013 or 1.3% of all metal cations. Annealing experiments showed that aluminium was kinetically trapped on the site and segregated into ZnAl2O4 upon further heating. This shows that lower calcination temperatures such as applied in catalyst synthesis conserve a higher aluminium doping concentration on that specific site than is expected thermodynamically.