Near-ambient Pressure X-ray Photoelectron Spectroscopy Research Articles

Disordered Au Nanoclusters for Efficient Ammonia Electrosynthesis.

The electrochemical nitrogen (N2 ) reduction reaction (N2 RR) under mild conditions is a promising and environmentally friendly alternative to the traditional Haber-Bosch process with high energy consumption and greenhouse emission for the synthesis of ammonia (NH3 ), but high-yielding production is rendered challenging by the strong nonpolar N≡N bond in N2 molecules, which hinders their dissociation or activation. In this study, disordered Au nanoclusters anchored on two-dimensional ultrathin Ti3 C2 Tx MXene nanosheets are explored as highly active and selective electrocatalysts for efficient N2 -to-NH3 conversion, exhibiting exceptional activity with an NH3 yield rate of 88.3±1.7 μg h-1 mgcat. -1 and a faradaic efficiency of 9.3±0.4 %. A combination of in situ near-ambient pressure X-ray photoelectron spectroscopy and operando X-ray absorption fine structure spectroscopy is employed to unveil the uniqueness of this catalyst for N2 RR. The disordered structure is found to serve as the active site for N2 chemisorption and activation during the N2 RR process.

Near ambient pressure–x-ray photoelectron spectroscopy spectra of lithium hexafluorophosphate in ethylene/dimethyl carbonate

Near ambient pressure–x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 5000 Pa. NAP-XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show the survey, Li 1s, P 2p, C 1s, N 1s, O 1s, and F1s NAP-XPS spectra of a Li-based electrolyte solution, which is a material that would be difficult to analyze by conventional XPS. The measurements were performed at 1000 Pa in ambient gas atmosphere to compensate for surface charging. Peak fits of the C 1s, O 1s, and F 1s narrow scans are presented.

Near ambient pressure–x-ray photoelectron spectroscopy spectra of lithium bis(trifluoromethane-sulfonyl) imide in propylene carbonate

Near ambient pressure–x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 5000 Pa. NAP-XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show the survey, Li 1s, S 2p, C 1s, N 1s, O 1s, and F 1s NAP-XPS spectra of a Li-based electrolyte solution, which is a material that would be difficult to analyze by conventional XPS. The measurements were performed at 200 Pa in ambient gas atmosphere to compensate for surface charging. Peak fits of the C 1s, O 1s, and F 1s narrow scans are presented.

Effect of Water Vapor on Oxidation Processes of the Cu(111) Surface and Sublayer.

Copper-based catalysts have different catalytic properties depending on the oxidation states of Cu. We report operando observations of the Cu(111) oxidation processes using near-ambient pressure scanning tunneling microscopy (NAP-STM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The Cu(111) surface was chemically inactive to water vapor, but only physisorption of water molecules was observed by NAP-STM. Under O2 environments, dry oxidation started at the step edges and proceeded to the terraces as a Cu2O phase. Humid oxidation of the H2O/O2 gas mixture was also promoted at the step edges to the terraces. After the Cu2O covered the surface under humid conditions, hydroxides and adsorbed water layers formed. NAP-STM observations showed that Cu2O was generated at lower steps in dry oxidation with independent terrace oxidations, whereas Cu2O was generated at upper steps in humid oxidation. The difference in the oxidation mechanisms was caused by water molecules. When the surface was entirely oxidized, the diffusion of Cu and O atoms with a reconstruction of the Cu2O structures induced additional subsurface oxidation. NAP-XPS measurements showed that the Cu2O thickness in dry oxidation was greater than that in humid oxidation under all pressure conditions.

Probing dynamic covalent chemistry in a 2D boroxine framework by in situ near-ambient pressure X-ray photoelectron spectroscopy.

Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where high crystallinity is achieved through reversible bond formation. Here, we exploit near-ambient pressure X-ray photoelectron spectroscopy to elucidate the reversible formation of a two-dimensional boroxine framework. By in situ mapping the pressure-temperature parameter space, we identify the regions where the rates of the condensation and hydrolysis reactions become dominant, being the key to enable the thermodynamically controlled growth of crystalline frameworks.

In situnear-ambient pressure X-ray photoelectron spectroscopy reveals the effects of water, oxygen and light on the stability of PM6:Y6 photoactive layers

To better understand the effects of water and oxygen on OSC materials’ stability,i.e.PM6 and Y6, we usedin situnear-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate the electronic structures.

Surface compositional dynamics in a PtNi bimetallic alloy under simulated operational conditions: Electrochemical and NAP-XPS Study

Platinum-based bimetallic alloys possess unique activities exceeding those of pure platinum. Nevertheless, as complex multi-component systems, they suffer from surface structural reorganization under operating conditions, strongly affecting their lifetime performance. This work reports an in situ electrochemical and spectroscopic study of surface compositional changes in a PtNi catalyst during repetitive oxidation/reduction cycles, simulating inherent working conditions for numerous redox reactions. Using cyclic voltammetry and near-ambient pressure X-ray photoelectron spectroscopy, quantitative surface characterization under realistic electrified liquid and gaseous environments are obtained and correlated. We show that regardless of the operating environment, PtNi undergoes a significant and irreversible change in compositional profile reflected in surface nickel enrichment and consequent catalyst deactivation, exemplary confirmed using a methanol electrooxidation reaction.

Operando NAP-XPS Studies of a Ceria-Supported Pd Catalyst for CO Oxidation

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22−) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1−xO2−δ solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1−xO2−δ phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.

In-situ near-ambient-pressure photoelectron spectroscopy investigations of high-work-function MoO3 on 4H-SiC(0001)

How to realize effective Ohmic contact to SiC represents one of the major challenges for high-power and high-frequency SiC electronics. Forming a high-work-function interfacial layer between metal electrode and semiconductor has been considered as a promising approach to realize effective Ohmic contact with wide bandgap semiconductors. In this work, a high-work-function (∼7.2 eV) interfacial molybdenum trioxide (MoO3) layer was grown on 4H-SiC(0001) using conventional vacuum thermal deposition process. We have also systematically investigated the evolution of surface properties of MoO3/4H-SiC(0001) via annealing under high vacuum and various gas environments (H2 and O2). The combination of in-situ near-ambient-pressure x-ray photoelectron spectroscopy (NAP-XPS) and ultra-high vacuum (UHV) ultraviolet photoelectron spectroscopy (UPS) investigations of the MoO3/4H-SiC(0001) reveal that the surface MoO3 tends to be reduced into Mo5+ and Mo4+ during annealing under UHV and 0.5 mbar H2. Most importantly, the surface work function of MoO3/4H-SiC(0001) was found to decrease accordingly together with the defect states originated from the transition from Mo+6 to Mo5+ and Mo4+. In contrast, the oxygen sites as well as its high work function are retained even after 400 °C annealing under 0.5 mbar O2, which indicates that the MoO3/4H-SiC(0001) interface is more thermally stable under oxygen treatment.

In Situ Spectroscopy and Microscopy Insights into the CO Oxidation Mechanism on Au/CeO2(111)

In this work, we prepared and investigated in ultra-high vacuum (UHV) two stoichiometric CeO2(111) surfaces containing low and high amounts of step edges decorated with 0.05 ML of gold using synchrotron-radiation photoelectron spectroscopy (SRPES) and scanning tunneling microscopy (STM). The UHV study helped to solve the still unresolved puzzle on how the one-monolayer-high ceria step edges affect the metal-substrate interaction between Au and the CeO2(111) surface. It was found that the concentration of ionic Au+ species on the ceria surface increases with increasing number of ceria step edges and is not correlated with the concentration of Ce3+ ions that are supposed to form on the surface after its interaction with gold nanoparticles. We associated this with an additional channel of Au+ formation on the surface of CeO2(111) related to the interaction of Au atoms with various peroxo oxygen species formed at the ceria step edges during the film growth. The study of CO oxidation on the highly stepped Au/CeO2(111) model sample was performed by combining near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), UHV-STM, and near-ambient-pressure STM (NAP-STM). This powerful combination provided comprehensive information on the processes occurring on the Au/CeO2(111) surface during the interaction with CO, O2, and CO + O2 (1:1) mixture at conditions close to the real working conditions of CO oxidation. It was found that the system demonstrates high stability in CO. However, the surface undergoes substantial chemical and morphological changes as the O2 is added to the reaction cell. Already at 300 K, gold nanoparticles begin to grow using a mechanism that involves the disintegration of small gold nanoparticles in favor of the large ones. With increasing temperature, the model catalyst quickly transforms into a system of primarily large Au particles that contains no ionic gold species.

Activity and segregation behavior of Pd75%Ag25%(1 1 1) during CO oxidation – An in situ NAP-XPS investigation

The surface reactivity during CO oxidation over Pd(111) and Pd75%Ag25%(111) single crystals has been investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under excess oxygen conditions (p ≈ 2 mbar, O2:CO=10). The (√6×√6) Pd5O4 surface oxide is present on Pd(111) when the surface is highly active under the applied conditions. The CO2 formation profile follows a hysteresis upon temperature cycling between 150 and 450 °C that depends on the temperature ramp rate. The behavior is different for Pd75%Ag25%(111), with the CO2 formation rate considerably lower, no surface oxide and only chemisorbed oxygen present at the surface at high temperature, reversible scrambling of the (near) surface Pd/(Pd+Ag) ratio, and no hysteresis behavior observed. The results suggest that the reactant activation is affected by both surface composition and surface termination, i.e. mechanistically different on Pd75%Ag25%(111) than Pd(111), and less efficient on (111) as compared to the (100) alloy counterpart.

Probing dynamic covalent chemistry in a 2D boroxine framework by in-situ near-ambient pressure X-ray photoelectron spectroscopy, Nanoscale, 2023,15, 1068-1075

Probing dynamic covalent chemistry in a 2D boroxine framework by in-situ near-ambient pressure X-ray photoelectron spectroscopy, Nanoscale, 2023,15, 1068-1075

Recycling of Glass Waste by Deposition of TiO2 for the Intensification of the Photocatalytic Effect in the Purification of Wastewater

In this study, the photocatalytic effect of TiO2 (1 wt. % and 3 wt. %) deposited on the surface of sintered cathode ray tube glass was examined, as well as its effect on an E. coli strain (BL21(DE3)). DTA analysis indicated the sintering temperature for samples to be 820 °C while scanning electron microscopy (SEM)showed an intimate contact and a strong interface between the support and photocatalyst. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to establish the chemical and bonding environment of the samples. The investigations of the bacterial viability were conducted using flow cytometry, a specific cellular viability assay, while bacterial growth was measured using the turbidimetric method. The experimental results show the influence of the TiO2 concentration on the bacterial inactivation process: higher concentrations (3% wt.) have a bactericidal effect in the long term, whereas lower concentrations (1% wt.) render them inactive for a shorter time in the exponential growth stage. The preliminary results were used to calculate the efficiency of microbial inactivation and the parameters of the kinetics of inactivation using ANOVA software. The results indicate that this material could be an effective solution for water disinfection.

Electronic Structure Evolution from Metallic Vanadium to Metallic VxOy: A NAPPES Study for O2 + V Gas–Solid Interaction

Gas–solid interactions between molecular oxygen and metallic vanadium surfaces and the systematic evolution in the electronic structure of vanadium oxide (VOx) surfaces have been explored in the present work by near-ambient pressure photoelectron spectroscopy (NAPPES). The current article studies the evolution of various oxides of vanadium as a function of partial pressure of O2 (ultrahigh vacuum to 1 mbar), temperature (298–875 K), and the exposure time to oxygen (up to 18 h). Valence-band (VB) and core-level spectral measurements recorded with UV (He–I = 21.2 eV) and Al Kα (1486.6 eV) photons, respectively, show interesting changes. (1) Oxidation is limited to the top layers of vanadium at 298 K and up to a partial pressure of 1 mbar O2. About 50% of vanadium gets oxidized, and the remaining amount exists as metal within the top 10 nm. (2) Metallic vanadium disappears above 625 K, and it is predominantly oxidized to a mixture of V4+ and V5+ oxidation states at a 0.1 mbar partial pressure of O2. Points 1 and 2 suggest the predominantly thermodynamically controlled nature of vanadium oxidation through oxygen diffusion into the subsurface and bulk layers. (3) The Fermi-level (EF) feature observed first at ≥725 K at a 0.1 mbar O2 pressure demonstrates the formation of metallic VO2; however, its metallic nature is preserved even at ambient temperature due to interweaving nanodomains of VOx with VO2. (4) Only partial conversion of surface layers to V5+ (V2O5) along with VO2 and V2O3 (within the probing depth of 8–10 nm by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)) was observed even after prolonged heating (18 h) in 1 mbar O2 pressure. (5) The nature of the surface changes between metal and semiconducting/insulator oxides is substantiated by the observation of changes in work function (ϕ) and EF features. Typical VB features and Fermi intensity of V-metal and vanadium oxides were observed, and the results were corroborated with core-level and VB spectra. The present results extend the capabilities of NAPPES to explore the electronic structure evolution as a function of reaction conditions and underscore its relevance to areas such as heterogeneous catalysis and sensing.

Strong Electronic Orbit Coupling between Cobalt and Single-Atom Praseodymium for Boosted Nitrous Oxide Decomposition on Co3O4 Catalyst.

Nitrous oxide (N2O) has gained increasing attention as an important noncarbon dioxide greenhouse gas, and catalytic decomposition is an effective method of reducing its emissions. Here, Co3O4 was synthesized by the sol-gel method and single-atom Pr was confined in its matrix to improve the N2O decomposition performance. It was observed that the reaction rate varied in a volcano-like pattern with the amount of doped Pr. A N2O decomposition reaction rate 5-7.5 times greater than that of pure Co3O4 is achieved on the catalyst with a Pr/Co molar ratio of 0.06:1, and further Pr doping reduced the activity due to PrOx cluster formation. Combined with X-ray photoelectron spectroscopy, X-ray absorption fine structure, density functional theory and in situ near-ambient pressure X-ray photoelectron spectroscopy, it was demonstrated that the single-atom doped Pr in Co3O4 generates the "Pr 4f-O 2p-Co 3d" network, which redistributes the electrons in Co3O4 lattice and increases the t2g electrons at the tetracoordinated Co2+ sites. This coupling between the Pr 4f orbit and Co2+ 3d orbit triggers the formation of a 4f-3d electronic ladder, which accelerates the electron transfer from Co2+ to the 3π* antibonding orbital of N2O, thus contributing to the N-O bond cleavage. Moreover, the energy barrier for each elementary reaction in the decomposition process of N2O is reduced, especially for O2 desorption. Our work provides a theoretical grounding and reference for designing atomically modified catalysts for N2O decomposition.

Oxidation of Aqueous H3PO3 on Pt Studied By X-Ray Spectroscopies

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) employing a phosphoric acid H3PO4-doped membrane are considered to be promising sustainable electrochemical energy storage. The high-temperature operation has several advantages, such as a higher tolerance to CO poisoning, allowing coupling of HT-PEMFCs with reformers [1-3], as well the possibility for heat and electric energy co-generation [1,2]. However, during operation, phosphorus oxo-acids (e.g.: H3PO3) are generated on the anode. These impurities adsorb on the Pt catalyst [4-6], thus possibly negatively affecting the HT-PEMFCs performance. A detailed understanding of the H3PO3-catalyst (Pt) interaction is hence necessary for further HT-PEMFC optimization. However, besides an investigation of the H3PO3 adsorption behavior on Pt [6,7], literature on the behavior of the H3PO3 in contact with Pt (with/without polarization) is scarce.In this work, the oxidation mechanism of H3PO3 was investigated using a combination of in situ x-ray spectroscopy techniques that directly probe the H3PO3/Pt interface interaction, complemented by ex situ x-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography (IEC). IEC gave insights into the effect of Pt on the stability of deaerated aqueous H3PO3 solutions. XPS was conducted on H3PO3/support structures (including Au and Pt supports) to determine to what extent the support affects the H3PO3 oxidation. Furthermore, in-situ dip and pull near-ambient pressure (NAP-)XPS was conducted to investigate the state of H3PO3 at the H3PO3/Pt interface and in solution bulk. It was observed that at the H3PO3/Pt interface, H3PO3 was chemically oxidized to H3PO4, while in the bulk solution it remains stable, as shown in Figure 1. Moreover, in situ x-ray absorption spectroscopy at the P K-edge was conducted at different concentrations of H3PO3 in aqueous solutions (i.e., different amounts of H2O) in contact with Pt, to determine the role of H2O in the oxidation of H3PO3. A higher degree of oxidation was observed for the less concentrated H3PO3, implying that H2O participates in the oxidation mechanism of H3PO3 to H3PO4.

Tuning Strong Metal–Support Interaction Kinetics on Pt-Loaded TiO2(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined Pt/TiO$_2$(110) catalysts at pressures of up to 0.1 Torr. By tuning the O$_2$ pressure, we can either selectively oxidize the TiO$_2$ support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10$^{-5}$ Torr O$_2$, but orders of magnitudes less effective at higher O$_2$ pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.

Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS.

Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10-1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7eV, with a relative intensity of ∼3:1, respectively. These features remained upon subsequent pumpdown to 10-9 Torr. The observed maximum N surface coverage was ∼1.5 × 1013cm-2-a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10-1 Torr is ∼25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree-Fock-based cluster calculations for N2-V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions.

CO2-Assisted Oxidative Dehydrogenation of Propane over VOx/In2O3 Catalysts: Interplay between Redox Property and Acid–Base Interactions

In this work, a series of VOx-loaded In2O3 catalysts were prepared, and their catalytic performance was evaluated for CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) and compared with In2O3 alone. The optimal composition is obtained on 3.4V/In2O3 (surface V density of 3.4V nm–2), which exhibited not only a higher C3H6 selectivity than other V/In catalysts and In2O3 under isoconversion conditions but also an improved reaction stability. To elucidate the catalyst structure–activity relationship, the VOx/In2O3 catalysts were characterized by chemisorption [NH3-temperature-programmed desorption (TPD), NH3-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), CO2-TPD, and CO2-DRIFTS], H2-temperature-programmed reduction (TPR), in situ Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, near-ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and further examined using density functional theory. The In–O–V structure and the extent of oligomerization, which play a crucial role in improving selectivity and stability, were identified in the VOx/In2O3 catalysts. In particular, the presence of surface VOx (i) inhibits the deep reduction of In2O3, thereby preserving the activity, (ii) neutralizes the excess basicity on In2O3, thus suppressing propane dry reforming and achieving a higher propylene selectivity, and (iii) introduces additional redox sites that participate in the dehydrogenation reaction by utilizing CO2 as a soft oxidant. The present work provides insights into developing selective, stable, and robust metal-oxide catalysts for CO2-ODHP by controlling the conversion of reagents via desired pathways through the interplay between acid–base interactions and redox properties.

Unraveling Catalytic Reaction Mechanism by In Situ Near Ambient Pressure X-ray Photoelectron Spectroscopy

Probing surface chemistry during reactions closer to realistic conditions is crucial for the understanding of mechanisms in heterogeneous catalysis. Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is one of the state-of-the-art surface-sensitive techniques used to characterize catalyst surfaces in gas phases. This Perspective begins with a brief overview of the development of the NAP-XPS technique and its representative applications in identifying the active sites at a molecular level. Next, recent in situ NAP-XPS investigations of several model catalysts in the CO2 hydrogenation reaction are mainly discussed. Finally, we highlight the major challenges facing NAP-XPS and future improvements to facilities for probing intermediates with higher resolutions under real ambient pressure reactions in heterogeneous catalysis.