Operando probing of the surface chemistry during the Haber-Bosch process.

The large-scale conversion of N2 and H2 into NH3 (refs. 1,2) over Fe and Ru catalysts3 for fertilizer production occurs through the Haber-Bosch process, which has been considered the most important scientific invention of the twentieth century4. The active component of the catalyst enabling the conversion was variously considered to be the oxide5, nitride2, metallic phase or surface nitride6, and the rate-limiting step has been associated with N2 dissociation7-9, reaction of the adsorbed nitrogen10 and also NH3 desorption11. This range of views reflects that the Haber-Bosch process operates at high temperatures and pressures, whereas surface-sensitive techniques that might differentiate between different mechanistic proposals require vacuum conditions. Mechanistic studies have accordingly long been limited to theoretical calculations12. Here we use X-ray photoelectron spectroscopy-capable of revealing the chemical state of catalytic surfaces and recently adapted to operando investigations13 of methanol14 and Fischer-Tropsch synthesis15-to determine the surface composition of Fe and Ru catalysts during NH3 production at pressures up to 1 bar and temperatures as high as 723 K. We find that, although flat and stepped Fe surfaces and Ru single-crystal surfaces all remain metallic, the latter are almost adsorbate free, whereas Fe catalysts retain a small amount of adsorbed N and develop at lower temperatures high amine (NHx) coverages on the stepped surfaces. These observations indicate that the rate-limiting step on Ru is always N2 dissociation. On Fe catalysts, by contrast and as predicted by theory16, hydrogenation of adsorbed N atoms is less efficient to the extent that the rate-limiting step switches following temperature lowering from N2 dissociation to the hydrogenation of surface species.

Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes.

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Demonstrating Pressure Jumping as a Tool to Address the Pressure Gap in High Pressure Photoelectron Spectroscopy of CO and CO2 Hydrogenation on Rh(211).

Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a "remnant" of the high pressure surface as well as the time dynamics during the re-equilibration to the new pressure. This methodology is demonstrated using the CO2 and the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows an adsorbate distribution which favors chemisorbed CHx adsorbates over chemisorbed CO. This contrasts against previous static operando spectra acquired at lower pressures. Furthermore, the pressure jumping method yields a faster acquisition and more detailed spectra than static operando measurements above 1 bar. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, and different hypotheses are presented regarding this observation.

In Situ Surface-Sensitive Investigation of Multiple Carbon Phases on Fe(110) in the Fischer-Tropsch Synthesis.

Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer-Tropsch synthesis process, converting CO and H2 into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron-carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst's surface in relation to the reaction.

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst.

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO2/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO2/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO2 methanol synthesis.

Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals.

The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.

The Structure of the Active Pd State During Catalytic Carbon Monoxide Oxidization.

Using grazing incidence X-rays and X-ray photoelectron spectroscopy during the mass transfer limited catalytic oxidation of CO, the long-range surface structure of Pd(100) was investigated. Under the reaction conditions of 50:4 O2 to CO, 300 mbar pressure, and temperatures between 200 and 450 °C, the surface structure resulting from oxidation and the subsequent oxide reduction was elucidated. The reduction cycle was halted, and while under reaction conditions, angle-dependent X-ray photoelectron spectroscopy close to the critical angle of Pd and modeling of the data was performed. Two proposed models for the system were compared. The suggestion with the metallic islands formed on top of the oxide island was shown to be consistent with the data.

A Novel Method to Maintain the Sample Position and Pressure in Differentially Pumped Systems Below the Resolution Limit of Optical Microscopy Techniques.

We present a new method to maintain constant gas pressure over a sample during in situ measurements. The example shown here is a differentially pumped high-pressure X-ray photoelectron spectroscopy system, but this technique could be applied to many in situ instruments. By using the pressure of the differential stage as a feedback source to change the sample position, a new level of consistency has been achieved. Depending on the absolute value of the sample-to-aperture distance, this technique allows one to maintain the distance within several hundred nanometers, which is below the limit of typical optical microscopy systems. We show that this method is well suited to compensate for thermal drift. Thus, X-ray photoelectron spectroscopy data can be acquired continuously while the sample is heated and maintaining constant pressure over the sample. By implementing a precise manipulator feedback system, pressure variations of less than 5% were reached while the temperature was varied by 400 ℃. The system is also shown to be highly stable under significant changes in gas flow. After changing the flow by a factor of two, the pressure returned to the set value within 60 s.

UL26 Attenuates IKKß-Mediated Induction of Interferon-Stimulated Gene (ISG) Expression and Enhanced Protein ISGylation during Human Cytomegalovirus Infection.

Viruses must negotiate cellular antiviral responses in order to replicate. Human cytomegalovirus (HCMV) is a prevalent betaherpesvirus that encodes a number of viral gene products that modulate cellular antiviral signaling. The HCMV UL26 gene has previously been found to attenuate cytokine-activated NF-κB signaling, yet the role that UL26 plays in modulating the host cell's global transcriptional response to infection is not clear. Here, we find that infection with a UL26 deletion virus (ΔUL26) induces a proinflammatory transcriptional environment that includes substantial increases in the expression of cytokine signaling genes relative to wild-type HCMV. These increases include NF-κB-regulated genes as well as interferon-stimulated genes (ISGs), such as ISG15 and bone marrow stromal cell antigen 2 (BST2). The ΔUL26 mutant-mediated induction of ISG15 expression was found to drive increases in global protein ISGylation during ΔUL26 mutant infection. However, short hairpin RNA (shRNA) and CRISPR-mediated targeting of ISG15 indicated that its induction does not restrict HCMV infection. In contrast, shRNA-mediated targeting of BST2 demonstrated that BST2 restricts HCMV cell-to-cell spread. In addition, the increased expression of both of these ISGs and the global enhancement in protein ISGylation were found to be dependent on the activity of the canonical inhibitor of NF-κB kinase beta (IKKß). Both CRISPR-based and pharmacologically mediated inhibition of IKKß blocked the induction of ISG15 and BST2. These results suggest significant cross-talk between the NF-κB and interferon signaling pathways and highlight the importance of IKK signaling and the HCMV UL26 protein in shaping the antiviral response to HCMV.IMPORTANCE Modulation of cellular antiviral signaling is a key determinant of viral pathogenesis. Human cytomegalovirus (HCMV) is a significant source of morbidity in neonates and the immunosuppressed that contains many genes that modulate antiviral signaling, yet how these genes contribute to shaping the host cell's transcriptional response to infection is largely unclear. Our results indicate that the HCMV UL26 protein is critical in preventing the establishment of a broad cellular proinflammatory transcriptional environment. Further, we find that the host gene IKKß is an essential determinant governing the host cell's antiviral transcriptional response. Given their importance to viral pathogenesis, continuing to elucidate the functional interactions between viruses and the cellular innate immune response could enable the development of therapeutic strategies to limit viral infection.

The IκB Kinases Restrict Human Cytomegalovirus Infection.

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that causes disease in immunosuppressed populations. HCMV has a complex relationship with innate immune signaling pathways. Specifically, HCMV has been found to block some aspects of inflammatory signaling while benefiting from others. Through analysis of knockout cell lines targeting the NF-κB regulatory kinases IκB kinase α (IKKα) and IKKß, we find that the IKKs are host restriction factors that contribute to cytokine-mediated resistance to viral infection, limit the initiation of HCMV infection, and attenuate viral cell-to-cell spread. The HCMV UL26 protein is a viral immune modulator important for HCMV infection that has been shown to inhibit host cell NF-κB signaling, yet it has remained unclear how UL26-mediated NF-κB modulation contributes to infection. Here, we find that UL26 modulation of NF-κB signaling is separable from its contribution to high-titer viral replication. However, we find that IKKß is required for the induction of cytokine expression associated with ΔUL26 infection. Collectively, our data indicate that the IKKs restrict infection but HCMV targets their signaling to modulate the cellular inflammatory environment.IMPORTANCE Innate immune signaling is a critical defense against viral infection and represents a central host-virus interaction that frequently determines the outcomes of infections. NF-κB signaling is an essential component of innate immunity that is extensively modulated by HCMV, a significant cause of morbidity in neonates and immunosuppressed individuals. However, the roles that various facets of NF-κB signaling play during HCMV infection have remained elusive. We find that the two major regulatory kinases in this pathway, IKKα and IKKß, limit the initiation of infection, viral replication, and cell-to-cell spread. In addition, our results indicate that these kinases contribute differently to the host cell response to infection in the absence of a virally encoded NF-κB inhibitor, UL26. Given the importance of NF-κB in viral infection, elucidating the contributions of various NF-κB constituents to infection is an essential first step toward the possibility of targeting this pathway therapeutically.

Who's Driving? Human Cytomegalovirus, Interferon, and NFκB Signaling.

As essential components of the host's innate immune response, NFκB and interferon signaling are critical determinants of the outcome of infection. Over the past 25 years, numerous Human Cytomegalovirus (HCMV) genes have been identified that antagonize or modulate the signaling of these pathways. Here we review the biology of the HCMV factors that alter NFκB and interferon signaling, including what is currently known about how these viral genes contribute to infection and persistence, as well as the major outstanding questions that remain.

Gas-cluster ion sputtering: Effect on organic layer morphology.

Analysis of the surface of thin Irganox 1010 films before and after sputtering with an argon gas-cluster ion beam was performed with AFM and XPS to determine the effect that Zalar rotation has on the chemistry and morphology of the surface. The analysis is based on the change in roughness of the surface by comparing the same location on the surface before and after sputtering. The ion beam used was an Arn+ of size n = 1000 and energy 4 keV. The XPS analysis agreed with previous results in which the ion beam did not cause measurable accumulation of damaged material. Based on the AFM results, the Irganox 1010 surface became rougher as a result of ion sputtering, and the degree of roughening was quantified, as was the sputter rate. Furthermore, Zalar rotation during ion sputtering did not have a significant effect on surface roughening, surprisingly.

Stealing the Keys to the Kitchen: Viral Manipulation of the Host Cell Metabolic Network.

Host cells possess the metabolic assets required for viral infection. Recent studies indicate that control of the host's metabolic resources is a core host-pathogen interaction. Viruses have evolved mechanisms to usurp the host's metabolic resources, funneling them towards the production of virion components as well as the organization of specialized compartments for replication, maturation, and dissemination. Consequently, hosts have developed a variety of metabolic countermeasures to sense and resist these viral changes. The complex interplay between virus and host over metabolic control has only just begun to be deconvoluted. However, it is clear that virally induced metabolic reprogramming can substantially impact infectious outcomes, highlighting the promise of targeting these processes for antiviral therapeutic development.