Following the structure of copper-zinc-alumina across the pressure gap in carbon dioxide hydrogenation

Abstract

Copper-zinc-alumina catalysts are the industrially-used formulation for methanol synthesis from carbon monoxide and carbon dioxide containing feedstock. Its high performance stems from synergies that develop between its components. This important catalytic system has been investigated with a myriad of approaches, however, no comprehensive agreement has emerged as to the fundamental source of its high activity. One potential source of the disagreements is the considerable variation in pressure used in studies to understand a process that is industrially performed at pressures above 20 bar. Here, by systematically studying the catalyst state during temperature-programmed reduction and under carbon dioxide hydrogenation with in situ and operando X-ray absorption spectroscopy over four orders of magnitude in pressure, we show how the state and evolution of the catalyst is defined by its environment. Especially below 1 bar, the structure of the catalyst shows a strong pressure dependence. As pressure gaps are a general problem in catalysis, these observations have wide-ranging ramifications.The improvement of heterogeneous catalysts is central to a sustainable development of energy conversion and the production of chemicals. Historically, such development relied heavily on trial-and-error based research. More recently, advances in characterization methods allowed the study of catalysts under pretreatment and catalytic conditions, thus in situ and operando. This has permitted the possibility to derive fundamental understanding of the state of the catalyst whilst it is actually working 1 . Ideally, a detailed comprehension of the reaction mechanisms of the desired catalytic reaction emerges. Many of these methods, such as electron microscopy and X-ray photoelectron spectroscopy, remain limited in their routine application to pressure regimes in the millibar range [2][3][4][5] ; others, however, such as X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), suffer no

Drastic Events and Gradual Change Define the Structure of an Active Copper‐Zinc‐Alumina Catalyst for Methanol Synthesis

“…At pressures of 5 bar and above, the reduction occurs at 423 K [29] . Figure 1a and b show the development of Zn K‐edge XANES and non‐phase corrected Fourier transformed (FT) EXAFS observed during in situ temperature‐programmed reduction (TPR) of the CZA catalyst under 10 bar of hydrogen up to 800 K [29] . Similar plots for the other pressures studied can be found in Supplementary Figure S1.…”

“…The evolution of the Cu K‐edge XANES and its interpretation in terms of transformation of the copper phase measured in the same experiment can be found in ref. [29]. It was found that the reduction of Cu II to metallic copper in hydrogen is a strong function of pressure.…”

“…At 1 mbar, the reduction event occurs at around 573 K. When the hydrogen pressure is increased, the temperature at which Cu II reduced continuously shifts to lower temperatures. At pressures of 5 bar and above, the reduction occurs at 423 K [29] . Figure 1a and b show the development of Zn K‐edge XANES and non‐phase corrected Fourier transformed (FT) EXAFS observed during in situ temperature‐programmed reduction (TPR) of the CZA catalyst under 10 bar of hydrogen up to 800 K [29] .…”

“…By comparison, we can evaluate the presence of these XANES features within our reference components (Table 1), as identified in ref. [29]. The pre‐edge feature [A] is present in spectra of both metallic zinc and in a copper‐zinc alloy, and the feature [B] in XANES of the zinc components having an oxidation state +2 and the precursor state zinc carbonate [29, 35, 36] .…”

“…In the Cu K-edge XANES minute changes occur (Supplementary Figure S6), which mainly correspond to a shift of the XANES to lower energy, which can be related to the transformation of any CuZn alloy present to metallic copper. [29] The derivative of the Zn K-edge XANES (Figure 3c) and the EXAFS (Figure 3d) indicate that the zinc structure also responds to the reactive switching. The difference spectra of the FT-EXAFS for the switch to and from catalytic conditions (Figure 3e) reveal a fully reversible process in the XAS.…”