A New Path to Carbon Dioxide Transformation

SCIENTIFIC ACHIEVEMENT

SIGNIFICANCE AND IMPACT

Rebalancing the carbon cycle

Fossil fuels are the lifeblood of modern societies, but their increased use releases carbon dioxide (CO₂)—a greenhouse gas—into the atmosphere faster than plants can recycle it through photosynthesis. One way to address this is to reprocess CO₂ into syngas, a feedstock for generating other useful chemicals. However, because CO₂ is highly nonreactive, catalysts such as copper and silver are needed to facilitate the transformation (“reduction”) of CO₂ into carbon monoxide (CO), a major component of syngas.

In searching for more efficient catalysts, scientists have computationally screened and lab-tested many materials. These approaches, however, have been based on preconceived notions about the relevant reaction mechanisms and have not produced dramatic successes. An atomic-level understanding of CO₂ catalysis is essential to designing optimally performing materials.

Collaboration on copper

The Joint Center for Artificial Photosynthesis (JCAP) is a Department of Energy (DOE) Research Hub that aims to mimic photosynthesis—to produce renewable fuel using sunlight, and water, and CO₂. In a previous collaboration, JCAP brought together theorists from Caltech and experimentalists from the ALS to study in detail what happens to CO₂ at a copper surface.

In that study, the researchers found that subsurface oxygen in the copper dramatically boosts the early stages of CO₂ catalysis. Bent CO₂ (b-CO₂) was shown to be a stable intermediate species in the presence of water. Such insights are essential to understanding, and eventually driving (using electrochemical processes), the reaction pathways to desired products.

A close-up of the silver lining

Now, a similar JCAP collaboration between the ALS and Caltech has expanded on the earlier work by investigating how CO₂ and water molecules interact with a silver surface. They combined ambient-pressure x-ray photoelectron spectroscopy (APXPS) at ALS Beamline 9.3.2 with quantum-mechanical calculations to obtain a comprehensive understanding of the initial steps of CO₂ adsorption and activation on silver.

Beamline 9.3.2 offers photons with a soft x-ray energy range that allows surface-sensitive probes, advancing investigations at solid/gas interfaces. The beamline endstation chamber allows excellent control of O₂, CO₂, and H₂O gas pressures and reaction temperatures, important for accurately predicting the stable chemical species on the surface through simulations. A vacuum chamber attached to the analyzer allows in situ sample preparation and direct, pristine-sample measurement.

A new reaction pathway

The results revealed a very different mechanism of CO₂ activation on silver versus copper. A stable intermediate chemical species (O=CO₂^δ−) forms on silver when exposed to CO₂ alone. In the presence of CO₂ and water, the intermediate attaches up to four water molecules, and two water molecules stabilize b-CO₂ as well. The new surface-chemisorbed species structurally resembles carbonic acid (H₂CO₃), but processes completely different electric properties. The (O=CO₂^δ−)–(H₂O)_n clusters on silver represent a significantly more favorable activation mechanism than b-CO₂ on copper.

These unprecedented and surprising results—made possible using a synergistic and systematic experimental and computational approach—raise numerous questions about subsequent steps that will drive many new studies. Overall, the results lay a foundation for understanding the differences between catalysts, with the goal of controlling CO₂ adsorption using additives or alloys and observing the reactions in process as they’re being electrochemically driven.

ALS SCIENCE HIGHLIGHT #396