Heterogeneous catalysis, where reactions occur on a solid catalyst surface, is crucial for many chemical processes. While desirable, achieving reaction site precision in heterogeneous catalysis, akin to homogeneous catalysis, for controlled activity, selectivity, and stability remains a challenge. Surface heterogeneity, influenced by particle size and exposed facets, often leads to inconsistent reactivity and selectivity, and instability over time. Ideally, a heterogeneous catalyst would consist of uniform, high surface area nanoparticles with stable structure. Metal phosphides, with their highly dispersed metal nanoclusters integrated into a phosphorus lattice, offer a potential solution. This paper explores their application in photocatalytic CO₂ hydrogenation, focusing on Ni₁₂P₅ as an example, and examining its potential as an active, selective, and stable catalyst for the photothermal reverse water gas shift (RWGS) reaction. The study also investigates the broader applicability of this concept to other metal phosphides, such as Co₂P, demonstrating the promise of these earth-abundant materials for sustainable CO₂ conversion.

The existing literature highlights the challenges in achieving site precision in heterogeneous catalysis, emphasizing the need for catalysts with uniform nanoparticle size, shape, and facet exposure. Several studies have explored metal phosphides for various catalytic applications, including electrochemical hydrogen evolution and hydro-processing. However, their potential for heterogeneous photocatalytic CO₂ hydrogenation remained largely untapped before this research. Previous work on CO₂ hydrogenation has focused on various catalysts and methodologies, including photochemical and photothermal approaches, but achieving high rates, selectivity, and stability simultaneously has proven difficult. This study builds on the existing knowledge by investigating a new class of materials with unique structural and electronic properties.

Nickel phosphide materials (Ni₁₂P₅ and SiO₂-supported Ni₁₂P₅) were synthesized using a temperature-programmed reduction method. The materials were characterized using various techniques: powder X-ray diffraction (XRD), UV-visible-NIR diffuse reflectance spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES), and X-ray absorption spectroscopy (XAS). Photocatalytic CO₂ hydrogenation was tested in a batch reactor using an unfiltered Xe lamp as the light source. Product gases were analyzed by gas chromatography (GC). Long-term stability tests were conducted in a flow reactor. Isotope tracing experiments were performed using ¹³CO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate the reaction mechanism. Similar synthetic and characterization methods were applied to Co₂P, a cobalt phosphide analog. The light intensity was controlled using neutral density filters and high-pass cutoff filters. The internal quantum yield (IQY) was estimated. Local temperature of the catalyst surface was estimated by ASPEN Plus software. A Ni/SiO₂ reference catalyst was prepared for comparison. The study also involved detailed data processing and analysis using Athena and WinXAS software packages.

The unsupported Ni₁₂P₅ nanoparticles exhibited a CO production rate of 156 ± 3 mmol g_cat⁻¹h⁻¹ with 99.5% selectivity towards CO in the RWGS reaction. Supporting Ni₁₂P₅ on SiO₂ significantly enhanced the catalytic activity, with the optimized 10.4 wt% Ni₁₂P₅/SiO₂ sample achieving a maximum rate of 960 ± 12 mmol g_cat⁻¹h⁻¹ and >99.7% selectivity. This is significantly higher than the rate reported for commercial iron-chrome based catalyst (63 mmol g_cat⁻¹ h⁻¹ under the same conditions). The catalyst demonstrated excellent stability over eight cycles and 100 hours of continuous operation. Isotope tracing confirmed that CO originated from the CO₂ feedstock. XAS and DRIFTS studies revealed the importance of the linearly bonded nickel-carbonyl-dominated reaction pathway, attributed to the ensemble effect of phosphorus atoms, resulting in the formation of well-dispersed, low-coordinate Ni nanoclusters. This unique pathway leads to near 100% selectivity towards CO, regardless of the CO₂/H₂ ratio or reaction temperature, in contrast to conventional nickel catalysts. This near-unity selectivity is attributed to the weak binding energy of linearly bonded CO compared to bridge-bonded CO. The photothermal nature of the catalysis was confirmed by the exponential relationship between CO production rate and light intensity, with the estimated local temperature reaching 401 °C. The Co₂P analog also showed high activity and selectivity, supporting the generalizability of the metal phosphide approach.

The high activity and near-unity selectivity of Ni₁₂P₅ and Co₂P for CO₂ hydrogenation to CO, achieved through photothermal catalysis, demonstrate the potential of metal phosphides as highly efficient and stable catalysts for this important reaction. The unique linearly bonded nickel-carbonyl-dominated reaction pathway, facilitated by the dispersed Ni nanoclusters, allows for high CO selectivity. The independence of selectivity from reaction conditions (CO₂/H₂ ratio and temperature) is a significant advantage, distinguishing these metal phosphide catalysts from other nickel-based catalysts. The photothermal mechanism, enabled by the broad light absorption of these materials, allows for efficient conversion of light energy into heat, driving the catalytic reaction. The findings are significant for advancing sustainable CO₂ conversion technologies, providing a cost-effective and high-performance catalytic material for the reverse water-gas shift reaction. Further exploration of other transition metal phosphides could lead to the discovery of even more efficient catalysts.

This study successfully demonstrated the high-performance photothermal catalytic capability of Ni₁₂P₅ for the reverse water gas shift reaction, achieving a remarkable CO production rate and near-unity selectivity. The key to this success lies in the unique structural features of metal phosphides, specifically the well-isolated Ni nanoclusters, leading to a linearly bonded nickel-carbonyl-dominated reaction pathway. The excellent light-harvesting capability across the solar spectrum further enhanced the photothermal activity. The success with Co₂P suggests that transition metal phosphides represent a promising class of catalysts for efficient and sustainable CO₂ conversion technologies. Future research can explore the optimization of metal phosphide catalysts for different reaction conditions and the integration of these catalysts into practical CO₂ conversion systems.

The study primarily focused on Ni₁₂P₅ and Co₂P, and further investigation is needed to determine the generality of this approach across a broader range of transition metal phosphides. While the long-term stability was impressive, even longer-term studies under various conditions would enhance confidence in the stability assessment. The ASPEN Plus estimation of local temperature is a theoretical calculation based on equilibrium conditions, and the actual local temperature during reaction may differ. The study mainly used batch and flow reactors, and scaling up to larger industrial-scale reactors may present its own challenges.