The team set out to investigate the stability of concentrated citrate-based Co–Mo and Ni–Mo impregnation solutions, focusing on the composition of precipitates that form unintentionally when citric acid concentrations are insufficient. This is a practical problem of considerable industrial weight. In catalyst preparation, concentrated impregnation solutions are prized because they allow high metal loadings in a single impregnation step, saving time and energy. But when the solution chemistry is not properly balanced, solids can drop out of solution, and until now there has been genuine uncertainty about what those solids actually are and whether they help or harm the catalyst that eventually results.

To answer these questions, the researchers prepared a series of solutions and precipitates and subjected them to a battery of characterization techniques: X-ray diffraction to identify crystalline phases, infrared spectroscopy to probe molecular structures, scanning electron microscopy paired with energy-dispersive X-ray spectroscopy to examine morphology and elemental composition, and X-ray photoelectron spectroscopy to determine surface chemical states. This multi-technique approach allowed them to pin down the identity of the offending solids with unusual confidence. The verdict was striking: the dominant compounds in the precipitates are Anderson-type heteropoly compounds, a distinctive family of polyoxometalates in which a central heteroatom is encased by a ring of six molybdenum oxide octahedra.

The chemistry turned out to differ sharply depending on the metal system involved. In nickel–molybdenum citrate solutions, the precipitates consist predominantly of Anderson-type ammonium 6-molybdonickelate, a compound in which nickel sits at the center of the molybdenum oxide cage. The cobalt–molybdenum system proved more nuanced. In the absence of alumina, cobalt–molybdenum citrate solutions form sparingly soluble polymolybdates rather than well-defined heteropoly compounds. However, when the solution contacts γ-alumina, the standard support material for hydrotreating catalysts, Anderson-type heteropoly compounds emerge, specifically ammonium 6-molybdocobaltate and ammonium 6-molybdoaluminate. The appearance of the aluminum-containing species is particularly telling, because it signals that the acidic citrate solution is dissolving aluminum from the support itself, a ligand-promoted dissolution process that has been documented in molybdate-on-alumina systems but whose consequences for citrate-based impregnation had not been fully mapped.

Why does citric acid concentration matter so much? The answer lies in competition for metal ions. Citrate is a powerful chelating agent, meaning it wraps around cobalt and nickel ions to form soluble coordination complexes. When enough citric acid is present, essentially all of the promoter metal is held in solution as citrate complexes, and the molybdate species remain dispersed as well. When citric acid falls short, free nickel or cobalt ions become available to react with molybdate anions, nucleating the Anderson-type heteropolyanions that then crystallize as ammonium salts. In effect, citric acid acts as a chemical referee that keeps the players separated; remove the referee, and the metals combine into new compounds on their own schedule, not the manufacturer’s.

The crucial next question was what these precipitates do to catalyst performance. The researchers evaluated the resulting catalysts in the hydrotreating of straight-run gas oil, the sulfur-rich fraction that feeds directly into diesel production. The results upended the intuitive assumption that precipitated solids are simply a defect to be avoided. During sulfidation, the activation step in which the oxide precursors are converted into the active molybdenum sulfide phases, the precipitated heteropoly compounds are transformed into sulfide species and may contribute to the overall hydrodesulfurization performance. In other words, the solids are not inert contaminants; they are convertible precursors that join the active phase population.

This finding reframes the entire problem. Catalysts prepared from solutions in which precipitate formation occurred showed higher activity, and the authors attribute this advantage mainly to increased actual metal loading. When solids form, they still end up deposited on the support during impregnation, carrying their full complement of molybdenum and promoter metal with them. The net effect is that more of the intended metal charge is retained in the final catalyst rather than being lost or unevenly distributed. The heteropoly compounds, once sulfided, deliver their metals into the Co–Mo–S or Ni–Mo–S active phases that perform the actual desulfurization chemistry.

At the same time, the study does not suggest that precipitation should be embraced as a manufacturing strategy. Solution stability remains a quality-control benchmark for reproducible catalyst production, because uncontrolled precipitation can vary batch to batch and complicate the uniform distribution of metals across support pellets. What the work provides instead is mechanistic clarity: the higher activity of precipitate-containing catalysts stems from metal loading rather than from any intrinsic superiority of the heteropoly-derived sulfide phases. That distinction matters for process engineers deciding whether to tune citric acid concentration upward to keep solutions clear or to accept and manage the solids that form.

The broader significance of this research extends across the catalyst preparation community. Citric acid is one of several chelating agents, alongside EDTA and nitrilotriacetic acid, used to enhance hydrotreating catalysts, and the literature stretching back decades documents its promotional effects on the active NiMoS and CoMoS phases. By identifying the specific Anderson-type heteropoly compounds that appear when citrate is insufficient, the Novosibirsk team has given researchers a concrete chemical handle for monitoring and diagnosing solution instability. Techniques such as Raman spectroscopy and X-ray diffraction can now be aimed at known targets, ammonium 6-molybdonickelate in Ni–Mo systems and ammonium 6-molybdocobaltate or 6-molybdoaluminate in Co–Mo systems contacting alumina, rather than treating precipitates as an undifferentiated nuisance.

For an industry under pressure to produce ultra-low-sulfur diesel with ever-greater efficiency, the message is one of controlled chemistry rather than luck. The boundary between a clear, stable impregnation solution and a precipitating one is a citric acid stoichiometry boundary, and on either side of that boundary the resulting catalysts differ in measurable, predictable ways. The Boreskov Institute team’s careful coupling of solution chemistry, solid-state characterization, and realistic hydrotreating tests demonstrates that even the mistakes in catalyst preparation have molecular identities worth understanding. In the quest for cleaner fuels, it turns out that the solids you did not plan for can still pull their weight, provided you know exactly what they are.

Subject of Research: Effect of citric acid concentration on the stability of citrate-based Co-Mo and Ni-Mo impregnation solutions and the performance of hydrotreating catalysts

Article Title: Citrate Impregnation Solutions for the Preparation of Co-Mo/Ni-Mo Hydrotreating Catalysts: How Does Citric Acid Concentration Affect Solution Stability and Catalyst Performance

Article References: Budukva, S. V., Parfenov, M. V., Revyakin, M. E., Suprun, E. A., Bykova, E. S., Uvarkina, D. D., & Kazakov, M. O. (2026). Citrate Impregnation Solutions for the Preparation of Co-Mo/Ni-Mo Hydrotreating Catalysts: How Does Citric Acid Concentration Affect Solution Stability and Catalyst Performance. Catalysis Letters, 156(10), Article 291. https://doi.org/10.1007/s10562-026-05535-w

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05535-w

Keywords: hydrotreating catalysts, citric acid, heteropoly compounds, hydrodesulfurization, impregnation solution, Co-Mo catalysts, Ni-Mo catalysts, Anderson-type polyoxometalates, alumina support, sulfidation, catalyst preparation, straight-run gas oil

The team, led by Xiaoxue Liu, Yazhong Chen, Haifeng Wu, Xiaolin Sun, Bing Li, Xueping Wu, and Xianlong Zhang, prepared a family of catalysts labeled Mnx/SFCCC, where x denotes the weight percent of manganese loaded onto the support. The support, SFCCC, is spent fluid catalytic cracking catalyst, the used zeolite-based material that petroleum refineries discard by the ton once it loses activity in cracking units. Rather than letting this spent material go to landfill, the researchers repurposed it as a scaffold for manganese oxide, dramatically cutting the cost of the final catalyst while giving the waste a second life.

Because the way a catalyst is made can matter as much as what it is made of, the group synthesized their materials by three distinct routes: in situ deposition, precipitation-deposition, and incipient wetness impregnation. In situ deposition forms manganese oxide directly on the support surface under reaction conditions, precipitation-deposition builds the oxide through controlled precipitation onto the support, and incipient wetness impregnation soaks the support pores with a precursor solution before drying and calcination. Each technique distributes the manganese differently, and those differences in dispersion, particle size, and surface chemistry translate directly into differences in catalytic behavior.

To understand why one preparation outperformed the others, the researchers characterized their samples with an extensive toolkit. Low-temperature nitrogen adsorption measured surface area and pore structure, X-ray diffraction and scanning electron microscopy probed crystalline phase and morphology, hydrogen temperature-programmed reduction and oxygen temperature-programmed desorption assessed redox behavior and oxygen mobility, and X-ray photoelectron spectroscopy and electron paramagnetic resonance revealed surface oxidation states and defect sites. Inductively coupled plasma optical emission spectrometry confirmed the actual manganese loading in each batch. Together these measurements connected synthetic route, physical structure, and chemical functionality in a single coherent picture.

The performance results were striking. The catalyst prepared by in situ deposition with 14 weight percent manganese, Mn14/SFCCC-SP, delivered the best outcomes of the entire series. In static testing with an initial formaldehyde concentration of 4.5 parts per million, it removed 90.0 percent of the gas within 120 minutes at room temperature. Under dynamic conditions in a fixed-bed reactor, more representative of a real air purification device, it achieved greater than 95 percent removal at a gas hourly space velocity of 34,000 milliliters per gram per hour with an initial formaldehyde concentration of 7.0 parts per million. The catalyst also showed good stability and consistent performance across repeated use cycles, a critical requirement for any practical air-cleaning product.

The characterization data explain this superiority. Mn14/SFCCC-SP possesses a high specific surface area of 158 square meters per gram together with substantial pore volume, providing abundant sites where formaldehyde molecules can adsorb and react. The spent FCC support itself contributes porosity and a chemically heterogeneous surface, while the in situ deposition method deposits manganese oxide in a highly dispersed, well-contacted form rather than in bulky crystalline aggregates that bury active sites. High dispersion means more manganese atoms sit at the interface where gas molecules can reach them.

Even more important is the catalyst’s oxygen chemistry. X-ray photoelectron spectroscopy and electron paramagnetic resonance showed that the best sample carries a high concentration of surface-adsorbed oxygen species and oxygen vacancies, defects in the oxide lattice where an oxygen atom is missing. These vacancies are generated through the Mn4+/Mn3+ redox couple: when manganese cycles between its +4 and +3 oxidation states, lattice oxygen is consumed and replenished, continuously creating and refilling vacancy sites. The redox couple also gives the material good low-temperature reducibility, meaning it can activate oxygen molecules from air and transfer them to adsorbed formaldehyde even without external heating.

This mechanism sits at the heart of why the result matters. In the Mars-van Krevelen-type process that governs manganese oxide catalysts, formaldehyde first adsorbs on the surface, reacts with reactive oxygen to form intermediates such as formate and dioxymethylene species, and is finally oxidized to carbon dioxide and water, while gas-phase oxygen refills the vacancies the reaction consumed. A catalyst rich in vacancies and easily exchanged oxygen keeps this cycle running briskly at ambient temperature. The Hefei team’s evidence that the Mn4+/Mn3+ redox couple generates the vacancies that drive the reaction provides a clear design rule: maximize the interfacial contact between well-dispersed manganese oxide and a porous, defect-friendly support.

The choice of support is what makes the work economically compelling. Spent FCC catalysts are generated worldwide in enormous quantities, and disposal is an ongoing cost and environmental burden for refiners. Previous studies have explored supports ranging from activated carbon and diatomite to palygorskite, halloysite, and montmorillonite, often with excellent catalytic results but at a price that hinders scale-up. Using a pre-existing industrial waste stream as the support flips the cost equation: the raw material is essentially free, widely available, and already engineered to have favorable particle properties from its first life in the refinery. The authors specifically highlight that SFCCC was chosen to significantly reduce the cost of the catalyst, and the strong performance of the in situ deposition route shows that low cost need not come at the expense of activity.

For consumers, the implications are tangible. A room-temperature formaldehyde oxidation catalyst that maintains over 95 percent removal under demanding flow conditions could be integrated into air purifiers, ventilation systems, and building materials, continuously eliminating a carcinogenic pollutant rather than temporarily trapping it. Because the active component is abundant manganese rather than precious platinum, silver, or gold, the materials cost of deployment falls by orders of magnitude. And because the support is recycled refinery waste, the catalyst’s own footprint shrinks, turning one industrial waste problem into a solution for another environmental one. The study, published in Catalysis Letters, demonstrates that careful attention to preparation method, manganese loading, and support chemistry can transform a discarded powder into a high-performing environmental catalyst. As indoor air quality gains attention as a public health issue, such cheap, stable, and efficient room-temperature catalysts may soon move from the laboratory bench into the devices quietly cleaning the air in homes, offices, and schools.

Subject of Research: Room-temperature catalytic oxidation of formaldehyde using manganese oxides supported on spent FCC catalyst

Article Title: Room-Temperature Catalytic Oxidation of Formaldehyde Over Manganese Oxides Supported on SFCCC Prepared via Different Methods

Article References: Liu, X., Chen, Y., Wu, H., Sun, X., Li, B., Wu, X., & Zhang, X. (2026). Room-Temperature Catalytic Oxidation of Formaldehyde Over Manganese Oxides Supported on SFCCC Prepared via Different Methods. Catalysis Letters, 156(10), Article 279. https://doi.org/10.1007/s10562-026-05513-2

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05513-2

Keywords: formaldehyde, manganese oxides, spent FCC catalyst, room temperature catalysis, oxygen vacancies, indoor air purification, catalyst preparation, redox couple, waste recycling, Catalysis Letters, Room-Temperature, Catalytic