An autocatalyst is the platinum-, palladium- and rhodium-containing catalyst inside a vehicle's catalytic converter that turns exhaust carbon monoxide, unburned hydrocarbons and nitrogen oxides (NOx) into carbon dioxide, water and nitrogen. It is not "autocatalysis," the unrelated chemistry term for a reaction accelerated by one of its own products. Johnson Matthey put global automotive demand for platinum, palladium and rhodium combined at 12,455,000 troy ounces in 2025, and this page sets out which metal does which chemical job, device by device, why rhodium has no substitute, and how many grams of each metal a vehicle carries.
What Is an Autocatalyst?#
An autocatalyst and a catalytic converter are the same emissions-control device viewed two ways: "autocatalyst" names the platinum-group-metal catalyst itself, and "catalytic converter" names the exhaust component that houses it. The catalyst sits as a washcoat, a thin porous layer of alumina or ceria loaded with platinum, palladium or rhodium particles, on the surface of a ceramic or metal honeycomb through which exhaust gas flows. That honeycomb structure gives the washcoat a large surface area in a small volume, so a few grams of metal contact billions of exhaust molecules a second. Vehicle manufacturers buy autocatalysts as finished parts from a small number of specialist suppliers, one of many uses of platinum group metals across industry.
Autocatalyst vs Autocatalysis: Two Different Things#
Autocatalysis is a chemical reaction accelerated by one of its own products, a general chemistry concept with no vehicle-specific meaning. Wikipedia's top-ranking "Autocatalysis" article covers that sense: a reaction such as the hydrolysis of an ester, where the acid produced speeds up the same hydrolysis, is autocatalytic because the product acts as its own catalyst. An autocatalyst, by contrast, is a manufactured part containing platinum, palladium or rhodium, and nothing in a three-way catalyst catalyzes itself. The two terms share a root word and nothing else, and confusing them sends a reader looking for exhaust chemistry into a page about reaction kinetics.
Three-Way Catalyst: How Platinum, Palladium and Rhodium Work Together#
A three-way catalyst (TWC) oxidizes carbon monoxide and hydrocarbons and reduces NOx simultaneously, using palladium and rhodium, with platinum where substituted in, on a honeycomb washcoat. "Three-way" names the three reactions it runs at once, not three metals, though a modern gasoline TWC typically does use all three. The reactions only run together within a narrow window around stoichiometric combustion, the exact air-to-fuel ratio at which fuel burns completely with no leftover oxygen or unburned fuel. A closed-loop oxygen sensor upstream of the catalyst reads exhaust oxygen continuously and signals the engine computer to hold combustion inside that window, because a lean (oxygen-rich) or rich (fuel-rich) excursion lets one of the three reactions fail even while the other two continue. This lambda-controlled design is why gasoline engines, which run near stoichiometric, use a TWC, while diesel engines, which always run lean, need a different device stack described below. The full definition of a three-way catalyst covers its washcoat chemistry in more depth.
Device by Device: TWC, DOC, DPF, SCR, ASC and LNT#
Table 1 lists six autocatalyst devices: which engine type each is fitted to, which platinum group metal, if any, it uses and what it does. A gasoline vehicle typically carries only a TWC, sometimes paired with a gasoline particulate filter (GPF) coated with TWC-type washcoat; a diesel vehicle carries a chain of devices because lean combustion cannot reduce NOx the way a TWC does.
| Device | Fitted to | PGM used | Function |
|---|---|---|---|
| Three-way catalyst (TWC) | Gasoline engines | Palladium + rhodium, platinum where substituted | Oxidizes CO and hydrocarbons, reduces NOx, at stoichiometric combustion |
| Diesel oxidation catalyst (DOC) | Diesel engines | Platinum-dominant with palladium | Oxidizes CO, hydrocarbons and NO to NO2, feeding the DPF and SCR downstream |
| Diesel particulate filter (DPF) | Diesel engines | Platinum + palladium washcoat | Traps soot; the PGM coating assists passive regeneration |
| Selective catalytic reduction (SCR) | Diesel engines | None | Reduces NOx to nitrogen using urea (AdBlue) over vanadium oxide or zeolite catalysts |
| Ammonia slip catalyst (ASC) | Diesel engines, after SCR | Platinum, very low loading | Oxidizes ammonia slip that passes the SCR unreacted |
| Lean NOx trap (LNT) | Diesel engines (SCR alternative) | Platinum + rhodium, with a barium oxide adsorbent | Stores NOx during lean running, reduces it in brief rich excursions |
The SCR unit is the exception worth marking: it uses no platinum group metal at all, running instead on vanadium oxide or copper- or iron-substituted zeolites with a urea dosing system, according to the US Department of Energy's 2016 PGM light-duty vehicle model.
Why Rhodium Cannot Be Substituted#
Rhodium is the three-way catalyst's NOx reduction metal, with no commercial substitute, because its electron levels transfer charge into NO's antibonding orbital in a way platinum and palladium's do not. In rhodium, the uppermost occupied electron levels sit above the partially vacant 2π* antibonding orbital of the nitric oxide molecule, so charge flows into that orbital, weakens the nitrogen-oxygen bond and lets it dissociate into nitrogen and oxygen atoms, according to the Open University's OpenLearn explainer on the three-way catalytic converter. Platinum and palladium have vacant metal levels that sit below that same orbital instead, so charge drains out of it and the bond strengthens rather than breaks. That single electronic difference is why engineers cannot simply swap in more platinum or palladium to do rhodium's job: the mechanism itself, not just the cost or supply of rhodium, is metal-specific. Rhodium also happens to be one of the most refractory of the three metals under close-coupled catalyst conditions, which is a separate reason it survives the hottest position in the exhaust line.
Platinum-for-Palladium Substitution in Gasoline Catalysts#
Platinum and palladium substitute for each other in gasoline three-way catalysts on an almost 1:1 gram basis, limited mainly by palladium's better thermal durability at the hottest, close-coupled position. The US Department of Energy's 2016 PGM light-duty vehicle model states plainly that gram-for-gram substitution between the two metals is technically feasible and that the choice mainly comes down to relative cost, and the World Platinum Investment Council confirmed on 24 January 2024 that platinum and palladium "can be substituted on an almost 1:1 basis in autocatalysis." The constraint is heat: palladium tolerates the highest exhaust temperatures better than platinum, so substitution happens more easily in the cooler underfloor brick than in the close-coupled brick nearest the engine. Substitution also runs against a structural limit that has nothing to do with either metal's chemistry, described next. Platinum-palladium substitution in autocatalysts covers the mechanism in more depth.
Reverse Substitution: Back Toward Palladium#
WPIC (September 2025) expects reverse substitution, palladium back into platinum's role, to reach 250,000 ounces by 2029, after platinum-for-palladium substitution added an estimated 720,000 ounces of platinum demand in 2024, according to Metals Focus data cited in the same WPIC report. Substitution is structurally slow in either direction: it occurs almost exclusively on new vehicle models, around 15% of the market in any given year according to WPIC's January 2024 analysis, and once a platform adopts a metal mix it stays locked in for that platform's roughly seven-year production life because of the cost and risk of changing an already-running line. That lock-in is why the direction of substitution can reverse for years before it shows up broadly across the vehicle fleet, and why a platform decision made today still carries its metal choice into the early 2030s.
How Much Platinum, Palladium and Rhodium Is in an Autocatalyst?#
A medium US gasoline light-duty vehicle carried an estimated 6.21 grams of platinum, palladium and rhodium in 2025 (0.35 g platinum, 5.52 g palladium, 0.35 g rhodium); a comparable diesel light-duty vehicle carried 8.51 grams, mostly platinum, per the US Department of Energy's 2016 PGM light-duty vehicle model. Table 2 sets the two vehicle types side by side. These figures are a 2016 projection for the model year 2025, not an observed measurement of any single vehicle sold that year, because catalyst formulations are commercially confidential and no analyst publishes an audited per-vehicle loading.
| Vehicle type (2025, medium case) | Total PGM (g) | Platinum (g) | Palladium (g) | Rhodium (g) |
|---|---|---|---|---|
| US gasoline light-duty vehicle | 6.21 | 0.35 | 5.52 | 0.35 |
| US diesel light-duty vehicle | 8.51 | 6.32 | 1.86 | 0.32 |
The gasoline vehicle's loading is almost 89% palladium by mass, while the diesel vehicle's is almost 74% platinum, a direct reflection of each engine's exhaust chemistry: palladium dominates the stoichiometric TWC and platinum dominates lean diesel oxidation. Within a diesel system specifically, a further choice moves the total loading by more than double, covered next.
What Sets the Loading: Diesel's SCR-or-LNT Choice#
A diesel engine's choice between selective catalytic reduction (SCR) and a lean NOx trap (LNT) swings a vehicle's PGM content by more than 2 to 1, because SCR uses no platinum group metal at all and removes rhodium from the system entirely, while an LNT still needs platinum and rhodium. Table 3 shows the swing on a 3.0-liter, 11.7-liter-catalyst diesel system, drawn from the same DOE model, which assumed an 80% SCR, 20% LNT mix across the US diesel fleet.
| NOx route (3.0 L diesel, 11.7 L catalyst) | Platinum (g) | Palladium (g) | Rhodium (g) | Total PGM (g) |
|---|---|---|---|---|
| LNT | 13.3 | 2.14 | 1.88 | 17.3 |
| SCR only | 5.79 | 2.14 | 0.00 | 7.93 |
| 80% SCR / 20% LNT blend | 7.29 | 2.14 | 0.38 | 9.81 |
An LNT-equipped system carries 17.3 grams of platinum, palladium and rhodium against 7.93 grams for an SCR-only system on the same displacement, a 2.2-times difference driven almost entirely by rhodium and platinum, since SCR needs neither the barium oxide NOx-storage chemistry nor the rhodium reduction step an LNT requires.
Ruthenium, Iridium and Osmium: Why They Are Not Used in Autocatalysts#
Of the six platinum group metals, only platinum, palladium and rhodium appear in vehicle emissions catalysts; ruthenium, iridium and osmium carry no automotive line in Johnson Matthey's demand tables. The reasons differ by metal, listed below.
- Ruthenium: Johnson Matthey's ruthenium demand table splits 2026 forecast demand of 1,193,000 ounces into chemical, electrical and electronics, electrochemical and other categories, with no automotive line at all.
- Iridium: Johnson Matthey's iridium demand table shows the same pattern, chemical, electrical and electronics, electrochemical and other, again with no automotive category; iridium's one automotive-adjacent role is spark plug electrodes, a durable-electrode application rather than catalysis, and it is not used inside a catalytic converter.
- Osmium: Johnson Matthey's PGM Market Report tracks only five metals, platinum, palladium, rhodium, ruthenium and iridium, and does not track osmium at all. The reason generally given, though not independently verified to a primary citation in this research, is that osmium oxidizes in hot, oxygen-rich exhaust to osmium tetroxide, a volatile and acutely toxic compound that would disqualify it from any open exhaust application.
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The Global Autocatalyst Market for PGMs#
Johnson Matthey put global automotive PGM demand, platinum, palladium and rhodium combined, at 12,455,000 ounces in 2025, forecasting an 11,911,000-ounce, -4.4% decline for 2026 as internal-combustion vehicle production falls. Rhodium alone accounted for 899,000 of those 2025 ounces, an 84.6% automotive share of total rhodium demand, just under Johnson Matthey's own quoted 85% to 95% band for rhodium's automotive share over the past decade. The full autocatalyst demand for platinum and palladium forecast, including the regional breakdown behind these totals, sits on its own page rather than here, since this page's job is the chemistry and engineering, not the market model. Both metals also trade on live spot markets: current platinum price today and palladium price today update continuously from over-the-counter quotes.
Selling, Recycling and Researching Autocatalysts#
Autocatalysts leave the vehicle fleet through scrap and recycling, which this page covers only by pointing to the pages that value and process them.
Selling a Catalytic Converter for Scrap#
Selling a used catalytic converter for scrap depends on its PGM content and grade, covered in full on DailyPlatinum's scrap pages, not here. The exact grams of how much platinum, palladium and rhodium is in a catalytic converter vary by device type and vehicle, following the same device logic set out above, and catalytic converter scrap prices turns that content into a payout. A buyer's first question, how much a catalytic converter is worth, depends on both figures together.
Autocatalyst Recycling#
Spent autocatalysts are the largest secondary source of platinum, palladium and rhodium, returning metal to the market through decanning, milling, smelting and refining, according to Johnson Matthey's May 2026 PGM Market Report. The full process, from decanning a converter to a refined bar, is set out on catalytic converter recycling, part of the wider scrap platinum group metals recovery chain that also includes jewelry and industrial scrap.
Related PGM Market and Application Pages#
Autocatalyst figures on this page draw on the same houses that publish the wider PGM market, and the PGM market reports: WPIC, Johnson Matthey and Metals Focus page lists what each one covers and how often it updates.
Autocatalyst FAQ#
How much rhodium is in a three-way catalyst?#
Rhodium makes up roughly 5% to 7% of a three-way catalyst's PGM mass, despite trading at four to six times the platinum price, because no substitute exists for its NOx-reduction role. That small mass share, combined with rhodium's 84.6% automotive demand share industry-wide, is why rhodium supply swings move autocatalyst economics more than either platinum's or palladium's larger gram counts.
Can platinum replace palladium in a catalytic converter?#
Platinum can replace palladium in a gasoline catalytic converter on an almost 1:1 gram basis, but only where the position runs cool enough for platinum's lower thermal durability. The close-coupled position nearest the engine stays palladium's stronghold for this reason, while the cooler underfloor brick is where platinum substitution happens first.
Why don't autocatalysts use ruthenium or iridium?#
Ruthenium and iridium are not used in autocatalysts because Johnson Matthey's demand tables carry no automotive category for either metal. Iridium appears elsewhere on the vehicle, in spark plug electrodes, but that is a durable-electrode application, not catalysis, and neither metal sits inside a catalytic converter's washcoat.
What are platinum's, palladium's and rhodium's symbol, atomic number and CAS number?#
Platinum has the symbol Pt, atomic number 78, CAS 7440-06-4; palladium Pd, atomic number 46, CAS 7440-05-3; rhodium Rh, atomic number 45, CAS 7440-16-6. Each element's fuller profile, including density, melting point and other industrial uses, is on platinum (Pt), element 78, palladium (Pd), element 46 and rhodium (Rh), element 45.